The ZnO thin films were used as an electron transport layer in solar cells to improve the photovoltaic performances. To explore the effects of ZnO film thick-.
Copyright © 2010 American Scientific Publishers All rights reserved Printed in the United States of America
Journal of Nanoelectronics and Optoelectronics Vol. 5, 1–4, 2010
Inverted Organic Solar Cells with ZnO Thin Films Prepared by Sol–Gel Method Hye-Jeong Park1 , Kang-Hyuck Lee1 , Brijesh Kumar1 , Kyung-Sik Shin1 , Soon-Wook Jeong2 ∗ , and Sang-Woo Kim1 3 ∗ 1
School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea 2 School of Advanced Materials and System Engineering, Kumoh National Institute of Technology, Gumi 730-701, Republic of Korea 3 SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nanotechnology (HINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea
Keywords: Inverted Organic Solar Cell, ZnO Thin Films, Buffer Layer, Sol–Gel Method.
1. INTRODUCTION Inverted organic solar cells (IOSCs) that are based on bulkheterojunction (BHJ) have attracted substantial attention due to their low cost, light-weight materials, compatibility with flexible plastic substrates, and ease of fabrication.1–3 However organic materials have the small excitons diffusion length and low carrier mobility. This causes the IOSCs to reduce the efficiency. An approach to reduce the problem is to insert a buffer layer between the metal and active layers as a path for photo-generated carriers to collect at the electrode. This buffer layer can also solve another problem with prolonged exposure to air that can lead to oxidation of the electrode and degradation of the active layer from oxygen and moisture.3–5 The zinc oxide (ZnO) buffer layer, among metal oxide semiconductor films, has been widely studied in recent years. ZnO is a good candidate for this application due to its high electron mobility and high transparency in visiblewavelength range. Additionally, zinc oxide have interesting properties such as excellent chemical and thermal ∗
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stability in different environments, non-toxicity, excellent substrate adherence, and they are inexpensive.6–8 A wide variety of techniques for the deposit method of high quality ZnO thin films have been reported, such as metalorganic chemical vapor deposition (MOCVD),9 pulsed laser deposition (PLD),10 sol–gel,11 12 sputtering,13 and electrochemical deposition.14 Among them, ZnO thin films on flexible substrate can be deposited by easily using the sol–gel method. The general advantage of the sol–gel method is that it can be used for large area deposition and processed at a low temperature. In addition, this method is easy in its composition control, it has a low cost and it provides uniformity of the film’s thickness.15–17 Many recent studies have focused on the control of the preferential orientation of ZnO thin films in IOSCs by using a relatively tractable sol–gel method. However, studies on the thickness of ZnO thin films have rarely been carried out. Therefore, in this paper, we used the sol–gel method to study the thickness of ZnO buffer layer in IOSCs. We studied the morphology-dependent influences of ZnO buffer layer on the photovoltaic performances in IOSCs, and the thickness effect of ZnO buffer layer was explored and optimized for the IOSCs. 1555-130X/2010/5/001/004
doi:10.1166/jno.2010.1079
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The influences of buffer layers on the performance of hybrid solar cells that are fabricated with an active layer blend of poly (3-hexylthiophene) (P3HT) and [6, 6]-phenyl-C61-buytyric acid methyl ester (PCBM), were investigated. The ZnO thin films were used as an electron transport layer in solar cells to improve the photovoltaic performances. To explore the effects of ZnO film thickness, ZnO thin films were deposited as an electron transport layer by spin coating them one to four times at 4000 rpm using zinc acetate dehydrate that was dissolved in a mixed solution of 2-methoxyethanol and ethanolamine. We fabricated IOSCs on PES substrate, consist of Au/MoO3 /PCBM:P3HT/ZnO/ITO. The optical property of the ZnO buffer layers was investigated. Furthermore, we found the optimum ZnO buffer layer by using the sol–gel method to enhance the performance of the solar cells.
