Esr study of degradation mechanism due to charge formation in polymer solar cells

Abstract

The degradation mechanism of polymer solar cells containing a lithium-fluoride (LiF) buffer layer created by a thermal annealing process at different temperatures was studied using an electron spin resonance (ESR) and current-voltage characteristics. The device structure was indium-tin-oxide (ITO)/poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS)/poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM)/LiF/Al. When the post-annealing temperature increased, the performance of the cells decreased and the ESR signal from PCBM radical anions was enhanced dramatically. The formation of a greater number of anions enhanced the electron scattering and caused an energy-level shift at the interface. These effects led to degradation in the device performance.