The effect of oxygen on interface microstructure evolution in the CdS/CdTe solar cells

Microstructural changes at the CdS/CdTe solar cell interface where close-spaced sublimation (CSS) is used as the growth technique to deposit the p-type CdTe absorber layer are studied by systematic layer characterization at various stages during heterojunction growth. US layers grown by both chemical bath deposition (CBD) and CSS provide a basis for determining the effects of CdS crystallinity, grain size, and oxygen content on the subsequent CdTe layer. As-grown CBD US films exhibit small grains and variations in optical properties attributed to film impurities. In contrast, CSS yields CdS films with good crystallinity, larger grains, and nearly ideal optical properties. The hexagonal nature of CSS-grown US is seen to nucleate hexagonal CdTe during the initial stages of CdTe film growth. Cubic US deposited by CBD in contrast promotes cubic CdTe nucleation. Oxygen anneals in the latter case can aid hexagonal CdTe nucleation. Auger electron spectroscopy (AES) and transmission electron microscopy (TEM) of the CdS/CdTe interface show CdS-dependent differences in interdiffusion at the interface. This interdiffusion appears to be determined by the oxygen level in the CdS. When low-oxygen-containing CSS CdS films are used, sulfur diffusion is substantial, leading to significant consumption of the US layer. When these same films are annealed in oxygen, the consumption is reduced. Te diffusion into the US layer is also observed to decrease with oxygen anneals. Optical modeling shows that Te alloying with the US layer can greatly reduce the short-circuit current of CdS/CdTe devices. Copyright (C) 2002 John Wiley Sons, Ltd.