Efficient charge generation and low open circuit voltage loss enable a PCE of 10.3% in small molecule donor and polymer acceptor organic solar cells

Abstract

Using a combination of polymer acceptors and small molecule donors (SM_D/P_A-type) is expected to exhibit easier morphology control because of the different diffusion speeds and solubilities due to their relatively large molecular weight difference, but lack of investigation. In this manuscript, we developed two novel small donor molecules Se-1 and Se-2, with alkyl chains in the π-bridge near the donor core or far away from the donor core. We investigated their effects on the molecular properties, morphologies and device performances of the new type of OSCs. The alkyl chains away from the donor core (Se-2) exhibit ordered molecular packing ability, longer exciton lifetime, and an induced face-on packing mode in the blend. Consequently, the Se-2/polymer acceptor blend, with a concurrently high charge generation and low open-circuit voltage loss, reached a best PCE of 10.3%, which is higher than that of the Se-1/polymer acceptor (9.4%). The result shows the highest PCE in the less investigated SM_D/P_A-type OSCs. Through comparison of other types of OSCs based on the two small donor materials, we find that the small molecules and PJ1 exhibited high miscibility, and we emphasize the importance of enhancing the Flory–Huggins interaction parameter and molecular crystallinity in further photovoltaic material design in SM_D/P_A-type OSCs.