Monte Carlo simulation of charge transport in amorphous selenium photoconductors

Abstract

The electronic properties of amorphous materials are greatly affected by the density of localized states in the mobility gap of these materials. The exact shape of the density of states (DOS) distribution in amorphous selenium (a-Se) is still unresolved despite decades of research. One of the most commonly employed methods to investigate charge transport properties in high resistivity materials is time-of-flight (TOF) transient photoconductivity experiment. The TOF transient photoconductivity technique is used to measure the induced photocurrent in the external circuit when the sample is photoexcited. Information pertaining to carrier mobility and other carrier parameters are deduced from the shape of the photocurrent. The investigation of the charge transport phenomenon is well known to be a complicated task. Monte Carlo (MC) simulation method has become a standard method for carrier transport studies in amorphous materials. The purpose of this research work is to develop a Monte Carlo simulation model for charge transport in typical TOF transient photoconductivity experiment to investigate the DOS distribution in a-Se. The MC simulations were first performed for relatively simpler models for which theoretical and analytical solutions were available. The MC model developed here is based on simulating the drift of carriers resulting from photogeneration, subject to the influence of an applied electric field and multiple trapping events. The free drift time of photocarriers and their dwell time in the traps are stochastic in nature, in accordance with the probabilities of these events. Electron time-of-flight transient photocurrents were calculated in amorphous selenium as a function of the electric field. The distribution of localized states (DOS) in a-Se has been investigated by comparing the experimentally measured and calculated transient photocurrents. The analysis of multiple-trapping transport has been done by the discretization of a continuous DOS. The DOS distribution has been optimized to produce the best agreement between the calculated and measured transient photocurrents. The resulting DOS has distinct features: A first peak at ~0.30 eV below Ec with an amplitude ~1017 eV–1 cm–3, a second small peak (or shoulder) at 0.45–0.50 eV below Ec with an amplitude 1014–1015 eV–1 cm–3, and deep states with an integral concentration 1011–1014 cm–3 lying below 0.65 eV, whose exact distribution could not be resolved because of the limitations of the available experimental data. The density of states (DOS) distribution in the vicinity of the valence band mobility edge in vacuum coated a-Se films has been investigated by calculating the MC hole transient photocurrents at different temperatures, and also the dependence of the drift mobility on the temperature and field. The calculated TOF transient photocurrents were compared with experimental data published elsewhere. It is shown that, analogous to electron transport in a-Si:H, the DOS near Ev is a featureless, monotonically decreasing distribution in energy up to Ev + 0.4 eV, without the 0.28 eV peak near the valence band which was thought to control the hole drift mobility. Such a DOS was able to account for hole TOF data reported previously by several authors to date.