New Insight of Li-Doped Cu₂ZnSn(S,Se)₄ Thin Films: Li-Induced Na Diffusion from Soda Lime Glass by a Cation-Exchange Reaction

In our recent report (<i>ACS Appl. Mater. Interfaces</i> <b>2016</b>, <i>8</i>, 5308), Li⁺ ions had been successfully incorporated into the lattice of the selenized Cu₂ZnSn­(S,Se)₄ thin film on a quartz substrate by substituting equivalent Cu⁺ ions, and Li⁺ ions was also found to have the little effect on the crystal growth and defect passivation. To further improve the cell performance of Li-doped CZTSSe devices, we conducted the same experiments on the sodium-rich soda-lime glass (SLG) substrate in this study, instead of sodium-free quartz substrate. Surprisingly, only trace amounts of Li (Li/Cu molar ratio ∼1 × 10^–4) were detected in the final CZTSSe thin films; meanwhile, a large amount of sodium was present on the surface and at the grain boundaries of the selenized thin films. A Li/Na exchange mechanism is used to explain this phenomenon. Only on the sodium-free substrate can Li⁺ ions enter the CZTSSe host lattice, and doping Li⁺ ions on the SLG substrate are nearly identical to doping Na⁺ ions

Large-Scale Synthesis of Well-Dispersed Copper Nanowires in an Electric Pressure Cooker and Their Application in Transparent and Conductive Networks

We present a novel large-scale synthetic method for well-separated copper nanowires (CuNWs) in a commercial electric pressure cooker under mild reaction conditions. CuNWs (∼2.1 g) can be prepared in a batch with the cost of $4.20/g. Well-dispersed polyvinylpyrrolidone-capped CuNWs were obtained via a ligand-exchange method. The transparent and conductive CuNW networks with excellent electrical conductivity and high optical transmittance (30 Ω/□ at 86% transmittance, respectively) were fabricated by a spin-coating process

A Novel and Versatile Strategy to Prepare Metal–Organic Molecular Precursor Solutions and Its Application in Cu(In,Ga)(S,Se)₂ Solar Cells

A novel and versatile metal–organic molecular precursor-based solution approach and the fabrication of high efficiency Cu­(In,Ga)­(S,Se)₂ solar cells are presented. Many types of metal oxides, hydroxides, and acetylacetonates (acac), such as Cu₂O, ZnO, SnO, Sb₂O₃, MnO, PbO, In­(OH)₃, Cd­(OH)₂, Ga­(acac)₃, and so forth, can be easily dissolved in butyldithiocarbamic acid, forming thermally degradable metal–organic molecular precursor solutions. By developing a simple and green ethanol solution-processed route and tuning the chemical composition of the Cu­(In,Ga)­(S,Se)₂ thin film, as-fabricated solar cells exhibit an average power conversion efficiency up to 8.8%

Tuning the Band Gap of Cu₂ZnSn(S,Se)₄ Thin Films via Lithium Alloying

Alkali metal doping plays a crucial role in fabricating high-performance Cu­(In,Ga)­(S,Se)₂ and Cu₂ZnSn­(S,Se)₄ (CZTSSe) thin film solar cells. In this study, we report the first experimental observation and characterizations of the alloyed Li_<i>x</i>Cu_2–<i>x</i>ZnSn­(S,Se)₄ thin films. It is found that Cu⁺ ions in Cu₂ZnSn­(S,Se)₄ thin films can be substituted with Li⁺ ions, forming homogeneous Li_<i>x</i>Cu_2–<i>x</i>ZnSn­(S,Se)₄ (0 ≤ <i>x</i> ≤ 0.29) alloyed thin films. Consequently, the band gap, conduction band minimum, and valence band maximum of Li_<i>x</i>Cu_2–<i>x</i>ZnSn­(S,Se)₄ thin films are profoundly affected by Li/Cu ratios. The band alignment at the Li_<i>x</i>Cu_2–<i>x</i>ZnSn­(S,Se)₄/CdS interface can be tuned by changing the Li/Cu ratio. We found that the photovoltaic parameters of the Li_<i>x</i>Cu_2–<i>x</i>ZnSn­(S,Se)₄ solar cell devices are strongly influenced by the Li/Cu ratios. Besides, the lattice constant, carrier concentration, and crystal growth of Li_<i>x</i>Cu_2–<i>x</i>ZnSn­(S,Se)₄ thin films were studied in detail

Scaling up the Aqueous Synthesis of Visible Light Emitting Multinary AgInS₂/ZnS Core/Shell Quantum Dots

