Processing and characterization of copper indium selenide for photovoltaic applications

Polycrystalline CuInSe₂ (CIS) and related materials, are promising candidates of low-cost high efficiency solar cells. This thesis describes the applications of phase diagrams to suggest alternative absorber precursor structures, explores the use of Rapid Thermal Processing (RTP), and elucidates the structure of CuInSe₂ and related materials.; A critical assessment of the thermochemical and phase diagram data for the Se unary, Cu-Se, and In-Se binary systems was performed. This information was used to give an evaluated Cu₂Se-In₂Se₃ pseudobinary phase diagram and suggest liquidus lines in the Cu-In-Se ternary system. The resulting information was used to propose a novel bi-layer precursor structure consisting of a co-deposited In-Se (50 to 55 Se at.%) top layer and a Cu-Se (50 to 55 Se. at.%) bottom layer. This precursor structure was designed to form CIS by reacting a liquid CuSe with a solid InSe phase in the process temperature window erature window 524°C to 611°C. Two different precursor structures were fabricated and tested. The first one consists of the three layers, In₄Se₃/CuInSe₂/Cu ₂Se, and the second one was a simple two-layer structure, InSe/CuSe. After RTP, the first precursor structure underwent a liquid-solid reaction and formed large grained CuInSe₂ films. It is believed the melting of top In₄Se₃ layer occurs through a monotetic reaction. The second precursor structure also lead to the formation of large grained CIS (∼5 μm) films, as suggested in the reaction pathway analysis.; X-ray Absorption Fine Structure (XAFS) measurements were performed on crystals of CuInSe₂ with different compositions close to the 1:1:2 stoichiometry, and CuIn₃Se₅. It was found that the Cu-Se bond length is independent of the Cu occupation number in this composition range within a length of 0.01 Å. Least-square fitting of the Se-K edge EXAFS spectra indicated CuInSe₂ consists of Se-centered tetrahedron with an average of 2 Cu and 2 In nearest neighbors. On the other hand, CuIn ₃Se₅ consists of Se-centered tetrahedron with an average of 0.8 Cu and 2.4 In nearest neighbors. The Cu-Se and In-Se bond lengths were determined to be 2.424 and 2.598 Å, respectively. This result directly proves that CuIn₃Se₅ is indeed a defect tetrahedral structure and suggests it consists of three types of local tetrahedral cationic clusters around each Se: 20% 2Cu + 2In (k = 8), and 40% □ + Cu + 2In (k = 7), and 40% □ + 3 In ( k = 9), where □ denotes vacancy and k denotes the sum of the cation valence electrons.