Thin Film Solar Cells with n-type CdTe Absorber and p-type ZnTe Window Layers

In this paper the effect of indium (In) doping on CdTe thin film solar cells was investigated. CdTe thin films were deposited using the elemental vapor transport (EVT) technique under various Cd/Te gas phase ratios and In vapor concentrations. Solar cells of the superstrate configuration (glass/TCO/CdS/n-CdTe/p-ZnTe/BC) have been fabricated and characterized. There was a correlation between the concentration of In in the vapor phase and net n-type doping for CdTe devices fabricated near Cd/Te stoichiometric ratio; increasing the amount of indium resulted in higher n-type doping. From C-V measurements doping levels >1016cm-3 were measured. Devices were also fabricated at various Cd/Te vapor ratios. Films deposited at lower Cd/Te vapor ratios (i.e., Te-rich) exhibited higher n-type doping. Lower Cd/Te ratios favor the creation of Cd-vacancies which are needed for substitutional In doping, which can explain why the net doping increases at lower Cd/Te ratios. Minority-carrier lifetimes of ~8ns were achieved for intrinsic and In-doped films deposited under Cd-rich conditions.