Fast Cu(In, Ga)Se2 formation by processing Cu-In-Ga precursors in selenium atmosphere

The selenization of metallic precursors is a widely used and investigated technique for the fabrication of Cu(In, Ga)Se₂ films on large areas. A vacuum process with Se supply from the gas phase can be a suitable way to achieve a homogeneous, fast and controllable selenization reaction. In this study in situ XRD measurements are employed to investigate the reaction path for this type of process. The experimental setup is based on a reaction box mounted at a white light beamline of the synchrotron facility BESSY. Diffraction signals as well as K_α fluorescence lines of Mo, In and Se can be measured with high time resolution via energy dispersive detection. To elucidate the influence of selenium on the metallic precursors upon heating a comparison of experiments with and without Se exposure is presented. For the Se-free process the phases In and a Cu_x(In, Ga)_y-phase are detected at room temperature. The solid In phase melts according to its melting point at approximately 150°C, the remaining metallic phase melts at significantly higher temperatures of approximately 600°C. In the selenization process the metallic phases behave similar to the Se-free annealing process. The first detectable Se-containing phases are indium selenides. The indium selenides and the metallic diffraction signals vanish when chalcopyrite is formed. The Se fluorescence intensity was utilized to evaluate Se incorporation in the layers. Solar cells made out of absorbers from this kind of process exhibit fill factors of up to 72% and efficiencies up to 14%. The open circuit voltage was comparatively low with 535 mV and QE measurements confirmed a low band gap of approximately 1.0 eV. Energy-dispersive X-ray spectroscopy (EDS) measurements on the cross section showed a significant Ga enrichment at the back of the film.