Intrinsic photodegradation of organic solar cells, particularly bulk heterojunction (BHJ), remains a key commercialization barrier. Two types of photogenerated defects in BHJ films and related systems have recently been explored via electron paramagnetic resonance (EPR): (a) deeply trapped holes and electrons in polyelectrolyte-fullerene assemblies  and (b) carbon dangling bonds (C DBs) ; the latter were invoked in support of simulations [3,4]. In both cases, the generated EPR defect signature observed in photodegraded films weakens over several days. This talk will present new results of broadly examined various donor/acceptor structures, including of BHJ blend films with a non-fullerene acceptor, to obtain a comprehensive understanding of photodegradation. Evidence for C DBs vs deeply trapped holes and electrons will be discussed, given that the defects are generated largely by blue/UV irradiation rather than longer wavelengths. This observation clearly supports C DB formation over deeply trapped charges. The role of “hot” polarons in donor:acceptor interface C DB formation will also be discussed.
 R. C. Huber et al., Science 348, 1340 (2015).
 F. Fungura et al., Adv. Ener. Mater. 7, 1601420 (2017).
 J. E. Northrup, Appl. Phys. Express 6, 121601 (2013).
 S. Shah and R. Biswas, J. Phys. Chem. C 119, 20265 (2015).