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Reactive Mechanism of Cu2ZnSnSe4 Thin Films Prepared by Reactive Annealing of the Cu/Zn Metal Layer in a SnSex + Se Atmosphere

Cu2ZnSnSe4 (CZTSe) thin films were prepared by a two-step process with the electrodeposition of a Cu/Zn metallic stack precursor followed by a reactive anneal under a Se + Sn containing atmosphere. We investigate the effect of the Sex and SnSex (x = 1,2) partial pressures and annealing temperature on the morphological, structural, and elemental distribution of the CZTSe thin films. Line scanning energy dispersive spectroscopy (EDS) measurements show the presence of a Zn-rich secondary phase at the back-absorber region of the CZTSe thin films processed with higher SnSex partial pressure and lower annealing temperatures. The Zn-rich phase can be reduced by lowering the SnSex partial pressure and by increasing the annealing temperature. A very thin MoSe2 film between the CZTSe and Mo interface is confirmed by X-ray diffraction (XRD) and grazing incidence X-ray diffraction (GIXRD) measurements. These measurements indicate a strong dependence of these process variations in secondary phase formation and accumulation. A possible reaction mechanism of CZTSe thin films was presented. In a preliminary optimization of both the SnSex partial pressure and the reactive annealing process, a solar cell with 7.26% efficiency has been fabricated.