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Inverted-structure polymer solar cells fabricated by sequential spraying of electron-transport and photoactive layers
Organic Electronics 15 (2014) 2337–2345
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Inverted-structure polymer solar cells fabricated by sequential spraying of electron-transport and photoactive layers Hye-Yun Park a, Dongchan Lim b, Seung-Hwan Oh c, Phil-Hyun Kang c, Giseop Kwak d,⇑, Sung-Yeon Jang a,⇑ a
Department of Chemistry, Kookmin University, 861-1, Jeongneung-Dong, Seongbuk-Gu, Seoul 136-702, Republic of Korea Surface Technology Division, Korea Institute of Materials Science, Changwon 641-010, Republic of Korea c Radiation Research Division for Industry and Environment, Korea Atomic Energy Research Institute (KAERI), 29 Geumgu-gil, Jeongeup-si, Jeollabuk-do 580-185, Republic of Korea d School of Applied Chemical Engineering, Major in Polymer Science and Engineering, Kyungpook National University 1370 Sankyuk-dong, Buk-ku, Daegu 702–701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 9 May 2014 Received in revised form 28 June 2014 Accepted 29 June 2014 Available online 9 July 2014 Keywords: Spray method Inverted polymer solar cell Electron transport layer Sol–gel ZnO Solvent assisted treatment
a b s t r a c t Inverted-structure polymer solar cells (I-PSCs) containing sequentially sprayed electrontransporting layers (ETLs) and photoactive layers were fabricated. Low-temperature sol– gel-derived ZnO thin films were used as the ETLs and films of a poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend were used as the photoactive layers. Nanoripples-containing ZnO ETLs could be successfully fabricated by controlling the spraying rate of the ZnO precursor solution and the subsequent annealing conditions. The P3HT/PCBM active layers sprayed on the ZnO ETLs were optimized using a unique solvent-assisted post-deposition treatment, namely, the sprayed solvent overlayer (SSO) treatment. The power conversion efficiency (PCE) of the I-PSCs based on the optimized ETLs and active layers was as high as 3.55%, which is comparable to that reported for I-PSCs fabricated using the conventional spin-coating method. The sprayed I-PSCs also exhibited high environmental stability, maintaining 80% of their PCE even after 40 days of aging in air under ambient conditions without encapsulation. The I-PSCs based on the P3HT/PCBM photoactive layers optimized using the SSO treatment displayed much higher stability than those based on photoactive layers optimized using a conventional thermal annealing treatment. This result indicated that the SSO treatment is a suitable post-deposition treatment method for improving the morphological stability of P3HT/PCBM active layers. Further, the fabrication technique investigated in this study is a high-throughput low-temperature one and is suitable for fabricating high-stability PSCs. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Polymer-based solar cells (PSCs) are being considered as the next-generation solar cells, and their development and ⇑ Corresponding authors. Tel.: +82 (53) 950 7558; fax: +82 (53) 950 6623 (G. Kwak). Tel.: +82 (2) 910 5768; fax: +82 (2) 910 4415 (S.-Y. Jang). E-mail addresses: [email protected] (G. Kwak), [email protected] (S.-Y. Jang). http://dx.doi.org/10.1016/j.orgel.2014.06.036 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
commercialization should lead to the realization of lowcost, disposable, and flexible energy devices. Improving the power conversion efficiency (PCE) of PSCs has been a major research goal. The PCE of such devices has been improved continuously, to up to 9%, through the design of novel semiconducting materials such as low-band-gap polymers and fullerene derivatives [1]. The development of devices with PCE values as high as this have made it feasible to consider the commercialization of PSCs for various
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large-scale applications. However, in spite of these improvements, there remain a few obstacles to be overcome. These are the poor long-term stability of PSCs in ambient conditions and the infeasibility of fabricating high-PCE PSCs using industrial manufacturing techniques [2,3]. Conventional PSCs generally have a structure in which a hole-transporting layer (HTL), usually poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS), is coated on a layer of indium-doped tin oxide (ITO), and a low-work-function metal such as Al is used as a cathode. These normal PSCs is known to have low environmental stability because the oxidation of the low-work-function cathode (Al) and the degradation of ITO by the acidic PEDOT:PSS HTL [4–7]. One of the simple routes to enhance the stability is to invert the charge collection direction, in which an electron transporting layer (ETL), regularly wide band-gap metal oxides, was deposited on ITO and a relatively high-work-function metals such as Ag or Au were used as anodes. This type of cells, known as inverted structured PSCs (I-PSCs) could considerably improve the environmental stability of conventional PSCs [8,9]. While the spin-coating method has been used almost exclusively as the fabrication method of choice for PSCs, other methods that are compatible with high throughputs and continuous processing have also been suggested [10]. These include screen printing [11,12], inkjet printing [13], and spray-based methods [14–19]. In most studies on these techniques, the polymer active layers were deposited using a number of different methods. The performances of the devices based on the thus-deposited layers were then
compared with those of devices based on spin-coated layers, as the performance of PSCs is determined primarily by the characteristics of the active layer. Controlling the morphology of the active layer at macroscopic level, along with the internal nanomorphology of the layer, by choosing the appropriate solvents, controlling the concentrations of the solutions used, and adjusting the solvent evaporation kinetics have been areas of intensive focus. Further, most of these studies have investigated conventional-structured PSCs alone. In order to allow for the commercialization of PSCs, the development of high-throughput processing techniques for fabricating I-PSCs is essential. In I-PSCs, the charge-selective layer between the active layer and the electrodes plays a crucial role in determining the open-circuit voltage (VOC) and the fill factor (FF) of the device. The ability to fabricate these layers using different deposition methods is a prerequisite. Given that the purported flexibility of PSCs is one of their most attractive qualities, the ability to fabricate I-PSCs using a highthroughput process at a low temperature is therefore of great importance. In this study, we fabricated I-PSCs in which the ETL and the photoactive layer were fabricated by a spray-based method. Sol–gel-derived ZnO thin films were used as the ETLs, and films of a poly(3-hexylthiophene) (P3HT)/[6,6]phenyl-C61-butyric acid methyl ester (PCBM) blend were used as the photoactive layers. The two types of layers were deposited using a gas-assisted spraying technique at relatively low temperatures. The embedded ZnO ETLs, whose surfaces contained nanoripples, could be deposited
Fig. 1. (a) Schematic showing the fabrication of I-PSCs by the spraying method. AFM images of the as-sprayed precursor films deposited using different spraying rates: (b) 1.2 ml min 1 and (c) 1.8 ml min 1. The root mean squares roughness (RRMS) values of the films were 5.5 nm and 2.89 nm, respectively.
