Effects of the position substrate upon the structural behaviour, electrical and optical properties of zinc-oxide films used in solar cells

The preparation by the rf sputtering technique and characterisation of ZnO thin films used as windows in solar cells are described. The electrical behaviour and structural spectra clearly show an important effect of the substrate position with respect to the target. In fact, among all the studied substrate positions, only the samples facing the target are randomly oriented having the mixed orientation (100), (002) and (101). All the others have the c(002)-axis orientation. The scanning electron-microscope observations confirm the X-ray analysis results. The last samples have a resistivity as low as 10(-3) Omega cm while the randomly-oriented, ones have a large resisivity of about 10(2)-10(3) Omega cm These latter show, in their transmittance characteristics, a slight shift towards higher wavelengths. However, no effect is noticed when the other samples are optically assessed. Consequently, the optical gap is found to be about 3.38 eV for the conducting films and 3.3 eV for the ones having a higher resistivity. The average transmittance in the visible range is around 85-90% for all the samples (C) 1999 Elsevier Science Ltd. All rights reserved.