Two-Layer Multiple Trapping Model for Charge Transport in Molecularly Doped Polymers

Organic materials are being investigated for their electronic properties. Such materials are especially attractive for lightweight, flexible, and low-cost solar cells and light emitting devices, as well as transistors and electrophotographic photoreceptors. Yet, even after 40 years of work and a large database, the physics and chemistry that determines the electronic properties of organic materials are not well understood. This paper briefly summarizes data obtained from a new experimental variant of the time of flight (TOF) technique called TOF1a, which are compared to the predictions of a two-layer multiple trapping model (MTM) with an exponential distribution of traps. In TOF1a the charge generation depth is varied continuously, from surface generation to bulk generation, by varying the energy of the electron-beam excitation source. This produces systematic changes in the shape of the current transient that can be compared to the predictions of the two-layer MTM. We find that we can semi-quantitatively fit current transient data over the whole time range of the experiment, but only by using theoretical parameters that lie in a narrow range, the extent of which we quantify here.