Density of states in pure and As doped amorphous selenium determined from transient photoconductivity using Laplace-transform method

A Laplace-Transform method is used to determine the localized density of states (DOS) in amorphous Selenium (a-Se) pure and a-Se doped with arsenic (As) from the transient photocurrents. The obtained results confirm the existence of two defect levels above the valence band edge in pure a-Se superimposed on an exponential tail. However, in As doped a-Se the shallower defect disappears while the deeper one is more pronounced than in pure a-Se. Using the well-known least square method the DOS of doped a-Se with 0.2 at% and 0.5 at% As are calculated. Using these calculated DOS we, successfully, reproduced the experimental photocurrents in these materials.