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 COC 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 CC 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.
Multiple resonance thermally activated delayed fluorescence (MR-TADF) compounds have set off an upsurge of research because of their tremendous application prospects in the field of wide color gamut display. Herein, we propose a novel MR-TADF molecular construction paradigm based on polycyclization of the multiple resonance parent core, and construct a representative multiple resonance polycyclic aromatic hydrocarbon (MR-PAH) based on the para-alignment of boron and nitrogen atoms into a six-membered ring (p-BNR). Through the retrosynthesis analysis, a concise synthesis strategy with wide applicability has been proposed, encompassing programmed sequential boron esterification, Suzuki coupling and Scholl oxidative coupling. The target model molecule BN-TP shows green fluorescence with an emission peak at 523 nm and a narrow full-width at half-maximum (FWHM) of 34 nm. The organic light-emitting diode (OLED) employing BN-TP as an emitter exhibits ultrapure green emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.26, 0.70), and achieves a maximum external quantum efficiency (EQE) of 35.1 %.
A series of ultrapure-blue thermally activated delayed fluorescence (TADF) emitters featuring through-space charge transfer (TSCT) have been constructed by close stacking between the donor and acceptor moieties in rigid heteroaromatic compounds. The obviously accelerated radiative transition of singlet excitons, the diminished vibrionic relaxation of ground and excited states, and the consequent reduced Stokes shift and the narrow emission are evident. The corresponding organic light-emitting diodes (OLEDs) based on AC-BO realize the best performance among all deep-blue TSCT-TADF emitters, with an external quantum efficiency (EQE max ) of 19.3 %. Furthermore, the OLEDs based on QAC-BO display an EQE max of 15.8 %, and achieve the first high-efficiency ultrapureblue TSCT-TADF material with an excellent Commission Internationale de L'Eclairage coordinate (CIE) of (0.145, 0.076) which perfectly matches the ultrapureblue CIE requirements (0.14, 0.08) defined by the National Television System Committee.
Highly efficient organic thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) emitters for organic light-emitting diodes (OLEDs) generally consist of a twisted donor-acceptor skeleton with aromatic amine donors. Herein, through introducing sulfur atoms into isomeric pentaphene and pentacene frameworks, we demonstrate a set of polycyclic luminophores exhibiting efficient TADF and RTP characters. The incorporation of sulfur atoms confirms a folded molecular plane, while intensifies singlet-triplet spin-orbit coupling. Further, the isomeric effect has a significant effect on the electronic structure of excited state, giving rise to the investigated compounds tunable luminescence mechanisms of TADF and RTP. With efficient triplet harvesting ability, maximum external quantum efficiencies up to 25.1 % and 8.7 % are achieved for the corresponding TADF and RTP OLEDs, verifying the great potential of sulfurbridged frameworks for highly efficient devices.
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 NI 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.
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