www.advenergymat.de 1903163 (1 of 10) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim FULL PAPER What is Killing Organic Photovoltaics: Light-Induced Crosslinking as a General Degradation Pathway of Organic Conjugated Molecules Olga R. Yamilova, Ilya V. Martynov, Allison S. Brandvold, Irina V. Klimovich, Alex H. Balzer, Alexander V. Akkuratov, Ilya E. Kusnetsov, Natalie Stingelin, and Pavel A. Troshin* DOI: 10.1002/aenm.201903163 1. Introduction Organic solar cells have a number of advan- tages compared to many of their inorganic counterparts, such as being lightweight, mechanically flexible and aesthetically appealing (e.g., because of their semitrans- parency), promising their application in multiple areas where conventional photo- voltaics cannot be employed, for instance, as PV modules for smart windows, energy- generating wallpapers and beyond. [1–6] Encouragingly, the light power conversion efficiency (PCE) of organic solar cells is rapidly increasing, and the best laboratory devices have recently surpassed the 17% threshold. [7] Moreover, organic solar cells may eventually be produced using high throughput roll-to-roll printing and coating technologies which would open a plat- form for low-cost production of the final product. [8,9] The most efficient solution-processible organic solar cells are based on blends of an electron donating conjugated polymer (p-type semiconductor) and an n-type material usually represented by fullerene derivatives or non- fullerene acceptors. [10,11] Rational design of novel material com- binations has enabled bringing the efficiency of single junction organic solar cells from ≈4%, routinely reported for benchmark devices based on poly(3-hexylthiophene) P3HT blends with the phenyl-C 61 butyric acid methyl ester (PC 61 BM), to the current >15% efficiency reached for the most advanced systems com- prised of low band gap conjugated polymers and nonfullerene acceptors. [12–16] While the development of new materials is mainly driven by the goal to increase the device efficiency, using materials design to improve the operation stability of organic solar cells remains to date too little investigated. This is somehow sur- prising as multiple degradation pathways can occur in organic solar cells under realistic operation conditions that severely restrict their lifetime and, therefore, represent one of the key obstacles for commercialization of this technology. [17–19] Generally, indeed, practically useful organic solar cells are encapsulated using appropriate barrier coatings to minimize the penetration of oxygen and moisture into the active layer of In view of a rapid development and increase in efficiency of organic solar cells, reaching their long-term operational stability represents now one of the main challenges to be addressed on the way toward commercialization of this photovoltaic technology. However, intrinsic degradation pathways occurring in organic solar cells under realistic operational conditions remain poorly understood. The light-induced dimerization of the fullerene-based acceptor materials discovered recently is considered to be one of the main causes for burn-in degradation of organic solar cells. In this work, it is shown that not only the fullerene derivatives but also different types of conjugated polymers and small molecules undergo similar light-induced crosslinking regard- less of their chemical composition and structure. In the case of conjugated polymers, crosslinking of macromolecules leads to a rapid increase in their molecular weight and consequent loss of solubility, which can be revealed in a straightforward way by gel permeation chromatography analysis via a reduction/loss of signal and/or smaller retention times. Results of this work, thus, shift the paradigm of research in the field toward designing a new gen- eration of organic absorbers with enhanced intrinsic photochemical stability in order to reach practically useful operation lifetimes required for successful commercialization of organic photovoltaics. O. R. Yamilova, I. V. Klimovich, Dr. P. A. Troshin Center for Energy Science and Technology (CEST) Skolkovo Institute of Science and Technology Nobel St. 3, 143026 Moscow, Russia E-mail: p.troshin@skoltech.ru O. R. Yamilova, I. V. Martynov, I. V. Klimovich, Dr. A. V. Akkuratov, I. E. Kusnetsov, Dr. P. A. Troshin Laboratory of Functional Materials for Electronics and Medicine (FMEM) Institute for Problems of Chemical Physics of Russian Academy of Sciences (IPCP RAS) Semenov Ave. 1, 142432 Chernogolovka, Moscow Region, Russia A. S. Brandvold, Prof. N. Stingelin School of Materials Science and Engineering Georgia Institute of Technology North Ave. NW, Atlanta, GA 30332, USA A. H. Balzer, Prof. N. Stingelin School of Chemical and Biomolecular Engineering Georgia Institute of Technology North Ave. NW, Atlanta, GA 30332, USA The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201903163. Adv. Energy Mater. 2020, 1903163