The strategies for alleviating the efficiency roll-off in MR-TADF OLEDs are as follows: (1) hyperfluorescence sensitization (HFS) by using TADF materials with highkRISC; (2) introducing "heavy atoms" like S or Se into the skeleton; (3) extending charge delocalization by fusing rigid skeleton. In 2019, Adachiet al. designed HFS OLEDs based onν-DABNA[9] and hetero-donor-type TADF material (HDT-1) with accelerated S1 energy transfer process[10]. A highkRISC (9.2 × 105 s–1) was obtained in doped ternary film. Compared with host-guest type devices, sensitized pure-blue TADF OLEDs showed higher EQE and small efficiency roll-off, and the EQE reached 32% at 1000 cd/m2. Later, Duanet al. fused aza-aromatics into B/N skeleton and synthesized a pure-green AZA-BN emitter (λPL = 522 nm, FWHM = 28 nm)[11] (Fig. 1(c)). Benefitting from efficient HFS mechanism, HFS OLEDs displayed a higher EQEmax of 31.6% and smaller efficiency roll-off than non-sensitized devices[12]. Obviously, through the intervention of TADF sensitizer, the ternary emitting layer showed a more efficient triplet-exciton up-conversion rate.
Thermally activated delayed fluorescence (TADF) emitter is a promising organic light-emitting diode (OLED) material due to low cost, wide luminous color gamut and 100% exciton utilization efficiency[1]. To achieve high TADF performance, a feasible strategy is to construct a twisted donor–acceptor (D–A) unit, decreasing the overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and minimizing the energy gap (∆EST) between the lowest singlet (S1) and triplet (T1) states[2,3]. However, this long-range charge transfer feature is often disadvantageous for achieving high oscillator strengths (f) and radiative transition rates (kr)[4] (Fig. 1(a)). Moreover, common TADF emitters always display broad electroluminescence spectra, whose full-widths at half-maximum (FWHMs) are 70–100 nm[5]. Therefore, it is necessary to realize a narrow-band emission system, which can improve the display quality greatly, with highkr and high rate constant of reverse intersystem crossing (kRISC).
![(Color online) (a) Traditional design strategy for TADF. Reproduced with permission[6], Copyright 2016, Wiley-VCH. (b) Design strategy for MR-TADF. Reproduced with permission[6], Copyright 2016, Wiley-VCH. (c) New MR-TADF skeletons with fused aza-aromatics. Reproduced with permission[11], Copyright 2020, Wiley-VCH.](/Images/icon/loading.gif)
Figure 1.(Color online) (a) Traditional design strategy for TADF. Reproduced with permission[6], Copyright 2016, Wiley-VCH. (b) Design strategy for MR-TADF. Reproduced with permission[6], Copyright 2016, Wiley-VCH. (c) New MR-TADF skeletons with fused aza-aromatics. Reproduced with permission[11], Copyright 2020, Wiley-VCH.
In 2015, Hetakeymaet al. developed a rigid polycyclic aromatic framework based on B/N system with opposite multiple-resonance (MR) effect for the first time, offering narrowband emission and efficient TADF performance[6] (Fig. 1(b)). In the emitter DABNA-1 with a highly rigid framework, the cofacial backbone resulted in short-range charge transfer, giving a high PLQY of 88% and a small FWHM of 30 nm in doped film. The corresponding OLEDs with 1 wt% doping offered a maximum external quantum efficiency (EQEmax) of 13.5% and a FWHM of 28 nm. Through modifying peripheral benzene ring and diphenylamine, the emission peak of DABNA-2 was slightly red-shifted and the OLEDs exhibited a FWHM of 28 nm and an EQEmax of 20.2%. In terms of device efficiency and color purity, it is superior to previous commercial blue emitters[7], and it also has potential to replace current commercial blue fluorescent materials as the core of OLEDs. Although MR-TADF emitters have achieved nearly full-color emission, this class of materials tends to exhibit poorkRISC values (~104 s–1) and severe efficiency roll-off at high current densities[8].
