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

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

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

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

Solution-Processed Highly Efficient Cu₂ZnSnSe₄ Thin Film Solar Cells by Dissolution of Elemental Cu, Zn, Sn, and Se Powders

Solution deposition approaches play an important role in reducing the manufacturing cost of Cu₂ZnSnSe₄ (CZTSe) thin film solar cells. Here, we present a novel precursor-based solution approach to fabricate highly efficient CZTSe solar cells. In this approach, low-cost elemental Cu, Zn, Sn, and Se powders were simultaneously dissolved in the solution of thioglycolic acid and ethanolamine, forming a homogeneous CZTSe precursor solution to deposit CZTSe nanocrystal thin films. Based on high-quality CZTSe absorber layer, pure selenide CZTSe solar cell with a photoelectric conversion efficiency of 8.02% has been achieved without antireflection coating

Homogeneous Synthesis and Electroluminescence Device of Highly Luminescent CsPbBr₃ Perovskite Nanocrystals

Highly luminescent CsPbBr₃ perovskite nanocrystals (PNCs) are homogeneously synthesized by mixing toluene solutions of PbBr₂ and cesium oleate at room temperature in open air. We found that PbBr₂ can be easily dissolved in nonpolar toluene in the presence of tetraoctylammonium bromide, which allows us to homogeneously prepare CsPbBr₃ perovskite quantum dots and prevents the use of harmful polar organic solvents, such as <i>N</i>,<i>N</i>-dimethylformamide, dimethyl sulfoxide, and <i>N</i>-methyl-2-pyrrolidone. Additionally, this method can be extended to synthesize highly luminescent CH₃NH₃PbBr₃ perovskite quantum dots. An electroluminescence device with a maximal luminance of 110 cd/m² has been fabricated by using high-quality CsPbBr₃ PNCs as the emitting layer

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

Versatile and Low-Toxic Solution Approach to Binary, Ternary, and Quaternary Metal Sulfide Thin Films and Its Application in Cu₂ZnSn(S,Se)₄ Solar Cells

We developed a versatile and environmentally friendly solution approach for the fabrication of a variety of metal sulfide nanocrystal thin films. Metal oxides, metal hydroxides, metal chlorides, metal acetates, and metal acetylacetonates can be used as the starting materials and dissolved in thioglycolic acid and ethanolamine, forming many types of metal–organic precursor solutions. High quality CdS, SnS, CuInS₂, CuSbS₂, Cu₂ZnSnS₄, Cu­(In_0.7Ga_0.3)­S₂, and luminescent Ag-doped Zn_<i>x</i>Cd_1–<i>x</i>S nanocrystal thin films have been successfully prepared by spin-coating their corresponding metal precursor solutions. Cu₂ZnSn­(S,Se)₄ thin film solar cell with a power conversion efficiency of 6.83% has been realized by this versatile method