Distant-Atom Mutation for Better Earth-Abundant Light Absorbers: A Case Study of Cu₂BaSnSe₄

Thin-film Cu(In,Ga)Se2 and CdTe solar cells have demonstrated high power conversion efficiencies, but they cannot provide a sustainable clean energy pathway because of the scarcity of Te and In. Here, we propose a distant-atom concept to mutate In by a group II element (Ba) and a group IV element (Sn) that are at rather different locations on the periodic table. Because of the very different electronic properties between the cations, the resultant earth-abundant orthorhombic Cu2BaSnSe4 absorber does not have the detrimental cation–cation disorder issue seen in the earth-abundant kesterite Cu2ZnSnSe4 absorber. We anticipate that Cu2BaSnSe4 solar cells should not have large open-circuit voltage deficits as seen in CuZnSnSe4 solar cells. Density functional theory calculation of the electronic and defect properties of Cu2BaSnSe4 confirms these expectations.