A diketopyrrolopyrrole-based low bandgap polymer with enhanced photovoltaic performances through backbone twisting

Abstract

A diketopyrrolopyrrole (DPP)-based π-conjugated polymer is a very promising low band gap electron donor material for polymer solar cells. We have incorporated alkyl groups into the 4-position of the thiophene rings connected to the DPP fragment, which is proven to be beneficial for improving the open-circuit voltage (V_OC) and short-circuit current density (J_SC). Two DPP-based polymers are synthesized with benzo[1,2-b:4,5-b′]dithiophene (BDT) as the electron-donating segment. Both the polymers show good solubility, slightly wide optical band gaps and deep HOMO energy levels when incorporating the alkyl groups. Heterojunction solar cells are fabricated with polymer:PC₇₁BM as the active layer. V_OC and J_SC are simultaneously enhanced compared to the device performance of traditional DPP-BDT alternating polymers. A power conversion efficiency (PCE) of 8.11% was obtained, which indicates that rational utilization of backbone torsion is a promising strategy to improve the photovoltaic performance.