an unforgettable Rock Werchter; Robert âUltimaâ Gehlhaar, for all the tears I shed laughing and for his help during the endless laser ordering process. Andriy,.
Arenberg Doctoral School of Science, Engineering & Technology Faculty of Engineering Department of Electrical Engineering
Performance improvement of organic solar cells by metallic nanoparticles
Bjoern NIESEN
Dissertation presented in partial fulfillment of the requirements for the degree of Doctor in Engineering
August 2012
Performance improvement of organic solar cells by metallic nanoparticles
Bjoern NIESEN
Jury: Prof. Dr. ir. Y. Willems, chair Prof. Dr. ir. P. Heremans, supervisor Prof. Dr. M. Van der Auweraer Prof. Dr. G. Borghs Prof. Dr. ir. J. Poortmans Prof. Dr. ir. P. Bienstman (Ghent University - imec) Prof. Dr. S. Maier (Imperial College London, UK)
Dissertation presented in partial fulfillment of the requirements for the degree of Doctor in Engineering
In collaboration with imec Kapeldreef 75 B-3001 Heverlee – Belgium
August 2012
© Katholieke Universiteit Leuven – Faculty of Engineering Kasteelpark Arenberg 10, B-3001 Heverlee (Belgium) Alle rechten voorbehouden. Niets uit deze uitgave mag worden vermenigvuldigd en/of openbaar gemaakt worden door middel van druk, fotocopie, microfilm, elektronisch of op welke andere wijze ook zonder voorafgaande schriftelijke toestemming van de uitgever. All rights reserved. No part of the publication may be reproduced in any form by print, photoprint, microfilm or any other means without written permission from the publisher. D/2012/7515/84 ISBN 978-94-6018-550-2
Acknowledgments I am very happy to acknowledge the people who, in one way or another, contributed to this thesis and made my four years at imec a highly enjoyable experience. First and foremost, I would like to thank my supervisor, Professor Paul Heremans for his guidance during the course of this thesis. I am especially indebted for his advice, not only on technical matters, but also on pursuing a career in research. I am grateful to the members of the examination board, Professors Mark Van der Auweraer, Gustaaf Borghs, Jozef Poortmans, Peter Bienstman, and Stefan Maier, for carefully reviewing the thesis manuscript and helping me to improve it with their constructive comments. I would also like to thank Professor Yves Willems for chairing both defenses. This thesis would not have been possible without the invaluable support I received from the members of the group formerly known as the Polymer and Molecular Electronics Group: I am greatly indebted to Barry, my daily supervisor, for sharing his vast knowledge during our, indeed, almost daily discussions, which were typically followed up by an email with the citations of the most important papers on the topic. Barry, your optimistic “don’t worry, this will work” attitude has always encouraged me to continue the quest for the plasmonically enhanced organic solar cell after failed experiments. Further, I would like to thank: Jan and Tom, for always being available when I had questions about purchase requests and HR-related issues; Inge and Chantal for very competently dealing with administrative issues; David, for measuring the optical constants of all materials used in this thesis, for writing the transfer matrix MATLAB script used in Chapter 5, and for finally taking me seriously; Sören, for many interesting dinner conversations and for the empirical proof that Dutch is basically German with somewhat different pronunciation; Luuk,
i
ii
ACKNOWLEDGMENTS
for keeping the labs running even in the most chaotic times, for introducing the mid-birthday to make even short-term internship students bring cakes, and for an unforgettable Rock Werchter; Robert “Ultima” Gehlhaar, for all the tears I shed laughing and for his help during the endless laser ordering process. Andriy, for his help in interpreting photoluminescence spectra and his support whenever the OMA system had a bad day; Afshin, for generously sharing the secrets and unwritten laws of polymer solution processing; Robert Müller, for making sure that I wouldn’t do anything stupid when mixing chemicals; Kurt. J., for his incredible machine; My cubicle mates, Peter and John, who both decorated the cubicle walls, and Manoj, who taught me the basic rules of Cricket. Thanks, Peter, for translating the abstract to Dutch! Paweł, thanks for all the friendly strangers I have met since you joined the group. Cédric, I’ve always expected the unexpected when talking to you. Thank you for still surprising me on a regular basis. Claudio, Dottore, your great sense of humor made having lunch with you the highlight of any day at imec. Grazie mille! Eszter, thanks for taming ICBA, and for your help while we simultaneously suffered through the process of writing a PhD thesis. Bregt, your unique way to be completely enthusiastic about your work while still being thoroughly relaxed is inspiring. Max, thanks for taking care of the OMA system. I am sure you will spend many happy hours behind the curtains. Jeffrey, thanks for spending your Sunday afternoons baking incredibly tasty cookies. I would like to thank Griet, Myriam, Erwin, Sarah, Marc, Tung Huei, Kjell, Karolien, Wan-Yu, Steve, Kris, Maarten Rockele, BKam, Adriantje, Ajay, Alexander, and the former PME Group members Dieter and Maarten Debucquoy, for the great atmosphere on the ground floor of imec 1 and for many pleasant chats during lunch, receptions, group events, and coffee breaks. During the course of this thesis, I had the pleasure to collaborate with many researchers from other groups at imec. I am especially grateful to: Pol Van Dorpe, for helping me whenever I had a question (and I had many) about any issue related to plasmonics or numerical simulations; Ounsi El Daif, for enabling regular interaction between the organic and the Si PV groups by coordinating the imec-internal PRIMA meetings; Jian Ye and Hilde Jans, for their advice on the synthesis of metal nanoparticles; Josine Loo and Alain Moussa, for pushing the SEM and AFM set-ups to the limit to image the tiny metal nanoparticles used for the near-field interaction study. Several results presented in this thesis were obtained in collaboration with external research groups. I would like to thank Eduard Fron for performing time-resolved fluorescence measurements and for his patience while explaining me how to analyze the resulting decay curves. I am grateful to Honghui Shen and Bjorn Maes for writing the COMSOL script on which the 3D numerical simulations shown in Chapters 2 and 3 are based on, and for many interesting
ACKNOWLEDGMENTS
iii
discussions about plasmonically enhanced solar cells. Special thanks go to Lianming Tong, who visited imec in February 2011 and helped me to transfer the hole-mask colloidal lithography process to organic semiconductor layers. Ich möchte mich von ganzem Herzen bei meinen Eltern, Geschwistern und Freunden bedanken. Ihr seid immer für mich da gewesen wenn mal wieder alle Experimente schief gegangen sind und habt mich jeweils entweder aus der Ferne (danke, Skype!) oder bei einem Besuch in Leuven wieder aufgemuntert. Anya, ich kann mich nicht genug für deine Unterstützung während der letzten vier Jahre bedanken. Ich freue mich auf viele weitere Jahre mit dir!
Abstract The aim of this thesis is to investigate the utilization of plasmonic metal nanostructures for efficiency enhancement of organic solar cells. This efficiency enhancement strategy exploits the strong near-field enhancement and highly efficient light scattering that originates from localized surface plasmon resonances (LSPRs) excited in metal nanostructures and leads to an increased absorption in the solar cell active layer. In the first part of this thesis, the near-field enhancement is systematically studied by employing a model system of metal nanoparticles (NPs) covered by thin-films of organic molecules. Absorption and both steady-state and transient photoluminescence measurements are used to probe the interactions between the LSPRs excited in the NPs and the excitons in the organic thin-film. By introducing a transparent spacer layer, the range of these near-field plasmon-exciton interactions is determined. The absorption enhancement is found to be accompanied by a strong reduction in exciton lifetime, which is detrimental to the device efficiency. Since the range of both interactions is comparable, isolating the NPs with a layer thick enough to prevent exciton quenching may simultaneously eliminate any beneficial effect due to the near-field absorption enhancement. These results demonstrate that the exploitation of near-field enhancement in organic solar cells is quite challenging. Far-field light scattering, on the other hand, shows promise for producing plasmonically-enhanced organic solar cells. This is demonstrated by equipping an organic solar cell with a plasmonic nanostructured Ag rear electrode. The LSPR of this electrode is tuned to the red absorption tail of the active layer and generates enhanced absorption by light scattering, as verified by experiments and numerical simulations. Due to this absorption enhancement, the plasmonic nanostructured cell exhibits an enhanced power conversion efficiency compared to an optimized, high performance organic solar cell.
v
Beknopte samenvatting Het doel van deze thesis is om het gebruik te onderzoeken van metalen nanostructuren voor de verbetering van de efficiëntie van organische zonnecellen. Deze efficiëntie verbeteringsstrategie gebruikt de sterke verbetering van het nabije veld en hoogefficiënte licht verstrooiing, afkomstig van lokale oppervlakte plasmon resonanties die geëxciteerd worden in metalen nanostructuren en die zal leiden tot een verhoogde absorptie in de actieve laag van de zonnecel. In het eerste deel zal de verbetering van het nabije veld systematisch bestudeerd worden door gebruik te maken van een model systeem van metaal nanopartikels die bedekt zijn met dunne films van organische moleculen. De interacties tussen de oppervlakte plasmons die exciteren in de nanopartikels en de excitons in de organische dunne film worden gemeten door absorptie enerzijds en steadystate en transitie fotoluminescentie anderzijds. Door de introductie van een transparante spacer laag kan het bereik van deze plasmon-excitons in het nabije veld bepaald worden. Er is vastgesteld dat de absorptie verbetering samenhangt met een sterke reductie van de exciton levensduur wat nadelig is voor de efficiëntie van de zonnecel. Wanneer het bereik van beide interacties gelijk is, kan het aanbrengen van een dikke laag rond de nanopartikels die de excitons behoedt voor quenching het voordelige effect van de verbeterde absorptie in het nabije veld teniet doen. Deze resultaten tonen aan dat de exploitatie van het verbeterde nabije veld in organische zonnecellen uitdagend is. Aan de andere kant is ook gebleken dat vorming van plasmonisch verbeterde zonnecellen ook gerealiseerd kan worden door licht verstrooiing in het verre veld. Hiervoor werd een zonnecel voorzien van een plasmonische nano gestructureerde zilver elektrode aan de achterkant. De lokale oppervlakte plasmon resonantie van deze elektrode is afgestemd op de uitloper in het rode absorptiespectrum van de actieve laag en genereert een verbeterde absorptie door licht verstrooiing zoals aangetoond wordt in experimenten en numerieke simulaties. Door deze absorptie verbetering is de plasmonische nano gestructureerde cel in staat om een verbeterde vermogen efficientie te genereren vergeleken bij een geoptimaliseerde, hoogperformante organische zonnecel.
vii
Abbreviations 2D 3D
two-dimensional three-dimensional
Ag Alq3 AM 1.5G Au
silver tris(8-hydroxyquinolinato)aluminium air mass 1.5 global gold
CuPc
copper(II) phthalocyanine
EQE
external quantum efficiency
FDTD
finite-difference time-domain
HCL HOMO Hyflon AD60X
hole-mask colloidal lithography highest occupied molecular orbital copolymer of 2 2 4-trifluoro-5-trifluorometoxy-1 3dioxole and tetra-fluoro-ethylene with a molar ratio of 0.6:0.4
IC ICBA IRF ISC ITO
internal conversion indene-C60 bis-adduct instrument response function intersystem crossing indium tin oxide
LSPR LUMO
localized surface plasmon resonance lowest unoccupied molecular orbital
MeO-TPD
N N N’ N’-tetrakis(4-methoxy-phenyl)benzidine
ix
x
ABBREVIATIONS
MoOx
molybdenum oxide
N2 NP
molecular nitrogen nanoparticle
O2
molecular oxygen
P3HT PCBM PDDA PEDOT:PSS
poly(3-hexylthiophene-2 5-diyl) [6 6]-phenyl C61 butyric acid methyl ester poly(diallyldimethylammonium chloride) poly(3 4-ethylenedioxythiophene): poly(styrenesulfonate) photoluminescence poly(methyl methacrylate) polystyrene 3 4 9 10-perylenetetracarboxylic-bisbenzimidazole
PL PMMA PS PTCBI
S Si SPP SubPc
singlet state silicon surface plasmon polariton chloroboron subphtalocyanine
T
triplet state
UV
ultraviolet
VR
vibronic relaxation
ZnO
zinc oxide
List of Symbols α
polarizability
η
solar cell power conversion efficiency
Γ0rad
radiative decay rate in absence of metal nanoparticles
ΓET−0
ΓET /Γ0rad
ΓET
non-radiative energy transfer rate to metal nanoparticles
Γnonrad
non-radiative decay rate
Γrad−0
Γrad /Γ0rad
Γrad
radiative decay rate
λ
wavelength
λLSPR
localized surface plasmon resonance wavelength
ω
angular frequency
τ
exciton lifetime
E
electric field
E0
electric field in absence of metal nanoparticles
ENP
electric field in presence of metal nanoparticles
k
wave vector
ε
relative permittivity
ε0
vacuum permittivity
εm
relative permittivity of the embedding medium xi
xii
LIST OF SYMBOLS
εNP
relative permittivity of the nanoparticle
Cabs
absorption cross-section
Cext
extinction cross-section
Csca
scattering cross-section
d
distance to metal nanoparticle surface
e
elementary charge
FF
fill factor
h
height of silver protrusions
J
current density
Jsc
short-circuit current density
k
wavenumber
l
multipole order
lmax
highest-order multipole
n
refractive index
Nph
photon flux density
P0
power density of incident light
Pabs
absorbed power per unit volume
Pmax
maximal power density
Q
normalized cross-section
q
quantum yield
q0
quantum yield in absence of metal nanoparticles
R
metal nanoparticle radius
s
center-to-center spacing of metal nanoparticles
t
organic layer thickness
V
voltage
Voc
open-circuit voltage
Contents Abstract
v
Contents
xiii
List of Figures
xvii
List of Tables
xxi
1 Introduction
1
1.1
Motivation
1.2
Organic molecules . . . . . . . . . . . . . . . . . . . . . . . . .
3
1.2.1
Molecular orbitals . . . . . . . . . . . . . . . . . . . . .
3
1.2.2
Optical transitions . . . . . . . . . . . . . . . . . . . . .
5
1.3
Organic solar cells . . . . . . . . . . . . . . . . . . . . . . . . .
8
1.4
Optical properties of metal nanoparticles . . . . . . . . . . . . .
12
1.4.1
Localized surface plasmon resonances . . . . . . . . . .
12
1.4.2
Light extinction by metal NPs . . . . . . . . . . . . . .
14
1.4.3
Metal NP-embedding medium Interactions
19
1.5
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . .
1
Plasmonic solar cells . . . . . . . . . . . . . . . . . . . . . . . . . 21 1.5.1
Plasmonic enhancement strategies . . . . . . . . . . . . . 21
xiii
xiv
CONTENTS
1.5.2
Literature review for plasmonic organic solar cells . . .
24
Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
2 Optical properties of metal nanoparticles covered by organic thinfilms
29
1.6
2.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
2.2
Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
2.3
Three-dimensional finite-element numerical simulations . . . . .
32
2.4
Results and discussion . . . . . . . . . . . . . . . . . . . . . . .
33
2.4.1
Morphology and optical properties of the Ag NP layer .
33
2.4.2
Excitation of multiple dipole resonances . . . . . . . . .
37
2.5
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3 Plasmon-exciton near-field interactions: absorption
43
3.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
43
3.2
Results and discussion . . . . . . . . . . . . . . . . . . . . . . .
44
3.2.1
Absorption enhancement in CuPc
44
3.2.2
Spectral dependence and influence of the permittivity . . 51
3.2.3
Effect of a spacer layer . . . . . . . . . . . . . . . . . . .
53
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
57
3.3
. . . . . . . . . . . .
4 Plasmon-exciton near-field interactions: emission
59
4.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
4.2
Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
60
4.3
Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
62
4.4
Results and discussion . . . . . . . . . . . . . . . . . . . . . . .
64
4.4.1
Analytical calculations . . . . . . . . . . . . . . . . . . .
64
4.4.2
Experimental results . . . . . . . . . . . . . . . . . . . . . 71
CONTENTS
4.4.3 4.5
xv
Comparison between experiment and theory . . . . . . .
77
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5 Organic solar cells with nanostructured metal rear electrode
83
5.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
83
5.2
Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
84
5.3
Results and discussion . . . . . . . . . . . . . . . . . . . . . . .
87
5.3.1
Reference device optimization . . . . . . . . . . . . . . .
89
5.3.2
Optical properties of the nanostructured Ag layer . . . .
92
5.3.3
Organic solar cells with nanostructured Ag rear electrode
97
5.4
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
6 General conclusions and outlook
103
Bibliography
107
Curriculum vitae
123
List of publications
125
List of Figures 1.1
Atomic orbitals . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
1.2
Bonding and antibonding molecular orbitals . . . . . . . . . . .
4
1.3
Jablonski diagram . . . . . . . . . . . . . . . . . . . . . . . . .
6
1.4
Schematic of photocurrent generation in an organic solar cell .
9
1.5
Current density-voltage characteristics of an organic solar cell .
10
1.6
Air mass 1.5 global spectrum . . . . . . . . . . . . . . . . . . . . 11
1.7
Schematic of a dipole localized surface plasmon resonance excited in a metal nanoparticle . . . . . . . . . . . . . . . . . . . . . . .
13
1.8
Relative permittivities of of gold and silver . . . . . . . . . . .
15
1.9
Dependence of the extinction of a silver nanoparticle on the permittivity of the embedding medium . . . . . . . . . . . . . .
16
1.10 Extinction, scattering and absorption efficiencies of spherical gold and silver nanoparticles . . . . . . . . . . . . . . . . . . . . . .
17
1.11 The scattering quantum yield for silver nanoparticles, depending on their radius and the permittivity of the embedding medium
18
1.12 Extinction spectra of spherical and rod-shaped gold nanoparticles 19 1.13 Electric field intensity near silver NPs . . . . . . . . . . . . . .
20
1.14 Design concepts for organic solar cells containing metal nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
1.15 Guided mode profiles in an organic solar cell
23
xvii
. . . . . . . . . .
xviii
LIST OF FIGURES
2.1
Structural formula of Hyflon AD60X . . . . . . . . . . . . . . .
2.2
Absorption spectra of several samples fabricated during a single run . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.3
Cross-section of the geometry used for 3D numerical simulations
32
2.4
Morphology of the silver nanoparticle layer . . . . . . . . . . .
33
2.5
Optical properties of the bare silver nanoparticle layer . . . . .
34
2.6
Absorption spectra of the silver nanoparticle layer covered by tthin-films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
Direct and diffuse transmission spectra of the silver nanoparticle layer covered by CuPc and SubPc . . . . . . . . . . . . . . . . .
36
Absorption spectra of neat SubPc thin-films and the silver nanoparticle layer covered by SubPc . . . . . . . . . . . . . . .
37
permittivities of silver and SubPc as well as the dipole polarizability of a spherical silver nanoparticle in SubPc . . . .
38
2.10 Results of three-dimensional numerical simulations of the silver nanoparticle layer covered by SubPc . . . . . . . . . . . . . . .
40
2.7 2.8 2.9
30
2.11 Simulated field distribution and charge density plots . . . . . . . 41 3.1
Experimentally measured and simulated absorption spectra of the silver nanoparticle layer covered by CuPc . . . . . . . . . .
45
Thickness dependence of the absorption enhancement in presence of silver nanoparticles . . . . . . . . . . . . . . . . . . . . . . .
47
3.3
Simulated electrical field intensity around a silver nanoparticle
48
3.4
Normalized measured and simulated absorption spectra of the silver nanoparticles covered by CuPc . . . . . . . . . . . . . . .
49
Simulated absorption spectra of silver nanoparticles and absorption enhancement in the embedding medium . . . . . . . . . . .
52
3.6
Relative permittivities of CuPc, SubPc, and Hyflon AD60X . .
53
3.7
Experimentally measured effect of a spacer layer on the absorption enhancement in SubPc and CuPc thin-films due to the presence of silver nanoparticles . . . . . . . . . . . . . . . . . . . . . . .
54
3.2
3.5
LIST OF FIGURES
3.8
Simulated effect of a spacer layer on the absorption in silver nanoparticles coated with SubPc and the absorption enhancement in SubPc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xix
56
4.1
Several steady-state photoluminescence spectra of a single sample 61
4.2
Schematic of the exciton decay processes in an isolated molecule and one that is near a metal nanoparticle . . . . . . . . . . . .
64
Calculated spectral dependence of decay rates and the emission quantum yield . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
Effect of the number of multipole modes taken into account when calculating decay rates . . . . . . . . . . . . . . . . . . . . . . .
66
Calculated distance dependence of decay rates and the emission quantum yield . . . . . . . . . . . . . . . . . . . . . . . . . . . .
67
Calculated spectral dependence of the decay rates and the emission quantum yield on the permittivity of the embedding medium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
68
Calculated dependence of the decay rates and the emission quantum yield on the nanoparticle radius . . . . . . . . . . . .
69
Comparison between the original and an extended Gersten-Nitzan model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
70
Absorption and scattering cross-sections of a silver nanoparticle depending on the permittivity of the embedding medium . . . .
70
4.3 4.4 4.5 4.6
4.7 4.8 4.9
4.10 Steady-state photoluminescence spectra of organic thin-films on glass and on the silver nanoparticle layer . . . . . . . . . . . . . . 71 4.11 Photoluminescence intensity and decay time ratios of samples with and without silver nanoparticles . . . . . . . . . . . . . . .
73
4.12 Absorption spectra of MeO-TPD, Alq3 , SubPc, and PTCBI thin-films deposited on glass and on the silver nanoparticle layer
74
4.13 Normalized time-dependent photoluminescence decay curves of thin-films deposited on glass and on the silver nanoparticle layer
76
4.14 Normalized steady-state photoluminescence spectra of SubPc and PTCBI thin-films . . . . . . . . . . . . . . . . . . . . . . .
78
4.15 Real part of the relative permittivities of MeO-TPD, Alq3 , SubPc, PTCBI, and Hyflon AD60X . . . . . . . . . . . . . . . . . . . .
79
xx
LIST OF FIGURES
5.1
Fabrication process flow for an organic solar cell containing a flat and a nanostructured rear electrode . . . . . . . . . . . . . . .
86
Comparison between the polystyrene bead distribution on PMMA and on the P3HT:ICBA active layer . . . . . . . . . . . . . . .
88
Short-circuit current density of reference and nanostructured organic solar cells . . . . . . . . . . . . . . . . . . . . . . . . . .
90
5.4
External quantum efficiency spectra of flat reference devices . .
90
5.5
Current density-voltage characteristics of reference and nanostructured organic solar cells . . . . . . . . . . . . . . . . . . . . . 91
5.6
Light scattering spectra of nanostructured layers on PMMA . .
93
5.7
Absorption spectrum of the P3HT:ICBA active layer . . . . . .
94
5.8
Comparison between the polystyrene bead distribution with a diameter of 140 and 200 nm on PMMA . . . . . . . . . . . . .
94
Measured and simulated light scattering spectra of nanostructured and flat silver layers on PMMA . . . . . . . . . . . . . .
95
5.10 The simulated electric field intensity around a silver protrusion of a nanostructured layer on PMMA . . . . . . . . . . . . . . .
96
5.11 External quantum efficiency, total reflectance, and light scattering spectra of organic solar cells with flat and nanostructured rear electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
98
5.12 Simulated electric field intensity around a silver protrusion of the nanostructured rear electrode . . . . . . . . . . . . . . . . . . .
99
5.13 Simulated absorption in the active layer for organic solar cells containing flat and nanostructured rear electrodes . . . . . . .
100
5.2 5.3
5.9
5.14 External quantum efficiency spectra of nanostructured organic solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
List of Tables 4.1
5.1
Fitting parameters for the photoluminescence decay curves of MeO-TPD, Alq3 , SubPc, and PTCBI thin-films . . . . . . . . .
