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

Abstract

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. CdS 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 CdS 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 CdS is seen to nucleate hexagonal CdTe during the initial stages of CdTe film growth. Cubic CdS 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 CdS layer. When these same films are annealed in oxygen, the consumption is reduced. Te diffusion into the CdS layer is also observed to decrease with oxygen anneals. Optical modeling shows that Te alloying with the CdS layer can greatly reduce the short‐circuit current of CdS/CdTe devices. Copyright © 2002 John Wiley & Sons, Ltd.

Accelerated stress testing and diagnostic analysis of degradation in CdTe solar cells

“…The Kirkendall effect predicts that such interdiffusion involving asymmetric atomic fluxes will result in a bulk movement of the interface into the CdS, leading to a general "consumption" of that layer, i.e., a 'thinning" of the CdS. This is indeed what is typically observed with CdS/CdTe interfaces joined at high temperature [19]. The Kirkendall effect also suggests that if such bulk movement cannot occur (perhaps at lower temperatures), voids will be introduced in the CdS layer as a consequence of nonequal diffusion rates.…”

Effect of deposition power in fabrication of highly efficient CdS:O/CdTe thin film solar cell by the magnetron sputtering technique

“…Recently, Paudel and Yan reported an improved efficiency of the CdTe solar cell (13.6-14.5%) via oxygen incorporation on the CdS layer [29]. Yan et al [30] and Albin et al [31] also suggested that higher oxygen atomic concentration in CdS film can help to reduce Te diffusion from the CdTe to CdS film, thereby improving device short-circuit current (Jsc) and efficiency. Consequently, many researchers conducted detailed studies on CdS:O films prepared by varying the concentration of oxygen in the deposition ambient for better performance of CdTe solar cells.…”

Graded-Bandgap Solar Cells Using All-Electrodeposited ZnS, CdS and CdTe Thin-Films

“…Reducing the thickness of the CdS window layer has been proposed as a step towards increasing the efficiency of CdS/CdTe solar cells due to the high absorption coefficient of CdS [6,12]. A substantial amount of light in the visible region of the solar spectrum is absorbed in the bulk of thick CdS layers without reaching the CdTe absorber material.…”

“…The problem associated with this idea however, is the complete intermixing of the very thin CdS layer with CdTe during post-deposition CdCl2 heat-treatment of the CdS/CdTe structure which results in the presence of pinholes and eventually causing the CdTe layer to come into direct contact with the transparent conducting oxide (TCO) front contact. This leads to short-circuiting between CdTe and the TCO resulting in low fill factors and open-circuit voltages [12]. It also leads to loss of the active junction between the CdS and CdTe layers assuming a p-n junction structure.…”