![(Color online) (a) Molecular structures of CzBO, CzBS, and CzBSe with different chalcogens, conventional TADF mechanism and ideal superimposed fluorescence (SF) mechanism. Reproduced with permission[16], Copyright 2022, Wiley-VCH. (b) Double resonance unit superposition strategy. Reproduced with permission[18], Copyright 2022, Wiley-VCH.](/Images/icon/loading.gif)
Figure 2.(Color online) (a) Molecular structures of CzBO, CzBS, and CzBSe with different chalcogens, conventional TADF mechanism and ideal superimposed fluorescence (SF) mechanism. Reproduced with permission[16], Copyright 2022, Wiley-VCH. (b) Double resonance unit superposition strategy. Reproduced with permission[18], Copyright 2022, Wiley-VCH.
According to Fermi’s golden rule, thekRISC in TADF systems mainly depends on spin-orbit coupling (SOC) and energy splitting between S1 and T1 states, as expressed in equation:kRISC |1|ĤSOC|T1>/ΔEST|2[13]. Recently, Yasuda group developed a fused-nonacyclic π-system (BSBS-N1), embedded with B, N, and S atoms. With “heavy atom” S[14], BSBS-N1 exhibited a big 1|ĤSOC|T1> value of 0.31 cm–1 and a highkRISC of 1.9 × 105 s–1. The corresponding OLEDs offered smaller efficiency roll-off than BBCz-SB LEDs[8]. Similarly, the strategy of using S to improve SOC was further confirmed by Yanget al.[15]kRISC over 105 s–1 was obtained in toluene solution and MR-TADF OLEDs showed smaller efficiency roll-off. To intuitively reflect the influence of heavy atom on RISC, Yasudaet al. doped Se atom into MR-TADF emitter (CzBSe)[16], yielding a recordkRISC exceeding 108 s–1 (Fig. 2(a)). Benefitting from its ultrafast triplet-exciton up-conversion, OLEDs with CzBSe offered an EQEmax of 23.9%, with narrow blue emission (λEL = 481 nm, FWHM = 33 nm) and significantly alleviated efficiency roll-off.
Extending charge delocalization by fusing rigid skeleton is an effective approach to solve efficiency roll-off of MR-TADF OLEDs. By fusing hole-transport units (carbazole, dibenzofuran) into B/N framework, Zhenget al. achieved two π-extended MR-TADF emitters (NBO and NBNP), peaking at 487 and 500 nm with narrow FWHMs of 27 and 29 nm in toluene solutions[17], respectively. ∆EST were reduced (0.12 eV for NBO, 0.09 eV for NBNP)via charge delocalization of frontier orbitals. Meanwhile, SOC values were further improved due to the introduction of O and N heteroatoms. As results,kRISC for NBO and NBNP are nearly an order of magnitude higher than that of BBCz-SB. Consequently, NBO- and NBNP-based OLEDs showed EQEmax of 26.1% and 28.0%, with low efficiency roll-off. To further enhance the CT state of MR-TADF emitters, Zhenget al. adopted double resonance unit superposition strategy and obtained two green MR-TADF emitters (VTCzBN and TCz-VTCzBN) based on indolo[3,2,1-jk]carbazole (ICz) unit and B/N skeletons[18] (Fig. 2(b)), and the emissions peaked at 496 and 521 nm with FWHMs of 34 and 29 nm, respectively. Benefitting from thorough charge delocalization within frontier molecular orbitals, ∆EST values were close to 0 eV and large 1|ĤSOC|T1> values were obtained. As a result, highkRISC values were also achieved, and VTCzBN and TCz-VTCzBN-based OLEDs showed EQEmax of 31.7% and 32.2%, with low efficiency roll-off, respectively. D-TCz-VTCzBN displayed ultra-pure green CIE of (0.22, 0.71), consistent with the green display standard of the National Television System Committee.
In short, enhancingkRISC of MR-TADF emitters is crucial for reducing efficiency roll-off of OLEDs. Some strategies are highlighted, like TADF sensitization, heavy atom introduction, extending charge delocalization. More efforts are needed to develop MR-TADF OLEDs with high EQE, low efficiency roll-off and narrow emission.