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  Fritsch, D.; Schorr, S.: On the ground state crystal structure of (Ag_0.5Cu_0.5)₂ZnSnSe₄. Thin Solid Films 738 (2021), p. 138957/1-5

10.1016/j.tsf.2021.138957

Abstract:
In our quest to improve the device performance of kesterite based thin film solar cells a recent combined neutron and x-ray diffraction study on the structural properties of powder samples of (Ag1-xCux)2ZnSnSe4 yielded surprising results. Although both end members, Ag2ZnSnSe4 and Cu2ZnSnSe4 crystallise in the kesterite crystal structure, in the intermediate region the structurally similar stannite crystal structure has been identified as the prevalent ground state crystal structure. Here, in order to understand this behaviour, we perform first-principles calculations based on density functional theory, combined with evolutionary crystal structure prediction investigation as implemented in the USPEX code. Our results show that (Ag0.5Cu0.5)2ZnSnSe4 crystallises in the kesterite crystal structure as well, with the Ag and Cu atoms being exclusively placed at the (2a) and (2c) Wyckoff positions, with no apparent mixing. A detailed structural analysis combined with electronic and optical properties will be discussed alongside experimental results.