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CdTe solar cells were fabricated by depositing an Au/Cu contact layer with various thicknesses and deposition conditions on polyerystalline CdTe/CdS/SnO2/glass structures. Cu has a dual effect on the cell performance; it helps in the formation of better ohmic contacts to CdTe and increases the acceptor doping concentration, but excess Cu could diffuse all the way to the CdTe/CdS interface to form recombination centers and shunt paths and degrade cell performance. Both secondary ion mass spectroscopy and capacitance-voltage measurements confirm the incorporation of Cu into the bulk of the CdTe films. Cd outdiffusion toward the surface of CdTe was observed during the Au/Cu deposition. The thickness of Cu plays a critical role in the CdTe solar cell performance because both the series and shunt resistances decrease with the increase in Cu thickness. Carrier transport analysis showed thai depletion region recombination dominates the current transport in the CdTe solar cells with an Au/Cu contact. The transport mechanism remains the same in spite of the amount of Cu incorporation into the bulk and interface. Higher Au/Cu deposition rates result in a greater excess of Cd at the CdTe surface, leaving more Cd vacant sites below the surface. This causes an increase in dopant concentration but also results in a higher defect density and reduced cell performance.

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CdTe/CdS solar cells were fabricated by depositing CdTe films on glass/SnO,/CdS substrates by MOCVD in different growth ambients with varying Te/Cd mole ratio in the range of 0.02 to 15. Short circuit current density (JJ showed a minimum at Te/Cd ratio of 0.1 and increased on both sides of this minimum iving rise to a "U" shaped curve. Open circuit voltage T V,) was highest for the Te-rich growth ambient and was low for the stoichiometric and the Cd-rich growth conditions. Such variation in J, and V, gave the best cell efficiency of 12% in this study at a Te/Cd ratio of SIX. The films own under Te-rich conditions form ptype CdTe, but tfe Cd-rich growth conditions produce ntype CdTe which require 40O0C/air anneal for the type conversion. Detailed measurements and analysis revealed a high degree of atomic interdiffusion at the interface when the CdTe films were grown in the Te-rich conditions. The enhanced interdiffusion reduces interface states which is further supported by higher effective lifetime in the Te-rich films, as well as rapid decrease in QE under forward bias for the Cd-rich cells.

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