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1 Copyright 218 by American Scientific Publishers All rights reserved. Printed in the United States of America /218/8/189/6 doi:1.1166/mex Air-processed semitransparent organic solar cells with tunable color Shunjiro Fujii and Yusuke Hara Research Institute for Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST) Central 5, Higashi, Tsukuba, Ibaraki , Japan ABSTRACT Semitransparent bulk-heterojunction (BHJ) solar cells have attracted considerable attention owing to their potential applications in photovoltaic devices. In this work, we demonstrate a novel approach to developing semitransparent and color-tunable solar cells using a MoO 3 /metal/moo 3 structure as the device electrode. By changing the type of metal used in transparent anode stack, the transmission spectra of the semitransparent cells could be tuned over the visible-light region IP: (4 75 nm). On: Fri, Semitransparent 15 Jun 218 2:13:11 solar cells with blue green, green, and greyish green color showed bifacial Copyright: energy generation American when Scientific illuminated Publishers from either the bottom or top side. Our Delivered by Ingenta semitransparent color-tunable solar cells fabricated in air showed an average efficiency of 3% under AM 1.5G illumination. Keywords: Organic Solar Cells, Semitransparent Solar Cells, Bulk-Heterojunction, Transparent Electrode, Oxide/Metal/Oxide. 1. INTRODUCTION Organic solar cells have emerged as a promising costeffective alternative to silicon-based solar cells because they can be fabricated by solution processes without the need for high-vacuum equipment. 1 Bulk-heterojunction (BHJ) solar cells have attracted particular attention owing to their compatibility with flexible plastic substrates and potential for low-cost processing over large surface areas. 2 In addition, interest in semitransparent BHJ solar 3 4 cells has increased because of their possible uses in applications such as vinyl greenhouses, power-generating windows, and multi-junction photovoltaic devices. Many inverted solar cells have been reported that feature transparent top anodes, such as thin metals, 5 oxide/metal/oxide trilayers, 6 1 carbon nanotube, graphene, PEDOT:PSS, and silver nano-networks Trilayer stacked structures of oxide/metal/oxide are a particularly Author to whom correspondence should be addressed. sh-fujii@aist.go.jp promising because of their good optical and electrical properties, and ease of fabrication by sequential deposition of three layers by vacuum evaporation. However, the transmission spectra of semitransparent organic solar cells is primarily determined by the materials used in the active layer. Typical BHJ organic solar cells based on poly(3-hexylthiophene):(6,6)-phenyl and C61 butyric acid methyl ester (P3HT:PCBM), appears to be red purple because of strong absorbance at approximately 5 nm, with a cut-off in the absorption spectrum at 65 nm. Consequently, the most common way to alter the transmittance spectrum and colors of semitransparent solar cells is to utilize absorbing materials of different colors used as the active layers. However, the methods used to alter solar cells color by incorporating different materials suffer from the complexity of synthesizing new materials with the desired absorption spectra while maintaining the same device performance. Up to now, there are no reports on the color-tuning of semitransparent organic solar cells with same active layer by using oxide/metal/oxide trilayer anode. Semitransparent P3HT:PCBM solar cells Mater. Express, Vol. 8, No. 2,

