Temperature dependence of charge carrier ranges in stabilized a-Se photoconductors

Using time-of-flight (TOF) and interrupted-field TOF we have measured the electron and hole drift mobilities, μe and μh, lifetimes, τe and τh, and hence carrier ranges, μeτe and μhτh, in stabilized a-Se as a function of temperature from −25 to 45 °C. Above room temperature, μeτe and μhτh show a slight increase with temperature and peak around 31 and 35 °C, respectively. The μeτe at 35 °C and μhτh at 40 °C are approximately the same as room temperature values. Below room temperature, down to −25 °C, both μe and μh decrease with decreasing temperature with activation energies Eμe and Eμh that are 0.38 and 0.21 eV. The electron and hole lifetimes, τe and τh, are also thermally activated but they increase with decreasing temperature with activation energies Eτe and Eτh that are 0.31 and 0.18 eV. These are quite close to corresponding activation energies for the drift mobilities within experimental errors. The electron and hole ranges, μeτe and μhτh, therefore exhibit only a weak temperature dependence, both increase slightly with temperature. These observations are consistent with shallow trap controlled transport in the presence of a set of deep traps. The X-ray sensitivity of a photoconductor, among other factors, depends on the charge collection efficiency, which depends on electron and hole ranges, μeτe and μhτh. Because these carrier ranges do not show any significant temperature dependence, one can conclude that the X-ray sensitivity of stabilized a-Se over the temperature range from −25 to 40 °C should not be affected by changes in the collection efficiency.