Spacer Cation Alloying in Ruddlesden-Popper Perovskites for Efficient Red Light-Emitting Diodes with Precisely Tunable Wavelengths

High Current Efficiency Red Perovskite Light-Emitting Diodes Meeting Rec. 2020 Standard

Cesium lead iodide light-emitting diodes (LEDs) are attractive for displays due to their Rec. 2020 red standard compliance. However, achieving high current efficiencies (CEs), which is important for displays, is challenging because their emission spectrum is near the tail of the photopic luminosity function. Substituting some iodine with bromine can improve CEs by enlarging the bandgap, but defects easily form in iodine-bromine mixed perovskites. Here, we successfully reduced defect formation by adding organic ammonium salts and zwitterions. The organic ammonium salts do not form low-dimensional perovskites under the hydrogen bonding interaction of zwitterions. Instead, they passivate the cesium vacancy by forming new hydrogen bonds after perovskite crystallization. This approach leads to a red perovskite LED with a high CE of 12.8 cd A-1 and a peak external quantum efficiency of 20.3%, meeting the Rec. 2020 standard. It can be extended to large-area devices (2500 mm2) without a significant efficiency loss.

Precise Control Light Emission of PVDF-CH3NH3PbBr3-xClx Nanocrystalline Films Using a Cl-(CH3OH)n System

Methylammonium lead halide perovskites with highly efficient pure-color or white-light generation have gained increasing scientific interest and promote the development of a great commercial opportunity in displays, lighting, and other applications. However, the poor stabilities, lead toxicity, and unfriendly solvents and ligands in the growth process severely restrict their commercial application. Here, we proposed a green method for preparing uniform and stable polymer-encapsulated photoluminescence (PL) tunable CH3NH3PbBr3-xClx NC thin films at room temperature. Utilizing the swelling effect between alcohol compounds and organic polymers and the ionization of NaCl in methanol solution, the anion exchange process can be achieved rapidly within 7 min. Moreover, the PL wavelengths of the CH3NH3PbBr3-xClx NCs films were precisely tuned with steps as fine as 2 nm. Experimental results showed that NaCl dissolved in methanol solution can form Cl-(CH3OH)n, which brings ionized Cl into the polymer-encapsulated CH3NH3PbBr3 NCs film for CH3NH3PbBr3-xClx NCs film growth. Based on the swelling and anion exchange dynamics, a modified NaCl-CH3OH-MABr solution system was developed to trigger CH3NH3PbBr3-xClx NCs film PL emission tuning from 528 to 463 nm with several-fold intensity enhancement. The realization of precisely controlled photoluminescence from the perovskite nanocrystal film would have wide applications in the optical and imaging fields.

Fabrication of red-emitting perovskite LEDs by stabilizing their octahedral structure

Light-emitting diodes (LEDs) based on metal halide perovskites (PeLEDs) with high colour quality and facile solution processing are promising candidates for full-colour and high-definition displays1-4. Despite the great success achieved in green PeLEDs with lead bromide perovskites5, it is still challenging to realize pure-red (620-650 nm) LEDs using iodine-based counterparts, as they are constrained by the low intrinsic bandgap6. Here we report efficient and colour-stable PeLEDs across the entire pure-red region, with a peak external quantum efficiency reaching 28.7% at 638 nm, enabled by incorporating a double-end anchored ligand molecule into pure-iodine perovskites. We demonstrate that a key function of the organic intercalating cation is to stabilize the lead iodine octahedron through coordination with exposed lead ions and enhanced hydrogen bonding with iodine. The molecule synergistically facilitates spectral modulation, promotes charge transfer between perovskite quantum wells and reduces iodine migration under electrical bias. We realize continuously tunable emission wavelengths for iodine-based perovskite films with suppressed energy loss due to the decrease in bond energy of lead iodine in ionic perovskites as the bandgap increases. Importantly, the resultant devices show outstanding spectral stability and a half-lifetime of more than 7,600 min at an initial luminance of 100 cd m-2.

