Optimization of Mo03 Buffer Layer Thickness for Short Circuit Current Enhancement in Ag Nanoparticle Incorporated CuPc Solar Cell Devika Kataria and S. Sundar Kumar Iyer Department of Electrical Engineering and Samtel Centre for Display Technologies, Indian Institute of Technology Kanpur, Kanpur, 208016, India Tel.:+91-512-259 6659, Fax:+91-512-259 6621, Email:
[email protected]@iitk.ac.in Abstract
Metal nano-particles (MNP) have been used for
-
light trapping in the photovoltaic material.
Concerns about the
excitons generated being quenched by the MNP exist. Thus organic bilayer solar cells have to be designed with a buffer layer between the MNP and the active layers. structure
used
in
this
study,
silver
MNP
In the device
were
thermally
evaporated over indium tin oxide coated glass substrate. layer of molybdenum oxide before
depositing
layer.
The thickness of
the
(Mo03)
copper-phthalocyanine
Mo03
A
was deposited on the MNP (CuPc)
active
layer was varied to study its
optical and electrical effects on the layer-stack.
The spectral
response of the different layer stacks with aluminum electrodes deposited
on
improvement
CuPc in
short
helped circuit
absorption enhancements.
quantify current
and
compare
possible
due
to
the the
Simulation has been carried out
In the work reported in this paper, the extent of enhancement of absorption of light in CuPc is estimated for Mo03 buffer layer between the Ag MNP and the CuPc active layer. This is accomplished by taking the absorption spectrum and as well as spectral response measurements on samples prepared with and without Ag MNP close to CuPc active layer for different Mo03 buffer layer thickness. The absorption and spectral response results were corroborated with the help of simulation based on finite element method (FEM) and analysis. With the help of these results, the optimum Mo03 thickness range to be used for Ag MNP to maximize photovoltaic response for CuPc active layer films has been estimated.
using finite element method to study role of near field and plasmonic scattering due to the MNP.
Mo03
identified.
Index Terms
-
silver nanoparticles, light trapping, short
circuit currents, spectroscopy, finite element analysis.
I.
II.
The thickness range of
that would maximize the short circuit current has been
INTRODUCTION
Small molecule organic solar cells are gaining popularity due to ease of synthesis and purification of the photovoltaic material, easy fabrication of devices and better reproducibility from batch to batch. Efficiencies of these small molecule solar cells are approaching the reported efficiencies for polymer solar cells [1, 2]. Increasing photon harvesting in the solar cell active layer by various techniques is one of the ways by which the cell efficiencies may be improved. Kyaw et al. have reported efficiency enhancement from 8% to 8.9% by using optical spacer to increase light harvesting in solution processed small molecule solar cells [3]. Stenzel et al. had shown that incorporating silver, gold or copper nano-c1usters directly in contact with copper-phthalocyanine (CuPc) results in enhancement is photocurrent currents by 1. 3, 2. 3 and 2.7 times respectively [4]. Lee et al. have analyzed the effect of embedding spherical metal nanoparticles (MNP) on absorption enhancement in CuPc and studied the effect of very thin encapsulation using material of higher refractive index on MNPs [5].
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EXPERIMENT AND SIMULATION
ITO coated glass substrates were cleaned thoroughly with soap water and dried in an oven. The substrates were subjected to ozone plasma for 20 minutes. Two sets of devices were fabricated - the control samples without any MNP (Fig. 1a) and the experimental samples with Ag MNP deposited on the ITO surface (Fig. 1b). CuPc
CuPc
ITO
ITO
(a)
(b)
Fig. I. Sample structure for absorption spectrum measurement (a) reference sample (b) samples with MNP and with 0,5,10 and 20 nm thickness of Mo03 .AI electrode is deposited on top of CuPc layer for taking spectral response measurements.
The Ag MNP were formed by targeting a deposition thickness of 2 nm film. The deposited Ag MNP were analyzed by FESEM to study their shape size and spacing. The Mo03 buffer layers of varying thickness of tMo03 0 nm (no Mo03 deposited), 5 nm, 10 nm and 20 nm, were prepared - for samples without as well as with Ag MNP on ITO. On top of these, the active layer CuPc of thickness 20 nm was =
deposited. All thermal deposition were carried out in a vacuum chamber with a base pressure of 5x 10-7 mbar. Absorption spectrum was carried out for the spectral range of light wavelength from 350 nm to 750 nm with the help of a Perkin Lambda 750 absorption spectrometer. To perform spectral response measurements, an aluminum (AI) electrode was thermally deposited on top of the CuPc active layer. The spectral measurements were taken by shining light of different wavelength from the glass side and measuring the current with a Keithley 1200 system. The measurement set up used for the spectral response measurement is described in detail in [6]. The size, spacing and shape of MNP were measured with the help of field effect scanning electron microscope (FESEM). The analysis of FESEM micrograph showed the median diameter of MNP to be 16 nm. The MNPs were closely packed and the median value of the spacing between the edges of the MNP was 11 nm. The micrograph also revealed that the MNP were sections of spheres with a contact angle of 100° with ITO.