2 have been reported based on MoO 3 /Au/MoO 8 3 and MoO 3 /Ag/MoO 9 3 as top anodes, which also appeared to have a red purple color. We have recently reported a semitransparent BHJ solar cell based on poly[[4,8-bis [(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b ]dithiophene-2,6-diyl] Bottom Top [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl]] (PTB7) 5 2 illumination illumination and [6,6]-phenyl-C71-butyricacid-methyl-ester (PC 71 BM) blend films. This solar cell was fabricated on an ITO/glass substrate with an Fig. 1. Schematic diagram of semitransparent solar cell structure. anode and exhibited a greenish color. 1 In this work, we have developed a simple method of Finally, a 1-nm-thick MoO 3 layer, a 2-nm-thick metal tuning the color of semitransparent PTB7:PC 71 BM solar layer, and a 2-nm-thick MoO 3 layer were sequentially cells through the use of different metal layers in the thermally deposited on the surface of the active layer with oxide/metal/oxide trilayer anode. Thin layers of Ag, Au or the use of a metal mask. The thermal evaporation rate Al/Cu were applied in a thin layer sandwiched by the two of MoO 3 and metal were.1.2 nm/s and.3.4 nm/s, transparent MoO 3 layers in the anode stack. These metal respectively. In this case, the 1 nm-thick MoO 3 layer was layers acted both as an optical spacer and electrode. By used as a buffer layer, whereas the 2 nm-thick capping combining the flat transmittance of a PTB7:PC 71 BM film MoO 3 layer was deposited to enhance transmittance in the with the MoO 3 /metal (Ag, Au or Al/Cu)/MoO 3 anodes, visible region. 19 The thin metal layers sandwiched by the color of the semitransparent cells could be tuned to the two transparent MoO 3 layers included: Ag (2 nm), be blue green, green, and greyish green. The present Au (2 nm), and Al (2 nm)/cu (18 nm). The Al layer was color-tuning method is superior to previously reported used as a buffer layer to prevent Cu diffusion. 21 The area methods to use absorbing materials of different colors, of the active layer was defined by the overlap of the ITO in terms of simplicity, scalability, cost, etc. Similar cell and top electrodes to be.4 cm 2. The current density performance were maintained among the different colored voltage characteristics of the solar cell devices were measured Jun with 218 a 2:13:11 Keithley 24 source meter under AM 1.5 cells with power conversion efficiencies as high as 3%. IP: On: Fri, 15 Copyright: American Scientific illumination Publishers (1 mw/cm 2 in a solar simulator. A spectral Ingenta response and an incident photon-to-electron Delivered by conversion 2. EXPERIMENTAL DETAILS Inverted BHJ solar cells were fabricated on ITO-coated glass substrates with sheet resistances of 1 /sq (Techno Print), to serve as the cathode in the devices. The solar cell structures fabricated in this work were: glass/ito/pfn/ptb7:pc 71 BM/MoO 3 /metal/moo 3. A schematic representation of the layered structure is shown in Figure 1. The fabrication process is based on our previously reported method. 1 The substrates were cleaned in acetone and isopropanol in an ultrasonic bath and then dried under a N 2 flow. Subsequently, UV-ozone cleaning was performed for 2 min. Next, a poly[(9,9-bis(3 -(N,N -dimethylamino)propyl)-2,7- fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) layer was deposited on the glass substrate by the following procedures. First, PFN (1-Materials) was dissolved in methanol containing a small amount of acetic acid. The PFN layers were applied by spin coating the PFN solution at 2 rpm for 12 s on the substrates. Then, the active layers of PTB7 (1-Material Inc.) and PC 71 BM (American Dye Source Inc.) were deposited via spin coating at 1 rpm for 1 s from a solution of PTB7:PC 71 BM = 1:1.5 in 1,2- dichlorobenzene, which contained 3% diiodooctane. The concentrations of PTB7 and PC 71 BM were 1 mg/ml and 15 mg/ml, respectively. All spin coatings were performed under ambient conditions in air. Pieces of the substrates coated with PTB7:PC 71 BM were stored in a Petri dish for 3 min and then transferred to a vacuum chamber. ITO/Glass PFN PTB7:PC 71 BM MoO 3 Thin metal efficiency measurement system was used to characterize the external quantum efficiency (EQE). All measurements were performed under ambient conditions in air. 3. RESULTS AND DISCUSSION 3.1. Optical Properties Figure 2 shows the optical transmittance spectra of the three different trilayer anodes on an ITO/glass substrate. For comparison, the spectrum of ITO on glass is also Transmittance(%) MoO 3 Wavelength (nm) Fig. 2. Optical transmittance spectra of semitransparent electrodes on ITO/glass substrate. 19 Mater. Express, Vol. 8, 218

28%, 44% and 29% at 55 nm, respectively. These results indicate that the trilayers based on different metals acted as both an optical spacer and electrode.

