YOU ET AL. VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX www.acsnano.org A C XXXX American Chemical Society Low-Temperature Solution-Processed Perovskite Solar Cells with High Efficiency and Flexibility Jingbi You, † Ziruo Hong, †, * Yang (Michael) Yang, † Qi Chen, † Min Cai, † Tze-Bin Song, † Chun-Chao Chen, † Shirong Lu, † Yongsheng Liu, † Huanping Zhou, †, * and Yang Yang †,‡, * † Department of Materials Science and Engineering and ‡ California NanoSystems Institute, University of California ; Los Angeles, Los Angeles, California 90095, United States P erovskite semiconductors have attracted tremendous attention 1,2 beginning with their incorporation into photovoltaic devices by Miyasaka et al. in 2009, 3 achieving an initial power conversion efficiency (PCE) of 4%. In 2012, signi ficant progress was realized in perovskite solar cells, with several groups reporting over 10% PCE by employing meso- porous nanostructures, 410 attracting great attention in the field of photovoltaics. 1116 More recently, it was observed that perovskite materials exhibit long charge carrier lifetimes and thus can be used for planar junctions. 17,18 In fact, both the solution and coevaporation processing approaches showed up to a 11.4 19 and 15% 20 PCE, respectively, incorporating a planar structure. However, it has been recog- nized that, in the perovskite-based photovol- taic cells, a high-quality condensed TiO 2 layer often requires high-temperature treatment above 450 °C. Such extreme processing con- ditions could limit the future development of perovskite solar cells, particularly in flexible formats. It is therefore critical to explore the possibility of fabricating high-performance perovskite-based solar cells at low tempera- ture. In addition, a low-temperature proces- sing approach offers a wider selection of potential substrates and electrode materials that could be used in devices, including poly- mer-based flexible substrates and solution- processed interfacial materials that could thereby be incorporated into perovskite photovoltaic cells. A few groups have at- tempted to fabricate all low-temperature processed perovskite solar cells using different transport layer processing. 2123 However, the resultant device efficiency is notably inferior compared to high-tempera- ture processed TiO 2 . The efficiency losses in the low-temperature processed perovskite solar cells originate mainly due to charge recombination at imperfect interfaces and structural or chemical defects in perovskite films. In this article, we explore the possibility of fabricating perovskite-based solar cells via a low-temperature (<120 °C) solution- processing approach. We obtain 11.5% PCE based on a glass/ITO rigid substrate, which is * Address correspondence to yangy@ucla.edu, zrhong@ucla.edu, happyzhou@ucla.edu. Received for review November 21, 2013 and accepted January 5, 2014. Published online 10.1021/nn406020d ABSTRACT Perovskite compounds have attracted recently great attention in photovoltaic research. The devices are typically fabri- cated using condensed or mesoporous TiO 2 as the electron transport layer and 2,2 0 7,7 0 -tetrakis-(N,N-dip-methoxyphenylamine)9,9 0 - spirobifluorene as the hole transport layer. However, the high- temperature processing (450 °C) requirement of the TiO 2 layer could hinder the widespread adoption of the technology. In this report, we adopted a low-temperature processing technique to attain high- efficiency devices in both rigid and flexible substrates, using device structure substrate/ITO/PEDOT:PSS/CH 3 NH 3 PbI 3x Cl x /PCBM/Al, where PEDOT:PSS and PCBM are used as hole and electron transport layers, respectively. Mixed halide perovskite, CH 3 NH 3 PbI 3x Cl x , was used due to its long carrier lifetime and good electrical properties. All of these layers are solution-processed under 120 °C. Based on the proposed device structure, power conversion efficiency (PCE) of 11.5% is obtained in rigid substrates (glass/ITO), and a 9.2% PCE is achieved for a polyethylene terephthalate/ITO flexible substrate. KEYWORDS: perovskite solar cells . low temperature . planar structure . flexible solar cells ARTICLE