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.

We report direct observation of the existence of buried thin wurtzite CdTe layers in nominally pure zinc-blende CdTe thin films using high-resolution transmission electron microscopy. The formation of the buried wurtzite layers is a result of the formation of high density of planar defects in the zinc-blende films—the wurtzite layers are formed by closely spaced lamellar twins. First-principles calculations reveal that the presence of the buried wurtzite layers may be responsible for the poor electrical properties of the polycrystalline zinc-blende CdTe films.

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Toward a high Cu2ZnSnS4 solar cell efficiency processed by spray pyrolysis method J. Renewable Sustainable Energy 5, 053137 (2013); 10.1063/1.4825253 Dependence of efficiency of thin-film CdS/CdTe solar cell on optical and recombination losses J. Appl. Phys. 113, 093105 (2013); 10.1063/1.4794201 Impedance spectroscopy of thin-film CdTe/CdS solar cells under varied illuminationIn this work, we found that the interdiffusion of the CdS and Zn 2 SnO 4 ͑ZTO͒ layers can occur either at high temperature (550-650°C) in Ar or at lower temperature (400-420°C) in a CdCl 2 atmosphere. By integrating a Zn 2 SnO 4 film into a CdS/CdTe solar cell as a buffer layer, this interdiffusion feature can solve several critical issues and improve device performance and reproducibility of both SnO 2 -based and Cd 2 SnO 4 -based CdTe cells. Interdiffusion consumes the CdS film from both the ZTO and CdTe sides during the device fabrication process and improves quantum efficiency at short wavelengths. The ZTO film acts as a Zn source to alloy with the CdS film, which results in increases in the band gap of the window layer and in short-circuit current density J sc . Interdiffusion can also significantly improve device adhesion after CdCl 2 treatment, thus providing much greater process latitude when optimizing the CdCl 2 process step. The optimum CdCl 2 -treated CdTe device has high quantum efficiency at long wavelength, because of its good junction properties and well-passivated CdTe film. We have fabricated a Cd 2 SnO 4 /Zn 2 SnO 4 / CdS/CdTe cell demonstrating an NREL-confirmed total-area efficiency of 15.8% (V oc ϭ844.3 mV, J sc ϭ25.00 mA/cm 2 , and fill factorϭ74.82%). This high-performance cell is one of the best thin-film CdTe solar cells in the world.

We report on a transmission electron microscopy and energy-dispersive x-ray spectroscopy study of S diffusion in polycrystalline CdS/CdTe heterojunctions. We find that grain boundaries significantly assist S diffusion in the CdTe layer when the CdTe is grown without the presence of oxygen, i.e., the S diffuses more easily along the grain boundaries than in the grains. However, grain boundaries do not enhance the S diffusion in CdTe when it is grown in the presence of oxygen. The reason is likely to be the formation of Cd–O bonds at the grain boundaries, which are resistance to the S diffusion.