Electroluminescent vertical tunneling junctions based on WSe2 monolayer quantum emitter arrays: Exploring tunability with electric and magnetic fields.

We experimentally demonstrate the creation of defects in monolayer WSe2 via nanopillar imprinting and helium ion irradiation. Based on the first method, we realize atomically thin vertical tunneling light-emitting diodes based on WSe2 monolayers hosting quantum emitters at deterministically specified locations. We characterize these emitters by investigating the evolution of their emission spectra in external electric and magnetic fields, as well as by inducing electroluminescence at low temperatures. We identify qualitatively different types of quantum emitters and classify them according to the dominant electron-hole recombination paths, determined by the mechanisms of intervalley mixing occurring in fundamental conduction and/or valence subbands.

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.

Regulating the Grain-Growth Surface for Efficient Near-Infrared Perovskite Light-Emitting Diodes.

Metal halide perovskites hold great potential for next-generation light-emitting diodes (PeLEDs). Despite significant progress, achieving high-performance PeLEDs hinges on optimizing the interface between the perovskite crystal film and the charge transport layers, especially the buried interface, which serves as the starting point for perovskite growth. Here, we develop a bottom-up perovskite film modulation strategy using formamidine acetate (FAAc) to enhance the buried interface. This multifaceted approach facilitates the vertical-oriented growth of high-quality perovskites with minimized defects. Meanwhile, the in situ deprotonation between FA+ and ZnO could eliminate the hydroxyl (-OH) defects and modulate the energy level of ZnO. The resulting FAPbI3-PeLED exhibits a champion EQE of 23.84% with enhanced operational stability and suppressed EQE roll-off. This strategy is also successfully extended to other mixed-halide PeLEDs (e.g., Cs0.17FA0.83Pb(I0.75Br0.25)3), demonstrating its versatility as an efficient and straightforward method for enhancing the PeLEDs' performance.

Matrix-Free Thermally Activated Delayed Fluorescent Carbon Dots-Based Electroluminescent Light-Emitting Diodes Exceeding 5.6% External Quantum Efficiency.

Carbon dots (CDs) are promising luminescent emission layer materials for next generation electroluminescent light emitting diodes (EL-LEDs) due to their many advantages, such as environmental friendliness, low cost, and high stability. However, limited by the spin-forbidden properties of the triplet transition, it is difficult to improve the external quantum efficiency (EQE) of fluorescent CDs-based EL-LEDs. Meanwhile, traditional thermally activated delayed fluorescent (TADF) CDs prepared using coating strategies are difficult to utilize in EL-LEDs due to the nonconductivity of the coating agent. Herein, we successfully developed matrix-free TADF CDs with yellow emission and achieved a device EQE of 5.68%, which is the highest value reported in CDs-based EL-LEDs. In addition, we also developed white EL-LEDs with an EQE of 1.70%. This study highlights the importance of interactions between precursors in modulating the electroluminescence properties of TADF emitters and provides an effective design principle for matrix-free TADF CDs.

Efficient Quasi-2D Perovskite Based Blue Light-Emitting Diodes with Carbon Dots Modified Hole Transport Layer.

Quasi-2D perovskites based blue light-emitting diodes (LEDs) suffer from its poor electroluminescence performance, mainly caused by the nonradiative recombination in in defect-rich low-n phases and the unbalanced hole-electron injection in the device. Here, we developed a highly efficient quasi-2D perovskite based sky-blue LEDs behaving recorded external quantum efficiency (EQE) of 21.07% by employing carbon dots (CDs) as additives in the hole transport layer (HTL). We ascribe the high EQE to the effective engineering of CDs: (1) The CDs at the interface of HTLs can suppress the formation of low-efficient n = 1 phase, resulting a high luminescence quantum yield and energy transfer efficiency of the mixed n-phase quasi-2D perovskites. (2) The CDs additives can reduce the conductivity of HTL, partially blocking the hole injection, and thus making more balanced hole-electron injection. The CDs-treated devices have excellent Spectral stability and enhanced operational stability and could be a new alternative additive in the perovskite optoelectronic devices.

Preferred Orientation and Phase Distribution of Quasi-2D Perovskite for Bifunctional Light-Emitting Photodetectors.

