Favorable Bonding and Band Structures of Cu2ZnSnS4 and CdS Films and Their Photovoltaic Interfaces

Thin-film photovoltaic cells using Cu₂ZnSnS₄ (CZTS, p-type) have many advantages, such as high photoconversion, low cost, and great tunability with earth-abundant, nontoxic elements, all of which are necessary to be long-term contributors to next-generation solar energy harvesting. Accurate measurements of bonding and band structures of both the thin-film materials and their interfaces are paramount to designing the solar devices layer-by-layer. Here, finely tuned 1 μm thick CZTS films, 50 nm thick CdS layers (n-type), and their 1 μm/2 nm p-n junction were fabricated inexpensively using our previously studied methods and investigated extensively for maximizing the key interface in the CZTS solar devices. Synthesized bulk CZTS and CdS were analyzed for structural deviations and crystal defects using synchrotron-based (SR) X-ray absorption fine structure (XAFS) along with simulated XAFS patterns. The structural properties of the two materials were designed to favor photovoltaic activity. Interface valence band structures of the CZTS/CdS p-n junction were measured through SR X-ray photoelectron spectroscopy (SR-XPS) and compared with the ones simulated using density functional theory. A full band diagram was constructed from XPS of the bulk films and SR-XPS of the interface, providing guidelines in optimizing charge-carrier extraction from the CZTS absorber to CdS buffer layer. It turns out that a small spike-like interface in the conduction band overlap was formed, maintaining a strong internal bias, while favoring a small energy barrier to prevent large-scale recombination from occurring. A large open-circuit voltage was obtained in the preliminary solar cell devices built on the small spike-like interface.

Cu2ZnSn(S,Se)4 thin-films prepared from selenized nanocrystals ink

For the first time, CZTS ink was formulated using low-temperature heating up synthesis of NCs. Besides, the influence of powder concentration on the properties of the films was examined. Subsequently, the CZTS films were annealed under a selenium (Se)/argon (Ar) atmosphere at different temperatures to enhance their properties. The influence of selenization temperature on the properties of CZTS films was examined in detail. Structural analysis showed a peak shift towards lower 2θ values for CZTSSe films because of Se incorporation, resulting in larger lattice parameters for CZTSSe than CZTS. As the selenization temperature increases, an increment in the grain size was observed and the band gap was decreased from 1.52 to 1.05 eV. Hall Effect studies revealed a significant improvement in the mobility and carrier concentration with respect to selenization temperatures. Moreover, film selenized at 550 °C exhibited higher photoconductivity as compared to other films, indicating their potential application in the field of low-cost thin-film solar cells.

For the first time, CZTS ink was formulated using low-temperature heating up synthesis of NCs.

"Green" Aqueous Synthesis and Advanced Spectral Characterization of Size-Selected Cu2ZnSnS4 Nanocrystal Inks

Structure, composition, and optical properties of colloidal mercaptoacetate-stabilized Cu2ZnSnS4 (CZTS) nanocrystal inks produced by a "green" method directly in aqueous solutions were characterized. A size-selective precipitation procedure using 2-propanol as a non-solvent allows separating a series of fractions of CZTS nanocrystals with an average size (bandgap) varying from 3 nm (1.72 eV) to 2 nm (2.04 eV). The size-selected CZTS nanocrystals revealed also phonon confinement, with the main phonon mode frequency varying by about 4 cm-1 between 2 nm and 3 nm NCs.

Resolving the effects of compositional change on structures in Cu2ZnSnS4 nanocrystals by X-ray absorption fine structure

Renewable energy sources, and solar energy in particular, are a high impact research topic in the push for sustainable, long-term energy alternatives to fossil fuels. Cu2ZnSnS4 (CZTS) is one of the attractive, cost-effective materials that meets these needs. The quaternary nature makes the structure prone to defects and crystal alignment disorder. Some of these defects create advantageous electronic effects through antisite substitutions of Zn for Cu, [Formula: see text]. Others such as Sn for Zn replacements are detrimental. Synchrotron-based X-ray absorbance fine structure (XAFS) analysis was used to identify specific patterns in the antisite contributions to the structure of low-cost CZTS films that produced the highest photoresponse in each of our samples. Correlations were found between the Cu/(Zn + Sn) ratio and advantageous antisite formations, though at the cost of increased alignment disorder. Similarly, the Zn/Sn ratio showed relationships between both advantageous and disadvantageous antisite and vacancy pairs. Variations in the local surroundings for each metal center were confirmed through X-ray absorption near-edge structures (XANES). Extended X-ray absorption fine structures (EXAFS), verified through FEFF fitting of the EXAFS, confirmed the patterns in crystal alignment disorder, and the effects each antisite had on the overall crystal structure. The precision and unique nature of such synchrotron techniques offers opportunities to identify these trends at each metal center, providing guidance to balance negative and positive structural components during fabrication. Each minor change in stoichiometry has been shown to affect several interactions within the structure.

