Fluorescence-based organic light-emitting diodes have continued to attract interest because of their long operational lifetimes, high colour purity of electroluminescence and potential to be manufactured at low cost in next-generation full-colour display and lighting applications. In fluorescent molecules, however, the exciton production efficiency is limited to 25% due to the deactivation of triplet excitons. Here we report fluorescence-based organic light-emitting diodes that realize external quantum efficiencies as high as 13.4-18% for blue, green, yellow and red emission, indicating that the exciton production efficiency reached nearly 100%. The high performance is enabled by utilization of thermally activated delayed fluorescence molecules as assistant dopants that permit efficient transfer of all electrically generated singlet and triplet excitons from the assistant dopants to the fluorescent emitters. Organic light-emitting diodes employing this exciton harvesting process provide freedom for the selection of emitters from a wide variety of conventional fluorescent molecules.
Metal complexes emitting thermally activated delayed fluorescence based on intra-ligand charge transfer and enhanced by metallization were synthesized. Organic light-emitting diodes using a thermally stable zinc complex processed by vacuum vapor deposition achieved an external quantum efficiency of nearly 20%.
A complete horizontal molecular orientation of a linear-shaped thermally activated delayed fluorescent guest emitter 2,6-bis(4-(10Hphenoxazin-10-yl)phenyl)benzo[1,2-d:5,4-d′] bis(oxazole) (cis-BOX2) was obtained in a glassy host matrix by vapor deposition. The orientational order of cis-BOX2 depended on the combination of deposition temperature and the type of host matrix. Complete horizontal orientation was obtained when a thin film with cis-BOX2 doped in a 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host matrix was fabricated at 200 K. The ultimate orientation of guest molecules originates from not only the kinetic relaxation but also the kinetic stability of the deposited guest molecules on the film surface during film growth. Utilizing the ultimate orientation, a highly efficient organic light-emitting diode with the external quantum efficiency of 33.4 ± 2.0% was realized. The thermal stability of the horizontal orientation of cis-BOX2 was governed by the glass transition temperature (Tg) of the CBP host matrix; the horizontal orientation was stable unless the film was annealed above Tg.
A strategy for designing highly efficient thermally activated delayed fluorescence (TADF) emitters was reported. TADF emitters with donoracceptordonor (DAD)-type structures showed highly efficient TADF because of their small singlet triplet energy gap and large oscillator strength. An organic lightemitting diode containing a DAD-type TADF emitter, cis-BOX2, exhibited a high external electroluminescence quantum efficiency of 17.6%.Organic compounds offer flexibility in molecular design, and a wide variety of organic semiconductors has been developed over the last two decades. In organic light-emitting diodes (OLEDs), fluorescent and phosphorescent materials have been used as electroluminescent materials. 15 In electroluminescence (EL) processes, singlet and triplet excitons are generated in a 1:3 ratio, i.e., 25% singlet and 75% triplet excitons. Consequently, when conventional fluorescent materials are used as dopants, the internal EL quantum efficiency (© int ) of OLEDs is at most 25%. However, using transition-metal-centered phosphorescent materials such as iridium(III) complexes, © int has been increased to nearly 100%. 5 Further, and most recently, we have reported that using thermally activated delayed fluorescence (TADF) emitters can achieve highly efficient OLEDs with external EL quantum efficiencies (EQEs) comparable to those obtained with phosphorescent dopants. 6 The use of TADF emitters has the advantage of realizing highly efficient OLEDs without rare metal complexes. Thus, much effort has been devoted to improve the luminescence efficiency of TADF emitters. 715In this study, we report a molecular design strategy for highly efficient TADF emitters. TADF emitters with linear donoracceptordonor (DAD) structures have a small energy difference between the excited singlet and triplet states and a large oscillator strength, which lead to high luminescence efficiency. The DAD-type TADF emitter 2,6-bis[4-(10H-phenoxazin-10-yl)phenyl]benzo[1,2-d:5,4-d¤]bis(oxazole) (cis-BOX2) has a photoluminescence quantum yield (PLQY) of 98% in a solid-state host layer. An OLED containing cis-BOX2 as a dopant exhibits an EQE of 17.6%.In TADF processes, because the lowest triplet state (T 1 ) is converted into the lowest singlet excited state (S 1 ) via thermal activation, minimizing the energy difference between S 1 and T 1 (¦E ST ) is necessary to induce efficient T 1 ¼ S 1 upconversion. In fact, small ¦E ST in TADF emitters has been realized by spatially separating their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). 16 However, decreasing the spatial overlap often reduces the transition dipole moment, i.e., the oscillator strength ( f ) between S 1 and S 0 , corresponding to a reduced rate of S 1 ¼ S 0 radiative decay (k r ). This trade-off between a small ¦E ST and large f should be solved to establish molecular design parameters for highly efficient TADF emitters. By extending a linear donoracceptor (DA)-type structure into a linear DAD-type, we achieved compatibility betwe...
Two novel electron-transport materials (ETMs), 2,3′-Bpy-TP and 2,4′-Bpy-TP, with high horizontal molecular orientation to substrates, were synthesized. It was shown by measuring the IR absorption spectra of their deposited thin films that C–H···N hydrogen bonds are formed between the 2,3′-Bpy-TP molecules and between the 2,4′-Bpy-TP molecules. It was also shown that there is a closed relationship between their molecular orientations and the driving voltages of electron-only devices (EODs) using them as electron-transport layers (ETLs).
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