BaCu2Sn(S,Se)4: Earth-Abundant Chalcogenides for Thin-Film Photovoltaics

Chalcogenides such as CdTe, Cu(In,Ga)(S,Se)(2) (CIGSSe), and Cu2ZnSn(S,Se)(4) (CZTSSe) have enabled remarkable advances in thin-film photovoltaic performance, but concerns remain regarding (i) the toxicity (CdTe) and (ii) scarcity (CIGSSe/CdTe) of the constituent elements and (iii) the unavoidable antisite disordering that limits further efficiency improvement (CZTSSe). In this work, we show that a different materials class, the BaCu2SnSexS4-x (BCTSSe) system, offers a prospective path to circumvent difficulties (i-iii) and to target new environmentally friendly and earth-abundant absorbers. Antisite disordering and associated band tailing are discouraged in BCTSSe due to the distinct coordination environment of the large Ba2+ cation. Indeed, an abrupt absorption edge and sharp associated photoluminescence emission demonstrate a reduced impact of band tailing in BCTSSe relative to CZTSSe. Our combined experimental and computational studies of BCTSSe reveal that the compositions 0 = x = 4 exhibit a tunable nearly direct or direct bandgap in the 1.6-2 eV range, spanning relevant values for single- or multiple-junction photovoltaic applications. For the first time, a prototype BaCu2SnS4-based thin-film solar cell has been successfully demonstrated, yielding a power conversion efficiency of 1.6% (0.42 cm2 total area). The systematic experimental and theoretical investigations, combined with proof-of-principle device results, suggest promise for BaCu2SnSexS4-x as a thin-film solar cell absorber.