75
Device parameters of reference and nanostructured organic solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
89
xxi
Chapter 1
Introduction 1.1
Motivation
The performance of photovoltaic devices has increased dramatically since the invention of the first solid state solar cell in 1954 [1, 2]. Recently, a record efficiency of 34% was achieved using a GaInP/GaInAs/Ge multijunction cell at 1 sun conditions [3]. As such multijunction cells are very expensive, the market for terrestrial, non-concentrator applications is dominated by the much cheaper wafer-based crystalline Si solar cells that exhibit cell efficiencies of up to 25% [4]. Despite the drastic reduction of Si solar cell module prizes from ∼4 $/Wp in the year 2000 down to ∼0.6-1.2 $/Wp in 2012 [5, 6], photovoltaic power generation is still not competitive with conventional energy sources in many countries. As a result, solar energy currently still accounts for only a small fraction of the total global energy production [5]. Moreover, a large share of the photovoltaics market growth in the last decade is due to government incentive programs [7, 8]. Therefore, to reach a significant market share, the price of solar cells has to be strongly reduced [9]. This issue has been addressed either by thinning down crystalline Si devices to reduce material costs or by employing active layers based on thin-films of materials that absorb light much more efficiently than Si. Both approaches do not necessarily lead to a reduced price of the electricity generated by these devices. Thinning down the crystalline Si active layer reduces the light absorption and thus the device performance, while the efficiency of thin-film solar cells is typically limited by the poor electrical properties of their active
1
2
INTRODUCTION
layer materials. Absorption enhancement strategies are therefore required in both cases: for crystalline Si solar cells to maintain a high efficiency while thinning down the active layer, for thin-film devices to enhance the absorption of active layers that are thin enough to allow for efficient charge extraction. Absorption enhancement strategies that have been proposed for this purpose include photonic crystals [10–13], optical cavities [14, 15], and plasmonic metal nanostructures [16–18]. In crystalline Si solar cells, surface recombination becomes increasingly important when thinning down the active layer, which is why it is crucial to combine light trapping strategies with highly effective surface passivation in this type of cells. Organic solar cells have attracted considerable interest during the last decade [19–21]. This thin-film solar cell technology employs organic semiconducting molecules to absorb light and generate charge carriers, in contrast to competing thin-film technologies such as solar cells based on amorphous and microcrystalline Si [22], cadmium telluride [23], and copper indium gallium selenide [24]. Organic semiconductors can be solution-processed at low temperatures, which allows for low-cost high-throughput fabrication of organic solar cells on flexible substrates [25]. Moreover, the chemical elements that are typically utilized to synthesize organic molecules are not toxic and do not suffer from limited availability as some of the inorganic materials (e.g. In, Cd, and Te). The two main challenges for the commercialization of organic solar cells are their limited efficiency and lifetime. Is has been estimated that organic solar cells reach grid-parity at 15% efficiencies and lifetimes of 20 years [26]. During the last few years, both parameters have been improved considerably, such that lifetimes of 5-10 years and efficiencies of over 10% have been reached [2, 3, 27–29]. A further improvement of the device performance is not only limited by the poor electrical properties of organic semiconductors but also by their relatively narrow absorption bands. For this reason, absorption enhancement schemes have to be specifically tailored to the organic semiconductors utilized in the active layer when applied in organic solar cells. In this thesis, the utilization of plasmonic metal nanostructures for efficiency enhancement of organic solar cells is investigated. In the remainder of this chapter, the theoretical concepts of light absorption and emission of organic molecules will be described, followed by an introduction to organic solar cell operation principles and fabrication methods, and the optical properties of metal nanoparticles. Further, several strategies to exploit plasmonic effects in solar cells will be discussed, including a literature review on plasmonic organic solar cells. Finally, an outline of this thesis is given at the end of this chapter.
ORGANIC MOLECULES
y
3
y
y z
s
z
z
x
z
x
x
px
y
py
x
pz
Figure 1.1: Atomic s orbital and three orthogonal p orbitals.
1.2
Organic molecules
The distinction between “organic” and “inorganic” chemical compounds originates from the believe that organic compounds can only be synthesized by living organisms. In 1828, Friedrich Wöhler proved this theory to be wrong by synthesizing urea from the inorganic salts potassium cyanate and ammonium sulfate in vitro. Nevertheless, the term “organic” is still commonly used to describe chemical compounds. There is no exact scientific definition of an organic compound. As a general rule, molecules containing carbon-carbon bonds are considered to be organic. Carbon-hydrogen bonds are also typical of organic compounds, with some exceptions (e.g. urea and Teflon). The field of organic electronics dates back to the mid-20th century, when organic compounds were found to conduct electricity [30]. Ever since, a large number of organic conductors and semiconductors has been identified and synthesized.
1.2.1
Molecular orbitals
In an isolated atom, wave functions, called atomic orbitals, can be used to describe the probability to find an electron at a given position around the nucleus [31–33]. In the absence of external electric or magnetic fields, the energy of an orbital is determined by its angular momentum and its principal quantum number that governs the size of the orbital and corresponds to the shell number in the Bohr atomic model. Atomic orbitals are categorized into several groups based on their angular momentum. The so-called s orbitals have no angular momentum due to their spherical symmetry and therefore they also posses the lowest energy (cf. Fig. 1.1).
4
INTRODUCTION
C
C C
C
* p
2py
2py
p Figure 1.2: Bonding (π) and antibonding (π ∗ ) molecular orbitals of an ethylene molecule, resulting from the interaction of the 2py atomic orbitals of two carbon atoms. The π orbital is filled with two electrons with opposite spins (indicated by arrows). The molecule and the atoms contain additional orbitals that are not shown.
Orbitals with one nodal plane, the p orbitals, posses an angular momentum and therefore have a higher energy. There are three orthogonal p orbitals with the same energy in the absence of external fields. Orbitals with two and three nodal planes, the d and f orbitals, respectively, have an even higher angular momentum and energy. The atomic orbitals are filled by electrons in a way that yields the lowest potential energy. Because electrons are fermions, the Pauli exclusion principle applies and each orbital can be occupied by maximally two electrons with opposite spins. The atoms of an organic molecule form covalent chemical bonds by sharing their electrons. A single bond between two atoms is formed when each of these atoms shares one of its electrons. When two or three electrons are shared, a double or triple bonds is formed, respectively. This interaction leads to the formation of molecular orbitals that arise from combinations of atomic orbitals. Two molecular orbitals are formed for each pair of interacting atomic orbitals: a bonding orbital with an energy that is lower than that of the atomic orbitals, and an antibonding orbital with a higher energy, as shown in Fig. 1.2. Due to this energy difference, the bonding orbital is filled first with the electrons of the atomic orbitals. Depending on the shape and the relative orientation of the two interacting atomic orbitals, molecular orbitals are formed that are either symmetrical (σ orbitals) or asymmetrical (π orbitals) with respect to rotation around the axis joining the two atomic nuclei.
ORGANIC MOLECULES
5
Double and triple bonds in organic molecules are formed by a σ and one or two π molecular orbitals. In molecules with multiple (i.e. double or triple) bonds, the energy difference between the bonding and antibonding π orbitals is typically smaller than that of the σ orbitals. Therefore, a bonding π orbital is usually the highest occupied molecular orbital (HOMO), with a lowest unoccupied molecular orbital (LUMO) that consists of an antibonding π ∗ orbital. There are, however, molecules where the HOMO consists of a nonbonding atomic orbital, for example a nonbonding p orbital of the oxygen in molecules containing carbonyl (C=O) groups. As electronic transitions can only occur from orbitals that are occupied by at least one electron into orbitals that are not already occupied by two electrons, many optical properties of organic molecules depend on the HOMO-LUMO energy gap. In molecules with alternating single and multiple bonds, the π orbitals overlap, such that they form a conjugated system with delocalized electrons. With increasing size of the conjugated system, the HOMO-LUMO gap decreases. In fully conjugated polymers such as poly(acetylene), the electrons could, in theory, be delocalized throughout the entire polymer chain and the HOMO-LUMO gap could thus become indefinitely small for large chain lengths. In reality, however, the non-perfect coplanarity of the polymer chain and other intrinsic effects limit the delocalization length such that the polymer exhibits semiconducting properties.
1.2.2
Optical transitions
A pair of electrons can either form a singlet (S) or a triplet (T) state depending on whether their spins are antiparallel and therefore result in a net spin of 0, or parallel with a net spin of 1, respectively [34, 35]. Two electrons occupying the same orbital must be in the S configuration due to the Pauli exclusion principle. When an organic molecule is in its electronic ground state, all bonding and nonbonding orbitals are typically filled with two electrons, whereas the antibonding orbitals of the valence shell remain empty. This means that the ground state is usually a singlet state, denoted by S0 . A notable exception is the O2 molecule with a T ground state. When a molecule absorbs a photon, an electron is excited from the HOMO to either the LUMO or an unoccupied orbital with a higher energy. As this excited electron does not share its orbital anymore, the Pauli exclusion principle does not apply. The pair of the excited electron and the one that remained in the HOMO can therefore form a S or T configuration. After a HOMO-LUMO transition, the electronic transition with the lowest energy difference, the molecule is thus either in the S1 or T1 state. The subscript indicates the energy ranking of the
6
INTRODUCTION
Sn Tn
S2 VR
IC
T2 ISC
T1
Phosphorescence
Fluorescence
Absorption
VR
S1
S0
Figure 1.3: Jablonski diagram of an organic molecule. Electronic states are depicted as solid lines whereas vibrational states are shown as dashed lines. Transitions between the states occur either radiatively (straight arrows) or non-radiatively (curved arrows).
excited state relative to S0 , such that the states with the next higher energies are denoted by S2 and T2 . The T1 state typically has a lower energy than the S1 state because the electrons in the T configuration are not allowed to occupy the same space and thus will feel less electron-electron repulsion. The intersystem crossing (ISC) between S and T states is spin-forbidden and requires spin-orbit coupling to conserve the total angular momentum. Intersystem crossing is typically only efficient in heavy atoms as the spin-orbit coupling strongly increases with increasing weight of the nucleus. Optical transitions are commonly depicted by a Jablonski diagram, as shown in Fig. 1.3, where the electronic states are shown as solid lines. Each electronic state comprises a manifold of vibrational states with an energy spacing in the
ORGANIC MOLECULES
7
order of 0.1 eV (dashed lines). The various transitions between electronic and vibrational states can be illustrated by the example of a molecule in the S0 ground state that absorbs a photon and undergoes a S0 → Sn transition, with n ≥ 1. According to Kasha’s rule that is applicable to most organic molecules, the molecule will non-radiatively relax to the vibrational ground state of S1 faster than any other measurable process. This relaxation includes vibronic relaxation (VR) to the vibrational ground state of Sn and the Sn → S1 internal conversion (IC) if n > 1. The molecule can then relax to S0 either radiatively by emitting a photon or nonradiatively by dissipating energy to the local environment. As the molecule was excited by absorbing a photon, the radiative decay is called photoluminescence (PL). The PL due to a direct S1 → S0 transition is called fluorescence, the PL originating from T1 → S0 via S1 → T1 ISC is called phosphorescence. Due to the ISC involved in phosphorescence, the lifetime of the T1 state is usually orders of magnitudes larger than that of S1 . Because PL typically occurs from the vibrational ground state, after the the excited molecule lost some of its energy due to VR, the PL emission band is usually red-shifted compared to the absorption band. This shift is known as Stokes-shift. An important figure of merit for PL is the emission quantum yield (q), which is given by q = Γrad /(Γrad + Γnonrad ) ,
(1.1)
where Γrad is the radiative decay rate of either fluorescence of phosphorescence, and Γnonrad is the sum of the decay rates of all non-radiative decay pathways. The S0 → S1 and S0 → T1 transitions can be described as the generation of an electrically neutral quasi-particle called Frenkel exciton, which consists of the excited electron in the LUMO that is bound by Coulomb interactions to the positively charged hole it left behind in the HOMO. In an isolated molecule, the exciton will eventually decay by recombination of the electron-hole pair, either radiatively or non-radiatively, as described above. In molecular media, the exciton can diffuse through the medium by hopping from molecule to molecule. This results in additional decay mechanisms such as exciton-exciton annihilation or dissociation into free charge carriers at the interface between two molecular media with an energy offset between their HOMO and LUMO levels. The latter is a basic operation principle of organic solar cells and will be discussed in the following section.
8
1.3
INTRODUCTION
Organic solar cells
Organic solar cells are thin-film photovoltaic devices that employ organic semiconductors to generate photocurrent [19–21]. They typically consist of an organic active layer that is placed between a reflecting metal electrode and a transparent electrode, through which the cell is illuminated. The active layer is either in direct contact with the electrodes or separated from them by layers that block excitons or selectively collect holes or electrons. As the photocurrent generation of thin-film solar cells critically relies on the optical interference pattern created by the reflecting metal electrode, these layers are also utilized as optical spacers to optimize the absorption within the active layer. Thin-films of the conductive polymer blend poly(3,4ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) are frequently used as hole collection layers. However, as recent results indicate that it significantly reduces the device lifetime [36], MoOx hole collection layers are increasingly employed to replace it. Traditionally, the transparent electrode consists of indium tin oxide (ITO) and acts as an anode, whereas the metallic rear electrode acts as a cathode. For this reason, solar cells with an ITO cathode and reflecting metal anode are referred to as “inverted” cells. These inverted cells employ metal oxides such as ZnO or TiOx as electron collection layers between the ITO cathode and the active layer [37, 38]. The active layer of virtually all efficient organic solar cells comprises two organic semiconductor materials. One of them acts as electron donor, the other one as electron acceptor. When a photon is absorbed in the active layer, an exciton is generated, as described in Section 1.2.2. As the binding energy of excitons in organic media exceeds the thermal energy at room temperature, the exciton has to diffuse to the donor-acceptor interface to be efficiently dissociated. This dissociation occurs by charge transfer of the electron to the acceptor and the hole to the donor due the offset of the donor and acceptor energy levels, as shown in Fig. 1.4. This requires that the donor HOMO lies between the acceptor energy levels and the acceptor LUMO lies between the donor energy levels. At the same time, the donor-acceptor HOMO-HOMO and LUMO-LUMO energy differences have to be at least ∼0.2-0.3 eV to allow for efficient charge transfer. After charge transfer, the hole travels through the donor and is finally collected at the anode, whereas the electron travels through the acceptor layer and is collected at the cathode. When the active layer consists of a donor/acceptor heterojunction with a planar donor-acceptor interface, its thickness is limited to a few tens of nm due to the small exciton diffusion length in polycrystalline and amorphous organic
ORGANIC SOLAR CELLS
9
3 1
2
4
5
Donor Acceptor Cathode Anode
Figure 1.4: Energy level diagram of a planar heterojunction organic solar cell. The upper energy levels of donor and acceptor represent their LUMOs, the lower levels their HOMOs. The first step of photocurrent generation in the active layer is the generation of an exciton by the absorption of a photon (1). The exciton then diffuses trough the organic semiconductor layer (2). Only when it reaches the donor-acceptor interface, the exciton can be dissociated efficiently into free charge carriers (3). The hole then travels trough the donor, and the electron trough the acceptor layer (4). Finally, they are collected at the electrodes (5).
semiconductors. As the optical absorption length is typically larger than the exciton diffusion length, the optimization of planar heterojunction organic solar cells involves a trade-off between maximizing absorption and maximizing the fraction of excitons that reach the donor-acceptor interface. To allow for thicker active layers, donor and acceptor can be blended into an interpenetrating network. The donor-acceptor interface in such a bulk heterojunction is greatly enhanced and most excitons have to diffuse only a few nm before reaching it, even for active layers that are hundreds of nm thick. Currently, the highest efficiencies are achieved utilizing C60 , C70 or their derivatives as acceptors. A much larger variety of efficient donor materials exist, including small-weight molecules [39–42] and polymers [43, 44]. Traditionally, polymer layers are deposited by solution-processing, whereas small-weight molecule thin-films are obtained by thermal evaporation. Recently, however, high performance organic solar cells with solution-processed smallweight molecules have been reported [45, 46]. Solar cells based on either of these two material types have reached efficiencies of ∼10% [2]. These record
10
INTRODUCTION
Dark Under illumination Voc J
0 Pmax Jsc
0 V Figure 1.5: Current density-voltage characteristics of a solar cell in the dark (dashed curve) and under illumination (solid curve). Under illumination, the maximal power density (Pmax ) generated by the cell is given by the area of the largest rectangle that can be fitted above the curve in the fourth quadrant (dark gray area). It is given by Jsc × Voc × FF. As FF is smaller than unity, Pmax is always smaller than Jsc × Voc (light gray area).
efficiencies were achieved by utilizing tandem solar cells that contain two active layers with complementary absorption spectra stacked on top of each other [47–49]. The performance of organic solar cell can be assessed by several types of optoelectronic measurements. The most basic one is the measurement of the current density-voltage (J-V ) characteristics of the cell. A typical J-V curve is shown in Fig. 1.5. When measured in the dark, the solar cell generates no photocurrent and thus behaves like a normal diode (dashed curve). Under illumination, the solar cell produces photocurrent, as evidenced by the negative current (solid curve). At short-circuit conditions, i.e. when no voltage bias is applied, the cell generates the short-circuit current density (Jsc ). When a positive voltage bias is applied, the photocurrent decreases until the bias reaches the open-circuit voltage (Voc ), where the photocurrent equals zero. The Voc obtained with a donor-acceptor material combination is to a certain extent
ORGANIC SOLAR CELLS
1 8
4 x 1 0
1 8
3 x 1 0
1 8
6 0
2 x 1 0
1 8
1 x 1 0
1 8
1 .5 G 2 0
E Q E (% )
4 0
A M
N
p h
( p h o to n s m
-2
s
-1
)
5 x 1 0
11
0 0 3 0 0
6 0 0
9 0 0
1 2 0 0
1 5 0 0
1 8 0 0
W a v e le n g th ( n m )
Figure 1.6: Comparison between the EQE spectrum of a typical organic solar cell (black dashed curve) and the AM 1.5G spectrum (gray solid curve, data taken from Ref. [53]).
determined by the HOMO of the donor and the LUMO of the acceptor, but is also influenced by other factors, such as charge transfer states and charge carrier recombination [50–52]. The maximal power density generated by the solar cell (Pmax ) is given by the area of the largest rectangle that can be fitted above the J-V curve in the fourth quadrant. The area of this rectangle is smaller than the product of Voc and Jsc , and therefore has to be corrected by the so-called fill factor (FF). A small FF is typically due to a high series and/or a low shunt resistance of the cell. The power conversion efficiency (η) of the solar cell is given by η=
Jsc Voc FF Pmax = , P0 P0
(1.2)
with the power density of the incident light (P0 ). The photocurrent generated by a solar cell can be resolved spectrally by external quantum efficiency (EQE) measurements. The EQE is the ratio of the number of collected electrons to the number of incident photons of monochromatic light with a given wavelength (λ). An EQE of 100% thus means that every incident photon contributes to the photocurrent at this wavelength. When the EQE is measured without applying a bias voltage, Jsc can be calculated from the EQE
12
INTRODUCTION
spectrum as Z Jsc = e
Nph (λ)EQE(λ)dλ ,
(1.3)
AM 1.5G
where e denotes the elementary charge and Nph the photon flux density of the incident light with an integrated intensity of 1000 W/m2 and an air mass 1.5 global (AM 1.5G) spectrum. The AM 1.5G spectrum is shown as a gray solid curve in Fig. 1.6 and corresponds to the total (i.e. direct and diffuse) light that reaches the earth surface when the sun light is incident at an angle of 48.2◦ relative to the zenith. The black dashed curve in the same graph shows the EQE spectrum of a typical organic solar cell. The comparison between the AM 1.5G and the EQE spectra demonstrates that organic solar cells typically only absorb a small fraction of the incident sunlight.
1.4
Optical properties of metal nanoparticles
Owing to their bright colors, metal nanoparticles (NPs) have been employed for centuries in church windows, mosaics, glass cups, and pottery [54–57]. This utilization was based on empirical knowledge, without an understanding of the physical origins of the metal NP colors. This however changed, when Gustav Mie published his pioneering work explaining the optical properties of metal NPs in the beginning of the 20th century [58]. After this theoretical framework had been developed, it took several decades until the advances in the field of nanotechnology led to a widespread use of metal NPs in various research areas. This resulted in potential applications in solar cells [16, 18, 59, 60], light emitting devices [61–63], chemical and biological sensors [64–66], surface enhanced Raman spectroscopy [67, 68], photochemistry [69], and lasers [70–72].
1.4.1
Localized surface plasmon resonances
The unique optical properties of metal NPs arise from the interaction of the NP conduction electrons with the incident light [73–77]. This interaction can be described by a simplified formalism, the so-called quasistatic approximation, for NPs with a size much smaller than the wavelength (λ) of the incident light. In this size regime, the electric field (E0 ) of the light wave is approximately constant across the entire particle. This spatially constant electric field exerts a force on the conduction electrons of the metal NP, which causes them to collectively move to the NP surface, as shown in Fig. 1.7. At the same time, the Coulomb attraction between these displaced electrons and the ionized metal
OPTICAL PROPERTIES OF METAL NANOPARTICLES
- - -
Metal NP +
+
13
+
E0
E0 +
+
+
- - Electron cloud
Time Figure 1.7: Schematic of a dipole LSPR excited in a metal NP with a size much smaller than the wavelength of the incident light with an electric field E0 . This external field exerts a force on the NP conduction electrons, resulting in a collective oscillation. Because of the small NP size, E0 is approximately spatially constant, resulting in a dipole resonance.
lattice leads to a restoring force. The combination of these two forces causes a collective oscillation of the electrons with a maximal amplitude at the so-called localized surface plasmon resonance (LSPR) frequency. In NPs that are small compared to λ, all electrons oscillate in-phase and therefore only dipole LSPRs are excited. These LSPRs should not be confused with surface plasmon polarities (SPPs), which are electromagnetic excitations propagating at the surface of planar metal layers that exhibit evanescent decay in the direction of the surface normal [78–80]. Unlike SPPs, LSPRs do not propagate (hence the localized in their name) and can be excited by a plane light wave without the need for specific incoupling schemes such as gratings and prisms. The optical properties of metal NPs are highly dependent on the relative permittivities of the NP material and the embedding medium, εNP and εm , respectively. In general, both of them are complex numbers with ε = ε1 + iε2 . According to the quasistatic approximation, we only need to know these two material properties as well as the NP radius (R) to calculate the dipole polarizability (α) of a spherical metal NP at a given frequency (ω) of the incident
14
INTRODUCTION
light: α(ω) = 4πR3
εNP (ω) − εm (ω) . εNP (ω) + 2εm (ω)
(1.4)
The response of the metal NP on the excitation by an external light source can then be described by the induced dipole moment p = ε0 εm αE0 , with the vacuum permittivity ε0 . From Eq. (1.4), it follows that α exhibits resonance behavior whenever |εNP + 2εm | = Minimum .
(1.5)
In a transparent embedding medium, i.e. ε2,m = 0, and for a small ε2,NP or ∂ε2,NP /∂ω, the dipole resonance condition is simplified to ε1,NP = −2εm
(1.6)
This implies that LSPRs can only be excited if either ε1,m or ε1,NP are negative, which necessitates that either the NP or the embedding medium consists of a metal (the latter would correspond to a void in a metal layer, which supports LSPRs as well [81, 82]). The ε1 of Ag and Au are both negative in the visible spectral range, as shown in Fig.1.8, such that for small spherical NP of these noble metals in vacuum, a dipole LSPR is excited at λ ∼ 360 nm and λ ∼ 500 nm for Ag and Au NPs, respectively.
1.4.2
Light extinction by metal NPs
Metal NPs exhibit efficient light extinction, which is the sum of light absorption and scattering. These optical properties are a consequence of the LSPRs excited in the metal NPs and have been studied extensively during the last decades. The NP extinction has been found to strongly depend on the NP composition, size, shape, and dielectric environment [83–89]. The light extinction, absorption, and scattering of a single metal NP are commonly described by their cross-sections, Cext , Cabs , and Csca , respectively, with Cext = Cabs +Csca . These cross-sections are related to the area within which the metal NP interacts with the incident light. According to the quasistatic approximation, both Csca and Cabs of spherical NPs are directly related to α by Cabs = k Im(α) Csca =
k4 2 |α| , 6π
and
(1.7) (1.8)
OPTICAL PROPERTIES OF METAL NANOPARTICLES
1 0
ε2
15
, A u
0
ε2 , A u
-2 0
ε1
, A g
, A g
εr
-1 0
ε1
-3 0
3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
9 0 0
W a v e le n g th ( n m ) Figure 1.8: Relative permittivities of Ag and Au, as measured by spectroscopic ellipsometry.
√ where k = 2π εm /λ denotes the wavenumber, which is the magnitude of the wave vector (k) of the incident light. It is convenient to normalize these cross-sections by the physical size of the metal NP, such that the normalized cross-sections (Q) for a spherical NP are given by Q = C/(R2 π). The quasistatic approximation is rather accurate for R < 20 nm. For NPs with a larger size, the approximation does not hold anymore because phase retardation of the external and internal fields lead to a red-shift of the LSPR and strong light scattering leads to radiation damping [90]. In addition, higher-order multipole resonances are excited. All these effects are taken into account in the more rigorous elecrodynamic Mie model, where Cext and Csca are given by a multipole expansion of the electromagnetic fields [91]: Cext
∞ 2π X = 2 (2l + 1)Re(al + bl ) k
and
(1.9)
l=1
Csca =
∞ 2π X (2l + 1)(|al |2 + |bl |2 ) . k2 l=1
(1.10)
16
INTRODUCTION
9 0 0
εm = 4
(n m
εm = 3
3 0 0
εm = 2
C
e x t
εm = 1 0
εm = 5
6 0 0
2
)
εm = 7
εm = 1
0 4 0 0
5 0 0
6 0 0
7 0 0
W a v e le n g th ( n m ) Figure 1.9: Extinction cross-section (Cext ) of a Ag NP with R = 3.5 nm for several εm , according to Mie theory.