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successfully by controlling the spraying rate of the ZnO precursor solution and the subsequent annealing conditions for the gelation process. The sprayed photoactive layers were optimized in terms of their nanomorphology by using a unique solvent-assisted post-deposition treatment technique, namely, the sprayed solvent overlayer (SSO) method. The optimized I-PSCs based on the sequentially sprayed ETLs and photoactive layers exhibited a PCE of 3.55% (VOC = 0.58 V, short-circuit current (JSC) = 10.12 mA cm 2, FF = 0.60), which is comparable to that of I-PSCs fabricated through the conventional spin-coating method [8,9,20–24]. The environmental stability of the sprayed I-PSCs was examined. It was found that the I-PSCs containing the active layers treated by the SSO method were more stable those based on active layers treated by a conventional thermal annealing (TA). This suggested that the SSO method is a more beneficial post-deposition treatment technique for enhancing the morphological stability of the active layers. 2. Results and discussion Fig. 1a shows a schematic description of the spraying technique used for depositing the ZnO ETLs and the P3HT/ PCBM photoactive layers; the structure of the fabricated
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I-PSCs is also shown. The spray gun used was composed of three distinct inlets: (i) a nitrogen gas inlet for atomization, (ii) a pressure-controlled lift nozzle, and (iii) an injection nozzle for the solutions of the materials to be deposited. To fabricate the ZnO ETLs, a dilute precursor solution (0.45 M of zinc acetate dehydrate) was sprayed on the ITO/glass substrate (sheet resistance <20 ohm/sq) from the injection nozzle, powered by N2 gas at 0.35 kgf cm 2. The sprayed precursor layers were annealed at 200 °C. When the precursor solution was sprayed at too low rates (<0.8 ml min 1), continuous films were not formed, because the atomized droplets dried completely before colliding with the substrate, resulting in poor droplet–substrate adhesion. However, at too high rates (>1.8 ml min 1), the films began to dewet, owing to the presence of excessive residual solvent for a prolonged duration. Continuous ZnO films were obtained when the spray rate of the precursor solutions was within a particular range (0.8–1.8 ml min 1). Thus, the surface morphology of the resulting ZnO films was significantly affected by the spraying rate of the precursor solution. The root mean square roughness (RRMS) values of the ZnO films obtained after subsequent annealing the precursor layers sprayed at rates of 1.2 ml min 1 and 1.8 ml min 1 were 5.5 nm and 2.9 nm, respectively (Fig. 1b and c). The surface
Fig. 2. SEM images of the ZnO films obtained by the (a) dynamic annealing (DA) and (b) static annealing (SA) of the precursor films deposited by the conventional spin-coating method. (c) Schematics describing the DA and SA modes. (d) Schematics showing the effects of the precursor spraying rate on the formation of nanoripples on the surfaces of the ZnO films during the annealing process (DA mode).
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Fig. 3. Topographic AFM images of the ZnO films prepared from the sprayed precursor films. The ZnO films were fabricated under the following conditions (annealing modes/precursor spraying rate): (a) SA/1.8 ml min 1, (b) DA/1.8 ml min 1, (c) SA/1.2 ml min 1, (d) DA/1.2 ml min 1, (e) DA/1.2 ml min 1, and (f) DA/0.8 ml min 1.
morphology of the ZnO films was also influenced by the annealing conditions. We annealed the sprayed precursor films using two different annealing modes (Fig. 2c): (i) a dynamic annealing (DA) mode, in which the films were gradually heated to 200 °C from room temperature at a heating rate of 40 °C min 1 and (ii) a static annealing (SA) mode, in which the films were heated at 200 °C for 5 min. It has been reported that ZnO films annealed using the DA mode usually contain nanoscaled ripple-like structures on their surfaces, whereas those annealed using the SA mode have a smoother, featureless surface [22,25,26]. This is because the DA mode allows for greater structural relaxation of the precursor molecules, permitting them to assemble and form nanoripples (see the scheme in Fig. 2d) [22,27]. On the other hand, in the SA mode, the solvent is instantly evaporated before the molecules can start to assemble, resulting in smooth domain edges [28].
Fig. 4. XRD patterns of the sprayed P3HT/PCBM blend films after they had been subjected to various post-deposition treatments.
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Fig. 5. Performances of the sprayed I-PSCs based on the different ZnO ETLs and P3HT/PCBM active layers: (a) J–V characteristics of the I-PSCs based on the optimized sprayed ZnO ETLs and the sprayed active layers subjected to different post-deposition treatments. (b) J–V characteristics of the I-PSCs based on the optimized sprayed active layers and the various ZnO ETLs prepared by the annealing of the sprayed precursor films. The spraying rate of the precursor solution and annealing modes for the resulting films were varied to optimize device performance.
