Exciton-Charge Annihilation in Organic Semiconductor Films

Time-resolved optical spectroscopy is used to investigate exciton-charge annihilation reactions in blended films of organic semiconductors. In donoracceptor blends where charges are photogenerated via excitons, pulsed optical excitation can deliver a sufficient density of temporally overlapping excitons and charges for them to interact. Transient absorption spectroscopy measurements demonstrate clear signatures of exciton-charge annihilation reactions at excitation densities of similar to 10(18) cm(-3). The strength of exciton-charge annihilation is consistent with a resonant energy transfer mechanism between fluorescent excitons and resonantly absorbing charges, which is shown to generally be strong in organic semiconductors. The extent of exciton-charge annihilation is very sensitive not only to fluence but also to blend morphology, becoming notably strong in donoracceptor blends with nanomorphologies optimized for photovoltaic operation. The results highlight both the value of transient optical spectroscopy to interrogate exciton-charge annihilation reactions and the need to recognize and account for annihilation reactions in other transient optical investigations of organic semiconductors.