Side-chain engineering improves molecular stacking and miscibility for efficient fullerene organic solar cells

Abstract

Side-chain engineering has been acknowledged as an ingenious method to regulate material crystallinity, miscibility and microstructure to achieve favorable photovoltaic performance. Subtle changes of the side chains would have a significant impact on the intermolecular interactions and molecular stacking. In this work, two conjugated polymers were synthesized based on BDT (D) and TT (A) units with different side chain substituents, thiophene sulfur, benzene sulfur and methyl, on the π-bridge. The PPSBTz-Me (with benzene sulfur side chains) exhibits a deeper HOMO energy level, and PTSBTz-Me (with thiophene sulfur side chains) exhibits strong intermolecular interactions, ordered stacking, and appropriate miscibility with fullerene materials, thus achieving an excellent photovoltaic performance in fullerene organic solar cells, with a high PCE of 10.36%, a V_OC of 0.89 V, a J_SC of 17.34 mA cm⁻², and a FF of 67.39%. Therefore, the subtle structure change has an important impact on the performance of organic photovoltaics, and the underlying mechanism needs to be carefully studied in depth.