In traditional optical wireless communication (OWC) systems, the simultaneous use of multiple sets of light-emitting diodes (LEDs) and photodetectors (PDs) increases the system complexity and instability. Here we report bifunctional light-emitting photodetectors (LEPDs) fabricated with quasi-2D perovskite (F-PEA)2Cs4Pb5I11Br5 as light-emitting/detecting layers for efficient, miniaturized, and intelligent bidirectional OWC. By simply changing the solvent composition of the precursor solution and using antisolvent engineering, we manipulated the crystal orientation and phase distribution of (F-PEA)2Cs4Pb5I11Br5, realizing high irradiance (4.36 µW cm-2) and a -3 dB refresh rate (0.21 MHz) of electroluminescence in LED mode as well as low noise (below 1 pA Hz-1/2) and high responsivity (0.1 A W-1) in PD mode. The rapid and accurate OWC process was demonstrated through interaction of LEPDs. We also demonstrated the high-fidelity compression and digitization of high-resolution (256 × 256 pixels) color images using the four-step phase shift method to realize intelligent encrypted image OWC.

Efficient, Color-Stable, Pure-Blue Light-Emitting Diodes Based on Aromatic Ligand-Engineered Perovskite Nanoplatelets.

Perovskite nanoplatelets (NPLs) show great potential for high-color-purity light-emitting diodes (LEDs) due to their narrow line width and high exciton binding energy. However, the performance of perovskite NPL LEDs lags far behind perovskite quantum dot-/film-based LEDs, owing to their material instability and poor carrier transport. Here, we achieved efficient and stable pure blue-emitting CsPbBr3 NPLs with outstanding optical and electrical properties by using an aromatic ligand, 4-bromothiophene-2-carboxaldehyde (BTC). The BTC ligands with thiophene groups can guide two-dimensional growth and inhibit out-of-plane ripening of CsPbBr3 NPLs, which, meanwhile, increases their structural stability via strongly interacting with PbBr64- octahedra. Moreover, aromatic structures with delocalized π-bonds facilitate charge transport, diminish band tail states, and suppress Auger processes in CsPbBr3 NPLs. Consequently, the LEDs demonstrate efficient and color-stable blue emissions at 465 nm with a narrow emission line width of 17 nm and a maximum external quantum efficiency (EQE) of 5.4%, representing the state-of-the-art CsPbBr3 NPL LEDs.

Solvent-Reconstructed Interface That Enhances Light Out-Coupling in Quasi-Two-Dimensional Perovskite Light-Emitting Diodes.

Light management is critical to maximizing the external quantum efficiency of perovskite light-emitting diodes (PeLEDs), but strategies for enhancing light out-coupling are typically complex and expensive. Here, using a facile solvent treatment strategy, we create a layer of lithium fluoride (LiF) nanoislands that serve as a template to reconstruct the light-extracting interfaces for PeLEDs. The nanoisland interface rearranges the near-field light distribution in order to maximize the efficiency of internal light extraction. With the proper adjustment of the nanoisland size and distribution, we have achieved an optimal balance between charge injection and light out-coupling, resulting in bright, pure-red quasi-two-dimensional PeLEDs with a 21.8% peak external quantum efficiency.

Origin of the Efficiency Roll-off in Quantum Dot Light-Emitting Diodes: An Electrically Excited Transient Absorption Spectroscopy Study.

In situ characterizations of charge injection dynamics, equilibrated concentration, and electric field distributions shed light on the critical mechanisms of quantum dot light-emitting diodes (QD-LEDs). In this work, we developed electrically excited transient absorption spectroscopy, which can provide the above key information, to investigate the efficiency roll-off of QD-LEDs. We found that the average electron populations per QD are low when QD-LEDs exhibit efficiency roll-off, excluding Auger recombination as the main cause. We also revealed that the weak electrical field inside the QD layer under forward biases has a negligible impact on the efficiency. Interestingly, we found that as the voltage increases the electron concentration in the QD layer saturates at very low levels. When combined with the concomitant efficiency roll-off, we propose electron leakage is the main loss at elevated driving voltages. We further demonstrate that increasing the electron confinement potential with the ZnS shell enables us to efficiently mitigate the efficiency roll-off.

Polarity-Mediated Antisolvent Control Enables Efficient Lanthanide-Based near-Infrared Perovskite LEDs.

