1656 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 6, NOVEMBER/DECEMBER 2010 Versatile Multilayer Transparent Electrodes for ITO-Free and Flexible Organic Solar Cells Seungchan Han, Student Member, IEEE, Sooyeon Lim, Hoyeon Kim, Hyunsu Cho, and Seunghyup Yoo, Member, IEEE (Invited Paper) Abstract—Multilayer transparent electrodes based on a dielectric–metal–dielectric structure are explored as an effective alternative to indium tin oxide (ITO) electrodes that can lead to ITO-free or highly flexible organic photovoltaic (OPV) cells with a performance comparable to conventional cells. The role of each layer in multilayer electrodes is discussed with an emphasis on the potential role of inner dielectric layer as a buffer layer, and the optimization strategy considering the overall optical structure is provided. Experimental results showing the OPV performance un- der dynamic flexibility test are also provided as a function of the number of bending cycles. Index Terms—Flexible structures, modeling, multilayered me- dia, optical materials, photovoltaic cells. I. INTRODUCTION O RGANIC photovoltaic (OPV) cells are rapidly gaining interest as an alternative to established photovoltaic tech- nologies. In order to become a viable and practical solution for OPVs, of premium importance is to define the strength of OPV technologies that can differentiate them from the conventional technologies. While there can be several advantages of OPVs, the potential of being low-cost and flexible PV technologies has been regarded as one of the most critical benefits that OPVs can offer. In this regard, indium tin oxide (ITO), most commonly used transparent electrodes in OPVs, are regarded problematic because 1) the limited supply of indiums and a heavy demand from flat-panel display industry tend to make them incompati- ble with low-cost solar cells [1] and 2) ITO films are known to perform quite poorly under repeated bending conditions [2]. In an effort to find a low-cost and flexible transparent elec- trode for OPVs [3]–[6], we explore the potential of mul- Manuscript received December 14, 2009; revised January 14, 2010; ac- cepted January 20, 2010. Date of publication March 1, 2010; date of current version December 3, 2010. This work was supported in part by Korea En- ergy Management Corporation under the New and Renewable Energy Research and Development Grant 2008-N-PV08-02, in part by Korea Institute of Energy Technology Evaluation and Planning, Ministry of Knowledge Economy, under the New and Renewable Energy Research and Development Grant N02090016, in part by the Energy, Environment, Water, and Sustainability Program of Korea Advanced Institute of Science and Technology, and in part by Korea Iron Steel Corporation. S. Han was with Department of Electrical Engineering, Korea Advanced In- stitute of Science and Technology, Daejeon 305-701, Korea. He is now with the Test and Package Center, Device Solution Business, Samsung Electronics, Asan 336-841, Korea. S. Lim, H. Kim, H. Cho, and S. Yoo are with the Department of Electrical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea (e-mail: syoo@ee.kaist.ac.kr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSTQE.2010.2041637 tilayer transparent electrodes (MTEs) based on adielectric– metal–dielectric (DMD) structure that Cho et al. have recently demonstrated as an effective solution for high-performance, highly flexible organic LEDs (OLEDs) [7]. In this paper, we try to shed light on a DMD-based MTE structure tailored to OPV device applications, and discuss its optimization strategy from the OPV standpoint, along with a careful consideration of their optical and electrical aspects. II. EXPERIMENTAL DMD transparent electrodes consisting of ZnS (Alfa Aesar, 99.99%), Ag (Alfa Aesar, 99.999%), and WO 3 (Alfa Aesar 99.99%) were deposited sequentially by vacuum thermal evap- oration (<10 −6 torr) on the precleaned polyethylene terephtha- late (PET) (PANAC Co., 250 μm) and glass substrates. Sub- strates were planarized with spin-coated polymeric films (SU-8, Kayaku Microchem) or with spin-on glass (SOG) (Honeywell). Previous study has shown that planarization of substrates is a key process in obtaining a uniform and continuous metal thin film, and thus, achieving a low sheet resistance while maintaining a high transmission [7]. After deposition of DMD layers, bulk heterojunction (BHJ) layers were spin-coated from a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C 71 butyric acid methyl ester (PC 70 BM) (20 mg/mL, 1:0.7 by weight) in dichlorobenzene. Devices were dried for 30 min at a room temperature, and then, annealed at 110 ◦ C for 10 min on a hot plate. Subsequently, they were loaded to a vacuum evaporation chamber for deposition of Ca/Al electrodes. Overall structure and fabrication proce- dure of MTEs-based organic solar cell are shown in Fig. 1. For comparison, ITO-based conventional cells using poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layers (Baytron P AI 4083, H.C. Starck, Inc.) as a buffer layer were also fabricated in the same batch. The active area of the fabricated devices was typically in the range of 0.05–0.10 cm 2 . Current density–voltage (J–V) characteristics were measured using Keithley 2400 source measure unit. The photovoltaic per- formances were characterized using a solar simulator (ABET Technologies) with air mass (AM) 1.5 G filters. Irradiance was measured each time using a Si photodiode, the response of which was cross checked with the calibrated reference cell. All the fab- rication and testing were done in an N 2 -filled glove box or in a vacuum-compatible chamber without exposure to an ambient atmosphere. For a bending cycle test, we used a custom-made 1077-260X/$26.00 © 2010 IEEE