I2–II–IV–VI4 (I = Cu, Ag; II = Sr, Ba; IV = Ge, Sn; VI = S, Se): Chalcogenides for Thin-Film Photovoltaics

Recent work has identified a non-zinc-blende-type quaternary semiconductor, Cu 2 BaSnS 4– x Se x (CBTSSe), as a promising candidate for thin-film photovoltaics (PVs). CBTSSe circumvents difficulties of competing PV materials regarding (i) toxicity (e.g., CdTe), (ii) scarcity of constituent elements (e.g., Cu(In,Ga)(S,Se) 2 /CdTe), and (iii) unavoidable antisite disordering that limits further efficiency improvement (e.g., in Cu 2 ZnSnS 4– x Se x ). In this work, we build on the CBTSSe paradigm by computationally scanning for further improved, earth-abundant and environmentally friendly thin-film PV materials among the 16 quaternary systems I 2 –II–IV–VI 4 (I = Cu, Ag; II = Sr, Ba; IV = Ge, Sn; VI = S, Se). The band structures, band gaps, and optical absorption properties are predicted by hybrid density-functional theory calculations. We find that the Ag-containing compounds (which belong to space groups I 222 or I 4̅2 m ) show indirect band gaps. In contrast, the Cu-containing compounds (which belong to space group P 3 1 / P 3 2 and Ama 2) show direct or nearly direct band gaps. In addition to the previously considered Cu 2 BaSnS 4– x Se x system, two compounds not yet considered for PV applications, Cu 2 BaGeSe 4 ( P 3 1 ) and Cu 2 SrSnSe 4 ( Ama 2), show predicted quasi-direct/direct band gaps of 1.60 and 1.46 eV, respectively, and are therefore most promising with respect to thin-film PV application (both single- and multijunction). A Cu 2 BaGeSe 4 sample, prepared by solid-state reaction, exhibits the expected P 3 1 structure type. Diffuse reflectance and photoluminescence spectrometry measurements yield an experimental band gap of 1.91(5) eV for Cu 2 BaGeSe 4, a value slightly smaller than that for Cu 2 BaSnS 4 .