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 beh aviour and structural spectra clearly show an important effect of the subst rate position with respect to the target. In fact, among all the studied su bstrate positions, only the samples facing the target are randomly oriented having the mixed orientation (100), (002) and (101). All the others have t he c(002)-axis orientation. The scanning electron-microscope observations c onfirm the X-ray analysis results. The last samples have a resistivity as l ow as 10(-3) Omega cm while the randomly-oriented, ones have a large resisi vity of about 10(2)-10(3) Omega cm These latter show, in their transmittanc e characteristics, a slight shift towards higher wavelengths. However, no e ffect is noticed when the other samples are optically assessed. Consequentl y, the optical gap is found to be about 3.38 eV for the conducting films an d 3.3 eV for the ones having a higher resistivity. The average transmittanc e in the visible range is around 85-90% for all the samples (C) 1999 Elsevi er Science Ltd. All rights reserved.