First‐principles study on alkali‐metal effect of Li, Na, and K in Cu2ZnSnS4 and Cu2ZnSnSe4

Abstract The substitution energies and migration energies of the alkali‐metals of Li, Na, and K atoms in Cu 2 ZnSnS 4 (CZTS) and Cu 2 ZnSnSe 4 (CZTSe) were investigated by first‐principles calculations. The migration energies of Li, Na, and K atoms in CZTS and CZTSe are obtained by a combination of linear and quadratic synchronous transit (LST/QST) methods and a nudged elastic band (NEB) method. For CZTS and CZTSe, the substitution energies of Na Cu and Na Zn are much smaller than that of Na Sn . The substitution energy of Na Zn is comparable to that of Na Cu , indicating that Na Cu and Na Zn are easily formed in CZTS and CZTSe. The substitution energies of the Li Cu and Li Zn atoms are smaller than those of Na Cu and Na Zn , while substitution energies of K Cu and K Zn atoms are much larger than those of Na Cu and Na Zn . Therefore, it is difficult to form K Cu and K Zn in CZTS and CZTSe. The theoretical migration energies of Na atom at Cu site to the nearest Cu vacancy (Na Cu →V Cu ) and Na atom at Zn site to the nearest Cu vacancy (Na Zn →V Cu ) are much smaller than those of (Na Sn →V Cu ). The migration energies of (Na Cu →V Cu ) and (Na Zn →V Cu ) in CZTS and CZTSe are comparable to that of Na atom at Cu site to Cu vacancy (Na Cu →V Cu ) in CIS. The mechanism for the alkali‐metal effect of Li, Na, and K in the Cu 2 ZnSn(S,Se) 4 films during the post‐deposition treatment of LiF, NaF, and KF is discussed on the basis of the calculated substitution and migration energies. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)