Tunable and Efficient Photoluminescence of Lanthanide-Doped Cs2NaScCl6 Double Perovskite Single Crystals toward Multifunctional Light-Emitting Diode Applications

White-Light Emission from Halide Perovskites Based on a Single Emissive Layer

In contemporary society, lighting has become indispensable in both daily life and industrial activities. Researchers are actively developing new materials and technologies to meet global goals for clean, sustainable energy. Among them, halide perovskite materials play a vital role in light-emitting diodes (LEDs) due to their high carrier mobility, low exciton binding energy, and tunable emission wavelength. White perovskite light-emitting diodes (WPeLEDs) based on a single emission layer (SEL) feature simpler production processes and clearer luminescence mechanisms compared with other layered and series devices, attracting extensive research interest and showing great potential in practical applications. This Account systematically provides an overview of the recent advances in SEL-WPeLEDs. The concepts of perovskite materials and the luminescent principles of PeLEDs are first elaborated, and the typical approaches for perovskite film deposition are then summarized. Subsequently, the implementation strategies toward high-performance SEL-WPeLEDs of electric driving, including Perovskite with Self-trapped excitons, perovskite-organic molecule coupling, and metal ion doping, are carefully discussed. Finally, challenges and perspectives for the further development of SEL-PeLEDs are proposed.

Novel rare-earth doped Cs2NaGdCl6:Sb3+ double perovskite nanocrystals toward good ability of color tuning with high quantum efficiency

All-inorganic halide double perovskites have attracted extensive attention as new luminescent materials owing to their high luminous efficiency and wide emission spectrum. In this work, blue-red bicolor Cs2NaGdCl6 (CNGC) double perovskite nanocrystals doped with Sb3+/Ho3+ were prepared using a hydrothermal method. The incorporation of Sb3+ significantly enhanced the intrinsic "self-trapped excitons (STEs)" with a wide-band bright blue emission centered at 470 nm. On this basis, different concentrations of Ho3+ were introduced to achieve color tunable emission from blue to red region, with extremely high quantum efficiencies of 96.53% and 92.17%, respectively. By the use of diverse methods, including spectroscopic analysis and fluorescence lifetime, the mechanism of efficient luminescence related to the different dopant concentrations was comprehensively explored. Results show that Sb3+ not only can enhance the intrinsic STEs luminescence of the CNGC but also has an efficient activation effect on the rare earth ion Ho3+, because it plays the role as a bridge for the two energy transfer channels between the luminescence centers, and the details of the mechanism and energy transfer efficiency were investigated. Thermal stabilities of the double perovskites were studied over the temperature range of 303-473 K and good performances were demonstrated from the fact that the PL intensities of CNGC:1%Sb3+ and CNGC:1%Sb3+/10%Ho3+ at 423 K were 75.1% and 70.1% compared to those at 303 K, respectively. With the CNGC:1%Sb3+ and CNGC:1%Sb3+/10Ho3+ used, the UV light-emitting diode activated devices were fabricated and characterized. The bright blue and red emissions suggest the materials' prominent potential in the application of plant lighting. Finally, to achieve white-light emission in the sole CNGC matrix, a strategy of tri-doping Tb3+ was proposed based on the principle of the three primary colors. The device fabrication with CNGC:1%Sb3+/10%Ho3+/2.5%Tb3+ sample and the characterization processes prove its high feasibility and then provide valuable insights in the design of Sb ion-doped lead-free double perovskites toward easily tunable color in a wide range which can be applied in various fields.

Exploring the structural and photophysical properties of tri-cation mixed halide double perovskites (Cs2AgIn0.85-XCeXBi0.15Cl6) for high-performance phosphor-based WLEDs

