The performance of CdTe solar cells is strongly limited by the thickness of CdS window layer.A higher
short-circuit current density might be achieved by decreasing the thickness of CdS layer as a result of
reducing the absorption losses that take place in this layer. However, it is difficult to obtain uniform
and pin-hole free CdS layers thinner than 50 nm. This problem can be solved through increasing the
band gap of the window layer by adding a wide band gap semiconductor such as ZnS. In this work, bilayer
ZnS/CdS film was studied as an improved window layer of ITO/ZnS/CdS/CdTe solar cell. The
total thickness of ZnS/CdS layer was taken about 60 nm. The effect of optical losses due to reflection at
different interfaces in the cell and absorption in ITO, ZnS, CdS as well as the recombination loss have
been studied. Finally, the effects of the recombination losses in the space-charge region and the
reflectivity from the back contact were taken into accounts. The results revealed that the optical losses
of 23% were achieved at 60 nmthickness of CdS and theses losses minimized to 18% when ZnS layer
of 30 nmthickness was added to CdS layer. The minimum optical and recombination losses of about
26% were obtained at 1 ns of electron life-time and∼0.4 μmwidth of the space-charge region. The
maximum efficiency of 18.5% was achieved for ITO/CdS/CdTe cell and the efficiency increased up to
20% for ITO/ZnS/CdS/CdTe cell.