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 %

In this work, difluoro and dicyano substituted terpolymers, P6TI-3N1F, P6TI-1N1F, and P6TI-1N3F, based on isoindigo -sexithiophene & pi;-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, mono -molecular 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.

Automated summary

In this work, difluoro and dicyano substituted terpolymers based on isoindigo-sexithiophene & pi;-conjugated polymers were designed and characterized. The champion terpolymer, P6TI-1N1F, demonstrated superior performance in terms of electron/hole mobility, nanofibril and its agglomeration, charge recombination, polymer chain alignment, and photoluminescence quenching in binary and ternary thin films. The binary polymer solar cells (PSCs) based on a bulk heterojunction (BHJ) of P6TI-1N1F:PC71BM or P6TI-1N1F:IT4F showed the highest power conversion efficiency (PCE) of 10.4% or 11.9%, respectively, while the ternary PSCs achieved PCE as high as 12.8% or 14.5%.