Lastly, I want to express my special thanks to my wife Huachao accompanying ... A series of mixed (porphyrinato)(phthalocyaninato) rare-earth double-decker ..... 4.11 Parameters of cells fabricated by double-decker tetrapyrrole complexes.
Synthesis, Characterization, and Photovoltaic Applications of Mesoscopic Phthalocyanine Structures
BY Yong Li
A dissertation submitted in partial fulfillment of the requirements for the Doctor of Philosophy Major in Electrical Engineering South Dakota State University 2011
iii
Acknowledgements
This material is based upon work supported by the National Science Foundation/EPSCoR Grant No. 0903804 and by the State of South Dakota. I would like to extend my gratitude to the NSF (ECCS-0723114), the NASA (NNX09AP67A), and the SDSU EE PhD program for partially supporting me financially during the last three and a half years. Without this support I would probably not have been here in the first place. I would also like to thank all the professors whose classes I took for providing stimulating courses and for sharing their expertise, insights and wisdom. At an individual level, there are many people that I have had the pleasure and the privilege to closely work with and learn from. My dissertation would not have been possible without their great support. First I want to thank my dissertation advisor Dr. Xingzhong Yan, who introduced me to the amazing area of organic photovoltaics from a pure chemistry background when I first came to the USA in March of 2008. I am grateful to him for sharing his wonderful ideas and experiences, for training me to think analytically and critically, for providing insightful suggestions about my work, and for always being helpful, encouraging and positive. I am thankful to my major advisor Dr. David W. Galipeau, who led me on the path to a professional academic career in Engineering. He provided the infrastructure
iv
needed for my research as well as an intellectually stimulating environment in which to perform this research. My master’s advisor Dr. Jianzhuang Jiang and my former colleague Dr. Yongzhong Bian at Shandong University (now both at University of Science and Technology Beijing), have been a source of inspiration. I am grateful to them for being my advisor and co-worker, for sharing with me their enthusiasm about science, and for providing strong and continuous support even after I had graduated from their lab in China and already started to pursue my PhD studies in the USA. I would also like to thank Dr. Qiquan Qiao, Dr. Mahdi Farrokh Baroughi, and Dr. Gary Taylor for accepting to be members and graduate faculty representative of my committee, and for their time to review my dissertation. I consider myself lucky because I have always been blessed with good friends as well as great lab partners at the same time in all these years. With my colleague buddies, Dr. Lixin Xiao, Qi Wang, Ming Yan, Peifen Lu, Minlin Jiang, Prem Thapaliya, Dr. Mukesh Kumar, Dr. Qiliang Chen, Dr. Jing Li, Yu Xie, Tingting Xu, Jun Wang, Prakash Joshi, Mukul Dubey, Mariyappan Shanmugam and Mahbube Siddiki, we all found us in the photovoltaic program of SDSU. Having them around has made coping with things of lab so much easier. Our common experiences and mutual support has helped our bond grow stronger and I am sure that it will stay like this for years to come.
v
Lastly, I want to express my special thanks to my wife Huachao accompanying with me all these years, my son Kevin who was born in Brookings, and my parents and brothers in China. They have always been there for me, giving me their unconditional love, encouraging and supporting me in all my decisions. My parents’ blessing has brought me so far and will carry me all through my life. Their affection has been a source of energy for me all the time.
vi
Abstract
Synthesis, Characterization, and Photovoltaic Applications of Mesoscopic Phthalocyanine Structures Yong Li 2011
Organic-based solar cells including organic solar cells and dye-sensitized solar cells (DSSCs) have drawn considerable attention worldwide as potential competitors to conventional crystalline silicon solar cells and thin film solar cells owing to their prospective advantages to produce large-area, flexible, low-cost, and light-weight devices using simple techniques. However, most energy of the photons from the near infrared (NIR) region to longer wavelengths (~49% of the solar energy) usually cannot be harvested by these devices due to a relatively high band gap of the organic photovoltaic materials. The goal of this work was to develop solution-processable low-band-gap organic photovoltaic materials for low-cost high-efficiency organic-based solar cells. This dissertation depicts in detail the synthesis, characterization, and photovoltaic applications of 17 novel mesoscopic phthalocyanine structures. Three hyperbranched phthalocyanines 1–3 (HBMPc-CN; M = H2, TiO, Cu) were fabricated into cell structures
vii
of indium tin oxide/TiO2/TiOx:HBMPc-CN/CuSCN/Au (or carbon) following the extremely-thin absorber concept, showing efficiencies of up to 0.23%. A series of nonaggregated hyperbranched phthalocyanine dyes 4–8 (HBMPc-COOH; M = H2, AlCl, Co, Cu, Zn) were utilized as sensitizers of TiO2 in DSSCs, displaying efficiencies of up to 1.15%. A series of mixed (porphyrinato)(phthalocyaninato) rare-earth double-decker complexes [MIIIH(TClPP){Pc(α-OC4H9)8}] (9–15; M = Y, Sm, Eu, Tb, Dy, Ho, Lu; TClPP = meso-tetrakis(4-chlorophenyl)porphyrinate; Pc(α-OC4H9)8 = 1,4,8,11,15,18,22,25-octakis(1-butyloxy)phthalocyaninate) and [YIII(TClPP)(Pc)] (16, Pc = unsubstituted phthalocyaninate), along with a heteroleptic phthalocyaninato yttrium double-decker complex [YIIIH(Pc){Pc(α-OC4H9)8}] (17), were utilized as broadband absorbers and electron donors, N,N’-bis(1-ethylhexyl)-3,4:9,10-perylenebis(dicarboximide) or [6,6]-phenyl-C61 butyric acid methyl ester was adopted as primary electron acceptor, and an in situ formed cheap inorganic network of nanoporous TiOx was used as secondary electron acceptor in solution-processed organic-inorganic hybrid solar cells with efficiencies of up to 0.82%. Mesoscopic phthalocyanine structures are low-cost photovoltaic materials with tunable absorption/photophysical properties. Future work should address cell efficiency improvement via material engineering and device engineering to make mesoscopic phthalocyanine materials and solar cells commercially valuable.
viii
Table of Contents
Page Acknowledgements ....................................................................................................
iii
Abstract ......................................................................................................................
vi
List of Tables .............................................................................................................
xv
List of Figures ............................................................................................................
xvii
Chapter 1.
2.
Introduction ..................................................................................................
1
1.1
Background ........................................................................................
1
1.2
Previous work ....................................................................................
9
1.3
Motivation ..........................................................................................
16
1.4
Objectives ..........................................................................................
17
References ..........................................................................................
18
Theory ...........................................................................................................
35
2.1
Solar cells and their operating principles ...........................................
35
2.1.1
35
Crystalline silicon p-n junction solar cells .......................
ix
2.1.2
Extremely-thin absorber (ETA) solar cells ......................
41
2.1.3
Dye-sensitized solar cells (DSSCs) .................................
46
2.1.4
Organic solar cells............................................................
49
Solar cell device characterization ......................................................
58
References ..........................................................................................
62
Experimental Procedures ............................................................................
66
2.2
3.
3.1
Synthesis and characterization of hyperbranched phthalocyanines 1–3 (HBMPc-CN; M = H2, TiO, Cu).................................................
66
3.1.1
General procedures ..........................................................
66
3.1.2
Synthesis of hyperbranched phthalocyanines 1–3 (HBMPc-CN; M = H2, TiO, Cu)......................................
3.1.3
Femtosecond time-resolved fluorescence measurement of HBCuPc-CN solution ..................................................
3.2
66
70
Synthesis and characterization of hyperbranched phthalocyanine dyes 4–8 (HBMPc-COOH; M = H2, AlCl, Co, Cu, Zn) ....................
72
3.2.1
General procedures ..........................................................
72
3.2.2
Synthesis of hyperbranched phthalocyanine dyes 4–8 (HBMPc-COOH; M = H2, AlCl, Co, Cu, Zn) .................
74
x
3.3
3.2.3
Femtosecond time-resolved fluorescence measurements
80
3.2.4
Electrochemical cyclic voltammetry measurements ........
80
Synthesis and characterization of sandwich-type porphyrinato/ phthalocyaninato rare earth(III) double-decker complexes 9–17 ......
81
3.3.1
General procedures ..........................................................
81
3.3.2
Synthesis of mixed (porphyrinato)(phthalocyaninato) rare-earth(III) double-decker complexes 9–15 [MIIIH(TClPP){Pc(α-OC4H9)8}; M = Y, Sm, Eu, Tb, Dy, Ho, Lu] ......................................................................
3.3.3
82
Synthesis of mixed (porphyrinato)(phthalocyaninato) yttrium(III) double-decker complex 16 [YIII(TClPP)(Pc)] .............................................................
3.3.4
3.4
91
Synthesis of heteroleptic phthalocyaninato yttrium(III) double-decker complex 17 (YIIIH(Pc){Pc(α-OC4H9)8}) ..
93
3.3.5
Femtosecond time-resolved fluorescence measurements
96
3.3.6
Electrochemical cyclic voltammetry measurements ........
97
Fabrication and characterization of extremely-thin absorber (ETA) solar cells ...........................................................................................
97
3.4.1
97
General procedures ..........................................................
xi
3.5
3.4.2
Fabrication procedures .....................................................
98
3.4.3
Current-voltage (I-V) characterization .............................
101
3.4.4
Femtosecond time-resolved fluorescence measurements of the HBCuPc-CN solid films ........................................
102
Fabrication of dye-sensitized solar cells (DSSCs) .............................
103
3.5.1
General procedures ..........................................................
103
3.5.2
Fabrication procedures .....................................................
103
3.5.3
I-V characterization ..........................................................
107
3.5.4
Incident photon to current conversion efficiency (IPCE) measurements ...................................................................
3.6
107
Fabrication of organic-inorganic hybrid bulk-heterojunction solar cells ....................................................................................................
108
3.6.1
General procedures ..........................................................
108
3.6.2
Synthesis of PDI ..............................................................
110
3.6.3
Fabrication procedures .....................................................
111
3.6.4
I-V characterization ..........................................................
113
3.6.5
IPCE measurement...........................................................
113
References ..........................................................................................
114
xii
4.
Results and Analysis .................................................................................... 4.1
118
Extremely-thin absorber (ETA) solar cells made from hyperbranched phthalocyanines 1–3 (HBMPc-CN; M = H2, TiO, Cu)......................................................................................................
118
4.1.1
Transmission and morphology of the n-type TiO2 films .
118
4.1.2
Absorption and morphology of the HBMPc-CN:TiOx films .................................................................................
4.1.3
Cross-section scanning electron microscopy (SEM) images of the solar cell devices .......................................
122
4.1.4
Solar cell fabrication and characterization .......................
124
4.1.5
Steady-state and ultrafast time-resolved fluorescence investigation of HBCuPc-CN (3) .....................................
4.2
120
129
Dye-sensitized solar cells (DSSCs) made from non-aggregated hyperbranched phthalocyanine dyes 4–8 (HBMPc-COOH; M = H2, AlCl, Co, Cu, Zn)............................................................................... 4.2.1
136
Synthesis of hyperbranched phthalocyanines HBMPc-CN (M = H2, AlCl, Co, Cu, Zn) and hyperbranched phthalocyanine dyes 4–8 (HBMPc-COOH; M = H2, AlCl, Co, Cu, Zn) .................
136
xiii
4.3
4.2.2
Steady-state absorption ....................................................
137
4.2.3
Steady-state fluorescence .................................................
140
4.2.4
Femtosecond time-resolved fluorescence ........................
143
4.2.5
Electrochemical properties...............................................
152
4.2.6
Solar cell fabrication and characterization .......................
156
Organic-inorganic hybrid bulk-heterojunction (BHJ) solar cells made form sandwich-type porphyrinato/phthalocyaninato rare
4.4
5.
earth(III) double-decker complexes (9–17) .......................................
165
4.3.1
Synthesis and characterization of 9–17............................
165
4.3.2
Steady-state absorption ....................................................
170
4.3.3
Steady-state emission .......................................................
180
4.3.4
Femtosecond time-resolved fluorescence ........................
184
4.3.5
Electrochemical properties...............................................
188
4.3.6
Solar cell fabrication and characterization .......................
189
Cost estimation of phthalocyanine materials in phthalocyaninebased solar cells .................................................................................
204
References ..........................................................................................
204
Conclusions ...................................................................................................
225
xiv
5.1
Summary ............................................................................................
220
5.2
Conclusions ........................................................................................
224
5.3
Future work ........................................................................................
226
xv
List of Tables
Table 3.1
Adopted pH values for the preparation of rare-earth acetylacetonates .............
4.1
Parameters of the ETA solar cells made from hyperbranched phthalocyanines 1–3 (HBMPc-CN; M = H2, TiO, Cu).....................................
4.2
84
129
Biexponential fitting parameters for the fluorescence dynamics of the three HBCuPc-CN (3) samples. The sign of minus represents a rising behavior ......
4.3
Page
136
Steady-state UV-visible absorption and fluorescence data for the hyperbranched phthalocyanine dyes HBMPc-COOH (M = H2, AlCl, Co, Cu, Zn; 4–8) in anhydrous DMF.......................................................................
4.4
139
Biexponential fitting parameters for the Soret band fluorescence dynamics of the hyperbranched phthalocyanine dyes 4–8 in anhydrous DMF with excitation at 400 nm and emission at 480 nm ...................................................
4.5
Biexponential fitting parameters for the Q-band fluorescence dynamics of 4–8 in anhydrous DMF with excitation at 400 nm ...........................................
4.6
4.7
145
149
Biexponential fitting parameters for the fluorescence dynamics of 8/TiO2 film with excitation at 400 nm ..........................................................................
149
Optical and redox properties of 4–8 in anhydrous DMF ..................................
154
xvi
4.8
Parameters of the DSSCs fabricated with 4–8 under 1-sun AM 1.5G illumination .......................................................................................................
4.9
161
Steady-state UV-visible absorption data for the complexes 9–17 in CHCl3 ................................................................................................................
174
4.10 Multi-exponential fitting parameters for the fluorescence dynamics of the complexes 9–15 in CH2Cl2 with emission at 655 nm and excitation at 420 nm... ..................................................................................................................
187
4.11 Parameters of cells fabricated by double-decker tetrapyrrole complexes 9–17 under 1-sun AM 1.5G illumination..........................................................