Alloying lanthanide ions (Yb3+) into perovskite quantum dots (Yb3+:CsPb(Cl1-xBrx)3) is an effective method to achieve efficient near-infrared (NIR) luminescence (>950 nm). Increasing the Yb3+ alloying ratio in the perovskite matrix enhances the luminescence intensity of Yb3+ emission at 990 nm. However, high Yb3+ alloying (>15%) results in vacancy-induced inferior material stability. In this work, we developed a polarity-mediated antisolvent manipulation strategy to resolve the incompatibility between a high Yb3+ alloying ratio and inferior stability of Yb3+:CsPb(Cl1-xBrx)3. Precise control of solution polarity enables increased uniformity of the perovskite matrix with fewer trap densities. Employing this strategy, we obtain Yb3+:CsPb(Cl1-xBrx)3 with the highest Yb3+ alloying ratio of 30.2% and a 2-fold higher electroluminescence intensity at 990 nm. We lever the engineered Yb3+:CsPb(Cl1-xBrx)3 to fabricate NIR-LEDs, achieving a peak external quantum efficiency (EQE) of 8.5% at 990 nm: this represents the highest among perovskite NIR-LEDs with an emission wavelength above 950 nm.

High-Performance Blue Perovskite Light-Emitting Diodes Enabled by Synergistic Effect of Additives.

While quasi-two-dimensional (quasi-2D) perovskites have good properties of cascade energy transfer, high exciton binding energy, and high quantum efficiency, which will benefit high-efficiency blue PeLEDs, inefficient domain distribution management and unbalanced carrier transport impede device performance improvement. Herein, (2-(9H-carbazol-9-yl)ethyl)phosphonic acid (2PACz) and methyl 2-aminopyridine-4-carboxylate (MAC) were simultaneously introduced to a blue quasi-2D perovskite film. Relying on the synergistic effect of 2PACz and MAC, it not only modulates the phase distribution inhibiting the n = 2 phase but also greatly improves the electrical property of the quasi-2D perovskite film. As a result, the as-modified blue quasi-2D PeLED demonstrated an external quantum efficiency (EQE) of 17.08% and a luminance of 10142 cd m-2. This study exemplifies the synergistic effect among dual additives and offers a new effective additive strategy modulating phase distribution and building balanced carrier transport, which paves the way for the fabrication of highly efficient blue PeLEDs.

Interfacial Passivation Engineering for Highly Efficient Quantum Dot Light-Emitting Diodes via Aromatic Amine-Functionalized Dipole Molecules.

Blue quantum dot (QD) light-emitting diodes (QLEDs) exhibit unsatisfactory operational stability and electroluminescence (EL) properties due to severe nonradiative recombination induced by large numbers of dangling bond defects and charge imbalance in QD. Herein, dipolar aromatic amine-functionalized molecules with different molecular polarities are employed to regulate charge transport and passivate interfacial defects between QD and the electron transfer layer (ETL). The results show that the stronger the molecular polarity, especially with the -CF3 groups possessing a strong electron-withdrawing capacity, the more effective the defect passivation of S and Zn dangling bonds at the QD surface. Moreover, the dipole interlayer can effectively reduce electron injection into QD at high current density, enhancing charge balance and mitigating Joule heat. Finally, blue QLEDs exhibit a peak external quantum efficiency (EQE) of 21.02% with an operational lifetime (T50 at 100 cd m-2) exceeding 4000 h.

Very Stable and Efficient Tandem Quantum-Dot Light-Emitting Diodes Enabled by IZO-Based Interconnecting Layers.

Theoretically, tandem quantum-dot light-emitting diodes (QLEDs) hold great promise for achieving both high efficiency and high stability in display applications. However, in practice, their operational stability remains considerably inferior to that of state-of-the-art devices. In this study, we developed a new tandem structure with optimal electrical and optical performance to simultaneously improve the efficiency and stability of tandem QLEDs. Electrically, upon development of a barrier-free interconnecting layer enabled by an indium-zinc oxide bridging layer and a conductive ZnMgO layer, the driving voltage of the tandem QLEDs is remarkably reduced. Optically, upon development of a top-emitting structure and optimization of the cavity length guided by a theoretical simulation, a maximum light extraction efficiency is achieved. As a result, the red tandem QLEDs exhibit a maximum external quantum efficiency of 49.01% and a T95 lifetime at 1000 cd/m2 of >50 000 h, making them one of the most efficient and stable QLEDs ever reported.

