How to control the stoichiometry of cadmium telluride thin film for photovoltaic applications

In this study, Bube's growth model for a cadmium telluride (CdTe) polycrystalline thin film was re-examined with a view of avoiding his assumptions that neglect the vapor pressures of Cd and Te₂ near the film. We proposed a new thermodynamic growth model based on the fact that there is an experimentally verified characteristic ratio (α) of equilibrium partial pressures P_Cd/2P_Te2 that depends on the temperature T and CdTe stoichiometry. By writing P_Cd (0)=2α(0)P_Te2 (0) and P_Cd (h)=2α(h)P_Te2 (h), where α(0) is determined by source stoichiometry, we can solve the equations for α(h) and thereby determine the stoichiometry of the CdTe thin film grown under physical vapor deposition (PVD) conditions. Simulation was performed to predict the stoichiometry of the CdTe thin film as a function of source stoichiometry for various source-film temperature combinations. The results show that for a typical CdTe PVD process with T_source > T_{thin film}: (1) stable deposition, without a non-stoichiometric composition shift, can be achieved at congruent-growth stoichiometry; (2) any stoichiometric deviation from the congruent sublimation point becomes more substantial (in the same direction) in the thin film than in the source; and (3) larger ΔT between the source and the thin film results in a more composition shift.