197
xvii
List of Figures
Figure 1.1
Best research cell power conversion efficiencies (from National Renewable Energy Laboratory, NREL) ..............................................................................
2.1
Page
2
Current-voltage characteristics of a p-n junction diode solar cell under dark and illuminated conditions, respectively...........................................................
37
2.2
Illustration of a crystalline silicon p-n junction solar cell.................................
38
2.3
Energy-band diagram of a crystalline silicon p-n junction solar cell under light illumination and open circuit condition ....................................................
2.4
Schematic diagram of an ETA solar cell showing a superstrate n-i-p arrangement on a conducting indium tin oxide (ITO) glass substrate ..............
2.5
39
42
Energy band diagram of an ETA device utilizing phthalocyanine (Pc) as absorber .............................................................................................................
43
2.6
Schematic overview of a dye-sensitized solar cell ...........................................
47
2.7
Operating principle and energy level diagram of dye-sensitized solar cells. Potentials are referenced to the normal hydrogen electrode (NHE) .................
48
2.8
Device architecture of a Schottky-barrier organic solar cell ............................
51
2.9
Energy level diagram of a Schottky-barrier organic solar cell .........................
52
xviii
2.10 Device architecture of a bilayer donor/acceptor heterojunction solar cell .......
53
2.11 Energy level diagram of a bilayer donor/acceptor heterojunction solar cell showing six steps associated in device operation .............................................
55
2.12 Device architecture of a bulk donor/acceptor heterojunction solar cell ...........
57
2.13 Illustration of the geometry used to derive the standard AM1.5 spectrum .......
58
2.14 Standard Solar Spectra for space and terrestrial use .........................................
59
3.1
Synthesis routine of hyperbranched phthalocyanines 1–3 (HBMPc-CN; M = H2, TiO, Cu) ......................................................................................................
67
3.2
Femtosecond fluorescence upconversion measurement system .......................
70
3.3
Synthesis routine of hyperbranched phthalocyanine dyes 4–8 (HBMPcCOOH; M = H2, AlCl, Co, Cu, Zn) ...................................................................
3.4
Synthesis routine of the mixed (porphyrinato)(phthalocyaninato) rareearth(Ш) double-decker complexes 9–15 .........................................................
3.5
83
Synthesis routine of the mixed (porphyrinato)(phthalocyaninato) rareearth(Ш) double-decker complex 16 ................................................................
3.6
73
92
Synthesis routine of the heteroleptic phthalocyaninato yttrium(Ш) doubledecker complex 17 ............................................................................................
94
3.7
Experimental setup for measuring the I-V response of the solar cells ..............
102
3.8
Experimental setup for measuring the IPCE of the solar cells .........................
108
xix
3.9
Schematic molecular structures of the double-decker complexes 9–17, PDI, and PCBM .........................................................................................................
4.1
Transmittance of the n-type TiO2 films made from TiO2 sol gel and Solaronix Ti-Nanoxide HT/SP..........................................................................
4.2
123
Cross-section SEM of glass/ITO/TiO2 (Solaronix)/TiOx:HBH2Pc-CN (1)/ CuSCN/Au ........................................................................................................
4.6
121
AFM images [(a), topography; (b) phase] of the films of HBH2Pc-CN (1): TiOx, HBTiOPc-CN (2):TiOx, and HBCuPc-CN (3):TiOx ...............................
4.5
119
UV-visible absorption spectra of the films of (a) HBH2Pc-CN (1):TiOx, (b) HBTiOPc-CN (2):TiOx, and (c) HBCuPc-CN (3):TiOx ...................................
4.4
118
Atomic force microscopy (AFM) images of the TiO2 films made from (a) TiO2 sol gel and (b) Solaronix Ti-Nanoxide HT/SP ..........................................
4.3
109
124
Current density–voltage (J-V) curves (empty: dark; filled: illuminated) for the cell structures of (a) glass/ITO/TiO2 (sol gel)/HBTiOPc-CN (2)/CuSCN/ Carbon, (b) glass/ITO/TiO2 (sol gel)/TiOx:HBH2Pc-CN (1)/CuSCN/Au, and (c) glass/ITO/TiO2 (sol gel)/TiOx:HBCuPc-CN (3)/CuSCN/Au, under 1-sun air mass 1.5 global (AM 1.5G) illumination .....................................................
4.7
Absorption and emission (top line, excited at 340 nm; bottom line, excited at 400 nm) spectra of HBCuPc-CN (3) in dimethylacetamide (DMAc)/CH2Cl2
125
xx
(1/1, v/v)............................................................................................................ 4.8
130
Fluorescence dynamics of (a) HBCuPc-CN (3) pristine film and (b) HBCuPc-CN (3):TiO2 film with emission at 520 nm and excitation at 400 nm. The lines are the plotted instrument response function and the fitted curves ................................................................................................................
4.9
132
Fluorescence dynamics of HBCuPc-CN (3) solution with excitation at 400 nm and emission at (a) 520 nm and (b) 710 nm. The lines are the plotted instrument response function and the fitted curves...........................................
133
4.10 UV-visible absorption spectra of 4–8 in anhydrous DMF ................................
138
4.11 Fluorescence spectra of 4–8 in anhydrous DMF ..............................................
142
4.12 Soret band fluorescence dynamics of 4–8 in anhydrous DMF with excitation of 400 nm and emission of 480 nm. The red lines are plotted instrument response function and the blue ones are the fitted curves...............
144
4.13 Q-Band fluorescence dynamics of 4–8 except 6 in anhydrous DMF with excitation of 400 nm. The red lines are plotted instrument response function and the blue ones are the fitted curves ..............................................................
147
4.14 Anisotropy decay profiles for the Q-band emissions of 4, 5 and 8 in anhydrous DMF with excitation at 400 nm. The red lines are plotted instrument response function ............................................................................
148
xxi
4.15 Fluorescence dynamics of 8/TiO2 film with excitation of 400 nm. The red lines are plotted instrument response function and the blue ones are the fitted curves ................................................................................................................
150
4.16 Reductive cyclic voltammograms of 4–8 in anhydrous DMF ..........................
153
4.17 J-V curves (empty: dark; filled: illuminated) of the DSSCs of 8 fabricated (a) with a double-layered mesoporous TiO2 (~10 µm) and a scattering TiO2 layer, and (b) with a single-layered mesoporous TiO2 (~5 µm) and without the scattering TiO2 layer under 1-sun AM 1.5G illumination ..........................
157
4.18 J-V curves (empty: dark; filled: illuminated) of the DSSCs of 4–8 fabricated with a double-layered mesoporous TiO2 and a scattering TiO2 layer under 1-sun AM 1.5G illumination .............................................................................
160
4.19 IPCEs of the DSSCs of 4–8 fabricated with a double-layered mesoporous TiO2 and a scattering TiO2 layer under 1-sun AM 1.5G illumination ..............
163
4.20 (a) Experimental and (b) simulated isotopic pattern for [MH]+ of 10 in the MALDI-TOF mass spectrum ............................................................................
168
4.21 IR spectra of the mixed-ring double-decker complexes 9–15. The spectra are stacked in the order of 10-11-12-13-9-14-15, following the ionic radius contraction sequence of the rare-earth(III) cations within these complexes.....
169
4.22 Absorption spectra of 9–15 in CH2Cl2 ..............................................................
172
xxii
4.23 Absorption spectra of the yttrium double-decker complexes 16 and 17, as well as PDI and PCBM, in CH2Cl2 ...................................................................
173
4.24 Absorption spectra of mixtures 9:PCBM (1/1, wt/wt), and double-decker complex (9, 16, and 17):PDI (1/2, molar ratio) in CH2Cl2 ...............................
176
4.25 Absorption spectra of mixtures of double-decker complex (10–15):PDI (1/2, molar ratio) in CH2Cl2 ......................................................................................
177
4.26 Absorption spectra of the active layers of 9:PCBM, 9:PCBM:TiOx, 9:PDI: TiOx, 16:PDI:TiOx, and 17:PDI:TiOx on ITO substrates ..................................
178
4.27 Absorption spectra of the active layers of (10–15):PDI:TiOx on ITO glass substrates ...........................................................................................................
179
4.28 Emission spectra of 10–11 under the 324 nm excitation in CH2Cl2 solution ...
181
4.29 Emission spectra of 10–11 under the 420 nm excitation in CH2Cl2 solution at room temperature ..............................................................................................
182
4.30 Emission spectrum of 17 under the 330 nm excitation in CH2Cl2 solution at room temperature ..............................................................................................
194
4.31 Fluorescence dynamics of the complexes 9–15 in CH2Cl2 with emission at 655 nm and excitation at 420 nm. The red lines are plotted instrument response function and the blue ones are the fitted curves .................................
185
4.32 Solar cells device architecture ..........................................................................
190
xxiii
4.33 Proposed working principle of the solar cells ...................................................
191
4.34 J-V curves (empty: dark; filled: illuminated) of the cells of glass/ITO/ PEDOT:PSS/9:PCBM/TiOx/Al, and Glass/ITO/PEDOT:PSS/9:PCBM:TiOx /LiF/Al under 1-sun AM 1.5G illumination ......................................................
193
4.35 AFM (a–c) topography images and (d–f) phase images of the active layers of 9:PCBM (a & d), 14:PDI:TiOx (b & e), and 17:PDI:TiOx (c & f), respectively. Area: 0.5 μm × 0.5 μm.................................................................
195
4.36 J-V curves (empty: dark; filled: illuminated) of the cells of glass/ITO/ PEDOT:PSS/double-decker complex (9–15):PDI:TiOx/Al, under 1-sun AM 1.5G illumination ..............................................................................................
198
4.37 J-V curves (empty: dark; filled: illuminated) of the cells of glass/ITO/ PEDOT:PSS/double-decker complex (11, 16, 17):PDI:TiOx/Al, under 1-sun AM 1.5G illumination ........................................................................................
200
4.38 IPCEs of the cells of glass/ITO/PEDOT:PSS/double-decker complex (11, 16, 17):PDI:TiOx/Al, under 1-sun AM 1.5G illumination ......................................
202
1
Chapter 1 Introduction
1.1 Background As an extremely flexible form of energy, electricity has become a necessity of everyday life. However, the traditional ways to generate electric power either by chemical combustion or by nuclear fission suffer from many critical problems, including green house gas emissions, and particulate and radioactive pollution [1]. Fossil energy crisis and environmental issues have led people to seek for renewable, sustainable, and environmentally clean energy sources. As considered one of the most promising candidates, solar electricity has been sparkled and stimulated in both scientific research and practical applications [2]. Solar cells are the optoelectronic devices that directly convert solar energy into electrical energy. Since sunlight is ubiquitous and widely available, solar cells can enable geographically diverse solar electric systems that are less vulnerable to international energy politics, volatile fossil fuel-based markets, and transmission failures. As the most important milestones in the historical development of solar cell technologies, the first photovoltaic effect was observed by Becquerel in 1839 [3], the first highly efficient solar cell was developed by Chapin et al. in 1954 using a diffused silicon
2
p-n junction with 6% efficiency [4], and the efficiency of a commercialized silicon solar cell PERL was reported to reach to 24.7% by Zhao et al. in 1999 [5]. Figure 1.1 shows the mainstream of the current solar cell technologies, including crystalline silicon solar cells, thin-film solar cells, multi-junction concentrators, organic solar cells (OSCs), and dye-sensitized solar cells (DSSCs), along with their best research cell power conversion efficiencies certified by the National Renewable Energy Laboratory (NREL).
Figure 1.1. Best research cell power conversion efficiencies (from National Renewable Energy Laboratory, NREL).
3
Thus far, global solar cell market has being primarily dominated by devices of crystalline silicon with a market share of 80-90% [6]. Crystalline silicon has proved convenient because it yields stable devices with good efficiencies as high as 25.0±0.5% (cell efficiency) and 22.9±0.6% (module efficiency), and uses processing technology which has been well-developed from the huge knowledge base of the microelectronics industry [6–7]. However, crystalline silicon is an indirect band-gap semiconductor, and thus is a relatively poor absorber of light. In order for sufficient light absorption, it requires a considerable thickness of up to several hundred microns of material [6]. It is worth noting that the unavoidable kerf loss during the slicing of silicon wafers out of an ingot results in large silicon loss and a consequently higher cost [8]. Besides, the high purity and quality requirement of the crystalline silicon wafers for solar cells is another reason for the high cost of these devices [8]. The high cost of crystalline silicon wafers has prompted the industry to look for less expensive semiconducting materials for thin film solar cells. The selected materials have all been strong light absorbers with direct band gap which only need to be as thin as a few microns for efficient light harvesting, so materials cost has been significantly reduced. The most commonly used materials for thin film solar cells are amorphous silicon (a-Si:H), cadmium telluride (CdTe), gallium arsenide (GaAs), and copper indium gallium (di)selenide (CIGS). The state-of-the-art power conversion efficiencies for
4
single-junction thin film solar cells have reached to 10.1±0.3% for a-Si:H, 16.7±0.5% for CdTe, 28.1±0.8% for GaAs, and 19.6±0.6% for CIGS [7]. GaAs based multi-junction devices are the most efficient solar cells to date, and a GaAs triple-junction metamorphic cell has reached a record high of 42.3% in October of 2010 [9]. The module efficiencies with thin film technologies have reached to 12.8±0.4% for CdTe, 21.1±0.6% for GaAs, and 15.7±0.5% for CIGS [7]. Thin film solar cells have now held 10–20% of the solar cell market share [6]. The biggest concern with these thin film materials is the requirement of expensive manufacturing equipments and technologies, such as physical or chemical vapour deposition. The thin film solar cells have other problems as well. Amorphous Si:H thin film solar cells suffer from a light-induced degradation effect (the so-called Staebler-Wronski effect) and stabilize only at lower efficiency values [10]. Cd element is toxic and CdTe is not environment-friendly [11]. The applications of GaAs solar cells are now only limited to power satellites and other spacecraft, because the cost of a single-crystal GaAs substrate is as high as $10,000 per square meter, which is too expensive for terrestrial use [12]. The scarcity of In and Se elements, both of which are less than 0.05 ppm in the earth’s crust, limits the super mass production of CIGS solar cells [13]. Therefore, it has become important to develop “green” and cost-effective photovoltaic materials as well as low-cost cell fabrication methodologies.
