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.
To achieve highly‐efficient organic light‐emitting diodes (OLEDs), great efforts have been devoted into constructing thermally activated delayed fluorescence (TADF) with high horizontal dipole ratios (Θ//). Here, we proposed a design strategy by integrating a rigid electron‐accepting oxygen‐bridged boron core with triple electron‐donating groups, which exhibited a “shamrock‐shape”, namely BO‐3DMAC and BO‐3DPAC. Benefiting from the rigid and large‐planar skeletons brought by shamrock‐shaped design, BO‐3DMAC and BO‐3DPAC exhibit high Θ// of 84%/70% and 93%/94% in neat/doped films, respectively, and finally furnish excellent external quantum efficiencies (EQEs) of up to 28.3% and 38.7% in 20 wt% doped OLEDs with sky‐blue emission, as well as adequate EQEs of up to 21.0% and 16.7% in nondoped OLEDs. This work unveils a promising strategy to establish high‐Θ// TADF emitters by constructing large‐planar molecular structures using shamrock‐shaped design.
Fast spin-flipping is the key to exploit the triplet excitons in thermally activated delayed fluorescence based organic light-emitting diodes toward high efficiency, low efficiency roll-off and long operating lifetime. In common donor-acceptor type thermally activated delayed fluorescence molecules, the distribution of dihedral angles in the film state would have significant influence on the photo-physical properties, which are usually neglected by researches. Herein, we find that the excited state lifetimes of thermally activated delayed fluorescence emitters are subjected to conformation distributions in the host-guest system. Acridine-type flexible donors have a broad conformation distribution or bimodal distribution, in which some conformers feature large singlet-triplet energy gap, leading to long excited state lifetime. Utilization of rigid donors with steric hindrance can restrict the conformation distributions in the film to achieve degenerate singlet and triplet states, which is beneficial to efficient reverse intersystem crossing. Based on this principle, three prototype thermally activated delayed fluorescence emitters with confined conformation distributions are developed, achieving high reverse intersystem crossing rate constants greater than 106 s−1, which enable highly efficient solution-processed organic light-emitting diodes with suppressed efficiency roll-off.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.