Field-Controlled Photogeneration and Free-Carrier Transport in Amorphous Selenium Films

Abstract

We have studied the photogeneration and transport of free carriers in amorphous selenium films under varying conditions of thickness, wavelength of the exciting radiation, and applied electric field. We have found that the efficiency of photogeneration is strongly field controlled, varying by two or more powers of 10 with the field changes reported. Using a technique based on time resolving the transport of free carriers across the film, we have measured deep-hole trapping times of 10-45 \ensuremath{\mu}sec and deep-electron trapping times of 40-50 \ensuremath{\mu}sec at room temperature. In addition, we have found hole-drift mobilities of 0.13-0.16 ${\mathrm{cm}}^{2}$/V sec and electron-drift mobilities of 6.0-8.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ ${\mathrm{cm}}^{2}$/V sec at room temperature, in good agreement with previous workers. These measurements allow an independent determination of the hole and electron range of (1.3-6.3)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ ${\mathrm{cm}}^{2}$/V and (2.4-3.1)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ ${\mathrm{cm}}^{2}$/V, respectively. The hole range measured in this new way is 20-100 times larger than previous determinations based on an inappropriate Hecht-type analysis, which is only a measure of the product of the generation and transport efficiency and not of the transport alone. The voltage dependence of the total transported charge cannot be described by a simple range limitation. The thickness scaling laws and the independent measurement of the range require that the photogeneration efficiency must depend on the applied electric field. It is suggested that the photogeneration step involves the field-aided thermal dissociation of a tightly bound hole-electron pair. This is indicated by the electric-field and wavelength dependence of the apparent quantum efficiency.