5
Organic semiconductors and dyes have emerged as promising alternatives for solar cells fabrication due to their advantages of (a) low cost of synthesis, (b) ease of manufacturing by vacuum or vapor phase deposition, solution casting or screen printing, and (c) high absorption coefficients which is in the order of 10-5 cm-1 [14–16]. Over the past few decades, organic-based solar cells including both OSCs and DSSCs have drawn considerable attention worldwide as potential competitors to conventional crystalline silicon solar cells and thin film solar cells owing to their prospective advantages to produce large-area, flexible, low-cost, and light-weight devices using simple techniques [14–16]. From the aspect of cell architecture, the research of OSCs started from Schottky-barrier solar cells with a single organic semiconductor sandwiched between two electrodes [17]. Schottky-barrier OSCs usually yield very low power conversion efficiencies (700 nm) photo-responsivity are needed. In addition, advanced cell designs that can significantly increase the interfacial area for efficient exciton dissociation and can provide pathways for electron and holes transport, are needed.
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1.4 Objectives The objectives of this work were to 1) develop novel mesoscopic phthalocyanine structures, including hyperbranched phthalocyanines, hyperbranched phthalocyanine dyes, and sandwich-type porphyrinato/phthalocyaninato rare earth(III) double-decker complexes; 2) develop solution processed organic-inorganic hybrid solar cells using hyperbranched phthalocyanine absorbers following the extremely-thin-absorber concept; 3) develop near infrared photon harvestable high-efficiency DSSCs using non-aggregated hyperbranched phthalocyanine dyes; 4) develop solution processed OSCs with broadband light harvesting capabilities using sandwich-type porphyrinato/phthalocyaninato rare earth(III) double-decker complexes; and 5) develop an understanding of photovoltaic material design. In order to accomplish these goals, the following specific tasks were established: 1) design and synthesize soluble hyperbranched phthlocyanines with varied metal centers; 2) design and synthesize non-aggregated hyperbranched phthalocyanine dyes with different metal centers; 3) design and synthesize sandwich-type porphyrinato/phthalocyaninato rare earth(III) double-decker complexes with different rare earth metal centers and different ligands; 4) study the optical and photophysical properties of the synthesized complexes to understand the exciton generation and charge transfer process within these complexes; 5) fabricate and characterize ETA solar cells,
18
DSSCs, and organic-inorganic hybrid BHJ solar cells for improved power conversion efficiencies.
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[89] L. Galmiche, A. Mentec, A. Pondaven and M. L’Her, “Charge Transfer Complexes Between Carbazole and Lutetium Bisphthalocyanine”, New J. Chem. 25, pp. 1148–1151, 2001.
[90] J. P. Collman, J. L. Kendall, J. L. Chen, K. A. Collins and J. C. Marchon, “Formation of Charge-Transfer Complexes from Neutral Bis(porphyrin) Sandwiches”, Inorg. Chem. 39, pp. 1661–1667, 2000.
[91] C. Videlot, D. Fichou and F. Garnier, Mol. Cryst. Liq. Cryst. 322, pp. 319–328, 1998.
[92] L. Liu and A. T. Hu, “Synthesis of Soluble Functional Dye Phthalocyanines and Perylene Tetracarboxylic Derivatives by Microwave-Irradiation and Their Photoelectric Performances”, J. Porphyrins Phthalocyanines 7, pp. 565–571, 2003.
[93] M. O. Liu and A. T. Hu, “Microwave-Assisted Synthesis of Phthalocyanine–Porphyrin Complex and Its Photoelectric Conversion Properties”, J. Organomet. Chem. 689, pp. 2450–2455, 2004.
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Chapter 2 Theory
2.1 Solar cells and their operating principles Solar cells generate electrical power by converting the energy of sunlight into direct current electricity using semiconductors that exhibit the photovoltaic effect which was first discovered by Becquerel in 1839 [1–2]. Photons within the incident light having energy greater than the band gap of the semiconductor(s) can be harvested by the semiconductor(s) and create electron-hole pairs, which are sequentially separated by a built-in electrical field and extracted to the two electrode terminals of the device [3].
2.1.1 Crystalline silicon p-n junction solar cells Crystalline silicon p-n junction solar cells are the most fundamental and most important inorganic solar cells. At the heart of a crystalline silicon p-n junction solar cell is the large-area p-n junction diode, which is formed when a p-type semiconductor and an n-type semiconductor are brought in direct contact with each other [4]. If the layers or regions of the p-type and the n-type are similar semiconductor materials with equal band gaps but are differently doped, a homojunction will be formed [5]; and if the p-type and the n-type are dissimilar semiconductor materials with unequal band gaps, a
36
heterojunction will be formed [6]. Crystalline silicon solar cells are homojunction devices. The p-type and the n-type of silicon are obtained by intentionally introducing different impurities into extremely pure (intrinsic) silicon for the purpose of modulating its electrical properties [7]. When doped with Group III elements such as boron, the intrinsic silicon becomes an electrically conductive p-type semiconductor, due to the missing of the fourth valence electron which creates freely movable "broken bonds" (holes) in the silicon lattice [7]. In the p-type silicon, positively charged carriers (holes) are the majority carriers that can move freely, and electrons are the minority carriers. When doped with Group V elements such as phosphorus, the intrinsic silicon becomes an electrically conductive n-type semiconductor, due to the added extra valence electrons which are unbounded from individual atoms [7]. In the n-type silicon, negatively charged carriers (electrons) are the majority carriers that can move freely, and holes are minority carriers. When the p-type silicon and the n-type silicon are first brought together, the holes and electrons from each side diffuse together, neutralizing each other, leaving a space charge region and a net electric field in the junction [4]. This happens until the Fermi level of the p-type silicon lines up with the Fermi level of the n-type silicon, i.e. reaching
37
to a thermal equilibrium [4]. At this point, there is an energy barrier (qVbi) for electrons to surmount in order to move from the n side to the p side of the junction [4].
Figure 2.1. Current-voltage characteristics of a p-n junction diode solar cell under dark and illuminated conditions, respectively.
Note that an applied voltage can either decrease this energy barrier under forward bias or increase the energy barrier under reverse bias [4]. Under a forward bias voltage
38
equal to ~67% of the band gap energy, the energy barrier is made small enough, and electrons start to flow from the n-type to the p-type [4]. This corresponds to the knee in the dark current versus voltage (I-V) curve of the diode as shown in Figure 2.1. Under reverse bias voltage, the energy barrier becomes greater than that under zero bias, and no current flows [4]. As a result, the p-n junction diode is a rectifier that only allows current flow in a one-way direction [4].
Figure 2.2. Illustration of a crystalline silicon p-n junction solar cell.
As shown in Figure 2.2, a typical crystalline silicon solar cell is essentially a large p/n junction diode with a metal grid on its front side facing the sun and a reflective metal
39
counter electrode on its backside, which can perform the function of converting the energy of sunlight photons to electrical power.
Figure 2.3. Energy-band diagram of a crystalline silicon p-n junction solar cell under light illumination and open circuit condition.
Figure 2.3 shows the energy-band diagram of a crystalline silicon p-n junction solar cell under light illumination at zero current (open circuit condition). These energy band edge diagrams illustrate how a crystalline silicon p-n junction solar cell works. First, photons with energy greater than the band gap (Eg) of silicon are absorbed, exciting
40
electrons from the valence band (VB) of silicon in the p-type and n-type region to the excited conduction band (CB) states [4]. This process represents the generation of electron-hole pairs. Electrons generated in the p-type region and holes generated in the n-type region are minority carriers. If their lifetime at the excited states is long enough, they can diffuse to the junction and fall down the potential barrier by the built-in electric field [4]. Then electrons transport through the quasi-neutral region of n-type Si to cathode, and holes transport through the quasi-neutral region of p-type Si to anode. Under this optical bias and open circuit condition, there is an open circuit voltage (VOC = EFn－EFp, where EFn is the electron quasi-Fermi level, and EFp is the hole quasi-Fermi level) between the two electrode terminals of the solar cell. If the anode and the cathode are connected through an external circuit, a direct current flows. As shown in Figure 2.1, the current resulting from this directional flow is superimposed upon the normal rectifying current-voltage characteristics of the p-n junction diode, displacing them downwards by an amount that depends on the light intensity [3]. A portion of the curve is forced into the fourth quadrant of the I-V coordinates, and power can be extracted from the solar cell electrode terminals, as from a normal electrochemical battery [3].
41
2.1.2 Extremely-thin absorber (ETA) solar cells ETA solar cells consist of an extremely-thin (tens to one or two hundred of nanometers) absorber layer embedded between two transparent charge transport layers [8]. The absorber is an intrinsic or lightly doped layer which is responsible for harvesting sunlight photons and generating electron-hole pairs [9]. The two charge transport layers serve to transfer the photo-generated carriers from the absorber layer towards the two planner contacts they border on [9]. Usually these two charge transport layers do not contribute to the light absorption of the ETA solar cells. Their role is mainly to accommodate the highly folded absorber layer and ensure efficient charge carrier transfer to the respective contacts under majority-carrier transport conditions [9]. Therefore, these charge transport layers must be both transparent and at least slightly p- and n-doped [9]. The interfaces between the transport layers and their respective contacts are planar [9]. The two contacts should be ohmic, transparent on the light-entry side, and reflective on the backside [9]. The absorber layer can be deposited on a porous n-type (or p-type) layer which is pre-coated on a transparent conducting oxide (TCO) glass, followed by coating in turn a p-type (or n-type) transparent layer in void-filling fashion and an optically reflective metal contact layer with a proper work function on the absorber [9]. This is to say, the deposition sequence for an ETA cell can be either n-i-p or p-i-n [9]. Figure 2.4 shows a
42
schematic diagram of a typical ETA solar cell with a non-planar porous geometry. It is a superstrate n-i-p cell, in which the absorber is conformally deposited on a deeply structured transparent n-type layer.
Figure 2.4. Schematic diagram of an ETA solar cell showing a superstrate n-i-p arrangement on a conducting indium tin oxide (ITO) glass substrate.
From the aspect of materials, the choices of the deposition sequence have been limited somewhat. Most deeply porous structured films come from n-type oxides, most prominently TiO2 and ZnO, rather than p-type materials [9]. As is known, TiO2 and ZnO can produce majority-carrier transport conditions only for electrons [9]. Besides, their
43
films usually have high transparency over most of the solar spectrum. Therefore, most ETA solar cells use TiO2 or ZnO as the light-entry side, and adopt the superstrate n-i-p architecture [9]. On the contrary, the choice of superstrate p-i-n architecture has seldom been adopted.
Figure 2.5. Energy band diagram of an ETA device utilizing phthalocyanine (Pc) as absorber.
44
Figure 2.5 shows the operating principle of an envisioned ETA cells. The absorber is embedded in a wide-gap p/n-heterojunction. The energy levels of the charge transport layers with respect to the light absorber layer, and the direction of the built-in electric field across the absorber layer, are clearly illustrated. The band offsets between the absorber layer and the two charge transparent layers are carefully considered in order to fulfill the possibility of thermalized majority-carrier injection from the absorber into the two charge transport layers. Photon absorption by the absorber results in the generation of electron-hole pairs (for inorganic semiconductor) or excitons (for organic semiconductor). The large built-in electric field support fast separation of the photo-generated carriers in the absorber layer [9]. Electrons are driven towards the n-type transparent electron-transport layer and subsequently collected by the n-contact, while holes can only enter the p-type hole-transport layer and then reach the p-contact. If the two electrode terminals are connected through an external circuit, current flows. Since the absorber layer is sandwiched between the two transparent charge transport layers, sunlight can pass through the highly folded absorber several times, and thus the local absorber thickness in the ETA configuration is much thinner than the total optical thickness [9]. When the total volume of the absorber is constant, the local thickness is inversely proportional to the surface enlargement of the substrate [9]. If unavoidable strong absorption and recombination losses occur at the interfaces, there will
45
be a trade-off between the reduction of the local absorber thickness and enlarged interface area [9]. However, for systems with bulk recombination as the main loss mechanism, for example, systems of those low quality inorganic semiconductors or organic semiconductors with short electron-hole pair or exciton diffusion lengths, a reduced local absorber thickness will be very advantageous [9]. In other words, the ETA cell architecture can significantly lower down the high quality/purity requirement of the absorber materials. Considering the thermalization within the absorber, there are several constrains on the band energy alignment between the different layers [9]. On the one hand, to permit electron injection into the n-type charge transport layer, its conduction band edge should be slightly (~0.3 eV) lower than the conduction band edge (or lowest unoccupied molecular orbital, LUMO) of the absorber layer [9]. Due to the large offset between the valence band (or highest occupied molecular orbital, HOMO) of the absorber and the valence band of n-type charge transport layer, hole injection at this interface is energetically suppressed [9]. On the other hand, a proper alignment of the valence band (or HOMO) of the absorber and the valence band of the p-type charge transport layer is required to ensure efficient hole injections from the absorber to the p-type charge transport layer [9]. And electron injection is not possible at this interface [9]. When only majority carriers are allowed to be injected into the transport layers, the interfaces can be
46
regarded as blocking filters of minority carriers [9]. This carrier filtering naturally supports the separation of the photo-generated carriers in solar cells [9]. If these charge transport layers are sufficiently doped, and only majority carriers can be injected, charge transport is essentially free of recombination [9]. A simple analysis has shown that large distances can be covered by the excess carriers [9]. Therefore, in order to ensure satisfactory charge collection, the charge transport layers can be comparably thick and their electronic quality does not need to be very high [9].
2.1.3 Dye-sensitized solar cells (DSSCs) Figure 2.6 shows the typical basic configuration of a dye-sensitized solar cell. At the heart of the device is a sintered mesoporous oxide layer composed of a network of nanometer-sized particles, in most cases anatase TiO2 [11]. This sintered TiO2 layer has a thickness of typically 5–20 μm, showing relatively high electronic conductivity [11]. Other wide band gap oxides like ZnO and Nb2O5, can be alternative materials for this layer as well [11]. The mesoporous TiO2 layer is deposited on a TCO glass substrate, usually fluorine-doped tin oxide (FTO) glass substrate. Attached to the surface of the nanocrystalline oxide film is a monolayer of organic dye with anchoring groups such as carboxylic acid groups in its molecular structure [12]. The dye-sensitized mesoporous TiO2film on the FTO glass substrate is called photoanode, while the other FTO glass
47
coated with a thin layer of platinum catalyst is called counter electrode. Between the photoanode and counter electrode is the electrolyte, usually an organic solution containing redox system such as an iodide/tri-iodide couple.
