This work reports the fabrication and characterization of superstrate-type Zn1-xMgxO/CdTe heterojunction solar cells on both CdxSnyO and commercial SnO2:F transparent conducting oxides (TCOs) in which the ZMO and CTO layers are produced for the first time by hollow cathode sputtering. The sputtering is conducted in a reactive mode using metal or alloyed metal targets fitted to a custom-made linear cathode. It is notable that the CdS buffer layer conventionally employed in CdTe solar cells is entirely replaced by the ZMO window layer. The use of ZMO is found to eliminate the blue loss associated with CdS optical absorption and further results in a higher open-circuit voltage. Key parameters were found to be the conduction band offset at the ZMO/CdTe interface and the ZMO thickness. It was discovered that the ZMO exhibits intense photoluminescence even at room temperature. Most of the solar cells were fabricated in the FTO/ZMO/CdTe configuration although CTO/ZMO/CdTe solar cells were also demonstrated. The CTO was produced with an electron mobility of 46 cm2 V-1s-1 without any post-deposition annealing or treatment.

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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 PCd/2PTe2 that depends on the temperature T and CdTe stoichiometry. By writing PCd(0)=2α(0)PTe2(0) and PCd(h)=2α(h)PTe2(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 Tsource>Tthin 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.