Probing the CZTS/CdS heterojunction utilizing photoelectrochemistry and x-ray absorption spectroscopy

The importance of renewable resources is becoming more and more influential on research due to the depletion of fossil fuels. Cost-effective ways of harvesting solar energy should also be at the forefront of these investigations. Cu₂ZnSnS₄ (CZTS) solar cells are well within the frame of these goals, and a thorough understanding of how they are made and processed synthetically is crucial. The CZTS/CdS heterojunction was examined using photoelectrochemistry and synchrotron radiation (SR) spectroscopy. These tools provided physical insights into this interface that was formed by the electrophoretic deposition of CZTS nanocrystals and chemical bath deposition (CBD) of CdS for the respective films. It was discovered that CBD induced a change in the local and long range environment of the Zn in the CZTS lattice, which was detrimental to the photoresponse. X-ray absorption near-edge structures and extended X-ray absorption fine structures (EXAFSs) of the junction showed that this change was at an atomic level and was associated with the coordination of oxygen to zinc. This was confirmed through FEFF fitting of the EXAFS and through IR spectroscopy. It was found that this change in both photoresponse and the Zn coordination can be reversed with the use of low temperature annealing. Investigating CZTS through SR techniques provides detailed structural information of minor changes from the zinc perspective.

A Hollow-Structured CuS@Cu2 S@Au Nanohybrid: Synergistically Enhanced Photothermal Efficiency and Photoswitchable Targeting Effect for Cancer Theranostics

It is of great importance in drug delivery to fabricate multifunctional nanocarriers with intelligent targeting properties, for cancer diagnosis and therapy. Herein, hollow-structured CuS@Cu₂ S@Au nanoshell/satellite nanoparticles are designed and synthesized for enhanced photothermal therapy and photoswitchable targeting theranostics. The remarkably improved photothermal conversion efficiency of CuS@Cu₂ S@Au under 808 nm near-infrared (NIR) laser irradiation can be explained by the reduced bandgap and more circuit paths for electron transitions for CuS and Cu₂ S modified with Au nanoparticles, as calculated by the Vienna ab initio simulation package, based on density functional theory. By modification of thermal-isomerization RGD targeting molecules and thermally sensitive copolymer on the surface of nanoparticles, the transition of the shielded/unshielded mode of RGD (Arg-Gly-Asp) targeting molecules and shrinking of the thermally sensitive polymer by NIR photoactivation can realize a photoswitchable targeting effect. After loading an anticancer drug doxorubicin in the cavity of CuS@Cu₂ S@Au, the antitumor therapy efficacy is greatly enhanced by combining chemo- and photothermal therapy. The reported nanohybrid can also act as a photoacoustic imaging agent and an NIR thermal imaging agent for real-time imaging, which provides a versatile platform for multifunctional theranostics and stimuli-responsive targeted cancer therapy.

Identifying barriers to charge-carriers in the bulk and surface regions of Cu2ZnSnS4 nanocrystal films by x-ray absorption fine structures (XAFSs)

Solar cell performance is most affected by the quality of the light absorber layer. For thin-film devices, this becomes a two-fold problem of maintaining a low-cost design with well-ordered nanocrystal (NC) structure. The use of Cu₂ZnSnS₄ (CZTS) NCs as the light absorber films forms an ideal low-cost design, but the quaternary structure makes it difficult to maintain a well-ordered layer without the use of high-temperature treatments. There is little understanding of how CZTS NC structures affect the photoconversion efficiency, the charge-carriers, and therefore the performance of the device manufactured from it. To examine these relationships, the measured photoresponse from the photo-generation of charge-carrier electron-hole pairs was compared against the crystal structure, as short-range and long-range crystal orders for the films. The photoresponse simplifies the electronic properties into three basic steps that can be associated with changes in energy levels within the band structure. These changes result in the formation of barriers to charge-carrier flow. The extent of these barriers was determined using synchrotron-based X-ray absorbance fine structure to probe the individual metal centers in the film, and comparing these to molecular simulations of the ideal extended x-ray absorbance fine structure scattering. This allowed for the quantification of bond lengths, and thus an interpretation of the distortions in the crystal lattice. The various characteristics of the photoresponse were then correlated to the crystallographic order and used to gain physical insight into barriers to charge-carriers in the bulk and surface regions of CZTS films.

Synthesis of ligand-free CZTS nanoparticles via a facile hot injection route

Single-phase, ligand-free Cu2ZnSnS4 (CZTS) nanoparticles that can be dispersed in polar solvents are desirable for thin film solar cell fabrication, since water can be used as the solvent for the nanoparticle ink. In this work, ligand-free nanoparticles were synthesized using a simple hot injection method and the precursor concentration in the reaction medium was tuned to control the final product. The as-synthesized nanoparticles were characterized using various techniques, and were found to have a near-stoichiometric composition and a phase-pure kesterite crystal structure. No secondary phases were detected with Raman spectroscopy or scanning transmission electron microscopy energy dispersive x-ray spectroscopy. Furthermore, high resolution transmission electron microscopy showed large-sized nanoparticles with an average diameter of 23 nm ± 11 nm. This approach avoids all organic materials and toxic solvents that otherwise could hinder grain growth and limit the deposition techniques. In addition the synthesis route presented here results in nanoparticles of a large size compared to other ligand-free CZTS nanoparticles, due to the high boiling point of the solvents selected. Large particle size in CZTS nanoparticle solar cells may lead to a promising device performance. The results obtained demonstrate the suitability of the synthesized nanoparticles for application in low cost thin film solar cells.

Significant enhancement in photocatalytic reduction of water to hydrogen by Au/Cu2 ZnSnS4 nanostructure

Enhanced photocatalytic activities by Au core Novel Au/Cu2 ZnSnS4 core/shell nanoparticles (NPs) are synthesized for the first time via wet chemistry approach. The insertion of Au core into CZTS NPs dramatically enhances light absorption due to surface plasmon resonance effect, especially in the Vis-NIR region. Au/CZTS core/shell NPs show much higher photocatalytic activities for hydrogen evolution compared with other CZTS nanostructures.