Numerical simulation of the transient photoconductivity in a-Si:H as a function of the excitation density

The dependence of the transient photoconductivity induced by pulsed excitation (TPC) on the excitation density is discussed with the help of numerical simulations. It is shown that recombination between excess mobile electrons and all excess holes (mainly localized) can explain the excitation density dependence of the TPC amplitude of standard a-Si:H at room temperature using a rate parameter k(BB) of 10(-8)cm(3)/s. This model leads to a decay faster than experimentally observed in the time range from 40ns to 1 mu s. A variation of the recombination model is presented that gives a better fit far the longer time range still showing the correct excitation density dependence in the short time range. Moreover comparison of the simulations with experimental data yields limits for the parameters of the conduction band tail. In particular, the time necessary to establish a dynamic equilibrium of excess electrons between delocalized states in the conduction band and localized states in the tail appears to be very informative.