Efficient pure-red perovskite light-emitting diodes with strong passivation via ultrasmall-sized molecules

Perovskite light-emitting diodes (PeLEDs) have attracted great attention in recent years; however, the halogen vacancy defects in perovskite notably hamper the development of high-efficiency devices. Previously, large-sized passivation agents have been usually used, while the effect of defect passivation is limited due to the weak bonding or the large space steric hindrance. Here, we predict that the ultrasmall-sized formate (Fa) and acetate (Ac) have more efficient passivation ability because of the stronger binding with the perovskite, as demonstrated by density functional theory calculation. We introduce ultrasmall-sized cesium salts (CsFa/CsAc) into buried interface, which can also diffuse into the bulk, resulting in both buried interface and bulk passivation. In addition, the improved perovskite growth has been found due to the enhanced hydrophily after introducing CsFa/CsAc as additive. According to these advantages, a pure-red PeLED with 24.2% efficiency at 639 nm has been achieved.

Large-n quasi-phase-pure two-dimensional halide perovskite: A toolbox from materials to devices

Despite their excellent environmental stability, low defect density, and high carrier mobility, large-n quasi-two-dimensional halide perovskites (quasi-2DHPs) feature a limited application scope because of the formation of self-assembled multiple quantum wells (QWs) due to the similar thermal stabilities of large-n phases. However, large-n quasi-phase-pure 2DHPs (quasi-PP-2DHPs) can solve this problem perfectly. This review discusses the structures, formation mechanisms, and photoelectronic and physical properties of quasi-PP-2DHPs, summarises the corresponding single crystals, thin films, and heterojunction preparation methods, and presents the related advances. Moreover, we focus on applications of large-n quasi-PP-2DHPs in solar cells, photodetectors, lasers, light-emitting diodes, and field-effect transistors, discuss the challenges and prospects of these emerging photoelectronic materials, and review the potential technological developments in this area.

Regulating Perovskite Crystallization through Interfacial Engineering Using a Zwitterionic Additive Potassium Sulfamate for Efficient Pure-Blue Light-Emitting Diodes

Quasi-two-dimensional (quasi-2D) perovskites are emerging as efficient emitters in blue perovskite light-emitting diodes (PeLEDs), while the imbalanced crystallization of the halide-mixed system limits further improvements in device performance. The rapid crystallization caused by Cl doping produces massive defects at the interface, leading to aggravated non-radiative recombination. Meanwhile, unmanageable perovskite crystallization is prone to facilitate the formation of nonuniform low-dimensional phases, which results in energy loss during the exciton transfer process. Here, we propose a multifunctional interface engineering for nucleation and phase regulation by incorporating the zwitterionic additive potassium sulfamate into the hole transport layer. By using potassium ions (K+ ) as heterogeneous nucleation seeds, finely controlled growth of interfacial K+ -guided grains is achieved. The sulfamate ions can simultaneously regulate the phase distribution and passivate defects through coordination interactions with undercoordinated lead atoms. Consequently, such synergistic effect constructs quasi-2D blue perovskite films with smooth energy landscape and reduced trap states, leading to pure-blue PeLEDs with a maximum external quantum efficiency (EQE) of 17.32 %, spectrally stable emission at 478 nm and the prolonged operational lifetime. This work provides a unique guide to comprehensively regulate the halide-mixed blue perovskite crystallization by manipulating the characteristics of grain-growth substrate.

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.

Homogeneous Phase Distribution in Q-2D Perovskites via Co-Assembly of Spacer Cations for Efficient Light-Emitting Diodes