Figure 2.6. Schematic overview of a dye-sensitized solar cell [10].
Figure 2.7 shows the operating principle of a dye-sensitized solar cell. First, absorption of sunlight photons leads to photo-excitation of the dye sensitizer. Electrons from the excited dye molecules are ultrafastly injected into the conduction band of the mesoporous TiO2, and subsequently collected by the TCO [11]. The extracted electrons travel to the cathode through an external circuit with a load. The dye molecules are
48
regenerated by the electrolyte. The redox system itself is in turn restored at the counter electrode by electrons passing through the load from the TCO [11]. The open circuit voltage of the solar cell under illumination can be estimated from the difference between the redox potential of the electrolyte and the Fermi level, as indicated with a dashed line in Figure 2.7, of the nanocrystallline TiO2 film [11].
Figure 2.7. Operating principle and energy level diagram of dye-sensitized solar cells. Potentials are referenced to the normal hydrogen electrode (NHE).
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On the whole a DSSC generates electrical power from sunlight absorption without suffering any permanent chemical transformation which usually happens to an electrochemical battery [11].
2.1.4 Organic solar cells Organic solar cells are photovoltaic cells using organic semiconductors including conjugated polymers and small organic molecules for light absorption and charge transport. Organic semiconductors and device architectures are both of utmost importance for the performance of organic solar cells [13]. Classified by the active materials, there are three types of organic solar cells, i.e. molecular solar cells with only organic molecular semiconductor(s), polymer solar cells with only organic semiconductors including at least one conjugated polymer, and organic-inorganic hybrid solar cells with both organic and inorganic semiconductors. Classified by the cell architectures, there are at least six types of organic solar cells: 1) Schottky-barrier organic solar cells, which are also called single-layer organic solar cells, 2) bilayer donor/acceptor heterojunction solar cells, 3) bulk donor/acceptor heterojunction solar cells, and 4) trilayer p-i-n heterojunction solar cells, 5) multilayer heterojunction solar cells, and 6) tandem solar cells. Among them, the first three types are the fundamental device configurations for organic solar cells, which will be discussed in detail in this section.
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2.1.4.1 Schottky-barrier organic solar cells As the earliest and simplest form of organic solar cells, Schottky-barrier organic solar cell is constructed by a single-component layer of organic semiconducting small molecule or conjugated polymer sandwiched between two different electrodes [14]. In such a solar cell, a Schottky (rectifying) contact is formed at one of the organic/electrode interfaces, with the other contact being ohmic [15]. Figure 2.8 shows an example diagram of such devices. The organic film is sandwiched between a transparent ITO glass substrate and a thermally evaporated Al layer. The ITO and Al electrodes are sometimes replaced by other materials. Figure 2.9 shows the working principle of a Schottky-barrier organic solar cell. Solar photons enter the cell through the transparent front ohmic contact and are absorbed by the organic film. Excitons are created in the organic film upon photon absorption. Some of the photons that are not absorbed in the film the first time may be reflected back by the optically reflective metal back contact surface and provide a second chance for light absorption by the film before leaving the device through the transparent front contact. However, in such a solar cell, only a very small portion of the incident light that is absorbed closely near the Schottky contact is effective in producing carriers, since charge separation occurs only at the rectifying (Schottky) junction with one electrode [16]. This is to say, the photoactive region is very thin, which limits the quantum yield of
51
charge photogeneration. Exciton quenching at organic/electrode interfaces can also reduce photocurrent yields [15]. Besides, since both positive and negative photo-generated charges travel through the same material, recombination losses are generally high [16]. As a result, Schottky-barrier organic solar cells are intrinsically inefficient [15].
Figure 2.8. Device architecture of a Schottky-barrier organic solar cell.
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Figure 2.9. Energy level diagram of a Schottky-barrier organic solar cell.
2.1.4.2 Bilayer donor/acceptor heterojunction solar cells Figure 2.10 shows the architecture of a bilayer donor/acceptor heterojunction solar cell. The donor/acceptor interface is necessary for efficient dissociation (almost 100% at the interface) of photo-generated excitons in the active layer into free charge carriers in organic solar cells [13]. The bilayer donor/acceptor architecture dramatically increases the power conversion efficiency of organic solar cells compared to the aforementioned
53
Schottky-barrier configuration that consists of only one organic material sandwiched between two electrodes [13]. As a basic and organized configuration as well as a straightforward and useful way to evaluate the photovoltaic performances of new organic semiconductors, the bilayer design has been applied to a wide variety of donor and acceptor species, showing its capability of yielding relatively high power conversion efficiencies [13]. In some cases, this architecture is even more advantageous over those disordered or complicated systems when designing experiments [13].
Figure 2.10. Device architecture of a bilayer donor/acceptor heterojunction solar cell.
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In a bilayer donor/acceptor heterojunction solar cell, as illustrated in Figure 2.11, the conversion of a photon into electrical power typically takes six steps as follows: 1) photon absorption (ηabs); 2) exciton generation (ηex); 3) exciton diffusion (ηdiff); 4) exciton dissociation (ηdiss); 5) charge transport (ηct); and 6) charge collection (ηcoll), where the η in the parentheses represents the efficiency of each step [17]. Firstly, absorption of sunlight photons leads to the generation of excitons. The photo-generated excitons then diffuse to the donor/acceptor interface where charge separation occurs. Separated free electrons transport through the acceptor layer by hopping mechanism to anode, at the same time the separated free holes transport within the donor layer by hopping mechanism to cathode [17]. Associated with the aforementioned six steps in a bilayer donor/acceptor heterojunction solar cell are losses including: (i) absorption loss due to spectral mismatch; (ii) thermalization loss; (iii) exciton loss; (iv) energy loss required for exciton dissociation; and (v) charge recombination [17]. These limitations and losses can reduce the power conversion efficiency of a bilayer donor/acceptor heterojunction solar cell. Besides, the bilayer architecture suffers from the short exciton diffusion length in most organic films [13]. On the one hand, the donor and acceptor layers need to be thick enough to absorb most photons. On the other hand, the donor and acceptor layers need to be thin enough (in the order of exciton diffusion length) to ensure that most
55
photo-generated excitons can reach the donor/acceptor interface before recombination. One practical solution is to engineer the heterointerface, or to adopt more advanced architectures with multiple or highly convoluted interfaces to allow for a thicker active layer while still maintaining a short path for exciton diffusion [13].
Figure 2.11. Energy level diagram of a bilayer donor/acceptor heterojunction solar cell showing six steps associated in device operation [17].
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2.1.4.3 Bulk donor/acceptor heterojunction solar cells Bulk heterojunction architecture conquers the drawback of planar bilayer donor/acceptor heterojunction solar cells due to the intrinsically short exciton diffusion length of organic semiconductors, and significantly improves the power conversion efficiency [13]. In a so-called bulk donor/acceptor heterojunction solar cell, as illustrated in Figure 2.12, the active layer is deposited by a co-evaporation technique, or by spin-coating of a solution of donor-acceptor blend, allowing for nano-scale donor and acceptor phase separation to form an interpenetrating bicontinuous network [13]. Numerous donor/acceptor interfaces are formed, and thus the distance for an exciton to travel to reach a donor/acceptor interface for efficient charge dissociation is significantly reduced [13]. The working principle of a bulk donor/acceptor heterojunction solar cell is very similar to that of a bilayer donor/acceptor heterojunction solar cell. First, absorbed photons generate excitons. The photo-generated excitons diffuse to the donor/acceptor interface where excitons dissociate into free carriers. And separated free electrons transport through the acceptor phase by hopping to anode, while the separated free holes transport within the donor phase by hopping to cathode. However, due to the random distribution of the donor and acceptor phases in such a thermodynamically driven system, charge trapping at bottlenecks and cul-de-sacs in the
57
conducting pathways to the electrodes is likely to happen because of the entropy of the interface formation process [13]. One possible solution is to tune the positions and orientations of the donor/acceptor interfaces by controlling the growth sequence or growth methods of the films to reduce contorted and resistive conducting pathways while maintaining the large interfacial area [13].
Figure 2.12. Device architecture of a bulk donor/acceptor heterojunction solar cell.
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2.2 Solar cell device characterization To evaluate new materials, new cell architectures, or new processing technologies, it is of vital importance to assure accurate efficiency measurements [13]. However, it is not always easy to accurately compare performances of different cells reported in literature, since testing conditions like the active area of the device, the spectral output of the light source, and the spectral sensitivity of the reference detector, may vary substantially and are not always comparable [13]. Therefore, it is essential to set up a standardized and consistent evaluation system.
Figure 2.13. Illustration of the geometry used to derive the standard AM1.5 spectrum [13].
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In the United States of America, the Cell Performance Laboratory at the National Renewable Energy Laboratory (NREL) offer services of certifying solar cell efficiency measurements and calibrating reference cells [13]. The efficiency measurement system used for this dissertation work was calibrated by the reference cells, which have been standardized by the NREL, every time immediately before the measurements of sample cells.
Figure 2.14. Standard Solar Spectra for space and terrestrial use [19].
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A number of factors need to be considered in order to accurately measure the efficiency of an organic solar cell. Foremost, it is required to compare the test light source to the solar spectrum [13]. The spectral irradiance of the sun outside the earth’s atmosphere is referred to as air mass 0 (AM0), closely matching that of a 5780 K blackbody spectrum [18]. Efficiencies of solar cells that are fabricated for space power applications, such as powering communication satellites, are usually measured with AM0 [18]. The spectral irradiance of the sun reaching the sea level of the earth after travelling through the atmosphere with the sun directly overhead is defined as AM1 [18]. AM1.5 corresponds to the solar spectral irradiance at the sea level of the earth with a solar zenith angle of θz = 48.19°(Figure 2.13) [13,18]. AM1.5 represents the overall yearly averaged solar spectrum at mid-latitudes of the earth where many of the world’s major population centers like United States of America, China, Europe, Japan etc. and hence where solar installations and industry are located [18]. Therefore, the solar industry generally uses AM1.5 for all standardized testing of terrestrial solar cells and modules [18]. Figure 2.14 shows the standard solar spectra for space and terrestrial use. AM1.5 Global reference solar spectrum has been admitted as the most accurate representation of the yearly averaged spectral output measured at the sea level in the northern hemisphere with a south facing slope that is tilted 37°to best estimate the average latitude of the United States of America [13], as illustrated in Figure 2.13. And AM1.5 Global spectrum at
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25 °C with an incident power density of 100 mW/cm2 represents the standard reporting conditions for testing solar cells [13]. The most important parameters to evaluate the photovoltaic performance of a solar cell are the open circuit voltage (VOC), short circuit current (ISC), fill factor (FF), power conversion efficiency (η), and incident photon to current conversion efficiency (IPCE). When there is no external load connected to a solar cell, the difference of the electrical potential between the two electrode terminals is the VOC [20]. When the two electrode terminals are connected together with conductive wire, the current passing through the circuit is the ISC. ISC and VOC can be obtained from the illuminated I–V curve as shown in Figure 2.1. The power conversion efficiency (ηP) of a solar cell is defined as the ratio of the maximum output power (Pmax) to the incident power (Pin) from the light source [13]. Pmax = ImVm, where Im and Vm are the current and voltage at the maximum power point. ηP = ImVm/Pin = ISCVOCFF/Pin
(Equation 2.1)
where FF is given by FF = ImVm/ISCVOC [13]. Fill factor is an indicator of the diode quality. It is influenced by charge carrier transport which is corresponding to series resistance (Rs), and charge recombination corresponding to shunt resistance (Rsh). Rs and Rsh of the devices can be estimated by the following equations [21].
62 V V VOC 0.2 J V Rsh V 0 J
Rs
(Equation 2.2) (Equation 2.3)
In practical, short circuit current density (JSC) is more often used than the short circuit current (ISC), for the purpose of performance comparison of different solar cells. JSC = ISC/A, where A is the effective solar cell area. JSC in a single junction solar cell under AM1.5 Global is given by: JSC =
(Equation 2.4)
where q is the elementary charge, IPCE(λ) is the incident photon to current conversion efficiency at the incident wavelength of λ, and bs(λ) is the photon flux density at the incident wavelength of λ [21]. IPCE is a very important parameter to describe the performance of a solar cell under different conditions [21]. IPCE is dependent on the absorption coefficient of the solar cell materials and the efficiency of charge separation, charge transport and charge collection [21].
References
[1] http://en.wikipedia.org/wiki/Solar_cell (accessed October, 2011).
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[2] A. E. Becquerel, “Recherches sur les Effets de la Radiation Chimique de la Lumière Solaire au Moyen des Courants Electriques”, Comptes rendus de L’Academie des Sciences 9, pp. 145–149, 1839.
[3] M. A. Green, “Solar Cells”, in Modern Semiconductor Device Physics (Ed.: S. M. Sze), John Wiley & Sons, Inc., New York, USA, Ch. 8, pp. 473–530, 1998.
[4] L. Fraas, “Solar Cells, Single-Crystal Semiconductors, and High Efficiency”, in Solar Cells and Their Applications, 2nd Edition (Eds.: L. Fraas and L. Partain), John Wiley & Sons, Inc., Hoboken, New Jersey, USA, Ch. 3, pp. 43–66, 2010.
[5] http://en.wikipedia.org/wiki/Homojunction (accessed October, 2011).
[6] http://en.wikipedia.org/wiki/Heterojunction (accessed October, 2011).
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[8] C. Lévy-Clément, R. Tena-Zaera, M. A. Ryan, A. Katty and G. Hodes, “CdSe-Sensitized p-CuSCN/Nanowire n-ZnO Heterojunctions,” Adv. Mater. 17, pp. 1512–1515, 2005.
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[9] R. Könenkamp, “Inorganic Extended-Junction Devices”, in Nanostructured and Photoelectrochemical Systems for Solar Photon Conversion (Eds.: M. D. Archer and A. J. Nozik), Imperial College Press, London, UK, Ch. 6, pp. 393–452, 2008.