The so-called Mie coefficients al and bl , which are given by al =
mψl (mx)ψl0 (x) − ψl (x)ψl0 (mx) mψl (mx)ξl0 (x) − ξl (x)ψl0 (mx)
(1.11)
ψl (mx)ψl0 (x) − mψl (x)ψl0 (mx) (1.12) ψl (mx)ξl0 (x) − mξl (x)ψl0 (mx) p where x = kR and m = εNP /εm , with Im(εm ) = 0. ψl and ξl are cylindrical (1) Riccati-Bessel functions, defined by ψl (z) = zjl (z) and ξl (z) = zhl (z), with (1) the spherical Bessel (jl (z)) and Hankel (hl ) functions. The prime indicates differentiation with respect to the argument in parentheses. The order of the multipole is given by l, where the lowest-order mode (l = 1) is a dipole. bl =
Equations (1.9) and (1.10) allow to extract several general trends for spherical metal NPs. The Cext spectra of a spherical Ag NP with R = 3.5 nm for several εm is shown in Fig. 1.9. Such small NPs (i.e. R λ) exhibit a single extinction band, originating from a dipole LSPR. It red-shifts with increasing εm , as expected from the quasistatic dipole resonance condition, Eq. (1.6), and the monotonically decreasing ε1 of Ag in the visible spectral range (cf. Fig. 1.8). The size dependence of extinction, scattering, and absorption of Ag NPs is shown in Fig. 1.10(a)-(c). For R = 5 nm, only the dipole LSPR is excited at λ = 360
OPTICAL PROPERTIES OF METAL NANOPARTICLES
(a ) 8
17
4 0 8
Q
e x t
Q
s c a
Q
a b s
A g , R = 5 n m
(b )
A g , R = 4 0 n m
(c )
A g , R = 1 0 0 n m
a b s
4
s c a
, Q
0
e x t
, Q
8
Q
4 0
(d ) 8
A u , R = 4 0 n m
4 0 3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
W a v e le n g th ( n m ) Figure 1.10: Extinction, scattering and absorption efficiencies of spherical (a)-(c) Ag and (d) Au NPs in air for several R, according to Mie theory.
nm [Fig. 1.10(a)]. As the NP size increases, the dipole resonance red-shifts due to phase retardation and becomes broader due to radiative damping [Fig. 1.10(b) and (c)]. For R = 100, for example, the dipole resonance is centered at λ ∼ 600 nm and is highly damped. At the same time, higher-order multipoles are excited at wavelengths smaller than that of the dipole LSPR. The comparison between Qscat and Qabs reveals that for the smallest Ag NPs no significant scattering is observed, whereas it dominates for the larger two NP sizes. Figure 1.11 shows the dependence of the Cscat /Cext ratio on R, according to Mie theory. The growing importance of light scattering with increasing R is also taken into account by the quasistatic approximation, where scattering scales with R6 , whereas absorption only scales with R3 [Eq. (1.7) and (1.8)]. The comparison between the Cscat /Cext ratio for εm = 1 (solid curve in Fig. 1.11)
18
INTRODUCTION
0 .1
εm = 1 0
εm = 1
0 .0 1
C
s c a
/ C
e x t
1
1 E -3 2 0
4 0
6 0
8 0
1 0 0
R a d iu s ( n m ) Figure 1.11: The Cscat /Cext ratios for a Ag NP with a radius R in an embedding medium with εm = 1 (solid curve) and εm = 10 (dashed curve), according to Mie theory.
and εm = 10 (dashed curve) shows that the importance of light scattering for a given R strongly increases with increasing εm . Finally, we can consider the role of the NP material composition itself. In the visible spectrum, Ag absorbs only very little light, resulting in a small ε2 (cf. Fig. 1.8). Gold, on the other hand, absorbs more strongly due to interband transitions that extend into the visible spectral range. This is evidenced by the much weaker resonance and less effective light scattering of Au NPs compared to Ag NPs of the same size, as shown in Fig. 1.10(d). So far, we have only discussed the optical properties of spherical metal NPs. Non-spherical metal NPs have optical properties that strongly depend on their shape [92–95]. Rod-shaped particles, for example, exhibit one dipole LSPR per non-degenerated axis [96], as shown by the experimentally measured spectra of Au NPs in Fig. 1.12. The optical properties of such rather simple non-spherical geometries can still be calculated analytically. For more complicated geometries, however, numerical simulations have to be employed. The quasistatic and Mie models described above both rely on ε as their only material-related parameter. Therefore, the results obtained from these models can only be as accurate as the ε values that are utilized for the calculations. Because ε is a macroscopic material property, the models neither take into
OPTICAL PROPERTIES OF METAL NANOPARTICLES
19
8 0
E x tin c tio n ( % )
R o d -s h a p e d 6 0
4 0
2 0
S p h e r ic a l 0 4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
9 0 0
W a v e le n g th ( n m )
Figure 1.12: Experimentally measured extinction spectra of colloidal dispersions of spherical Au NPs with R ∼ 5 nm (dashed curve) and rod-shaped Au NPs with a minor axis diameter of 10 nm and a 3:1 aspect ratio (solid curve).
account the damping of the electron oscillation when the NP size is smaller than the mean free path length of the electrons nor nonlocal effects at the metaldielectric interface, that both become increasingly important with decreasing NP size. Although models exist that take these effects into account [74], they require the detailed knowledge about microscopical parameters such as the exact morphology of the metal NP/embedding medium interface and the shape and size distribution of the NPs. As in real-life experiments, these parameters are mostly unknown or could only be roughly estimated, the benefit of utilizing models that correct for surface scattering and nonlocal effects is not obvious.
1.4.3
Metal NP-embedding medium Interactions
The excitation of LSPRs in a metal NP leads to a strong enhancement of the electric field intensity (|E|2 ) near its surface. This enhancement depends on the metal NP shape and the position relative to the E0 vector of the incident light wave, as shown by Fig. 1.13. In the quasistatic approximation, the near-field intensity enhancement (|ENP |2 /|E0 |2 ) at the distance d to the surface of a metal
20
INTRODUCTION
(b)
(a)
10 nm
|E|2(a.u.)
1
0
k E0
Figure 1.13: Normalized electric field intensity (|E|2 ) near Ag NPs with a (a) triangular and (b) circular cross-section, as obtained by 2D numerical simulations. The plane light wave is incident from the bottom, with its E0 and k vectors as indicated by arrows.
NP with radius R is given by [97] 2 2 α α |ENP |2 2 sin2 θ , = 1 + cos θ + −1 + 2 3 3 |E0 | 2π(R + d) 4π(R + d)
(1.13)
where |ENP |2 denotes the electric field intensity outside of the metal NP and θ the angle between the E0 vector of the incident light and the vector pointing from the NP center to the point of interest. The maximal field enhancement is obtained for θ = 0, π. At these angles, Eq. (1.13) can be simplified to 2 |ENP |2 α . = 1+ (1.14) |E0 |2 max 2π(R + d)3 As a result of this near-field enhancement, the absorption in the embedding medium can be strongly increased due to the presence of metal NPs. Below saturation, this absorption enhancement equals |ENP |2 /|E0 |2 [98–101]. In addition to near-field enhancement, light scattering by metal NPs can also lead to absorption enhancement in thin-films due to light trapping, as will be discussed in Section 1.5.1. Light scattering by metal NPs only becomes efficient for NPs with a size above ∼30-50 nm, whereas the near-field enhancement can already be very strong even for NPs with a size of only a few nm. The presence of metal NPs does not only affect light absorption in the embedding medium but also the PL of nearby emitters such as organic molecules and
PLASMONIC SOLAR CELLS
21
semiconductor nanocrystals. Metal NPs have been found to either strongly quench or enhance the PL of nearby emitters, depending on the size, shape, composition, and dielectric environment of the metal NPs [102–106], the natural quantum yield of the emitter [106, 107], the distance between the NPs and the emitter [108–112], and the spectral overlap of the LSPR and the emission [113–117]. The effect of the presence of metal NPs on the PL of nearby emitters originates from both the absorption enhancement that corresponds to an excitation enhancement in the context of PL measurements, as well as a modulation of the emission quantum yield and a reduction of the exciton lifetime [77, 101, 118–121]. The near-field absorption enhancement will be discussed in detail in Chapter 3, whereas the modulation of the emission quantum yield and the reduction of the exciton lifetime will be the subject of Chapter 4.
1.5 1.5.1
Plasmonic solar cells Plasmonic enhancement strategies
The aim of exploiting surface plasmons in solar cells lies in enhancing light absorption in their active layer. The necessity for absorption enhancement in thin-film solar cells is due to the poor electrical properties of their active layer materials that limit the device thickness, resulting in incomplete light absorption especially in the absorption tails. For crystalline Si solar cells, on the other hand, absorption enhancement is necessary to maintain a high efficiency while thinning down the Si active layer to save costs. Several design concepts have been proposed for solar cells containing plasmonic nanostructures such as metal NPs, gratings, and nanostructured electrodes (cf. Fig. 1.14) [16, 59, 122]. Enhancement schemes based on metal nanostructures placed at the front of the cell mainly exploit far-field light scattering [Fig. 1.14(a)]. This scheme enhances light incoupling into the cell as the nanostructures scatter light preferentially into the high-index substrate and therefore reduce reflectance [123]. Moreover, the light is scattered in various directions, which leads to lighttrapping in the active layer [60]. The mechanisms of light trapping are well understood in layers characterized by a thickness much larger than the wavelength of the incident light and a weak absorption, such that only a small fraction of the light is absorbed when it passes the layer once. Both conditions are fulfilled in wafer-based crystalline Si solar cells. In such cells, light trapping can be described by ray optics. If the angle of the scattered light is beyond the critical angle for total internal
22
INTRODUCTION
(a) Active layer
Metal electrode
(b) SPP
(c)
Figure 1.14: Design concepts for solar cells containing metal nanostructures. The metal nanostructures are either (a) located at the front of the cell, (b) placed the rear side, or (c) embedded into the active layer. Several plasmonic enhancement mechanisms are indicated, including near-field enhancement, light trapping by scattering at shallow angles, and coupling to SPPs at the active layer/metal electrode interface.
reflection, the light can be reflected many times at either interface of the layer until it is completely absorbed, thus strongly enhancing its pathlength in the active layer. For a random texturing of the active layer surface, resulting in isotropic light scattering, the maximal absorption enhancement is given by the so-called Yablonovitch limit of 4n2 , where n is the active layer refractive index [124]. For such thick layers, light can be scattered into the layer at any angle. When the device thickness is reduced to ∼λ, ray optics can not be applied anymore, and
PLASMONIC SOLAR CELLS
23
Glass (n=1.5)
115 nm ITO (n=1.7)
230 nm Active layer (n=2)
130 nm MoOx (n=2)
Ag
|E|2 (a.u.)
1
/
SPP TM TE
0.5
0
Figure 1.15: Normalized guided mode profiles at λ = 650 nm in an organic solar cell, as obtained by numerical simulation employing a mode solver. The SPP as well as transverse electric (TE) and transverse magnetic (TM) “photonic” and mixed modes are shown. The refractive index (n) of the individual layers is given in brackets.
scattered light can only couple into a finite number of guided modes within the thin-film device [122, 125]. These guided modes are bound states that resemble photonic modes in a dielectric waveguide and are characterized by a vanishing electrical field intensity outside the thin-film solar cell layer stack. Several models that have been developed to describe the fundamental light trapping limit in thin-film solar cells predict that the 4n2 limit could be surpassed in these cells [126, 127]. Enhancement schemes based on plasmonic nanostructures located at the rear side of the cell typically employ nanopatterned metal electrodes, as shown in Fig. 1.14(b). A major advantage of plasmonic rear-side features lies in the reduced parasitic absorption losses and the elimination of destructive interference between the incident and scattered light [128, 129]. Plasmonic nanostructures at the rear of the solar cell allow for efficient light trapping by scattering light into the active layer at shallow angles and coupling into guided modes, as described above for nanostructures located at the front of the cell. In addition, the LSPRs excited in a nanostructured metal electrode can efficiently couple to SPPs that propagate at the electrode/semiconductor interface [16]. The excitation of SPPs results in an exponentially decaying near-field that can enhance the absorption in the active layer if the spacer between electrode and active layer is thin enough. As an example of the various modes that are
24
INTRODUCTION
supported in thin-film solar cells, the SPP and “photonic” modes in an organic solar cell with Ag rear electrode and an oxide spacer layer between the active layer and the electrode are shown in Fig. 1.15. Finally, plasmonic enhancement schemes utilizing metal nanostructures embedded in the active layer have been proposed, as shown in Fig. 1.14(c). This enhancement strategy most efficiently exploits plasmonic near-field enhancement due to the large interface between the metal nanostructures and the active layer material. In addition, light scattering can contribute to absorption enhancement in the active layer if nanostructures with high scattering efficiencies are chosen. The practical application of this concept is very challenging for many solar cell technologies and has so far mainly been applied in solution-processed organic solar cells.
1.5.2
Literature review for plasmonic organic solar cells
As described in the previous section, several strategies have been proposed to enhance the efficiency of solar cells by plasmonic metal nanostructures. Enhancement schemes based on these strategies have been implemented into various cell types, including crystalline [130–132] and amorphous Si [133–136], III/V [137, 138], dye sensitized [139, 140], and organic solar cells [141]. The application of plasmonic enhancement strategies in organic solar cells has gained increasing interest during the last decade, resulting in a substantial number of publications that will be the topic of this non-exhaustive literature review. The most simple experimental realization of an organic solar cell containing metal nanostructures is to blend metal NPs into the active layer by mixing a solution containing the dissolved organic semiconductor materials with a colloidal dispersion of the NPs. Using this strategy, efficiency enhancement of polymer:fullerene solar cells has been reported using Au [142–145], Ag [143, 146, 147], and Pt [148] NPs. Another strategy lies in distributing metal NPs in the hole/electron collection or exciton blocking layer between the active layer and the electrode, either by means of thermal evaporation [149, 150] or solution-processing [144, 151–155]. This also includes the use of metal NPs in the recombination layer of tandem solar cells [98, 156]. Finally, organic solar cells with nanostructured electrodes have been realized by either patterning the (semi) transparent front electrode [157–161] or the reflecting rear electrode [162–164]. The experimental implementation of plasmonic rear-side features is challenging as the processing of organic solar cells usually begins with the transparent electrode. The nanostructures thus
PLASMONIC SOLAR CELLS
25
have to be applied after the deposition of the organic active layer that is readily damaged by the harsh processing conditions often involved in nanofabrication or nanopatterning. As a result, metal nanostructures are usually placed at the front side of organic solar cells, and the realization of cells containing nanostructured rear electrodes that demonstrate state-of-the-art efficiencies has only recently been reported [165]. These experimental results reported in literature demonstrate that the efficiency of organic solar cells can be enhanced by implementing any of the design concepts presented in Section 1.5.1 into organic solar cells. However, the fact that the presence of metal nanostructures enhances the cell efficiency does not automatically mean that this enhancement occurs due to plasmonic effects. For example, Topp et al. [166] have shown that metal NPs stabilized by alkylthiols blended into the active layer can enhance the device efficiency by improving the phase-separation of polymers and fullerenes due to the presence of free alkylthiols in the NP dispersion. This is in-line with the strong efficiency enhancement of low-bandgap polymer solar cells when adding a small amount of additives [167]. In addition, an increased conductivity of polymer layers due to the incorporation of metal NPs has been reported [168]. Another factor that complicates the interpretation of the experimental results is the control over the metal NP dispersion in polymer layers. Obtaining a homogeneous distribution is not trivial as NPs and polymers tend to form separate phases, especially when the mixing of the molecules that coat the metal NPs and the polymer host is entropically or enthalpically not favored [169, 170]. Typical examples of such phase-separating blends are alkylthiols and aromatic polymers. As many of the above mentioned papers use alkylthiol-coated metal NPs and aromatic host polymers without providing any characterization of the NP dispersion (e.g. a cross-sectional electron-micrograph or a time-of-flight secondary ion mass spectrogram), it is questionable whether the NPs are welldispersed or form aggregates. Recently, Wang et al. [147] made a virtue out of necessity and fabricated organic solar cells with aggregated Ag NPs in the active layer to enhance charge transport. Non-aggregated metal NPs, on the other hand, might lead to charge trapping [171]. As a result of the complexity of organic solar cells containing metal nanostructures, experimental results reported in literature vary strongly. For example, a significant absorption [144, 152, 153, 156] and charge carrier generation [99] enhancement in presence of metal NPs was reported in some papers whereas efficiency enhancement without an increased absorption has been reported as well [148, 166, 168]. Moreover, the external quantum efficiency was either enhanced only at the plasmon resonance wavelength [150, 152] or it was increased over the full absorption spectrum of the active layer materials [143, 153, 156].
26
INTRODUCTION
The diversity of reported results is certainly also partially due to the difficulty to experimentally distinguish between parasitic absorption by metal nanostructures and absorption enhancement in the active layer. This typically also makes it challenging to experimentally determine the relative importance of plasmonic near-field enhancement and far-field light scattering. Therefore, it is crucial to complement experiments with numerical simulations to gain a better understanding of the enhancement mechanisms involved in organic solar cells containing metal nanostructures. By employing numerical simulations, absorption enhancement was predicted for organic solar cells with metal NPs embedded in the active layer [100, 128, 172, 173] and with nanostructured front or rear metal electrodes [174–178]. This suggests that an efficiency enhancement exclusively originating from plasmonic absorption enhancement might be achievable. However, as the models employed in these papers do only take into account absorption, but not all the subsequent steps in photocurrent generation, such as exciton diffusion and dissociation as well as charge carrier collection, it is not clear to what extent the beneficial effect of this absorption enhancement is diminished due to exciton quenching and charge trapping by the metal nanostructures. Models that combine optical and electrical simulations have recently been developed for inorganic solar cells [179, 180]. Such models have also been applied to plasmonic organic solar cells, however, without taking into account the impact of the metal NPs on the exciton lifetime [181]. In summary, this literature review shows that even though a large number of papers reported an efficiency enhancement of organic solar cell by placing a variety of metal nanostructures at virtually any position within the cell, there is still a lack of fundamental understanding concerning the most promising plasmonic enhancement strategy (i.e. near-field enhancement vs. far-field scattering). Moreover, an unambiguous demonstration of a purely plasmonicallyenhanced organic solar cell is still missing.
1.6
Thesis outline
This thesis aims to investigate plasmonic enhancement strategies for organic solar cells. A crucial element of this investigation lies in the identification and characterization of the various near- and far-field plasmonic effects at work. In addition, all non-plasmonic contributions to efficiency enhancement due to the presence of metal nanostructures (cf. Section 1.5.2) have to be identified and eliminated. Both these elements are key in determining the most promising
THESIS OUTLINE
27
plasmonic enhancement strategy and unambiguously demonstrating plasmonic efficiency enhancement of organic solar cells. In the first part of the thesis, plasmonic near-field enhancement is systematically studied by employing a model system of a Ag NP layer covered by thin-films of organic semiconductors. The size of the NPs is chosen such that far-field light scattering is negligibly small compared to absorption. This model system is introduced in Chapter 2, where the morphology and optical properties of the Ag NPs are presented, followed by a discussion of the effect of the embedding medium on the LSPRs excited in the NPs. The results shown in this chapter demonstrate that the optical properties of metal NPs are strongly influenced by the highly dispersive permittivity of absorbing organic media. Notably, these interactions might even result in the excitation of multiple dipole LSPRs in spherical metal NPs. In Chapters 3 and 4, this model system is utilized to assess the exploitation of plasmonic near-field enhancement in organic solar cells. The near-field interactions between LSPRs excited in the metal NPs and excitons generated in the organic thin-film are probed by absorption measurements as well as steadystate and time-resolved PL measurements. Experiments, numerical simulations, and an analytical model are employed to probe various aspects of the near-field interactions such as their range and spectral dependence as well as the role of the permittivity of the embedding medium. Chapter 3 focuses on plasmonic absorption enhancement in the organic embedding medium, whereas the effect of the presence of metal NPs on the decay rates of nearby emitters is discussed in Chapter 4. The absorption enhancement is found to be accompanied by a strong reduction in exciton lifetime, which is detrimental to the device efficiency. Since the range of both interactions is found to be comparable, isolating the NPs with a layer thick enough to prevent exciton quenching may simultaneously eliminate any beneficial effect due to near field absorption enhancement. This leads to the conclusion that the exploitation of near field enhancement in organic solar cells appears challenging. The subject of Chapter 5 is the exploitation of plasmonic far-field light scattering in organic solar cells. A plasmonic enhancement scheme based on an organic solar cell equipped with a nanostructured Ag rear electrode is presented. A bottom-up nanofabrication method is employed to pattern the rear electrode on top of the organic active layer without affecting the optical and electrical properties of the organic active layer materials. The LSPR of this nanostructured electrode is tuned to the red absorption tail of the active layer and generates enhanced absorption by light scattering, as verified by experiments and numerical simulations. Moreover, the plasmonic
28
INTRODUCTION
nanostructured cell exhibits an enhanced power conversion efficiency compared to an optimized, high performance organic solar cell. This demonstrates that the nanostructured electrode enables to enhance the efficiency of an organic solar cell beyond the efficiency that can be achieved by optimization of any other parameter. In addition, plasmonic near-field enhancement is exclusively found in the spacer layer separating the rear electrode from the active layer, and is therefore unlikely to significantly contribute to the efficiency enhancement. Based on these findings, we conclude that the efficiency enhancement demonstrated here unambiguously originates from plasmonic light scattering. Finally, in Chapter 6, the main conclusions of this thesis are summarized and the outlook for plasmonic organic solar cells is discussed.
Chapter 2
Optical properties of metal nanoparticles covered by organic thin-films 2.1
Introduction
The optical properties of metal NPs are highly sensitive to the local dielectric environment, as described in Chapter 1.4. When metal NPs are embedded in strongly absorbing molecular media, for example to exploit plasmonic effects in organic solar cells, the LSPRs excited in the NPs will be affected by the dispersive permittivity of the embedding medium (εm ). It is therefore important to understand the interactions between the LSPRs and the embedding medium. For this study, a model system is employed that consists of Ag NPs covered by thin-films of organic media. The size of the NPs is chosen such that far-field light scattering is negligibly small compared to absorption, allowing to exclusively probe near-field interactions. The same model system will also be utilized in Chapters 3 and 4, where the influence of LSPRs on the absorption and emission of nearby organic molecules will be studied.
29
30
OPTICAL PROPERTIES OF METAL NANOPARTICLES COVERED BY ORGANIC THIN-FILMS
2.2
Experiments
Copper(II) phthalocyanine (CuPc) and chloro-boron subphtalocyanine (SubPc) were purchased from Sigma-Aldrich and the Luminescence Technology Corp, respectively. Both materials were purified twice by thermal gradient sublimation prior to use. Hyflon AD60X, a copolymer of 2,2,4-trifluoro-5-trifluorometoxy1,3-dioxole and tetra-fluoro-ethylene with a molar ratio of 0.6:0.4, was obtained from Solvay-Solexis and used as received. The structural formula of Hyflon AD60X is shown in Fig. 2.1. Molybdenum(VI) oxide was purchased from Sigma-Aldrich. As the exact oxygen content in thin-films obtained by thermal evaporation of molybdenum(VI) oxide is not known, their material composition will be denoted by MoOx . Silver with a purity of 99.99% was purchased from the Kurt J. Lesker Company.
Figure 2.1: Structural formula of Hyflon AD60X.
Eagle XG glass (purchased from Corning Inc.) and Si/SiO2 substrates were cleaned by subsequent sonication in soapy water, deionized water, acetone, and isoproyl alcohol, followed by exposure to ultraviolet(UV)-O3 for 15 min. Silver, CuPc, MoOx , and SubPc were deposited by thermal evaporation at a chamber pressure below 2 × 10−6 Torr. Silver was deposited at a rate of 0.01 nm/s, CuPc, SubPc, and MoOx were deposited at a rate of 0.1 nm/s. Hyflon AD60X thin-films were deposited in the same chamber by means of vacuum pyrolysis [182], at a pressure below 5 × 10−6 Torr and a rate of 0.05 nm/s. During all depositions, the rate and film thickness were monitored by a quartz-crystal oscillator. The substrate remained at room temperature and was rotating during the deposition, leading to a thickness inhomogeneity of < 5% over an area of 100 cm2 , as determined by spectroscopic ellipsometry. A computer-controlled retractable shadow mask allowed for the deposition of layers with six different thicknesses in a single run.
EXPERIMENTS
31
The direct light transmission of thin-films on glass substrates was measured using a Shimadzu UV-1601PC spectrophotometer, whereas the total (i.e. the sum of direct and diffuse) transmission and reflectance were measured using a Bentham PVE 300 photovoltaic device characterization system equipped with an integrating sphere. As will be discussed in Section 2.4.1, far-field light scattering and thus also the diffuse transmission and reflectance of the thin-films with and without Ag NPs were found to be negligible. Therefore, the light absorption was defined as absorption = 1 - direct transmission - total reflectance. The reason for using the direct instead of the total transmission was the significantly higher noise level in the total transmission spectra due to the integrating sphere. The absorption of the glass substrate was subtracted from all absorption spectra. 5 0
A b s o r p tio n ( % )
4 0 3 0 2 0 1 0 0 3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
W a v e le n g th ( n m )
Figure 2.2: Absorption spectra of 12 samples comprising either a pure layer of 10 nm SubPc (gray solid curves) or a 10 nm SubPc layer deposited on the Ag NP layer (black solid curves). All samples were fabricated during a single run. The absorption of the glass substrate (dashed black curve) was subtracted from all spectra.
The inaccuracy of the absorption spectra of samples deposited during a single run, resulting from the thickness inhomogeneity and the inaccuracy of the measurement set-ups, was estimated as ± 0.5% for λ > 350 nm and ± 2% for λ < 350 nm, as shown in Fig. 2.2. The morphology of Ag NPs was determined by scanning electron microscopy (Hitachi SU8000) and atomic force microscopy in tapping mode (Veeco Dimension 3100 system with Nanoscope IV controller). The size-distribution of the Ag NPs was determined from scanning electron micrographs using Gwyddion 2.18. The relative permittivities of all materials were determined from their thin-films on Si/SiO2 substrates, using a SOPRA GESP-5 spectroscopic ellipsometer.
