Comprehensive studies of response characteristics of organic photodetectors based on rubrene and C₆₀

We studied the transient response characteristics of organic photodetectors composing of high mobility materials of rubrene and C₆₀, respectively, as donor and acceptor. It was found that the response speed was limited by the delay time of both exciton diffusion and transit, and an anomalous phenomenon that the bandwidth decreases as the reverse bias increases was found for the first time. The investigation of frequency dependence at different device structures and light excitations demonstrated that the light absorption of rubrene prevents the photodetector from obtaining a fast response. With the help of magnetic field effect study, it was clearly elucidated that the slow diffusion time of the long lifetime triplet excitons generated from singlet fission in rubrene limited the bandwidth of the device. Moreover, the simulation of the response of photodetector under transient and steady state by exciton transport-diffusion equation showed that the exciton dissociation efficiency in rubrene increases more quickly than that in C₆₀, which should account for the negative dependence of bandwidth on the reverse bias in rubrene-based device.