Difluoro and dicyano substituted sexithiophene − isoindigo terpolymers enabling ternary polymer solar cells with a bulk heterojunction thickness 100 nm → 300 nm and a minor decline of efficiency 14.5 % → 14.0 %

Abstract

In this work, difluoro and dicyano substituted terpolymers, P6TI-3N1F, P6TI-1N1F, and P6TI-1N3F, based on isoindigo − sexithiophene π-conjugated polymers have been designed and characterized. Together with two only cyano and only fluoro substituted copolymers, P6TIN and P6TIF, we have demonstrated that P6TI-1N1F is the champion terpolymer in terms of the ratio of electron/hole mobility, nanofibril and its agglomeration, monomolecular and bimolecular charge recombination, polymer chain face-on alignment, and photoluminescence quenching in a binary thin film of polymer:PC71BM or polymer:IT4F. In the binary polymer solar cells (PSCs) based on a bullk heterojunction (BHJ) of either polymer:PC71BM or polymer:IT4F, P6TI-1N1F exhibits the highest fill factor of 67.62 % or 76.86 %, and hence the highest power conversion efficiency (PCE) of 10.4 % or 11.9 %. Corresponding ternary PSCs with a 100 nm thick BHJ of P6TI-1N1F:IT4F:PC71BM or P6TI-1N1F:Y6:PC71BM exhibit PCE as high as 12.8 % or 14.5 %. It is rarely seen in literature that the PCE of the ternary PSCs shows only minor decline to 12.4 % or 14.0 % when the thickness of BHJ increases from 100 nm to 300 nm.