The mobility spatial distribution function: Turn-on dynamics of polymer photocells

Abstract

To better understand the transport in thin film devices and the role of the excitation density we have measured the photocurrent dynamic response of a photocell upon the switch on of light excitation. Unlike the standard time of flight methods that utilize a pulse excitation we employ a step function that is more compatible with real device operating conditions. The fact that the steady state of the step function is the cw operation of the device allows us to examine the role of charge density and use device analysis tools. To explain the broadened features measured we introduce a physical scheme by which the dispersive nature is due to spatial inhomogeneity, and the semiconducting layer is considered to be comprised of a distribution of parallel pathways (on the nanometer scale) each having a different, but well defined, mobility value. This insight allows us to extract, in a unique way, a mobility distribution function. The mean values of this distribution function are in good agreement with those deduced by previous measurements made on the same sample in which the mobility was determined from the excitation power dependence of the photocurrent quantum efficiency.