Curvature effects of electron-donating polymers on the device performance of non-fullerene organic solar cells

Abstract In this study, we investigate the effects of polymer backbone curvature on the performance of non-fullerene organic solar cells (NFA-OSCs) fabricated using an electron-donating π-conjugated polymer and a non-fullerene electron-acceptor (NFA), 3,9-bis[2-methylene(3-(1,1-dicyanomethylene)-indanone)]-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2ʹ,3ʹ-dʹ]-s-indaceno[1,2-b:5,6-bʹ]dithiophene (ITIC). The NFA-OSCs are fabricated using structurally similar bis(pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione) (BPPD)-based π–conjugated polymers, i.e., P1 and P2, and they indicate almost identical absorption and energy levels but dissimilar backbone curvatures. P1 comprises two-dimensional conjugated benzodithiophene (BDTT) and thiophene π-bridged BPPD derivatives, and P2 comprises BDTT and thieno[3,2-b]thiophene π-bridged BPPD derivatives. The absorption maximum and highest occupied/lowest unoccupied molecular orbital levels of P1 and P2 are ~505 nm and ~˗5.4/˗3.4 eV, respectively. However, P1 shows a twisted backbone, whereas P2 exhibits a well-controlled wavy backbone owing to the presence of slightly different-structured π-bridges, such as thiophene and thieno[3,2-b]thiophene, on their backbones. The NFA-OSCs are fabricated with the structures of glass-ITO/PEDOT:PSS/P1 and P2:ITIC+0.5 vol% 1,8-diiodoctane (DIO)/PFN-Br/Al provide quite different device performances with power conversion efficiency of 0.88% and 6.25%, respectively. In general, the curvature of the polymer backbone significantly affects the nanoscale network formation between the polymer and ITIC, thereby resulting in a significant difference in the carrier transport and photovoltaic performances of the NFA-OSCs.