Time-resolved investigation of Cu(In,Ga)Se2 growth and Ga gradient formation during fast selenisation of metallic precursors

Ga segregation at the backside of Cu(In,Ga)Se-2 solar cell absorbers is a commonly observed phenomenon for a large variety of sequential fabrication processes. Here, we investigate the correlation between Se incorporation, phase formation and Ga segregation during fast selenisation of Cu-In-Ga precursor films in elemental selenium vapour. Se incorporation and phase formation are analysed by real-time synchrotron-based X-ray diffraction and fluorescence analysis. Correlations between phase formation and depth distributions are gained by interrupting the process at several points and by subsequent ex situ cross-sectional electron microscopy and Raman spectroscopy. The presented results reveal that the main share of Se incorporation takes place within a few seconds during formation of In-Se at the top part of the film, accompanied by outdiffusion of In out of a ternary Cu-In-Ga phase. Surprisingly, CuInSe2 starts to form at the surface on top of the In-Se layer, leading to an intermediate double graded Cu depth distribution. The remaining Ga-rich metal phase at the back is finally selenised by indiffusion of Se. On the basis of a proposed growth model, we discuss possible strategies and limitations for the avoidance of Ga segregation during fast selenisation of metallic precursors. Solar cells made from samples selenised with a total annealing time of 6.5min reached conversion efficiencies of up to 14.2 % (total area, without anti-reflective coating). The evolution of the Cu(In,Ga)Se-2 diffraction signals reveals that the minimum process time for high-quality Cu(In,Ga)Se-2 absorbers is limited by cation ordering rather than Se incorporation. Copyright (c) 2014 John Wiley & Sons, Ltd.