[10] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, “Dye-Sensitized Solar Cells”, Chem. Rev. 110, pp. 6595–6663, 2010.
[11] M. Grätzel, “Dye-Sensitized Solar Cells”, J. Photoch. Photobio. C 4, pp. 145–153, 2003.
[12] H. Imahori, T. Umeyama and S. Ito, “Large π-Aromatic Molecules as Potential Sensitizers for Highly Efficient Dye-Sensitized Solar Cells”, Acc. Chem. Res. 42, pp 1809–1818, 2009.
[13] A. W. Hains, Z. Liang, M. A. Woodhouse and B. A. Gregg, “Molecular Semiconductors in Organic Photovoltaic Cells”, Chem. Rev. 110, 6689–6735, 2010.
[14] C. Y. Kwong, A. B. Djurišiĉ, P. C. Chui, L. S. M. Lam and W. K. Chan, “Improvement of the Efficiency of Phthalocyanine Organic Schottky Solar Cells with ITO Electrode Treatment”, Appl. Phys. A 77, pp. 555–560, 2003.
[15] P. A. Lane and Z. H. Kafafi, “Solid-State Organic Photovoltaics: A Review of Molecular and Polymeric Devices”, in Organic Photovoltaics: Mechanisms, Materials,
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and Devices (Eds.: S.-S. Sun and N. S. Sariciftci), CRC Press, Boca Raton, Florida, USA, Ch. 4, pp. 49–106, 2005.
[16] K. Petritsch, “Organic Solar Cell Architectures”, PhD Thesis, Technische Universität Graz, Austria, July 2000.
[17] M. K. Siddiki, J. Li, D. Galipeau and Q. Qiao, “A Review of Polymer Multijunction Solar Cells”, Energy Environ. Sci. 3, pp. 867–883, 2010.
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[21] J. Nelson, The physics of solar cells, Imperial College Press, 2003.
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Chapter 3 Experimental Procedures
3.1 Synthesis and characterization of hyperbranched phthalocyanines 1–3 (HBMPc-CN; M = H2, TiO, Cu) 3.1.1 General procedures Dichloromethane (CH2Cl2) for spectroscopic studies was freshly distilled from CaH2 under N2. All other reagents and solvents were used as received from vendors. 1
H NMR spectra were recorded on a Varian 500 NMR spectrometer in CDCl3.
Mass spectra were taken on a Bruker TOF mass spectrometer. Absorption spectra were recorded on an Agilent 8453 UV-Visible Spectrophotometer. Steady-state emission spectra were measured with an Edinburgh FS920 Fluorescence Spectrophotometer. Solutions for absorption and fluorescence measurements were prepared in dimethylacetamide (DMAc)/CH2Cl2 (1/1, v/v).
3.1.2 Synthesis of hyperbranched phthalocyanines 1–3 (HBMPc-CN; M = H2, TiO, Cu) Hyperbranched phthalocyanines HBMPc-CN 1–3 (M = H2, TiO, Cu) were prepared by the cyclotetramerization reaction of 1,3-bis(3,4-dicyanophenoxy)benzene with or without the corresponding metal salts in n-pentanol under the catalysis of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), following the routine shown in Figure 3.1.
67
Figure 3.1. Synthesis routine of hyperbranched phthalocyanines 1–3 (HBMPc-CN; M = H2, TiO, Cu).
3.1.2.1 Preparation of 1,3-bis(3,4-dicyanophenoxy)benzene
1,3-Bis(3,4-dicyanophenoxy)benzene was prepared following a similar method to that of 1,2-bis(3,4-dicyanophenoxy)benzene [1]. 4-Nitrophthalonitrile (2.00 g, 11.6 mmol) and 0.64 g (5.8 mmol) of resorcinol (1,3-dihydroxybenzene) were dissolved in 30 mL of dimethyl sulfoxide (DMSO). Then 1.60 g (11.6 mmol) of potassium carbonate
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(K2CO3) was added into the flask, and the mixture was stirred at room temperature for 24 h. The mixture was poured into 600 mL of methanol/water (1/1, v/v) and stirred for 30 min. The precipitate was collected by filtration, washed with 1,000 mL of water, and dried under vacuum. The pure product was obtained by recrystallization in methanol. The powdery product was collected by filtration and dried under vacuum for 24 h, affording 1.77 g of 1,3-bis(3,4-dicyanophenoxy)benzene. Yield: 84.1%. 1
H NMR (500 MHz, CDCl3): δ 7.76-7.82 (d, 2H, -C6H4), 7.53-7.58 (t, 1H, -C6H4),
7.30-7.36 (m, 4H, -C6H3), 7.00-7.03 (m, 2H, -C6H3), 6.83-6.86 (t, 1H, -C6H4). MS: m/z 362.1 [Calcd. for C22H10N4O2: [M]+ 362.3].
3.1.2.2 Preparation of hyperbranched phthalocyanine 1 (HBH2Pc-CN) 1,3-Bis(3,4-dicyanophenoxy)benzene (36.23 mg, 0.1 mmol) and a few drops of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were dissolved in 3.5 mL of 1-pentanol, and the mixture was heated at ~100oC for ~4 h under a slow stream of nitrogen. The resulting green solution was cooled to room temperature and then poured into 100 mL of CH3OH and stirred for 30 min. The precipitate obtained by filtration was sequentially washed by CH3OH (10 mL), deionized water (50 mL), and CH3OH (10 mL). The dark green power was dried in air for 6 h and then kept under vacuum for another 12 h. Yield: ~99%.
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3.1.2.3 Preparation of hyperbranched phthalocyanine 2 (HBTiOPc-CN) 1,3-Bis(3,4-dicyanophenoxy)benzene (36.23 mg, 0.1 mmol), 17.02 mg (0.05 mmol) of Ti(OC4H9)4, and a few drops of DBU were dissolved in 3.5 mL of 1-pentanol, and then the mixture was heated at ~90oC for ~2 h under a slow stream of nitrogen. The resulting green solution was cooled to room temperature and then poured into 100 mL of CH3OH and stirred for 30 min. The precipitate obtained by filtration was sequentially washed by CH3OH (10 mL), deionized water (50 mL), and CH3OH (10 mL). The dark green power was dried in air for 6 h and then kept under vacuum for another 12 h. Yield: ~99%.
3.1.2.4 Preparation of hyperbranched phthalocyanine 3 (HBCuPc-CN) 1,3-Bis(3,4-dicyanophenoxy)benzene (36.23 mg, 0.1 mmol), 13.09 mg (0.05 mmol) of Cu(acac)2, and a few drops of DBU were dissolved in 3.5 mL of 1-pentanol, and then the mixture was heated at ~100oC for ~4 h under a slow stream of nitrogen. The resulting blue solution was cooled to room temperature and then poured into 100 mL of CH3OH and stirred for 30 min. The precipitate obtained by filtration was sequentially washed by CH3OH (10 mL), deionized water (50 mL), and CH3OH (10 mL). The dark green power was dried in air for 6 h and then kept under vacuum for another 12 h. Yield: ~99%.
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Figure 3.2. Femtosecond fluorescence upconversion measurement system.
3.1.3 Femtosecond time-resolved fluorescence measurement of HBCuPc-CN solution Time-resolved fluorescence measurement for 3 (HBCuPc-CN) in solution was carried out by using a femtosecond fluorescence upconversion (FFU) technique with system as shown in Figure 3.2. The solution was prepared in DMAc/CH2Cl2 (1/1, v/v) at a concentration of ∼0.5 mg/mL. The FFU system used in this work is a FOG 100 system (CDP, Russia) with a mode-locked Ti:sapphire laser (Tsunami, Spectra Physics) light source pumped by a 10-W CW Nd:YVO4 laser (Millennia, Spectra-Physics). The sample was excited by the second harmonic light (400 nm) generated by frequency doubling of a fundamental light at the wavelength of ~800 nm from the
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mode-locked Ti:sapphire laser with a pulse width of ~57 fs and a pulse repetition rate of 86 MHz, through a β-barium borate nonlinear crystal. The output spectrum of the laser pulse was monitored with a spectrum analyzer (Ocean Optics), to ensure pure mode-locking regime of the femtosecond laser. The polarization of the excitation beam for anisotropy measurements was controlled by a Berek’s plate. The sample was rotated to avoid possible photo-degradation and other accumulative effects. The fluorescence emitted from the sample was collected with an achromatic lens and then directed onto another β-barium borate nonlinear crystal. The fundamental light passed through a motorized optical delay line and then combined with the sample emission in the nonlinear crystal to generate a sum frequency light. The sum frequency light was dispersed by a monochromator and detected via a photomultiplier tube (Hamamatsu R1527P). The instrument response function (IRF) was estimated to be ~188 fs (full width at half maximum, FWHM). The fluorescence data were fitted with a multi-exponential decay/rise model, in which the fluorescence signal F(t) can be theoretically expressed by a convolution of the IRF r(t) with a molecule-response function f(τ),
F (t ) r (t ) f ( )d 0
(Equation 3.1)
where r(t) is a Gaussian function with laser pulse width and f(τ)is given by: f ( ) Ai exp i i
(Equation 3.2)
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where the factor A represents the relative weights (or amplitudes) of the corresponding components, whose sign can distinguish rising or decay process; and τi is the rising or decay time constant.
3.2 Synthesis and characterization of hyperbranched phthalocyanine dyes 4–8 (HBMPc-COOH; M = H2, AlCl, Co, Cu, Zn) 3.2.1 General procedures N,N-dimethylformamide (DMF) was distilled from anhydrous MgSO4. All other reagents and solvents were used as received from vendors. 1
H NMR spectra were recorded on a Varian 500 NMR spectrometer or on a
Bruker Avance 400 NMR spectrometer in CDCl3 or DMSO-d6. Mass spectra were taken on a Bruker TOF mass spectrometer. IR spectra were recorded in KBr pellets using a Nicolet 6700 FT-IR spectrometer with a spectral resolution of 4 cm-1. Absorption spectra were recorded on an Agilent 8453 UV-Visible Spectrophotometer. Steady-state fluorescence spectra were recorded with an Edinburgh FS920 Fluorescence Spectrophotometer. Solutions for absorption and fluorescence measurements were prepared using anhydrous DMF. AFM images were taken in air under ambient conditions using the intermittent contact mode on an Agilent 5500 AFM/SPM microscope.
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Figure 3.3. Synthesis routine of hyperbranched phthalocyanine dyes 4–8 (HBMPc-COOH; M = H2, AlCl, Co, Cu, Zn).
74
3.2.2 Synthesis of hyperbranched phthalocyanine dyes 4–8 (HBMPc-COOH; M = H2, AlCl, Co, Cu, Zn) Hyperbranched phthalocyanine dyes 4–8 (HBMPc-COOH; M = H2, AlCl, Co, Cu, Zn) were synthesized following the routine shown in Figure 3.3.
3.2.2.1 Preparation of 1,3-bis(3,4-dicyanophenoxy)benzene The precursor 1,3-bis(3,4-dicyanophenoxy)benzene was synthesized following the same procedures as described in detail in section 3.1.2.1.
3.2.2.2 Preparation of hyperbranched phthalocyanine HBH2Pc-CN 1,3-Bis(3,4-dicyanophenoxy)benzene (221.90 mg, 0.61 mmol) was dissolved in 5 mL of 2-dimethylaminoethanol (DMAE), and then the mixture was refluxed for 75 min under a slow stream of nitrogen. The mixture was poured into 100 mL of water/methanol (10/1, v/v). The precipitate was collected by filtration, washed with water, and dried under vacuum. After refluxing in methanol twice, the product was filtered and rinsed with cold methanol three times. The dark green powdery product was dried under vacuum, affording 73.94 mg of HBH2Pc-CN.
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IR (KBr, cm-1): 3300, 3070, 2950, 2870, 2830, 2770, 2560, 2440, 2390, 2230, 1910, 1770, 1730, 1660, 1590, 1520, 1470, 1400, 1360, 1310, 1260, 1220, 1180, 1120, 1090, 1050, 1010, 972, 937, 881, 870, 833, 785, 746, 717, 688, 525.
3.2.2.3 Preparation of hyperbranched phthalocyanine dye 4 (HBH2Pc-COOH)[2] HBH2Pc-CN (51.76 mg) and 7.76 mg (15 wt.-%) of potassium hydroxide were dissolved in 6 mL of water/ethanol (1/1, v/v) mixed solvent. The mixture was refluxed for 24 h until the evolution of ammonia ceased. The green clear solution was poured into 50 mL of water and the pH value of the solution was adjusted to 3-4. The dark blue precipitate was collected by filtration and rinsed with dilute hydrochloric acid and then with water. HBH2Pc-COOH (48.92 mg) was obtained.
3.2.2.4 Preparation of hyperbranched phthalocyanine HBAlClPc-CN 1,3-Bis(3,4-dicyanophenoxy)benzene (230.33 mg, 0.64 mmol) and 42.37 mg (0.32 mmol) of AlCl3 were dissolved in 5 mL of DMAE, and then the mixture was refluxed for 2 h under a slow stream of nitrogen. The mixture was poured into 120 mL of water. The precipitate was collected by filtration, washed with 3 mol/L of HCl and then with water, and dried under vacuum. After refluxing in methanol twice, the product was
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filtered and rinsed with cold methanol three times. The dark blue powdery product was dried under vacuum, affording 73.49 mg of HBAlClPc-CN. IR (KBr, cm-1): 3070, 2730, 2230, 1770, 1720, 1670, 1590, 1480, 1410, 1360, 1310, 1270, 1220, 1170, 1130, 1080, 1050, 976, 881, 841, 791, 758, 750, 687, 646, 559, 526.
3.2.2.5 Preparation of hyperbranched phthalocyanine dye 5 (HBAlClPc-COOH) [2] HBAlClPc-CN (49.03 mg) and 7.35 mg (15 wt.-%) of potassium hydroxide were dissolved in 6 mL of water/ethanol (1/1, v/v) mixed solvent. The mixture was refluxed for 24 h until the evolution of ammonia ceased. The blue clear solution was poured into 30 mL of water and the pH value of the solution was adjusted to 3-4. The dark blue precipitate was collected by filtration and rinsed with dilute hydrochloric acid and then with water. HBAlClPc-COOH (47.78 mg) was obtained.