High-Efficiency and Stable Colloidal One-Dimensional Core/Shell Nanorod Light-Emitting Diodes.

Anisotropic nanocrystals such as nanorods (NRs) display unique linearly polarized emission, which is expected to break the external quantum efficiency (EQE) limit of quantum dot-based light-emitting diodes (LEDs). However, the progress in achieving a higher EQE using NRs encounters several challenges, primarily involving a low photoluminescence quantum yield (PLQY) of NRs and imbalanced charge injection in NR-LEDs. In this work, we investigated NR-LEDs based on CdSe/CdZnS/ZnS rod-in-rod NRs with a high PLQY and higher linear polarization compared to those of dot-in-rod NRs. The balanced charge injection is achieved using ZnMgO nanoparticles as the electron transport layer and poly-TPD {poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine]} as the hole transport layer. Therefore, the NR-LEDs exhibit a maximum EQE of 21.5% and a maximum luminance of >120 000 cd/m2 owing to the high level of in-plane transitions with a dipole moment of 90%. The NR-LEDs also have greatly inhibited droop in EQE under a high current density as well as outstanding operation lifetime and cycle stability.

Modulation of Nucleation and Growth Kinetics of Perovskite Nanocrystals Enables Efficient and Spectrally Stable Pure-Red Light-Emitting Diodes.

Perovskite light-emitting diodes (PeLEDs) based on CsPb(Br/I)3 nanocrystals (NCs) usually suffer from severe spectral instability under operating voltage due to the poor-quality PeNCs. Herein, zeolite was utilized to prepare high-quality CsPb(Br/I)3 NCs via promoting the homogeneous nucleation and growth and suppressing the Ostwald ripening of PeNCs. In addition, the decomposed zeolite interacted strongly with PeNCs through Pb-O bonds and hydrogen bonds, which inhibited the formation of defects and suppressed halide ion migration, leading to an improved photoluminescence quantum yield (PLQY) and enhanced stability of PeNCs. Moreover, the strong binding affinity of decomposed zeolite to PeNCs contributed to the formation of homogeneous perovskite films with high PLQY. As a result, pure-red PeLEDs with Commission International de I'Eclairage (CIE) coordinates of (0.705, 0.291) were fabricated, approaching the Rec. 2020 red primary color. The devices achieved a peak external quantum efficiency of 23.0% and outstanding spectral stability.

High Efficiency and Low Roll-Off Pure-Red Perovskite LED Enabled by Simultaneously Inhibiting Auger and Trap Recombination of Quantum Dots.

CsPbI3 perovskite quantum dots (QDs) could achieve pure-red emission by reducing their size, but the increased exciton binding energy (EB) and surface defects for the small-sized QDs (SQDs) cause severe Auger and trap recombinations, thus worsening their electroluminescence (EL) performance. Herein, we utilize the dangling bonds of the SQDs as a driving force to accelerate KI dissolution to solve its low solubility in nonpolar solvents, thereby allowing K+ and I- to bond to the surface of SQDs. The EB of the SQDs was decreased from 305 to 51 meV because of the attraction of K+ to electrons, meanwhile surface vacancies were passivated by K+ and I-. The Auger and trap recombinations were simultaneously suppressed by this difunctional ligand. The SQD-based light-emitting diode showed a stable pure-red EL peak of 639 nm, an external quantum efficiency of 25.1% with low roll-off, and a brightness of 5934 cd m-2.

Dimensional and Doping Engineering of Chiral Perovskites with Enhanced Spin Selectivity for Green Emissive Spin Light-Emitting Diodes.

