On the nano-scale characterization of kesterite thin-films

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Cu2ZnSnSe4 (CZTSe, kesterite) serves as a polycrystalline absorber material for thin-film solar cells, but its fabrication is hindered by secondary phase formation, which poorly impacts cell efficiency. Identifying and characterizing these phases is challenging due to their structural similarities with CZTSe, making conventional X-ray diffractometry unreliable. To address this, atom probe tomography (APT) and transmission electron microscopy were employed to explore nanoscale compositional fluctuations and phase formation that influence absorber layer properties. APT findings indicate that regardless of growth conditions, single CZTSe grains maintain a consistent Cu-poor composition and nearly stoichiometric Zn/Sn ratio. Cu-Sn-Se compounds, only a few nanometers in size, were found in the space charge region of films produced under Cu-rich conditions and subsequently annealed in a SnSe atmosphere, contributing to solar cell failure. Compositional fluctuations at CZTSe grain boundaries were quantified, revealing Cu-enrichment before annealing and Zn-enrichment or Na and K impurities afterward. A complex nanosized ZnSe network, doped with Cu and Sn, was detected, acting as a barrier that reduces electron and hole mobility, potentially causing high series resistance at low temperatures. DFT calculations suggest that Cu and Sn doping in ZnSe leads to the formation of CuZn and SnZn antisites, impacting p-type doping. APT also ident