Direct Observation of High Densities of Antisite Defects in Ag2ZnSnSe4

The limited efficiency of Cu2ZnSn(SSe)4 (CZTSSe) solar cells has been widely attributed to band tailing due to high densities of CuZn and ZnCu antisite defects. It has been proposed that partial replacement of Cu by Ag should reduce the antisite defect density, leading to reduced band tailing and increased cell efficiencies. This paper examines antisite defects in Ag2ZnSnSe4 (AZTSe) crystals grown at high temperatures from a stoichiometric mixture of elements by scanning transmission electron microscopy (STEM), X-ray diffraction (XRD) and photoluminescence (PL). The elemental distribution was examined directly by atomic resolution STEM energy-dispersive X-ray (EDX) mapping and simultaneous annular dark field (ADF) imaging. EDX mapping suggested complete intermixing of Ag and Zn on the Wyckoff 2c and 2d sites in the AZTSe kesterite unit cell, and around 14% substitution of Zn for Ag on the 2a site, while ADF images showed evidence for local nanometer-sized regions within the disordered matrix with partial ordering of Zn and Ag on the 2c and 2d sites. These observations show that the AZTSe had a high density of AgZn and ZnAg antisite defects, in contrast with room temperature photoluminescence showing relatively narrow emission lines close to the band edge and thus minimal band tailing. The interpretation of these results, and their wider significance for understanding the role of antisite defects in CZTSSe and Ag-substituted CZTSSe cells, are discussed.