3.2.2.6 Preparation of hyperbranched phthalocyanine HBCoPc-CN 1,3-Bis(3,4-dicyanophenoxy)benzene (248.85 mg, 0.69 mmol) and 81.70 mg (0.34 mmol) of CoCl2.6H2O were dissolved in 5 mL of DMAE, and then the mixture was refluxed for 2 h under a slow stream of nitrogen. The mixture was poured into 100 mL of water. The precipitate was collected by filtration, washed with water, and dried under
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vacuum. After refluxing in methanol twice, the product was filtered and rinsed with cold methanol three times. The dark blue powdery product was dried under vacuum, affording 269.28 mg of HBCoPc-CN. IR (KBr, cm-1): 3060, 2750, 2600, 2440, 2380, 2230, 1910, 1770, 1720, 1590, 1520, 1470, 1410, 1390, 1360, 1310, 1270, 1220, 1180, 1130, 1100, 1060, 1000, 974, 881, 839, 783, 754, 694, 646, 559, 528.
3.2.2.7 Preparation of hyperbranched phthalocyanine dye 6 (HBCoPc-COOH) [2] HBCoPc-CN (109.55 mg) and 16.43 mg (15 wt.-%) of potassium hydroxide were dissolved in 12 mL of water/ethanol (1/1, v/v) mixed solvent. The mixture was refluxed for 24 h until the evolution of ammonia ceased. The blue clear solution was poured into 60 mL of water and the pH value of the solution was adjusted to 3-4. The dark blue precipitate was collected by filtration and rinsed with dilute hydrochloric acid and then with water. HBCoPc-COOH (100.79 mg) was obtained.
3.2.2.8 Preparation of hyperbranched phthalocyanine HBCuPc-CN [1–2] 1,3-Bis(3,4-dicyanophenoxy)benzene (206.91 mg, 0.57 mmol) and 18.92 mg (0.19 mmol) of CuCl were dissolved in 5 mL of DMAc, and then the mixture was stirred at 160oC for 4 h. The mixture was poured into 100 mL of water. The precipitate was
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collected by filtration, washed with water, and dried under vacuum. After refluxing in methanol twice, the product was filtered and rinsed with cold methanol three times. The dark blue powdery product was dried under vacuum, affording 182.07 mg of HBCuPc-CN. IR (KBr, cm-1): 3220, 3070, 2940, 2860, 2740, 2660, 2620, 2580, 2420, 2230, 1920, 1850, 1770, 1720, 1590, 1500, 1480, 1400, 1360, 1340, 1310, 1260, 1220, 1170, 1130, 1090, 1050, 1000, 974, 949, 879, 837, 787, 775, 748, 694, 681, 640, 615, 577, 552, 536, 525.
3.2.2.9 Preparation of hyperbranched phthalocyanine dye 7 (HBCuPc-COOH) [2] HBCuPc-CN (104.70 mg) and 15.71 mg (15 wt.-%) of potassium hydroxide were dissolved in 12 mL of water/ethanol (1/1, v/v) mixed solvent. The mixture was refluxed for 24 h until the evolution of ammonia ceased. The dark blue clear solution was poured into 30 mL of water and the pH value of the solution was adjusted to 3-4. The dark blue precipitate was collected by filtration and rinsed with dilute hydrochloric acid and then with water. HBCuPc-COOH (44.87 mg) was obtained.
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3.2.2.10 Preparation of hyperbranched phthalocyanine HBZnPc-CN 1,3-Bis(3,4-dicyanophenoxy)benzene (236.03 mg, 0.65 mmol) and 66.60 mg (> 0.33 mmol) of ZnCl2.xH2O were dissolved in 5 mL of DMAE, and then the mixture was refluxed for 75 min under a slow stream of nitrogen. The mixture was poured into 100 mL of water/methanol (10/1, v/v). The precipitate was collected by filtration, washed with water, and dried under vacuum. After refluxing in methanol twice, the product was filtered and rinsed with cold methanol three times. The dark blue powdery product was dried under vacuum, affording 194.17 mg of HBZnPc-CN. IR (KBr, cm-1): 3070, 2600, 2550, 2400, 2230, 1910, 1770, 1720, 1600, 1480, 1390, 1360, 1340, 1310, 1270, 1220, 1180, 1130, 1090, 1050, 1000, 974, 945, 887,877, 833, 789, 775, 762, 746, 694, 559, 528.
3.2.2.11 Preparation of hyperbranched phthalocyanine dye 8 (HBZnPc-COOH) [2] HBZnPc-CN (174.50 mg) and 26.18 mg (15 wt.-%) of potassium hydroxide were dissolved in 12 mL of water/ethanol (1/1, v/v) mixed solvent. The mixture was refluxed for 24 h until the evolution of ammonia ceased. The blue clear solution was poured into 60 mL of water and the pH value of the solution was adjusted to 3-4. The dark blue precipitate was collected by filtration and rinsed with dilute hydrochloric acid and then with water. HBZnPc-COOH (109.87 mg) was obtained.
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3.2.3 Femtosecond time-resolved fluorescence measurements A FFU technique was employed to investigate the fluorescence dynamics of these hyperbranched phthalocyanine dyes 4–8. The solutions were prepared with anhydrous, deoxygenated DMF at a concentration of ~1.0 mg/mL. One-minute ultrasonication treatment was performed on the solutions to accelerate the dissolution process of the hyperbranched phthalocyanines. The solutions were kept sealed and still at room temperature for at least 2 days. The clear solution over the precipitate was removed for test. The measurements were carried out using the same system and procedures as described in detail in section 3.1.3. The IRF was estimated to be ~250 fs. The fluorescence data were fitted with a multi-exponential decay/rise model. The fluorescence signal F(t) can be theoretically expressed by Equation 3.1, the convolution of the IRF r(t) with the molecule-response function f(τ), in which r(t) is a Gaussian function with laser pulse width and f(τ)is given by Equation 3.2.
3.2.4 Electrochemical cyclic voltammetry measurements Electrochemical cyclic voltammetry measurements were performed on a VersaSTAT 3 electrochemical working station (Princeton Applied Research). The cell comprised inlets for a platinum working electrode (MF-2013) of 1.6 mm in diameter and a platinum wire counter electrode. The reference electrode was Ag/AgCl electrode (a 3.5
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M KCl aqueous solution). Its potential was internally calibrated using ferrocene/ferrocenium (Fc/Fc+) redox couple [E1/2(Fc/Fc+) = +567 mV]. Typically, a 0.2 M solution of [NBu4][PF6] (tetrabutylammonium hexafluorophosphate) in anhydrous DMF containing sample was purged with nitrogen for at least 10 min, and then the voltammograms were recorded at ambient temperature. The scan rate was 50 mV.s-1. The HOMO and LUMO energy levels were calculated by the following equations. HOMO = －(Eox － E1/2(Fc/Fc+) ＋ 4.8) (eV); LUMO = －(Ered － E1/2(Fc/Fc+) ＋ 4.8) (eV).
3.3 Synthesis and characterization of sandwich-type porphyrinato/phthalocyaninato rare earth(III) double-decker complexes 9–17 3.3.1 General procedures n-Octanol was distilled from sodium under nitrogen. CH2Cl2 for spectroscopic studies was freshly distilled from CaH2 under N2. All other reagents and solvents were used as received from vendors. Column chromatography was carried out on silica gel (Merck, Kieselgel 60, 70–230 mesh) columns with the indicated eluents to isolate and purify the title complexes. 1
H NMR spectra were recorded on a Bruker DPX 300 spectrometer or a Bruker
Avance 400 NMR spectrometer in CDCl3/DMSO-d6 (1/1, v/v) in the presence of ca. 1% (by volume) hydrazine hydrate. Spectra were referenced internally to the residual solvent
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resonance at chemical shift (δ) of 2.50 (DMSO-d6, 5 peaks with J = 1.9 Hz). MALDI-TOF mass spectra were taken on a Bruker BIFLEX III ultrahigh-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer with α-cyano-4-hydroxycinnamic acid as a matrix. Elemental analyses were performed on Flash EA 1112 Elemental Analyzer. IR spectra were recorded in KBr matrix pellets on a BIORAD FTS-165 spectrometer with a spectral resolution of 2 cm-1. Absorption spectra were recorded on a Hitachi U-4100 spectrophotometer or on an Agilent 8453 UV-Visible Spectrophotometer. Steady-state emission spectra were recorded with an Edinburgh FS920 Fluorescence Spectrophotometer. Solutions for absorption and emission measurements were prepared using CH2Cl2 with concentration of ~5.0 μM for 9–17.
3.3.2 Synthesis of mixed (porphyrinato)(phthalocyaninato) rare-earth(III) double-decker complexes 9–15 [MIIIH(TClPP){Pc(α-OC4H9)8}; M = Y, Sm, Eu, Tb, Dy, Ho, Lu] The mixed (porphyrinato)(phthalocyaninato) rare-earth(III) double-decker complexes 9–15 [MIIIH(TClPP){Pc(α-OC4H9)8}; M = Y, Sm, Eu, Tb, Dy, Ho, Lu] were synthesized following the routine shown in Figure 3.4.
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Figure 3.4. Synthesis routine of the mixed (porphyrinato)(phthalocyaninato) rare-earth(Ш) double-decker complexes 9–15.
3.3.2.1 Preparation of rare-earth(III) acetylacetonates [MIII(acac)3.nH2O; M = Y, Sm, Eu, Tb, Dy, Ho, Lu] [3]
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Table 3.1. Adopted pH values for the preparation of rare-earth acetylacetonates. M
Y
Sm
Eu
Tb
Dy
Ho
Lu
pH
6.4
6.5
6.5
6.4
6.4
6.4
6.2
Rare earth oxide (M2O3; M = Y, Sm, Eu, Tb, Dy, Ho, Lu. 1.00 g, 4.43 mmol for M = Y; 2.87 mmol for M = Sm; 2.84 mmol for M = Eu; 2.73 mmol for M = Tb; 2.68 mmol for M = Dy; 2.65 mmol for M = Ho; 2.51 mmol for Lu) was dissolved in a minimum amount of dilute hydrochloric acid with concentration of 2 M, and the pH value of the solution was adjusted to ~5.0 by adding dilute ammonium hydroxide. A solution of ammonium acetylacetonate was prepared by adding concentrated ammonium hydroxide together with sufficient water for solution to an amount [3.99 g (39.9 mmol) for M = Y; 2.58 g (25.8 mmol) for M = Sm; 2.56 g (25.6 mmol) for M = Eu; 2.46 g (24.6 mmol) for M = Tb; 2.41 g (24.1 mmol) for M = Dy; 2.39 g (23.9mmol) for M = Ho; 2.26 g (22.6 mmol) for M = Lu] of freshly distilled acetylacetone which was 50% in excess of that required for complete reaction with the rare earth oxide. The solution of ammonium acetylacetonate was added slowly with stirring to the rare earth chloride solution. The pH value of the mixture was adjusted to corresponding value as shown in Table 3.1 by the addition of either dilute ammonium hydroxide or hydrochloric acid as required. The
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mixture was stirred for 12 h. Precipitates of the rare earth acetylacetonates were filtered, dried in air for 24 h, and then dried under vacuum for 4 h.
3.3.2.2 Preparation of meso-tetrakis(4-chlorophenyl)porphyrin (H2TClPP) [4]
Pyrrole (5.6 mL, 0.08 mol) and 11.2 g (0.08 mol) of 4-chlorobenzaldehyde were simultaneously added to 300 mL of refluxing propionic acid. The mixture was stirred under reflux for 30 min. After the mixture was cooled to room temperature, the precipitate was filtered and thoroughly washed in turn with methanol, water, and methanol. The product was dried in air for 24 h, and then dried under vacuum for 4 h, yielding 3.1 g of purple glistening crystals. Yield: 21%.
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3.3.2.3 Preparation of 3,6-dibutoxy-1,2-dicyanobenzene [5]
2,3-Dicyano-1,4-dihydroquinone (2.40 g, 15 mmol) was dissolved in 175 mL of dry acetone. Anhydrous potassium carbonate (9.0 g, 65 mmol) and 13.8 g (75 mmol) of n-butyl iodide were added into the solution with stirring. The mixture was refluxed for 60 h. After a brief cooling, the mixture was poured into 450 mL of water, and the precipitate was collected by filtration. The crude product was recrystallized from CHCl3/CH3OH, collected by filtration, and dried first in air for 24 h and then under vacuum for 4 h, yielding 1.58 g of white powder. Yield: 38%.
3.3.2.4 Preparation of 1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine [H2Pc(α-OC4H9)8] [5] 3,6-Dibutoxy-1,2-dicyanobenzene (545 mg, 2 mmol) and 139 mg (20 mmol) of lithium were added into 6 mL of n-butanol. The mixture was heated to reflux under a
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slow stream of nitrogen for ca. 80 min. After cooled down to room temperature, 100 mL of methanol was added to the mixture. The formed precipitate was collected by filtration, and redissolved in 100 mL of toluene. Acetic acid (2-3 mL) was added into the toluene solution with stirring. The solution was further stirred for 1 h, and then evaporated under reduced pressure to remove the solvent. The residue was subjected to chromatography on a silica gel column. The column was eluted with CHCl3/MeOH (98/2, v/v), and a green band containing the metal-free phthalocyanine was developed and collected. After removing the solvent by evaporation under reduced pressure, the crude product was purified by a repeated chromatography followed by recrystallization from CHCl3/MeOH. The final product was collected by filtration, and dried first in air for 12 h and then under vacuum for 4 h, yielding dark green crystalline powder. Yield: ~60%.
