Cu–Ga–In thermodynamics: experimental study, modeling, and implications for photovoltaics

Both experimental measurements and assessment of phase equilibria are reported in the Cu–Ga–In material system, which is an important constituent in growth of the thin film photovoltaic absorber Cu(In,Ga)Se2 (CIGS). Characterization of four different alloys using inductively coupled plasma atomic emission spectroscopy, X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, differential thermal analysis, and differential scanning calorimetry has been conducted, and high-temperature equilibration studies have been performed on 2 of those. The new data are qualitatively consistent with the previous nonequilibrium thin film Cu–Ga–In observations. A thermodynamic assessment of the ternary system has also been performed using a CALPHAD approach after re-assessing the Cu–In constituent. The model fits the ternary data well in addition to the Cu–Ga, Cu–In, and Ga–In binary data. Practical applications of the model to metal precursors in a CIGS selenization process are discussed. Using the assessed parameters, the model predicts that at temperatures typically used in CIGS processes, Cu–Ga–In films should undergo equilibrium phase transformations. Slight changes in composition are found to determine whether or not these transitions occur, and at what temperature. Equilibrated precursor films are calculated to have high γ-Cu9(Ga,In)4 phase content, where reducing equilibration and γ-Cu9(Ga,In)4 formation have previously been found to improve photovoltaic performance.