Owing to their superior optoelectronic properties, lead-free halide double perovskites (HDPs) have been extensively studied for a wide range of optoelectronic applications, especially for fabricating white light-emitting diodes (WLEDs). Considering white light emission, the HDP structure's dual octahedral configuration facilitates greater lattice distortion, thereby fostering strong electron-phonon coupling-derived self-trapped exciton (STE) emission upon photoexcitation. Herein, we propose facile fabrication of a highly feasible phosphor-converted white light LED and an intensive analysis of the structural, compositional and photophysical properties of a tri-cation mixed halide double perovskite. We chose Cs2AgIn0.85Bi0.15Cl6 as a potential candidate for electroluminescent-based white light LED devices as its composition exhibits high stability, direct-allowed transition, and a notable photoluminescence quantum yield. However, we incorporated a lanthanide ion (Ce3+) into this cubic HDP structure via tri-cation mixing at the B'' site (Cs2AgIn0.85-XCeXBi0.15Cl6) to internally disturb structural periodicity and further enhance STE emission. Initially, powder XRD revealed the lattice expansion induced by Ce3+ incorporation, while XPS and TEM verified the substitution of Ce3+ at the In3+ site. Meanwhile, compositional and optical studies established the role of Ce3+ in retaining the direct allowed transition by effectively replacing the In3+ site. Urbach energy (EU), a measure of energetic disorderness at band edges, was found to be significantly reduced, showing a value of 135 meV for the Ce-5% sample. Most significantly, PL emission studies revealed an appreciable enhancement in the PL intensity with a prolonged STE lifetime of 670 ns for Cs2AgIn0.80Ce0.05Bi0.15Cl6, indicating improved radiative recombination. Besides, excitation-dependent Pl and PLE studies revealed that the emission solely came from the STE states. Elaboratively, vibrational studies elucidated that the Ce-5% sample exhibited a restabilized elpasolite structure and enhanced lattice phonons, which ultimately helped in boosting STE emission, as proven by the Huang-Rhys factor (S = 13). Finally, an efficient and durable phosphor-converted WLED was fabricated, and its performance was assessed, revealing CIE coordinates of (0.35,0.32), a CCT of 4368 K, and an extremely high CRI (Ra) of 92. Thus, our work provides an exclusive strategy to enhance the STE emission for potential application in electroluminescent-based WLED devices.

Opportunities and challenges of lead-free metal halide perovskites for luminescence

Metal halide perovskites (MHPs) have been developed rapidly for application in light-emitting diodes (LEDs), lasers, solar cells, photodetectors and other fields in recent years due to their excellent photoelectronic properties, and they have attracted the attention of many researchers. Perovskite LEDs (PeLEDs) show great promise for next-generation lighting and display technologies, and the external quantum efficiency (EQE) values of polycrystalline thin-film PeLEDs exceed 20%, which is undoubtedly a big breakthrough in lighting and display fields. However, the toxicity and instabilities of lead-based MHPs remain major obstacles limiting their further commercial applications. The exploration and development of lead-free MHPs (LFMHPs) are regarded as the most facile strategies to solve these problems. Compared with lead-based perovskites, LFMHPs exhibit better stabilities and broadband emission. With continuous development of LFMHPs, their photoluminescence quantum yields (PLQYs) have reached 99%, facilitating their use as ideal emitters. In this review, the structures and features of LFMHPs are analyzed, and the preparation methods of LFMHPs with various structures and configurations are discussed. Then, the mechanisms and strategies for improving the emission performance of white LEDs based on LFMHPs are demonstrated. Finally, their challenges in commercial production and perspectives are prospected.

Full-color, time-valve controllable and Janus-type long-persistent luminescence from all-inorganic halide perovskites

Long persistent luminescence (LPL) has gained considerable attention for the applications in decoration, emergency signage, information encryption and biomedicine. However, recently developed LPL materials - encompassing inorganics, organics and inorganic-organic hybrids - often display monochromatic afterglow with limited functionality. Furthermore, triplet exciton-based phosphors are prone to thermal quenching, significantly restricting their high emission efficiency. Here, we show a straightforward wet-chemistry approach for fabricating multimode LPL materials by introducing both anion (Br-) and cation (Sn2+) doping into hexagonal CsCdCl3 all-inorganic perovskites. This process involves establishing new trapping centers from [CdCl6-nBrn]4- and/or [Sn2-nCdnCl9]5- linker units, disrupting the local symmetry in the host framework. These halide perovskites demonstrate afterglow duration time ( > 2,000 s), nearly full-color coverage, high photoluminescence quantum yield ( ~ 84.47%), and the anti-thermal quenching temperature up to 377 K. Particularly, CsCdCl3:x%Br display temperature-dependent LPL and time-valve controllable time-dependent luminescence, while CsCdCl3:x%Sn exhibit forward and reverse excitation-dependent Janus-type luminescence. Combining both experimental and computational studies, this finding not only introduces a local-symmetry breaking strategy for simultaneously enhancing afterglow lifetime and efficiency, but also provides new insights into the multimode LPL materials with dynamic tunability for applications in luminescence, photonics, high-security anti-counterfeiting and information storage.

