No abstract

The formation chemistry and growth dynamics of thin-film CuInSe2 grown by physical vapor deposition have been considered along the reaction path leading from the CuxSe:CuInSe2 two-phase region to single-phase CuInSe2. The (Cu2Se)β(CuInSe2)1−β (0<β≤1) mixed-phase precursor is created in a manner consistent with a liquid-phase assisted growth process. At substrate temperatures above 500 °C and in the presence of excess Se, the film structure is columnar through the film thickness with column diameters in the range of 2.0–5.0 μm. Films deposited on glass are described as highly oriented with nearly exclusive (112) crystalline orientation. CuInSe2:CuxSe phase separation is identified and occurs primarily normal to the substrate plane at free surfaces. Single-phase CuInSe2 is created by the conversion of the CuxSe into CuInSe2 upon exposure to In and Se activity. Noninterrupted columnar growth continues at substrate temperatures above 500 °C. The addition of In in excess of that required for conversion produces an In-rich near-surface region with a CuIn3Se5 surface chemistry. A model is developed that describes the growth process. The model provides a vision for the production of thin-film CuInSe2 in industrial scale systems. Photovoltaic devices incorporating Ga with total-area efficiencies of 14.4%–16.4% have been produced by this process and variations on this process.

No abstract

No 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.

No abstract

No abstract

The absorption coefficient (α) and fundamental transition energies of thin-film CuInSe2 were determined by spectrophotometry in the near-infrared (NIR) and visible wavelength regions from 500 to 2000 nm for a wide range of compositions. The results suggest a relationship between the constituent specie fluxes and substrate temperature, and the resulting polycrystalline nature of the film which dominates the optical properties. Near-stoichiometric and Cu-rich films appear to crystallize in larger grain sizes in comparison with Cu-poor films, with a Cu2−δ Se secondary phase at grain boundaries and free surfaces. Correspondingly, significant variations in the absorption coefficient among different film compositions exist in the neighborhood of the band edge. At energies well above the gap, all films behave similarly with α’s of (1–2)×105 cm−1 at 500 nm. Similarly, continuous dispersion curves for the index of refraction have only been derived for single phase Cu-poor material by an iterative technique. The absorption data are substantiated through spectral response simulations that accurately reproduce measured device data. The range of primary and secondary transition energies, respectively, is 0.95–1.01 and 1.17–1.22 eV. These values indicate a valence-band splitting of 0.20–0.24 eV, in good agreement with single-crystal values.