Bromine-Terminated Additives for Phase-Separated Morphology Control of PTB7:PC₇₁BM-Based Polymer Solar Cells

Trace amounts of solvent additive can effectively regulate the phase-separated morphology of the active layer composed of donor and acceptor materials for improved power conversion efficiency (PCE) of polymer solar cells (PSCs). However, applicable solvent additives for PSCs are still limited, and it is difficult to rationally design or select appropriate solvent additives for optimal morphology control of the active layer, mainly due to the lack of sufficient understanding of the morphological regulation mechanism. Here, on the basis of a series of bromine-terminated additives with different chain lengths, we systematically investigated the relations between properties of solvent additives, active layer morphology, and photovoltaic performance of PTB7:PC₇₁BM bulk heterojunction PSCs. In addition to the widely acknowledged requirements of solvent additives with selective solubility toward one of the components in the active layer and remarkably higher boiling point than that of the host solvent, it was found that additives should also have suitable solubility parameters for the formation of nanoscale phase-separated morphology and pure PTB7 domains simultaneously. Therefore, the PTB7:PC₇₁BM-based PSCs using a small amount (3 vol %) of specific bromine-terminated additive show significant PCE enhancement up to 55% in comparison with that of additive-free devices. These results illustrate clearly the positive effects of solvent additive-induced phase-separated morphology for high photovoltaic performance, providing important understanding of morphology control and valuable clues for the rational selection and development of suitable additives for high-performance PSCs.