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. Lowdimensional 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 Cs2AgInCl6 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 MA2CuCl3 (MA = CH3NH3+) that meets the requirements of i) nontoxicity, ii) high PLQY, and iii) dopant‐free is presented. Both single crystals and thin films of MA2CuCl3 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 MA2CuCl3 thin films and single crystals, respectively, which show bright luminescence with decent efficiencies and operational stability. These findings suggest that MA2CuCl3 has a great potential for the single‐component indoor lighting and display applications.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/advs.202203596

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

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. This article is protected by copyright. All rights reserved.

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

Hybrid metal halides (HMHs) are a class of materials that combine extraordinary photophysical properties and excellent processability. Their chemical variability allows for the solid-liquid transition toward melt-processable HMHs. Herein, we report the design and synthesis of zero-dimensional HMHs [M(DMSO)6][SbCl6], where the isolated octahedra of [M(DMSO)6]3+ and [SbCl6]3- are alternatively aligned in the crystal structure. The luminescent center of [SbCl6]3- enables the photogeneration of self-trapped excitons, resulting in broadband photoluminescence with a large Stokes shift and a nearly 100% quantum yield. Meanwhile, the release of DMSO ligands from [M(DMSO)6]3+ is controlled by the M-O coordination and thus a low melting point of ∼90 °C is achieved for HMHs. Interestingly, the glass phase is obtained by melt quenching, with a sharp change in photoluminescence colors compared to the crystal phase of melt-processable HMHs. The robust crystal-liquid-glass transition opens a new avenue to tailoring structural disorder and optoelectronic performance in organic-inorganic materials.

Crystal-Liquid-Glass Transition and Near-Unity Photoluminescence Quantum Yield in Low Melting Point Hybrid Metal Halides.

A phosphor emitting both white light and broad near-infrared (NIR) radiation can simultaneously provide visual inspection and early signs of rotting of food products. The broad NIR emission is absorbed by the vibrational overtones of water molecules present in food items, providing the non-invasive image contrast to assess the food freshness. Here we design a phosphor, namely, Cr3+-Bi3+-codoped Cs2Ag0.6Na0.4InCl6, that simultaneously emit warm white light and broad NIR (1000 nm) radiation with quantum yield 27%. This dual emitter is designed by combining the features of s2-electron (Bi3+) and d3-electron (Cr3+) doping in a weak crystal field of the halide perovskite host. 6s2 to 6s16p1 excitation of Bi3+, using a commercial 370 nm ultraviolet light-emitting-diodes (UV-LED), yields both the emissions. A fraction of the excited Bi3+ dopants emit the warm white light, and the other fraction transfers its energy non-radiatively to Cr3+. Then the Cr3+ de-excites emitting broad NIR emission. Temperature dependent (6.4 - 300 K) photoluminescence in combination with Tanabe-Sugano diagram show that the Cr3+ experiences a weak crystal field (Dq/B = 2.2), yielding the 4T2 to 4A2 NIR emission. As a proof of concept, we fabricated a panel containing 122 phosphor-converted LEDs, demonstrating its capability to inspect food products.

Broad Dual Emission by Codoping Cr3+ (d-d) and Bi3+ (s-p) in Cs2Ag0.6Na0.4InCl6 Double Perovskite.

Designing and synthesizing cuprous halide phosphors unifying efficient low-energy emission and a broad excitation band is still a great challenge. Herein, by rational component design, three novel Cu(i)-based metal halides, DPCu4X6 [DP = (C6H10N2)4(H2PO2)6; X = Cl, Br, I], were synthesized by reacting p-phenylenediamine with cuprous halide (CuX), and they show similar structures, consisting of isolated [Cu4X6]2− units separated by organic layers. Photophysical studies uncover that the highly localized excitons and rigid environment give rise to highly efficient yellow-orange photoluminescence in all compounds with the excitation band spanning from 240 to 450 nm. The bright PL in DPCu4X6 (X = Cl, Br) originates from self-trapped excitons due to the strong electron–phonon coupling. Intriguingly, DPCu4I6 features a dual-band emissive characteristic, attributed to the synergistic effect of halide/metal-to-ligand charge-transfer (X/MLCT) and triplet cluster-centered (3CC) excited states. Benefiting from the broadband excitation, a high-performance white-light emitting diode (WLED) with a high color rendering index of 85.1 was achieved using single-component DPCu4I6 phosphor. This work not only unveils the role of halogens in the photophysical processes of cuprous halides, but also provides new design principles for high-performance single-component WLEDs.

