Large-Grained Cu2BaSnS4 Films for Photocathodes

Raman study of colloidal Cu2ZnSnS4 nanocrystals obtained by "green" synthesis modified by seed nanocrystals or extra cations in the solution

The method of affordable colloidal synthesis of nanocrystalline Cu₂ZnSnS₄ (CZTS) is developed, which is suitable for obtaining bare CZTS nanocrystals (NCs), cation substituted CZTS NCs, and CZTS-based hetero-NCs. For the hetero-NCs, the synthesized in advance NCs of another material are introduced into the reaction solution so that the formation of CZTS takes place preferably on these "seed" NCs. Raman spectroscopy is used as the primary method of structural characterization of the NCs in this work because it is very sensitive to the CZTS structure and allows to probe NCs both in solutions and films. Raman data are corroborated by optical absorption measurements and transmission electron microscopy on selected samples. The CdTe and Ag NCs are found to be good seed NCs, resulting in a comparable or even better quality of the CZTS compound compared to bare CZTS NCs. For Au NCs, on the contrary, no hetero-NCs could be obtained under the given condition. Partial substitution of Zn for Ba during the synthesis of bare CZTS NCs results in a superior structural quality of NCs, while the introduction of Ag for partial substitution of Cu deteriorates the structural quality of the NCs.

Emerging Chalcogenide Thin Films for Solar Energy Harvesting Devices

Chalcogenide semiconductors offer excellent optoelectronic properties for their use in solar cells, exemplified by the commercialization of Cu(In,Ga)Se₂- and CdTe-based photovoltaic technologies. Recently, several other chalcogenides have emerged as promising photoabsorbers for energy harvesting through the conversion of solar energy to electricity and fuels. The goal of this review is to summarize the development of emerging binary (Sb₂X₃, GeX, SnX), ternary (Cu₂SnX₃, Cu₂GeX₃, CuSbX₂, AgBiX₂), and quaternary (Cu₂ZnSnX₄, Ag₂ZnSnX₄, Cu₂CdSnX₄, Cu₂ZnGeX₄, Cu₂BaSnX₄) chalcogenides (X denotes S/Se), focusing especially on the comparative analysis of their optoelectronic performance metrics, electronic band structure, and point defect characteristics. The performance limiting factors of these photoabsorbers are discussed, together with suggestions for further improvement. Several relatively unexplored classes of chalcogenide compounds (such as chalcogenide perovskites, bichalcogenides, etc.) are highlighted, based on promising early reports on their optoelectronic properties. Finally, pathways for practical applications of emerging chalcogenides in solar energy harvesting are discussed against the backdrop of a market dominated by Si-based solar cells.

Structural, Optical, and Electronic Properties of Two Quaternary Chalcogenide Semiconductors: Ag2SrSiS4 and Ag2SrGeS4

Quaternary chalcogenide materials have long been a source of semiconductors for optoelectronic applications. Recent studies on I₂-II-IV-X₄ (I = Ag, Cu, Li; II = Ba, Sr, Eu, Pb; IV = Si, Ge, Sn; X = S, Se) 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, Ag₂SrSiS₄ and Ag₂SrGeS₄, each found to form in the tetragonal I4̅2m structure of Ag₂BaGeS₄. During the synthesis targeting the title compounds, two additional materials, Ag₂Sr₃Si₂S₈ and Ag₂Sr₃Ge₂S₈, have also been identified. These cubic compounds represent impurity phases during the synthesis of Ag₂SrSiS₄ and Ag₂SrGeS₄. We show through hybrid density functional theory calculations that Ag₂SrSiS₄ and Ag₂SrGeS₄ 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 Ag₂SrSiS₄ and Ag₂SrGeS₄ powders show frequency-doubling capabilities in the near-infrared range.

Fully stoichiometric Cu2BaSn(S1-x Se x )4 solar cells via chemical solution deposition

Cu₂BaSn(S_1-x Se _x )₄ has shown great prospects in the photoelectric field due to Earth-abundance, low toxicity, cost efficiency, direct bandgap, high absorption coefficient (>10⁴ cm^-1) and reduced anti-site disorder relative to Cu₂ZnSn(S_1-x Se _x )₄. A fully-tunable ratio of S/Se is the key to broaden the bandgap of Cu₂BaSn(S_1-x Se _x )₄. Here, we introduce a thionothiolic acid metathesis process to readily tune the stoichiometry of Cu₂BaSn(S_1-x Se _x )₄ films for the first time. Different stoichiometric Se/(S + Se) of Cu₂BaSn(S_1-x Se _x )₄ from zero to one can vary the bandgap range from 2 to 1.68 eV. The grain size of Cu₂BaSn(S_1-x Se _x )₄ films can be grown more than 10 μm. The optimized bandgap and high-quality growth of Cu₂BaSn(S_1-x Se _x )₄ films ensure the best power conversion efficiency of 2.01% for solution-processed Cu₂BaSn(S_1-x Se _x )₄ solar cells. This method provides an alternative solution-processed way for the synthesis of fully stoichiometric Cu₂BaSn(S_1-x Se _x )₄.