Mechanism of carrier photogeneration in amorphous selenium: Fast transient photoconductivity

Studies of fast transient photoconductivity in amorphous selenium reveal two distinct transport mechanisms. The first is a short-lived one due to carrier dynamics at extended band states and possibly states near the band edges where the carriers tunnel progressively into lower states. The second is a long-lived one due to phonon assisted multiple trapping at band tails. We find the quantum efficiency in this prototypic low-mobility system independent of temperature and electric field up to the maximum applied field of 5X10(5) V/cm, inconsistent with previous models of carrier photogeneration such as the Onsager geminate recombination model. In order to reconcile the controversial issue of the carrier photogeneration mechanism in this material, we examine in detail the processes underlying various transient photoconductivity measurements. Our analysis shows that in time-of-flight and xerographic-discharge measurements the external field may modify the extent of carrier recombination and thereby determines the carrier supply yield, whereas measurements utilizing the microstripline Auston switch configuration are better suited for investigating the intrinsic properties of the quantum efficiency.