Quasi-2D (Q-2D) perovskites are promising candidates to apply in light-emitting diodes (LEDs). However, delicate control on crystallization kinetics is needed to suppress severe phase segregation. Here, the crystallization kinetics of Q-2D perovskites are investigated via in situ absorbance spectroscopy and for the first time find the multiphase distribution is governed by the arrangement, rather than diffusion, of spacer cations at the nucleation stage, which associate with its assembling ability determined by molecular configuration. A "co-assembly" strategy is conceived by combining co-cations with different configuration characteristics, where bulky cations disturb the assembling between slender cations and lead-bromide sheet, contributing to homogeneous emitting phase with effective passivation. Correspondingly, in the phenylethylammonium (PEA+ )-based Q-2D perovskites ( = 3), homogeneous phase distribution is achieved by incorporating co-cation triphenylmethaneammonium (TPMA+ ), the branching terminals of which suppress cations assembling into low-n phases and afford adequate cations as passivating ligands. Therefore, the champion external quantum efficiency of the LED device reaches 23.9%, which is among the highest performance of green Q-2D perovskite LEDs. This work reveals that the arrangement of spacer cations determines the crystallization kinetics in Q-2D perovskites, providing further guidance on the molecular design and phase modulation of Q-2D perovskites.

Efficient Red-Emissive Circularly Polarized Electroluminescence Enabled by Quasi-2D Perovskite with Chiral Spacer Cation

Perovskites are promising environmentally sustainable materials for circularly polarized electroluminescence (CPEL). While another chiral nonemissive layer is required for the developed perovskite-based CPEL, we report herein a highly efficient circularly polarized electroluminescence based on a single layer of quasi-2D perovskite with achiral phenethylammonium iodide (PEAI) and chiral S/R-1-(1-naphthyl)ethylammonium iodide (S/R-NEAI) as dual spacer cations. The quasi-2D perovskite was further passivated by carbazole-functionalized phosphonium. The as-fabricated film exhibits not only a circular dichroism (CD) signal but also prominent circularly polarized luminescence (CPL) activity with a maximum photoluminescence dissymmetry factor (g_lum) of ∼2.1 × 10^-3. More importantly, a highly efficient, spin-polarized light-emitting diode (LED) was fabricated based on the in situ passivated quasi-2D perovskite with a peak external quantum efficiency of 3.7% and a maximum electroluminescence dissymmetry factor (g_EL) of ∼4.0 × 10^-3.

Photoluminescence Modulation of Ruddlesden-Popper Perovskite via Phase Distribution Regulation

The intrinsic chaotic phase distribution in Ruddlesden-Popper Perovskite (RPP) hinders its further improvement of photoluminescence (PL) emission and limits its application in optical devices. In this work, we achieve the phase distribution regulation of RPP by varying the composition ratio of organic bulky spacer cations 1-naphthylmethylamine (NMA) and phenylethyl-ammonium (PEA), which is controllable and nondestructive for structures of RPP. By suppressing the small n-phase, the PL intensity emission of RPP is further improved. Through the time-resolved PL (TRPL) measurements, we find the PL lifetime of the sample with 66% PEA concentration increases with the temperature initially and possesses the highest values of τ1 and τ2 at ~255 K, indicating the immediate state assisting exciton radiative recombination, and it can be modulated by phase manipulation in RPP. The immediate state may outcompete other non-radiative decay channels for excited carriers, leading to the PL enhancement in RPP, and broadening its further application.

Suppressing High-Order Phase for Efficient Pure Red Quasi-2D Perovskite Light-Emitting Diodes

Quasi-two-dimensional (quasi-2D) perovskites are promising for the realization of spectrally stable pure red perovskite light-emitting diodes (PeLEDs) with a single iodide component, because they avoid the halide separation that red three-dimensional perovskites of mixed halides have faced. However, the distribution of high-order phases in solution-processed quasi-2D perovskite films causes the spectral shift away from the pure red region. Here, we introduced a simple approach of adding excessive ligand combinations to redistribute the phase distribution of quasi-2D perovskite and to inhibit the high-order phase. Appropriate excess organic ligands will not affect charge injection but will keep the efficient energy funneling and passivate the defect. The narrowed phase distribution reduced the band tail state and restrained reverse charge transfer, resulting in enhanced radiation recombination. We obtained efficient and spectrally stable pure red PeLEDs at 638 nm (approaching the Rec. 2020 specification) with a peak EQE of 11.8% and maximum luminance of 1688 cd/cm². This study provides guidance for future developments of highly efficient pure red PeLEDs.

