Optimisation of ZnO:Al films by change of sputter gas pressure for solar cell application

The structural, electrical and optical properties of ZnO:Al films prepared by rf magnetron sputtering have been characterised. At substrate temperature of 250 °C and rf power of 100 W, Ar gas pressure is changed from 0.2 to 3.2 Pa to optimise the properties of ZnO:Al films for solar cell application. The X-ray diffraction (XRD) studies show that the films are highly oriented with their crystallographic c-axis perpendicular to the substrate almost independent on Ar pressure. Measurements of transmission spectra reveal that ZnO:Al films have an average transmission of over 83% in visible wavelength range, slightly depending on Ar pressure. The roughness and morphology of films are very sensitive to change of Ar pressure and microstructure is in agreement with the trend given by Thornton’s structure zone model. Furthermore, the use of optimum ZnO:Al film as a window layer and heterojunction partner in ZnO:Al/n-Si heterojunction solar cells has been explored. The best cell exhibits a stable conversion efficiency of 8.2%.