Efficient Interface Engineering Enhances Photovoltaic Performance of a Bulk-Heterojunction PCDTBT:PC₇₁BM System

High-performance conventional polymer solar cells based on a poly[N-9''-heptadecanyl-2,7-carbazole-alt- 5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT): [6,6]-phenyl C71-butyric acid methyl ester (PC $_71$ BM) system are achieved via cathodic interfacial engineering by a bispyridinium salt small molecule (FPyBr). The bispyridinium salt can form a directed dipole at the cathode interface and thus decrease the cathode work function, leading to an improved built-in potential and open-circuit voltage. The good energy level alignment and hole-blocking ability of FPyBr at the cathode may suppress the charge recombination and promote the electron extraction process to improve the device performance. A smooth and uniform FPyBr film can be formed on the active layer, resulting in a good interfacial contact. As a result, a power conversion efficiency of 7.68% can be realized with FPyBr, which is significantly higher than for bare Al, solvent-treated and poly[(9,9-bis(3'-(N,N-dimethylamino)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]-modified devices. To the best of our knowledge, this result is one of the highest photovoltaic performance based on PCDTBT:PC $_71$ BM system. Therefore, FPyBr is a promising cathode modifier for high-performance polymer solar cells.