Band Gap Tailoring and Structure-Composition Relationship within the Alloyed Semiconductor Cu₂BaGe_1–xSn_xSe₄

Recently, the I2–II–IV–VI4 (I = Cu, Ag; II = Ba, Sr; IV = Ge, Sn; VI = S, Se) materials family was identified as a promising source of potential new photovoltaic (PV) and photoelectrochemical (PEC) absorbers. These materials avoid the pitfalls of the successful photovoltaic semiconductors Cu(In,Ga)(S,Se)2 and CdTe, as they do not contain scarce (In, Te) or toxic (Cd) elements. Furthermore, ionic sizes and coordination preferences are very different for the I, II, and IV cations in the I2–II–IV–VI4 family, providing an intriguing avenue to avoid intrinsic antisite disordering that limits efficiency improvement in Cu2ZnSn(S,Se)4 (where Cu and Zn can easily substitute for one another). Here, we experimentally and computationally explore alloys Cu2BaGe1–xSnxSe4 (CBGTSe, 0 ≤ x ≤ 1) to fine-tune the structural, optical, and electronic properties for the relatively large band gap (Eg = 1.91(5) eV) unalloyed compound Cu2BaGeSe4 (CBGSe). We show that CBGTSe maintains the P31 crystal structure type of the parent CBGSe up to x ≤ 0.70. A minimum band gap value of 1.57(5) eV can be reached at x = 0.70 before the structure transforms to the Ama2 structure type. The experimental and theoretical investigations demonstrate the potential of CBGTSe for thin-film PV and PEC absorbers.