Promoting Energy Transfer Between Quasi-2D Perovskite Layers Toward Highly Efficient Red Light-Emitting Diodes

Although tremendous progress has recently been made in quasi-2D perovskite light-emitting diodes (PeLEDs), the performance of red PeLEDs emitting at ≈650-660 nm, which have wide prospects for application in photodynamic therapy, is still limited by an inefficient energy transfer process between the quasi-2D perovskite layers. Herein, a symmetric molecule of 3,3'-(9H-fluorene-9,9-diyl)dipropanamide (FDPA) is designed and developed with two functional acylamino groups and incorporated into the quasi-2D perovskites as the additive for achieving high-performance red PeLEDs. It is demonstrated that the agent can simultaneously diminish the van der Waals gaps between individual perovskite layers and passivate uncoordinated Pb²⁺ related defects at the surface and grain boundaries of the quasi-2D perovskites, which truly results in an efficient energy transfer in the quasi-2D perovskite films. Consequently, the red PeLEDs emitting at 653 nm with a peak external quantum efficiency of 18.5% and a maximum luminance of 2545 cd m^-2 are achieved, which is among the best performing red quasi-2D PeLEDs emitting at ≈650-660 nm. This work opens a way to further improve the electroluminescence performance of red PeLEDs.

Planar defect-free pure red perovskite light-emitting diodes via metastable phase crystallization

Solution-processable all-inorganic CsPbI_3-xBr_x perovskite holds great potential for pure red light-emitting diodes. However, the widely existing defects in this mixed halide perovskite markedly limit the efficiency and stability of present light-emitting diode devices. We here identify that intragrain Ruddlesden-Popper planar defects are primary forms of such defects in the CsPbI_3-xBr_x thin film owing to the lattice strain caused by inhomogeneous halogen ion distribution. To eliminate these defects, we develop a stepwise metastable phase crystallization strategy to minimize the CsPbI_3-xBr_x perovskite lattice strain, which brings planar defect-free CsPbI_3-xBr_x thin film with improved radiative recombination, narrowed emission band, and enhanced spectral stability. Using these high-quality thin films, we fabricate spectrally stable pure red perovskite light-emitting diodes, showing 17.8% external quantum efficiency and 9000 candela meter^-2 brightness with color coordinates required by Rec. 2020.

Strong two-photon absorption induced by energy funneling in chiral quasi-2D perovskites

Quasi-2D Ruddlesden-Popper-type perovskites (RPPs) exhibit excellent nonlinear optical properties due to their multiple quantum well structures with large exciton binding energy. Herein, we introduce chiral organic molecules into RPPs and investigate their optical properties. It is found that the chiral RPPs possess effective circular dichroism in the ultraviolet to visible wavelengths. Two-photon absorption (TPA)-induced efficient energy funneling from small- to large-n domains is observed in the chiral RPP films, which induces strong TPA with a coefficient up to 4.98 cm MW^-1. This work will broaden the application of quasi-2D RPPs in chirality-related nonlinear photonic devices.

High-Efficiency Pure-Red Perovskite Quantum-Dot Light-Emitting Diodes

It is still challenging to achieve high-efficiency pure-red (620-650 nm wavelength) perovskite light-emitting diodes (PeLEDs). Herein, we report pure-red PeLEDs with Commission Internationale de l'Eclairage coordinates (0.703, 0.297) meeting the Rec. 2020, an external quantum efficiency of 20.8%, and a luminance of 3775 cd/m². This design is based on the strong quantum confinement CsPbI₃ quantum dots (QDs) capped by composite ligands of 3-phenyl-1-propylamine and tetrabutylammonium iodide. This strategy stabilized the structure of the strong-confined QDs and reduced the influence of the electric field-induced Stark effect on the PeLEDs. Furthermore, the exciton binding energy of the QDs was decreased by the composited ligands to suppress Auger recombination within the devices. Additionally, the valence-band maximum of the QDs was lifted to match the hole-transport layer, thus balancing charge injection in the PeLEDs. Our device also demonstrated a stable electroluminescence spectrum and a lifetime of 5.6 times longer than the control device.

