Defect Origin of Emission in CsCu2I3 and Pressure-Induced Anomalous Enhancement

Why do similar 1D-polyhedron-chain copper chloride semiconductors have 2-order-distinct luminescence quantum efficiencies?

The "green" copper halides with one-dimensional polyhedron chains are very interesting novel semiconductors. These weakly interacting parallel quantum wires (1D polyhedron chains) play key roles in their photophysical properties. Unlike Cs3Cu2I5, which has been much investigated, its homologous compounds Cs3Cu2Cl5 and CsCu2Cl3 remain less studied and their properties are controversial. Both of them are composed of specific 1D-polyhedron-chains. We report the synthesis and comparatively study the photophysical properties of the single crystals of Cs3Cu2Cl5 and CsCu2Cl3. They exhibit green and orange emissions, respectively. Surprisingly, their luminescence quantum efficiencies have a giant difference of over two orders of magnitude (96.7% vs 0.7%). The CsCu2Cl3 crystals exhibit much slower radiative transition and substantially faster nonradiative transition. The experiment in combination with the density functional theory calculation reveals that their 1D-polyhedron-chains have distinct bonding structures and degrees of distortion. This leads to different distributions of electron wave functions and different concentrations of carrier-trapping chlorine vacancies, which account for their highly contrasted quantum efficiencies. The CsCu2Cl3 and Cs3Cu2Cl5 crystals exhibit easy phase transition between each other driven by the changed temperature or ethanol erosion owing to their resembling skeleton structures of 1D polyhedral chain.

High-pressure effects on the electronic properties and photoluminescence of Ag-doped CsCu2I3

CsCu2I3 is a popular lead-free metal halide perovskite with good thermal and air stability. To facilitate its applications in optoelectronics, Ag doping and high pressure are employed in this work to improve the optoelectronic properties of CsCu2I3. Using first-principles calculations and experiments, the structural phase change of 10% Ag-doped CsCu2I3 is found to occur at about 4.0 GPa. This reveals the regulation of band structures by hydrostatic pressure. In addition, the high pressure not only increases the emission energy of photoluminescence of 10% Ag-doped CsCu2I3 by more than 0.2 eV, but also increases the emission intensity by multiple times. Finally, the origin of luminescence in 10% Ag-doped CsCu2I3 is attributed to the I vacancies. This work provides insight into the structure and optoelectronic properties of 10% Ag-doped CsCu2I3, and offers significant guidance for the design and manufacturing of future luminescence devices.

A zero-dimensional hybrid copper(I) bromide single crystal with highly efficient green emission

Lead-free metal halides are considered as alternatives to lead-based perovskites due to their low toxicity, rich structural diversity, and high luminescence properties. We report millimeter-sized single crystals of a new zero-dimensional (0D) copper(I)-based hybrid material, (AEP)2Cu2Br6·2Br·2H2O (AEP = C6H18N33+), which exhibits bright broadband green photoluminescence (PL) at 510 nm with a Stokes shift of 220 nm and a PL lifetime of 121.1 μs. Density functional theory (DFT) calculations and experimental studies reveal that the green light can be attributed to self-trapping exciton (STE) emission. It is worth mentioning that this crystal has a high photoluminescence quantum yield (PLQY) of 90.5%, which is higher than most copper halides.

Recent Advances and Opportunities of Eco-Friendly Ternary Copper Halides: A New Superstar in Optoelectronic Applications

Recently, the newly-emerging lead-free metal-halide materials with less toxicity and superior optoelectronic properties have received wide attention as the safer and potentially more robust alternatives to lead-based perovskite counterparts. Among them, ternary copper halides (TCHs) have become a vital group due to their unique features, including abundant structural diversity, ease of synthesis, unprecedented optoelectronic properties, high abundance, and low cost. Although the recent efforts in this field have made certain progresses, some scientific and technological issues still remain unresolved. Herein, a comprehensive and up-to-date overview of recent progress on the fundamental characteristics of TCH materials and their versatile applications is presented, which contains topics such as: i) crystal and electronic structure features and synthesis strategies; ii) mechanisms of self-trapped excitons, luminescence regulation, and environmental stability; and iii) their burgeoning optoelectronic devices of phosphor-converted white light-emitting diodes (WLEDs), electroluminescent LEDs, anti-counterfeiting, X-ray scintillators, photodetectors, sensors, and memristors. Finally, the current challenges together with future perspectives on the development of TCH materials and applications are also critically described, which is considered to be critical for accelerating the commercialization of these rapidly evolving technologies.

