Isoindigo-dicyanobithiophene-Based Copolymer for High Performance Polymer-Fullerene Solar Cells Reaching 1.06 V Open Circuit Voltage and 8.36% Power Conversion Efficiency

To investigate the effect of substitution of cyano groups (CN) on D-pi-A type conjugated copolymer in photophysical and photovoltaic properties, a non-CN-substituted P4TIH and a CN-substituted P4TIN isoindigo-based copolymers were synthesized and characterized. Having dicyano-substituted bithiophene as electron-donating segment and isoindigo as electron-accepting segment, P4TIN exhibits a deeper highest occupied molecular orbital energy level (HOMO) than that of the non-CN-substituted P4TIH. Benefiting from the improved solubility via copolymer side-chain substituent (2-decylteradecyl), inverted solar cells fabricated with a thick (similar to 200 nm) active layer (P4TIN:PC61BM, 1:2.0) have achieved a very high open circuit voltage of 1.06 V. High power conversion efficiency of 8.36% can be reached without thermal annealing treatment or processing solvent additives.