2.3
Three-dimensional finite-element numerical simulations s Organic Ag y
7 nm glass x
t
5 nm
k
Periodic boundary
OPTICAL PROPERTIES OF METAL NANOPARTICLES COVERED BY ORGANIC THIN-FILMS
Periodic boundary
32
E0
Figure 2.3: Cross-section of the geometry used for 3D numerical simulations. The Ag NPs are represented by truncated spheroids with a height of 5 nm and a lateral dimension of 7 nm. Periodic boundaries in both lateral dimensions result in an infinite two-dimensional array of truncated Ag spheroids with center-to-center spacing s. An organic layer with a thickness t and a flat surface covers the Ag NPs. The light wave is incident through the semi-infinite glass substrate with the wave vector (k) and electric field vector (E0 ) as indicated by arrows.
We utilized the software package COMSOL Multiphysics 3.5a to perform 3D finite-element numerical simulations. This software allows to monitor the absorption in each component individually and therefore to deconstruct the absorption spectra of composite layers. The geometry used for these simulations was motivated by our microscopy study of the Ag NP layer that will be discussed further in Section 2.4.1. Its cross-section is shown in Fig. 2.3. The Ag NPs are represented by truncated spheroids with a height of 5 nm and a lateral dimension of 7 nm. Periodic boundary conditions in the lateral directions lead to a Ag NP array with a center-to-center spacing denoted by s. An organic layer with a thickness t and a flat surface covers the Ag NPs. The light is simulated by a plane wave with the wave vector (k) parallel to the y-axis and the electric field vector (E0 ) parallel to the x-axis. It is incident through the semi-infinite glass substrate with ε = 2.25. As a reference, neat organic films without Ag NPs were simulated. When the simulation results from such a reference film are compared to those of an organic film on Ag NPs, the thickness of the film on Ag NPs is chosen slightly larger than that of the neat reference film such that both organic layers have the same volume, compensating for the volume occupied by the Ag NPs.
RESULTS AND DISCUSSION
2.4 2.4.1
33
Results and discussion Morphology and optical properties of the Ag NP layer
The deposition of a Ag layer with a nominal thickness of 1 nm on a glass substrate by means of thermal evaporation spontaneously leads to the formation of a dense Ag NP layer. From above, the Ag NPs approximately have a circular shape, as shown by the top-view scanning electron micrograph in Fig. 2.4(a). The particle diameter distribution obtained from this micrograph has an average and maximum value of 6.6 nm and 12 nm, respectively, with a standard deviation of 1.4 nm [cf. Fig. 2.4(c)]. From atomic force microscopy measurements [cf. Fig. 2.4(b)], an average NP height of 5 nm was determined. 120
Particle count
(a)
(c)
100 80 60 40 20
100 nm
0
5.5 nm
(b)
0
2
4
6
8
10
12
14
Diameter (nm)
Height (nm)
100 nm
2.0 nm
6 4 2 0
50
100 150 Distance (nm)
200
250
Figure 2.4: (a) Top-view scanning electron micrograph of the Ag NP layer on a Si/SiO2 substrate. (b) Atomic force micrograph (top) of the Ag NPs on a glass substrate, with a height-profile (bottom) taken along the dashed line indicated by the arrow. An average NP height of 5 nm was determined from this micrograph. The lateral size of the NPs is overestimated due to convolution of the atomic force microscope tip and the sample features. (c) Histogram of the Ag NP size-distribution obtained from 474 particles. The average diameter is 6.6 nm, with a standard deviation of 1.4 nm.
34
OPTICAL PROPERTIES OF METAL NANOPARTICLES COVERED BY ORGANIC THIN-FILMS
2 0 8
3
α / (4πR ) ]
1 0
Im [
4
2
5
0
0 3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
A b s o r p tio n ( % )
1 5 6
8 0 0
W a v e le n g th ( n m )
Figure 2.5: Experimentally measured absorption spectrum of the bare Ag NP layer (solid curve) and the imaginary part of the dimensionless fraction of the polarizability, α/(4πR3 ), for a spherical Ag NP in vacuum (dashed curve).
The absorption spectrum of the bare Ag NP layer is shown in Fig. 2.5 as a solid curve and features a single band centered at λ = 430 nm. The small size of the Ag NPs compared to this wavelength allows us to apply the quasistatic approximation to determine whether a dipole LSPR is expected at this spectral position, as discussed in Section 1.4. Indeed, from Eq. (1.4), a single resonance at λ = 360 nm is obtained for a spherical Ag NP in vacuum (dashed curve in Fig. 2.5), using the relative permittivity of Ag shown in Fig. 2.9(a). The experimentally measured absorption band can therefore be attributed to a dipole LSPR, red-shifted from the calculated value due to the presence of the substrate, interactions between the NPs, and the deviation of the NP shape from a perfect sphere [183, 184]. The absence of any further absorption bands is expected for spherical Ag NPs of this size and indicates that no higher order LSPR modes are excited [185]. The absorption band extending from λ =300-320 nm originates from Ag interband transitions [186]. According to the dipole resonance condition, Eq. (1.6), the LSPR of the Ag NP layer is expected to red shift when εm increases. This is because ε1,Ag monotonically decreases at λ > 320 nm [cf. Fig. 2.9(a)]. The spectral position of the LSPR, according to the quasistatic approximation, only depends on εNP and εm . It is therefore only accurate for a metal NP in an infinite, homogeneous medium. The system studied here, however, is more complex and the LSPR of the Ag NP layer is influenced by additional parameters, such as the presence of
RESULTS AND DISCUSSION
35
H y f lo n A D 6 0 X G la s s M o O A ir
A b s o r p tio n ( % )
M o O x
( ε ≅ 4.5)
2 0 1 5
1 0 n m
1 0 5 0 n m 0
A b s o r p tio n ( % )
x
(a )
2 5
H y f l o n A D 6 0 X ( ε = 1.8)
(b )
2 0 1 5 1 0
1 0 n m 5 0
0 n m 3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
W a v e le n g th ( n m ) Figure 2.6: Experimentally measured absorption spectra of the Ag NP layer covered by thin-films of (a) MoOx and (b) the block copolymer Hyflon AD60X, with thicknesses of 1, 2, 5, and 10 nm. The absorption spectrum of the bare Ag NP layer is shown as solid curve. The vertical dashed lines indicate the spectral position of the dipole LSPR of a spherical Ag NP in air (ε = 1), Hyflon AD60X (ε = 1.8), glass (ε = 2.25), and MoOx (ε = 4.5), according to the quasistatic approximation.
the substrate and the thickness of the thin-film covering the NPs. As a result, the spectral position of the LSPR obtained with Eq. (1.4) will be most accurate for thick layers with a ε higher than that of the substrate. This is demonstrated by covering the NPs with MoOx with ε ∼ 4.5 [Fig. 2.6(a)]. The LSPR peak red-shifts with increasing MoOx layer thickness until it almost exactly reaches the spectral position predicted for an Ag NP in an infinite MoOx medium. On the other hand, when the NPs are coated with Hyflon AD60X, a material with a lower ε than the glass substrate, the LSPR does not shift, as its spectral position is mainly determined by the substrate [Fig. 2.6(b)]. In general, when describing the optical properties of metal NPs, both absorption
36
OPTICAL PROPERTIES OF METAL NANOPARTICLES COVERED BY ORGANIC THIN-FILMS
as well as light scattering have to be taken into account, such that the extinction is given by the sum of absorption and scattering. According to electrodynamic Mie theory [91], for spherical Ag NPs with a diameter of less than 12 nm in air, the ratio of scattering to extinction remains below 7 × 10−3 (cf. Fig.1.11). This ratio increases with increasing εm and reaches values of up to 0.16 for εm = 10. Light scattering thus only becomes significant for the extreme cases of the largest particles in the NP layer and very high εm . 1 0 0
T r a n s m is s io n ( % )
9 0 8 0 1 5 n m
C u P c
7 0 6 0 5 0
1 0 n m 4 0 0
5 0 0
S u b P c 6 0 0
7 0 0
8 0 0
9 0 0
W a v e le n g th ( n m )
Figure 2.7: Direct (gray curves) and total, i.e. the sum of direct and diffuse, (black curves) transmission spectra of a 10 nm thick SubPc layer and 15 nm thick CuPc layer deposited either on glass (solid curves) or on the Ag NP layer (dashed curves). These spectra were measured versus a clean glass reference sample. The direct transmission does not include scattered light, whereas, when measuring the total transmission, scattered light is captured using an integrating sphere.
This low scattering efficiency was confirmed experimentally by comparing the direct and total transmission of the Ag NPs coated with organic media, as shown in Fig. 2.7. The direct transmission, which does not include scattered light, was identical to the total transmission, which includes scattering, even when the NPs were covered by SubPc with a ε that reaches values of up to 10.5. Throughout this chapter and Chapter 3, the light extinction is therefore approximated by the absorption, and far-field scattering is neglected. As we will see later, this approximation does not hold when discussing the effect of the Ag NPs on light emission of nearby molecules in Chapter 4.
RESULTS AND DISCUSSION
2.4.2
37
Excitation of multiple dipole resonances
∆A b s o r p tio n ( % )
A b s o r p tio n ( % )
A b s o r p tio n ( % )
As experimentally verified above for transparent embedding media, spherical metal NPs typically exhibit a single dipole LSPR. In contrast to transparent materials with a spectrally independent ε, the highly absorptive organic semiconductors employed in the active layer of organic solar cells can exhibit a highly dispersive ε due to their intense and narrow absorption bands. To probe the effect of such highly dispersive materials on the LSPR excited in metal NPs, we chose SubPc, an organic semiconducting molecule with very strong absorption in limited absorption bands and applications in organic solar cells [187, 188].
6 0
(a ) 3 0 n m
4 0 2 0
6 n m 0 6 0
(b )
4 0
3 0 n m
2 0
6 n m 0
(c )
2 0
6 n m 1 0
3 0 n m 0 3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
W a v e le n g th ( n m )
Figure 2.8: (a) Absorption spectra of Ag NPs covered by SubPc with a layer thickness of 6, 10, 14, 18, 22, and 30 nm. (b) Absorption spectra of SubPc layers on glass, with the same thicknesses as in (a). (c) Difference between the absorption spectra of thin-films containing SubPc and Ag NPs shown in (a) and those of pure SubPc layers shown in (b) for each SubPc layer thickness.
38
OPTICAL PROPERTIES OF METAL NANOPARTICLES COVERED BY ORGANIC THIN-FILMS
1 0
(a ) 4 2
, A g
1
-1 0
ε2
3
ε1
, A g
0
-2 0 0
(b ) u b P c
8
εS
4
2 0
Im [
3
α / (4πR ) ]
(c )
2 0 1 6
1 0
1 2 8 0 4 0
∆A b s o r p tio n ( % )
0 2 4
-1 0 3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
W a v e le n g th ( n m )
Figure 2.9: (a,b) Real (ε1 , solid curve) and imaginary part (ε2 , dashed curve) of the relative permittivity of (a) Ag and (b) SubPc. (c) The imaginary part of the dimensionless fraction of the polarizability, α/(4πR3 ), for a spherical Ag NP embedded in SubPc is shown as a solid curve. For comparison, the absorption difference between a thin-film consisting of the Ag NP layer covered by 10 nm of SubPc and a pure SubPc layer with a thickness of 10 nm is shown (open circles).
The absorption spectra of thin-films consisting of the Ag NP layer introduced in Section 2.4.1, covered by 6 to 30 nm of SubPc are shown in Fig. 2.8(a), whereas the spectra of pure SubPc thin-films with the same thicknesses are shown in Fig. 2.8(b). In both cases, the strengths of the high energy Soret band (λ = 320 nm) and the low energy Q band (450 nm < λ < 630 nm) increase with increasing SubPc layer thickness. In order to determine the impact of the presence of the Ag NPs to the absorption of these layers, each absorption spectrum of the pure SubPc layers was subtracted
RESULTS AND DISCUSSION
39
from that of the thin-film consisting of Ag NPs and SubPc with the same layer thickness. These absorption differences feature two bands centered at λ = 445 nm and λ = 610-630 nm, as shown in Fig. 2.8(c). The intensity of both bands decreases with increasing SubPc thickness. It should be noted that for all SubPc layer thicknesses the peak wavelengths of the Soret and the Q bands are not altered by the presence of the Ag NPs. The low energy band observed in Fig. 2.8(c) rather results from an extension of the lowest energy absorption tail of the composite film shown in Fig. 2.8(a). The analysis of this absorption difference is complicated by the fact that in addition to the absorption in the Ag NPs, contributions from enhanced absorption in the SubPc layer via the increased electromagnetic field surrounding the NPs are expected (see Section 1.4.3). The wavelengths at which absorption in the Ag NPs contributes to the absorption difference can be estimated by calculating the dipole polarizability (α) for a spherical Ag NP embedded in SubPc, using Eq. (1.4). The relative permittivity of SubPc is shown in Fig. 2.9(b). It is highly dispersive, with its real (ε1,SubPc ) and imaginary part (ε2,SubPc ) featuring an intense peak at λ = 603 nm and λ = 591 nm, respectively, corresponding to the strong light absorption of SubPc at these wavelengths. Interestingly, as a result of the highly dispersive permittivity of SubPc, two dipole LSPRs at λ = 430 and 624 nm are obtained from Eq. (1.4), as shown by the solid curve in Fig. 2.9(c). This agrees well with the observed absorption differences [open circles in Fig. 2.9(c)] and suggests that, indeed, dipole LSPRs contribute to both absorption bands. In order to obtain further evidence for the nature of the absorption difference, 3D numerical simulations were performed as described in Section 2.3. The difference between the simulated absorption in the SubPc covering the truncated Ag spheroids and the pure SubPc layer is shown as a solid curve for several SubPc thicknesses in Fig. 2.10(a). These spectra show a broad band with two peaks at λ = 452 and 624 nm, and can be attributed to the plasmonic absorption enhancement in SubPc. The comparison with the simulated absorption spectra of a pure SubPc layer [dashed curve in Fig. 2.10(a)] reveals that this plasmonic absorption enhancement, which will be discussed in more detail in Chapter 3, peaks at the blue and red absorption tail of the low energy Q band. The simulated spectra of light absorbed in the truncated Ag spheroids when covered by SubPc layers with several thicknesses are shown as solid curves in Fig. 2.10(b). These spectra feature two distinct bands centered at λ = 446 and 632 nm. For the absorption in the Ag as well as for the absorption difference in SubPc, the intensity of all bands decreases with increasing SubPc layer thickness.
OPTICAL PROPERTIES OF METAL NANOPARTICLES COVERED BY ORGANIC THIN-FILMS
∆A b s o r p tio n in S u b P c ( a .u .)
2 .0
5
(a ) 4
1 .5 3
A b s o r p tio n in S u b P c ( a .u .)
40
1 .0 2
1 0 n m 0 .5 1 3 0 n m
A b s o r p tio n in A g ( a .u .)
0 .0
0
(b ) 5 4 3 2 1 0 n m 1
3 0 n m 0 4 0 0
5 0 0
6 0 0
7 0 0
W a v e le n g th ( n m )
Figure 2.10: Results of 3D numerical simulations. The cross-section of the simulation geometry is shown in Fig. 2.3. (a) Difference between the simulated absorption in the SubPc layer covering the truncated Ag spheroids and the absorption in the neat SubPc layer with the same volume of the organic material for layer thicknesses of 10, 14, 18, 22, and 30 nm (solid curves). The simulated absorption of a pure SubPc layer with a thickness of 10 nm is shown as a dashed curve. (b) Simulated absorption in the uncovered truncated Ag spheroids (dashed curve) and in the truncated Ag spheroids covered by SubPc layers with the same thicknesses as in (a) (solid curves). A center-to-center distance (s) of 14 nm was chosen for these simulations.
The fact that the low energy peak in the simulated Ag absorption spectra is more intense than the high energy peak is in-line with the resonance strengths obtained from Eq. (1.4) for a spherical Ag NP in SubPc. It is however in contrast to the experimentally observed absorption differences between SubPc thin-films with and without Ag NPs, which will be further discussed in Chapter 3. Qualitatively, however, these results are well in agreement with the experimentally observed results and clearly confirm the presence of two plasmon resonances in the Ag
CONCLUSIONS
41
SubPc
1 2 |E| (a.u.)
Ag
0
Glass
+
y
-
+
-
-1
x
l = 446 nm
1 Div(E) (a.u.)
l = 632 nm
Figure 2.11: Surface plots of the simulated electric field intensity (top) and charge density (bottom) at λ = 446 nm (left) and 632 nm (right) for a Ag NP covered by 10 nm of SubPc. A center-to-center distance (s) of 14 nm was chosen for these simulations.
NPs surrounded by SubPc. Furthermore, as observed experimentally, the low energy resonance peak is not present in the case of the uncovered truncated Ag spheroids [dashed curve in Fig. 2.10(b)]. Finally, the simulated charge density in the truncated Ag spheroid covered by 10 nm of SubPc as well as the electric field intensity surrounding the spheroid, both shown in Fig. 2.11, confirm that both LSPRs are indeed dipolar in nature.
2.5
Conclusions
We examined the influence of the embedding medium on the LSPR excited in Ag NPs covered by thin-films. For transparent media with a high, spectrally independent ε, we observed the expected red-shift of the dipole LSPR. In composite layers of the Ag NPs covered by the strongly absorbing organic semiconductor SubPc, the presence of the Ag NPs led to two absorption bands. By means of numerical simulations, plasmonic absorption enhancement in the SubPc layer as well as absorption in the Ag NPs was found to contribute to both bands. By considering the simulated electric field surrounding the Ag NPs as well as calculations applying the quasistatic approximation, we were able to attribute the absorption in the Ag NPs at both absorption bands to dipole
42
OPTICAL PROPERTIES OF METAL NANOPARTICLES COVERED BY ORGANIC THIN-FILMS
LSPRs. Given this, we conclude that embedding spherical metal NPs in a highly dispersive medium can give rise to multiple absorption bands originating from dipole resonances, which is otherwise only exhibited by non-spherical NPs [94], and could be employed to broaden the spectral range of metal NP-based sensors. Furthermore, as the increase in absorption in SubPc benefits from the multiple LSPRs of the Ag NPs [cf. Fig 2.10(a)], this feature might be beneficial for a potential application in organic solar cells, where enhanced absorption in the red tail of the absorption spectrum of the organic semiconductor should give rise to a corresponding increase in photocurrent. These results demonstrate that the interactions between metal NPs an the embedding medium can give rise to unexpected phenomena. Therefore, when designing a metal NP-organic molecule system with a specific aim, for example for absorption or emission enhancement, these interactions have to be taken into account.
Chapter 3
Plasmon-exciton near-field interactions: absorption 3.1
Introduction
The light absorption and thus also the exciton generation in an organic medium can be increased in presence of metal NPs due to plasmonic near-field enhancement (cf. Section 1.4.3). At the same time, the presence of metal NPs decreases the lifetime of nearby excitons. When metal NPs are applied in organic solar cells to increase the cell performance, beneficial absorption enhancement should be maximized, while exciton quenching should be minimized. To that end, we study the near-field interactions between LSPRs excited in metal NPs and excitons in organic thin-films by employing the Ag NP layer introduced in Chapter 2. As the Ag NPs possess a size of less than 12 nm, they are in a size-regime where far-field scattering is negligibly small compared to absorption (cf. Section 2.4.1), which allows to exclusively probe near-field interactions. In this chapter, we examine the effect of the Ag NP layer on the absorption of organic molecules. In Chapter 4, we probe the impact of the Ag NP layer on the emission of organic molecules by steady-state and time-resolved photoluminescence measurements. All the experimental details of this chapter are described in Section 2.2.
43
44
3.2 3.2.1
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: ABSORPTION
Results and discussion Absorption enhancement in CuPc
We chose CuPc, a well-characterized organic semiconductor, as a representative material to study the effect of the Ag NP layer on the absorption of organic molecules. A major advantage of this material choice is the complementary nature of the absorption spectra of CuPc and the Ag NP layer. The absorption spectrum of the Ag NP layer is shown in as a dotted curve in Fig. 3.1(a). It features a single absorption band centered at λ = 430 nm due to a dipole LSPR, as discussed in Section 2.4.1. The absorption spectrum of a 10 nm thick CuPc layer on glass is shown as a dashed curve. It exhibits two absorption bands: the Soret band centered at λ = 345 nm and the low energy Q band with peaks at λ = 625 and 690 nm. The absorption spectrum of 10 nm CuPc deposited on the Ag NP layer (solid curve) exhibits the same features as well as the LSPR absorption band of the Ag NPs. The LSPR is slightly red-shifted to λ = 450 nm compared to that of the bare Ag NP layer due to the change in the dielectric environment. The absorption difference between the sample with and without Ag NPs is shown as a dash-dot-dot curve. It features a band at 550 nm < λ < 800 nm that is not present in the absorption spectrum of the uncovered Ag NP layer. This increase in absorption has been attributed to an absorption enhancement in the CuPc medium caused by the LSPR excited in the Ag NPs [98, 189]. In the following paragraphs, this absorption enhancement will be characterized in detail and it will be discussed whether it exclusively originates from the Ag NP LSPR. To this end, 3D finite-element numerical simulations were made as described in Section 2.3, with a geometry that was motivated by our microscopy study of the Ag NP layer shown in Section 2.4.1. In short, the Ag NPs are represented by truncated spheroids with a height of 5 nm and a lateral dimension of 7 nm. Periodic boundary conditions in the lateral directions lead to a Ag NP array with a center-to-center spacing denoted by s. An organic layer with a thickness t and a flat surface covers the Ag NPs. When the simulation results from a neat organic film are compared to those of an organic film on Ag NPs, the thickness of the film on Ag NPs is chosen slightly larger than that of the neat reference film such that both organic layers have the same volume, compensating for the volume occupied by the Ag NPs. The software package used for these simulations, COMSOL Multiphysics 3.5a, allowed us to monitor the absorption in each component individually and therefore to deconstruct the absorption spectra of composite layers as shown in Fig. 3.1(b). The simulated absorption of a pure 10 nm thick CuPc layer
RESULTS AND DISCUSSION
45
3 0
A b s o r p tio n ( % )
2 0
S im u la te d a b s o r p tio n ( a .u .)
(a )
1 .2
A g N P s C u P c
A g N P s /C u P c ∆A b s o r p t i o n
1 0
0
(b )
B a re A P la in C A g N P s /C u P A g N P C u P c
0 .9
g N P s u P c la y e r c : s
0 .6 0 .3 0 .0 3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
W a v e le n g th ( n m ) Figure 3.1: (a) Absorption spectra of the bare Ag NP layer (dotted curve), the Ag NP layer coated by 10 nm of CuPc (solid curve), and a 10 nm thick CuPc layer on glass (dashed curve). The absorption difference between the CuPc layers with and without Ag NPs is shown as dash-dot-dot curve. (b) Results obtained from 3D numerical simulations (details are given in Section 2.3). This graph shows the simulated absorption in the bare Ag NPs (dotted curve), a pure 10 nm thick CuPc layer (dashed curve), a CuPc layer covering the Ag NPs with t equivalent to the pure CuPc layer (solid curve), and the Ag NPs when covered by that layer (dash-dot-dot curve). For this simulation, s was set to 18 nm.
and that of a CuPc layer with equivalent t covering the Ag NPs are shown as dashed and solid curves, respectively. The absorption in the Q band is significantly enhanced in presence of the Ag NPs, whereas the Soret band remains unchanged. The simulated absorption in the Ag NPs when covered by CuPc (dash-dot-dot curve) exhibits a sharp peak at λ = 460 nm, red-shifted compared to the absorption peak of the uncovered NPs at λ = 390 nm (dotted
46
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: ABSORPTION
curve). In addition, it also features a very weak absorption band in the CuPc Q band spectral region, which is absent for the uncovered NPs. Combined, these simulated spectra of the individual components accurately describe the experimentally obtained spectra shown in Fig. 3.1(a), including the absence of absorption enhancement at λ smaller than the LSPR wavelength (λLSPR ). Moreover, these results suggest that most of the absorption in the spectral region of the CuPc Q band occurs within the organic medium and not in the NPs, which is in-line with previously reported results from analytical calculations [189]. The slight mismatch of λLSPR and the narrower absorption band compared to the experimentally observed results can be explained by the uniform size and shape of the simulated particles and the uncertainty about the exact NP shape. In order to determine the range of the absorption enhancement, samples with various CuPc layer thicknesses were fabricated. The absorption spectra of the samples with and without Ag NPs, shown in Fig. 3.2(a) as solid and dashed curves, respectively, clearly show a decreasing influence of the Ag NPs with increasing CuPc layer thickness. The absorption enhancement at λ = 690 nm obtained from these measurements is shown in Fig. 3.2(b) as black squares. It decreases from 1.44 ± 0.08 for a 8 nm thick CuPc layer to 1.06 ± 0.03 for a 50 nm thick layer. This experimentally measured thickness dependence could be reproduced by numerical simulations [gray area in Fig. 3.2(b)]. Quantitative agreement was obtained when s was set to 18 nm for the thinnest and to 14 nm for the thickest CuPc layer. Both these values are in-line with typical center-to-center distances observed in Fig. 2.4(a). A possible cause for the deviation between experiment and simulation could be the roughness of the CuPc layer surface, which is not taken into account in the simulations and becomes increasingly important for thinner layers. The absorption enhancement obtained with these simulations is equivalent to the enhancement of the E field intensity due to the Ag NPs, |ENP |2 /|E0 |2 , where E0 and ENP denote the E field without NPs and in their presence, respectively. The contour-plot of |ENP |2 /|E0 |2 at λ = 690 nm in the x-y plane cutting through the center of the simulated Ag NP therefore yields direct information about the range of the absorption enhancement [cf. Fig. 3.3(a)]. The absorption is strongly enhanced between the NPs, whereas it is reduced above the NPs as evidenced by |ENP |2 /|E0 |2 < 1. The border between enhanced and reduced absorption (the |ENP |2 /|E0 |2 = 1 contour) extends from the top of the NP to the border of the simulation halfway to the neighboring NP where it levels off at a height of 12 nm.
