Balanced Electric Field Dependent Mobilities: A Key to Access High Fill Factors in Organic Bulk Heterojunction Solar Cells

The compositions of most lab-based bulk-heterojunction (BHJ) solar cells are optimized by their power conversion efficiencies (PCEs). In this report, we suggest that the compositions should be optimized by their fill-factors (FFs) instead. With the optimized-FF approach, BHJ cells tend to have higher acceptor contents and possess better thermal and operational stabilities. Three model systems, namely, PTB7:PC₇₁BM, PTB7-Th:ITIC, and PBDB-T:ITIC BHJs, are chosen as case studies. Charge carrier transport measurements are used to reveal the origin of the enhanced FFs of these BHJ solar cells. We demonstrate that these acceptor-rich BHJs possess better balanced field-dependent electron-to-hole mobility ratios due to improved electron mobilities near open-circuit conditions. We introduce a new parameter, known as charge imbalance factor (Δ), to quantify the impact of field dependent mobilities on the FF of the OPV cells. The improved mobility ratio (reduced Δ) suppresses carrier recombinations (especially at the open-circuit conditions). Despite having slightly reduced PCEs, the FF-optimized cells enjoy a much better stability. Our results suggest that FF-optimized BHJ cells with higher acceptor contents should be considered for practical applications, due to better thermal and operational stability.