Chiral perovskites play a pivotal role in spintronics and optoelectronic systems attributed to their chiral-induced spin selectivity (CISS) effect. Specifically, they allow for spin-polarized charge transport in spin light-emitting diodes (LEDs), yielding circularly polarized electroluminescence at room temperature without external magnetic fields. However, chiral lead bromide-based perovskites have yet to achieve high-performance green emissive spin-LEDs, owing to limited CISS effects and charge transport. Herein, we employ dimensional regulation and Sn2+-doping to optimize chiral bromide-based perovskite architecture for green emissive spin-LEDs. The optimized (PEA)x(S/R-PRDA)2-xSn0.1Pb0.9Br4 chiral perovskite film exhibits an enhanced CISS effect, higher hole mobility, and better energy level alignment with the emissive layer. These improvements allow us to fabricate green emissive spin-LEDs with an external quantum efficiency (EQE) of 5.7% and an asymmetry factor |gCP-EL| of 1.1 × 10-3. This work highlights the importance of tailored perovskite architectures and doping strategies in advancing spintronics for optoelectronic applications.

Efficient Deep-Blue Light-Emitting Diodes from Highly Luminescent Eu2+-Doped Alkali Metal Halide Nanocrystals via Lattice Field Modulation.

Lead-halide perovskite nanocrystals (NCs) are promising for fabricating deep-blue (<460 nm) light-emitting diodes (LEDs), but their development is plagued by low electroluminescent performance and lead toxicity. Herein, the synthesis of 12 kinds of highly luminescent and eco-friendly deep-blue europium (Eu2+)-doped alkali-metal halides (AX:Eu2+; A = Na+, K+, Rb+, Cs+; X = Cl-, Br-, I-) NCs is reported. Through adjustment of the coordination environment, efficient deep-blue emission from Eu-5d → Eu-4f transitions is realized. The representative CsBr:Eu2+ NCs exhibit a high photoluminescence quantum yield of 91.1% at 441 nm with a color coordinate at (0.158, 0.023) matching with the Rec. 2020 blue specification. Electrically driven deep-blue LEDs from CsBr:Eu2+ NCs are demonstrated, achieving a record external quantum efficiency of 3.15% and half-lifetime of ∼1 h, surpassing the reported metal-halide deep-blue NCs-based LEDs. Importantly, large-area LEDs with an emitting area of 12.25 cm2 are realized with uniform emission, representing a milestone toward commercial display applications.

Gradient Alloy Shell Enabling Colloidal Quantum Wells Light-Emitting Diodes with Efficiency Exceeding 22.

Colloidal quantum well (CQW) based light emitting diodes (LEDs) possess extra-high theoretical efficiency, but their performance still lags far behind conventional LEDs due to severe exciton quenching and unbalanced charge injection. Herein, we devised a gradient composition CdxZn1-xS shell to address these issues. The epitaxial shell with gradient composition was achieved through controlling competition between Cd2+ and Zn2+ cations to preferentially bind to the anions S2-. Thus, exciton quenching was suppressed greatly by passivating defects and reducing nonradiative recombination, thereby achieving near-unity photoluminescence quantum yield (PLQY). The gradient energy level of the shell reduced the hole injection barriers and increased the hole injection efficiency to balance the charge injection of LEDs. As a result, the LEDs achieved a high external quantum efficiency (EQE) of 22.83%, luminance of 111,319 cd/m2 and a long operational lifetime (T95@100 cd/m2) over 6,500 h, demonstrating the state-of-the-art performance for the CQW based LEDs.

Entropy-Driven Strongly Confined Low-Toxicity Pure-Red Perovskite Quantum Dots for Spectrally Stable Light-Emitting Diodes.

Spectrally stable pure-red perovskite quantum dots (QDs) with low lead content are essential for high-definition displays but are difficult to synthesize due to QD self-purification. Here, we make use of entropy-driven quantum-confined pure-red perovskite QDs to fabricate light-emitting diodes (LEDs) that have low toxicity and are efficient and spectrum-stable. Based on experimental data and first-principles calculations, multiple element alloying results in a 60% reduction in lead content while improving QD entropy to promote crystal stability. Entropy-driven QDs exhibit photoluminescence with 100% quantum yields and single-exponential decay lifetimes without alteration of their morphology or crystal structure. The pure-red LEDs utilizing entropy-driven QDs have spectrally stable electroluminescence, achieving a brightness of 4932 cd/m2, a maximum external quantum efficiency of over 20%, and a 15-fold longer operational lifetime than the CsPbI3 QD-based LEDs. These achievements demonstrate that entropy-driven QDs can mitigate local compositional heterogeneity and ion migration.