The formation of nanoripples on the surfaces of ZnO films has been an effective strategy for improving their performance as ETLs for I-PSCs, owing to the increase in the contact area between the ETL and the photoactive layer [22,25,29]. However, in all the previous studies on this topic, the precursor solutions were deposited using the conventional spin-coating method, which usually yields smooth precursor films [22,25,30]. The field-emission
scanning electron microscopy (FESEM) images of the ZnO films obtained by the dynamic annealing (DA, Fig. 2a) and static annealing (SA, Fig. 2b) of the precursor films deposited by the conventional spin-coating method were in keeping with the results reported in the literature [22,30]. When the spin-coating method was used, precursor layers of high uniformity could be deposited easily because the evaporation of the solvent is relatively even throughout the entire film. In contrast, spray deposition was not a very suitable deposition technique, in that the deposition of the precursor material occurred through the sequential piling-up of nearly dried droplets. This often resulted in the formation of coffee-stain-ring-like droplet edges [31,32]. Recently, there have been attempts to fabricate I-PSCs based on ZnO ETLs using the spraying method [18,33]. However, only ZnO ETLs with flat surfaces could be obtained even after annealing at high temperatures (>300 °C). Thus, if it were to become possible to fabricate ZnO ETLs with nanorippled surfaces using the spraying method, it would result in further improvements in the performance of sprayed I-PSCs. Tapping-mode atomic force microscopy (AFM) images of the ZnO films obtained using various fabrication conditions (spraying rates and annealing modes) are shown in Fig. 3. The fabrication conditions had a significant effect on the surface morphologies of the ZnO films. The use of the SA mode resulted in featureless, flat ZnO films at all spraying rates (Fig. 3a and c). However, when the DA mode was used, the surface morphology was determined by the spraying rate. In the case of the samples prepared at low spraying rates (<1.2 ml min 1), the formation of nanoripples was significantly suppressed. Nanoripples were observed only in a few regions, and most of them were relatively underdeveloped (Fig. 3d–f). In the ZnO films deposited at a relatively high rate (1.8 ml min 1), nanoripples were formed throughout the entire films (Fig. 3b). The nanoripples were 200 nm wide and 20 nm high; these dimensions are similar to those reported for spin-coated films [18,33]. The RRMS values of the ZnO films formed using the DA and SA modes at a spraying rate of 1.8 ml min 1 were 5.8 nm and 1.2 nm, respectively. These values indicated that nanoripples-containing ZnO films could be obtained using sprayed precursor layers when the spraying rate and the choice of the annealing mode were optimized. Next, the P3HT/PCBM photoactive layers were sequentially sprayed on the ZnO ETLs. A P3HT/PCBM blend solution (5 mg ml 1) was sprayed at a rate of 0.18 ml min 1 using pressurized N2. The spray nozzle tip was kept 25–30 cm from the substrate. The thickness of the
Table 1 Results of analysis of J–V characteristics of the fabricated I-PSCs. Spraying rate for ZnO solution (ml/min)
Annealing ZnO ETL
Post treatment for photoactive layers
VOC (V)
Jsc (mA/ cm2)
FF (%)
PCE (%)
RSh (X cm2)
RS (X cm2)
1.8
DA DA DA SA DA DA
SSO,TA SSO TA SSO,TA SSO,TA SSO,TA
0.58 0.59 0.57 0.58 0.57 0.57
10.12 9.11 9.02 10.13 9.32 8.61
60.40 59.46 52.83 55.57 60.22 58.4
3.55 3.18 2.73 3.25 3.21 2.86
729 578 377 541 633 621
10 10.9 11.9 10.4 10.1 12.1
1.8 1.2 0.8
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resulting photoactive layers was 250 nm. We had previously reported the fabrication of conventional-structured PSCs that contained sprayed photoactive layers [19,20]. In that study, a P3HT/PCBM blend solution was sprayed on films of PEDOT:PSS, a very popular choice of material for HTLs. The surface morphology and internal nanomorphology of the films had been optimized through external solvent-assisted post-deposition treatments. In this work, we deposited the active layers on the sprayed ZnO ETLs and attempted to optimize the nanomorphology of the active layers using a solvent-assisted treatment technique, that is, the SSO treatment [20]. 1,2-dichlorobenzene (DCB) was sprayed in a short burst (5 s) onto the P3HT/PCBM active layers. The SSO method has been demonstrated to be a simple and fast strategy for optimizing the surface morphology and internal nanomorphology of sprayed polymer active layers under ambient conditions at room temperature. The active layers treated using the SSO method exhibited an increased donor/acceptor interfacial area, which resulted in greater exciton dissociation and more bicontinuous charge-transport pathways [20,34]. The X-ray diffraction patterns shown in Fig. 4 indicated that the crystallinity of P3HT increased after the SSO treatment. The self-assembled crystalline P3HT, evaluated on the basis of the relative intensity of the (1 0 0) plane peak, which occurred at 2h = 5.4° in the out-of-plane XRD
spectrum, was similar to that noticed in conventionalstructured P3HT/PCBM based PSCs [17,19,20,35]. The crystallinity of P3HT could be increased further by subjecting the active layers to TA, which is a commonly used method for optimizing P3HT/PCBM-based PSCs. This increase in the crystallinity of P3HTshould have a significant effect on the performance of I-PSCs [17,19,20,35]. Fig. 5a shows the J–V characteristics of the I-PSCs based on the sequentially sprayed ZnO ETLs and P3HT/PCBM active layers. The J–V curves were measured under AM 1.5G one sun illumination (100 mW cm 2). That the active layers were indeed optimized by the post-deposition treatments (SSO and TA) was evident from the PCE of the resulting I-PSCs (3.55%, VOC = 0.58 V, JSC = 10.12 mA cm 2, FF = 0.60). The SSO treatment improved the cell performance considerably (PCE = 3.18%), while the TA treatment resulted in further improvements, as evidenced by the results of the XRD analyses. The various parameters of the fabricated cells are summarized in Table 1. The effects of the post-deposition treatments on the characteristics of the P3HT/PCBM active layers were investigated further; these are discussed later in the article. The effects of the various ZnO ETLs on cell performance were investigated using the optimized P3HT/PCBM active layers (Fig. 5b). As shown in Fig. 3, the surface morphology of the ZnO ETLs was affected by the spraying rate of the
Fig. 6. Normalized device performances of the I-PSCs after being stored for 40 days in air under ambient conditions. Three types of I-PSCs based on P3HT/ PCBM active layers subjected to different post-deposition treatments were tested. (d: SSO, j: TA, and N: SSO/TA.)
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precursor solutions and the choice of the annealing mode. The I-PSCs based on the sprayed ZnO ETLs containing uniformly distributed nanoripples exhibited the highest performance (PCE = 3.55%, Table 1). As the spraying rate was increased from 0.8 ml min 1 to 1.8 ml min 1, the performance of the ZnO ETLs continued to improve. For the same spraying rate, the ZnO ETLs annealed using the DA mode exhibited better performance than did those annealed using the SA mode; this was owing to the presence of well-formed nanoripples in the case of the former (see the results listed in Table 1). Finally, the long-term stability of the I-PSCs based on the sequentially sprayed ZnO ETLs and P3HT/PCBM active layers was evaluated in air under ambient conditions. After 40 days of aging, the PCE values of the optimized I-PSCs decreased by only <25%, indicating that the devices had high intrinsic environmental stability (Fig. 6). It should be noted that the stability of the I-PSCs containing active layers subjected only to the SSO treatment exhibited better long-term stability than those subjected to the TA treatment as well as those subjected to both treatments. This was an intriguing phenomenon and one not noticed in our previous studies on conventional-structured PSCs fabricated by the spraying method, probably because these PSCs had exhibited poor stabilities [20]. The effects of the TA treatment on the internal nanomorphology of the P3HT/PCBM active layers have been investigated extensively [16,36–38]. Recently, Wang et al. reported that TA can enhance the mesoscale segregation of PCBM in amorphous PCBM-rich/P3HT domains, in addition to accelerating macroscale PCBM aggregation. This increase in the degree of phase segregation can potentially lower the stability of the active layers [38]. The stability values shown in Fig. 6 indicated that the SSO treatment resulted in the formation of a more stable internal nanomorphology in the case of the P3HT/PCBM active layers than did the TA treatment, retarding the segregation of the constituent moieties. Thus, it can be concluded that the SSO treatment is a more beneficial one for improving the long-term stability of I-PSCs.
