Zinc oxide/<i>n</i>-Si junction solar cells produced by spray-pyrolysis method

Abstract

Zinc oxide (ZnO)/n-Si junction solar cells were fabricated by a spray-pyrolysis method and high short-circuit photocurrent densities and relatively high photovoltages were obtained by adjusting the conditions of the deposition and the post-deposition heat treatment. Consequently, relatively high conversion efficiencies ranging between 6.9% and 8.5% were obtained. The efficiency of the solar cells with ZnO/n-Si structure decreases slowly with time when they are kept in air in the dark because of the increase in the thickness of the silicon oxide layer between Si and the ZnO film. This degradation can be avoided by forming an indium-tin-oxide (ITO) overlayer on the ZnO film, indicating that the silicon oxide layer grows through the reaction of Si with oxygen diffusing from the atmosphere, not with ZnO. The efficiency of the ZnO/n-Si junction solar cells decreases rapidly with the illumination time. Capacitance-voltage measurements show that this degradation is caused by a decrease in the work function of the ZnO film. The decrease in the work function is caused by desorption of O−2 from the grain boundaries of the ZnO films. When incident light contains no ultraviolet (UV) component, this degradation does not occur, indicating that the desorption is caused by the acceptance of holes generated by UV light.