Theoretical and Experimental Advances in High-Pressure Behaviors of Nanoparticles

Using compressive mechanical forces, such as pressure, to induce crystallographic phase transitions and mesostructural changes while modulating material properties in nanoparticles (NPs) is a unique way to discover new phase behaviors, create novel nanostructures, and study emerging properties that are difficult to achieve under conventional conditions. In recent decades, NPs of a plethora of chemical compositions, sizes, shapes, surface ligands, and self-assembled mesostructures have been studied under pressure by in-situ scattering and/or spectroscopy techniques. As a result, the fundamental knowledge of pressure-structure-property relationships has been significantly improved, leading to a better understanding of the design guidelines for nanomaterial synthesis. In the present review, we discuss experimental progress in NP high-pressure research conducted primarily over roughly the past four years on semiconductor NPs, metal and metal oxide NPs, and perovskite NPs. We focus on the pressure-induced behaviors of NPs at both the atomic- and mesoscales, inorganic NP property changes upon compression, and the structural and property transitions of perovskite NPs under pressure. We further discuss in depth progress on molecular modeling, including simulations of ligand behavior, phase-change chalcogenides, layered transition metal dichalcogenides, boron nitride, and inorganic and hybrid organic-inorganic perovskites NPs. These models now provide both mechanistic explanations of experimental observations and predictive guidelines for future experimental design. We conclude with a summary and our insights on future directions for exploration of nanomaterial phase transition, coupling, growth, and nanoelectronic and photonic properties.

Organic-Inorganic Hybrid Cuprous-Based Metal Halides for Warm White Light-Emitting Diodes

Single-component emitters with stable and bright warm white-light emission are highly desirable for high-efficacy warm white light-emitting diodes (warm-WLEDs), however, materials with such luminescence properties are extremely rare. Low-dimensional lead (Pb) halide perovskites can achieve warm white photoluminescence (PL), yet they suffer from low stability and PL quantum yield (PLQY). While Pb-free air-stable perovskites such as Cs2 AgInCl6 emit desirable warm white light, sophisticated doping strategies are typically required to increase their PL intensity. Moreover, the use of rare metal-bearing compounds along with the typically required vacuum-based thin-film processing may greatly increase their production cost. Herein, organic-inorganic hybrid cuprous (Cu+ )-based metal halide MA2 CuCl3 (MA = CH3 NH3 + ) that meets the requirements of i) nontoxicity, ii) high PLQY, and iii) dopant-free is presented. Both single crystals and thin films of MA2 CuCl3 can be facilely prepared by a low-cost solution method, which demonstrate bright warm white-light emission with intrinsically high PLQYs of 90-97%. Prototype electroluminescence devices and down-conversion LEDs are fabricated with MA2 CuCl3 thin films and single crystals, respectively, which show bright luminescence with decent efficiencies and operational stability. These findings suggest that MA2 CuCl3 has a great potential for the single-component indoor lighting and display applications.

Photoluminescence and Raman Spectra of One-Dimensional Lead-free Perovskite CsCu2I3 Single-Crystal Wires

Lead-free highly luminescent CsCu₂I₃ perovskite has attracted much attention recently, but agreements on basic optical properties have remained unsettled. By correlating X-ray diffraction with the photoluminescence (PL) of CsCu₂I₃ single-crystal wires, we first show that blue PL at 420 nm originates from CuI. We then exclude defect states as a source for the broadband emission centered at 570 nm from the lack of defect absorption, PL under sub-bandgap photoexcitation, observations of a linear dependence of PL intensity on excitation laser power, and a strong spectral blueshift under mild hydrostatic pressure. Finally, using a model of the self-trapped exciton and the associated coordinate configuration diagram, we explain pressure evolutions of PL energy, intensity, and lifetime. Single-crystal wires also enable us to obtain polarization-dependent Raman spectra down to 10 cm^-1 and confirm their respective ambient crystal structure of orthorhombic Cmcm and phase transition to Pbnm at ∼5 GPa.