Influence of Ligands on the Formation of Kesterite Thin Films for Solar Cells: A Comparative Study

Solution-Based Synthesis and Characterization of Cu2ZnSnS4 (CZTS) Thin Films

Cu2ZnSnS4 (CZTS) ink was synthesized from metal chloride precursors, sulfur, and oleylamine (OLA), as a ligand by a simple and low-cost hot-injection method. Thin films of CZTS were then prepared by spin coating, followed by thermal annealing. The effects of the fabrication parameters, such as ink concentration, spinning rate, and thermal treatment temperatures on the morphology and structural, optical, and electrical properties of the films were investigated. As expected, very thin films, for which the level of transmittance and band-gap values increase, can be obtained either by reducing the concentration of the inks or by increasing the rate of spinning. Moreover, the thermal treatment affects the phase formation and crystallinity of the film, as well as the electrical conductivity, which decreases at a higher temperature.

Controlling Morphology in Polycrystalline Films by Nucleation and Growth from Metastable Nanocrystals

Solution processing of polycrystalline compound semiconductor thin film using nanocrystals as a precursor is considered one of the most promising and economically viable routes for future large-area manufacturing. However, in polycrystalline compound semiconductor films such as Cu₂ZnSnS₄ (CZTS), grain size, and the respective grain boundaries play a key role in dictating the optoelectronic properties. Various strategies have been employed previously in tailoring the grain size and boundaries (such as ligand exchange) but most require postdeposition thermal annealing at high temperature in the presence of grain growth directing agents (selenium or sulfur vapor with/without Na, K, etc.) to enlarge the grains through sintering. Here, we show a different strategy of controlling grain size by tuning the kinetics of nucleation and the subsequent grain growth in CZTS nanocrystal thin films during a crystalline phase transition. We demonstrate that the activation energy for the phase transition can be varied by utilizing different shapes (spherical and nanorod) of nanocrystals with similar size, composition, and surface chemistry leading to different densities of nucleation sites and, thereby, different grain sizes in the films. Additionally, exchanging the native organic ligands for inorganic surface ligands changes the activation energy for the phase change and substantially changes the grain growth dynamics, while also compositionally modifying the resulting film. This combined approach of using nucleation and growth dynamics and surface chemistry enables us to tune the grain size of polycrystalline CZTS films and customize their electronic properties by compositional engineering.

Investigation into the Selenization Mechanisms of Wurtzite CZTS Nanorods

Here, we report the first detailed investigation into the selenization mechanism of thin films of wurtzite copper zinc tin sulfide (CZTS) nanorods (NRs), giving particular emphasis to the role of the long-chain organic ligands surrounding each NR. During selenization, the NRs undergo a selenium-mediated phase change from wurtzite to kesterite, concurrent with the replacement of sulfur with selenium in the lattice and in situ grain growth, along with the recrystallization of larger copper zinc tin selenide kesterite grains on top of the existing film. By utilizing a facile ligand removal technique, we demonstrate that the formation of a large-grain overlayer is achievable without the presence of ligands. In addition, we demonstrate an elegant ligand-exchange-based method for controlling the thickness of the fine-grain layer. This report emphasizes the key role played by ligands in determining the structural evolution of CZTS nanocrystal films during selenization, necessitating the identification of optimal ligand chemistries and processing conditions for desirable grain growth.

Fabrication of Cu2ZnSn(S,Se)4photovoltaic devices with 10% efficiency by optimizing the annealing temperature of precursor films

Fabrication of a champion solar cell with 10.04% efficiency by optimizing the annealing temperature of precursor films.

Copper-Zinc-Tin-Sulfide Thin Films via Annealing of Ultrasonic Spray Deposited Nanocrystal Coatings

Thin polycrystalline films of the solar absorber copper-zinc-tin-sulfide (CZTS) were formed by annealing coatings deposited on molybdenum-coated soda lime glass via ultrasonic spraying of aerosol droplets from colloidal CZTS nanocrystal dispersions. Production of uniform continuous nanocrystal coatings with ultrasonic spraying requires that the evaporation time is longer than the aerosol flight time from the spray nozzle to the substrate such that the aerosol droplets still have low enough viscosity to smooth the impact craters that form on the coating surface. In this work, evaporation was slowed by adding a high boiling point cosolvent, cyclohexanone, to toluene as the dispersing liquid. We analyzed, quantitatively, the effects of the solvent composition on the aerosol and coating drying dynamics using an aerosol evaporation model. Annealing coatings in sulfur vapor converts them into polycrystalline films with micrometer size grains, but the grains form continuous films only when Na is present during annealing to enhance grain growth. Continuous films are easier to form when the average nanocrystal size is 15 nm: using larger nanocrystals (e.g., 20 nm) sacrifices film continuity.

Compound Copper Chalcogenide Nanocrystals

This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), energy storage (lithium-ion batteries, hydrogen generation), emissive materials (plasmonics, LEDs, biolabelling), sensors (electrochemical, biochemical), biomedical devices (magnetic resonance imaging, X-ray computer tomography), and medical therapies (photochemothermal therapies, immunotherapy, radiotherapy, and drug delivery). The confluence of advances in the synthesis, assembly, and application of these NCs in the past decade has the potential to significantly impact society, both economically and environmentally.

Oxide-induced grain growth in CZTS nanoparticle coatings

Oxidizing the surface of ligand-free CZTS nanoparticles promotes the formation of uniform, large-grain thin films after sulfurization.