CdS thin films synthesized by changing the complexing agents, the Cd source and the nominal S/Cd ratio are investigated.

CdS:Cu showed an efficiency improvement in CIGSe solar cell in comparison with the non-doped one.

By using Cd(NO₃)₂ as Cd source in CdS deposition, an 8.03% CZTSe solar cell efficiency was achieved.

Higher V_oc values are obtained for thinner and more resistive CdS layers.

It is demonstrated that CdS/CIGSe and CdS/CZTSe solar cells operate rather as a MIS structure.