Dispersive transport in the temperature dependent transient photoresponse of organic photodiodes and solar cells

Abstract

The nanosecond transient photoresponse of organic solar cells and photodiodes based on a conjugated polymer (poly(3-hexylthiophene-2,5-diyl)) blended with a fullerene derivative ([6,6]-phenyl C61-butyric acid methyl ester) exhibits a strong temperature dependence, whose origin can be traced back to charge carrier transport phenomena. In the framework of a drift-diffusion model including multiple-trapping, the temperature dependence of effective mobilities arises naturally without the need of using a temperature dependent parameterization of the mobilities. Furthermore, the extended drift-diffusion simulation reproduces the measured change of slope of the transient current density from j(t)∼t(−1+α) to j(t)∼t(−1−α), indicating dispersive charge carrier transport influenced by an exponential trap distribution characterized by the dimensionless parameter α. A second kink is identified to be the point in time of the crossover from electron to hole dominated charge carrier transport, enabling for the determination of the donor and acceptor transport properties independent of each other.