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Multifunctional SWCNT-ZnO Nanocomposites for Enhancing Performance and Stability of Organic Solar Cells Won Hyun Shim, Sun-Young Park, Mi Yeong Park, Hyun Ook Seo, Kwang-Dae Kim, Young Tae Kim, Yang Do Kim, Jae-Wook Kang, Kyu Hwan Lee, Yongsoo Jeong, Young Dok Kim,* and Dong Chan Lim* Photovoltaic systems have been extensively studied and, among them, organic solar cells (OSCs) have attracted particular attention due to their low price and the possibility of using them in flexible devices.[1–4] One of the disadvantages of OSCs is their low chemical stability, which is due to the oxidation of their interfaces by oxygen and water and the photodegradation of the active layers.[5–9] In order to increase their stability, various methods have been employed. Oxygen and water diffusion barriers were employed to protect the interfaces of OSCs from degradation.[10,11] Recently, an OSC with an inverted structure was developed, which was shown to be more stable than conventional solar cells.[12,13] In conventional structures, holes are injected into the transparent conducting electrode (TCE) and, in many cases, materials such as PEDOT: PSS with a high capability of hole injection are deposited on the TCE. In the inverted structure, in contrast, electrons are injected into the TCE. In the past, the improved stability of the inverted structure based on n-type ZnO layers on a TCE with respect to that of conventional OSCs was reported.[12,13] Carbon nanotubes (CNTs) have been widely used in photovoltaic systems. In many cases, bare CNTs and CNTs combined with C60, semiconductive and metallic nanoparticles were incorporated in the active layers, and the high capability of CNTs for electron transport has been exploited.[14–20] On the other hand,

W. H. Shim, S.-Y. Park, M. Y. Park, Y. T. Kim, Dr. J.-W. Kang, Dr. K. H. Lee, Dr. Y. Jeong, Dr. D. C. Lim Materials Processing Division Korea Institute of Materials Science 641–010 Changwon, Republic of Korea E-mail: [email protected] W. H. Shim, H. O. Seo, K.-D. Kim, Prof. Dr. Y. D. Kim Department of Chemistry Sungkyunkwan University 440–746 Suwon, Republic of Korea E-mail: [email protected] Y. T. Kim, Prof. Y. D. Kim School of Materials Science and Engineering Pusan National University 609–735 Pusan, Republic of Korea S.-Y. Park, M. Y. Park Division of Applied Chemical Engineering Department of Polymer Engineering Pukyong National University 608–739 Pusan, Republic of Korea

DOI: 10.1002/adma.201003083

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due to their semitransparent nature, CNTs have also been regarded as an alternative to indium tin oxide (ITO), which is one of the materials the most extensively used as the TCE.[21–23] In the present work, a novel fabrication process for the efficient incorporation of single-walled carbon nanotubes (SWCNTs) into ZnO layers was developed, and this technique was used for constructing OSCs having a ZnO-based inverted structure with embedded SWCNTs. Figure 1 displays a schematic description of the structure of the OSCs used in the present work. The photovoltaic devices consisted of a stack of ITO coated glass, ZnO (or ZnO+SWCNT), active layers consisting of P3HT and PCBM (1:0.7), and a NiO/Ag electrode. Valence band edge of NiO is located 5 eV below the vacuum level, which is lower than that of P3HT.[24] On the other hand, 5eV is higher than the work function of Ag (4.8eV) so that NiO can facilitate hole transfer from P3HT to Ag.[25] Figure 2 shows the Atomic Force Microscopy (AFM) images of the ZnO and ZnO+SWCNT structures deposited on ITO glasses. The ZnO+SWCNT structure showed a significantly higher roughness and larger mean grain size than the pure ZnO structure. The surface roughness in Figure 2a and b were estimated to be 4.23 and 8.86nm, respectively, i.e. by SWCNT the surface roughness was significantly increased. These results confirm that ZnO and ZnO+SWCNT exhibit completely different geometric structures, that is, the SWCNTs were efficiently combined with the ZnO. As shown in Figure 3a, the performance of the photovoltaic cell consisting of the ZnO+SWCNT layers was found to be much better than that of the pure ZnO structure. The open-circuit voltage, short-circuit current, fill-factor, and power conversion efficiency of the pure ZnO-cell were 0.56 V,

Figure 1. Schematic description of the OSCs with an inverted structure fabricated in the present work.

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Figure 3. a) The photovoltaic performances of the OSCs with pure ZnO and ZnO+SWCNT layers are compared. b) The carrier mobilities of the bare and SWCNT-incorporated ZnO layers on ITO are compared.

Figure 2. AFM images of a) pure ZnO layers and b) ZnO+SWCNT layers deposited on ITO.

5.048 mA cm−2, 0.414, and 1.173%, respectively. By using the ZnO+SWCNT layers instead of bare ZnO, the short-circuit current and efficiency were increased by a factor ∼2, whereas the open-circuit voltage was virtually unchanged by the addition of the SWCNTs. By changing the method of deposition of the ZnO+SWCNT layers and the P3HT:PCBM ratio, a power conversion efficiency of 2.6% could be achieved (Supporting Information 6). The higher surface roughness of ZnO+SWCNT is related to a better performance of the photovoltaic cell (Figure 2, Supporting Information 14), since a higher surface roughness can result in a larger contact area with active layers. Beside the surface roughness, carrier (electron) mobility of ZnO/ITO electrode was increased by SWCNT (Figure 3b). For comparison multi-walled carbon nanotube (MWCNT) was incorporated in the inverted OSC instead of SWCNT. ZnO+MWCNT showed even higher surface roughness than that of ZnO+SWCNT; however, no significant improvement in the photovoltaic performance and carrier mobility was found by

