Dithienoquinoxaline-quaterthiophene wide bandgap donor polymers with strong interchain aggregation for efficient organic solar cells processed with a non-halogenated solvent

Abstract

High-performing and easily synthesized oligothiophene-based polymer donors that can be processed with non-halogenated solvents are an important concern to meet massive production of organic solar cells (OSCs). In this work, two dithieno[3,2-f:2′,3′-h]quinoxaline-quaterthiophene-based polymers, PQx4T and PQx4T-2F, with wide bandgaps of 1.95 eV, were designed and synthesized to reveal the relation between the fluorinated backbone structure and photovoltaic performance. Compared to PQx4T, the polymer PQx4T-2F with fluorination on the central bithiophene could show a deeper HOMO energy level, more planar backbone, stronger interchain aggregation, and higher hole mobility. With non-fullerene Y14 as the acceptor, chloroform-processed PQx4T-2F:Y14 active layers showed a much higher power conversion efficiency (PCE) of 15.95% compared with that of the PQx4T based OSCs of 13.30%. Using toluene as the non-halogenated solvent could further elevate the efficiency of the PQx4T-2F:Y14 active layer to 16.82% when a solution temperature of 60 °C was selected. This is the highest device performance for OSCs with simple oligothiophene-based polymer donors when fabricated with non-halogenated solvents. Moreover, the polymer PQx4T-2F with strong interchain aggregation was applied to construct thick-film OSCs, whose toluene-processed 200 and 300 nm thick active layers also achieved PCEs of 16.39% and 15.90%, respectively. The results suggest that the oligothiophene-based polymer PQx4T-2F is promising for the OSC application.