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conditions. The I-PSCs based on the active layers subjected to the SSO treatment showed improved stability (20% decrease) when compared to those subjected to the TA treatment (35% decrease). This indicated that the SSO treatment is a more beneficial post-deposition treatment for enhancing the morphological stability of P3HT/PCBM active layers. 4. Experimental 4.1. Preparation of sprayed ZnO ETLs The ZnO precursor was prepared by dissolving zinc acetate dihydrate (Zn(CH3COO)22H2O)/ethanolamine (NH2CH2CH2OH) in 2-methoxyethanol (0.45 M) under vigorous stirring for 12 h at 60 °C in air, which resulted in a hydrolysis reaction. The ZnO precursor solution was stored for 24 h under ambient conditions and then diluted with methanol (ZnO sol–gel precursor:methanol = 1:5). Next, the diluted ZnO precursor solution was sprayed on indium-doped tin oxide-coated glass (ITO/glass) substrates using a gas-spray gun, which was powered by pressurized N2 gas at 0.35 kgf cm 2. The distance between the ITO/ glass substrates and the air-spray gun was 40 cm. The ITO/glass substrates (sheet resistance <20 X/h, transparency >85%) were subjected to an ultraviolet (UV)/ozone treatment for 20 min before use. The sol–gel precursor solution-feeding rate was adjusted to 0.8, 1.2, and 1.8 ml s 1 to control the amount of solvent in the sprayed films. The nanoripples-containing ZnO films were obtained through the DA treatment, which involved increasing the temperature of the coated substrates from 25 to 200 °C and immediately removing them from the hot plate as soon as the temperature reached 200 °C. In the case of the planar ZnO films, the substrates were placed onto the hot plate, which was heated to 200 °C, and kept there for 5 min. The thickness of the ZnO films was approximately 40–50 nm. 4.2. Fabrication of I-PSCs
3. Conclusions We fabricated I-PSCs in which the ETLs and the polymer active layers were sequentially deposited using the spraying method. Sol–gel-derived ZnO thin films deposited by spraying were used as the ETLs, and their characteristics were optimized in terms of the spraying and annealing conditions. ZnO ETLs with a desirable surface morphology, that is, films whose surfaces were uniformly covered with nanoripples, could be deposited by using the correct spraying rate (1.8 ml min 1) and annealing mode (DA mode). The sprayed P3HT/PCBM active layers were optimized by being subjected to the SSO treatment. The highest PCE for the I-PSCs based on the nanoripples-containing ZnO ETLs and optimized active layers was 3.55%, which is comparable to that of I-PSCs fabricated by the conventional spin-coating method. The sprayed PSCs exhibited high environmental stability, exhibiting a decrease of only <25% in the PCE after 40 days of aging in air under ambient
To prepare the blend solution, 1 mg of P3HT (Rieke Metals) and 1 mg of PC60BM (Nano C) were dissolved in 1 ml of chlorobenzene (CB). The solutions were stirred for more than 5 h at room temperature before use. A commercial spray gun assembly (Lumina automatic spray gun, nozzle size: 1.0 mm) was used to deposit the active materials and the agents used in the SSO treatment. During the spraying process, the P3HT/PCBM blend solution was loaded into a fluid cup and sprayed onto the ZnO-coated ITO/glass substrate at a rate of 0.18 ml min 1 through the air-spray gun, which was powered by pressurized N2 gas at 0.8 kgf cm 2. The thickness of the photoactive film was 250 nm. The spray nozzle tip was kept 25–30 cm from the substrate during the deposition of the active layers. For the SSO treatment, 1,2-dichlorobenzene (DCB) was sprayed onto the active layers for 5 s at a rate of approximately 2 ml min 1 through the g-spray nozzle. The distance between the substrates and the air-spray nozzle was 30 cm. After the spraying of DCB on the photoactive
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films, the wet films were left for 10 min to allow the solvent to evaporate slowly. During the TA treatment, the devices were thermally annealed at 135 °C for 10 min in air, before the deposition of the anode. The coated samples were then transferred to a vacuum thermal evaporation chamber for the deposition of the Ag (1000 Å) top electrodes. The Ag layer was deposited using a shadow mask under high vacuum (10 7 Torr.). The active area of the devices was 0.09 cm2. 4.3. Device characterization The J–V characteristics of the devices were measured using a Keithley 2400 source unit. A class-A solar simulator with a 150 W Xenon lamp (Newport) served as the light source, and the light intensity was adjusted using a KG-5 filter-covered monosilicon detector calibrated by the National Renewable Energy Laboratory (NREL) in order to minimize the spectral mismatch. The illumination used had an intensity of approximately AM 1.5 G one sun. 4.4. Morphology characterization of ZnO and P3HT/PC60BM films The surface morphologies of the ZnO films on the ITO/ glass substrates were characterized using AFM (SPA 300 HV, Seiko Instrument Inc.) and SEM (SUPRA 55VP, Carl Zeiss); the accelerating voltage was 2 kV. A surface profilometer (Alpha-Step IQ Profilers) was used to measure the film thicknesses. The XRD patterns of the P3HT/PC60BM films were obtained using a high-resolution X-ray diffractometer (RIGAKU, Cu anode, 40 kV–200 mA). Acknowledgments The authors acknowledge financial support from the Basic Science Research Program and Nuclear R&D program (2014M2A2A6021006) through the National Research Foundation of Korea (NRF, 2012045675), and New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20133030000210). Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.orgel.2014.06.036. References [1] H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, G. Li, Polymer solar cells with enhanced open-circuit voltage and efficiency, Nat. Photon. 3 (2009) 649. [2] Y.Y. Liang, L.P. Yu, A new class of semiconducting polymers for bulk heterojunction solar cells with exceptionally high performance, Acc. Chem. Res. 43 (2010) 1227. [3] C.J. Brabec, S. Gowrisanker, J.J.M. Halls, D. Laird, S.J. Jia, S.P. Williams, Polymer–fullerene bulk-heterojunction solar cells, Adv. Mater. 22 (2010) 3839.
