Photoelectronic behavior of a -Se and some a -Se: As alloys in their glass transition regions

Coordinated measurements of dielectric response, hole and electron drift mobilities, photogeneration efficiencies, and space-charge-limited currents have all been carried out on a-Se and some a-Se: As alloys under experimental conditions in which temperature is continuously scanned at a preselected, constant rate over a range encompassing their respective glass transitions. In addition to the time-temperature profile of sample films during measurement, preparatory procedure and thermal history prior to measurement are also fixed. The single dielectric loss peak observed exhibits a temperature and frequency dependence in conformity with the phenomenological Williams-Landel-Ferry equation but the spectra recorded during the initial heating of a well-rested sample film are found to differ systematically from spectra obtained in subsequent scans carried out in a program of repeated thermal cycling experiments. This latter behavior is attributed to a fully reversible thermal hysteresis in molecular packing and its characteristic relaxation time is determined to be days at 294 K. The impact of this structural transition on photoelectronic behavior is readily identified, but is itself superimposed on the marked change in temperature dependence of drift mobility and photogeneration efficiency which is always found to accompany the glass-transition process. Two trapping phenomena also reflect the glass transition. The first is manifested at T₆ as the sudden appearance of an abrupt range limitation on electron transport and is hysteresis-free on our experimental time scale. The second which shows a pronounced thermal hysteresis can be experimentally related to the fluctuation in molecular packing and is manifested as an abrupt drop in trap-limited space-charge-limited current during the first heating cycle through T₆.