Effects of incorporating different chalcogenophene comonomers into random acceptor terpolymers on the morphology and performance of all-polymer solar cells

Abstract

A new family of naphthalenediimide (NDI)-bithiophene-based random terpolymers (PNDI-Fu10, PNDI-Th10, and PNDI-Se10) was prepared by incorporating a small amount (10 mol%) of different chalcogenophene units, namely furan (Fu), thiophene (Th), and selenophene (Se), into a poly((N,N’-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5′-(2,2′-bithiophene)) (P(NDI2OD-T2)) backbone. The terpolymers all exhibited negligible optical property and frontier energy level differences. Interestingly, the blend film morphology and photovoltaic performance of all-polymer solar cells (all-PSCs), comprising a random terpolymer series as an acceptor and poly(6-fluoro-2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-dyl-alt-thiophene-2,5-diyl) (FTQ) as a donor, were strongly dependent on the type of chalcogenophene. Consequently, the all-PSCs revealed a clear variation in short-circuit current density (J_SC) and fill factor (FF) while retaining identical open-circuit voltages (V_OC). The greatest power-conversion efficiency of 5.88% with a high J_SC of 10.77 mA cm⁻² was observed for PNDI-Th10:FTQ because of a synergistic contribution from a more preferential π-face-on orientation and suppression of bimolecular/geminate recombination. This was confirmed by a detailed investigation on the morphology and charge dynamic structural properties. In addition to understanding their effects on photovoltaic characteristics, this study demonstrates that introducing a small amount of the chalcogenophenes into a well-performing polymer is a simple and effective method to improve J_SC values while maintaining V_OCs of the parent polymers.