Recent work has identified a non-zinc-blendetype 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 thinfilm 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 I222 or I4̅ 2m) show indirect band gaps. In contrast, the Cu-containing compounds (which belong to space group P3 1 /P3 2 and Ama2) 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 (P3 1 ) and Cu 2 SrSnSe 4 (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 Cu 2 BaGeSe 4 sample, prepared by solid-state reaction, exhibits the expected P3 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 .

Recent work on quaternary semiconductors Cu 2 BaSn(S,Se) 4 and Ag 2 BaSnSe 4 for photovoltaic and thermoelectric applications, respectively, has shown the promise of exploring the broader family of defect-resistant I 2 -II-IV-X 4 materials (where I, II, and IV refer to the formal oxidation state of the metal cations and X is a chalcogen anion) with tetrahedrally coordinated I/IV cations and larger II cations (i.e., Sr, Ba, Pb, and Eu) for optoelectronic and energy-related applications. Chemical dissimilarity among the II and I/IV atoms represents an important design motivation because it presents a barrier to antisite formation, which otherwise may act as electronically harmful defects. We herein show how all 31 experimentally reported I 2 -II-IV-X 4 examples (with large II cations and tetrahedrally coordinated smaller I/IV cations), which form within five crystal structure types, are structurally linked. Based on these structural similarities, we derive a set of tolerance factors that serve as descriptors for phase stability within this family. Despite common usage in the wellstudied perovskite system, Shannon ionic radii are found to be insufficient for predicting metal−chalcogen bond lengths, pointing to the need for experimentally derived correction factors as part of an empirically driven learning approach to structure prediction. We use the tolerance factors as a predictive tool and demonstrate that four new I 2 -II-IV-X 4 compounds, Ag 2 BaSiS 4 , Ag 2 PbSiS 4 , Cu 2 PbGeS 4 , and Cu 2 SrSiS 4 , can be synthesized in correctly predicted phases. One of these compounds, Ag 2 PbSiS 4 , shows potentially promising optoelectronic properties for photovoltaic applications.

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–x Sn x Se4 (CBGTSe, 0 ≤ x ≤ 1) to fine-tune the structural, optical, and electronic properties for the relatively large band gap (E g = 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.

1 of 6) 1600568 dielectric function. Alloying of these noble metals has been applied to tune the material dielectric function, where the LSPR can be modulated progressively from the UV (pure Ag) to the NIR (pure Au). [12][13][14][15] Thus, metallic nanostructures composed of Ag-Au can enable the rational design of building blocks for different applications, such as metamaterials, [16,17] hot carrier devices, [18] light absorption improvement in photovoltaics, [19,20] colored glasses, [21] displays, [22,23] and catalysis. [24] To date, different fabrication techniques have been successfully utilized to realize Ag x Au 1−x alloyed NPs. They can be formed by colloidal synthesis via the reduction of precursors containing metals in solution, [25,26] and by the sequential pulsed laser deposition of Ag and Au targets, [27,28] which can yield large amounts of NPs with narrow size distribution. However, the overall size of the NPs cannot be varied beyond 150 nm. [29] Alternatively, nanolithographic methods enable full control of NPs size, shape, and distribution. [13] Nevertheless, this technique is constrained to specific applications due to its high cost and very limited scalability. The dewetting of metallic thin films has also been used to fabricate pure [21,[30][31][32] and alloyed [19] metal NPs. In this simple and effective fabrication route, a very thin layer of metal (<50 nm) is initially deposited onto a substrate. Then, when the thin-film sample is annealed under a controlled environment (oxygen free), surface diffusion takes place and results in the formation of nanostructures to minimize the energy of the system. [33][34][35][36] This method has been particularly useful for optoelectronic devices, where these metallic NPs act as light scattering centers that ultimately increase light absorption within the semiconductor. [4,37] In this work, we fabricate fully alloyed Ag x Au 1−x NPs with controlled chemical composition by dewetting thin films and characterize their optical response at the macro-and nano-scale. Surprisingly, we find that the NPs' distribution heavily depends on the thin-film chemical composition, irrespective of the original film thickness. Simultaneously, we measure a shift of the LSPR due to the NPs' composition variation, which defines their optical response. We map the elemental distribution of Ag and Au and confirm that the NPs are fully alloyed, forming a solid solution at the nanoscale. To further illustrate how the chemical composition affects the material optical response, we perform a detailed analysis of the optical characteristics of fully alloyed Ag 0.5 Au 0.5 nanostructures in the visible range of the spectrum. For that, we combine spectrally dependent NSOM measurements and finite-difference time-domain (FDTD) simulations to locally resolve the optical response of individual NPs. Our results of the near-field light-matter interactions for Ag 0.5 Au 0.5 nanostructures reveal an electric field enhancement of 30 times in the visible range of the spectrum under the NPs Combining meta...

materials have shown particular versatility and promise among these compounds. These semiconductors take advantage of a diverse bonding scheme and chemical differences among cations to target a degree of antisite defect resistance. Within this set of compounds, the materials containing both Ag and Sr have not been experimentally studied and leave a gap in the full understanding of the family. Here, we have synthesized powders and single crystals of two Ag-and Sr-containing compounds, Ag 2 SrSiS 4 and Ag 2 SrGeS 4 , each found to form in the tetragonal I4̅ 2m structure of Ag 2 BaGeS 4 . During the synthesis targeting the title compounds, two additional materials, Ag 2 Sr 3 Si 2 S 8 and Ag 2 Sr 3 Ge 2 S 8 , have also been identified. These cubic compounds represent impurity phases during the synthesis of Ag 2 SrSiS 4 and Ag 2 SrGeS 4 . We show through hybrid density functional theory calculations that Ag 2 SrSiS 4 and Ag 2 SrGeS 4 have highly dispersive band-edge states and indirect band gaps, experimentally measured as 2.08(1) and 1.73(2) eV, respectively. Second-harmonic generation measurements on Ag 2 SrSiS 4 and Ag 2 SrGeS 4 powders show frequencydoubling capabilities in the near-infrared range.