CdTe solar cells were fabricated by depositing an Au/Cu contact layer with various thicknesses and deposition conditions on polycrystalline 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 that 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.