Real time spectroscopic ellipsometry of first stage CuIn1−xGaxSe2: Indium-gallium selenide co-evaporation

In this study, real time spectroscopic ellipsometry (RTSE) has been applied for in-situ monitoring of the first stage of copper indium-gallium diselenide (CuIn_1-xGa_xSe₂; CIGS) thin film deposition by the three-stage co-evaporation process used for high efficiency photovoltaic (PV) devices. The first stage entails the growth of indium-gallium selenide (In_1-xGa_x)₂Se₃ (IGS) at a temperature of 400°C on substrates consisting of soda lime glass coated with thin film molybdenum (Mo). This is a critical stage of CIGS deposition because a large fraction of the final film thickness is deposited, and as a result, precise composition and thickness control is desired in order to achieve the optimum open circuit voltage (V_oc) and fill-factor (FF) of the resulting CIGS solar cell. In fact, RTSE has been applied broadly in previous studies for the characterization of complicated thin film deposition processes used in PV device fabrication - with the potential for process monitoring and control in many situations. In the case of the first-stage IGS deposition of this study, RTSE has been used to characterize the time evolution of (i) the Mo/IGS interface filling fraction, (ii) the IGS surface roughness layer thickness, and (iii) the IGS bulk layer thickness, as the depositing layer covers the rough Mo surface. In addition, one can extract the evolution of the bulk layer optical properties, expressed in the form of the complex dielectric function, which can serve as a fingerprint for IGS composition and also provide information on relative void vol.% and grain size (or defect density) in the IGS. Overall the structural and compositional information can assist in understanding the growth of three-stage CIGS absorbers for solar cells and in optimizing cell performance.