Input Port LIght coupling
ITO
Periodic boundaries (Four sIdes)
CuPe
Fig. 2. scale).
Device structure used for simulation in COM SOL (not to
Simulations, to calculate optical intensity enhancement due to presence of MNP, were carried out using FEM software COMSOL [7]. The structure simulated with the 2D simulator is shown in Fig. 2. Periodic boundary conditions were used to simulate an array of MNP. The size shape and spacing between the MNP were taken according to the FESEM analysis results. Simulation was carried out for white light of 1 W intensity entering through top port of the structure. The propagation vector for the simulation was set along the y direction. The polarization vector was taken in x direction. Optical constant values of Ag for the simulation were taken from reference [8]. III.
RESULTS
A. Absorption Spectrum
The absorption spectrum measurements of the samples were performed for the spectral range 350 to 750 nm and are shown in Fig. 3. The absorption spectrum for reference sample (without MNP) shows that CuPc absorbs very little
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light in the spectral region 350-600 nm however, has significant absorbance in spectral region 600-750 nm. The samples where MNP are present have an additional absorption peak in region 350-600 nm. These absorption values correspond to the absorption of the MNP itself There is also enhancement in the absorption observed in the range 600-750 nm in the spectrum where the CuPc absorbs. The samples where MNP were directly in contact with CuPc, show the highest absorbance enhancement in the spectral region 600-750 nm. The absorbance in samples with Mo03 thickness 5, 10 and 20 nm monotonically decreases. 0.5 ....----, c without MNP • with MNP & OnmMo0 3
...
with MNP & 5nm Mo03
T
with MNP &
lOnmMo03
O.l 0.0 +--..-----r--r"-�-_.__-_r__-.--___I 350 450 550 650 750
Wavelength(nm) Fig. 3. Absorption spectrum of reference sample and samples with MNP and with different thickness of Mo03 B. Spectral Response
The absorption spectrum shows the combined absorption by the MNP as well as the CuPc layer. The absorption in the MNP, however, does not help in photovoltaic action, but only dissipates the optical energy as heat. The absorption in the CuPc active layer only has scope to be useful for a photovoltaic response. The spectral response measurement allows us to identity the optical power that is getting absorbed in the CuPc active layer and helping create useful electron hole pairs. The spectral responses in the wavelength range of 350-750 nm for samples with Mo03 buffer layer thickness of 0, 5, 10 and 20 nm are shown in Fig. 4. The samples which contain MNP show higher external quantum efficiency (EQE) in the spectral range 550-750 nm as compared to samples that do not contain MNP for all cases of Mo03 thickness. The almost zero current in the wavelength range of 350-550 nm in the EQE results, although the absorption spectrum values were high, confirm that the absorption was predominantly in the MNP. The enhancement in EQE in the spectral range 550-750 nm, confirms that the optical intensity enhancement
occurs in this range and this causes the increase in the absorption in the CuPc layer. It is observed that of all the devices, the samples with Mo03 thickness of 5 nm (Fig. 4b)
1.0 0.8 ,.-,
0.6
� � 0.4 0 r.Ll
(a) OnmMo03
have the highest current response. The current response improves further with the incorporation of MNP.
Filled: with MNP Unfilled: w/o MNP
•
tc,
• • •
o
¢
0
•
4
..
(c) lOnmMo03
(d) 20nmMo03
0.2 0.0 1.0 0.8
,.-,
0.6
� � 0.4 o r.Ll 0.2 o.o
�_�� �� 450 550 650 750
350
Wavelength(nm)
350
550 650 450 Wavelength(nm)
750
Fig. 4. EQE spectrum for samples with and without MNP with Mo03 thickness of 0,5,I° and 20 nm. The samples with and without and with 5 nm Mo03 show the highest current response.
