Growth and characterization of copper indium diselenide polycrystalline thin films for photovoltaic applications

The incorporation of sulfur into CuInSe₂ thin films was quantitatively investigated to establish a scientific and engineering basis for the fabrication of homogeneous and compositionally graded CuIn(Se,S)₂ thin films. The motivation for this work was to develop a means of controlling the band gap for this class of materials to achieve improved performance in photovoltaic devices. The approach taken was the reaction of thin film Cu/In layers and CuInSe₂ thin films in H₂S and/or H₂Se gasses at atmospheric pressure.; The reaction of Cu/In layers in a mixture of H₂S and H ₂Se as a function of gas phase composition was investigated to quantify the relationship between the gas and solid phase compositions in the formation of homogeneous CuIn(Se,S)₂ thin films. A reaction model, accounting for mixing and the presence of oxygen, was developed and regressed to the experimental data. This work then led to the development of a phenomenological model for the inhomogeneous incorporation of sulfur into CuInSe₂ thin films by a surface reaction followed by diffusion.; X-ray diffraction line profiles, grain size distribution, and grain boundary width were used in conjunction with a quantitative diffusion model to estimate the bulk and grain boundary diffusion of sulfur into CuInSe₂. Diffusion coefficients were determined at multiple temperatures and activation energies were estimated. The analysis was also applied to the diffusion of CdS into CdTe that occurs during the post deposition thermochemical treatments that are necessary to achieve high performance CdTe solar cells. Bulk and grain boundary diffusion coefficients and activation energies for CdS-CdTe were estimated.; Based on the analysis of both equilibrium chemistry and diffusion, a well defined process for the fabrication of homogeneous and graded CuIn(Se,S) ₂ thin films was developed. This process provides a method of band gap engineering that has application in both the fabrication of wide band gap devices for use in tandem solar cells and the control of compositional grading for use in graded band gap devices.