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Inverted-structure polymer solar cells fabricated by sequential spraying of electron-transport and photoactive layers Hye-Yun Park a, Dongchan Lim b, Seung-Hwan Oh c, Phil-Hyun Kang c, Giseop Kwak d,⇑, Sung-Yeon Jang a,⇑ a
Department of Chemistry, Kookmin University, 861-1, Jeongneung-Dong, Seongbuk-Gu, Seoul 136-702, Republic of Korea Surface Technology Division, Korea Institute of Materials Science, Changwon 641-010, Republic of Korea c Radiation Research Division for Industry and Environment, Korea Atomic Energy Research Institute (KAERI), 29 Geumgu-gil, Jeongeup-si, Jeollabuk-do 580-185, Republic of Korea d School of Applied Chemical Engineering, Major in Polymer Science and Engineering, Kyungpook National University 1370 Sankyuk-dong, Buk-ku, Daegu 702–701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 9 May 2014 Received in revised form 28 June 2014 Accepted 29 June 2014 Available online 9 July 2014 Keywords: Spray method Inverted polymer solar cell Electron transport layer Sol–gel ZnO Solvent assisted treatment
a b s t r a c t Inverted-structure polymer solar cells (I-PSCs) containing sequentially sprayed electrontransporting layers (ETLs) and photoactive layers were fabricated. Low-temperature sol– gel-derived ZnO thin films were used as the ETLs and films of a poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend were used as the photoactive layers. Nanoripples-containing ZnO ETLs could be successfully fabricated by controlling the spraying rate of the ZnO precursor solution and the subsequent annealing conditions. The P3HT/PCBM active layers sprayed on the ZnO ETLs were optimized using a unique solvent-assisted post-deposition treatment, namely, the sprayed solvent overlayer (SSO) treatment. The power conversion efficiency (PCE) of the I-PSCs based on the optimized ETLs and active layers was as high as 3.55%, which is comparable to that reported for I-PSCs fabricated using the conventional spin-coating method. The sprayed I-PSCs also exhibited high environmental stability, maintaining 80% of their PCE even after 40 days of aging in air under ambient conditions without encapsulation. The I-PSCs based on the P3HT/PCBM photoactive layers optimized using the SSO treatment displayed much higher stability than those based on photoactive layers optimized using a conventional thermal annealing treatment. This result indicated that the SSO treatment is a suitable post-deposition treatment method for improving the morphological stability of P3HT/PCBM active layers. Further, the fabrication technique investigated in this study is a high-throughput low-temperature one and is suitable for fabricating high-stability PSCs. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Polymer-based solar cells (PSCs) are being considered as the next-generation solar cells, and their development and ⇑ Corresponding authors. Tel.: +82 (53) 950 7558; fax: +82 (53) 950 6623 (G. Kwak). Tel.: +82 (2) 910 5768; fax: +82 (2) 910 4415 (S.-Y. Jang). E-mail addresses: [email protected] (G. Kwak), [email protected] (S.-Y. Jang). http://dx.doi.org/10.1016/j.orgel.2014.06.036 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
commercialization should lead to the realization of lowcost, disposable, and flexible energy devices. Improving the power conversion efficiency (PCE) of PSCs has been a major research goal. The PCE of such devices has been improved continuously, to up to 9%, through the design of novel semiconducting materials such as low-band-gap polymers and fullerene derivatives [1]. The development of devices with PCE values as high as this have made it feasible to consider the commercialization of PSCs for various
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large-scale applications. However, in spite of these improvements, there remain a few obstacles to be overcome. These are the poor long-term stability of PSCs in ambient conditions and the infeasibility of fabricating high-PCE PSCs using industrial manufacturing techniques [2,3]. Conventional PSCs generally have a structure in which a hole-transporting layer (HTL), usually poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS), is coated on a layer of indium-doped tin oxide (ITO), and a low-work-function metal such as Al is used as a cathode. These normal PSCs is known to have low environmental stability because the oxidation of the low-work-function cathode (Al) and the degradation of ITO by the acidic PEDOT:PSS HTL [4–7]. One of the simple routes to enhance the stability is to invert the charge collection direction, in which an electron transporting layer (ETL), regularly wide band-gap metal oxides, was deposited on ITO and a relatively high-work-function metals such as Ag or Au were used as anodes. This type of cells, known as inverted structured PSCs (I-PSCs) could considerably improve the environmental stability of conventional PSCs [8,9]. While the spin-coating method has been used almost exclusively as the fabrication method of choice for PSCs, other methods that are compatible with high throughputs and continuous processing have also been suggested [10]. These include screen printing [11,12], inkjet printing [13], and spray-based methods [14–19]. In most studies on these techniques, the polymer active layers were deposited using a number of different methods. The performances of the devices based on the thus-deposited layers were then
compared with those of devices based on spin-coated layers, as the performance of PSCs is determined primarily by the characteristics of the active layer. Controlling the morphology of the active layer at macroscopic level, along with the internal nanomorphology of the layer, by choosing the appropriate solvents, controlling the concentrations of the solutions used, and adjusting the solvent evaporation kinetics have been areas of intensive focus. Further, most of these studies have investigated conventional-structured PSCs alone. In order to allow for the commercialization of PSCs, the development of high-throughput processing techniques for fabricating I-PSCs is essential. In I-PSCs, the charge-selective layer between the active layer and the electrodes plays a crucial role in determining the open-circuit voltage (VOC) and the fill factor (FF) of the device. The ability to fabricate these layers using different deposition methods is a prerequisite. Given that the purported flexibility of PSCs is one of their most attractive qualities, the ability to fabricate I-PSCs using a highthroughput process at a low temperature is therefore of great importance. In this study, we fabricated I-PSCs in which the ETL and the photoactive layer were fabricated by a spray-based method. Sol–gel-derived ZnO thin films were used as the ETLs, and films of a poly(3-hexylthiophene) (P3HT)/[6,6]phenyl-C61-butyric acid methyl ester (PCBM) blend were used as the photoactive layers. The two types of layers were deposited using a gas-assisted spraying technique at relatively low temperatures. The embedded ZnO ETLs, whose surfaces contained nanoripples, could be deposited
Fig. 1. (a) Schematic showing the fabrication of I-PSCs by the spraying method. AFM images of the as-sprayed precursor films deposited using different spraying rates: (b) 1.2 ml min 1 and (c) 1.8 ml min 1. The root mean squares roughness (RRMS) values of the films were 5.5 nm and 2.89 nm, respectively.
