A reliable optical method for in situ process control for deposition of Cu(In,Ga)Se2 thin layers for photovoltaics

Abstract

A new method for optical process control of the three-stage co-evaporation of Cu(In,Ga)Se2 thin films is presented. Precise control of the deposition process is desirable as the field of process parameters is rather complex. In an enhancement to laser light scattering (LLS) with a single photo-detector, the diffuse part of the scattered laser light is now used to a larger extent. In consequence, it is possible to deduce compositional information (e.g., the Ga/III-ratio) for the deposited layer with high accuracy. This is demonstrated in a series of experiments on Mo-coated float glass and titanium foil substrates where the final Ga content of the Cu(In,Ga)Se2 thin film has been intentionally varied. As an additional benefit of the enhanced LLS system, the new system can also be used for process control, in cases where previously the intensity of scattered component of light has not been sufficient for reliable interpretation. The information from this new monitoring technique was used to set up an optical model for semitransparent, coevaporated InxGaySez-layers of various compositions. Using this model, an evaluation of phases formed during the process and adjustment of deposition parameters is possible. The knowledge of phases formed on glass and titanium substrates is important since the Cu(In,Ga)Se2 formation depends on properties of the InxGaySez-layer evaporated in stage 1 of the three-stage process. Break-off experiments at different points within stage 1 were carried out to test and improve the model. Depth profiling by means of x-ray fluorescence (XRF) and microstructural studies by means of x-ray diffraction (XRD) also deliver valuable information for the optical model.