Preparation and Properties of Evaporated CdTe Films Compared with Single Crystal CdTe: Annual Report, 1 February 1983 - 31 January 1984; A Subcontract Report

Variation of CdS/CdTe/graphite thick film solar cell properties was investigated as a function of temperature for CdS film deposition. A maximum open-circuit voltage of 0.67 V was found for a deposition temperature of 160 degrees C,corresponding to a CdS film resistivity of 150 ohm-cm. The effect is not due to avoidance of higher temperature annealing of the CdTe film in higher temperature CdSfilm depositions nor to the diffusion of In from the outermost CdS:In layer. The effect of coating the graphite before CdTe deposition with Au or Cu was also investigated. Although high concentrations of both Au or Cu could be determined after CdTe deposition, CdTe films grown on this coated graphite had lower hole densities than films grown on uncoated graphite. Photovoltaic parameters of thinfilm CdS/CdTe/graphite solar cells were investigated as a function of storage time to check the stability of these cells. Initial degradation of parameters (especially fill factor) could be reversed by heat treatment in hydrogen, with subsequent properties being stable. Heat treatment of CdS/CdTe/graphite solar cells in air increases cell resistivity and decreases fill factor; heat treatment inhydrogen produces the reverse effect. The hole density is not affected by these heat treatments, suggesting that effects are associated with grain boundaries in the film.