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successfully by controlling the spraying rate of the ZnO precursor solution and the subsequent annealing conditions for the gelation process. The sprayed photoactive layers were optimized in terms of their nanomorphology by using a unique solvent-assisted post-deposition treatment technique, namely, the sprayed solvent overlayer (SSO) method. The optimized I-PSCs based on the sequentially sprayed ETLs and photoactive layers exhibited a PCE of 3.55% (VOC = 0.58 V, short-circuit current (JSC) = 10.12 mA cm 2, FF = 0.60), which is comparable to that of I-PSCs fabricated through the conventional spin-coating method [8,9,20–24]. The environmental stability of the sprayed I-PSCs was examined. It was found that the I-PSCs containing the active layers treated by the SSO method were more stable those based on active layers treated by a conventional thermal annealing (TA). This suggested that the SSO method is a more beneficial post-deposition treatment technique for enhancing the morphological stability of the active layers. 2. Results and discussion Fig. 1a shows a schematic description of the spraying technique used for depositing the ZnO ETLs and the P3HT/ PCBM photoactive layers; the structure of the fabricated
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I-PSCs is also shown. The spray gun used was composed of three distinct inlets: (i) a nitrogen gas inlet for atomization, (ii) a pressure-controlled lift nozzle, and (iii) an injection nozzle for the solutions of the materials to be deposited. To fabricate the ZnO ETLs, a dilute precursor solution (0.45 M of zinc acetate dehydrate) was sprayed on the ITO/glass substrate (sheet resistance <20 ohm/sq) from the injection nozzle, powered by N2 gas at 0.35 kgf cm 2. The sprayed precursor layers were annealed at 200 °C. When the precursor solution was sprayed at too low rates (<0.8 ml min 1), continuous films were not formed, because the atomized droplets dried completely before colliding with the substrate, resulting in poor droplet–substrate adhesion. However, at too high rates (>1.8 ml min 1), the films began to dewet, owing to the presence of excessive residual solvent for a prolonged duration. Continuous ZnO films were obtained when the spray rate of the precursor solutions was within a particular range (0.8–1.8 ml min 1). Thus, the surface morphology of the resulting ZnO films was significantly affected by the spraying rate of the precursor solution. The root mean square roughness (RRMS) values of the ZnO films obtained after subsequent annealing the precursor layers sprayed at rates of 1.2 ml min 1 and 1.8 ml min 1 were 5.5 nm and 2.9 nm, respectively (Fig. 1b and c). The surface
Fig. 2. SEM images of the ZnO films obtained by the (a) dynamic annealing (DA) and (b) static annealing (SA) of the precursor films deposited by the conventional spin-coating method. (c) Schematics describing the DA and SA modes. (d) Schematics showing the effects of the precursor spraying rate on the formation of nanoripples on the surfaces of the ZnO films during the annealing process (DA mode).
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Fig. 3. Topographic AFM images of the ZnO films prepared from the sprayed precursor films. The ZnO films were fabricated under the following conditions (annealing modes/precursor spraying rate): (a) SA/1.8 ml min 1, (b) DA/1.8 ml min 1, (c) SA/1.2 ml min 1, (d) DA/1.2 ml min 1, (e) DA/1.2 ml min 1, and (f) DA/0.8 ml min 1.
morphology of the ZnO films was also influenced by the annealing conditions. We annealed the sprayed precursor films using two different annealing modes (Fig. 2c): (i) a dynamic annealing (DA) mode, in which the films were gradually heated to 200 °C from room temperature at a heating rate of 40 °C min 1 and (ii) a static annealing (SA) mode, in which the films were heated at 200 °C for 5 min. It has been reported that ZnO films annealed using the DA mode usually contain nanoscaled ripple-like structures on their surfaces, whereas those annealed using the SA mode have a smoother, featureless surface [22,25,26]. This is because the DA mode allows for greater structural relaxation of the precursor molecules, permitting them to assemble and form nanoripples (see the scheme in Fig. 2d) [22,27]. On the other hand, in the SA mode, the solvent is instantly evaporated before the molecules can start to assemble, resulting in smooth domain edges [28].
Fig. 4. XRD patterns of the sprayed P3HT/PCBM blend films after they had been subjected to various post-deposition treatments.
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Fig. 5. Performances of the sprayed I-PSCs based on the different ZnO ETLs and P3HT/PCBM active layers: (a) J–V characteristics of the I-PSCs based on the optimized sprayed ZnO ETLs and the sprayed active layers subjected to different post-deposition treatments. (b) J–V characteristics of the I-PSCs based on the optimized sprayed active layers and the various ZnO ETLs prepared by the annealing of the sprayed precursor films. The spraying rate of the precursor solution and annealing modes for the resulting films were varied to optimize device performance.