Multifunctional Phosphor with High-Efficient Near-Infrared Emission Based on Antimony-Zinc Halides

Metal halide-based broadband near-infrared (NIR) luminescent materials face problems such as complicated preparation, high cost, low photoluminescence quantum yield, and high excitation energy. Here, incorporating Sb3+ and Br- into (C20H20P)2ZnCl4 crystals allowed for the achievement of efficient broadband near-infrared emission under 400 nm excitation while maintaining satisfactory environmental and thermal stability. The compounds exhibit a broad range of emission bands from 550 to 1050 nm, with a photoluminescence quantum yield of 93.57%. This is a groundbreaking achievement for organic-inorganic hybrid metal halide NIR luminescent materials. The near-infrared emission is suggested to originate from [SbX5]2-, as supported by the femtosecond transient absorption spectra and density-functional theory calculations. This phosphor-based NIR LEDs successfully demonstrate potential applications in night vision, medical imaging, information encryption, and anticounterfeiting.

Cs2MnCl4:Eu2+: A Near-Violet Light-Triggered Single-Component White Light Emitter

Single-component white-light luminescent materials are considered an economical and facile choice for phosphor-converted white light-emitting diodes (pc-WLEDs). Here, a new single-component white-light-emitting material Cs2MnCl4:Eu2+ based on the combination of a lead-free halide structure and a rare-earth ion is first reported. Benefiting from the smart dilution-sensitization design strategy, white light composed of dual broad emission originating from Eu2+ (blue light, 444 nm, 4f65d1 → 4f7) and Mn2+ (yellow light, 566 nm, 4T1g → 6A1g) was successfully realized under near-ultraviolet light (404 nm) radiation with a high photoluminescence quantum yield of 66%. Based on the single-source Cs2MnCl4:Eu2+ phosphor, a pc-WLEDs device with "eye-friendly" white light production was successfully fabricated. The pc-WLEDs exhibit suitable color coordinates of (0.3294, 0.2746) and a high color rendering index of 82.3, demonstrating the potential in the future health-conscious illumination application by reducing the risk of eye strain and high-energy blue-light damage. This work achieves a new single-component white-light-emitting Mn-based halide phosphor and provides a new path for the design of single-component white light sources in Mn-based halides.

Near-Infrared Light Emitting Metal Halides: Materials, Mechanisms, and Applications

Near-Infrared (NIR) light emitting metal halides are emerging as a new generation of optical materials owing to their appealing features, which include low-cost synthesis, solution processability, and adjustable optical properties. NIR-emitting perovskite-based light-emitting diodes (LEDs) have reached an external quantum efficiency (EQE) of over 20% and a device stability of over 10,000 h. Such results have sparked an interest in exploring new NIR metal halide emitters. In this review, several different types of NIR-emitting metal halides, including lead/tin bromide/iodide perovskites, lanthanide ions doped/based metal halides, double perovskites, low dimensional hybrid and Bi3+/Sb3+/Cr3+ doped metal halides, are summarized, and their recent advancement is assessed. The characteristics and mechanisms of narrow-band or broadband NIR luminescence in all these materials are discussed in detail. Also, the various applications of NIR-emitting metal halides are highlighted and an outlook for the field is provided.

Cs2NaGdCl6:Tb3+─A Highly Luminescent Rare-Earth Double Perovskite Scintillator for Low-Dose X-ray Detection and Imaging