RESULTS AND DISCUSSION
47
A b s o r p tio n ( % )
5 0
(a )
4 0 3 0
5 0 n m
2 0 1 0
1 0 n m 0 5 5 0
6 0 0
6 5 0
7 0 0
7 5 0
8 0 0
1 4 1 .6
(b )
E x p e r im e n t S im u la tio n
s (n m )
A b s o r p tio n e n h a c n e m e n t
W a v e le n g th ( n m )
1 8 1 .4
1 .2
1 .0
λ= 6 9 0 n m 1 0
2 0
3 0
4 0
5 0
C u P c th ic k n e s s ( n m )
Figure 3.2: (a) Absorption spectra of CuPc layers deposited on glass (dashed curves) or on the Ag NP layer (solid curves) for CuPc layer thicknesses of 10, 20, 30, and 50 nm. (b) Absorption enhancement due to the Ag NP layer at λ = 690 nm determined by dividing the absorption of the Ag NP layer coated with CuPc by that of pure CuPc layers for CuPc layer thicknesses between 8 and 50 nm (black squares). The gray area shows the absorption enhancement at the same wavelength and for the same CuPc thickness range obtained by dividing the simulated absorption in the CuPc layer covering the Ag NPs by that of a CuPc layer on glass with equivalent t. Center-to-center spacing (s) values of 14 and 18 nm were used for these simulations.
The very large field enhancement close to the Ag NP surface observed in this graph might possibly not contribute to absorption enhancement in real-world experiments as the distance between a Ag surface and the first CuPc monolayer is about 0.3 nm [190]. However, as the exact morphology of the Ag NP-CuPc
48
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: ABSORPTION
y (nm)
8
2
|E NP | / |E 0|
(b)
2
λ = 690 nm 1.0
0.8
0.6
10 nm
10
(a)
50 nm
12
CuPc
6 3.0
4 2 0 0
2.0
Ag NP
2
4
x (nm)
6
8
1.0
1.5 2
2
2.0 2
|E NP | / |E 0|, |E 0| (a.u)
Figure 3.3: (a) Simulated |ENP |2 /|E0 |2 around a Ag NP when covered by a 50 nm thick CuPc layer. (b) Simulated average |ENP |2 /|E0 |2 value integrated over a slice of CuPc medium in the x-z plane versus the height along the y-axis for a 10 nm (gray solid curve) and 50 nm (black dashed curve) thick layer. The dotted black curves show the simulated |E0 |2 versus the y-axis in neat CuPc layers with thicknesses of 10, 15, 20, 30, and 50 nm. s was set to 18 nm for all simulations shown in this figure. The simulation results shown in this figure were obtained at λ = 690 nm.
interface is not known and the volume of the void is very small compared to the total CuPc layer volume, we did not take this metal-molecule separation into account, which means that the simulated absorption enhancement will be slightly overestimated. Figure 3.3(b) shows the average |ENP |2 /|E0 |2 value integrated over a slice of CuPc medium in the x-z plane versus the height along the y-axis for CuPc layer thicknesses of 10 nm (gray solid curve) and 50 nm (black dashed curve). |ENP |2 /|E0 |2 is nearly identical for both layer thicknesses, with |ENP |2 /|E0 |2 > 1
RESULTS AND DISCUSSION
49
A b s o r p tio n ( n o r m a liz e d )
1 .0 0 .9 0 .8 0 .7 0 .6 0 .5 5 6 0
6 0 0
6 4 0
6 8 0
7 2 0
W a v e le n g th ( n m ) Figure 3.4: Normalized measured absorption spectra of 10 nm CuPc deposited on glass (solid gray triangles) and on the Ag NP layer (open black squares), normalized simulated absorption in a 10 nm thick CuPc layer on glass (dashed gray curve) and in a CuPc layer with equivalent t covering the Ag NPs (solid black curve), and the absorption spectrum obtained by normalizing the product of the experimentally measured absorption of 10 nm CuPc on glass and |ENP |2 /|E0 |2 max for a Ag NP in CuPc obtained by Eq. (1.14) (dotted black curve). The values of d and R used to calculate |ENP |2 /|E0 |2 max are not important as the results were finally normalized.
up to a height of 7 nm. Above this height, |ENP |2 /|E0 |2 converges to a value of 0.94. These simulation results allow us to explain the experimentally observed CuPc layer thickness dependence shown in Fig. 3.2. As only the first 7 nm of the CuPc medium benefit from the absorption enhancement, increasing the CuPc layer thickness beyond that height will lead to a reduction in absorption enhancement relative to the total absorption of the layer. Moreover, |E0 |2 decreases with increasing CuPc layer thickness [black dotted curves in Fig. 3.3(b)], which means that for the same |ENP |2 /|E0 |2 value, |ENP |2 − |E0 |2 and thus also the absorption difference due to the Ag NPs will be smaller for thicker layers. A rather subtle feature of the absorption enhancement can be observed when analyzing the CuPc Q band spectral region (550 nm < λ < 800 nm) of the absorption spectra shown in Fig. 3.1. The normalized experimentally measured absorption spectra of the samples with and without Ag NPs are shown in Fig. 3.4
50
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: ABSORPTION
as black squares and gray triangles, respectively. The normalized simulated spectra with and without Ag NPs are shown as black solid curve and gray dashed curve, respectively. The relative intensities of the peaks at λ = 690 nm in the experimental and simulated spectra are enhanced by a similar amount in presence of the Ag NPs. Considering the experimental results only, this change in relative peak intensity might be interpreted as a change in the CuPc layer morphology when grown on the Ag NP substrate, as this ratio is an indicator of the degree of molecular aggregation within the CuPc layer [191]. An increased relative intensity of the peak at λ = 690 nm might indicate a decrease in CuPc aggregation. However, the numerical simulations, that do not take into account any change in the interactions between the CuPc molecules or between the molecules and the substrate when introducing the NPs, accurately reproduce this change in relative peak intensities. Therefore, we conclude that the spectral shape of the absorption enhancement can be reproduced by only taking into account the wavelength-dependent enhancement of the electromagnetic field intensity around the NPs. In addition to numerical simulations, the absorption enhancement due to the Ag NPs can also be described analytically. Because of the small Ag NP size relative to the wavelengths of the measured spectra, the E field across an illuminated NP is approximately constant and the quasistatic approximation can be employed. According to this approximation, the dipole polarizability (α) of a spherical metal NP with radius R in a homogeneous medium is given by Eq. (1.4). In a non-absorbing medium, the maximal E field intensity enhancement, |ENP |2 /|E0 |2 max , at the distance d to the surface of the NP is given by Eq (1.14). As Eq. (1.14) is only exact for non-absorbing media, |ENP |2 /|E0 |2 max can merely be considered as an upper limit for the E field enhancement in absorbing media, where an accurate treatment requires employing an electrodynamic formalism [189, 192]. For this reason, and because it assumes a spherical geometry, Eq. (1.14) will only yield qualitative information when applied to the thin-film system studied here. We calculated |ENP |2 /|E0 |2 max for Ag NPs embedded in CuPc by inserting the relative permittivity (ε) values of Ag and CuPc shown in Fig. 2.9 and Fig. 3.6 into Eq. (1.4) and Eq. (1.14). The resulting values were then multiplied with the experimentally measured absorption spectrum of the neat 10 nm thick CuPc layer and finally normalized. This calculated absorption spectrum is shown as a dotted black curve in Fig. 3.4. It is nearly identical to the simulated spectrum of the composite layer and thus also well in agreement with the experimental data. This means that the spectral shape of the absorption enhancement in the CuPc Q band is directly related to the relative permittivities of the Ag NPs
RESULTS AND DISCUSSION
51
(εNP ) and the embedding medium (εm ). This also suggests that the influence of small deviations of the NP shape from a perfect sphere as well as particle-substrate and particle-particle interactions are negligible. The limited influence of the substrate can be explained by its low ε compared to that of CuPc in this spectral region, such that the LPSR is more strongly affected by the organic medium. The negligible contribution of particle-particle interactions is expected for a Ag NP layer where the distance between the NPs is similar to their diameter, as such interactions would only become relevant for denser NP layers [193].
3.2.2
Spectral dependence and influence of the permittivity
After having confirmed the validity of numerical simulations to describe the absorption enhancement in CuPc, we extend these simulations to other embedding materials with spectrally-independent ε = ε1 + iε2 , employing the same geometry as used for CuPc. Figures 3.5(a) and (b) show the simulated absorption in the Ag NPs covered by these various materials. When ε2 of the material is set to 1 and ε1 is altered from 1 to 8, the LSPR absorption peak of the Ag NPs red-shifts from λ = 390 nm to 660 nm while its intensity increases by almost one order of magnitude [cf. Fig. 3.5(a)]. For constant ε1 = 3, a typical value for many organic materials, an increase in ε2 dampens the resonance intensity such that the absorption peak completely vanishes for ε2 ≥ 4, whereas the resonance frequency is not affected [cf. Fig. 3.5(b)]. The simulated absorption enhancement in these embedding media due to the Ag NPs is shown in Figures 3.5(c) and (d). It scales with the intensity of the LSPR and is strongest at λ slightly larger than λLSPR . For this set of materials, values of up to 3.2 are reached for ε = 3 + 0.5i and ε = 8 + 1i. For all of these materials, the enhancement steeply decreases at λ < λLSPR , whereas at λ > λLSPR , it exhibits an extended tail with values above unity up to λ = 800 nm. For highly absorbing media (ε2 ≥ 2), the absorption enhancement only reaches values below 1.5, with an off-resonance enhancement that is comparable or even stronger than that at λLSPR . This strong dependence of the absorption enhancement on ε2 is consistent with previously reported results obtained using an analytical model [194]. In summary, the strongest absorption enhancement is thus reached for materials with a large ε1 and a weak absorption, resulting in a small ε2 . Typically, the enhancement is strongest slightly red-shifted to λLSPR . However, exploiting the off-resonance enhancement rather than the stronger enhancement at λLSPR can
52
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: ABSORPTION
( a ) ε2 = 1 , ε1 =
A b s o r p tio n e n h a n c e m e n t in la y e r
A b s o r p tio n in A g N P ( a .u .)
4 3
1
3 2
5
(b )
ε1 = 3 , ε2 = 0 .5 1
8 2
2
4
ε2
ε1 1
8
0 3 .0
(c )
ε2 = 1 , ε1 = 1
ε1 = 3 , ε2 =
(d ) 8
2 .5
0 .5
2
4
3
2 .0
8
ε1 5
ε2
1 .5 1 .0 3 0 0
2
1
4 0 0
5 0 0
6 0 0
7 0 0
W a v e le n g th ( n m )
4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
W a v e le n g th ( n m )
Figure 3.5: (a,b) Simulated absorption in the Ag NPs when covered by a layer with a volume identical to that of a 10 nm thick neat layer for various (a) ε1 and (b) ε2 . (c,d) Simulated absorption enhancement in the layer covering the Ag NPs compared to a 10 nm thick plain layer. s was set to 18 nm for all these simulations.
still be beneficial as the absorption in the Ag NPs is much smaller far away from the resonance. This effect is especially relevant for applications where reducing parasitic absorption of the metal NPs is advantageous. The fact that the absorption enhancement in the embedding medium is strongest slightly red-shifted to the Ag NP absorption peak indicates that it originates from near-field components of light scattered by the NPs [189, 195]. This near-field scattering can be quite strong even for poor far-field scatterers, such as the NPs used here. We can apply the knowledge gained from these simulation results to explain the spectral shape of the CuPc Q band absorption enhancement. For a material with constant ε ≈ 3 + 2i, the absorption enhancement is expected to slightly decrease with increasing λ for λ > λLSPR . However, in the case of CuPc, this decrease is outweighed by a larger ε1 and smaller ε2 at λ = 690 nm compared to the ε at λ = 625 nm, leading to a stronger absorption enhancement at
RESULTS AND DISCUSSION
1 2
53
(a ) C u P c S u b P c H y flo n A D 6 0 X 9
ε1
6 3 0
(b ) 8
C u P c S u b P c H y flo n A D 6 0 X 6
ε2
4 2 0 3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
W a v e le n g th ( n m ) Figure 3.6: (a) Real and (b) imaginary part of the relative permittivity of CuPc (solid curves), SubPc (dashed curves), and Hyflon AD60X (dotted curves).
λ = 690 nm (solid curves in Fig. 3.6). This effect is even more pronounced for SubPc, a material with a highly dispersive ε (dashed curves in Fig. 3.6). When deposited on the Ag NP layer, the relatively low value of ε1 at 400 nm < λ < 550 nm results in the excitation of a LSPR at λ = 445 nm. Due to the narrow and intense absorption band of SubPc at λ = 590 nm (cf. Fig. 2.8), ε1 steeply increases at λ = 570-600 nm and reaches values of up to 10.5. This leads to the excitation of a second dipole LSPR at λ = 610 nm, as shown in Section 2.4.2. These two LSPR are evidenced by an additional absorption band peaking at λ = 445 nm, as well as an extension of the red tail of the SubPc absorption band centered at λ = 590 nm.
3.2.3
Effect of a spacer layer
We studied the effect of a spacer layer on the plasmon-exciton interactions for both CuPc and SubPc by introducing a layer of Hyflon AD60X between the
54
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: ABSORPTION
(a)
Absorption enhancement
∆ Absorption (%)
25 20 15
λ = 690 nm
1.4 1.2 1.0 0
10
0 nm
5
10
Hyflon AD60X thickness (nm)
20
20 nm
0
(b) Spacer thickness (nm) 0 1 3 5 10 20
∆ Absorption (%)
25 20 15 10 5 0 300
400
500
600
700
800
Wavelength (nm)
Figure 3.7: Absorption differences between samples with and without Ag NPs. These samples were made by depositing Hyflon AD60X layers with thicknesses of 1, 3, 5, 10, and 20 nm either on the Ag NP layer or on glass, followed by a 10 nm thick layer of either (a) CuPc or (b) SubPc. For both CuPc and SubPc, the absorption differences of samples without a Hyflon AD60X layer are shown as solid curves. The inset to (a) shows the absorption enhancement ratio at λ = 690 nm versus the Hyflon AD60X layer thickness for the CuPc samples.
Ag NP layer and the organic medium. Thin-films of Hyflon AD60X were found to be completely transparent in the measured spectral range [cf. Fig. 3.6(b)]. Moreover, because of the small ε1 of Hyflon AD60X, the LSPR of the Ag NPs was only slightly altered when covered with it (cf. Fig. 2.6 and Fig. 3.6). We fabricated samples without a spacer and ones that had spacer layers with
RESULTS AND DISCUSSION
55
thicknesses of 1 to 20 nm. For each spacer layer thickness, we fabricated a sample with and one without Ag NPs. For the reference samples without Ag NPs, Hyflon AD60X was directly deposited on the glass substrate. The spectra of the absorption differences between the samples with and without Ag NPs are shown in Fig. 3.7(a) and (b) for 10 nm thick layers of CuPc and SubPc, respectively. The absorption difference in the CuPc Q band rapidly decreases with increasing spacer layer thickness and finally vanishes for a 5 nm thick spacer, as shown by Fig. 3.7(a). For spacer layer thicknesses of 5-20 nm, the absorption differences thus resemble the absorption curve of the Ag NP layer covered only by Hyflon AD60X. The absorption enhancement at λ = 690 nm shown in the inset quantifies this rapid decrease, which is much steeper than that observed for an increasing CuPc layer thickness without spacer layer [cf. Fig. 3.2(b)]. The SubPc samples [cf. Fig. 3.7(b)] also show a vanishing absorption difference for spacer layers thicknesses beyond 3 nm, but with a notably different behavior for the three thinnest spacer layers compared to the CuPc samples. In particular, the peak observed at λ = 610 nm without a spacer layer gives way to a shoulder at λ = 600 nm and a peak at λ = 595 nm for Hyflon AD60X layer thicknesses of 1 and 3 nm, respectively. To gain a better understanding of these results, we conducted numerical simulations using the same geometry as shown in Fig. 2.3, but with an additional conformal Hyflon AD60X layer separating the Ag NPs from the SubPc layer. The simulated absorption in the Ag NPs and the increase in absorption in the SubPc layer caused by the NPs are shown in Fig. 3.8(a) and (b), respectively. Without the spacer layer, the absorption in the Ag NPs shows two bands centered at λ = 445 nm and 630 nm, respectively, due to the two dipole LSPR. These two bands lead to strong absorption enhancement in the SubPc layer at these wavelengths. The intensity of the Ag NP absorption band at λ = 630 nm rapidly decreases with increasing spacer layer thickness and completely vanishes at a spacer thickness of 1 nm. The increase in absorption in the SubPc layer, on the other hand, remains for all simulated spacer layer thicknesses and features the shoulder-to-peak transition around λ = 600 nm observed experimentally [cf. Fig. 3.7(b)]. For both CuPc and SubPc, the threshold spacer thickness for absorption enhancement is between 3 and 5 nm, which is about the same as the Ag NP height. These results confirm that the absorption enhancement occurs within a few nanometers from the Ag NP surface independent of the embedding medium. In addition, the effect of εm on the absorption enhancement can indirectly be observed. By increasing the thickness of a spacer layer with a low
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: ABSORPTION
∆A b s o r p tio n in S u b P c ( a .u .) A b s o r p tio n in A g N P
(a .u .)
56
1 4
(a ) S p a c e r th ic k n e s s (n m ) 0 0 .2 0 .5 1 3
1 2 1 0 8 6 4 2 0
(b ) 4 3 2 1 0 4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
6 5 0
W a v e le n g th ( n m ) Figure 3.8: (a) Simulated absorption spectra in the Ag NPs when covered by a SubPc layer with t equivalent to a 10 nm thick neat layer, with and without a conformal spacer layer of Hyflon AD60X separating the NPs from the SubPc layer. (b) Difference between the absorption in the SubPc layer covering the Ag NPs and a 10 nm thick neat SubPc layer, with and without Hyflon AD60X spacer layer.
ε, the effective εm gradually decreases from that of the initial organic medium to that of the spacer layer. For CuPc with ε1 < 4.5 , this transition is not too dramatic. For SubPc with ε1 values of up to 10.5, on the other hand, it leads to large changes in the spectral shape. Especially the excitation of the LSPR at λ = 610 nm is extremely sensitive to the embedding medium and is suppressed by a spacer layer of only 1 nm. The LSPR at λ = 445 nm on the other hand is much less effected by the spacer layer as at this wavelength the ε of SubPc is similar to that of Hyflon AD60X. It is noteworthy that the simulated absorption in the Ag NPs for a spacer layer
CONCLUSIONS
57
thickness of 0.2 nm describes the experimentally obtained absorption difference for SubPc without spacer layer much more accurately than the simulation without spacer layer. As already discussed for the numerical simulation of the Ag NPs embedded in CuPc, this distance roughly corresponds to the distance between the Ag surface and the first molecular adlayer (cf. Section 3.2.1). For SubPc where the low energy LSPR strongly depends on εm , the presence of this small gap makes a considerable difference.
3.3
Conclusions
We studied the absorption of thin-films comprising Ag NPs that show negligible far-field scattering covered by organic media by means of experiment, numerical simulations, and analytical calculations. For CuPc, we observed an increased intensity of the absorption Q band due to the Ag NP layer with absorption enhancement values of up to 1.44 ± 0.08 at λ = 690 nm for an 8 nm thick CuPc layer. A strong dependence of this absorption enhancement on the CuPc layer thickness was found. We could quantitatively reproduce these experimental results by numerical simulations. From these simulations, the absorption enhancement was found to mainly occur in the CuPc medium, caused by an enhanced E field intensity in the interstices of the Ag NPs. The short range of this enhancement was confirmed experimentally by introducing a transparent spacer layer between the NPs and the CuPc layer, which led to a rapid decrease of the absorption enhancement with increasing spacer thickness. By employing an analytical model, the spectral shape of the CuPc absorption enhancement could be directly linked to the ε values of the NP material and the embedding medium, while neglecting particle-particle and particle-surface interactions. We studied the effect of εm on the plasmon-exciton interactions by means of numerical simulations of materials with spectrally independent ε and found absorption enhancement at λ > λLSPR , which extends to the near-infrared for ε values typical of molecular media. When introducing a low-index spacer between the Ag NPs and a layer of SubPc, we observed a complex behavior of the spectral shape of the absorption enhancement due to the Ag NPs, which we attribute to the decrease of the effective εm with increasing spacer thickness. Our results show that care has to be taken on both the geometrical features and the material choice when designing metal NP-organic molecule structures with the aim of enhancing the absorption in the organic medium. For example, by coating metal NPs with a transparent layer of only a few nanometers, the
58
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: ABSORPTION
absorption enhancement in an absorbing embedding molecular medium due to the NPs can be strongly affected, and perhaps eliminated completely. In spite of the complex behavior of the plasmon-exciton interactions we show that at least in the case of CuPc and SubPc, the absorption enhancement in the embedding molecular medium due to metal NPs can be predicted accurately by numerical simulations that do not take the organic layer morphology into account. Furthermore, if the NPs are well separated, a simple analytical formula provides quantitative information about the spectral shape of the absorption enhancement. In Chapter 4, we will build upon the knowledge gained in this chapter, and extend our work on plasmon-exciton interactions to the effect of the Ag NP layer on the emission of organic molecules.
Chapter 4
Plasmon-exciton near-field interactions: emission 4.1
Introduction
When an organic molecule absorbs a photon, an exciton is generated by the transition of an electron from the highest occupied molecular orbital (HOMO) into an unoccupied one. The exciton decay due to the relaxation of the electron back to the HOMO can either occur radiatively by emitting a photon, or nonradiatively by heat dissipation, as described in detail in Section 1.2.2. In the presence of nearby metal NPs, both exciton generation and decay can be strongly modified by the LSPRs excited in the metal NPs [101, 118–120]. The influence of these plasmon-exciton interactions on the molecular absorption and thus also exciton generation was discussed in Chapter 3, whereas this chapter focuses on the effect of these interactions on the exciton decay. We utilize the Ag NPs introduced in Chapter 2 to probe the impact of near-field plasmon-exciton interactions on the emission of organic molecules by means of steady-state and time-resolved photoluminescence (PL) spectroscopy. At the end of this chapter, the application of near-field plasmon-exciton interactions for efficiency enhancement of organic solar cells will be assessed based on the results presented in this chapter and Chapter 3.
59
60
4.2
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: EMISSION
Experiments
N,N,N’,N’-tetrakis(4-methoxy-phenyl)benzidine (MeO-TPD, purchased from Luminescence Technology Corp.), chloroboron subphtalocyanine (SubPc, Luminescence Technology Corp.), tris(8-hydroxyquinolinato)aluminium (Alq3 , SigmaAldrich), and 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI, Sensient Imaging Technology GmbH) were purified at least once by thermal gradient sublimation prior to use. Hyflon AD60X, a copolymer of 2,2,4-trifluoro-5trifluorometoxy-1,3-dioxole and tetra-fluoro-ethylene with a molar ratio of 0.6:0.4, was obtained from Solvay-Solexis and used as received. Silver with a purity of 99.99% was purchased from the Kurt J. Lesker Company. 3 cm x 3 cm quartz and Si/SiO2 substrates were cleaned by subsequent sonication in soapy water, deionized water, acetone, and isoproyl alcohol, followed by exposure to UV-ozone for 15 min. Silver, MeO-TPD, Alq3 , SubPc, and PTCBI were deposited by thermal evaporation at a chamber pressure below 2 × 10−6 Torr. The deposition rate and film thickness were monitored by a quartz-crystal oscillator. The organic molecules were deposited at a rate of 0.1 nm/s, Ag at a rate of 0.01 nm/s. Hyflon AD60X layers were deposited in the same chamber by means of vacuum pyrolysis [182], at a chamber pressure below 5 × 10−6 Torr and a rate of 0.05 nm/s. During all depositions, the substrate was rotated and remained at room temperature due to passive cooling and the large distance (∼1 m) between the source and the substrate. A thickness inhomogeneity of 1 at the dipole resonance at λ = 446 nm and values close to unity off-resonance. The narrow dip at λ = 413 nm is due to the higher-order modes of ΓET−0 that are absent in Γrad−0 . Figure 4.5(a) shows the distance dependence of ΓET−0 and Γrad−0 both at the dipole resonance as well as off-resonant at λ = 800 nm. In both cases, the rates rapidly decay with ΓET−0 converging to zero and Γrad−0 to unity. Once both rates have reached their convergence values, the presence of the Ag NP does not influence the molecular emission anymore, and therefore q = q 0 , as shown in Fig. 4.5(b). At the resonance wavelength, Γrad−0 reaches convergence at d ∼ 15 nm, whereas it converges already at d < 5 nm off-resonant. This spectrally dependent decay is also observed for ΓET−0 . However, as it is orders of magnitudes larger than Γrad−0 at the metal NP surface, it requires up to 30 nm to reach values 1. As a result, ΓET−0 dominates the distance-dependence of q/q 0 for q 0 = 1
RESULTS AND DISCUSSION
1 0 0 0 T -0
, ΓE
1 0
Γr a
d -0
1 0 0
ΓE
67
Γr a
/
4 5 0 8 0 0 /
d -0
1 0 .1 q
1 .0
0
(b )
= 0 .0 1
λ(n m )
0
q / q
(a )
λ(n m ) T -0
0 .5 q
0
/
= 1
4 0 0 4 5 0 8 0 0 / /
0 .0 0
5
1 0
1 5
2 0
2 5
3 0
d (n m ) Figure 4.5: Calculated distance dependence of (a) ΓET−0 (gray curves) and Γrad−0 (black curves), and (b) the q/q 0 ratio for q 0 = 0.01 (gray curves) and q 0 = 1 (black curves), at several emission wavelengths.