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3.3.2.5 Preparation of mixed (porphyrinato)(phthalocyaninato) rare-earth(III) double-decker complexes 9–15 (MIIIH(TClPP){Pc(α-OC4H9)8}; M = Y, Sm, Eu, Tb, Dy, Ho, Lu) [6] [MIII(acac)3]nH2O (M = Y, Sm, Eu, Tb, Dy, Ho, or Lu; 0.06 mmol) and 37.6 mg (0.05 mmol) of H2TClPP were dissolved in 4 mL of n-octanol, and the mixture was heated to reflux under a slow stream of nitrogen for ca. 6 h. The mixture was cooled to room temperature, and then treated with 54.6 mg (0.05 mmol) of H2[Pc(-OC4H9)8] under reflux for another 70 minutes. After a brief cooling, the mixture was evaporated under reduced pressure and the residue was subjected to chromatography on a silica gel column. A small amount of unreacted H2TClPP was separated by using CHCl3/hexane (5/1, v/v). The column was eluted with CHCl3/MeOH (98/2, v/v), and a small green band containing the metal-free phthalocyanine was developed and separated. The column was further eluted with CHCl3/MeOH (95/5, v/v), another green band containing the protonated double-decker was developed, collected and evaporated. No nonprotonated counterpart was detected during the chromatography. The crude product was purified by repeated chromatography followed by recrystallization from CHCl3/MeOH. The final product was collected by filtration, and dried first in air for 12 h and then under vacuum for 4 h.
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For 9 [6]: yield: 52%. 1H NMR (300 MHz, CDCl3/DMSO-d6 (1/1, v/v) with ca. 1% hydrazine hydrate): δ 7.92 (s, 8H, Por, Hβ), 7.49 (s, 8H, Pc, Hβ), 7.48-7.62 (m, 4H, Por, Haryl), 7.06-7.24 (m, 8H, Por, Haryl), 6.42-6.54 (m, 4H, Por, Haryl), 5.01-5.10 [m, 8H, Pc(α-OC4H9)8, -OCH2-], 4.92-4.98 [m, 8H, Pc(α-OC4H9)8, -OCH2-], 2.06-2.15 [m, 16H, Pc(α-OC4H9)8, -CH2-], 1.65-1.76 [m, 16H, Pc(α-OC4H9)8, -CH2-], 1.08 [t, J = 7.5 Hz, 24H, Pc(α-OC4H9)8, -CH3]. MS (MALDI-TOF): an isotopic cluster peaking at m/z 1930.4 [Calcd. for (MH)+ 1930.6]. Anal. Calcd. (%) for C108H105O8N12Cl4Y: C, 67.22; H, 5.48; N, 8.71. Found: C, 66.60; H, 5.32; N, 8.70. For 10: yield: 52%. 1H NMR (400 MHz, CDCl3/DMSO-d6 (1/1, v/v) with ca. 1% hydrazine hydrate, 294.2 K): δ 8.22 (s, 8H, Por, Hβ), 7.68 (s, 8H, Pc, Hβ), 7.40-7.60 (m, 4H, Por, Haryl), 7.04-7.06 (m, 8H, Por, Haryl), 6.82 (m, 4H, Por, Haryl), 5.95 (m, 4H, Por, Haryl), 4.75-5.40 [m, 16H, Pc(α-OC4H9)8, -OCH2-], 1.40-1.80 [m, 32H, Pc(α-OC4H9)8, -CH2-], 0.90 [m, 24H, Pc(α-OC4H9)8, -CH3]. MS (MALDI-TOF): an isotopic cluster peaking at m/z 1992.4 [Calcd. for (MH)+ 1992.6]. Anal. Calcd. (%) for C108H105O8N12Cl4Sm.0.25CHCl3: C, 64.33; H, 5.25; N, 8.32. Found: C, 64.25; H, 5.38; N, 8.33. For 11: yield: 63%. 1H NMR (400 MHz, CDCl3/DMSO-d6 (1/1, v/v) with ca. 1% hydrazine hydrate, 300.0 K): δ 8.22 (s, 8H, Por, Hβ), 7.68 (s, 8H, Pc, Hβ), 7.29 (m, 4H, Por, Haryl), 6.90 (m, 8H, Por, Haryl), 6.60 (m, 4H, Por, Haryl), 6.25 (m, 4H, Por, Haryl),
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4.65-5.40 [m, 16H, Pc(α-OC4H9)8, -OCH2-], 1.40-1.80 [m, 32H, Pc(α-OC4H9)8, -CH2-], 1.00-1.20 [m, 24H, Pc(α-OC4H9)8, -CH3]. MS (MALDI-TOF): an isotopic cluster peaking at m/z 1993.2 [Calcd. for (MH)+ 1993.6]. Anal. Calcd. (%) for C108H105O8N12Cl4Eu.0.25CHCl3: C, 64.28; H, 5.24; N, 8.31. Found: C, 64.43; H, 5.40; N, 8.39. For 12: yield: 61%. MS (MALDI-TOF): an isotopic cluster peaking at m/z 2000.1 [Calcd. for (MH)+ 2000.6]. Anal. Calcd. (%) for C108H105O8N12Cl4Tb.0.25CHCl3: C, 64.06; H, 5.23; N, 8.28. Found: C, 64.13; H, 5.27; N, 8.21. For 13: yield: 57%. MS (MALDI-TOF): an isotopic cluster peaking at m/z 2004.8 [Calcd. for (MH)+ 2004.6]. Anal. Calcd. (%) for C108H105O8N12Cl4Dy: C, 64.75; H, 5.28; N, 8.39. Found: C, 65.32; H, 5.46; N, 8.41. For 14: yield: 45%. MS (MALDI-TOF): an isotopic cluster peaking at m/z 2006.3 [Calcd. for (MH)+ 2006.6]. Anal. Calcd. (%) for C108H105O8N12Cl4Ho.0.25CHCl3: C, 63.87; H, 5.21; N 8.26. Found: C, 63.67; H, 5.32; N, 8.38. For 15: yield: 37%. 1H NMR (400 MHz, CDCl3/DMSO-d6 (1/1, v/v) with ca. 1% hydrazine hydrate, 300.0 K): δ 7.85 (m, 8H, Por, Hβ), 7.45-7.60 (m, 4H, Por, Haryl), 7.43 (s, 8H, Pc, Hβ), 7.08 (s, 4H, Por, Haryl), 7.06 (s, 4H, Por, Haryl), 6.35 (m, 8H, Por, Haryl), 4.60-5.40 [m, 16H, Pc(α-OC4H9)8, -OCH2-], 1.40-1.80 [m, 32H, Pc(α-OC4H9)8, -CH2-], 1.07 [m, 24H, Pc(α-OC4H9)8, -CH3]. MS (MALDI-TOF): an isotopic cluster peaking at
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m/z 2016.5 [Calcd. for (MH)+ 2016.6]. Anal. Calcd. (%) for C108H105O8N12Cl4Lu: C, 64.35; H, 5.25; N, 8.34. Found: C, 64.04; H, 5.32; N, 8.38.
3.3.3 Synthesis of mixed (porphyrinato)(phthalocyaninato) yttrium(III) double-decker complex 16 [YIII(TClPP)(Pc)] The mixed (porphyrinato)(phthalocyaninato) yttrium(III) double-decker complex 16 [YIII(TClPP)(Pc)] was synthesized following the routine shown in Figure 3.5.
3.3.3.1 Preparation of yttrium acetylacetonate Y(acac)3.nH2O Y(acac)3.nH2O was synthesized following the same procedures as described in detail in section 3.3.2.1.
3.3.3.2 Preparation of H2TClPP H2TClPP was synthesized following the same procedures as described in detail in section 3.3.2.2.
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Figure 3.5. Synthesis routine of the mixed (porphyrinato)(phthalocyaninato) rare-earth(Ш) double-decker complex 16.
3.3.3.3 Preparation of mixed (porphyrinato)(phthalocyaninato) yttrium(III) double-decker complex 16 [YIII(TClPP)(Pc)] [7] [YIII(acac)3]nH2O (0.10 mmol) and 75 mg (0.10 mmol) of H2TClPP were dissolved in 4 mL of n-octanol, and the mixture was heated to reflux under a slow stream of nitrogen for ca. 6 h. The mixture was cooled to room temperature, and then 77 mg (0.60 mmol) of phthalonitrile and a few drops of DBU were added. The mixture was heated at 120oC for 12 h under nitrogen to give a dark green solution. After a brief cooling, 60 mL of methanol was mixed with the solution. The precipitate formed was collected by filtration and washed with methanol. The crude product was dissolved in a
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small amount of CHCl3, exposed to the air for several hours, and subjected to chromatography on a silica gel column. A small amount of unreacted H2TClPP was separated by using CHCl3/hexane (1/1, v/v). The column was further eluted with CHCl3, and a brown band containing the Y(TClPP)(Pc) was developed, collected and evaporated. The crude product was purified by repeated chromatography followed by recrystallization from CHCl3/MeOH. The final product was dried in air for 12 h and then under vacuum for 4 h. Yield: 36%.
3.3.4 Synthesis of heteroleptic phthalocyaninato yttrium(III) double-decker complex 17 (YIIIH(Pc){Pc(α-OC4H9)8}) The heteroleptic phthalocyaninato yttrium(III) double-decker complex 17 (YIIIH(Pc){Pc(α-OC4H9)8}) was synthesized following the routine shown in Figure 3.6.
3.3.4.1 Preparation of Y(acac)3.nH2O Y(acac)3.nH2O was synthesized following the same procedures as described in detail in section 3.3.2.1.
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Figure 3.6. Synthesis routine of the heteroleptic phthalocyaninato yttrium(Ш) double-decker complex 17.
3.3.4.2 Preparation of half-sandwich complex YIII(Pc)(acac) Y(acac)3.nH2O (0.8 mmol), 615 mg (4.8 mmol) of 1,2-dicynobenzene and 16 drops of DBU were dissolved in 5 mL of n-pentanol, and then the mixture was heated at ~100oC for ~2 h under a slow stream of nitrogen. The resulting green solution was cooled to room temperature, then mixed with 100 mL of hexane, and stirred for 30 min. The precipitate obtained by filtration was dissolved in a minimum amount of CH2Cl2 and purified by chromatography on a silica gel column using CH2Cl2/methanol. The crude
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product was recrystallized from CHCl3/hexane, collected by filtration, and dried in air for 24 h and then kept under vacuum for another 6 h. Yield: 50-60%.
3.3.4.3 Preparation of H2Pc(α-OC4H9)8 H2Pc(α-OC4H9)8 was synthesized following the same procedures as described in detail in section 3.3.2.4.
3.3.4.4 Preparation of heteroleptic phthalocyaninato yttrium(III) double-decker complex 17 (YIIIH(Pc){Pc(α-OC4H9)8}) [8] YIII(Pc)(acac) (0.05 mmol) and 54.6 mg (0.05 mmol) of H2[Pc(-OC4H9)8] were dissolved in 5 mL of n-octanol, and the mixture was heated to reflux under a slow stream of nitrogen for ca. 8 h. After cooled to room temperature, the mixture was mixed with ~40 mL of methanol. The precipitate formed was filtered, washed with methanol, and redissolved with a minimum amount of CHCl3 for chromatography on a silica gel column. The column was eluted with CHCl3, and a small blue band containing YIII(Pc)2 was developed and separated. The column was further eluted with CHCl3/MeOH (97/3, v/v), another grey blue band containing the protonated double-decker was developed, collected and evaporated. No nonprotonated counterpart was detected during the chromatography. The crude product was purified by repeated chromatography followed by recrystallization
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from CHCl3/MeOH. The final product was collected by filtration, and dried first in air for 12 h and then under vacuum for 4 h. Yield: ~15%.
3.3.5 Femtosecond time-resolved fluorescence measurements Time-resolved fluorescence measurements for the mixed (porphyrinato)(phthalocyaninato) rare-earth(III) double-decker complexes 9–15 in solution were carried out by using the FFU technique as described in detail in section 3.1.3. The solutions were prepared with anhydrous, deoxygenated CH2Cl2 at a concentration of ~0.2 mg/mL. The samples were excited by the second harmonic light (420 nm) generated by doubling a fundamental light at a wavelength of ~840 nm from the mode-locked Ti:sapphire laser with a pulse width of ~57 fs and a pulse repetition rate of 86 MHz. The IRF was estimated to be ~250 fs. The fluorescence data were fitted with a multi-exponential decay/rise model, and the fluorescence signal F(t) can be theoretically expressed by Equation 3.1, the convolution of the IRF r(t) with the molecule-response function f(τ), in which r(t) is a Gaussian function with laser pulse width and f(τ)is given by: f ( ) Ai exp Aoffset i i
(Equation 3.3)
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where the factor A represents the relative weights (or amplitudes) of the corresponding components, whose sign can distinguish rising or decay process; τ is the rising or decay time constant, and Aoffset is a small constant of 2%.
3.3.6 Electrochemical cyclic voltammetry measurements Electrochemical cyclic voltammetry measurements were performed on a VersaSTAT 3 electrochemical working station (Princeton Applied Research). The cell comprised inlets for a platinum working electrode (MF-2013) of 1.6 mm in diameter and a platinum wire counter electrode. The reference electrode was Ag/AgCl electrode (a 3.5 M KCl aqueous solution). Its potential was internally calibrated using ferrocene/ferrocenium (Fc/Fc+) redox couple [E1/2(Fc/Fc+) = +0.76 V]. Typically, a 0.2 M solution of [NBu4][PF6] (tetrabutylammonium hexafluorophosphate) in anhydrous CH2Cl2 containing sample was purged with nitrogen for at least 10 min, and then the voltammograms were recorded at ambient temperature. The scan rate was 50 mV.s-1.
3.4 Fabrication and characterization of extremely-thin absorber (ETA) solar cells 3.4.1 General procedures The hyperbranched phthalocyanines 1–3 (HBMPc-CN; M = H2, TiO, Cu) which were synthesized in section 3.1 were utilized as sunlight absorbers in the ETA solar cells.
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Indium tin oxide (In2O3:SnO2, ITO) pre-coated glass purchased from Delta Technologies with a sheet resistance of 8-12 Ω/sq. was used for substrates of solar cells. All other reagents and solvents were used as received. Absorption spectra were recorded on an Agilent 8453 UV-Visible Spectrophotometer. AFM images were taken in air under ambient conditions using the intermittent contact mode on an Agilent 5500 AFM/SPM microscope. SEM images were obtained using a Hitachi S-3400N scanning electron microscope. Unless otherwise noted, all the films or devices for either optical or morphology investigation were fabricated with the same procedures as for the fabrication of solar cell devices.
