Effects of conjugated bridges on the photovoltaic properties of ortho- functionalized perylene diimides for non- fullerene polymer solar cells

Perylene diimide (PDI) derivatives have great potential to replace the commonly used fullerene derivatives in bulk heterojunction (BHJ) polymer solar cells (PSCs). However, PDI-based molecules have a strong tendency to aggregate in thin films, which prevents the formation of appropriate phase separation. To alleviate this problem, much effort has been devoted to developing twisted PDI dimers with functional bridges at bay positions. Unfortunately, the bay-functionalized PDI dimers exhibit relatively low electron mobilities due to the excessive twist geometry. To solve this dilemma, an efficient method is to functionalize the PDI at less sterically-encumbered ortho-positions. In this article, we synthesized two novel ortho-functionalized PDI dimers, namely PDI-2Th-PDI and PDI-ThFTh-PDI, and investigated the influence of the conjugated bridge length on the physical and photovoltaic properties of PDIs. After introducing a fluorene bridge between the thiophene units, PDI-ThFTh-PDI with the extended conjugate length possessed a relatively low-crystalline nature and therefore exhibited one order of magnitude lower electron mobility compared to PDI-2Th-PDI. As a result, the power conversion efficiency (PCE) of solar cells fabricated with PDI-2Th-PDI as the electron acceptor and PTB7-Th as the electron donor is almost twice that of the solar cell with PDI-ThFTh-PDI as the acceptor. Our findings provide important guidelines for the design of more efficient ortho-functionalized PDI acceptors.