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 [1] and (b) carbon dangling bonds (C DBs) [2]; 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.
[1] R. C. Huber et al., Science 348, 1340 (2015).
[2] F. Fungura et al., Adv. Ener. Mater. 7, 1601420 (2017).
[3] J. E. Northrup, Appl. Phys. Express 6, 121601 (2013).
[4] S. Shah and R. Biswas, J. Phys. Chem. C 119, 20265 (2015).
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