[cf. Fig. 4.5(b)], with q/q 0 = 0 at the metal NP interface and a value approaching unity when ΓET−0 1. For a small q 0 , the growing importance of Γrad−0 leads to a steeper increase of q/q 0 , especially at the dipole resonance. For all calculations made so far, we assumed εm = 3. However, the decay rates strongly depend on εm , as shown in Fig. 4.6(a) for d = 5 nm. When εm increases from 1 to 10, the dipole LSPR shifts from λ = 357 nm to 692 nm. Moreover, the multipole resonances of ΓET−0 red-shift in a similar manner and additional higher-order modes become visible. At the same time, the peak values of both ΓET−0 and Γrad−0 increase. This increase is rather small for ΓET−0 with a factor of only ∼1.4. The peak value of Γrad−0 , on the other hand, increases by almost a factor of 20, such that the ΓET−0 /Γrad−0 peak intensity ratio decreases from 360 to 26. This relative enhancement of Γrad−0 has a strong impact on q/q 0 , especially for small q 0 , as shown in Fig. 4.6(b). For example, when q 0 = 0.01, q/q 0 1 for εm = 1 at the dipole LSPR wavelength, whereas q/q 0 = 3.3 for εm = 10. Finally, we calculated the influence a modulation of R has on the emission. The size dependences of Γrad−0 , ΓET−0 , and q/q 0 for several q 0 are shown in Fig. 4.7
68
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: EMISSION
εm = 1
5 3
2
7
1 0
(a ) Γ
1 0 0
E T -0
1 0
Γ
E T -0
, Γr a
d -0
1 0 0 0
2
5 3
1
7
1 0
Γ
ra d -0
1
(b )
εm
3 1
5 2
7 3
1 0
0
q / q
2 1 0
4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
W a v e le n g th ( n m ) Figure 4.6: Calculated spectral dependence of (a) ΓET−0 (solid curves) and Γrad−0 (dashed curves), and (b) the q/q 0 ratio for q 0 = 0.01, d = 5 nm, and εm = 1-10.
at the dipole resonance wavelength of 446 nm for εm = 3 and d = 5 nm. Γrad−0 rapidly increases with increasing R, and finally Γrad−0 > ΓET−0 for R > 17 nm. For the combination of large R and small q 0 , this leads to high q/q 0 ratios, whereas q is typically reduced for R < 5 nm. The GN model used here does not take into account radiation damping and the red-shift of the LPSR due to dynamic depolarization [90]. Implementing these two corrections [198] modifies q/q 0 for R >∼ 10 nm, but does not have a significant influence for Ag NPs with R = 3.5 nm (cf. Fig. 4.8). In summary, the results obtained from the GN model predict that, for the Ag NPs used in this study, q/q 0 is generally 1 for high εm combined with q 0 1. Because of the strong dependence of the scattering quantum yield on R, q/q 0 can be more strongly enhanced by larger NPs. This means that by increasing R, we can change from a regime where generally absorption dominates and q is typically reduced to a regime where scattering is dominant and q can be enhanced orders of magnitudes (if q 0 is sufficiently small, as q ≤ 1). It is noteworthy that the higher-order LSPR modes of ΓET−0 are commonly excited even for the small Ag NPs used in this study. This is a major difference to the results obtained with the Mie model, where these modes are not excited for such small Ag NPs (cf. Fig. 4.9). This discrepancy is due to a difference in the excitation. In the GN model the metal NPs are excited from a nearby dipole source (i.e. the emitting molecule), whereas in the Mie model, they are excited by a plane wave originating from a far-field light source. This is also the reason why, when experimentally measuring the absorption spectra of these Ag NPs, which employs a far-field source, higher-order modes were not observed, as described in Chapter 2. This necessity to include higher-order modes for any size of metal NPs when calculating their influence on an emitting molecule is in-line with literature, where this aspect has been discussed recently [199].
70
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: EMISSION
R
(n m ) m o d e l 3 .5 1 0 2 0 E n h a n c e d G N 3 .5 1 0 2 0 G N
6 q
0
= 0 .1 d = 5 n m εm = 3 5
0
4
q / q
3
m o d e l
2 1 0 3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
W a v e le n g th ( n m ) Figure 4.8: Comparison between the original GN model [197] (solid curves) and one that was modified in order to account for radiation damping and dynamic depolarization, according to Mertens and Polman [198] (dashed curves), for an Ag NP with radius R in a medium with εm = 3. The emitter has a q 0 = 0.1 and a distance d to the Ag NP of 5 nm.
1 0 0
5 3
7
1 0
εm = 1
C
1 0
, C
a b s
(n m
2
)
1 0 0 0
1
C
s c a
C
a b s
s c a
0 .1 0 .0 1 4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
W a v e le n g th ( n m ) Figure 4.9: Absorption and scattering cross-sections (C abs and C sca , respectively) for a Ag NP with R = 3.5 nm in an embedding medium with a relative permittivity εm , according to Mie theory (cf. Section 1.4).
RESULTS AND DISCUSSION
4.4.2
71
Experimental results
In order to experimentally probe the near-field plasmon-exciton interactions between Ag NPs and organic molecules by PL measurements, we coated the Ag NP layer introduced in Chapter 2 with layers of either MeO-TPD, Alq3 , SubPc or PTCBI. The distance dependence of these interactions was determined by employing a spacer layer consisting of the transparent, low-index fluorocopolymer Hyflon AD60X that was introduced between the Ag NP layer and the organic emitter. The optical and morphological properties of the Ag NP layer are described in detail in Section 2.4.1. In short, a dense layer of Ag NPs with an average lateral diameter of 7 nm and an average height of 5 nm was obtained by depositing 1 nm of Ag (as determined by a crystal-monitor) on a substrate by means of vapor-phase deposition. The bare Ag NP layer exhibits a single absorption band centered at λ = 420 nm that originates from a dipole plasmon resonance. The NPs do not show any significant far-field light scattering, even when embedded in a high-index medium. Moreover, neither the bare Ag NP layer nor the Hyflon AD60X thin-films show any detectable PL.
S te a d y -s ta te P L (a .u .)
1 .0
M e O -T P D
A lq 3
S u b P c
P T C B I
0 .5
0 .0 4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
W a v e le n g th ( n m ) Figure 4.10: Steady-state PL spectra of thin-films of MeO-TPD, Alq3 , SubPc, and PTCBI, with Ag NPs (dashed curves) and without Ag NPs (solid curves), in absence of a spacer layer. The spectra of each emitter are normalized with respect to the spectrum of the sample without Ag NPs. The thickness of all emitter layers is 10 nm.
72
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: EMISSION
Two series of samples were fabricated for each of the four emitting molecules. One series consists of samples where the Ag NP layer, deposited on a quartz substrate, is coated either directly by the organic emitter or separated from it by a Hyflon AD60X spacer layer with thicknesses from 1 to 20 nm. The thickness of the organic emitter was kept constant at 10 nm for all these samples. The samples of the other series contain the same organic layer structure (0-20 nm Hyflon AD60X/10 nm emitter), deposited on plain quartz substrates. The steady-state PL spectra of the samples without a spacer layer are shown in Fig. 4.10. The samples with Ag NPs are shown as dashed curves, these without Ag NPs are shown as solid curves. The spectra of each emitter are normalized with respect to the peak intensity of the sample without Ag NPs. The organic molecules all emit at different wavelengths, with the PL band of MeO-TPD, Alq3 , SubPc, and PTCBI centered at λ = 420, 525, 610, and 810 nm respectively. In the presence of the Ag NPs, the PL intensities of all emitters are strongly reduced. From these measurements, we determined the ratio of the PL peakintensity of the sample with Ag NPs to that of the sample without Ag NPs for each emitter and spacer layer thickness [cf. Fig. 4.11(a)]. The reduction in PL intensity due to the presence of the Ag NPs without a spacer layer is evidenced by peak intensity ratios between 0.2 and 0.6. When introducing the spacer layer, the impact of the presence of the Ag NPs rapidly decreases and the ratios converge to values close to unity. For MeO-TPD, Alq3 , and PTCBI, the ratios increases monotonically, with the steepest slope for PTCBI and the slowest recovery for MeO-TPD. The ratio of the SubPc samples, on the other hand, steeply increases when introducing a spacer layer, and reaches a peak value of 1.25 ± 0.05 at a spacer layer thickness of 3 nm. The ratio then rapidly decreases to reach unity at a spacer thickness of 5 nm. When analyzing the results of these steady-state PL measurements, we have to consider the effect of the plasmon-exciton interactions on the excitation as well as on the emission, as described by Eq.(4.1). The excitation enhancement is caused by plasmonic near-field enhancement due to the Ag NPs. This was discussed in Chapter 3, where we observed absorption enhancement only at λ > λLSPR . For these steady-state PL measurements, we excited the molecules at λ = 337 nm, thus at a wavelength well below λLSPR . Indeed, as shown in Fig. 4.12, we did not observe an absorption enhancement at the excitation wavelength for any of the materials used here. Therefore, we conclude that the effect of the plasmon-exciton interactions on the excitation can be neglected. There are several other influences that could affect the steady-state PL intensity, such as variations in the emitter layer thickness, the rotation of the sample relative to the laser beam and the detector, the sample-detector distance,
P L p e a k in te n s ity r a tio
RESULTS AND DISCUSSION
1 .2 5
73
(a )
1 .0 0 0 .7 5
M e O -T P D A lq 3
0 .5 0
S u b P c P T C B I
0 .2 5 0 .0 0
a v
0 .5 0
(b )
0
0 .7 5
τa v / τ
1 .0 0
M e O -T P D A lq 3
0 .2 5
S u b P c
0 .0 0 0
5
1 0
1 5
2 0
S p a c e r la y e r th ic k n e s s ( n m ) Figure 4.11: (a) Ratios of the PL peak intensities of thin-films with Ag NPs to those of thin-films without Ag NPs, determined from steady-state 0 PL measurements. (b) The τav /τav ratios determined from time-resolved PL measurements. Both steady-state and time-resolved PL were measured from samples that contain an emitter layer either in direct contact with the Ag NPs or separated from them by a spacer layer with a thickness of 1-20 nm. The thickness of the emitter layer is 10 nm for all samples.
and photo-oxidation of the emitter during the measurement. By carefully controlling the measurement conditions, we were able to minimize these effects (see experimental section). Assuming that these measurement uncertainties as well as the excitation modulation due to the Ag NPs are negligible, we can attribute any change in the PL intensity to a change in the emission, and therefore, according to Eq. (4.1), the peak intensity ratios shown in Fig. 4.11(a) are equivalent to q/q 0 .
74
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: EMISSION
(a )
A b s . (% )
3 0
M e O -T P D
2 0
P la in o r g a n ic la y e r W ith A g N P s
1 0 0
A b s . (% )
(b )
A lq
2 0
3
1 0
∆A b s . ( % )
A b s . (% )
A b s . (% )
0 4 5
(c )
S u b P c
(d )
P T C B I
(e )
M e O -T P D A lq 3
3 0 1 5 0 2 0 1 0 0 3 0 2 0
S u b P c P T C B I
1 0 U n c o a te d A g N P s
0 3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
W a v e le n g th ( n m ) Figure 4.12: (a)-(d) Absorption spectra of 10 nm thick organic layers deposited either on a quartz substrate (dashed curves) or on the Ag NP layer (solid curves). The absorption was obtained from reflectance and transmission measurements, as described in Section 2.2. (e) Absorption differences of the samples with and without Ag NPs. The absorption of the uncoated Ag NP layer on quartz is shown as a gray solid curve. The vertical black dotted line indicates the excitation wavelength of λ = 337 nm when measuring the steady-state PL of these samples.
RESULTS AND DISCUSSION
τ10 (ps) A01 0 τ2 (ps) A02 0 τ3 (ps) A03 0 τAv (ps) χ2
75
MeO-TPD 78 0.324 324 0.629 640 0.048 259 1.10
Alq3 814 0.174 11890 0.826 9963 1.06
SubPc 94 0.735 326 0.230 1442 0.036 196 1.00
Table 4.1: The normalized amplitudes (A0i ) and decay times (τi0 ) used to fit the time-resolved PL decay of 10 nm thick organic emitter thin-films deposited on 0 quartz with a multi-exponential decay function. The average decay time (τav ) P 0 0 0 Ai τi . The amplitude of the fastest decay component (A01 ) is given by τav = i
might contain components of decay processes that are faster than the IRF of ∼ 32 ps. The χ2 value of all three fits is close to 1. The emission wavelength was 420 nm, 525 nm, and 610 nm for MeO-TPD, Alq3 , and SubPc, respectively.
In addition to the steady-state PL, we measured the time-resolved PL of these samples. The decay curves of neat MeO-TPD, Alq3 , and SubPc layers on quartz are shown in Fig. 4.13(a). The PL decay of PTCBI could not be measured as its PL signal was beyond the spectral limit of the set-up. The instrument response function (IRF) is shown as a gray curve and has a full width at half-maximum of 32 ps, determined by measuring the light scattering of a non-emitting sample. The gray curves plotted on top of the experimentally measured data show the exponential fit for the decay curves. Due to intermolecular interactions, the PL decay in thin-films differs from the mono-exponential decay observed for molecules in dilute solutions, which the use of multi-exponential P necessitates decay fits in the form of I(t) = Ai e−t/τi , where I(t) is the PL intensity at i P time t, τi are the lifetimes, and Ai the normalized amplitudes with Ai = 1. i
The average decay time (τavP ) is then given by the weighted average of the individual decay times τav = Ai τi . i 0 values of From the decay curve fits of the neat emitter layers, we obtained τav 259 ± 21 ps, 10.0 ± 0.5 ns, and 196 ± 10 ps for MeO-TPD, Alq3 , and SubPc, respectively. The fitting parameters for these samples are shown in Table 4.1. The PL decay curves for the MeO-TPD, Alq3 , and SubPc samples with Ag NPs are shown in Figures 4.13(b)-(d). As a reference, the decay curves of the neat
76
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: EMISSION
(a)
(b)
(c)
(d)
Figure 4.13: Normalized time-dependent PL curves of (a) neat layers of MeOTPD, Alq3 , and SubPc, and (b)-(d) these emitter layers deposited on the Ag NP layer, either in direct contact, or separated by it from a Hyflon AD60X spacer layer with thicknesses of 1-20 nm. In (b)-(d), the decay curves of the neat layers are shown as black curves as a reference. The gray curves plotted on top of the experimentally measured data in (a) show the multi-exponential fit to the decay curves. The instrument response function (IRF) is shown as a gray curve.
RESULTS AND DISCUSSION
77
emitter layers without Ag NPs are shown as black curves. 0 The τav /τav ratios obtained from these graphs are shown in Fig. 4.11(b).1 For all three organic emitters, τav is significantly smaller in the presence of the 0 Ag NPs without a spacer layer, resulting in τav /τav ratios below 0.4. When 0 introducing a spacer layer between the Ag NPs and the organic emitter, τav /τav increases with increasing spacer layer thickness. The most rapid increase is shown by SubPc, where a ratio close to one is already observed for a spacer 0 layer thickness of 10 nm. For MeO-TPD and Alq3 , the τav /τav ratios increase more gradually. The slope is similar for both materials, and the values of the MeO-TPD ratios remain below those of the Alq3 samples for all spacer layer thicknesses. P For any decay process, τ = 1/( Γi ), with the individual decay rates Γi of all i 0 decay channels involved in the decay. The τav and τav values obtained from the time-resolved PL measurements are thus equivalent to the inverse of the total decay rates, 1/Γ0tot = 1/(Γ0rad + Γ0nonrad ) in the absence of metal NPs and 1/Γtot = 1/(Γrad + Γ0nonrad + ΓET ) in the presence of metal NPs. The 0 τav /τav ratios shown in Fig. 4.11(b) therefore equal Γ0tot /Γtot . Since ΓET ≥ 0 0 and Γrad−0 ≥ 1, Γ0tot /Γtot ≥ 1, and thus the τav /τav ratio is expected to be ≤ 1, which we indeed observe for all organic emitters regardless of the spacer layer thickness.
4.4.3
Comparison between experiment and theory
The comparison between the experimental results and those obtained with the GN model will be rather qualitative than quantitative, as the model assumes a spherical symmetry and an embedding medium which is transparent, homogeneous, and of infinite size. In reality, there are up to two organic layers with finite thicknesses, one of them absorbing, the presence of a planar substrate, and a NP shape that deviates from a perfect sphere. These issues could be partially solved using a transfer-matrix approach [200], where it is possible to calculate the effect of multiple layers coating the Ag NPs, or by using numerical simulations, where any arbitrary geometry can be employed [201, 202]. Moreover, the GN model does not take into account nonlocal effects that could become important for metal NPs with a size of only a few nanometers [203]. However, as we will show in the following paragraphs, the relatively simple GN model is sufficient to qualitatively describe and explain the experimental results. 1 The decay of the SubPc sample with Ag NPs without a spacer layer was only slightly slower than the IRF, such that the measured τav value has to be considered as an upper limit for the actual τav .
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: EMISSION
P h o to lu m in e s c e n c e ( a .u .)
78
1 .0
S u b P c P T C B I
0 .8 0 .6 0 .4 0 .2 0 .0 4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
W a v e le n g th ( n m ) Figure 4.14: Steady-state PL spectra of 10 nm thick thin-films of SubPc (dashed curve) and PTCBI (solid curve). Both curves were first normalized with respect to the emission band of the quartz substrate at λ = 430 nm, then multiplied with the spectral response of the detector and finally corrected for the absorption of the organic layers at the excitation wavelength of λ = 337 nm. This allows for a rough estimation of the relative q 0 of both organic emitters. The ratio of the SubPc q 0 to that of PTCBI was found to be ∼5.
For this comparison, we can divide the four emitters used in this study into two groups. Thin-films of MeO-TPD and Alq3 emit quite efficiently with a q 0 ∼ 0.25-0.4 [204, 205]. Thin-films of SubPc and PTCBI, on the other hand, emit rather inefficiently with q 0 ∼ 4 × 10−3 -2 × 10−2 [206] (As no q 0 value for PTCBI could be found in literature, we estimated it by comparing the emission spectra of SubPc and PTCBI, as shown in Fig. 4.14). As already established in Section 4.4.2, the peak intensity ratios extracted from steady-state PL measurements are equivalent to q/q 0 . We can thus directly compare these experimentally obtained q/q 0 ratios with those obtained with the GN model. We have to keep in mind though when comparing the distance dependence of these ratios that the experimental results were obtained from 10 nm thick emitting layers. This means that we always measure the average impact of the Ag NPs on the emission of the molecules in this layer. We chose for this relatively large emitter layer thickness to ensure that the layers possess a well-defined ε value, which might not be the case for layers that are only a few molecules thick.
RESULTS AND DISCUSSION
79
1 2 M e O -T P D A lq 3
1 0
S u b P c P T C B I H y flo n A D 6 0 X 8
ε1
6 4 2 0 3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
W a v e le n g th ( n m ) Figure 4.15: The real part of the relative permittivity (ε1 ) of the four emitters used in this study, as well as that of the spacer layer material Hyflon AD60X. The peak wavelength of the emission band of each organic emitter is indicated by a filled circle.
Taking into account only the influence of q 0 , the emission wavelength, and d, we find a good qualitative agreement between the GN model and the experiment for MeO-TPD, Alq3 , and PTCBI. For these materials, q/q 0 ≤ 1, with a strong distance dependence and a range that is ≤ 30 nm. The impact of the presence of the Ag NPs increases with increasing q 0 , such that MeO-TPD and Alq3 are more influenced than PTCBI. At the same time, the impact decreases with decreasing overlap between the emission band and the LPSR, such that the strength of the plasmon-exciton interactions is MeO-TPD > Alq3 > PTCBI, which emit from resonant with the LSPR to partially resonant and completely off-resonance, respectively. The impact of the Ag NPs on the emission of SubPc, on the other hand, follows a different trend. It has a q 0 comparable to PTCBI and also emits off-resonance, however, the measured q/q 0 ratios of both materials differ considerably. In addition, SubPc is the only emitter where we observe a PL peak intensity ratio of >1 (i.e. q/q 0 > 1), which can only be reached if we have significant enhancement of Γrad due to Ag NP light scattering [cf. Eq.(4.2)]. To understand this behavior, we consider the ε values of the emitters at their emission wavelengths (cf. Fig. 4.15). In principle, we have to take into account
80
PLASMON-EXCITON NEAR-FIELD INTERACTIONS: EMISSION
both the real and the imaginary parts of ε. However, as organic emitters typically do not absorb strongly at their emission wavelength, and because the GN model assumes a transparent embedding medium, we will neglect the imaginary part. Unlike the other emitters that possess ε1 ∼ 3-4 at their emission wavelengths, SubPc has a ε1 ∼ 10 at its emission wavelength of λ = 610 nm. At the same time, SubPc has a q 0 1. As discussed above, this high εm -low q 0 combination are the conditions under which the GN model predicts significant emission enhancement for such small Ag NPs. The total distance dependence of the SubPc PL intensity ratios can then be explained as follows (cf. Fig. 4.6): Without a spacer layer, q/q 0 1 as for εm = 10, the higher-order modes of ΓET−0 strongly reduce q at the emission wavelength of λ = 610 nm. When we introduce a spacer layer with a low ε1 , we reduce the average εm of the medium surrounding the Ag NPs, and thus blue-shift the LSPR, such that we reach a significant q enhancement for εm ∼ 7. As the spacer layer thickness increases, the average εm decreases and the dipole LSPR blue-shifts further, meaning that the enhancement eventually ceases to overlap with the emission wavelength. At the same time, we decrease the influence of the Ag NPs due to an increasing distance to the emitter, such that in total we expect a peak with q/q 0 > 1 followed by very fast convergence to q/q 0 = 1, which is exactly what we observe in our experiment. It is important to note that the significance of the far-field scattering for the SubPc emission originates from its low q 0 , which strongly increases the importance of Γrad−0 relative to ΓET−0 in Eq. (4.2). On the other hand, when the light scattering of the Ag NP/SubPc samples is measured directly, it is still negligibly small compared to the absorption (cf. Fig. 2.7). So far, we only considered the steady-state PL measurements during this discussion. The time-resolved PL measurements also yield valuable information, with the major advantage that they provide direct proof that the presence of the Ag NPs influences the decay rates. Many parameters that might influence the steady-state PL intensity, such as the excitation enhancement due to the Ag NPs, changes in the measurement set-up when switching between samples or small variations in the emissive layer thickness will much less affect τav . We can therefore use the time-resolved data to verify the steady-state PL measurements. A direct comparison between the two measurement types is possible when we 0 0 rewrite the τav /τav ratio as τav /τav = q/q 0 × 1/Γrad−0 . In the regime where Γrad−0 is ∼ 1, i.e. light scattering by the Ag NPs is not significant, the peak intensity ratios and PL decay time ratios are thus expected to be similar. Comparing Fig. 4.11(a) with Fig. 4.11(b), we observe nearly identical behavior for MeO-TPD and Alq3 . For SubPc, on the other hand, the comparison reveals significant differences for spacer layer thicknesses below 10 nm. Therefore, this
CONCLUSIONS
81
indicates that for MeO-TPD as well as for Alq3 , light scattering is insignificant while in the case of SubPc it clearly is significant. These findings confirm the comparison between the experiments and the GN model made above and therefore suggest that both the time-resolved and the steady-state PL measurements yield accurate information about the decay rates and q.
