Influence of the dipole moment on the photovoltaic performance of polymer solar cells employing non-fullerene small molecule acceptor

Abstract

A new narrow bandgap non-fullerene acceptor with a DPP core and dicyanorhodanine terminals linked by selenophene spacers has been synthesized. Optical and electrochemical properties of MPU5 were examined and compared to that of its comparable MPU2, which contains thiophene linkers instead of selenophene. MPU5 has a lower optical bandgap (1.38 eV) than MPU2 (1.41 eV), this may be due to the higher electron richness and lower aromatici character of selenophene, comparative to thiophene, favoring a quinoidal structure and intramolecular charge transfer between push–pull moieties. MPU5 was employed as the electron acceptor in conjunction with a conjugated polymer (PCDTBT) as donor with a complementary absorption (350–650 nm) and high HOMO and low LUMO energy levels (–5.4 eV and –3.6 eV, respectively) for the fabrication of polymer solar cells using a bulk heterojunction active layer. The MPU5-based polymer solar cell enabled an overall power conversion efficiency (PCE) of 10.10% with an Eloss of 0.43 eV with an optimized bulk heterojunction active layer. These values are better than those for the MPU2 counterpart (PCE = 8.04%, Eloss = 0.56 eV). The higher PCE obtained for the MPU5-based polymer can be correlated with better balanced charge transport, appropriate active layer nanoscale morphology, efficient charge extraction and longer charge carrier lifetime, resulting in an improvement in the Jsc and FF values. The superior Voc value for the MPU5-based polymer may be related to the lower energy loss caused by the larger dipole moment and dielectric constant of MPU5, which result in a smaller exciton binding energy in the active layer.