The Effect of Source Microstructure on the Close-Space Sublimation of CdTe Thin Films for Solar Cell Applications

ABSTRACTThe microstructure of CdTe sources grown on bare 7059 and SnO2-coated 7059 glass substrates used in the close-space sublimation (CSS) growth of CdTe films, is affected by nucleation conditions during their fabrication. For CdTe sources grown on bare glass substrates, denser source microstructures with smaller grains are promoted by low-temperature nucleation conditions. Growth rates of CdTe films deposited using these source plates are inversely proportional to the packing density of the grains. CdTe sources grown on glass undergo significant texture changes during their use in subsequent CSS deposition. CdTe sources grown on SnO2-coated glass substrates exhibit dense structures both with and without low-temperature nucleation and increased grain size with low-temperature nucleation. These source plates show less texture change associated with thermal annealing than their bare-glass counterparts, however, they are much more susceptible to CdO formation during subsequent CSS deposition processes in which oxygen is present. Source oxidation results in a decrease of the (111)/(220) x-ray diffraction intensity. Analysis of the (111)/(220) intensity ratio with and without source oxidation suggests the preferential formation of CdO on the (111) planes. In general, source oxidation appears to be a inversely proportional to the grain size of the source material.