3 28%, 44% and 29% at 55 nm, respectively. These results indicate that the trilayers based on different metals acted as both an optical spacer and electrode. In this work, the capping MoO3 layer (2 nm) was deposited to enhance transmittance in the visible region. It has been reported that the optical interference effect arising from the two transparent MoO3 layers sandwiching the thin metal layer is a significant cause for the higher transparency of the MoO3 /metal/moo3 structure than that of a single thin metal layer The MoO3 /metal/moo3 trilayer anodes in this work are semitransparent and the shade of color changes with the intermediate metal. 22 As a result, the transmission spectrum and colors of whole solar cells in Figure 3 are determined by the intrinsic optical properties of a thin layer of metals. Transmittance (%) 3.2. Performance Next, we characterized the photoresponse of the semitransparent solar cells based on different anodes. Figure 4(a) shows the current density versus voltage (J V ) characteristics for the semitransparent solar cells measured via top illumination under AM 1.5G. Table I summarizes the solar cell parameters of these devices. The short-circuit current density (Jsc and the PCE of the Au-trilayer anode were highest (5.7 ma/cm2, 2.1%) followed by those of thejun Al/Cu-trilayer (4.8 ma/cm2 and 1.7%) and Ag-trilayer IP: On: Fri, :13:11 2 (3.5 ma/cm and 1.3%, respectively). The variation in percopyright: American Scientific Publishers (a) Delivered byformance Ingenta of the cells with different metals in the trilayer anode under top illumination can be attributed to the different transparencies of the Au, Ag, and Al/Cu-trilayer anodes. Different performances in the same cells illuminated from the top- and bottom-sides could be attributed the differences in transparency of the PFN/ITO/glass and MoO3 /metal/moo3 layers. 1 1 mm The corresponding EQE spectra of these two devices are shown in Figure 4(b). We observed considerable dif5 (b) ferences in the three EQE spectrum. For the case of 45 the Ag-trilayer anode, a maximum EQE of 3% was confirmed in the region where PC71 BM absorbs light, 4 and the EQE was higher than those of the Au and 35 Cu-trilayer anodes in the range 3 4 nm. This is 3 likely because of the relatively high transmittance in 25 the near-ultraviolet region, as described in Figure 2(a), 2 which resulted in higher light absorption in the active MoO3/Ag/MoO3 15 layer at short wavelengths. Therefore, less light was MoO3/Au/MoO3 absorbed in the active layer in the visible region, result1 ing in a lower Jsc and PCE compared with that of MoO /Al/Cu/MoO the Au and Al/Cu-trilayer anodes. The trend of the EQE spectrum, shown in Figure 5(b) was consistent Wavelength (nm) with the transmittance spectra of the trilayer anodes (Fig. 2). Fig. 3. (a) Photograph (top view) of color-tuned semitransparfigure 4(c) shows the J V characteristics of the semient solar cells over text on paper based on MoO3 /Au/MoO3 transparent solar cells measured via bottom illumination (left), MoO3 /Ag/MoO3 (middle), MoO3 /Al/Cu/MoO3 (right) electrodes. (b) Optical transmittance spectra of semitransparent cells including under AM 1.5G. Table II summarizes the solar cell paramito/glass substrate. eters of these devices. The device featuring an Au-trilayer Mater. Express, Vol. 8, shown. The MoO3 /thin metal/moo3 anodes showed differences in their optical transmittance spectra, which originated from the optical characteristics of the constituent metals (Au, Ag, and Al/Cu). For the case the Ag-trilayer, transmittance of light in the near-ultraviolet region was observed; for the case of the Al/Cu-trilayer, transmittance in the near-infrared region was observed. The Au-trilayer showed transmittance of both near-infrared and nearultraviolet light. When looking through a PTB7:PC71 BM blend film, we did not perceive any notable alteration in the color of an image. By combining the flat transmittance