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 (∆E_ST) 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 (k_r)^[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 high k_r and high rate constant of reverse intersystem crossing (k_RISC).

In 2015, Hetakeyma et 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 (EQE_max) 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 EQE_max 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 poor k_RISC values (~10⁴ s^–1) and severe efficiency roll-off at high current densities^[8].

The strategies for alleviating the efficiency roll-off in MR-TADF OLEDs are as follows: (1) hyperfluorescence sensitization (HFS) by using TADF materials with high k_RISC; (2) introducing "heavy atoms" like S or Se into the skeleton; (3) extending charge delocalization by fusing rigid skeleton. In 2019, Adachi et al. designed HFS OLEDs based on ν-DABNA^[9] and hetero-donor-type TADF material (HDT-1) with accelerated S₁ energy transfer process^[10]. A high k_RISC (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, Duan et al. fused aza-aromatics into B/N skeleton and synthesized a pure-green AZA-BN emitter (λ_PL = 522 nm, FWHM = 28 nm)^[11] (Fig. 1(c)). Benefitting from efficient HFS mechanism, HFS OLEDs displayed a higher EQE_max 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.

According to Fermi’s golden rule, the k_RISC in TADF systems mainly depends on spin-orbit coupling (SOC) and energy splitting between S₁ and T₁ states, as expressed in equation: k_RISC $\propto$ |<S₁|Ĥ_SOC|T₁>/ΔE_ST|^{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 <S₁|Ĥ_SOC|T₁> value of 0.31 cm^–1 and a high k_RISC 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 Yang et al.^[15] k_RISC 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, Yasuda et al. doped Se atom into MR-TADF emitter (CzBSe)^[16], yielding a record k_RISC exceeding 10⁸ s^–1 (Fig. 2(a)). Benefitting from its ultrafast triplet-exciton up-conversion, OLEDs with CzBSe offered an EQE_max of 23.9%, with narrow blue emission (λ_EL = 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, Zheng et 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. ∆E_ST 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, k_RISC for NBO and NBNP are nearly an order of magnitude higher than that of BBCz-SB. Consequently, NBO- and NBNP-based OLEDs showed EQE_max of 26.1% and 28.0%, with low efficiency roll-off. To further enhance the CT state of MR-TADF emitters, Zheng et 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, ∆E_ST values were close to 0 eV and large <S₁|Ĥ_SOC|T₁> values were obtained. As a result, high k_RISC values were also achieved, and VTCzBN and TCz-VTCzBN-based OLEDs showed EQE_max 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, enhancing k_RISC 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.

Acknowledgements: This work was supported by the National Natural Science Foundation of China (21975119). L. Ding thanks the open research fund of Songshan Lake Materials Laboratory (2021SLABFK02) and the National Natural Science Foundation of China (51922032 and 21961160720).