Inverted Organic Solar Cells with ZnO Thin Films Prepared by Sol–Gel Method
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2. EXPERIMENTAL DETAILS We first prepared ZnO sol using zinc acetate (Zn(CH3 COO)2 ) as a source, 2-methoxy ethanol (C3 H8 O2 as a solvent, monoethanolamine (C2 H7 NO) as a stabilizing agent. These were stirred for one hour at 60 C. The molar concentration of zinc acetate in the solution was 0.05 M. The ZnO thin films were deposited using spin coating at 4000 rpm for 50 seconds on flexible substrate (ITO/PES). The deposited films were thermally treated at 150 C for 30 minutes in each deposition. Secondly, we prepared a P3HT:PCBM-blended solution for active layer deposition by spin coating it at 2000 rpm for 120 seconds. Poly(3hexylthiophene) (P3HT), used as a p-type material, and [6,6]-phenyl C61 butyric acid methyl ester (PCBM), used as an n-type material, were dissolved into chlorobenzene in a weight ratio of 1:1 for one day, this making P3HT:PCBM blended in chlorobenzene. After annealing an active layer for 30 minutes at a temperature of 150 C, an MoO3 layer of 20 nm as a hole-elective layer2 and a 100 nm Au layer were deposited by thermal evaporation through a shadow mask. The structure of the IOSC was PES/indiumtin-oxide (ITO)/ZnO/P3HT: PCBM/MoO3 /Au stacked from bottom to top, as shown in Figure 1. The optical transmittance spectra were recorded by using an UV-VIS spectrophotometer (Shimadza UV-3600). The thickness and microstructures of ZnO thin films were measured by scanning electron microscopy (SEM). Moreover, we measured the photovoltaic performance.
3. RESULTS AND DISCUSSION The cross section FE-SEM images of the ZnO thin films with the depositions from one to four (1–4) times are
Fig. 1. Device structure of the flexible inverted OSCs. including the short circuit current density (Jsc ), open circuit voltage (Voc ), fill factor (FF), power conversion efficiency (PCE) and series resistance (Rs ) by AM 1.5G, 100 mW/cm2 solar simulator.
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shown in Figure 2. The thickness of the ZnO thin films were estimated to be 53–100 nm, 93–107 nm, 120– 127 nm, and 133 nm as shown in Figures 2(a)–(d), respectively. The structure observed from the one-time deposit ZnO films had small wrinkles as shown in Figure 2(a). The thickness of this ZnO films was approximately 53 nm except for the wrinkles and approximately 100 nm with wrinkles. The thickness of the one-deposit ZnO films on the sample was a minimum of approximately 93 nm as shown in Figure 2(b). This value is similar with measuring value of thickness with wrinkles in Figure 2(a). The thickness error range of the two-deposit ZnO thin films was large due to a little curvature of the surface. The thickness error range of three-deposit ZnO thin films was reduced to deposit a littler curvature of the surface, as shown in Figure 2(c). The samples of the four-deposit ZnO thin films were measured at 133 nm of uniform thickness. Figure 3 shows the UV-visible optical transmittance spectra of the ZnO films between 300 and 800 nm wavelengths. All of the ZnO films were highly transparent in the UV-VIS region of 400 nm to 800 nm and a sharp fall in transmittance was observed below 400 nm due to bandgap absorption.18 All of the films had a higher transmittance than the ITO/PES substrate. Transmittance from the one-deposit ZnO thin films improved approximately 10% more than the ITO/PES substrate. The transmittance considerably improved up to three times deposition. Furthermore, the transmittance of the four-deposit ZnO thin films slightly decreased as shown in Figure 3. Compared to the surface of the ITO and ZnO thin films, the surface of ZnO thin films was relatively of higher roughness than the surface of the ITO/PES. We performed an X-ray diffraction (XRD) measurement of an ITO/PES substrate and a ZnO film-deposited ITO/PES substrate. A weak feature of indium oxide In2 O3 was observed from the ZnO-deposited film on the ITO/PES sample. However, we could not observe an In2 O3 peak from the ITO/PES substrate. From the XRD result, we suggest that a few of the indium sites were present in the ITO. It was interrupting the light to transmit through the ITO-coated substrate. These indium sites transformed into In2 O3 (wide bandgap semiconductor) during ZnO film deposition on the ITO/PES substrate, which allowed the transmission of the visible light through the ZnOdeposited ITO/PES sample, hence higher transmittance was observed. We fabricated IOSCs to confirm the solar power performances based on various thickness of the ZnO buffer layer. J –V measurements were extracted using a solar simulator under an AM 1.5G solar illumination. The J –V characteristics are summarized in Figure 4 and Table I, respectively. There was almost no change in the open circuit voltage (Voc of these devices. The short circuit current (Jsc increased when the thickness of ZnO thin films was increased. The Jsc ranges varied from 6.51 mA/cm2 J. Nanoelectron. Optoelectron. 5, 1–4, 2010
Park et al.