IV. DISCUSSION A. Analysis of Absorption Enhancement
In order to understand the effect of Mo03 buffer layers on the enhancement in optical absorption in the CuPc layer, the absorption spectra of the samples with MNP have been normalized with respect to the absorption of reference sample, in the spectral range 600 to 750 nm. These are shown in Fig. 5. The normalized graphs exhibit that the absorption enhancement is highest for no buffer layer, followed closely by a buffer layer thickness of 5 nm. However, the values drop significantly over the different wavelengths considered for buffer layer thicknesses of 10 nm and 20 nm. The results indicate that much of the optical
978-1-4799-7944-8/15/$31.00 ©2015 IEEE
MNP
enhancement takes place close to the MNP. Once the thickness of the active layer becomes larger, the enhancement is a small fraction of what it is capable of absorbing in the thicker layer. The cause for the absorption enhancement according to the FEM simulations using COMSOL indicated an optical intensity enhancement in CuPc due to near field of pIasmonic oscillations in MNP. The intensity enhancement is computed as
IntensityEnhanced
=
If
2 EW
llhMNP dx.dy / S
E2 WililOlIIM
NP
(1)
where S is the average surface area of CuPc conformal layer as shown in Fig. 2. The intensity enhancement computed by simulation has been plotted in Fig. 6.
Q) u c m .0
1.6
....
g1.4
.0
«
"0
. � 1.2 m
E ....
o z
1.0
... •
·
••
.u. . ••AU.. • .A ... . . ... ...... ... ....... ... ....... ... ... ... ... • OnmMo0 3 ... ,. ... • S nmMo0 3 ............... .. .... lOnmMo0 • ...
600 Fig. S.
1:
. . • .• • . �.• . • •••••••••••••• • •• • •• • ••••• • + • •••••• • • ••• • • • • •••
650
20nmMo0
ITO substrate. As before periodic boundary conditions are used in the x and y directions. Perfectly matched layers were used in the z direction. Two electromagnetics nodes were used in the simulation. The first node took the incident field as background field and computed the total field due to the MNP. This total field was then coupled to the second electromagnetic node which solved for the scattered field, which was a perturbation to the total field caused by the MNP. The simulations showed that as and aA of MNP increased as thickness of Mo03 increased to 10 nm and 20 nm. The ratio of these cross-sections for 10 nm and 20 nm thickness of Mo03 are shown in Fig. 7.
3 3
750
700
Wavelength(nm)
Normalized absorbance for samples with sample with
MNP with Mo03 thickness of 0,5,I0 and 20 nm.
The results show that the near field enhancement exists only for buffer layer Mo03 thickness less than or equal to S nm and is highest for 0 nm case. As per the simulation results, there is hardly any near field enhancement seen for buffer layer thicknesses of 10 nm and 20 nm.
.....
6
s::
•
Onm Mo03
•
S nm Mo03
...
(J)
a (J)
1Onm Mo03 20nm Mo03
..,
84 �
• •
(J)
1:3 2 (J) .... . s:: .... .. 0 600
• •
• •
•
•
•
•
*
*
•
;;.. ..... .....
•
•
•
•
*
650
•
•
Fig. 7. Ratio of scattering to absorption cross-section for different thickness of Mo03 as simulated using COM SOL.
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
700
750
Wavelength (run) Fig. 6. Intensity enhancement for different thickness of Mo03 as simulated using COM SOL.
Simulations were also done for absorption and scattering cross-sections for various thicknesses of Mo03 using the FEM software [7]. The scattering cross-section as and the absorption cross-section aA were computed for the MNP on
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The ratio shows that for wavelengths larger than 600 nm, scattering takes place for all thicknesses of Mo03. The results in Fig. 6 and Fig. 7 indicate that for samples with 10 nm and 20 nm Mo03 the enhancement in absorbance is not due to direct effect of the near field but because of scattering of the longer wavelength of light. B. Analysis of EQE Enhancement
The spectral response shown in shown in Fig. 4 was found to be best for the device structure ITO-Snm Mo03 -CuPc BCP_ITO. The EQE for this device structure was analyzed by simulating the structure and opto-electric phenomenon with the help of COMSOL and MATLAB [9]. The average Poynting vector was computed in the CuPc stack for the structures without MNP and with-MNP on ITO. The wavelength dependent energy absorption was calculated using the formulae [10] Q(x,y,ll)
=
a(ll) * fo * Pavg(x,y,ll)
(2)
where a(A.) is the wavelength dependent absorption coefficient for pristine CuPc, 10 is the incident AM1.5G photon flux and Pavg is the average pointing vector in CuPc stack determined as
The short circuit current density and the EQE were calculated by assuming that the exciton dissociation takes place only at the electrodes. The total current density was calculated using the formula [6]
(3)
S 3x1O � N'2x1O E
s
• •
...
•
� 2X1Os
I
� � 1x1Os 5x1O
7
V
