Recent work has identified a non-zinc-blende-type quaternary semiconductor, Cu₂BaSnS_4–xSe_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 Cu₂ZnSnS_4–xSe_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 I₂–II–IV–VI₄ (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 I222 or I4̅2m) show indirect band gaps. In contrast, the Cu-containing compounds (which belong to space group P3₁/P3₂ and Ama2) show direct or nearly direct band gaps. In addition to the previously considered Cu₂BaSnS_4–xSe_x system, two compounds not yet considered for PV applications, Cu₂BaGeSe₄ (P3₁) and Cu₂SrSnSe₄ (Ama2), 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 Cu₂BaGeSe₄ sample, prepared by solid-state reaction, exhibits the expected P3₁ structure type. Diffuse reflectance and photoluminescence spectrometry measurements yield an experimental band gap of 1.91(5) eV for Cu₂BaGeSe₄, a value slightly smaller than that for Cu₂BaSnS₄.

中文翻译：

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I ₂ –II–IV–VI ₄（I = Cu，Ag; II = Sr，Ba; IV = Ge，Sn; VI = S，Se）：薄膜光伏用硫属化物

最近的工作已确定非锌混合型四元半导体Cu ₂ BaSnS _{4– x} Se _x（CBTSSe），是薄膜光伏（PVs）的有希望的候选者。CBTSSe绕开了竞争性PV材料的难题，涉及到（i）毒性（例如CdTe），（ii）组成元素稀缺（例如Cu（In，Ga）（S，Se）₂ / CdTe）和（iii）不可避免的抗位点限制进一步提高效率的无序性（例如，在Cu ₂ ZnSnS _{4– x} Se _x中）。在这项工作中，我们以CBTSSe范例为基础，通过计算扫描16种四元体系中进一步改良的，富含地球和环保的薄膜光伏材料₂ –II–IV–VI ₄（I = Cu，Ag； II = Sr，Ba； IV = Ge，Sn； VI = S，Se）。通过混合密度泛函理论计算来预测能带结构，能带隙和光吸收特性。我们发现，含Ag的化合物（其属于空间群我222或我42米）示出了间接带隙。相反，含铜化合物（属于空间群P 3 ₁ / P 3 ₂和Ama 2）显示出直接或几乎直接的带隙。除了先前考虑的Cu ₂ BaSnS _{4– x} Se _x在系统中，尚未考虑用于光伏应用的两种化合物Cu ₂ BaGeSe ₄（P 3 ₁）和Cu ₂ SrSnSe ₄（Ama 2）分别显示了1.60和1.46 eV的拟准直接/带隙。对于薄膜光伏应用（单结和多结），最有前途。通过固相反应制备的Cu ₂ BaGeSe ₄样品表现出预期的P 3 ₁结构类型。扩散反射率和光致发光光谱法测量得出Cu ₂ BaGeSe ₄的实验带隙为1.91（5）eV，该值略小于Cu ₂ BaSnS _4的值。