Mainz, R.; Weber, A.; Rodriguez-Alvarez, H.; Levcenko, S.; Klaus, M.; Pistor, P.; Klenk, R.; Schock, H.-W.: Time-resolved investigation of Cu(In,Ga)Se2 growth and Ga gradient formation during fast selenization of metallic precursors. Progress in Photovoltaics 23 (2015), p. 1131–1143
Open Access Version
Ga segregation at the backside of Cu(In,Ga)Se2 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 6.5 min reached conversion efficiencies of up to 14.2 % (total area, without anti-reflective coating). The evolution of the Cu(In,Ga)Se2 diffraction signals reveals that the minimum process time for high-quality Cu(In,Ga)Se2 absorbers is limited by cation ordering rather than Se incorporation.