Transient charge carrier dynamics in organic solar cell devices studied by simultaneous optical and electric detection

For the clarification of dynamics of photogenerated carriers in practical organic solar cell devices, we have developed a methodology to simultaneously acquire reflection-mode transient optical absorption (ΔA) and transient electric current (Δi) signals. For a typical polythiophene:fullerene bulk heterojunction solar cell device, both the ΔA and Δi signals due to the photogenerated carriers are characterized by the power-law decays of ∝t-α, which are interpreted by detrapping-limited recombination at earlier times than ∼1 μs and trap-free diffusion/drift at later times. Furthermore, we have succeeded in observing switching of the power index α for ΔA signals as well as for Δi signals; the time at which switching occurs indicates the extraction of carriers by electrodes (transit times). From the transit times for ΔA and Δi, transit mobilities μtrΔA and μtrΔi are obtained, which are on the same order. It has been found from the comparison of the cell parameters among several devices fabricated under similar conditions that the device-to-device variation of photon energy conversion efficiency (0.5%-2%) is strongly correlated with the ratio μtrΔA/μtrΔi. It is considered that the charge accumulation at the active layer/electrode interfaces induces a delay between the carrier transport and electrode collection, which significantly lowers the power conversion efficiency. Our simultaneous optical and electrical detection thus allows us to diagnose carrier dynamics in individual devices that affect the solar cell performance.