4.5
Conclusions
We probed the near-field plasmon-exciton interactions between Ag NPs and organic molecules by steady-state and time-resolved PL measurements. We could directly relate the PL decay times and peak intensities resulting from these measurements to the modulation of q by the Ag NPs. The steady-state and time-resolved measurements were found to be in good agreement with each other. Furthermore, we found good qualitative agreement with analytical calculations based on the Gersten-Nitzan model. From this, we conclude that both time-resolved and steady-state PL measurements are suitable means to obtain accurate information on plasmon-exciton interactions. By using a transparent, non-emitting spacer layer between the organic emitter and the Ag NPs, we could determine the distance dependence of the plasmonexciton interactions, and found a range of typically < 30 nm, which confirms the near-field nature of these interactions. In addition, we employed several organic emitters and found a strong impact of q 0 , ε, and the spectral overlap between the emission band and the LSPR. For the Ag NP layer used in this study with R ∼ 3.5 nm, we generally found a strong decrease in q in the presence of the Ag NPs. This reduction of q due to the Ag NPs could be attributed to the small scattering quantum yield of Ag NPs of this size. One noteworthy exception is SubPc, where the combination of a high ε1 and a low q 0 led to an emission enhancement due to the Ag NPs of up to 25%. This demonstrates that even for these small metal NPs, light scattering can become important. Combining the results found here with these presented in Chapter 3, we can assess the application of near-field plasmon-excitation interactions for efficiency enhancement of organic solar cells. The use of metal NPs in organic solar cells involves the trade-off between absorption enhancement and the reduction of the exciton lifetime, which in turn leads to a shorter exciton diffusion length. According to our results, significant absorption enhancement can be reached with metal NPs, however, in the case of the small Ag NPs used in our study with a very short range of ∼ 5 nm. At the same time, we found a strong reduction in exciton lifetime in the presence of metal NPs with a range that is at least comparable to that of the absorption enhancement.
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PLASMON-EXCITON NEAR-FIELD INTERACTIONS: EMISSION
As the plasmon-exciton interactions strongly depend on the spectral overlap between the LSPR and the molecular absorption and emission bands, it might be beneficial to tune the LSPR to the spectral region where absorption enhancement is desired. This would maximize the absorption enhancement and lead to a significantly smaller impact on the exciton lifetime because the emission band in this case lies red-shifted to the LSPR and therefore the weaker off-resonance interaction is expected. This approach is especially interesting for materials with a large Stokes-shift. However, this strategy would also maximize parasitic absorption losses, which is especially detrimental when the organic medium already strongly absorbs at this spectral region. Our results also demonstrate, that for dipole LSPRs, no near-field enhancement is obtained above and below the NPs (assuming light that is incident normal to the substrate surface). This means that the metal NPs have to be embedded directly in the active layer. The exciton lifetime reduction due to the presence of the metal NPs, on the other hand, depends on the relative orientation of the molecular emission dipole and the metal NP, but not on the molecular position relative to the (spherical) metal NP. This means that when non-scattering metal NPs are placed adjacent to the active layer, only the detrimental effects of exciton quenching remain. These findings suggest that great care has to be taken when designing organic solar cells that exploit plasmonic near-field enhancement. The necessity to isolate the NPs with a layer thick enough to prevent exciton quenching might potentially eliminate any beneficial effect due to near-field enhancement even in optimized metal NP-molecule composite systems. In addition, because the NPs have to be embedded in the active layer, they might lead to charge trapping and disturb the typically highly optimized and sensitive morphology of the organic active layer. We therefore conclude that the application of near-field plasmon-exciton interactions in organic solar cells appears challenging.
Chapter 5
Organic solar cells with nanostructured metal rear electrode 5.1
Introduction
In Chapters 3 and 4, plasmonic near-field enhancement and exciton quenching were discussed with the conclusion that the exploitation of near-field plasmonic effects to enhance the efficiency of organic solar cells appears challenging. Therefore, in this chapter, we focus on far-field plasmon-exciton interactions via light scattering of metal nanostructures. The nanostructures employed in this chapter are part of the solar cell’s nanopatterned Ag rear electrode. This has the advantage that charge trapping by the nanostructures is not a concern, which would be expected for metal nanostrucutres that are separated from the electrode. We chose for a nanostructured rear electrode because plasmonic rear-side features are generally preferable to front-side ones, which suffer from parasitic light absorption and destructive interference effects between the incident and the scattered light. As the device processing begins with the transparent electrode, the nanostructures have to be applied after the deposition of the organic active layer. Therefore, we have to take great care to perform the nanopatterning without compromising the active layer optical and electrical properties. As will be demonstrated in this chapter, this can be achieved by hole-mask colloidal
83
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ORGANIC SOLAR CELLS WITH NANOSTRUCTURED METAL REAR ELECTRODE
lithography (HCL) [207], a bottom-up nanofabrication technique that does not involve high temperatures or organic solvents. In solar cells containing nanostructured metal electrodes, LSPRs as well as SPPs can contribute to absorption enhancement in the active layer (cf. Section 1.5.1). As a result, the optical properties of such structures can in general not be fully described by taking into account only the LSPRs. However, as will be described in 5.2, the features of the nanostructured Ag electrode utilized in this chapter lack periodicity, which means that light incoupling into SPPs via the LSPRs of the nanosized features only occurs incoherently and therefore not very efficiently. In addition, the Ag layer in the nanostructured solar cell used here is separated from the active layer by more than 100 nm, such that even if SPPs are excited, they mostly decay within the spacer layer and are therefore not expected to significantly contribute to absorption enhancement in the active layer. Therefore, only LSPRs will be taken into account in this chapter, with the main focus on the question whether the efficiency of an optimized organic solar cell can be enhanced by light scattering due to LSPRs of metal nanostructures. As the aim of this chapter lies in demonstrating unambiguous plasmonic efficiency enhancement, we employ a fully-optimized reference cell with a flat Ag rear electrode, as will be described in Section 5.3.1. This reference device features nearly saturated absorption over a broad wavelength range of 350 nm < λ < 620 nm. In such an optimized cell, any enhancement of absorption can thus only be effective in the absorption tail of the active layer, in our case at λ = 620-780 nm. Therefore, the plasmonic light scattering by the nanostructured metal rear electrode must be maximized in this wavelength range. This can readily be achieved by altering the feature size of the electrode, as will be described in Section 5.3.2. After determining the optimal feature size, solar cells containing the optimized nanostructured rear electrode are fabricated and their efficiency is compared to the optimized reference with a flat rear electrode. Finally, numerical simulations and experiments employing a model system allow for the identification of the plasmonic effects at work.
5.2
Experiments
Substrates were cleaned by sonication in soapy water, deionized water, acetone, and 2-propanol, followed by an UV/O3 -treatment for 15 min. The fabrication of both the flat reference and the nanostructured organic solar cells began with the deposition of a ZnO layer on a glass/indium tin oxide (ITO, 115 nm thickness) substrate with a sheet resistance of 15 Ω sq−1 . This ZnO layer was obtained by spin coating a solution of 60 mmol L−1 zinc acetate dihydrate (Sigma-Aldrich)
EXPERIMENTS
85
and 60 mmol L−1 ethanolamine (Sigma-Aldrich) in a mixture of 16 vol% 2ethoxyethanol and 84 vol% ethanol, and converting the Zn acetate to ZnO by baking the sample at 300◦ C in air for 10 min. The 230 ± 20 nm thick active layer was spin coated from a solution of regioregular poly(3-hexylthiophene2,5-diyl) (P3HT, Sepiolid P200 purchased from Rieke Metals) and indene-C60 bis-adduct (ICBA, Luminescence Technology Corp.) with a weight ratio of 1:1.2 in ortho-dichlorobenzene. The resulting thin-film was then dried under a petri dish for 30 min and heated to 170◦ C for 10 min in a N2 atmosphere. The flat reference devices were then completed by depositing MoOx and 200 nm of Ag by means of thermal evaporation of molybdenum(VI) oxide (SigmaAldrich) and Ag (99.99% purity, Kurt J. Lesker Co.) at a pressure below 2 × 10−6 Torr and a deposition rate of 0.1 and 0.6 nm/s, respectively. During the deposition, the substrate was rotated and kept at a temperature of -5◦ C. Silver was deposited through a shadow mask, resulting in a device area of 0.033 cm2 . The patterning of the rear electrode of the nanostructured devices was achieved by hole-mask colloidal lithography (HCL) on the active layer surface that was rendered hydrophilic by an O2 plasma at a power of 100 W for 6 s. An aqueous solution of 0.2 wt% poly(diallyldimethylammonium chloride) (PDDA, average molecular weight 100k-200k, purchased from Sigma-Aldrich) was drop cast on the active layer, followed by the drop casting of an aqueous dispersion of 0.2 wt% polystyrene (PS) nanobeads (sulfate latex, Invitrogen) with a nominal diameter of either 60, 110, 140 or 200 nm. In both cases, the solution/dispersion remained on the sample for 30 s and was removed by rinsing with deionized water for 10 s, followed by the drying of the sample with a N2 gun. This resulted in an array of well-separated PS beads on the active layer surface due to electrostatic repulsion between the negatively charged beads and attraction between the beads and the positively charged PDDA. During the deposition of MoOx , the PS beads acted as a shadow mask, such that upon the removal of the beads by a residue-free adhesive tape, the MoOx layer featured conical holes with sloped side walls and a bottom diameter slightly smaller than the diameter of the beads due to the lateral offset between the MoOx source and the sample during the deposition with substrate rotation. The fabrication was finished with a blanket deposition of 5 nm of MoOx to ensure a complete MoOx surface coverage of the active layer, followed by 200 nm of Ag. The poly(methyl methacrylate) (PMMA, purchased from Allresist) layers used for optical measurements were deposited by spin coating 1.5% PMMA in chlorobenzene on a clean glass substrate and were dried in N2 at 180◦ C for 10 min. The flat and nanostructured MoOx /Ag layers were then deposited on top of the PMMA layer as described above.
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ORGANIC SOLAR CELLS WITH NANOSTRUCTURED METAL REAR ELECTRODE
P3HT:ICBA (230 nm) ZnO (< 10 nm ) ITO (115 nm) Glass substrate
5 µm
1 µm
1 µm
1 µm
Drop cast PS beads
Deposit MoOx Deposit MoOx
Remove PS beads
Deposit 5 nm MoOx and 200 nm Ag
Deposit 200 nm Ag
Ag MoOx P3HT:ICBA ZnO ITO
200 nm
Nanostructured device
Glass substrate
Reference device
Figure 5.1: Fabrication process flow for an organic solar cell containing a nanostructured rear electrode (left) and a reference device with a flat electrode (right). The scanning electron micrographs show the nanostructured device at different stages during the fabrication. The fabrication of both types of devices begins with the deposition of a ZnO layer on a glass/ITO substrate, followed by the P3HT:ICBA active layer. On the active layer surface of the nanostructured device, a non-closed-packed layer of PS beads is applied. These PS beads act as a shadow mask during the deposition of the MoOx layer. After the PS beads have been removed, an additional 5 nm thick MoOx layer is deposited to prevent direct contact between the active layer and the metal electrode. Finally, 200 nm of Ag are deposited. The MoOx /Ag anode of the flat reference device is deposited directly on the active layer.
RESULTS AND DISCUSSION
87
Reflectance and external quantum efficiency (EQE) spectra were measured using a Bentham PVE 300 photovoltaic device characterization system. The total reflectance was measured by placing the sample at a port of an integrating sphere and illuminating it through the substrate with monochromatic light entering the sphere through a port at its opposite side. As a measure for the scattered light, the diffuse reflectance was measured by removing the specularly reflected light from the integrating sphere through an additional port. The current density-voltage (J-V ) characteristics were measured using a calibrated lamp with an air mass 1.5 global spectrum and an integrated intensity of 1000 W/m2 (equivalent to 1 sun). The EQE spectra and J-V characteristics were measured in a N2 atmosphere. Scanning electron micrographs were obtained by a Philips XL 30. Cross-sectional micrographs of the completed nanostructured device were obtained by dicing the sample using a diamond knife. Film thicknesses were measured using a Veeco Dektak D150 surface profiler. Three-dimensional finite-difference time-domain (FDTD) simulations were performed using the Lumerical 7.5 software package. An individual Ag protrusion was simulated by using perfectly matched boundary layers in all dimensions. The Ag protrusion was modeled as a conic surface with a radius of curvature of 35 nm and a conic constant of -1.2. A total-field/scattered-field plane-wave source was used, originating in the semi-infinite glass substrate with the total-field volume comprising the complete simulated layer stack. The simulated scattered light was monitored in the glass substrate outside the totalfield volume. After convergence testing, the mesh size was set to 1.5 nm in the vicinity of the Ag protrusion and to 5 nm in the rest of the simulated volume. A 2D power monitor was used to extract |E|2 . The absorbed power per unit volume (Pabs ) was calculated as Pabs = 0.5 ω|E|2 Im(ε), with the frequency ω and the relative permittivity ε.
5.3
Results and discussion
We fabricated organic solar cells with nanostructured Ag rear electrodes and reference cells with flat electrodes (see experimental section and Fig. 5.1 for fabrication details). Both have the same architecture, with ITO as a transparent cathode, ZnO as an electron collection material, an active layer consisting of a 230 ± 20 nm thick blend of the organic semiconducting polymer poly(3hexylthiophene-2,5-diyl) (P3HT) as an electron donor and indene-C60 bis-adduct (ICBA) as an electron acceptor, MoOx as a hole collection material, and Ag as a reflecting anode. The choice of ICBA instead of the more commonly used [6,6]-phenyl C61 butyric acid methyl ester (PCBM) is motivated by its higher open-circuit voltage (Voc ) in combination with P3HT [208]. The fabrication of
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ORGANIC SOLAR CELLS WITH NANOSTRUCTURED METAL REAR ELECTRODE
(a)
(b)
5m m
5m m
Figure 5.2: Scanning electron micrographs of PS nanobeads applied by hole-mask colloidal lithography on (a) the P3HT:ICBA active layer and (b) a thin-film of PMMA. The contrast of the micrograph in (b) is reduced by the strong charging of the PMMA layer by the electron beam during the measurement.
both types of devices begins with the deposition of the ZnO layer on top of the ITO/glass substrate, followed by the 230 ± 20 nm thick active layer obtained by spin coating. The reference device is then completed by depositing MoOx and 200 nm of Ag by means of thermal evaporation. The patterning of the nanostructured Ag layer on top of the active layer is achieved by HCL. In short, PS nanobeads dispersed in an aqueous solution are drop cast on the active layer surface. Their negative surface charge leads to electrostatic repulsion, which results in a layer of well-separated beads on the active layer surface. During the deposition of MoOx , the PS beads act as a shadow mask. Upon removal of the beads, the MoOx layer features holes with cone-shaped sidewalls and a bottom diameter slightly smaller than the diameter of the beads due to the lateral offset between the MoOx source and the sample during the deposition with substrate rotation. Finally, 5 nm of MoOx is deposited to ensure a complete MoOx surface coverage of the active layer, followed by the deposition of 200 nm Ag. The cross-sectional migrograph in Fig. 5.1 shows that the Ag completely fills in the holes in the MoOx layer. By means of this fabrication process we therefore obtain a nano-patterned Ag rear electrode with protrusions that have the inverted shape of the holes in the MoOx layer and thus have a size that scales with the diameter of the PS beads and the MoOx thickness. The tips of these Ag protrusions are separated from the active layer by 5 nm of MoOx . As the highly absorbing active layer of these solar cells only allows to obtain very limited information on the LSPR of the nanostructured Ag electrode, we
RESULTS AND DISCUSSION
Reference (5 nm MoOx ) Processed reference (5 nm MoOx ) Nanostructured (135 nm MoOx )
89
Jsc 2 (mA/cm ) 9.86 9.93 10.29
Voc (mV) 784 772 790
FF (%) 68.3 67.0 68.4
η (%) 5.28 5.14 5.56
Table 5.1: Short-circuit current density (Jsc ), open-circuit voltage (Voc ), fill factor (FF), and power conversion efficiency (η) of flat reference devices with a 5 nm thick MoOx layer, and the nanostructured device with a total MoOx layer thickness of 135 nm. One of the reference devices was stored in N2 after the active layer processing, the other one, the ‘Processed reference’, was exposed to all processing conditions required to nanopattern the rear electrode (air, O2 plasma, PDDA solution drop casting, deionized water rinsing). The current density-voltage characteristics of these devices are shown in Fig. 5.5.
employed a model system with the same nanostructured MoOx /Ag layers on a transparent PMMA layer to perform optical measurements. Scanning electron micrographs confirm that the HCL process leads to an identical distribution of the PS beads on PMMA and the P3HT:ICBA active layer (cf. Fig. 5.2). Moreover, the LSPR is expected to be dominated by the MoOx medium that completely surrounds the Ag protrusions (cf. Fig. 5.1), such that the refractive index difference between P3HT:ICBA and PMMA should not lead to dramatic resonance shifts.
5.3.1
Reference device optimization
In a first optimization step, the interference pattern within the reference devices with a flat rear electrode were optimized. This optimization began with cells that already contain an active layer with optimal thickness, composition, and morphology, as described in Ref. [208]. The interference pattern in the active layer generated by the reflective rear electrode can be tuned by spacer layers, such as in our case MoOx , inserted between the active layer and the rear electrode. We experimentally measured the dependence of the short-circuit current density (Jsc ) of flat reference devices on the MoOx spacer thickness, as shown by the open black triangles in Fig. 5.3. The highest Jsc is obtained with the thinnest MoOx layer with a thickness of 5 nm. At a thickness of 135 nm, a local Jsc maximum limits the drop in Jsc to about 4% with respect to that of the thinnest layer.
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ORGANIC SOLAR CELLS WITH NANOSTRUCTURED METAL REAR ELECTRODE
1 0 .5
9 .5
9 .0
J
s c
(m A /c m
2
)
1 0 .0
8 .5 0
4 0
8 0
M o O x
1 2 0
1 6 0
th ic k n e s s ( n m )
Figure 5.3: The Jsc of P3HT:ICBA solar cells with MoOx layer thicknesses between 5 nm and 185 nm. The measured Jsc values of devices with a flat Ag rear electrode are shown as open black triangles, those of devices with a nanostructured Ag rear electrode are shown as filled gray squares. The solid curve shows the calculated Jsc for devices with a flat Ag rear electrode, according to a transfer-matrix model (assuming an overall collection efficiency of 70%).
R e fe re n c e
6 0
M o O
(n m ) x
E Q E (% )
5 1 1 5 1 3 5 1 5 5 1 8 5
4 0
2 0
0 3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
W a v e le n g th ( n m ) Figure 5.4: External quantum efficiency (EQE) spectra of flat reference devices.
RESULTS AND DISCUSSION
91
C u r r e n t d e n s ity ( m A /c m
2
)
0
R e fe re n c e (5 n m ) P ro c e s s e d re fe re n c e (5 n m ) N a n o s tru c tu re d (1 3 5 n m )
-2 -4 -6 -8 -1 0 0 .0
0 .2
0 .4
0 .6
0 .8
V o lta g e ( V ) Figure 5.5: Current density-voltage characteristics of flat reference devices with a 5 nm thick MoOx layer, and the nanostructured device with a total MoOx layer thickness of 135 nm. One of the reference devices was stored in N2 after the active layer processing, the other one, called the ‘Processed reference’, was exposed to all processing conditions required to nanopattern the rear electrode (air, O2 plasma, PDDA solution drop casting, deionized water rinsing). The current densities of these devices were measured in the dark as well as when they were illuminated by a lamp with an air mass 1.5 global (AM 1.5G) spectrum at an intensity equivalent to one sun.
Utilizing a transfer-matrix model [209] and assuming an overall collection efficiency of 70%, we could reproduce the Jsc of the device with the thinnest MoOx layer (black solid curve). This model also reproduces the experimentally obtained modulation of Jsc , which suggests that it indeed originates from the change in the interference pattern when altering the spacer layer thickness. This is confirmed by external quantum efficiency (EQE) spectra of these flat devices that are shown in Fig. 5.4, where the impact of the MoOx thickness on the interference pattern is evidenced by a modulation of the shape of the spectra. The optimized reference device with a MoOx thickness of 5 nm exhibits a power conversion efficiency of η = 5.26%, with J-V characteristics and device parameters as shown in Fig. 5.5 and Table 5.1, respectively. As described in the introduction, the degradation of the active layer is a
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ORGANIC SOLAR CELLS WITH NANOSTRUCTURED METAL REAR ELECTRODE
major concern when performing nanolithography on top of it. Because all the device parameters of the reference cell are optimized, any degradation of the electrical or optical properties of the active layer will immediately be noticed by a sharp reduction in device performance. All processing steps of the HCL method employed were optimized in order to limit the impact on the active layer as much as possible. As a result, a device with the same layer structure as the optimized reference that had been exposed to all processing conditions required to nanopattern the rear electrode but without applying the PS bead dispersion showed a comparable η and a nearly identical Jsc (‘Processed reference’ in Table 5.1 and Fig. 5.5). Notably, this result demonstrates that the nanopatterning processing does not harm device performance.
5.3.2
Optical properties of the nanostructured Ag layer
As discussed in Section 1.4, the optical properties of metal nanostructures strongly depend on their size and shape. Therefore, the optical properties of the nanostructured Ag layer can be tuned by changing the size of the PS beads as well as the thickness of the MoOx layer. This tunability is evidenced by the scattering spectra of the nanostructured Ag layer on PMMA as shown in Fig. 5.6 for several PS bead sizes and MoOx layer thicknesses. With increasing PS bead size as well as with increasing MoOx layer thickness, the scattering bands red-shift and become more intense. As this nanostructured Ag layer will be applied to a solar cell with a P3HT:ICBA active layer with saturated absorption at wavelengths of λ < 620 nm, we need to maximize the light scattering in the spectral region of the active layer red absorption tail at 620 nm < λ < 780 nm (cf. Fig. 5.7). In order to obtain strong light scattering, a MoOx thickness of at least ∼50 nm is required. For such thick MoOx layers, an optimal thickness of 135 nm was found regarding the interference pattern within the solar cell stack (cf. Section 5.3.1). The PS beads with diameters of 140 and 200 nm both show strong scattering in the desired spectral range for this MoOx thickness. The peak intensities resulting from both bead sizes are comparable but the 200 nm beads result in significantly broader bands. A possible reason for this peak broadening is the aggregation observed for the 200 nm beads, in contrast to the 140 nm beads that are all well-separated (cf. Fig. 5.8). This aggregation occurs because with increasing PS bead diameter, the capillary forces during the drying of the bead dispersion become stronger and eventually start to outweigh the electrostatic attraction between the beads and the polymer layer surface. As the aggregation is still limited to a small number of beads in the case of the 200 nm beads, it is in principle not detrimental to the device performance.
RESULTS AND DISCUSSION
93
P S b e a d d ia m e te r ( n m ) /M o O 6 0 /7 0
S c a tte r e d lig h t ( % )
2 5
1 1 0 /7 0 1 1 0 /1 3 0
1 4 0 /7 0 1 4 0 /1 3 0
x
(n m ) 2 0 0 /7 0 2 0 0 /1 3 0
2 0 1 5 1 0 5 4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
W a v e le n g th ( n m ) Figure 5.6: Measured light scattering spectra of nanostructured MoOx /Ag layers on PMMA. The PS beads used during the fabrication of these nanostructured layers had a diameter of either 60, 110, 140 or 200 nm. The layer thickness of the first MoOx deposition was either 70 nm or 130 nm. In both cases, 5 nm of MoOx was deposited after the removal of the PS beads. The 60 nm PS beads were too small to be removed after the deposition of 130 nm MoOx which formed a continuous blanket layer over the smallest diameter beads.
However, we preferred the 140 nm beads because of their well-defined behavior, which allows for a better interpretation of the results as well as for a more accurate modeling when employing numerical simulations. In addition, the benefit of using the 200 nm beads instead of the 140 nm ones for the fabrication of nanostructured P3HT:ICBA solar cells is not expected to be too dramatic as the highest potential for absorption enhancement is at 650 nm < λ < 700 nm, where both bead sizes result in a similar scattering intensity. When employing low-bandgap organic semiconductors, the use of the 200 nm beads with their broad scattering bands that reach far into the near-infrared might be preferable. To analyze the LSPR evolution of the nanostructured Ag layer in more detail, we utilized the 140 nm PS beads and modulated the height of the Ag protrusions between 40 and 180 nm via the MoOx thickness, while keeping the distance between their tips and the PMMA/MoOx interface constant at 5 nm. The scattering spectra of these samples are shown in Fig. 5.9(a), together with that of a flat PMMA/MoOx /Ag reference sample that did not show any significant light scattering (black dotted curve).
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ORGANIC SOLAR CELLS WITH NANOSTRUCTURED METAL REAR ELECTRODE
A b s o r p tio n ( % )
8 0 6 0 4 0 2 0 0 4 0 0
5 0 0
6 0 0
7 0 0
W a v e le n g th ( n m ) Figure 5.7: Absorption spectrum of the 230 ± 20 nm thick active layer that was obtained by spin coating a solution of P3HT:ICBA with a weight ratio of 1:1.2 in ortho-dichlorobenzene on a glass substrate. The resulting thin-film was then dried under a petri dish for 30 min and heated to 170◦ C for 10 min in a N2 atmosphere. The absorption was calculated as absorption = 1 total transmission - total reflectance. The absorption of the glass substrate was subtracted to obtain the absorption spectrum of the active layer. These measurements were done in the absence of a metal back reflector. When the active layer is employed in an organic solar cell, the optical interference pattern due to the presence of the back reflector will modulate the absorption spectrum of the active layer shown here.
