Efficient Large-Area (81 cm2) Ternary Copper Halides Light-Emitting Diodes with External Quantum Efficiency Exceeding 13% via Host-Guest Strategy.

Ternary copper (Cu) halides are promising candidates for replacing toxic lead halides in the field of perovskite light-emitting diodes (LEDs) toward practical applications. However, the electroluminescent performance of Cu halide-based LEDs remains a great challenge due to the presence of serious nonradiative recombination and inefficient charge transport in Cu halide emitters. Here, the rational design of host-guest [dppb]2Cu2I2 (dppb denotes 1,2-bis[diphenylphosphino]benzene) emitters and its utility in fabricating efficient Cu halide-based green LEDs that show a high external quantum efficiency (EQE) of 13.39% are reported. The host-guest [dppb]2Cu2I2 emitters with mCP (1,3-bis(N-carbazolyl)benzene) host demonstrate a significant improvement of carrier radiative recombination efficiency, with the photoluminescence quantum yield increased by nearly ten times, which is rooted in the efficient energy transfer and type-I energy level alignment between [dppb]2Cu2I2 and mCP. Moreover, the charge-transporting mCP host can raise the carrier mobility of [dppb]2Cu2I2 films, thereby enhancing the charge transport and recombination. More importantly, this strategy enables a large-area prototype LED with a record-breaking area up to 81 cm2, along with a decent EQE of 10.02% and uniform luminance. It is believed these results represent an encouraging stepping stone to bring Cu halide-based LEDs from the laboratory toward commercial lighting and display panels.

Ligand-Induced Cation-π Interactions Enable High-Efficiency, Bright, and Spectrally Stable Rec. 2020 Pure-Red Perovskite Light-Emitting Diodes.

Achieving high-performance perovskite light-emitting diodes (PeLEDs) with pure-red electroluminescence for practical applications remains a critical challenge because of the problematic luminescence property and spectral instability of existing emitters. Herein, high-efficiency Rec. 2020 pure-red PeLEDs, simultaneously exhibiting exceptional brightness and spectral stability, based on CsPb(Br/I)3 perovskite nanocrystals (NCs) capping with aromatic amino acid ligands featuring cation-π interactions, are reported. It is proven that strong cation-π interactions between the PbI6 -octahedra of perovskite units and the electron-rich indole ring of tryptophan (TRP) molecules not only chemically polish the imperfect surface sites, but also markedly increase the binding affinity of the ligand molecules, leading to high photoluminescence quantum yields and greatly enhanced spectral stability of the CsPb(Br/I)3 NCs. Moreover, the incorporation of small-size aromatic TRP ligands ensures superior charge-transport properties of the assembled emissive layers. The resultant devices emitting at around 635 nm demonstrate a champion external quantum efficiency of 22.8%, a max luminance of 12 910 cd m-2 , and outstanding spectral stability, representing one of the best-performing Rec. 2020 pure-red PeLEDs achieved so far.

High-Efficiency and Stable Long-Persistent Luminescence from Undoped Cesium Cadmium Chlorine Crystals Induced by Intrinsic Point Defects.

Application of long-persistent luminescence (LPL) materials in many technological fields is in the spotlight. However, the exploration of undoped persistent luminescent materials with high emission efficiency, robust stability, and long persistent duration remains challenging. Here, inorganic cesium cadmium chlorine (CsCdCl3 ) is developed, featuring remarkable LPL characteristics at room temperature, which is synthesized by a facile hydrothermal method. Excited by ultraviolet light, the CsCdCl3 crystals exhibit an intense yellow emission with a large photoluminescence quantum yield of ≈90%. Different from the reported systems with lanthanides or transition metals doping, the CsCdCl3 crystals without dopants perform yellow LPL with a long duration of 6000 s. Joint experiment-theory characterizations reveal the intrinsic point defects of CsCdCl3 act as the trap centers of excited electrons and the carrier de-trapping process from such trap sites to localized emission centers contributes to the LPL. Encouraged by the attractive fluorescence and persistent luminescence as well as good stability of CsCdCl3 against environment oxygen/moisture (75%), heat (100 °C for 10 h), and ultraviolet light irradiation, an effective dual-mode information storage-reading application is demonstrated. The results open up a new frontier for exploring LPL materials without dopants and provide an opportunity for advanced information storage compatible for practical applications.