Inverted Organic Solar Cells with ZnO Thin Films Prepared by Sol–Gel Method
(a)
(b)
(c)
(d)
Fig. 2. FE-SEM images of ZnO thin films derived from different number of deposition: (a) 1 time, (b) 2 times, (c) 3 times, and (d) 4 times. Red and green bar means ZnO layer and ITO layer, respectively.
Fig. 3. The transmission spectra of ZnO thin films for four different depositions. ZnO thin films of A, B, C, and D were derived from 1, 2, 3, and 4 depositions.
J. Nanoelectron. Optoelectron. 5, 1–4, 2010
the maximum absorption is for P3HT:PCBM21 could produce a larger number of excitons, which resulted in a large photocurrent. The highest PCE, 2.2% with Jsc and Voc of 8.6 mA/cm2 and 0.55 V, respectively, was obtained by three-deposit ZnO thin films. These results suggest that photovoltaic PCE of IOSCs is affected by the optimum thick and transparent of the ZnO buffer layer. Figure 5 shows the J –V curve in a dark condition. The value of the sheet resistance (Rs ) increases by the
Fig. 4. The J –V characteristics of the ZnO thin films derives from a different number of deposition under simulated solar irradiation of AM 1.5G normalized 100 mW/cm2 . ZnO thin films of A, B, C, and D were derived from 1, 2, 3, and 4 depositions.
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to 8.85 mA/cm2 depending on the thickness and transmittance of the ZnO thin films. This trend could be the result of the formation of more percolation pathways of the ZnO buffer layer that increase the photocurrent and improve the transport of electrons. The FF ranged from 38.6% to 45.4% depending on the thickness of ZnO thin films. The FF of IOSC fabricated with ZnO thin films at three-deposit ZnO buffer layers is higher. The ZnO buffer layer revealed an enhancement in PCE from 1.1% to 2.2%. The enhanced photovoltaic performance was attributed to a higher transmittance in the visible wavelength around 550 nm, where
Inverted Organic Solar Cells with ZnO Thin Films Prepared by Sol–Gel Method Table I. Summary of device performance.
Device A B C D
Deposition number
Thickness (nm)
Jsc (mA/cm2 )
Voc (V)
FF (%)
PCE (%)
1 2 3 4
53–100 93–107 120–127 133
6.551 7.801 8.857 6.612
0.465 0.534 0.548 0.554
38.656 41.363 45.478 41.159
1.177 1.724 2.207 1.508
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Acknowledgments: This paper was supported by the Research Fund of Green Energy System Education Center, Kumoh National Institute of Technology (2009-130-012) and by the Basic Science Research Program through the NRF funded by the Ministry of Education, Science and Technology (2010-0015035).
References and Notes
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Fig. 5. In a condition, the J –V curve of the ZnO thin film devices from different numbers of depositions. ZnO thin films of A, B, C, and D were derived from 1, 2, 3, and 4 depositions.
increasing deposition number from two times to four times. The curve of sample A was unexpectedly in the middle of B and C. This could be due to the highest surface roughness of sample A and fairly large wrinkles as shown in Figure 2(a). This high surface roughness was also the reason why Jsc was lower in sample A.
4. CONCLUSIONS In this paper, surface morphologies and optical qualities of ZnO thin films that were prepared by the sol–gel method were investigated and optimized for inverted inorganic– organic solar cell structures. The transmittance from onedeposit ZnO thin films improved approximately 10% more than the ITO/PES substrate. The transmittance improved approximately 10% more with up to three times deposit. We found the optimum thickness and deposited transparent ZnO buffer layer using the sol–gel method. The ZnO buffer layer influenced Jsc of the IOSCs due to the combined factors of electron mobility and light harvesting.
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Received: 30 April 2010. Accepted: 16 June 2010.
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