High lying energy of charge-transfer states and small energetic offsets enabled by fluorinated quinoxaline-based alternating polymer and alkyl-thienyl side-chain modified non-fullerene acceptor

Abstract

Abstract Significant driving forces are the prerequisite to achieve fast and efficient charge separation in fullerene derivatives-based polymer solar cells to achieve high power conversion efficiency (PCE). However, the large driving forces both in photo-induced hole transfer (PHT) and in photo-induced electron transfer (PET) processes lead to significant energy losses, resulting in low open-circuit voltage in the devices. Recent studies indicate the driving forces in non-fullerene acceptors-based devices can be reduced to very low values but still with high PCE and low energy losses. Herein, we report a new donor:acceptor system with high lying energy of charge-transfer excitons (ECT) of 1.50 eV and very small driving forces (PHT of 0.28 eV and PET of 0.11 eV), in which a fluorinated quinoxaline-based alternating polymer (FTQ) and an alkyl-thienyl side-chain modified small molecule (ITIC-Th) are taken as the donor material and non-fullerene acceptor material, respectively. A high power conversion efficiency (PCE) of 8.19% with maximal external quantum efficiency of 71% are achieved successfully in FTQ:ITIC-Th-based device after appropriate thermal annealing treatment, indicating FTQ can be further applied as donor materials with other highly efficient NF-acceptors to achieve enhanced performances and low energy losses.