3.4.2 Fabrication procedures 3.4.2.1 Preparation of TiO2 sol gel [9] Ti(OC4H9)4 (1 g) was dissolved in 1 g of diethanolamine [(HOCH2CH2)2NH] and stirred for ~10 min. Ethanol (19.4 mL, 15 g) was mixed with 10 mL (10 g) of water, and added to the former solution drop by drop in 30-60 min. The mixture was then stirred for ~22 h at room temperature. Note that the prepared TiO2 sol gel was for immediate use only.
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3.4.2.2 Preparation of CuSCN solution [10] CuSCN (0.2 g) was added into 10 mL of dipropyl sulfide [(CH3CH2CH2)2S]. The suspension was stirred for ~24 h at room temperature, and then kept still for at least another 24 h. The clear solution over the precipitate was removed for use.
3.4.2.3 Cleaning of ITO glass substrates The ITO glass was cut into 2 cm by 2 cm pieces, cleaned in an ultrasonic bath sequentially by hot detergent, hot deionized water, toluene, acetone, and isopropyl alcohol, each for 15 minutes, and then dried in a nitrogen stream.
3.4.2.4 Fabrication of solar cells To prepare an n-type TiO2 layer, the TiO2 sol gel was spin-coated on ITO glass at 2,500 rotations per minute (rpm) for 20 s. The resulting film was sintered first at 100oC for 15 min and then at 450oC for 45 min. As an alternative fabrication method to prepare this n-type TiO2 layer, a nanoparticle TiO2 paste (Ti-Nanoxide HT/SP, Solaronix) was coated onto ITO surface by doctor-blading, and sintered first at 100oC for 15 min and then at 450oC for 45 min. A mixture of hyperbranched phthalocyanine HBMPc-CN (1 or 2 or 3) (10 mg/mL) and a small portion of TiO2 sol gel (controlling the volume ratio of HBMPc-CN to TiOx at ~1:1) in n-pentanol, was spin-coated onto the TiO2 surface, dried
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in air, and then annealed at 80oC under vacuum for 12 h. The saturated solution of CuSCN in dipropyl sulfide was then impregnated by spin-coating, and dried under vacuum for 2 days. Gold, or a blend of carbon nanopowder ( 7.8% in the range of 350–800 nm. The cell of 17 shows large IPCE at 605–800 nm, and the IPCE maximum is located at around 745 nm, which is the location of the maximum of the Q band absorption of the phthalocyaninato complex. All
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the IPCE spectra did not fade away at λ = 800 nm, indicating the existence of photocurrent contribution at longer wavelengths. The significant contribution of PDI to IPCE at 450–540 nm in these cells has also distinctly revealed, indicating that the excitons generated in PDI can be effectively utilized by these cells as well.
Figure 4.38. IPCEs of the cells of glass/ITO/PEDOT:PSS/double-decker complex (11, 16, 17):PDI:TiOx/Al, under 1-sun AM 1.5G illumination.
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In summary, the photovoltaic properties of bis(tetrapyrrole) complexes can be tuned either by changing the central metal ions or by varying the chemical structures of the macrocyclic ligands. The changing of the rare earth metal centers in the mixed (porphyrinato)(phthalocyaninato) complexes (9–15), caused a few to tens of nm blue or red shift of their absorption peaks, resulting in different light harvesting capability of these complexes at certain wavelengths to some extent. The rare-earth metal ions affect the energy transfer properties of these highly π-conjugated sandwiched systems by adjusting the distance between the adjacent tetrapyrrolic macrocycles. The fluorescence dynamics study revealed an ultra-fast energy transfer process between the macrocyclic ligands, and the generated excitons can be delocalized between the macrocycles due to the strong π-π interactions. The VOC of complexes 9–15 showed much similarity, while the highest cell efficiency came from the most stable europium counterpart in this work. On the other hand, changing the substituent groups or the species of the ligands may affect the stable existence form (protonated or neutral or both) of these bis(tetrapyrrole) complexes which has been discussed in details previously [35,40]. Complexes in protonated form and complexes in neutral form show much difference on their absorption profiles, and energy levels. All these can significantly change the spectral coverage and VOC of the solar cell devices.
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4.4 Cost estimation of phthalocyanine materials in phthalocyanine-based solar cells The phthalocyanine material cost for solar cell fabrication can be estimated by a simple calculation. In a typical organic solar cell of CuPc/C60, the CuPc layer is ~20 nm thick [75]. The density of CuPc film is ~1.62 g/cm3 or ~1.62 * 106 g/m3 [76]. For 1 m2 of CuPc film with thickness of 20 nm, 1 m2 * (20 * 10-9 m) * (1.62 * 106 g/m3) = 3.24 * 10-2 g of CuPc is needed. The price of CuPc is $3000.00/ton [77]. So the CuPc material cost for 1 m2 of CuPc is (3.24 * 10-2 g) * ($3000/ton * 10-6 ton/g) = $0.0000972. That is to say, the CuPc material cost is only $0.0000972/m2. And 1 ton of CuPc can cover an area of (106 g)/[( 1.62 * 106 g/m3) * (20 * 10-9 m)] = 3.09 * 107 m2 = 30.9 km2. Or in other words, from the aspect of CuPc material cost, it takes only $3000.00 for making CuPc solar cells to cover an area of 30.9 km2.
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Chapter 5 Conclusions
5.1 Summary The energy crisis and environmental issues have led people to seek renewable, sustainable, and environmentally clean energy sources. As one of the most promising candidates, solar cells directly convert solar energy into electrical energy by the photovoltaic effect. However, the high cost of crystalline silicon solar cells and thin film solar cells has hindered their widespread proliferation. Organic-based solar cells including organic solar cells (OSCs) and dye-sensitized solar cells (DSSCs) have drawn considerable attention owing to their prospective advantages to produce large-area, flexible, low-cost, and light-weight devices using simple techniques. Efficient OSCs have been achieved either by solution processed conjugated polymer-fullerene blends or by vacuum or organic vapour phase deposited small molecular materials in recent years. The state-of-the-art power conversion efficiency has steadily increased to 8.41% and 8.3% for single-junction polymer solar cells and double-junction small-molecule solar cells, respectively. The absorption band of most semiconducting polymers and small molecules used in existing OSCs and organic dyes used in existing DSSCs is in visible range (380–650 nm). Most energy of the photons from NIR to longer wavelengths (~49% of the solar energy) usually cannot be harvested by these devices due to a relatively high band
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gap of organic photovoltaic materials. On the other hand, phthalocyanines are emerging as promising low-band-gap light absorbers in OSCs and TiO2 sensitizers in DSSCs. The goal of this work was to develop solution-processable low-band-gap organic photovoltaic materials for low-cost high-efficiency organic-based solar cells. To accomplish this goal, the objectives were to: 1) develop novel mesoscopic phthalocyanine structures, including hyperbranched phthalocyanines, hyperbranched phthalocyanine dyes, and sandwich-type porphyrinato/phthalocyaninato rare earth(III) double-decker complexes; 2) develop solution processed organic-inorganic hybrid solar cells using hyperbranched phthalocyanine absorbers following the extremely-thin absorber (ETA) concept; 3) develop near infrared photon harvestable high-efficiency DSSCs using non-aggregated hyperbranched phthalocyanine dyes; 4) develop solution processed OSCs with broadband light harvesting capabilities using sandwich-type porphyrinato/phthalocyaninato rare earth(III) double-decker complexes; and 5) develop an understanding of photovoltaic material design. Solar cells generate electrical power by converting the energy of sunlight into direct current electricity using semiconductors that exhibit the photovoltaic effect. In an ETA solar cell, photon absorption by the absorber results in the generation of electron-hole pairs (for inorganic semiconductor) or excitons (for organic semiconductor). A large built-in electric field between p-type and n-type semiconductors supports fast
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separation of the photo-generated carriers in the absorber layer. Separated electrons are driven towards the n-type transparent electron-transport layer and subsequently collected by the n-contact, while holes can only enter the p-type hole-transport layer and then reach the p-contact. In a DSSC, absorption of photons leads to photo-excitation of dye, electrons from the excited dye are quickly injected into the conduction band (CB) of mesoporous TiO2, and subsequently collected by a transparent conductive oxide (TCO). The dye is regenerated by electrolyte, while the redox system itself is in turn restored at counter electrode by the electrons passing through the load from the TCO. In a bulk donor/acceptor heterojunction solar cell, absorbed photons generate excitons, and the photo-generated excitons diffuse to the donor/acceptor interface where excitons dissociate into free carriers. The separated free electrons transport through the acceptor phase by hopping to anode, while the separated free holes transport within the donor phase by hopping to cathode. Three novel hyperbranched phthalocyanines 1–3 (HBMPc-CN; M = H2, TiO, Cu), five novel non-aggregated hyperbranched phthalocyanine dyes 4–8 (HBMPc-COOH; M = H2, AlCl, Co, Cu, Zn), and nine sandwich-type porphyrinato/phthalocyaninato rare earth(III) double-decker complexes (9–17) were synthesized and characterized by UV-visible absorption, steady-state and femtosecond time-resolved fluorescence, Fourier transform infrared, proton nuclear magnetic resonance, element analysis, and cyclic
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votammetry. These mesoscopic phthalocyanine structures were fabricated into ETA solar cells (for 1–3), DSSCs (for 4–8), and organic-inorganic hybrid solar cells (for 9–17). The fabricated films/devices were characterized by atomic force microscopy, scanning electron microscopy, current-voltage characteristics, and photoelectrical properties. The absorption peaks of HBMPc (1–3):TiOx films were broadened due to aggregation of phthalocyanine rings and shifted due to variation of metal centers. ETA solar cell of 1 showed the best photovoltaic performance among the ETA cells of 1–3, and a highest efficiency of 0.23% was achieved from a cell structure of ITO/TiO2 (sol gel)/TiOx:1/CuSCN/Au. The lower efficiency for the ETA cell of 3 was likely due to the energy loss in the formation of triplet states with an intersystem crossing time of 0.76 ps. Absorption, fluorescence, fluorescence dynamics, electrochemistry, and photovoltaic performances of the hyperbranched phthalocyanine dyes 4–8 were found to be significantly affected by the use of different metal centers. Ultrafast multi-phasic electron injection from both the Soret band and Q band of 8 to CB of TiO2 was revealed. A power conversion efficiency of 1.15% was achieved from the Zn substituted hyperbranched phthalocyanine dye (8) sensitized solar cells, a value of at least ~3 times higher than that of any other metal centered counterpart sensitized solar cells in this work. The changing to rare earth metal centers in 9–15 caused a few to tens of nanometers blue or red shift of their absorption peaks, resulting in different light harvesting capability of these
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complexes at certain wavelengths to some extent. The fluorescence dynamics study revealed an ultra-fast energy transfer process between the macrocyclic ligands, and the generated excitons can be delocalized between the macrocycles due to the strong π-π interactions. Solution-processed organic-inorganic hybrid solar cells of 9–17 showed capability of broadband solar photon harvesting over the ultraviolet-visible-near-infrared spectral range. Solar cells utilizing PDI as primary acceptor showed higher values of open circuit voltage, fill factor, and power conversion efficiency than those cells using PCBM as primary acceptor. With cell area of 0.36 cm2, efficiencies of up to 0.82% were achieved by the double-decker complex (9–17):PDI:TiOx blends under 1-sun air mass 1.5 global illumination.
5.2 Conclusions Novel mesoscopic hyperbranched phthalocyanines demonstrated successful photovoltaic applications in solution processed ETA solar cells with efficiencies of up to 0.23%. The further chemical reaction of metal-free phthalocyanine rings with TiO2 precursor during the annealing procedure and the solvent effect of dipropyl sulfide can promote the formation of ETA configuration. In principle, the ETA cell design is applicable to other organic semiconductors (molecules or polymers).With broader absorption and proper energy alignment, higher power conversion efficiencies of ETA
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solar cells with organic semiconductors can be anticipated. For HBCuPc-CN (3), the formation of the triplet states is a competitive process with electron injection. For further optimization of HBCuPc-CN (3) cell, it is necessary to lower down the CB of TiO2 by doping in the n-type matrices or to use other n-type semiconductors with lower CB instead of TiO2. Novel mesoscopic non-aggregated hyperbranched phthalocyanine dyes have demonstrated successful photovoltaic applications in near infrared photon harvestable DSSCs with efficiencies of up to 1.15%. A new avenue of utilizing hyperbranched structure to solve the aggregation issue of phthalocyanine dyes on TiO2 which hinders phthalocyanine dyes from high performances in DSSCs was developed. DSSCs with higher efficiency are anticipated by increasing the spectral coverage of hyperbranched phthalocyanine dyes in the visible range through chemical structure modification. Novel mesoscopic sandwich-type porphyrinato/phthalocyaninato rare earth(III) double-decker complexes (9–17) have demonstrated successful photovoltaic applications in solution-processed organic-inorganic hybrid solar cells with capability of broadband solar photon harvesting over the ultraviolet-visible-near-infrared spectral range. The addition of secondary electron acceptor (TiOx) significantly suppressed the back electron transfer between the double-decker complexes and the primary electron acceptor (PCBM or PDI). The applicability of the complexes 9–17 for broadband light harvesting solar
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cells has been ensured in spite of the difficulties to gain the exact HOMO/LUMO energy levels of protonated tetrapyrrole double-decker complexes through electrochemical study. The solution processed bulk heterojunction solar cells of these double-decker complexes showed efficiencies of up to 0.82%, comparable to some vacuum deposited devices of single-ring phthalocyanines. Mesoscopic phthalocyanines are promising low-cost photovoltaic materials with tunable absorption/photophysical properties. The cell efficiencies achieved in this work are not promising for practical applications, and further cell optimization is needed in future work.
5.3 Future work From the aspect of materials, modify the chemical structures of mesoscopic hyperbranched phthalocyanine structures for broadened spectral coverage toward more efficient ETA solar cells and DSSCs. From the aspect of device fabrication, 1) lower down CB of TiO2 by doping in n-type matrices or use other n-type semiconductors with lower CB instead of TiO2 to improve ETA cell efficiency of HBCuPc-CN (3); 2) improve power conversion efficiencies of OSCs of 9–17 by using other nanostructured oxide as secondary acceptor, changing components ratio, tuning film thickness, engineering surface morphology and
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optimizing interface of nanocomposites; 3) apply mesoscopic phthalocyanine structures and solar cell devices for solar modules/panels with capability of broad-band light harvesting.