Approximately 3 g of water-soluble AgInS₂/ZnS core/shell quantum dots (AIS/ZnS QDs) with a maximum photoluminescence quantum yield of up to 39.1% was synthesized in an aqueous solution of gelatin and thioglycolic acid (TGA). The composition of the AIS QDs could be readily adjusted by controlling the molar ratio of the starting Ag/In precursors in the reaction solution, which leads to a tunable emission ranging from 535 to 607 nm. The as-prepared core/shell QDs exhibit excellent photostability and water/buffer stability. More importantly, these cadmium-free hydrophilic AIS/ZnS core/shell QDs are biocompatible and can be directly utilized in cancer cell imaging

Warm White Light Emitting Diodes with Gelatin-Coated AgInS₂/ZnS Core/Shell Quantum Dots

Cadmium-free and water-soluble AgInS₂/ZnS core/shell quantum dots (QDs) with a cost of 2.5 $/g are synthesized in an electric pressure cooker. The QD powders with different Ag/In ratios exhibit bright yellow, orange, and orange-red luminescence under UV light. Their absolute photoluminescence quantum yields (PLQYs) can reach as high as 50.5, 57, and 52%, respectively. Because gelatin is used as the capping agent, the concentrated QDs/gelatin solution can be directly utilized as phosphor for the fabrication of white light-emitting diodes (LEDs) by a simple drop-drying process without the need of resin package. Warm-white LEDs are obtained by combining orange-emitting QDs with blue InGaN chip. As-fabricated warm-white LED exhibits a luminous efficacy of 39.85 lm/W, a correlated color temperature (CCT) of 2634 K and a color rendering index (CRI) of 71 at a drive current of 20 mA. Furthermore, the electroluminescence (EL) stability of LED device and thermal stability of as-prepared QDs are evaluated

Facile and Low-Cost Sodium-Doping Method for High-Efficiency Cu₂ZnSnSe₄ Thin Film Solar Cells

We present a simple and low-cost sodium-doping method for Cu₂ZnSnSe₄ (CZTSe) thin film solar cells. In this method, a piece of soda-lime glass (SLG) is served as the sodium source and is placed on top of the CZTSe precursor thin film during selenization. It was observed that the grain growth and the hole-carrier concentration can be significantly improved by the diffusion of sodium from the top SLG. Through this approach, high-quality CZTSe absorber layer is obtained after the selenization, and the photoelectric conversion efficiencies (PCE) of 7.51% and 6.09% are achieved for CZTSe thin film solar cells deposited on a Mo-coated SLG substrate and a Mo-coated quartz substrate, respectively. The difference in PCE on SLG and quartz substrate revealed that Na diffusion from the bottom SLG substrate and the top SLG was most effective for the high-performance of CZTSe solar cell devices

Significantly Enhancing Grain Growth in Cu₂ZnSn(S,Se)₄ Absorber Layers by Insetting Sb₂S₃, CuSbS₂, and NaSb₅S₈ Thin Films

The Cu₂ZnSn­(S,Se)₄ (CZTSSe) has gained extensive attention in thin film solar cells due to their potential as a nontoxic, low-cost, and earth-abundant absorber material, and a rapid increase in power conversion efficiencies has been demonstrated in laboratory. Compared with the most successful hydrazine-based solution process, the nanocrystal-based ink method and non-hydrazine molecular precursor solution approach are more eco-friendly for fabricating high-efficiency CZTSSe solar cells. However, it is hard to obtain a complete large-grain CZTSSe absorber thin film which can facilitate the transport of photogenerated carriers while minimize grain boundary recombination. Here, we present a simple and effective strategy to significantly enhance grain growth of CZTSSe absorber layers by insetting Sb₂S₃, CuSbS₂, and NaSb₅S₈ thin films. The incorporation of Sb-based thin films can induce grain growth in the selenization process, and did not produce the impurity phase confirmed by XRD patterns and Raman spectra. It was found that the order of the crystal growth promotion ability is Sb₂S₃ > CuSbS₂ > NaSb₅S₈ under the same experimental conditions. The presented approach can be extended to other solution processes of fabricating CZTSSe solar cells to enhance their microstructural properties, which are critical for applications in CZTSSe absorbers with fine-grain layers

Fabrication of Cu₂ZnSn(S,Se)₄ Solar Cells via an Ethanol-Based Sol–Gel Route Using SnS₂ as Sn Source

Cu₂ZnSn­(S,Se)₄ semiconductor is a promising absorber layer material in thin film solar cells due to its own virtues. In this work, high quality Cu₂ZnSn­(S,Se)₄ thin films have been successfully fabricated by an ethanol-based sol–gel approach. Different from those conventional sol–gel approaches, SnS₂ was used as the tin source to replace the most commonly used SnCl₂ in order to avoid the possible chlorine contamination. In addition, sodium was found to improve the short-circuit current and fill factor rather than the open-circuit voltage due to the decrease of the thickness of small-grained layer. The selenized Cu₂ZnSn­(S,Se)₄ thin films showed large densely packed grains and smooth surface morphology, and a power conversion efficiency of 6.52% has been realized for Cu₂ZnSn­(S,Se)₄ thin film solar cell without antireflective coating