Rare-earth-based double perovskite (DP) X-ray scintillators have gained significant importance with low detection limits in medical imaging and radiation detection owing to their high light yield (LY) and remarkable spatial resolution. Herein, we report the synthesis of 3D double perovskite (DP) crystals, namely, Cs2NaGdCl6 and Tb3+-Cs2NaGdCl6 using hydrothermal reaction. Cs2NaGdCl6 DP single crystals exhibited a blue self-trapped exciton (STE) emission at 470 nm under ultraviolet (265 nm) excitation with a photoluminescence quantum yield (PLQY) of 8.4%. Introducing Tb3+ ions into Cs2NaGdCl6 has resulted in quenching of STE emission and enhancing green emission at 549 nm attributed to the 5D4 → 7F5 transition of Tb3+, suggesting efficient energy transfer (ET) from STE to Tb3+. This ET process is evidenced by the appearance of Tb3+ bands in the excitation spectra of the host, the shortening of the STE lifetimes in the presence of Tb3+ ions, and the enhancement of PLQY (72.6%). Furthermore, Cs2NaGdCl6:5%Tb3+ films of various thicknesses (0.1-0.6 mm) were synthesized and their X-ray scintillating performance has been examined. The Cs2NaGdCl6:5%Tb3+ film with 0.4 mm thickness has exhibited an excellent linear response to the X-ray dose rate with a low detection limit of 41.32 nGyair s-1, an LY of 39,100 photons MeV-1, and excellent radiation stability. Benefiting from the strong X-ray excited luminescence (XEL) of Cs2NaGdCl6:5%Tb3+, we developed a Cs2NaGdCl6:5%Tb3+ X-ray scintillator screen with a least thickness (0.1 mm), exhibiting remarkable imaging ability with a spatial resolution of 10.75 lp mm-1. These results suggest that Cs2NaGdCl6:Tb3+ can be a potential candidate for low-dose and X-ray imaging applications.

Double Perovskite Single Crystals with High Laser Irradiation Stability for Solid-State Laser Lighting and Anti-counterfeiting

Laser lighting devices, comprising an ultraviolet (UV) laser chip and a phosphor material, have emerged as a highly efficient approach for generating high-brightness light sources. However, the high power density of laser excitation may exacerbate thermal quenching in conventional polycrystalline or amorphous phosphors, leading to luminous saturation and the eventual failure of the device. Here, for the first time, we raise a single-crystal (SCs) material for laser lighting considering the absence of grain boundaries that scatter electrons and phonons, achieving high thermal conductivity (0.81 W m-1 K-1) and heat-resistance (575 °C). The SCs products exhibit a high photoluminescence quantum yield (89%) as well as excellent stability toward high-power lasers (>12.41 kW/cm2), superior to all previously reported amorphous or polycrystalline matrices. Finally, the laser lighting device was fabricated by assembling the SC with a UV laser chip (50 mW), and the device can maintain its performance even after continuous operation for 4 h. Double perovskite single crystals doped with Yb3+/Er3+ demonstrated multimodal luminescence with the irradiation of 355 and 980 nm lasers, respectively. This characteristic holds significant promise for applications in spectrally tunable laser lighting and multimodal anticounterfeiting.

The luminescence modulation of rare earth-doped/containing lead-free double perovskites toward multifunctional applications: a review

Lead-free double perovskites (DPs) with superior environmental stability and high defect tolerance have attracted considerable attention and exhibit great promise in photodetectors, solar cells, lighting devices, etc. However, achieving optical modulation and high photoluminescence quantum yield using this kind of material remains a challenge. Rare earth ions feature abundant energy levels and outstanding photophysical properties. Incorporating rare earth ions into lead-free DPs is an effective strategy to improve their optical performances, which have great effects on night-vision and light emitting diodes. Consequently, in this mini-review, we summarize the synthesis methods, optical properties, issues, and multifunctional applications of lead-free DPs described in recent years. The performances of DPs can be modulated via rare earth doping, which involves the extension of luminescence range, the improvement of PLQY, the realization of multi-mode excitation, and the regulation of luminescence color. We hope that this review will provide some insights into luminescence modulation and applications of lead-free DPs.

Multiwavelength Excitation in Ho3+-Doped All-Inorganic Double Perovskites Achieved by Codoping Mn2+ for Warm-White LEDs and Plant Growth

Doping lanthanide ions is an efficient method to modify the optical properties of lead-free double-perovskite halides. However, most lanthanide-doped double perovskites show a low luminescence efficiency and require a high excitation energy. Here, we have successfully prepared a series of Ho3+-doped Cs2NaBiCl6 microcrystals through a simple hydrothermal method and obtained strong characteristic emissions of Ho3+ at 492 and 657 nm under low-energy excitation (449 nm). After codoping Mn2+, apart from the characteristic emissions from Ho3+ under 450 nm wavelength excitation, the orangish-red luminescence consisting of the emission band centered at 591 nm from Mn2+ and a sharp emission peak at 657 nm from Ho3+ is obtained under 355 nm UV light excitation. Photoluminescence (PL) emission and excitation spectra, along with the PL decay curves, confirm the existence of an energy-transfer channel from Cs2NaBiCl6 to Mn2+ and then from Mn2+ to Ho3+. The enhanced absorption efficiency (10.5 → 70.7%) suggests that the codoping of Mn2+ overcomes the low absorption efficiency caused by f-f forbidden transitions of Ho3+. Finally, the diverse luminescent performance within the Cs2NaBiCl6:Ho3+, Mn2+ phosphor is realized by altering the excitation wavelength, thereby enabling its application in warm-white-light-emitting diodes and plant growth in this work.