of the PTB7:PC71 BM film with the MoO3 /metal (Au, Ag, and Al/Cu)/MoO3 anodes, the color of the semitransparent cells could be tuned to be green, blue green or grayish green. A photograph of the cells is shown in Figure 3(a). Figure 3(b) shows the corresponding optical transmittance spectra for the complete semitransparent cell structures including the ITO/glass substrate. The transmittance values of the blue green and green cells showed a similar trend to those of those of electrodes. However, the transmittance of the gray cell showed the weakest wavelength dependence in the visible and near-infrared regions. The three different semitransparent cells (blue green, green, and greyish green) showed total transmittance values of Materials Express

4 (a) 4 (c) 4 Current Density (ma/cm 2 ) Current Density (ma/cm 2 ) Voltage (V) Voltage (V) (b) (d).4.4 EQE IP: On: Fri, 15 Jun 218 2:13:11 Copyright: American Scientific Publishers Wavelength (nm) Delivered by Ingenta Wavelength (nm) EQE Fig. 4. (a) J V characteristics of semitransparent PTB7:PC 71 BM solar cells with top side illuminated; (b) Corresponding EQE spectra (top side illuminated); (c) J V characteristics of semitransparent PTB7:PC 71 BM solar cells with bottom side illuminated; (d) Corresponding EQE spectra (bottom side illuminated). anode delivered a J sc of 7.9 ma/cm 2, V oc of.67 V, and a fill factor (FF) of.58. By comparison, the device with the Ag-trilayer anode delivered a J sc of 7.7 ma/cm 2,aFF of.56, and a V oc of.65 V. The device with the Al/Cutrilayer anode delivered a J sc of 8.6 ma/cm 2, a FF of.49, and a V oc of.72 V. The PCEs of solar cells based on the Au, Ag, and Al/Cu-trilayer anodes were 3.1, 2.9, and 3.1%, respectively. It has been reported that V oc of inverted P3HT:PCBM solar cells with MoO 3 is independent of the work function of the top electrodes. 23 In this work, a thin MoO 3 layer was also inserted between the active layer and top trilayer anodes. Therefore, the V oc of the semitransparent solar cells did not show significant difference. Notably the three different anodes maintained similar cell performance with only small differences in their PCE although their colors appeared to be different. The data shown in Figure 4 clearly demonstrated bifacial energy generation. The corresponding EQE spectra of these devices are shown in Figure 4(d). The maximum EQE of each cell showed a maximum of 4% in range of 3 7 nm. Some differences from the results of bottom illumination, shown in (Fig. 4(b)), were observed. For the Ag-trilayer anode, the maximum EQE was confirmed to be in the range of the polymer absorption; however, this device featured a lower EQE than those of the Au and Al/Cu-trilayer anodes in the range of 3 5 nm. This result can likely be attributed to the relatively high transmittance, and low Table I. Comparison of solar cell parameters for devices with different electrodes under top-side illumination. Electrode J sc (ma/cm 2 V oc (V) FF PCE (%) Au Ag Al/Cu Table II. Comparison of solar cell parameters for devices based on different electrodes with illumination from bottom side. Electrode J sc (ma/cm 2 V oc (V) FF PCE (%) Au Ag Al/Cu Mater. Express, Vol. 8, 218

5 Materials Express reflectivity in the near-ultraviolet region. The reflectance of the metal electrode plays an important role in trapping light for the active layer to reabsorb. The low reflectivity of the Ag-trilayer in the 3 5 nm wavelength range likely contributed to a lower EQE of this cell. However, the differences in the EQE profiles were small; hence, the integrated photocurrents from the EQE spectra of the solar cells were similar and the J sc values among the devices were almost the same. These data support the results of Figure 4(c), which indicated that the three different anodes maintained similar cell performance. The fabricated semitransparent PTB7:FPC 71 BM solar cells showed a PCE of 3.1%, which is comparable to that of a