Highly Efficient Thermally Activated Delayed Fluorescence Emitters with a Small Singlet–Triplet Energy Gap and Large Oscillator Strength

Abstract

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...

Highly efficient electroluminescence from purely organic donor–acceptor systems

“…DAC-BTZ is a purely organic donor-acceptor-type molecule, in which N 3 ,N 3 ,N 6 ,N 6 -tetraphenyl-9H-carbazole-3,6-diamine (DAC-II) serves as an electron-donating unit and 2-phenylbenzo[d]thiazole (BTZ) acts as an electron-accepting unit. BTZ has good electron-accepting ability and appears in other TADF emitters [36,37]. DAC-II has great potential as an electron-donating unit to realize highly efficient TADF emitters because it improves the compatibility between fast k RISC and k r , and enhances TADF [41].…”

“…3a). ΔE ST of TADF emitters have been elucidated by various methods including the energy difference between the onsets of fluorescence and phosphorescence spectra [17, 18, 20, 23-25, 29-31, 34, 37, 38, 41, 43, 44], the difference between peak wavelengths of fluorescence and phosphorescence spectra [11,40,42,58], the temperature dependence of k RISC [7,12,21,36], and Berberan-Santos plots [14,15,32]. The method based on shift between fluorescence and phosphorescence spectra is valid when the energies of the FranckCondon states reached in 1* CT → S 0 and 3* CT → S 0 emissions are same.…”

Light blue and green thermally activated delayed fluorescence from 10H-phenoxaborin-derivatives and their application to organic light-emitting diodes

“…In order to improve η int of the OLEDs, phosphorescent materials consisting of organic-heavy metal complexes have developed because intersystem crossing (ISC) from singlet excited states to triplet excited states and radiative triplet decay rates are effectively enhanced by high spin-orbit coupling induced by the heavy metal, resulting in theoretical 100% η int . 10 As all of the excitons generated by electrical excitation can be converted into fluorescent luminescence, TADF materials such as cuprous complexes, 11 zinc complexes, 12 and metal-free organic molecules [13][14][15][16][17][18][19][20] have been developed in the past few years. 7 Hybridized local and charge-transfer (HLCT), which harvest triplet excitons via the reverse intersystem crossing (RISC) between high-lying triplet CT excited states ( 3 CT x ) and singlet CT excited state ( 1 CT x ), can realize theoretically 100% η int .…”

Review of organic light-emitting diodes with thermally activated delayed fluorescence emitters for energy-efficient sustainable light sources and displays

“…4, the vast majority of all published organic TADF emitters contain substituted carbazole- [56][57][58][59] or arylamine-type donors. [60][61][62] In most cases, the connection to the acceptors is realized via the aromatic nitrogen. On the acceptor-side (Fig.…”