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.

Carbazole-Containing Polymer-Assisted Trap Passivation and Hole-Injection Promotion for Efficient and Stable CsCu2 I3 -Based Yellow LEDs.

Perovskite light-emitting diodes (LEDs) are emerging light sources for next-generation lighting and display technologies; however, their development is greatly plagued by difficulty in achieving yellow electroluminescence, environmental instability, and lead toxicity. Copper halide CsCu2 I3 with intrinsic yellow emission emerges as a highly promising candidate for eco-friendly LEDs, but the electroluminescent performance is limited by defect-related nonradiative losses and inefficient charge transport/injection. To solve these issues, a hole-transporting poly(9-vinlycarbazole) (PVK)-incorporated engineering into CsCu2 I3 emitter is proposed. PVK with carbazole groups is permeated at the grain boundaries of CsCu2 I3 films by interacting with the uncoordinated Cu+ , reducing the CuCs and CuI antisite defects to increase the radiative recombination and enhancing the hole mobility to balance the charge transport/injection, resulting in substantially enhanced device performances. Eventually, the yellow LEDs exhibit an 8.5-fold enhancement of external quantum efficiency, and the half-lifetime reaches 14.6 h, representing the most stable yellow LEDs based on perovskite systems reported so far.

Regulating the Singlet and Triplet Emission of Sb3+ Ions to Achieve Single-Component White-Light Emitter with Record High Color-Rendering Index and Stability.

The rapid development of solid-state lighting technology has attracted much attention for searching efficient and stable luminescent materials, especially the single-component white-light emitter. Here, we adopt a facile ion-doping technology to synthesize vacancy-ordered double perovskite Cs2ZrCl6:Sb. The introduction of Sb3+ ions with a 5s2 active lone pair into Cs2ZrCl6 host stimulates the singlet (blue) and triplet (orange) states emission of Sb3+ ions, and their relative emission intensity can be tuned through the energy transfer from singlet to triplet states. Benefiting from the dual-band emission as a pair of perfect complementary colors, the optimum Cs2ZrCl6:1.5%Sb exhibits a high-quality white emission with a color-rendering index of 96. By employing Cs2ZrCl6:1.5%Sb as the down-conversion phosphor, stable single-component white light-emitting diodes with a record half-lifetime of 2003 h were further fabricated. This study puts forward an effective ion-doping strategy to design single-component white-light emitter, making practical applications of them in lighting technologies a real possibility.

Room-temperature synthesis of blue-emissive zero-dimensional cesium indium halide quantum dots for temperature-stable down-conversion white light-emitting diodes with a half-lifetime of 186 h.

Recently, the newly-emerging lead halide perovskite quantum dots (QDs) have attracted intensive research interest for lighting and display applications owing to their remarkable optoelectronic properties. It is regrettable that the toxicity and instability of lead largely hinder their practical applications. Here, zero-dimensional (0D) cesium indium halide (Cs3InX6) QDs were synthesized for the first time using a modified ligand-assisted reprecipitation method, and the emission wavelength can be tuned facilely via an anion exchange reaction. Typically, the Cs3InBr6 QDs showed broadband blue emission with a high photoluminescence (PL) quantum yield of 46%. First-principles calculations were performed and temperature-dependent PL was studied to investigate the emission mechanisms of the Cs3InBr6 QDs; the 0D nature of Cs3InBr6 enhances the localization of excitons, resulting in a large exciton binding energy. It is worth noting that the strong electron-phonon coupling of Cs3InBr6 indicates that the broadband emission comes from self-trapped exciton emission. Moreover, the Cs3InX6 QDs exhibit excellent stability against moisture, ultraviolet light and heat degradation, significantly better than for conventional lead halide perovskites. Subsequently, the white light-emitting diodes (WLEDs) prepared using blue-emissive Cs3InBr6 QD powder used as the phosphor showed an excellent working stability with a record half-life (T50) of 186 h. Even if the operating temperature is as high as 106.9 °C, the LED can still operate well and reach a T50 of 50 h. These results highlight the huge advantages and application potential of 0D Cs3InX6 QDs as an environmentally friendly emitter in the field of solid-state lighting.