previously reported low-bandgap polymer/fullerene semitransparent solar cells with inverted structure (for example, 4% 5% visible-light transmittance and 2.6% 3.4% PCE Many of these efficiencies reported for semitransparent solar cells, are based on devices fabricated and measured in an inert gas-filled glovebox to maintain their performance. However, the PCE reported here was obtained with fabrication, processing, and measurements performed in air (humidity: 3% 4%), resulting in the degradation of the active layer through exposure to oxygen and water during the fabrication process. It is likely that fabrication of solar cells in a nitrogen-filled 6, (216). glovebox could further improve theip: performance of On: thesefri, 15 Jun 218 2:13:11 devices. Copyright: American Scientific Publishers Delivered by Ingenta 4. CONCLUSION In this work, we have demonstrated the color-tunability of semitransparent PTB7:PC 71 BM solar cells. The specific device structure was glass/ito/pfn/ptb7:pc 71 BM/ MoO 3 /metal/moo 3. We conclude that the color of the cells could be broadly tuned to be green, blue green, or greyish green by changing the metal (Au, Ag, and Al/Cu) in the trilayer anode rather than the active layer materials. Notably, the fabricated semitransparent PTB7:PC 71 BM solar cells with different colors demonstrated similar performance. The average PCE value of the cells was 3% when illuminated from the bottom side. Acknowledgment: We thank Takeshi Saito and Yuki Kuwahara, Ph.D. s, from Nanomaterials Research Institute, AIST for helpful discussions. This work was supported by JSPS KAKENHI, grant no. 16K References and Notes 1. B. C. Thompson and J. M. J. Frechet; Organic photovoltaics Polymer-fullerene composite solar cells; Angew. Chem. Int. Ed. 47, 58 (28). 2. G. Dennler, M. C. Scharber, and C. J. 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Shiratori; Semitransparent polymer-based solar cells via simple wet lamination process with TiO 2 layer using automatic spray layerby-layer method; RSC Advances 5, (215). 17. M. Makha, P. Testa, S. B. Anantharaman, J. Heier, S. Jenatsch, N. Leclaire, J. N. Tisserant, A. C. Veron, L. Wang, F. Nuesch, and R. Hany; Ternary semitransparent organic solar cells with a laminated top electrode; Sci. Tech. Adv. Mater. 18, 68 (217). 18. S. Y. Ryu, J. H. Noh, B. H. Hwang, C. S. Kim, S. J. Jo, J. T. Kim, H. S. Hwang, H. K. Baik, H. S. Jeong, C. H. Lee, S. Y. Song, S. H. Choi, and S. Y. Park; Transparent organic light-emitting diodes consisting of a metal oxide multilayer cathode; Appl. Phys. Lett. 92, 2336 (28). 19. W. R. Cao, Y. Zheng, Z. F. Li, E. Wrzesniewski, W. T. Hammond, and J. G. Xue; Flexible organic solar cells using an oxide/metal/oxide trilayer as transparent electrode; Org. Electron. 13, 2221 (212). Mater. Express, Vol. 8,

6 2. Y.Y.Liang,Z.Xu,J.B.Xia,S.T.Tsai,Y.Wu,G.Li,C.Ray, and L. P. Yu; For the bright future-bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%; Adv. Mater. 22, E135 (21). 21. I. P. Lopez, L. Cattin, D. T. Nguyen, M. Morsli, and J. C. Bernede; Dielectric/metal/dielectric structures using copper as metal and MoO 3 as dielectric for use as transparent electrode; Thin Solid Films 52, 6419 (212). 22. V. V. Travkin, A. Yu. Luk yanov, M. N. Drozdov, E. A. Vopilkin, P. A. Yunin, and G. L. Pakhomov; Ultrathin metallic interlayers in vacuum deposited MoO x /metal/moo x electrodes for organic solar cells; Appl. Surf. Sci. 39, 73 (216). 23. C. Tao, G. H. Xie, C. X. Liu, X. D. Zhang, W. Dong, F. X. Meng, X. Z. Kong, L. Shen, S. P. Ruan, and W. Y. Chen; Performance improvement of inverted polymer solar cells with different top electrodes by introducing a MoO 3 buffer layer; Appl. Phys. Lett. 93, (28). Received: 28 September 217. Revised/Accepted: 1 February 218. IP: On: Fri, 15 Jun 218 2:13:11 Copyright: American Scientific Publishers Delivered by Ingenta 194 Mater. Express, Vol. 8, 218

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