Quasi‐2D perovskites have long been considered to have favorable “energy funnel/cascade” structures and excellent optical properties compared with their 3D counterparts. However, most quasi‐2D perovskite light‐emitting diodes (PeLEDs) exhibit high external quantum efficiency (EQE) but unsatisfactory operating stability due to Auger recombination induced by high current density. Herein, a synergetic dual‐additive strategy is adopted to prepare perovskite films with low defect density and high environmental stability by using 18‐crown‐6 and poly(ethylene glycol) methyl ether acrylate (MPEG‐MAA) as the additives. The dual additives containing COC bonds can not only effectively reduce the perovskite defects but also destroy the self‐aggregation of organic ligands, inducing the formation of perovskite nanocrystals with quasi‐core/shell structure. After thermal annealing, the MPEG‐MAA with its CC bond can be polymerized to obtain a comb‐like polymer, further protecting the passivated perovskite nanocrystals against water and oxygen. Finally, state‐of‐the‐art green PeLEDs with a normal EQE of 25.2% and a maximum EQE of 28.1% are achieved, and the operating lifetime (T50) of the device in air environment is over ten times increased, providing a novel and effective strategy to make high efficiency and long operating lifetime PeLEDs.

low-cost preparation process, metal halide perovskites are widely considered as a promising class of materials for light emission, [1][2][3][4] among which quasi-2D perovskites are particularly prominent for highly efficient perovskite light-emitting diodes (PeLEDs). [5][6][7][8] Early studies indicate that efficient energy transfer process from wide (low n values) to narrow band gap (high n values) is a main reason for the high PLQY and excellent device performance in quasi-2D perovskites. [9,10] However, it remains a great challenge to control the n values of the quasi-2D perovskites accurately with solution process. Due to the self-aggregation of ligands, the perovskites tend to form smaller n values to bring about inefficient energy transfer, strong electron-phonon coupling, and thus reduced radiative exciton recombination. [11][12][13][14] Meanwhile, the unfavorable 3D perovskites with more defects can also be formed on the upper surface of the vertically non-uniform quasi-2D perovskite film due to the lack of ligand coordination. [15] Furthermore, the aggregated ligands show lower electrical conductivity and stronger charge confinement, resulting in more pronounced Joule heating and Auger recombination which are detrimental to the operational lifetime of PeLEDs. [16,17] Therefore, it is desirable to suppress Quasi-2D perovskites show great promise for light-emitting diodes owing to suppressed non-radiative losses enabled by the energy funneling/cascading nanostructures. However, for red emission quasi-2D perovskites, these ideal energy landscapes for efficient perovskite light-emitting diodes (PeLEDs) can rarely be achieved due to detrimental aggregation of the low-dimensional ligands in perovskite precursors, leading to poor device efficiency and stability. Here, a ligand-modulated dimensionality control strategy is explored to achieve uniform phase distribution and reduce defect density for efficient light emission. In contrast to the model phenethylammonium iodide 2D ligand, the formation of small-n phases can be inhibited by a structurally similar phenoxyethylammonium iodide ligand owing to the weakened aromatic stacking between ligands. Besides, the oxygen atoms can interact with the uncoordinated Pb 2+ ions and promote the NI coordination in the perovskites, which greatly reduces the non-radiative recombination defects in the ionic lattice. With this simple and effective approach, deep-red quasi-2D PeLEDs with record-high external quantum efficiency of 21.6% and decent operational stability are achieved without the need for additional additives. These results highlight the potential of ligand-modulated dimensionality control to achieve highly efficient and stable PeLEDs with a facile fabrication process.

The band-edge electronic structure of lead halide perovskites (ABX 3 ) is composed of the orbitals of B and X components and can be tuned through the composition and structure of the BX 6 octahedron. Although A-site cations do not directly contribute to near-edge states, the bandgap of 3D metal halide perovskites can be affected by A-cations through BX 6 octahedron tilting or lattice size variation. Here, as confirmed by the Rietveld refinement results of X-ray diffraction characterization, the competition between lattice expansion and octahedral tilting is identified for the first time in emission wavelength tuning when introducing a large A-site cation (C 2 H 5 NH 3 + , EA + ) into 1-naphthylmethylammonium iodide-passivated CsPbI 3 system. The former dominates spectral redshift, while the latter leads to a blueshift of emission peak, which broadens the way to tune the emission wavelength. In addition, excess cations can also passivate the perovskites, leading to a photoluminescence (PL) quantum yield as high as 61%, increased average PL lifetime of 74.7 ns, and a high radiative and non-radiative recombination ratio of 15.7. Eventually, spectral-stable deep-red perovskite light-emitting diode with a maximum external quantum efficiency of 17.5% is realized, which is one of the highest efficiencies without using any light outcoupling and anti-solvent techniques.

We demonstrate a kind of perovskite/organic hybrid white electroluminescent device, where an ultrathin doping-free organic phosphorescent interlayer is embedded between a p-type hole transport layer and a ntype electron transport layer to give an organic p−i−n heterojunction unit, which is superimposed layer by layer onto a quasi-two-dimensional perovskite layer. The unique carrier transport character of the p-type hole transport layer leads to a broad carrier recombination region approaching the p−i−n heterojunction unit. As a result, pure-red emission from the perovskite layer and sky-blue emission from the organic p−i−n heterojunction were simultaneously achieved to generate white emission with a peak external quantum efficiency of 7.35%, Commission Internationale de L'Eclairage coordinates of (0.424, 0.363), and a low correlated color temperature of 2868 K. More importantly, excellent spectral stability and a greatly enhanced operating lifetime (10-fold longer than those of perovskite-only LEDs) are simultaneously achieved, providing a new path for the development of high-performance white LEDs.

High‐quality hosts are indispensable for simultaneously realizing stable, high efficiency, and low roll‐off blue solution‐processed organic light‐emitting diodes (OLEDs). Herein, three solution processable bipolar hosts with successively reduced triplet energies approaching the T1 state of thermally activated delayed fluorescence (TADF) emitter are developed and evaluated for high‐performance blue OLED devices. The smaller T1 energy gap between host and guest allows the quenching of long‐lived triplet excitons to reduce exciton concentration inside the device, and thus suppresses singlet‐triplet and triplet‐triplet annihilations. Triplet‐energy‐mediated hosts with high enough T1 and better charge balance in device facilitate high exciton utilization efficiency and uniform triplet exciton distribution among host and TADF guest. Benefited from these synergetic factors, a high maximum external quantum efficiency (EQEmax) of 20.8%, long operational lifetime (T50 of 398.3 h @ 500 cd m−2), and negligible efficiency roll‐off (EQE of 20.1% @ 1000 cd m−2) are achieved for bluish‐green TADF OLEDs. Additionally introducing a narrowband emission multiple‐resonance TADF material as terminal emitter to accelerate exciton dynamic and improve exciton utilization, a higher EQEmax of 23.1%, suppressed roll‐off and extended lifetime of 456.3 h are achieved for the sky‐blue sensitized OLEDs at the same brightness.