Review Recent progress in degradation and stabilization of organic solar cells Huanqi Cao a , Weidong He a, * , Yiwu Mao b , Xiao Lin c , Ken Ishikawa d, * , James H. Dickerson e , Wayne P. Hess f, * a School of Energy Science and Engineering, University of Electronic Science and Technology, Chengdu, Sichuan 611731, PR China b Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, PR China c School of Physics, University of Chinese Academy of Sciences, Beijing 100049, PR China d Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1-S8-28 O-okayama, Meguro, Tokyo 152-8552, Japan e Department of Physics, Brown University, Providence, RI 02912, USA f Chemical and Materials Science Division, Pacific Northwest National Laboratory, P.O Box 999, Richland, WA 99352, USA highlights  Reviews degradation of organic solar cells caused by water, oxygen and light irradiation.  Describes mechanisms of a few promising ways of stabilizing organic solar cells.  Overviews essential recent literature in stabilization of organic solar cells. article info Article history: Received 3 February 2014 Received in revised form 11 April 2014 Accepted 16 April 2014 Available online Keywords: Organic solar cells Stability Degradation Interface Diffusion abstract Stability is of paramount importance in organic semiconductor devices, especially in organic solar cells (OSCs). Serious degradation in air limits wide applications of these flexible, light-weight and low-cost power-generation devices. Studying the stability of organic solar cells will help us understand degra- dation mechanisms and further improve the stability of these devices. There are many investigations into the efficiency and stability of OSCs. The efficiency and stability of devices even of the same photoactive materials are scattered in different papers. In particular, the extrinsic degradation that mainly occurs near the interface between the organic layer and the cathode is a major stability concern. In the past few years, researchers have developed many new cathodes and cathode buffer layers, some of which have astonishingly improved the stability of OSCs. In this review article, we discuss the recent developments of these materials and summarize recent progresses in the study of the degradation/stability of OSCs, with emphasis on the extrinsic degradation/stability that is related to the intrusion of oxygen and water. The review provides detailed insight into the current status of research on the stability of OSCs and seeks to facilitate the development of highly-efficient OSCs with enhanced stability. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Organic solar cells (OSCs) are considered to be a green solar energy technology. OSCs are attractive mainly because they can be fabricated from various organic compounds with modifiable structures and exhibit excellent flexibility, high power conversion efficiency (PCE) under low light irradiation, and promising see- through power-generation windows. Over the course of the past few years, various aspects of organic solar cells have been extensively studied, including synthesis and application of new materials, modeling of physical processes, large-scale manufacturing, stability improvement, etc [1]. The research tra- jectories of OSCs have been described in various reviews [2e6]. The ultimate aim of organic solar cell research is to realize efficient solar energy conversion. In the past two decades, OSC ef- ficiency has been remarkably improved through new approaches such as using fullerene electron acceptors [7,8], and broadening OSC absorption bands within the solar spectra [9]. The total energy output of a fabricated solar cell equals the product of its efficiency and lifetime. Therefore, stability is an important OSC property that impacts the value (yield over cost) of an OSC system as current energy production is largely limited by the low durability. Thanks to persistent efforts, our understanding of the degradation of OSCs * Corresponding authors. Tel./fax: þ86 2861831252. E-mail addresses: weidong.he@uestc.edu.cn (W. He), iken@op.titech.ac.jp (K. Ishikawa), wayne.hess@pnnl.gov (W.P. Hess). Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour http://dx.doi.org/10.1016/j.jpowsour.2014.04.080 0378-7753/Ó 2014 Elsevier B.V. All rights reserved. Journal of Power Sources 264 (2014) 168e183