Se Activity and Its Effect on Cu(In,Ga)Se2 Photovoltaic Thin Films

We study some physical properties of CuIn _1-xGa _xSe ₂ thin-films fabricated by evaporation from elemental sources under various Selenium environments. Specifically, thin-films were fabricated under growth conditions such as Se deficiency, near stoichiometry and excess Se during coevaporation to in-vestigate the impact of the Se environment on absorber film properties and ultimately the device performance. We deter-mine the chemical activity of Se in the evaporation process has a strong influence on film macrostructure (prefered orien-tation) and microstructure, particularly at the grain and grain boundary level. It is shown that the optoelectronic properties at grain boundaries are affected by the Se environment used resulting in absorber thin-films with distictive defect distribu-tion and defect density. Consequently, the performance of the solar cells fabricated from those films is also affected by the Se environment. These effects on solar cell performance and absorber properties are reported in a (i) structural analysis of the CuIn _1-xGa _xSe ₂/Mo/glass samples by X-ray and electron backscattering techniques; (ii) optolectronic radiative charac-teristics of the absorbers by cathode luminescence and photo-luminescence studies and (iii) current-voltage, quantum effi-ciency and capacitance-voltage measurements for the solar cells made from the absorbers fabricated under the pre pre-scribed Se growth conditions.