Optimizing the energy levels and crystallinity of 2,2′-bithiophene-3,3′-dicarboximide-based polymer donors for high-performance non-fullerene organic solar cells

Abstract

The energy levels and crystallinity of the polymer donor greatly affect the active layer morphology and device performance in non-fullerene organic solar cells (OSCs). In this work, a new series of polymer donors consisting of dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT) and 2,2′-bithiophene-3,3′-dicarboximide (BTI) units were synthesized and named PDTBDT, PDTBDT-T, and PDTBDT-T-Cl. The BTI unit shows a strong electron withdrawing ability to reduce the highest occupied molecular orbital (HOMO) level and a good planarity to reduce the steric hindrance effectively. The combination of BTI and DTBDT units with high planarity and rigidity magnificently maintains the crystallinity of the polymer, improves that of the acceptor, and obtains a preferred face-on orientation, thereby greatly improving the charge transfer efficiency. The introduced thiophene π-bridges and chlorine atoms in the conjugated structure further optimized light absorption and the energy level of the polymers. With the small molecule Y6 as the acceptor, the optimized OSCs based on PDTBDT-T-Cl:Y6 achieved a power conversion efficiency of 15.63%, which is remarkably higher than those of PDTBDT-T:Y6 (12.71%) and PDTBDT:Y6 (8.22%). These results show that BTI-based polymer donors can achieve comparable performance to the classic polymer donor of PM6 by designing their molecular structure to enhance the crystallinity, promote the stacking of the acceptor, optimize the phase separation morphology, and improve the performance of the device.