The formation of nanoripples on the surfaces of ZnO films has been an effective strategy for improving their performance as ETLs for I-PSCs, owing to the increase in the contact area between the ETL and the photoactive layer [22,25,29]. However, in all the previous studies on this topic, the precursor solutions were deposited using the conventional spin-coating method, which usually yields smooth precursor films [22,25,30]. The field-emission
scanning electron microscopy (FESEM) images of the ZnO films obtained by the dynamic annealing (DA, Fig. 2a) and static annealing (SA, Fig. 2b) of the precursor films deposited by the conventional spin-coating method were in keeping with the results reported in the literature [22,30]. When the spin-coating method was used, precursor layers of high uniformity could be deposited easily because the evaporation of the solvent is relatively even throughout the entire film. In contrast, spray deposition was not a very suitable deposition technique, in that the deposition of the precursor material occurred through the sequential piling-up of nearly dried droplets. This often resulted in the formation of coffee-stain-ring-like droplet edges [31,32]. Recently, there have been attempts to fabricate I-PSCs based on ZnO ETLs using the spraying method [18,33]. However, only ZnO ETLs with flat surfaces could be obtained even after annealing at high temperatures (>300 °C). Thus, if it were to become possible to fabricate ZnO ETLs with nanorippled surfaces using the spraying method, it would result in further improvements in the performance of sprayed I-PSCs. Tapping-mode atomic force microscopy (AFM) images of the ZnO films obtained using various fabrication conditions (spraying rates and annealing modes) are shown in Fig. 3. The fabrication conditions had a significant effect on the surface morphologies of the ZnO films. The use of the SA mode resulted in featureless, flat ZnO films at all spraying rates (Fig. 3a and c). However, when the DA mode was used, the surface morphology was determined by the spraying rate. In the case of the samples prepared at low spraying rates (<1.2 ml min 1), the formation of nanoripples was significantly suppressed. Nanoripples were observed only in a few regions, and most of them were relatively underdeveloped (Fig. 3d–f). In the ZnO films deposited at a relatively high rate (1.8 ml min 1), nanoripples were formed throughout the entire films (Fig. 3b). The nanoripples were 200 nm wide and 20 nm high; these dimensions are similar to those reported for spin-coated films [18,33]. The RRMS values of the ZnO films formed using the DA and SA modes at a spraying rate of 1.8 ml min 1 were 5.8 nm and 1.2 nm, respectively. These values indicated that nanoripples-containing ZnO films could be obtained using sprayed precursor layers when the spraying rate and the choice of the annealing mode were optimized. Next, the P3HT/PCBM photoactive layers were sequentially sprayed on the ZnO ETLs. A P3HT/PCBM blend solution (5 mg ml 1) was sprayed at a rate of 0.18 ml min 1 using pressurized N2. The spray nozzle tip was kept 25–30 cm from the substrate. The thickness of the
Table 1 Results of analysis of J–V characteristics of the fabricated I-PSCs. Spraying rate for ZnO solution (ml/min)
Annealing ZnO ETL
Post treatment for photoactive layers
VOC (V)
Jsc (mA/ cm2)
FF (%)
PCE (%)
RSh (X cm2)
RS (X cm2)
1.8
DA DA DA SA DA DA
SSO,TA SSO TA SSO,TA SSO,TA SSO,TA
0.58 0.59 0.57 0.58 0.57 0.57
10.12 9.11 9.02 10.13 9.32 8.61
60.40 59.46 52.83 55.57 60.22 58.4
3.55 3.18 2.73 3.25 3.21 2.86
729 578 377 541 633 621
10 10.9 11.9 10.4 10.1 12.1
1.8 1.2 0.8
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resulting photoactive layers was 250 nm. We had previously reported the fabrication of conventional-structured PSCs that contained sprayed photoactive layers [19,20]. In that study, a P3HT/PCBM blend solution was sprayed on films of PEDOT:PSS, a very popular choice of material for HTLs. The surface morphology and internal nanomorphology of the films had been optimized through external solvent-assisted post-deposition treatments. In this work, we deposited the active layers on the sprayed ZnO ETLs and attempted to optimize the nanomorphology of the active layers using a solvent-assisted treatment technique, that is, the SSO treatment [20]. 1,2-dichlorobenzene (DCB) was sprayed in a short burst (5 s) onto the P3HT/PCBM active layers. The SSO method has been demonstrated to be a simple and fast strategy for optimizing the surface morphology and internal nanomorphology of sprayed polymer active layers under ambient conditions at room temperature. The active layers treated using the SSO method exhibited an increased donor/acceptor interfacial area, which resulted in greater exciton dissociation and more bicontinuous charge-transport pathways [20,34]. The X-ray diffraction patterns shown in Fig. 4 indicated that the crystallinity of P3HT increased after the SSO treatment. The self-assembled crystalline P3HT, evaluated on the basis of the relative intensity of the (1 0 0) plane peak, which occurred at 2h = 5.4° in the out-of-plane XRD
spectrum, was similar to that noticed in conventionalstructured P3HT/PCBM based PSCs [17,19,20,35]. The crystallinity of P3HT could be increased further by subjecting the active layers to TA, which is a commonly used method for optimizing P3HT/PCBM-based PSCs. This increase in the crystallinity of P3HTshould have a significant effect on the performance of I-PSCs [17,19,20,35]. Fig. 5a shows the J–V characteristics of the I-PSCs based on the sequentially sprayed ZnO ETLs and P3HT/PCBM active layers. The J–V curves were measured under AM 1.5G one sun illumination (100 mW cm 2). That the active layers were indeed optimized by the post-deposition treatments (SSO and TA) was evident from the PCE of the resulting I-PSCs (3.55%, VOC = 0.58 V, JSC = 10.12 mA cm 2, FF = 0.60). The SSO treatment improved the cell performance considerably (PCE = 3.18%), while the TA treatment resulted in further improvements, as evidenced by the results of the XRD analyses. The various parameters of the fabricated cells are summarized in Table 1. The effects of the post-deposition treatments on the characteristics of the P3HT/PCBM active layers were investigated further; these are discussed later in the article. The effects of the various ZnO ETLs on cell performance were investigated using the optimized P3HT/PCBM active layers (Fig. 5b). As shown in Fig. 3, the surface morphology of the ZnO ETLs was affected by the spraying rate of the
Fig. 6. Normalized device performances of the I-PSCs after being stored for 40 days in air under ambient conditions. Three types of I-PSCs based on P3HT/ PCBM active layers subjected to different post-deposition treatments were tested. (d: SSO, j: TA, and N: SSO/TA.)
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precursor solutions and the choice of the annealing mode. The I-PSCs based on the sprayed ZnO ETLs containing uniformly distributed nanoripples exhibited the highest performance (PCE = 3.55%, Table 1). As the spraying rate was increased from 0.8 ml min 1 to 1.8 ml min 1, the performance of the ZnO ETLs continued to improve. For the same spraying rate, the ZnO ETLs annealed using the DA mode exhibited better performance than did those annealed using the SA mode; this was owing to the presence of well-formed nanoripples in the case of the former (see the results listed in Table 1). Finally, the long-term stability of the I-PSCs based on the sequentially sprayed ZnO ETLs and P3HT/PCBM active layers was evaluated in air under ambient conditions. After 40 days of aging, the PCE values of the optimized I-PSCs decreased by only <25%, indicating that the devices had high intrinsic environmental stability (Fig. 6). It should be noted that the stability of the I-PSCs containing active layers subjected only to the SSO treatment exhibited better long-term stability than those subjected to the TA treatment as well as those subjected to both treatments. This was an intriguing phenomenon and one not noticed in our previous studies on conventional-structured PSCs fabricated by the spraying method, probably because these PSCs had exhibited poor stabilities [20]. The effects of the TA treatment on the internal nanomorphology of the P3HT/PCBM active layers have been investigated extensively [16,36–38]. Recently, Wang et al. reported that TA can enhance the mesoscale segregation of PCBM in amorphous PCBM-rich/P3HT domains, in addition to accelerating macroscale PCBM aggregation. This increase in the degree of phase segregation can potentially lower the stability of the active layers [38]. The stability values shown in Fig. 6 indicated that the SSO treatment resulted in the formation of a more stable internal nanomorphology in the case of the P3HT/PCBM active layers than did the TA treatment, retarding the segregation of the constituent moieties. Thus, it can be concluded that the SSO treatment is a more beneficial one for improving the long-term stability of I-PSCs.
