Bilayer phosphine oxide modification toward efficient and large-area pure-blue perovskite quantum dot light-emitting diodes

Tailored large-particle quantum dots with high color purity and excellent electroluminescent efficiency

High-quality quantum dots (QDs) possess superior electroluminescent efficiencies and ultra-narrow emission linewidths are essential for realizing ultra-high definition QD light-emitting diodes (QLEDs). However, the synthesis of such QDs remains challenging. In this study, we present a facile high-temperature successive ion layer adsorption and reaction (HT-SILAR) strategy for the growth of precisely tailored Zn1-xCdxSe/ZnSe shells, and the consequent production of high-quality, large-particle, alloyed red CdZnSe/Zn1-xCdxSe/ZnSe/ZnS/CdZnS QDs. The transitional Zn1-xCdxSe/ZnSe shells serve to effectively suppress heavy hole energy level splitting and weaken the exciton-longitudinal optical phonon coupling of QDs, thus facilitating the formation of highly luminescent QDs with a near-unity photoluminescence quantum yield of 97.8% and narrow emission with a full width at half maximum of 17.1 nm. In addition, the introduction of transitional shells can extend the particle size of QDs to 19.0 nm, which is beneficial for efficient carrier recombination and reduced Joule heating in QD-based LEDs. As a result, the fabricated QLEDs can achieve a record external quantum efficiency of 38.2%, luminance over 120,000 cd m-2, and exceptional operational stability T95 (tested at 1,000 cd m-2) of 24,100 h. These findings provide new avenues for synthesizing high-quality QDs with high color purity.

Strategies for Stabilizing Metal Halide Perovskite Light-Emitting Diodes: Bulk and Surface Reconstruction of Perovskite Nanocrystals

Light-emitting colloidal lead halide perovskite nanocrystals (PeNCs) are considered promising candidates for next-generation vivid displays. However, the operational stability of light-emitting diodes (LEDs) based on PeNCs is still lower than those based on polycrystalline perovskite films, which requires an understanding of defect formation in PeNCs, both inside the crystal lattice ("bulk") and at the surface. Meanwhile, uncontrollable ion redistribution and electrochemical reactions under LED operation can be severe, which is also related to the bulk and surface quality of PeNCs, and a well-designed device architecture can boost carrier injection and balance radiative recombination. In this review, we consider bulk and surface reconstruction of PeNCs by enhancing the crystal lattice rigidity and rationally selecting the surface ligands. Degradation pathways of PeNCs under applied voltage are discussed, and strategies are considered to avoid both undesirable ion migration and electrochemical reactions in the PeNC films. Subsequently, other critical issues hindering the commercial application of PeNC LEDs are discussed, including the toxicity of Pb in lead halide perovskites, scale-up deposition of PeNC films, and design of active-matrix prototypes for high-resolution LED modules.

Strongly Anchored Dion-Jacobson Perovskite for Efficient Blue Light-Emitting Diodes

Dion-Jacobson (DJ) perovskites are promising alternatives for Ruddlesden-Popper (RP) perovskites to fabricate blue perovskite light-emitting diodes (PeLEDs) due to their favorable structural and charge properties. However, the relatively weak hydrogen bond between the bridging diammonium group and perovskite poses huge challenges for regulating crystallization and defect density, leading to an undesirable film quality and device performance. Herein, we report the successful optimization of DJ perovskite films by introducing a new type of cesium octafluoroadipate (CsOFAA) precursor, which could strongly anchor the perovskite through coordination bonds and halogen-halogen bonds. Such an interaction between CsOFAA and perovskite significantly stabilizes the DJ perovskite structure and suppresses the nonradiative recombination, leading to high-performance DJ perovskite films with efficient and stable blue emission. Accordingly, high external quantum efficiency values of 15.2%, 10.0%, and 8.3% are achieved for PeLEDs with emission peaks at 490, 485, and 479 nm, respectively, representing the most efficient blue DJ PeLEDs.

Buried interface modification and light outcoupling strategy for efficient blue perovskite light-emitting diodes

Perovskite light-emitting diodes (PeLEDs) exhibit remarkable potential in the field of displays and solid-state lighting. However, blue PeLEDs, a key element for practical applications, still lag behind their green and red counterparts, due to a combination of strong nonradiative recombination losses and unoptimized device structures. In this report, we propose a buried interface modification strategy to address these challenges by focusing on the bottom-hole transport layer (HTL) of the PeLEDs. On the one hand, a multifunctional molecule, aminoacetic acid hydrochloride (AACl), is introduced to modify the HTL/perovskite interface to regulate the perovskite crystallization. Experimental investigations and theoretical calculations demonstrate that AACl can effectively reduce the nonradiative recombination losses in bulk perovskites by suppressing the growth of low-n perovskite phases and also the losses at the bottom interface by passivating interfacial defects. On the other hand, a self-assembly nanomesh structure is ingeniously developed within the HTLs. This nanomesh structure is meticulously crafted through the blending of poly-(9,9-dioctyl-fluorene-co-N-(4-butyl phenyl) diphenylamine) and poly (n-vinyl carbazole), significantly enhancing the light outcoupling efficiency in PeLEDs. As a result, our blue PeLEDs achieve remarkable external quantum efficiencies, 20.4% at 487 nm and 12.5% at 470 nm, which are among the highest reported values. Our results offer valuable insights and effective methods for achieving high-performance blue PeLEDs.

