Relaxation in the electrical properties of amorphous selenium based photoconductors

Abstract

Time-of-Flight (TOF) and Interrupted-Field Time-of-Flight (IFTOF) measurements were performed repeatedly on several different samples of amorphous Selenium (a-Se) alloys as they aged from deposition or after annealing above the glass transition temperature (Tg) in order to examine the relaxation of the electrical properties. The mobility was found to relax slightly, but the relaxation did not fit well to a stretched exponential. The increase in the mobility for electrons was significantly more than the increase in mobility for holes in all sample compositions measured. For electrons, the mobility increased by 20-40%, whereas for holes, the mobility only increased by less than 10%. The relaxation of the lifetime, on the other hand, fit well to a stretched exponential. Furthermore, the overall increase in lifetime as it relaxed was much greater than the increase in the mobility. The average increase in lifetime was 85% for holes and 45% for electrons. The stretched exponential fits consisted of two important factors: the structural relaxation time τsr and the stretching factor β. For a given a-Se alloy, τsr was approximately the same for relaxation from both immediately after sample deposition, and annealing above Tg, indicating that the relaxation is readily repeatable and has the same physical origin. The relaxation was found to be dependent on the a-Se alloy composition. While the general shape of the relaxation was consistently a stretched exponential, τsr increased with increasing arsenic (As) concentration in the alloy, while β remained constant between 0.6-0.7. Additionally, τsr was found to be the same for both electron and hole relaxations for a given composition. Thus, the relaxation in both the electron and hole lifetime seems to be controlled by the same structural relaxation process, that is, the electron and hole traps are structural in origin.