(a)
(b)
2m m
2m m
Figure 5.8: Top-view scanning electron micrographs of nanostructured MoOx /Ag layers on PMMA with a total MoOx thickness of 135 nm. These layers were fabricated utilizing PS beads with a diameter of either (a) 140 nm or (b) 200 nm.
RESULTS AND DISCUSSION
95
20
(a) Scattered light (%)
16
110 12
130 150 70 180
8
40 4
Reference 0
400
500
600
700
800
Wavelength (nm)
Scattered light (a.u.)
1.0
(b)
100
y
Ag
x
0.8
h 5
0.6
MoOx
E0
0.4 0.2 0.0
k
PMMA
40
150 110 130
70
Reference
400
500
600
700
800
900
1000
Wavelength (nm)
Figure 5.9: (a) Measured and (b) simulated light scattering spectra of flat (‘Reference’) and nanostructured MoOx /Ag layers on glass/PMMA. The labels of the curves in (a) indicate the thickness of the first MoOx layer, deposited following the HCL process of the nanostructured samples. The thickness of the MoOx layer deposited after the removal of the PS beads was kept constant at 5 nm (cf. Fig. 5.1 for fabrication details). The simulation results were obtained with 3D FDTD simulations using a cross section as shown in the inset to (b). The numbers in the inset indicate distances in nm. An individual Ag protrusion was simulated by employing perfectly matched boundary layers in all dimensions. The labels of the curves in (b) denote the height, h, of the Ag protrusion in nm. The plane-wave is incident through the PMMA layer, with its electric field (E0 ) and propagation (k) vector as indicated by arrows.
ORGANIC SOLAR CELLS WITH NANOSTRUCTURED METAL REAR ELECTRODE
y (nm)
120 80
Ag
MoOx
40
PMMA
0 -100
0
1 2 |E| (a.u.)
1 Div(E) (a.u.)
120
y (nm)
96
0
80 40
+-
0
100
-100
x (nm)
y (nm)
80 40
120
y (nm)
7 2 |E| (a.u.) 0
0 0
80
-
40 -100
100
80 40
0
0 100
1 Div(E) (a.u.)
120
y (nm)
y (nm)
-2
0
x (nm)
1 2 |E| (a.u.)
x (nm)
+
0
100
120
0
2 Div(E) (a.u.)
-
+
x (nm)
-100
100
x (nm)
120
-100
-1
0
-
80
+
40
-1
0 -100
0
100
x (nm)
Figure 5.10: The simulated |E|2 (left) and charge density (right) of a Ag protrusion at the spectral positions indicated by roman numbers in Fig. 5.9(b) for h = 130 nm.
In order to explain these scattering spectra, we employed 3D finite-difference time-domain (FDTD) numerical simulations of an individual Ag protrusion, with a cross section as shown in the inset to Fig. 5.9(b). From these simulations, the experimental scattering spectra could be reproduced qualitatively [cf. Fig. 5.9(b)]. By analyzing the simulated electric field intensity (|E|2 ) around the Ag protrusion, the individual scattering bands could be identified (cf. Fig. 5.10). For the thinnest MoOx layers, a highly localized resonance at the tip of the Ag protrusion as well as a dipole resonance centered at its side walls are excited. The tip resonance contributes to the scattering band centered at λ = 420 nm in the experiment and at 410 nm < λ < 460 nm in the FDTD simulations. The dipole resonance is evidenced by a broad band that red-shifts from λ = 550 nm to λ = 700 nm in the experiment and from λ = 500 nm to λ = 800 nm in the FDTD simulations as the MoOx thickness increases. Finally, for the thicker layers, a narrow, intense quadrupole resonance appears.
RESULTS AND DISCUSSION
97
In the FDTD simulations, it is clearly separated from the tip resonance and is centered between λ = 500 nm and λ = 580 nm, depending on the MoOx thickness. In the experiment, this resonance is centered between λ = 420 nm and λ = 480 nm and is damped by size and shape inhomogeneities, which causes an overlap with the tip resonance. For all of these resonances, the near-field enhancement is within the MoOx layer surrounding the Ag protrusion, and does not extend into the organic medium. As a consequence, the near-field enhancement is not expected to lead to a significant increase in the device performance when this nanostructured Ag layer is employed as the rear electrode of a solar cell.
5.3.3
Organic solar cells with nanostructured Ag rear electrode
The results presented in Sections 5.3.1 and 5.3.2 show that the relatively thick MoOx layers needed to obtain strong light scattering in the desired wavelength range inevitably lead to less favorable optical interference patterns with the Ag rear electrode. From this consideration, it is clear that for any thin-film solar cell, in which the optimum performance critically depends on the tuning of optical spacers, a conflict appears in the optimization of an optical spacer that at the same time satisfies the size requirements of plasmonic nanostructures and that of optical interference. It follows that it is a priori not obvious that the overall efficiency of such a thin-film cell can be increased when equipping it with plasmonic nanostructures. To investigate whether an efficiency enhancement can be reached by employing the nanostructured Ag rear electrode, we fabricated nanostructured P3HT:ICBA solar cells based on the knowledge gained in Sections 5.3.1 and 5.3.2, using PS beads with a diameter of 140 nm and total MoOx layer thicknesses between 115 and 185 nm. The Jsc values of these devices are shown as filled gray rectangles in Fig. 5.3. For all MoOx thicknesses, the Jsc of the nanostructured devices is larger than that of the flat reference devices with the same thickness. Moreover, the nanostructured device with a total MoOx layer thickness of 135 nm exhibits a Jsc that is enhanced by 4.4% in comparison to that of the optimized flat reference cell with 5 nm of MoOx . The η of this plasmon-enhanced cell is 5.56% with a Voc and a fill factor (FF) that are nearly identical to those of the optimized reference device (cf. Table 5.1 and Fig. 5.5). This demonstrates that the efficiency of an optimized organic solar cell can indeed be improved via an increase in Jsc due to a nanostructured rear electrode.
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ORGANIC SOLAR CELLS WITH NANOSTRUCTURED METAL REAR ELECTRODE
7 0
(a )
6 0
4 0 3 0 R e fe re n c e N a n o s tru c tu re d
S c a tte r e d lig h t ( % ) T o ta l r e fle c ta n c e ( % )
1 0 0 8 0
(b )
2 .0
N a n o s tru c tu re d / R e fe re n c e R e fe re n c e
6 0 4 0
1 .5
2 0
1 .0
E Q E r a tio
2 0
0 4
( c ) E x p e r im e n t:
1 2
R e fe re n c e N a n o s tru c tu re d S im u la tio n : N a n o s tru c tu re d
8 4
3 2 1 0
0 3 0 0
4 0 0
5 0 0
6 0 0
S im u la te d lig h t s c a tte r in g ( a .u .)
E Q E (% )
5 0
7 0 0
W a v e le n g th ( n m )
Figure 5.11: (a) External quantum efficiency (EQE) spectra of the optimized nanostructured device with a total MoOx layer thickness of 135 nm and the optimized flat reference devices with a MoOx layer thickness of 5 nm. (b) The total reflectance of the reference device (left axis), and the ratio of the EQE spectrum of the nanostructured device to that of the reference device (right axis). (c) The measured light scattering spectra of the reference device and the nanostructured device (left axis), and the light scattering spectrum of the nanostructured device obtained via FDTD simulation of the complete device layer stack (right axis, cf. Fig. 5.13(b) for a cross-section of the simulated geometry).
RESULTS AND DISCUSSION
99
1
80 40 0
Ag MoOx -100
Active layer
0
1 Div(E) (a.u.)
120 2
|E| (a.u.)
y (nm)
y (nm)
120
-
80
+
40
-1
0 0
x (nm)
100
-100
0
100
x (nm)
Figure 5.12: Simulated electric field intensity (left) and charge density (right) at a wavelength of λ = 717 nm around a Ag protrusion of a nanostructured solar cell device with a total MoOx thickness of 135 nm. For this simulation, the complete solar cell layer stack as shown in Fig. 5.1 was simulated, with the light incident through the semi-infinite glass substrate.
The comparison of the EQE, total reflectance and scattering spectra of the optimized reference and nanostructured devices, shown in Fig. 5.11, are key to determine whether this enhancement originates from plasmonic light scattering. The optimized flat reference device has a constant, low reflectance indicating a saturated absorption in the wavelength range of 350 nm < λ < 620 nm. As a result, it has a high EQE in this wavelength range. The EQE of the optimized nanostructured device is nearly identical to that of the reference at λ < 620 nm but with significant enhancement of up to a factor of 2.1 between 620 nm < λ < 780 nm, as shown in Fig. 5.11(b). Comparing this EQE enhancement with the total reflectance of the reference device [gray dashed curve in Fig. 5.11(b)] confirms that the enhancement occurs where targeted, i.e. coinciding with the red absorption tail of the active layer materials (the absorption spectrum of the active layer is shown in Fig. 5.7). At the same time, the reference device shows very little light scattering with no significant features [Fig. 5.11(c)]. The nanostructured device, on the other hand, exhibits strong light scattering in a band that is cut off on its high-energy side by the absorption of the active layer materials. This scattering band could be reproduced by a FDTD simulation of the full nanostructured solar cell stack, as shown by a dash-dot curve in Fig. 5.11(c). Moreover, this scattering band can clearly be identified as the same dipole resonance as observed from the PMMA samples (cf. Fig. 5.12). These results therefore suggest that the main contribution to the EQE enhancement is caused by light scattering induced by a dipole resonance. This interpretation is confirmed by the simulated absorption per unit volume
100
ORGANIC SOLAR CELLS WITH NANOSTRUCTURED METAL REAR ELECTRODE
(a)
Ag
MoOx
y (nm)
200 150 100 50
0
1
Abs. / Unit vol. (a.u.)
Active layer
0 -200
-100
0
100
200
x (nm)
(b) MoOx
Ag
y (nm)
200 150 100 50
Active layer
0 -200
-100
0
100
200
x (nm)
Figure 5.13: The simulated absorption per unit volume in the active layer at λ = 646 nm is for the (a) reference device with a 5 nm thick MoOx layer and (b) the nanostructured device with h = 130 nm. Both contour plots share the color scale shown in (a). For these two simulations, the complete solar cell layer stack as shown in Fig. 5.1 was simulated, with the light incident through the semi-infinite glass substrate.
in the red absorption tail of the active layer, as shown in Fig. 5.13(a) and 5.13(b) for the optimized reference and nanostructured devices, respectively. A comparison between these two contour plots clearly demonstrates significant absorption enhancement in the active layer underneath the Ag protrusions of the nanostructured device. As we observed strong light scattering in this wavelength range by both experiment and simulation, and because the near-field enhancement for all identified LSPRs mostly lies within the MoOx layer, we conclude that the plasmonic effect responsible for the enhanced absorption, and hence efficiency, is light scattering rather than near-field enhancement.
CONCLUSIONS
101
Nanostructured 60
EQE (%)
MoOx (nm) 115 135 155 185
40 20
20
0 300
Nanostructured
0 Reference 650
700
750
400
500
600
700
Wavelength (nm) Figure 5.14: External quantum efficiency (EQE) spectra of nanostructured devices. The inset shows the direct comparison between the nanostructured devices and the optimized flat reference with a MoOx thickness of 5 nm (black solid curve).
Further, these figures demonstrate that the disruption of the interference pattern by the presence of the Ag protrusions is outweighed by their intense light scattering. This is in agreement with the EQE spectra of nanostructured devices with total MoOx layer thicknesses between 115 and 185 nm shown in Fig. 5.14. The comparison of the EQE spectra in the spectral region of the active layer red absorption tail demonstrates that each nanostructured device performs better than the optimized flat reference in this wavelength range (inset to Fig. 5.14).
5.4
Conclusions
We have demonstrated a strategy and implementation for efficiency enhancement of an optimized, high performance organic solar cell by a plasmonic nanostructured Ag rear electrode. We utilized a bottom-up nanofabrication method that allowed for the patterning of the rear electrode on top of the
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ORGANIC SOLAR CELLS WITH NANOSTRUCTURED METAL REAR ELECTRODE
organic active layer without harming the device efficiency. The LSPR of this electrode was tuned to the red absorption tail of the active layer, and generated enhanced absorption by light scattering, as verified by characterizing and simulating the optical properties of nanopatterned and flat reference structures. Plasmonic near-field enhancement, on the other hand, was exclusively found in the MoOx spacer layer and is therefore unlikely to significantly contribute to the efficiency enhancement. Based on these findings, we conclude that the efficiency enhancement demonstrated here unambiguously originates from plasmonic light scattering. In addition, the plasmonic efficiency enhancement principle employed here is more generally valid to thin-film solar cells, as the LSPR of the electrode can readily be adjusted to coincide with the absorption edge of semiconductors within the visible and near-infrared spectrum. It is noteworthy that the nanostructured electrode described here could also be fabricated by other high-throughput fabrication methods such as nanoimprint lithography.
Chapter 6
General conclusions and outlook The application of plasmonic enhancement strategies for efficiency enhancement of organic solar cells was investigated by individually assessing the potential of near- and far-field plasmonic effects. Plasmonic near-field enhancement was studied by employing a model system of a Ag NP layer covered by thin-films of organic semiconductors. The Ag NPs of this model system that was introduced in Chapter 2 had a diameter in a size regime where far-field light scattering is negligibly small compared to absorption. Their optical properties were found to be strongly influenced by the dispersive permittivity of the absorbing organic thin-film covering them. For organic absorbers with strong and narrow absorption bands, such as SubPc, the highly dispersive permittivity led to the excitation of multiple dipole LSPRs in these spherical Ag NPs, which otherwise only occurs in non-spherical metal NPs. In Chapters 3 and 4, this model system was utilized to assess the exploitation of plasmonic near-field enhancement in organic solar cells. Absorption enhancement in molecular thin-films was observed in the presence of Ag NPs by experiments as well as numerical simulations. This enhancement was strongest at λLSPR , in weak absorbers with a high ε1 . However, the exploitation of the weaker off-resonance enhancement at λ > λLSPR could be interesting when parasitic absorption losses have to be minimized. By introducing a transparent spacer layer between the NPs and the absorbing molecular thin-film, the absorption was found to be enhanced within a range of about 5 nm from the Ag NP surface in the interstices of the NPs.
103
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GENERAL CONCLUSIONS AND OUTLOOK
The near-field absorption enhancement was found to be accompanied by a strong reduction in exciton lifetime in the presence of the Ag NPs. This lifetime reduction was strongest at λLSPR and occurred within a range of between 5 and 30 nm from the NP surface. The presence of the Ag NPs typically led to the reduction of the quantum yield of nearby emitters. However, when the Ag NPs were coated with a thin-film of a material with a high ε1 and a low intrinsic quantum yield, the relative importance of light scattering became important even for these small NPs, which led to an enhanced PL intensity of nearby organic emitters. The results obtained with this model system led to the conclusion that the exploitation of plasmonic near-field enhancement for efficiency enhancement of organic solar cells appears challenging. This is mainly because of the comparable range of absorption enhancement and exciton quenching, which implies that isolating the NPs with a layer thick enough to prevent exciton quenching may simultaneously eliminate any beneficial effect due to the near-field absorption enhancement. In addition, the LSPRs excited in metal NPs and thus also all the plasmonic near-field interactions strongly depend on the choice of the organic embedding medium. Therefore, an enhancement scheme based on plasmonic near-field interactions has to be designed specifically for a certain active layer material combination and cannot easily be adjusted to other material systems. Far-field light scattering, on the other hand, was found to be promising for producing plasmonically-enhanced organic solar cells. This was demonstrated in Chapter 5 by equipping an organic solar cell with a nanostructured Ag rear electrode. As the solar cells were fabricated starting with the transparent ITO electrode, the nanopatterning of the rear electrode had to be performed on top of the organic active layer without hurting the device performance. This was achieved by employing hole-mask colloidal lithography, a bottom-up nanofabrication method. The LSPR of this nanostructured electrode could readily be adjusted by altering the size of the Ag nanostructures. When the LSPR was tuned to the red absorption tail of the active layer, the nanostructured device exhibited an enhanced efficiency compared to an optimized reference cell with a flat rear electrode. Experiments and numerical simulations verified that this efficiency enhancement originated from an increased absorption in the active layer due to plasmonic far-field light scattering. At the same time, the near-field enhancement was found to be exclusively located in the spacer layer between the rear electrode and the active layer. As a result, near-field interactions are unlikely to significantly contribute to the efficiency enhancement. In addition, non-plasmonic contributions to the efficiency enhancement could also be excluded as the reference cell was fully optimized and the active layers of both the reference and the nanostructured cells were identical. Therefore, these results unambiguously demonstrate a
GENERAL CONCLUSIONS AND OUTLOOK
105
plasmonic efficiency enhancement of an optimized organic solar cell by light scattering. Moreover, the plasmonic efficiency enhancement principle employed here is more generally valid to thin-film solar cells, as the LSPR of the electrode can readily be adjusted to coincide with the absorption edge of semiconductors within the visible and near-infrared spectrum. The optimal active layer thickness of a bulk heterojunction organic solar cell is typically a compromise between maximizing absorption while still allowing for efficient charge carrier extraction. The P3HT:ICBA active layer employed here possesses a relatively high charge carrier mobility, which leads to an optimal thickness of ∼230 nm. This active layer thickness results in absorption saturation at λ < 620 nm at fill factors as high as ∼70%. As a result, the relative overall efficiency enhancement due to the plasmonic nanostructured rear electrode shown here is limited to 5%, even though the quantum efficiency in the red absorption tail of the active layer is increased by up to a factor of 2.1. By further optimization of the plasmonic structures, the efficiency enhancement of such absorption saturated cells might be increased to a certain extend. However, dramatically higher enhancement factors cannot be expected. The situation is different in the case of certain low-bandgap polymers with poor electrical properties, which leads to an optimal active layer thickness below the optical absorption length. In solar cells with such thin active layers, a broadband absorption enhancement and thus a significantly higher increase in efficiency might be reached. However, the active layer thickness of these cells has to be controlled very accurately as an increase of only a few nm would significantly reduce the fill factor. For this reason, these materials are not suitable for large area solution-processing. As a result, low-bandgap polymers with improved electrical properties have recently been developed. The plasmonic absorption enhancement achievable in solar cells based on these low-bandgap polymers will be comparable to that observed in P3HT:ICBA cells, with the difference that it occurs in absorption tails farther in the near-infrared. Currently, progress in organic photovoltaics is mostly driven by the development of improved active layer materials, which led to a steep increase in efficiencies during the last few years. As the technology matures, this development will slow down at a certain moment and the situation might eventually become similar to that of crystalline Si solar cells where an efficiency improvement of one-tenth of a percent is noteworthy. In such a competitive environment, the additional ∼0.5-1 mA/cm2 that a plasmonic enhancement scheme, as the one presented here, could add to the photocurrent of an organic solar cell with fully optimized material composition, morphology and layer thicknesses, might represent an important contribution. As organic solar cells have not yet reached a significant market share, it is
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GENERAL CONCLUSIONS AND OUTLOOK
nearly impossible to predict whether the efficiency gain due to plasmonic enhancement strategies will outweigh the additional fabrication/material costs for this technology. Recently, this uncertainty was demonstrated by wafer-based crystalline Si solar cells, a technology that has already been established for several decades. The main motivation for applying plasmonics in crystalline Si solar cells lies in maintaining a high absorption in the active layer while reducing the material cost by thinning down the cell. This strategy seemed promising in 2008, when the Si price on the spot market reached a value of ∼500 $/kg. Since then, the prize has fallen to less than 30 $/kg at the end of 2011 [210], which strongly reduced the possibility to save costs by thinning down the solar cells. As a result, plasmonic enhancement strategies are very unlikely to be applied in crystalline Si solar cells in the near future.
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Curriculum vitae Bjoern Niesen was born on July 15, 1983, in St. Gallen, Switzerland. From 2003 to 2008 he attended the University of Basel where he earned a Master of Science degree in Nanosciences. The topic of his master thesis was the fabrication and characterization of chemical sensors based on ion-sensitive Si nanowire field-effect transistors. As a part of his studies, he spent six months as an intern at Philips Research/Polymer Vision in Eindhoven, The Netherlands, where he worked on organic thin-film transistors. He commenced his PhD research in 2008 at the interuniversity microelectronics center (imec) to pursue a PhD degree at the Department of Electrical Engineering (ESAT) of the Katholieke Universiteit Leuven in Belgium. His current research interests include plasmonics for light management in optoelectronic devices as well as novel device architectures for solar cells.
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List of publications Journal publications • B. Niesen, B. P. Rand, P. Van Dorpe, D. Cheyns, H. Shen, B. Maes, and P. Heremans, Near-Field Interactions Between Metal Nanoparticle Surface Plasmons and Molecular Excitons. Part I: Absorption, submitted. • B. Niesen, B. P. Rand, P. Van Dorpe, D. Cheyns, E. Fron, M. Van der Auweraer, and P. Heremans, Near-Field Interactions Between Metal Nanoparticle Surface Plasmons and Molecular Excitons. Part II: Emission, submitted. • B. Niesen, B. P. Rand, P. Van Dorpe, D. Cheyns, L. Tong, A. Dmitriev, and P. Heremans, Plasmonic Efficiency Enhancement of High Performance Organic Solar Cells with a Nanostructured Rear Electrode, submitted. • C. Rolin, K. Vasseur, B. Niesen, M. Willegems, R. Müller, S. Steudel, J. Genoe, P. Heremans, Vapor Phase Growth of Functional Pentacene Films at Atmospheric Pressure, Adv. Func. Mater., DOI: 10.1002/adfm.201200896. • N. P. Sergeant, A. Hadipour, B. Niesen, D. Cheyns, P. Heremans, P. Peumans, and P. Rand, A Transparent Conducting Anode for Enhanced Light Harvesting in Organic Solar Cells, Adv. Mater. 24, 728-732 (2012). • B. Niesen, B. P. Rand, P. Van Dorpe, H. Shen, B. Maes, J. Genoe, and P. Heremans, Excitation of Multiple Dipole Surface Plasmon Resonances in Spherical Silver Nanoparticles, Opt. Express 18, 19032-19038 (2010).
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Conference presentations • B. Niesen, B. P. Rand, P. Van Dorpe, A. Dimitriev, P. Heremans, Plasmonic Organic Solar Cells: Near-Field Absorption Enhancement versus Far-Field Scattering, E-MRS Spring Meeting, Strasbourg, France (2012). • B. Niesen, P. Van Dorpe, A. Dimitriev, P. Heremans, B. P. Rand, Near- Versus Far-Field Plasmonic Absorption Enhancement Strategies for Organic Solar Cells, MRS Spring Meeting, San Francisco, CA (2012). • B. Niesen, B. P. Rand, P. Van Dorpe, L. Tong, A. Dmitriev, and P. Heremans, A Plasmonic Nano- Patterned Rear Contact for Organic Solar Cells, MRS Fall Meeting, Boston, MA (2011). • B. Niesen, B. P. Rand, P. Van Dorpe, E. Fron, and P. Heremans, Impact of Metal Nanoparticles on Excitons in Organic Thin-Films, MRS Fall Meeting, Boston, MA (2011). • N. P. Sergeant, A. Hadipour, B. Niesen, D. Cheyns, P. Heremans, P. Peumans, B. P. Rand, and S. Fan, The Origin of Enhanced Light Harvesting in Organic Solar Cells with ITO-Free MoO3/Ag/MoO3 Transparent Anode, MRS Fall Meeting, Boston, MA (2011). • B. Niesen, B. P. Rand, P. Van Dorpe, E. Fron, and P. Heremans, Impact of Metal Nanoparticles on Excitons in Organic Semiconductor Thin-Films, Summer School on Plasmonics 2, Porquerolles Island, France (2011). • B. P. Rand, D. Cheyns, B. Niesen, K. Vasseur, B. Verreet, J. Genoe, and P. Heremans, Probing The Influence of Molecular Orientation on Planar Heterojunction Solar Cell Performance, E-MRS Spring Meeting, Nice, France (2011). • N. P. Sergeant, A. S. Liu, B. Niesen, C. Stoessel, J. Kozak, L. Boman, B. P. Rand, P. Heremans, M. D. McGehee, P. Peumans, Multilayer Transparent Electrodes for Flexible Thin-Film Organic Solar Cells, 8th International Conference on Electroluminescence and Organic Optoelectronics (ICEL2010), Ann Arbor (2010). • B. Niesen, B. P. Rand, P. Van Dorpe, H. Shen, B. Maes, J. Genoe, and P. Heremans, Interactions of Excitons with Localized Surface Plasmons in Organic Semiconductor-Metal Nanoparticle Thin-Films, Passion for Knowledge Workshop, San Sebastian, Spain (2010).
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• B. P. Rand, B. Niesen, P. Van Dorpe, D. Cheyns, P. Heremans, The Interaction of Surface Plasmons with Organic Semiconductors, SPIE Optics + Photonics, San Diego, CA (2010). • B. Niesen, B. P. Rand, A. Kadashchuk, P. Van Dorpe, T. Aernouts, J. Genoe, P. Heremans, Exciton-Surface Plasmon Interactions in Organic Semiconductor-Metal Nanoparticle Thin-Films and Their Application to Solar Cells, SPIE Photonics Europe, Brussels, Belgium (2010).
Arenberg Doctoral School of Science, Engineering & Technology Faculty of Engineering Department of Electrical Engineering INSYS Kasteelpark Arenberg 10 B-3001 Heverlee
