Polycrystalline thin film CdTe continues to be a leading material for the development of cost effective and reliable photovoltaics. The two key properties of this material are its band gap (1.5 eV), close to the ideal for photovoltaic conversion efficiency (1.45 eV), and its high optical absorption coefficient. Thin film CdTe solar cells are typically hetero-junctions with Us being the n-type partner, or window layer. Efficiencies as high as 16.5% have been achieved, but still there is some potential for increasing them. We make an analysis of the typical CdS/CdTe superstrate solar cell, and from it we establish critical issues and different lines of research in order to improve the current efficiencies. We also show that present record efficiencies are very close to the practical efficiency limit for a CdS/CdTe hetero-junction cell. (c) 2005 Elsevier Ltd. All rights reserved.