Diindenocarbazole-based large bandgap copolymers for high-performance organic solar cells with large open circuit voltages

Three donor-acceptor alternating copolymers abbreviated as PC1, PC2, and PC3, respectively, were designed and synthesized using diindenocarbazole (DIC) and dithienylbenzothiadiazole (DTBT) units. Through backbone manipulation, copolymers with large bandgaps (similar to 2.0 eV) and deep-lying HOMO energy levels (below -5.41 eV) were obtained. The side chains were also investigated to tune the intermolecular interactions and morphology of the copolymers blended with PC71BM. Polymer solar cells (PSCs) based on PC2 : PC71BM exhibit an outstanding power conversion efficiency (PCE) of 7.26%, which represents one of the highest PCEs ever reported for PSCs while combining an open-circuit voltage (V-oc) of 0.93 V and a large optical bandgap of 2.01 eV. Under similar device fabrication conditions, regular PSCs based on PC1 and PC3 achieve PCEs of 2.45% and 6.68%, respectively. Moreover, inverted PSCs derived from PC2 also exhibit an attractive PCE of 6.17% with a high V-oc of 0.92 V. In view of its similar optical profiles to P3HT, but a deeper-lying HOMO energy level, PC2 should be a promising candidate as a short wavelength absorbing material for tandem solar cells.