Enhancing crystal integrity and structural rigidity of CsPbBr3 nanoplatelets to achieve a narrow color-saturated blue emission

Quantum-confined CsPbBr3 perovskites are promising blue emitters for ultra-high-definition displays, but their soft lattice caused by highly ionic nature has a limited stability. Here, we endow CsPbBr3 nanoplatelets (NPLs) with atomic crystal-like structural rigidity through proper surface engineering, by using strongly bound N-dodecylbenzene sulfonic acid (DBSA). A stable, rigid crystal structure, as well as uniform, orderly-arranged surface of these NPLs is achieved by optimizing intermediate reaction stage, by switching from molecular clusters to mono-octahedra, while interaction with DBSA resulted in formation of a CsxO monolayer shell capping the NPL surface. As a result, both structural and optical stability of the CsPbBr3 NPLs is enhanced by strong covalent bonding of DBSA, which inhibits undesired phase transitions and decomposition of the perovskite phase potentially caused by ligand desorption. Moreover, rather small amount of DBSA ligands at the NPL surface results in a short inter-NPL spacing in their closely-packed films, which facilitates efficient charge injection and transport. Blue photoluminescence of the produced CsPbBr3 NPLs is bright (nearly unity emission quantum yield) and peaks at 457 nm with an extremely narrow bandwidth of 3.7 nm at 80 K, while the bandwidth of the electroluminescence (peaked at 460 nm) also reaches a record-narrow value of 15 nm at room temperature. This value corresponds to the CIE coordinates of (0.141, 0.062), which meets Rec. 2020 standards for ultra-high-definition displays.

Charge Injection and Auger Recombination Modulation for Efficient and Stable Quasi-2D Perovskite Light-Emitting Diodes

The inefficient charge transport and large exciton binding energy of quasi-2D perovskites pose challenges to the emission efficiency and roll-off issues for perovskite light-emitting diodes (PeLEDs) despite excellent stability compared to 3D counterparts. Herein, alkyldiammonium cations with different molecular sizes, namely 1,4-butanediamine (BDA), 1,6-hexanediamine (HDA) and 1,8-octanediamine (ODA), are employed into quasi-2D perovskites, to simultaneously modulate the injection efficiency and recombination dynamics. The size increase of the bulky cation leads to increased excitonic recombination and also larger Auger recombination rate. Besides, the larger size assists the formation of randomly distributed 2D perovskite nanoplates, which results in less efficient injection and deteriorates the electroluminescent performance. Moderate exciton binding energy, suppressed 2D phases and balanced carrier injection of HDA-based PeLEDs contribute to a peak external quantum efficiency of 21.9%, among the highest in quasi-2D perovskite based near-infrared devices. Besides, the HDA-PeLED shows an ultralong operational half-lifetime T50 up to 479 h at 20 mA cm‒2, and sustains the initial performance after a record-level 30 000 cycles of ON-OFF switching, attributed to the suppressed migration of iodide anions into adjacent layers and the electrochemical reaction in HDA-PeLEDs. This work provides a potential direction of cation design for efficient and stable quasi-2D-PeLEDs.

Efficient Quantum Dot Light-Emitting Diode Enabled by a Thick Inorganic CdS Interfacial Modification Layer

Ultrathin (∼10 nm) insulating polymer films are commonly employed as an interfacial modification layer (IML) to improve charge balance and suppress interfacial exciton quenching in quantum dot light-emitting diodes (QLEDs). However, because the thickness is smaller than the energy transfer distance, interfacial exciton quenching is only partially suppressed, leading to the degrading of device performance. In this work, a thick (35 nm) inorganic CdS film is developed to serve as the IML of CdSe quantum-dot-based QLED. Benefiting from relatively low electron mobility and well-matched energy level, the CdS IML can effectively improve charge balance. In addition, because the thickness is larger than the energy transfer distance, interfacial exciton quenching can be completely blocked. As a result, the QLEDs with CdS IML exhibit a maximum EQE of 21.2% and a peak current efficiency of 24.2 cd A-1, which are about 1.32- and 1.4-fold higher than 16.1% and 17.3 cd A-1 of the devices without CdS IML, respectively. Our work offers an efficient method to completely block interfacial exciton quenching, which may open a new avenue for developing higher-performance QLEDs.