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conditions. The I-PSCs based on the active layers subjected to the SSO treatment showed improved stability (20% decrease) when compared to those subjected to the TA treatment (35% decrease). This indicated that the SSO treatment is a more beneficial post-deposition treatment for enhancing the morphological stability of P3HT/PCBM active layers. 4. Experimental 4.1. Preparation of sprayed ZnO ETLs The ZnO precursor was prepared by dissolving zinc acetate dihydrate (Zn(CH3COO)22H2O)/ethanolamine (NH2CH2CH2OH) in 2-methoxyethanol (0.45 M) under vigorous stirring for 12 h at 60 °C in air, which resulted in a hydrolysis reaction. The ZnO precursor solution was stored for 24 h under ambient conditions and then diluted with methanol (ZnO sol–gel precursor:methanol = 1:5). Next, the diluted ZnO precursor solution was sprayed on indium-doped tin oxide-coated glass (ITO/glass) substrates using a gas-spray gun, which was powered by pressurized N2 gas at 0.35 kgf cm 2. The distance between the ITO/ glass substrates and the air-spray gun was 40 cm. The ITO/glass substrates (sheet resistance <20 X/h, transparency >85%) were subjected to an ultraviolet (UV)/ozone treatment for 20 min before use. The sol–gel precursor solution-feeding rate was adjusted to 0.8, 1.2, and 1.8 ml s 1 to control the amount of solvent in the sprayed films. The nanoripples-containing ZnO films were obtained through the DA treatment, which involved increasing the temperature of the coated substrates from 25 to 200 °C and immediately removing them from the hot plate as soon as the temperature reached 200 °C. In the case of the planar ZnO films, the substrates were placed onto the hot plate, which was heated to 200 °C, and kept there for 5 min. The thickness of the ZnO films was approximately 40–50 nm. 4.2. Fabrication of I-PSCs
3. Conclusions We fabricated I-PSCs in which the ETLs and the polymer active layers were sequentially deposited using the spraying method. Sol–gel-derived ZnO thin films deposited by spraying were used as the ETLs, and their characteristics were optimized in terms of the spraying and annealing conditions. ZnO ETLs with a desirable surface morphology, that is, films whose surfaces were uniformly covered with nanoripples, could be deposited by using the correct spraying rate (1.8 ml min 1) and annealing mode (DA mode). The sprayed P3HT/PCBM active layers were optimized by being subjected to the SSO treatment. The highest PCE for the I-PSCs based on the nanoripples-containing ZnO ETLs and optimized active layers was 3.55%, which is comparable to that of I-PSCs fabricated by the conventional spin-coating method. The sprayed PSCs exhibited high environmental stability, exhibiting a decrease of only <25% in the PCE after 40 days of aging in air under ambient
To prepare the blend solution, 1 mg of P3HT (Rieke Metals) and 1 mg of PC60BM (Nano C) were dissolved in 1 ml of chlorobenzene (CB). The solutions were stirred for more than 5 h at room temperature before use. A commercial spray gun assembly (Lumina automatic spray gun, nozzle size: 1.0 mm) was used to deposit the active materials and the agents used in the SSO treatment. During the spraying process, the P3HT/PCBM blend solution was loaded into a fluid cup and sprayed onto the ZnO-coated ITO/glass substrate at a rate of 0.18 ml min 1 through the air-spray gun, which was powered by pressurized N2 gas at 0.8 kgf cm 2. The thickness of the photoactive film was 250 nm. The spray nozzle tip was kept 25–30 cm from the substrate during the deposition of the active layers. For the SSO treatment, 1,2-dichlorobenzene (DCB) was sprayed onto the active layers for 5 s at a rate of approximately 2 ml min 1 through the g-spray nozzle. The distance between the substrates and the air-spray nozzle was 30 cm. After the spraying of DCB on the photoactive
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films, the wet films were left for 10 min to allow the solvent to evaporate slowly. During the TA treatment, the devices were thermally annealed at 135 °C for 10 min in air, before the deposition of the anode. The coated samples were then transferred to a vacuum thermal evaporation chamber for the deposition of the Ag (1000 Å) top electrodes. The Ag layer was deposited using a shadow mask under high vacuum (10 7 Torr.). The active area of the devices was 0.09 cm2. 4.3. Device characterization The J–V characteristics of the devices were measured using a Keithley 2400 source unit. A class-A solar simulator with a 150 W Xenon lamp (Newport) served as the light source, and the light intensity was adjusted using a KG-5 filter-covered monosilicon detector calibrated by the National Renewable Energy Laboratory (NREL) in order to minimize the spectral mismatch. The illumination used had an intensity of approximately AM 1.5 G one sun. 4.4. Morphology characterization of ZnO and P3HT/PC60BM films The surface morphologies of the ZnO films on the ITO/ glass substrates were characterized using AFM (SPA 300 HV, Seiko Instrument Inc.) and SEM (SUPRA 55VP, Carl Zeiss); the accelerating voltage was 2 kV. A surface profilometer (Alpha-Step IQ Profilers) was used to measure the film thicknesses. The XRD patterns of the P3HT/PC60BM films were obtained using a high-resolution X-ray diffractometer (RIGAKU, Cu anode, 40 kV–200 mA). Acknowledgments The authors acknowledge financial support from the Basic Science Research Program and Nuclear R&D program (2014M2A2A6021006) through the National Research Foundation of Korea (NRF, 2012045675), and New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20133030000210). Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.orgel.2014.06.036. References [1] H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, G. Li, Polymer solar cells with enhanced open-circuit voltage and efficiency, Nat. Photon. 3 (2009) 649. [2] Y.Y. Liang, L.P. Yu, A new class of semiconducting polymers for bulk heterojunction solar cells with exceptionally high performance, Acc. Chem. Res. 43 (2010) 1227. [3] C.J. Brabec, S. Gowrisanker, J.J.M. Halls, D. Laird, S.J. Jia, S.P. Williams, Polymer–fullerene bulk-heterojunction solar cells, Adv. Mater. 22 (2010) 3839.
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