High-Performance Fullerene Free Polymer Solar Cells Based on New Thiazole -Functionalized Benzo[1,2-b:4,5-b′]dithiophene D-A Copolymer Donors

Two new wide bandgaps D-A copolymers compromising of same benzo [1,2-b:4, 5-b ']dithiophene functionalized with thiazole side-chain donor unit and different acceptors units, i. e., fluorinated benzotriazole (FBTA) (PBDTTZ-FBTA) and bis-thiophene-thieno-benzothiazole (BTZ) (PBDTTZ-BTZ) were synthesized and their optical and electrochemical properties were investigated. These copolymers exhibit appropriate frontier energy levels and complementary absorption to most middle and narrow bandgap non-fullerene small molecule non-fullerene acceptors. We have explored using these two copolymers as donors combined with a narrow bandgap non-fullerene acceptor, namely BThIND-Cl, for the construction of polymer solar cells. The optimized polymer solar cells employing PBDTTZ-FBTA: BThIND-Cl showed overall power conversion efficiency of 14.96 % (with energy loss of 0.51 eV), which is higher than that for PBDTTZ-BTZ: BThIND-Cl (11.68 % with energy loss of 0.56 eV). Although the PBDTTZ-BTZ exhibits a broader absorption profile than that of PBDTTZ-FBTA, the higher power conversion efficiency of the later copolymer may be correlated with the high charge carrier mobility, suppression of the bimolecular and trap-assisted recombination due to appropriate phase separation and compact pi-pi stacking distance in the active layer and low energy loss.