Improved Photovoltaic Performance of Polymer Solar Cells via a Volatile and Nonhalogen Additive to Optimize Crystallinity

Optimizing the photoactive layer morphology in polymer solar cells (PSCs) using solvent additives is an effective and influential approach to improve photovoltaic performance. Traditional halogen-containing additives, such as 1,8-diiodooctane (DIO), 1-chloronaphthalene (CN), etc., limited further development due to toxicity, thereby the demand of volatile and halogen-free additives without sacrificing the device performance increases continuously. In this study, a nontoxic halogen-free additive, ethyl 2-hydroxybenzoate (EHB), was introduced and added to chlorobenzene (CB) solution to fabricate efficient PSCs. The device studies indicated that the additive incorporation could increase the efficiency of PTB7-Th:PC71BM devices from 5.27 to 9.64%. Photophysical studies showed that additives could promote charge extraction and inhibit charge recombination, which in turn improves power conversion efficiency (PCE). In addition, morphology studies demonstrated better molecular ordering in active layers, which typically resulted in stronger crystallinity but reduced fullerene aggregation, thus positively affecting layer deposition and improving carrier transport and donor-acceptor surface contact in the photoactive layer. This work proposes a strategy to design halogen-free additives to replace conventional additives in favor of PSC's scalability in future.