Stability of Perovskite Light-Emitting Diodes: Existing Issues and Mitigation Strategies Related to Both Material and Device Aspects

Metal halide perovskites combine excellent electronic and optical properties, such as defect tolerance and high photoluminescence efficiency, with the benefits of low-cost, large-area, solution-based processing. Composition- and dimension-tunable properties of perovskites have already been utilized in bright and efficient light-emitting diodes (LEDs). At the same time, there are still great challenges ahead to achieving operational and spectral stability of these devices. In this review, the origins of instability of perovskite materials, and reasons for their degradation in LEDs are considered. Then, strategies for improving the stability of perovskite materials are reviewed, such as compositional engineering, dimensionality control, defect passivation, suitable encapsulation matrices, and fabrication of core/shell perovskite nanocrystals. For improvement of the operational stability of perovskite LEDs, the use of inorganic charge-transport layers, optimization of charge balance, and proper thermal management are considered. The review is concluded with a detailed account of the current challenges and a perspective on the key approaches and opportunities on how to reach the goal of stable, bright, and efficient perovskite LEDs.

Red Perovskite Light-Emitting Diodes with Efficiency Exceeding 25% Realized by Co-Spacer Cations

Perovskite light-emitting diodes (PeLEDs) have received great attention in recent years due to their narrow emission bandwidth and tunable emission spectrum. Efficient red emission is one of most important parts for lighting and displays. Quasi-2D perovskites can deliver high emission efficiency due to the strong carrier confinement, while the external quantum efficiencies (EQE) of red quasi-2D PeLEDs are inefficient at present, which is due to the complex distribution of different n-value phases in quasi-2D perovskite films. In this work, the phase distribution of the quasi-2D perovskite is finely controlled by mixing two different large organic cations, which effectively reduces the amount of smaller n-index phases, meanwhile the passivation of lead and halide defects in perovskite films is realized. Accordingly, the PeLEDs show 25.8% EQE and 1300 cd m^-2 maximum brightness at 680 nm, which exhibits the highest performance for red PeLEDs up to now.

Diammonium-Mediated Perovskite Film Formation for High-Luminescence Red Perovskite Light-Emitting Diodes

3D mixed-halide perovskite-based red emitters combine excellent charge-transport characteristics with simple solution processing and good film formation; however, light-emitting diodes (LEDs) based on these emitters cannot yet outperform their nanocrystal counterparts. Here the use of diammonium halides in regulating the formation of mixed bromide-iodide perovskite films is explored. It is found that the diammonium cations preferentially bond to Pb-Br, rather than Pb-I, octahedra, promoting the formation of quasi-2D phases. It is proposed that the perovskite formation is initially dominated by the crystallization of the thermodynamically more favorable 3D phase, but, as the solution gets depleted from the regular A cations, thin shells of amorphous quasi-2D perovskites form. This leads to crystalline perovskite grains with efficiently passivated surfaces and reduced lattice strain. As a result, the diammonium-treated perovskite LEDs demonstrate a record luminance (10745 cd m^-2 ) and half-lifetime among 3D perovskite-based red LEDs.

Multifunctional π-Conjugated Additives for Halide Perovskite

Additive is a conventional way to enhance halide perovskite active layer performance in multiaspects. Among them, π-conjugated molecules have significantly special influence on halide perovskite due to the superior electrical conductivity, rigidity property, and good planarity of π-electrons. In particular, π-conjugated additives usually have stronger interaction with halide perovskites. Therefore, they help with higher charge mobility and longer device lifetime compared with alkyl-based molecules. In this review, the detailed effect of conjugated molecules is discussed in the following parts: defect passivation, lattice orientation guidance, crystallization assistance, energy level rearrangement, and stability improvement. Meanwhile, the roles of conjugated ligands played in low-dimensional perovskite devices are summarized. This review gives an in-depth discussion about how conjugated molecules interact with halide perovskites, which may help understand the improved performance mechanism of perovskite device with π-conjugated additives. It is expected that π-conjugated organic additives for halide perovskites can provide unprecedented opportunities for the future improvement of perovskite devices.