Light-Induced Degradation Mechanism in Poly(3-hexylthiophene)/Fullerene Blend Solar Cells

The mechanism of light-induced degradation in organic solar cells based on regioregular poly(3-hexylthiophene) and indene-C-60 bisadduct is studied by transient absorption (TA) and electron spin resonance (ESR) measurements. After 45 h light exposure under simulated solar illumination at 100 mW cm(-2), the short-circuit current density, open-circuit voltage, and fill factor are all degraded by about 20%-30% relative to the initial photovoltaic parameters. For the assignment of limiting conversion processes in the degraded solar cells, exciton diffusion into a donor/acceptor interface, charge transfer at the interface, charge dissociation into free charge carriers, and charge collection to each electrode are observed before and after the light exposure by the TA measurement. As a result, it is found that the charge collection deteriorates after the light exposure because of light-induced charge trap formation in the bulk of the active layer. The origin of charge traps is further discussed on the basis of ESR measurements and density functional theory calculation.