Lead-Free Metal Halide Perovskites and Perovskite Derivatives as an Environmentally Friendly Emitter for Light-Emitting Device Applications.

Recently, newly emerging lead halide perovskites have attracted great attention as a new class of light emitters in luminescent devices because of their superior photoluminescence quantum yield, adjustable emission wavelength, high charge-carrier transport ability, and low-temperature processing technique. However, the poor stability and lead toxicity of such materials severely restrict their practical applications and future commercialization. Therefore, recent efforts have been devoted to developing lead-free metal halide perovskites and their derivatives to address the above hurdles. In this Perspective, we first review the recent progress on the lead-free metal halide materials and their optical properties. We then discuss the stability issues of lead-free perovskites against heat, ultraviolet light, oxygen, and moisture. Further, we give a demonstration of the preliminary achievements and limitations in lead-free material-based light-emitting devices. Finally, we present current existing challenges and possible development opportunities in this field based on lead-free material systems.

Colloidal Synthesis of Ternary Copper Halide Nanocrystals for High-Efficiency Deep-Blue Light-Emitting Diodes with a Half-Lifetime above 100 h.

Currently, the blue perovskite light-emitting diodes (PeLEDs) suffer from a compromise in lead toxicity and poor operation stability, and most previous studies have struggled to meet the crucial blue NTSC standard. In this study, electrically driven deep-blue LEDs (∼445 nm) based on zero-dimensional (0D) Cs3Cu2I5 nanocrystals (NCs) were demonstrated with the color coordinates of (0.16, 0.07) and a high external quantum efficiency of ∼1.12%, comparable with the best-performing blue LEDs based on lead-halide perovskites. Encouraged by the remarkable stability of Cs3Cu2I5 NCs against heat and environmental oxygen/moisture, the proposed device was operated in a continuous current mode for 170 h, producing a record half-lifetime of ∼108 h. The device stability was further verified by an aggressive thermal cycling test (300-360-300 K) and a 35-day storage test. Together with the eco-friendly features and facile colloidal synthesis technique, the 0D Cs3Cu2I5 NCs can be therefore regarded as a promising candidate for deep-blue LEDs applications.

Stable Yellow Light-Emitting Devices Based on Ternary Copper Halides with Broadband Emissive Self-Trapped Excitons.

Great successes have been achieved in developing perovskite light-emitting devices (LEDs) with blue, green, red, and near-infrared emissions. However, as key optoelectronic devices, yellow-colored perovskite LEDs remain challenging, mainly due to the inevitable halide separation in mixed halide perovskites under high bias, causing undesired color change of devices. In addition to this color-missing problem, the intrinsic toxicity and poor stability of conventional lead-halide perovskites also restrict their practical applications. We herein report the fabrication of stable yellow LEDs based on a ternary copper halide CsCu2I3, addressing the color instability and toxicity issues facing current perovskite yellow LED's compromise. Joint experiment-theory characterizations indicate that the yellow electroluminescence originates from the broadband emission of self-trapped excitons centered at 550 nm as well as the comparable and reasonably low carrier effective masses favorable for charge transport. With a maximum luminance of 47.5 cd/m2 and an external quantum efficiency of 0.17%, the fabricated yellow LEDs exhibit a long half-lifetime of 5.2 h at 25 °C and still function properly at 60 °C with a half-lifetime of 2.2 h, which benefits from the superior resistance of CsCu2I3 to heat, moisture, and oxidation in ambient environmental conditions. The results obtained promise the copper halides with broadband light emission as an environment-friendly and stable yellow emitter for the LEDs compatible with practical applications.