Effect of Stoichiometry on the Lifetime and Doping Concentration of Polycrystalline CdTe

Abstract

The effect of gas phase Cd/Te deposition ratio on the minority carrier lifetime and doping concentration of polycrystalline cadmium telluride (CdTe) was investigated. CdTe thin films were deposited by the elemental vapor transport (EVT) with various Cd/Te gas phase ratios. The films were fabricated into solar cells, and characterized by current–voltage ( JV ) and spectral response measurements. The minority carrier lifetime was measured using time-resolved photoluminescence measurements. Capacitance–voltage ( CV ) measurements indicated a change in doping concentration as a function of gas phase Cd/Te ratio. Films deposited under Te-rich deposition conditions exhibited superior electrical characteristics. Minority carrier lifetimes up to 7 ns and doping concentrations on the order of 1015 cm–3 were measured for CdCl2 heat-treated Te-rich deposited films. The addition of controlled amounts of Cu increased the doping concentration up to 1016 cm–3, however lowered the carrier lifetimes to less than 1 ns. EVT deposited CdTe solar cells exhibited $V_{{\rm{OC}}}$ of 800 mV without Cu and up to 850 mV with Cu doping.