Tunable dual-emission of Sb3+, Ho3+Co-doped Cs2NaScCl6single crystals for light-emitting diodes

Lead-free halide double perovskites are considered as one of the most promising materials in optoelectronic devices, such as solar cells, photodetectors, and light-emitting diodes (LEDs), due to their environmental friendliness and chemical stability. However, the extremely low photoluminescence quantum yield (PLQY) of self-trapped excitons (STEs) emission from lead-free halide double perovskites impedes their applications. Herein, Sb3+ions were doped into rare-earth-based double perovskite Cs2NaScCl6single crystals (SCs), resulting in a large enhancement of PLQY from 12.57% to 87.37%. Moreover, by co-doping Sb3+and Ho3+into Cs2NaScCl6SCs, the emission color can be tuned from blue to red, due to an efficient energy transfer from STEs to Ho3+ions. Finally, the synthesized sample was used in multicolor LED, which exhibited excellent stability and optical properties. This work not only provides a new strategy for improving the optical properties of Cs2NaScCl6SCs, but also suggests its potential application in multicolor LEDs.

Multimode Luminescence Tailoring and Improvement of Cs2 NaHoCl6 Cryolite Crystals via Sb3+ /Yb3+ Alloying for Versatile Photoelectric Applications

Lead-free halide double perovskites are currently gaining significant attention owing to their exceptional environmental friendliness, structural adjustability as well as self-trapped exciton emission. However, stable and efficient double perovskite with multimode luminescence and tunable spectra are still urgently needed for multifunctional photoelectric application. Herein, holmium based cryolite materials (Cs2 NaHoCl6 ) with anti-thermal quenching and multimode photoluminescence were successfully synthesized. By the further alloying of Sb3+ (s-p transitions) and Yb3+ (f-f transitions) ions, its luminescence properties can be well modulated, originating from tailoring band gap structure and enriching electron transition channels. Upon Sb3+ substitution in Cs2 NaHoCl6 , additional absorption peaking at 334 nm results in the tremendous increase of photoluminescence quantum yield (PLQY). Meanwhile, not only the typical NIR emission around 980 nm of Ho3+ is enhanced, but also the red and NIR emissions show a diverse range of anti-thermal quenching photoluminescence behaviors. Furthermore, through designing Yb3+ doping, the up-conversion photoluminescence can be triggered by changing excitation laser power density (yellow-to-orange) and Yb3+ doping concentration (red-to-green). Through a combined experimental-theoretical approach, the related luminescence mechanism is revealed. In general, by alloying Sb3+ /Yb3+ in Cs2 NaHoCl6 , abundant energy level ladders are constructed and more luminescence modes are derived, demonstrating great potential in multifunctional photoelectric applications.

An Er3+-Doped Cs2NaScCl6 Lead-Free Double Perovskite with Efficient Broadband Visible to Near-Infrared Emission and Multimodal Upconversion Luminescence

Lanthanide ions are widely used as dopants for halide perovskites for their broad energy level coverage from the visible to near-infrared (NIR) range. In this work, Cs2NaScCl6:Er3+ was synthesized by an improved solid-state reaction method, which showed effective NIR emission under ultraviolet excitation. Through calculations based on density functional theory and Bader charge analysis, it is shown that [ErCl6]3- octahedra show a strong localization effect in the Cs2NaScCl6:Er3+ lattice, which is conducive to the charge transfer process of Cl-Er3+, and charge transfer sensitization is responsible for the efficient visible to NIR luminescence of Er3+, where the NIR emission around λem = 1540 nm originated from the Er3+:4I13/2 → 4I15/2 transition with an ultrahigh photoluminescence quantum yield that reached ∼28.3%. Notably, Cs2NaScCl6:Er3+ also exhibited bright upconversion luminescence of green light (at 540 nm) under excitation by a variety of NIR laser diodes (808, 980, and 1550 nm) via self-sensitization processes.