Investigation of density of states and charge carrier mobility in amorphous semiconductors via time-of-flight photocurrent analysis
F. Serdouk, A. Boumali, M. L. Benkhedir, Y. Goutal
TL;DR
This paper tackles how to quantify the density of localized states (DOS) and hole mobility in amorphous selenium (a-Se) using time-of-flight (ToF) measurements and numerical simulation. The authors apply a Laplace-transform analysis of transient photocurrents within a trap-controlled multiple-trapping model to reconstruct the conduction-band DOS without relying on assumed functional forms. They identify two distinct trap features near $E-E_c \approx 0.30$ eV (shallow) and $0.45$-$0.50$ eV (deeper) and demonstrate symmetry between conduction- and valence-band tails, consistent with field-assisted thermally activated transport. The mobility shows Arrhenius-type temperature dependence with activation energies decreasing from about $0.28$ eV at zero field to $\sim 0.18$ eV at higher fields, in agreement with a Poole–Frenkel mechanism and with simulated data based on the extracted DOS. Overall, the work provides a physically grounded, efficient method to probe disorder-induced transport in amorphous semiconductors and informs device-level modeling of a-Se photoconductors.
Abstract
The present study examines the electronic transport characteristics of amorphous semiconductors through TOF measurements and numerical simulations. The primary objective is to determine the DOS in amorphous selenium (a-Se) and to assess the temperature and electric field dependence of the hole mobility. A comprehensive investigation of localized states within the mobility gap is performed using Laplace transform analysis of ToF photocurrent transients, combined with the multiple trapping model. This approach enables accurate reconstruction of the DOS across a wide temperature range, allowing clear identification of shallow and deep trap levels and revealing thermally activated transport mechanisms. Simulated ToF currents are also used to evaluate the hole drift mobility under various thermal and field conditions. Activation energies are extracted from Arrhenius plots of the mobility data. The results support a physically consistent description of the electronic structure in a-Se and validate the applicability of Laplace-based techniques for probing charge transport in disordered semiconductors.