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MWCNT (Supporting Information 8–10). Even though MWCNT shows a high conductivity, incorporation of MWCNT results in a larger thickness (<100 nm) of the ZnO-based film on ITO, and increase in the film thickness should be disadvantageous for carrier mobility and photovoltaic performances (Supporting Information 15). By using both SW and MWCNT, UV absorbance was increased (Supporting Information 7,9). Since only SWCNT enhanced photovoltaic performance, enhanced UV absorbance by CNT alone does not correlate to the increase in the photovoltaic performance. Various tests were performed to examine the stability of the devices prepared in the present work. The change in the photovoltaic performance as a function of time under the irradiation of UV light (2000 mJ cm−2) was measured for the two different devices based on the ZnO and ZnO+SWCNT layers, respectively. For the first 30minutes, a decrease in the power conversion efficiency by 13.0% was observed in the case of the ZnO-based device. When the ZnO+SWCNT layers were used instead of ZnO, a decrease in the power conversion efficiency by 6.4% was observed after 30minutes, suggesting that the device based on the ZnO+SWCNT showed higher stability (Figure 4). Probably, the ZnO+SWCNT also act as a UV protection medium, thus reducing the photodegradation of the active layers. In order to acquire information on the stability of the devices towards oxidation by oxygen and water penetrating into the interfaces, the changes in the short-circuit current of both devices were measured under atmospheric pressure for 5 days (Figure 5). The device with SWCNT showed a higher stability of the photovoltaic performance that the bare ZnO-based inverted OSC. It is worth mentioning that the conventional OSCs with

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Al electrodes became completely inactive within 2 days (Supporting Information 11).[12] Inverted OSC with MWCNT did not show such a high stability (Supporting Information 12). In summary, the power conversion efficiency of the ZnObased OSCs with the inverted structure was significantly enhanced by the incorporation of SWCNTs into the ZnO films. Use of SWCNTs provides increased surface roughness without significantly altering film thickness, increasing contact area between positive electrode and active layer. Carrier mobility of ZnO/ITO was enhanced by SWCNT. The OSCs fabricated in the present work were also shown to be highly resistant towards (photo-) degradation, suggesting that the strategy of solar cell fabrication introduced herein is of significant importance for various applications.

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For the preparation of the ZnO sol-gel solution, zinc acetate (16.46 mg) was dissolved in methoxyethanol (100 mL) using a magnetic stirrer. Ethanolamine (5 mL), a stabilizer of ZnO particles, was then added to the zinc acetate solution and the resulting solution was kept at 60 °C for 1 hour. As shown in Figure 1A of the supporting information, a transparent sol-gel solution was formed after these procedures. For the synthesis of the ZnO-SWCNT mixture, SWCNTs (P3-SWCNTs, Carbon solutions, Inc., 1 × 10−4g ml−1) were dispersed in the solution of 1 A using ultrasonic treatment. When this solution was kept for 1 hour, SWCNTs decorated with ZnO were selectively precipitated (1 B). Since SWCNT was already functionalized with –COOH groups, a selective precipitation with Zn2+ could take place. Centrifugal separation was performed at a speed of 15000 rpm for the purpose of selectively filtering the ZnO+SWCNT structures. When ZnO+SWCNT(5.6 mg) structures were added to the solution of 1 A and the solution was ultrasonicated, a transparent solution without any precipitate was obtained (1 C). In this solution, the ZnO-SWCNT structures were highly dispersed. The solutions of 1 A and C of the Supporting Information were used for the fabrication of the photovoltaic devices. These solutions were deposited onto the ITO-coated glasses by spin coating and the thicknesses of the ZnO or ZnO+SWCNT layers were 70 nm. For spin coating, 2000 rpm was used. Alternative to the spin coating, spray coating was also used for depositing ZnO+SWCNT layers. For the spray coating, the sample-nozzle distance, and voltage used were 10 cm and 12 kV, respectively, and the deposition was carried out for 30 sec with an injection rate of 0.1 μl min−1. After the deposition of these layers on the ITO-coated glass, the glass was treated at 300° under atmospheric conditions for 10 minutes. Then, a P3HT/PCBM mixture was deposited with a thickness of 500 nm using spin coating (400 rpm, 60 sec.). As solvent, DCB:CB (Dichlorobenzene:Chlorobenzene) mixture with a volume ratio of 3:2 was used (21mg of P3HT/PCBM per 1 mL of solvent). The photovoltaic cell was completed by additionally depositing a p-type semiconductor (NiO) using spin coating and an Ag electrode by the evaporation method. Before deposition of NiO, the active layers were annealed at 80 °C. For deposition of NiO, NiO powder (particle size < 50 nm; aldrich) was dispersed in isopropyl alcohol by ultrasonic treatment (300 W) (Supporting Information 13). The concentration of NiO solution was 1 × 10−3 M, and this solution was used for the spin coating of NiO with 1000 rpm. The thickness of NiO was 60 nm. It is worth emphasizing that, except for the electrode evaporation, all of the fabrication procedures of the present work took place in solution under atmospheric pressure. The final structure of the photovoltaic cell was annealed at 150 °C under atmospheric pressure. The active surface area of the cell was 0.38 cm2 and efficiencies of the solar cells with masks were tested using a solar simulator illuminated with a photointensity of 100 mW cm−2 (AM 1.5 spectrum). Hall effects measurements were performed using ECOPIA NMS-3000 for determining carrier mobility.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

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Figure 5. For the OSCs with ZnO (a) and ZnO+SWCNT (b) structures on ITO electrodes, the short-circuit currents were measured as a function of time, and no reduction of the Jsc value was found after 5 days in the presence of SWCNT.

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This research was supported by Korea Institute of Materials Science (KIMS) and a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea.

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Received: August 25, 2010 Published online: November 10, 2010

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