/pdf/photophysical-studies-for-cu-i-based-halides-broad-2nx1cjrp.pdf

Photophysical studies for Cu(i)-based halides: broad excitation bands and highly efficient single-component warm white-light-emitting diodes

Double perovskites (DPs) are one of the most promising candidates for developing white light-emitting diodes (WLEDs) owing to their intrinsic broadband emission from self-trapped excitons (STEs). Translation of three-dimensional (3D) DPs to one-dimensional (1D) analogues, which could break the octahedral tolerance factor limit, is so far remaining unexplored. Herein, by employing a fluorinated organic cation, we report a series of highly luminescent 1D DP-inspired materials, (DFPD)2MIInBr6 (DFPD = 4,4-difluoropiperidinium, MI = K+ and Rb+). Highly efficient warm-white photoluminescence quantum yield of 92 % is achieved by doping 0.3% Sb3+ in (DFPD)2KInBr6. Furthermore, single-component warm-WLEDs fabricated with (DFPD)2KInBr6:Sb yield a luminance of 300 cd/m2, which is one of the best-performing lead-free metal-halides WLEDs reported so far. Our study expands the scope of In-based metal-halides from 3D to 1D, which exhibit superior optical performances and broad application prospects.

Highly Luminescent One-Dimensional Organic-Inorganic Hybrid Double-Perovskite-Inspired Materials for Single-Component Warm White-Light-Emitting Diodes.

Low-dimensional lead (Pb)-free perovskite derivatives have recently been regarded as promising candidates for various optoelectronic applications because of their merits of easy preparation, nontoxicity, and broadband self-trapped exciton emission. However, similar to those Pb-based metal halide perovskites, most of these materials undergo an irreversible structural damage upon exposure to aqueous medium, which imposes restrictions on cyclic utilization in both academic and industrial fields. Herein, we report pure Mn-based metal halide, Cs3MnI5, which readily recovers from aqueous solution under mild heat stress and maintains excellent optical properties with high photoluminescence quantum yields of 73%. Meanwhile, this recyclable halide material demonstrates a sensitive X-ray response with a low detection limit of 0.4 μGy/s, high light yield of 33600 ph/MeV, and spatial resolution of 7.45 lp/mm, being suitable for X-ray imaging. 

Highly Reversible Cesium Manganese Iodine for Sensitive Water Detection and X-ray Imaging

Self-referencing optical thermometry based on the fluorescence intensity ratio (FIR) have drawn extensive attention as a result of their high sensitivity and non-invasively fast response to temperature. However, it is a great challenge for luminescent materials to achieve simultaneously high absolute and relative temperature sensitivity based on the FIR technique. Herein, we developed a novel optical thermometer by designing hybrid lead-free metal halide (TTPhP)2MnCl4:Sb3+ (TTPhP+ = tetraphenylphosphonium cation) single crystals with multimodal photoluminescence (PL). The large TTPhP+ organic chain resulted in isolated [MnCl4]2- and [SbCl5]2- in the single crystal, which leads to a negligible energy trasfer process within them. Therefore, the two PL bands (band 1 from [MnCl4]2-) with a peak at 518 nm and band 2 (from [SbCl5]2) with a peak at 640 nm exhibit different thermal-quenching effects, which resulted in excellent temperature sensitivity, with the maximum absolute and relative sensitivities reaching 0.236 K-1 and 3.77% K-1 in a temperature range from 300 to 400 K. Both the absolute and relative sensitivities are among the highest values for luminescence thermometry.

Ultrasensitive Optical Thermometry via Inhibiting the Energy Transfer in Zero-Dimensional Lead-Free Metal Halide Single Crystals.

Strong carrier‐phonon coupling‐induced self‐trapped excitons (STEs) limit the application of 2D double‐perovskites in optoelectronic devices because the separation and collection of the highly localized STEs are challenging. In this study, antimony (Sb) is incorporated into (BA)4AgBiBr8 (BA = CH3(CH2)3NH3+) to form (BA)4AgSbxBi1‐xBr8 (0 ≤ x ≤ 1) trivalent‐metal alloyed 2D double‐perovskites. It is found that introducing Sb into the (BA)4AgBiBr8 2D double‐perovskite will decrease the carrier‐phonon coupling strength and change STEs from “dark” states to “bright” states at room temperature. Combining detailed spectroscopic analysis with density functional theory calculation, it is concluded that STEs localized at [SbBr6] octahedron in (BA)4AgSbxBi1‐xBr8 give the broadband bright emission. The “bright” STEs with moderate carrier‐phonon coupling opens the possibility to efficiently separate and collect charge‐carriers in (BA)4AgSbBr8 2D double‐perovskites. Photodetectors based on (BA)4AgSbBr8 crystals are further fabricated. The photodetectors exhibit high responsivity up to 1368 A W−1 and an external quantum efficiency of 4.7 × 105. These figures are over three times higher than that in (BA)4AgBiBr8‐based photodetectors. This study provides a unique strategy for developing high‐performance optoelectronic devices based on STEs.

Tuning Self‐Trapped Exciton States via Trivalent‐Metal Alloying in Lead‐Free 2D Double‐Perovskites

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

Highly Luminescent One-Dimensional Organic-Inorganic Hybrid Double-Perovskite-Inspired Materials for Single-Component Warm White-Light-Emitting Diodes.

Highly Reversible Cesium Manganese Iodine for Sensitive Water Detection and X-ray Imaging

Ultrasensitive Optical Thermometry via Inhibiting the Energy Transfer in Zero-Dimensional Lead-Free Metal Halide Single Crystals.

Tuning Self‐Trapped Exciton States via Trivalent‐Metal Alloying in Lead‐Free 2D Double‐Perovskites