Thermochemistry and phase diagram studies in the copper-(indium,gallium)-selenium system

Polycrystalline Cu(In,Ga)Se₂ and related semiconductors show great potential as alternative materials in production of high efficiency solar cells. This dissertation reports the experimental determination of Gibbs energy changes and phase diagram calculations for selected sections of the Cu-Ga-In-Se system. The Gibbs energy changes were measured with solid-state electrochemical cells and this data along with selected literature data were assessed and model parameters suggested.; The homogeneity range of β-Cu_2−xSe was measured by coulometric titration and the thermodynamic properties for defect species estimated. The composition difference between the Se-rich and the Cu-rich boundaries was measured at 900K. A defect model was developed based on vacancy formation on the Cu sublattice. The gas phase equilibrium data for Cu-Se system and the results of a recent assessment of selenium unary system were used to predict defect concentrations.; A thermodynamic description of the Cu₂Se-In₂Se ₃ was obtained by optimization of the available phase equilibrium and thermodynamic information along with the direct results of EMF experiments. The Gibbs energy of formation of α-CuInSe₂ was directly measured by a solid oxide galvanic cell experiment. The transformation enthalpy and Gibbs energy data for CuIn₃Se₅ and CuIn₅Se ₈ were estimated. The Redlich-Kister model with a 3-coefficient expression was employed to define the Gibbs energy of the liquid phase. The intermediate β-CuIn ₃Se₅ and γ-CuIn₅Se₈ phases were modeled with a 2-coefficient expansion of the Redlich-Kister model. The α and δ modifications of CuInSe₂ phases were modeled with a specific sublattice model. A reasonable agreement between the model calculated values and the thermodynamic phase equilibrium data was achieved.; The thermochemistry and phase diagram of GaSe system was critically studied. The activity of Ga was measured along the liquidus between 800–1000K. Selected invariant phase transition temperatures were measured and transition enthalpies were calculated from the EMF measurements. A self-consistent thermodynamic data was obtained. The associated and sublattice models were used to represent the Gibbs energy of the liquid and α-Ga₂Se₃ phases, respectively. The Gibbs energy of formation of CuGaSe₂ was measured by an EMT experiment. The phase diagram of Cu-Ga system was calculated and the liquid phase Ga activity measurements was measured for 2 Ga rich compositions.