Synthesis and Post-Annealing of Cu2ZnSnS4 Absorber Layers Based on Oleylamine/1-dodecanethiol

Modulating temperature for Cu2ZnSnS4 (CZTS) synthesis via hot injection method and studying the photocatalytic efficiencies for the degradation of rhodamine 6G and methylene blue pollutants

Cu2ZnSnS4 (CZTS) was synthesized following hot injection method and the process was optimized by varying temperature conditions. Four samples at different temperatures viz., 200, 250, 300 and 350 °C were prepared and analyzed using different characterization techniques. Based on the correlation between XRD, Raman and XPS, we conclude that the formation of ZnS and SnS2 occurs at 350 °C but at 200 °C there is no breakdown of the complex as per XRD. According to Raman and XPS analysis, as the temperature rises, the bonds between the metals become weaker, which is visibly seen in Raman and XPS due to the minor peaks of copper sulfide. Scanning electron microscopic analysis confirmed nanometric particles which increase in size with temperature. The photocatalytic evaluation showed that CZTS synthesized at 200 °C performed efficiently in the removal of the two colorants, methylene blue and Rhodamine 6G, achieving 92.80% and 90.65%, respectively. The photocatalytic degradation efficiencies decreased at higher temperatures due to bigger sized CZTS particles as confirmed by SEM results. Computational simulations confirm that CZTS has a highly negative energy -25,764 Ry, confirming its structural stability and higher covalent than ionic character.

Mechanochemical Synthesis of Sustainable Ternary and Quaternary Nanostructured Cu2SnS3, Cu2ZnSnS4, and Cu2ZnSnSe4 Chalcogenides for Thermoelectric Applications

Copper-based chalcogenides have emerged as promising thermoelectric materials due to their high thermoelectric performance, tunable transport properties, earth abundance and low toxicity. We have presented an overview of experimental results and first-principal calculations investigating the thermoelectric properties of various polymorphs of Cu₂SnS₃ (CTS), Cu₂ZnSnS₄ (CZTS), and Cu₂ZnSnSe₄ (CZTSe) synthesized by high-energy reactive mechanical alloying (ball milling). Of particular interest are the disordered polymorphs of these materials, which exhibit phonon-glass-electron-crystal behavior-a decoupling of electron and phonon transport properties. The interplay of cationic disorder and nanostructuring leads to ultra-low thermal conductivities while enhancing electronic transport. These beneficial transport properties are the consequence of a plethora of features, including trap states, anharmonicity, rattling, and conductive surface states, both topologically trivial and non-trivial. Based on experimental results and computational methods, this report aims to elucidate the details of the electronic and lattice transport properties, thereby confirming that the higher thermoelectric (TE) performance of disordered polymorphs is essentially due to their complex crystallographic structures. In addition, we have presented synchrotron X-ray diffraction (SR-XRD) measurements and ab initio molecular dynamics (AIMD) simulations of the root-mean-square displacement (RMSD) in these materials, confirming anharmonicity and bond inhomogeneity for disordered polymorphs.

Topological Anderson Insulator in Cation-Disordered Cu2ZnSnS4

We present the first candidate for the realization of a disorder-induced Topological Anderson Insulator in a real material system. High-energy reactive mechanical alloying produces a polymorph of Cu₂ZnSnS₄ with high cation disorder. Density functional theory calculations show an inverted ordering of bands at the Brillouin zone center for this polymorph, which is in contrast to its ordered phase. Adiabatic continuity arguments establish that this disordered Cu₂ZnSnS₄ can be connected to the closely related Cu₂ZnSnSe₄, which was previously predicted to be a 3D topological insulator, while band structure calculations with a slab geometry reveal the presence of robust surface states. This evidence makes a strong case in favor of a novel topological phase. As such, the study opens up a window to understanding and potentially exploiting topological behavior in a rich class of easily-synthesized multinary, disordered compounds.

Synthesis and Characterization of Cu2ZnSnS4 Thin Films Obtained by Combined Magnetron Sputtering and Pulsed Laser Deposition

Cu2ZnSnS4 (CZTS) is a complex quaternary material, and obtaining a single-phase CZTS with no secondary phases is known to be challenging and dependent on the production technique. This work involves the synthesis and characterization of CZTS absorber layers for solar cells. Thin films were deposited on Si and glass substrates by a combined magnetron sputtering (MS) and pulsed laser deposition (PLD) hybrid system, followed by annealing without and with sulfur powder at 500 °C under argon (Ar) flow. Three different Cu2S, SnS2, and ZnS targets were used each time, employing a different target for PLD and the two others for MS. The effect of the different target arrangements and the role of annealing and/or sulfurization treatment were investigated. The characterization of the absorber films was performed by grazing incidence X-ray diffraction (GIXRD), X-ray reflectometry (XRR), Raman spectroscopy, scanning electron microscopy, and regular transmission spectroscopy. The film with ZnS deposited by PLD and SnS2 and Cu2S by MS was found to be the best for obtaining a single CZTS phase, with uniform surface morphology, a nearly stoichiometric composition, and an optimal band gap of 1.40 eV. These results show that a new method that combines the advantages of both MS and PLD techniques was successfully used to obtain single-phase Cu2ZnSnS4 films for solar cell applications.

Thermoelectric properties of CZTS thin films: effect of Cu-Zn disorder

Cu-Zn disorder is known to deeply affect kesterite (Cu2ZnSnS4, CZTS) due to the low temperature order-disorder phase transition, leading to a random occupation of the two cations in the shared crystallographic planes. This defect complex has been extensively studied in the thin film photovoltaic sector, with considerable efforts in developing methods to quantify disorder. In this study, a preliminary investigation of thermoelectric properties in temperature for thin film CZTS is presented. It is found that Cu-Zn disorder enhances both electrical conductivity and Seebeck coefficient. This can positively affect the thermoelectric performance, showing a mechanism of potential interest for a broad class of quaternary chalcogenides. The order-disorder transition is clearly visible in the electronic properties. This feature is repeatable, with samples from different preparations and groups showing consistent results, qualitatively suggesting electronic measurements as possible methods to quantify disorder. Furthermore, the reversibility of the transition allows the electronic properties to be tuned via specific thermal treatments, pointing to interesting applications in tunable electronics.

Significantly Improving the Crystal Growth of a Cu2ZnSn(S,Se)4 Absorber Layer by Air-Annealing a Cu2ZnSnS4 Precursor Thin Film

The crystal quality of a Cu₂ZnSn(S,Se)₄ (CZTSSe) thin film is crucially important to a high-performance CZTSSe solar cell. After selenization, a bilayer CZTSSe thin film consisting of a large-grain top layer and a small-particle bottom layer is usually observed according to the literature. In this work, a facile air-annealing pretreatment is conducted for a Cu₂ZnSnS₄ precursor thin film prior to selenization, which can lead to sodium diffusion into the CZTS precursor thin film and surface oxidization of the CZTS thin film. Our experimental results revealed that the Na prediffusion and the surface oxidation of the CZTS precursor thin film can significantly promote the crystal growth of the CZTSSe thin film, which can completely remove the small-particle bottom layer and form a large-grain-spanned CZTSSe thin film. As a result, a photoelectric conversion efficiency of 9.80% was achieved by this method.