Dinaphtho-s-indacene-based copolymers for inverted organic solar cells with high open-circuit voltages

We report on the synthesis and characterization of a series of donor–acceptor copolymers (PF1, PF2, PF3 or PF4) based on a dinaphtho-s-indacene (DNI) donor unit and four different acceptor units. The molecular weights of the copolymers were determined by using gel permeation chromatography, and their electrochemical properties were investigated by cyclic voltammetry. All four copolymers showed deep-lying highest occupied molecular orbital energy levels. Inverted bulk heterojunction solar cells were fabricated by using the synthesized copolymers as the electron donor material and 6,6-phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor material. Inverted solar cells based on PF1:PC71BM (1:4, w/w) exhibited a power conversion efficiency (PCE) of 3.07%, a high open-circuit voltage (Voc) of 0.99 V, a short-circuit current density (Jsc) of 7.85 mA/cm2, and a fill factor of 39.5% under the AM1.5G illumination. With the same fabrication method, the inverted devices based on PF2, PF3 and PF4 showed PCEs of 2.62, 1.18 and 1.32%, and Voc values of 0.97, 0.91 and 0.80 V, respectively.