Phenomenological model of anomalously high photovoltages generated in obliquely deposited semiconductor films

Abstract

Photovoltage in obliquely deposited films of semiconductors is found to be much higher in magnitude than the corresponding band gap of the semiconductor. Also, the magnitude of the photovoltage depends on the angle of deposition, the separation between the electrodes, the wavelength of the incident light, the intensity of the illumination and temperature, but this behavior is not understood well. In this work a phenomenological model of the generation of photovoltage along the horizontal plane of the obliquely deposited films on transparent substrates is presented for the first time. The model is based on the presence of obliquely grown grains separated by parallel grain boundaries and the existence of grain boundary potential barriers across the grain boundaries. In the case of semiconductor films with a high absorption coefficient for short wavelengths in the visible range, there occurs a relatively large photogeneration of carriers on the front side of the grain boundaries rather than on the back side, and this gives rise to a net photocurrent and a photovoltage across each grain boundary. The horizontal components of this photovoltage get added up due to a large number of grain boundaries lying between the electrodes on the front surface and produce a large photovoltage due to the series combination. An expression has been derived based on this model which shows that for low intensities of illumination the horizontal component V¯och of the photovoltage increases linearly with electrode separation, resistivity of the film, and the incident light intensity. It depends on the angle of deposition and under certain conditions it is expected to be maximum when the angle of deposition is 45°. The analysis can be applied to study the effect of increase in intensity or decrease in temperature on V¯och. It also shows that the photovoltage across the thickness of the obliquely deposited film V¯ocv is minimum for normal deposition and increases with the increase in the angle of oblique deposition.