Operando ESR elucidation of charge accumulation and molecular orientation in ternary polymer solar cell materials using organic electrochemical transistor structures

In recent years, polymer solar cells have been investigated extensively because of their cost-effectiveness and flexibility. Notably, inverted type polymer solar cells using PTzBT((2,5-bis(3-(2-butyloctyl)thiophen-2-yl)thiazolo[5,4-d]thiazole)-alt-(2,5-bis(3-(2-hexyldecyl)thiophen-2-yl)thiazolo[5,4-d]thiazole)) have gained prominence because of their superior conversion efficiency and stability, particularly with the incorporation of non-fullerene acceptor ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene) into the active layer. Nevertheless, a comprehensive understanding of charge accumulation states and molecular orientation within PTzBT remains elusive. For this study, electron spin resonance (ESR) spectroscopy was used to clarify the issues above in conjunction with organic electrochemical transistor structures, which are recognized for their low-voltage operation and flexibility. Our operando ESR investigation revealed the accumulation of positive holes within the PTzBT molecules, simultaneously revealing anisotropy in the ESR spectra upon altering the external magnetic field direction. Intriguingly, an additional observation surfaced: angle variation of the g -factor exhibited discernible changes related to the gate voltage. This finding demonstrates that charges are injected into distinct orientations in PTzBT molecules depending on the amount of accumulated charge, thereby contributing to improvement of solar cell performance.