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Kan Li, Hongyu Zhen,* Liyong Niu, Xu Fang, Yaokang Zhang, Ruisheng Guo, You Yu, Feng Yan, Haifeng Li, and Zijian Zheng*

Flexible organic solar cells (OSCs) have attracted remarkable attention in the past two decades because of their advantages in low-cost materials preparation, highly scalable roll-to-roll device fabrication, light weight, and superb mechanical flexibility.[1–9] In this research community, one major focus is to improve the photovoltaic efficiency by developing new organic semiconductors, optimizing device architectures, and understanding device physics.[10–13] Indeed, state-of-the-art OSC devices have reached the a power conversion efficiency (PCE) over 10%,[14,15] and it is expected to overcome the 15% hurdle in the next few years.[16] On the other hand, of equally paramount importance is engineering OSC materials into solution processable forms so as to be compatible with full-solution roll-to-roll printing, which is critical to meet the commercial viability in flexible OSC fabrication.[17–22] Among all functional materials (including electrodes, transporting layers, and active layers) in flexible OSCs, printable bottom (next to substrate) back (opaque and reflective) electrodes which can replace those made by conventional vacuum deposition remain to be the bottleneck challenge yet to tackle.[18,23,24] Bottom back electrodes are critical for most flexible OSCs because they can induce twice light absorption in OSCs, which can significantly improve the photovoltaic efficiency. In addition, compared with top back electrodes, they are more stable because they are most far away from the air. From production point of view, the use of bottom back electrodes can K. Li, H. Zhen, X. Fang, H. Li State key laboratory of Modern Optical Instrumentation Zhejiang University Hangzhou, 310027, China E-mail: [email protected] K. Li, H. Zhen, L. Niu, Y. Zhang, R. Guo, Y. Yu, Z. Zheng Nanotechnology Center Institute of Textiles and Clothing The Hong Kong Polytechnic University Hong Kong SAR, China E-mail: [email protected] L. Niu, Y. Zhang, Z. Zheng Advanced Research Centre for Fashion and Textiles The Hong Kong Polytechnic University Shenzhen Research Institute Shenzhen, 518000, China F. Yan Department of Applied Physics The Hong Kong Polytechnic University Hong Kong SAR, China

DOI: 10.1002/adma.201403494

Adv. Mater. 2014, 26, 7271–7278

COMMUNICATION

Full-Solution Processed Flexible Organic Solar Cells Using Low-Cost Printable Copper Electrodes

also save materials usage because the printed top back electrodes are typically much thicker. To date, Ag-based conductors including Ag pastes, Ag nanowires/nanoparticles inks, and Agconducting polymer composites are the best-performed and mostly used printable electrode materials.[25–29] However, electrodes made of Ag pastes and Ag nanowires/nanoparticles inks show disadvantage in high surface roughness and the need for thermal treatment, while composite electrodes lack high reflectance. As a consequence, although being used as transparent electrodes or top (next to air) back electrodes, those materials are not suitable for serving as bottom back electrodes of OSCs. More importantly, Ag is still considered expensive for largearea applications. Cu, on the other hand, possesses the second highest electrical conductivity (σ = 5.96 × 107 S/m) among metals and prices only 1% of Ag. Although Cu-based nanomaterials can be formulated into printable inks similar to those of Ag, they oxidize much more rapidly and significantly. As a result, high temperature (200–600 °C) sintering under reducing atmosphere (such as H2) is needed to regain the conductivity after printing.[31] This has been a major obstacle of using printable Cu inks for flexible OSC applications because all organic substrates shall deform or melt under such high temperatures. In addition, they also face the same problem in high roughness as of Ag inks. Not until very recently, there are reports on transparent electrodes using Cu nanowires networks made by lowtemperature chemical methods.[32,33] However, no device study was performed. Krebs reported the use of commercial kapton/ Cu foils (50 µm) as substrates and vacuum-sputtered Ti electrodes (100 nm) as bottom back electrodes of OSCs in roll to roll vacuum system.[30] Up till now, it is still a significant scientific challenge on how to print smooth, conductive, and inexpensive Cu bottom back electrodes of flexible OSCs in ambient conditions. To address above-mentioned challenge, we report herein, for the first time, the development of full-solution processed, indium tin oxide (ITO)-free, flexible OSCs on the basis of printable Cu electrodes that are fabricated at mild temperature (

Full-solution processed flexible organic solar cells using low-cost printable copper electrodes.

Full-solution-processed flexible organic solar cells (OSCs) are fabricated using low-cost and high-quality printable Cu electrodes, which achieve a po...
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