Reaction path for formation of Cu₂SnSe₃ film by selenization of Cu-Sn precursor

Reaction path for fabrication of Cu₂SnSe₃ (CTSe) film by selenization of Cu-Sn precursor was investigated via in-situ X-ray diffraction (XRD) as well as glazing incident XRD (GIXRD) measurements. Cross-sectional scanning electron microscopy (SEM)-energy dispersive spectrometry (EDS) and transmission electron microscope (TEM) analyses revealed the element and phase distribution along the depth direction. Based on these results, a proposed growth model was concluded below: first, the Se atoms from evaporation source reacted with Cu and Sn atoms to produce Cu_2-xSe and SnSe₂ phases. Noticeably, resulting film presented bilayer feature with Cu_2-xSe located at the surface and SnSe₂ located at bottom. Second, CTSe phase formed at the interface of Cu_2-xSe and SnSe₂ as the increasing temperature. The Cu_2-xSe was depleted by Sn-related secondary phases when the Cu/Sn ratio was smaller than 1.72. The secondary phases of SnSe₂ and SnSe were coexisted with CTSe phase independent of Cu/Sn ratio in metallic precursor, which was attributed to the weak diffusion ability of Sn and Sn-related secondary phases in the CTSe film. The origins for high carrier concentration in CTSe films were ascribed to the Cu_2-xSe and intrinsic acceptor concentration and effective approach to reduce the value was explored. An attempt of solar cell with CTSe as absorber was performed and photocurrent of 9.9 mA/cm² was detected.