Near-IR organic light-emitting diodes (NIR-OLEDs) are potential light-sources for various sensing applications as OLEDs have unique features such as ultraflexibility and low-cost fabrication. However, the low external electroluminescence (EL) quantum efficiency (EQE) of NIR-OLEDs is a critical obstacle for potential applications. Here, we demonstrate a highly efficient NIR emitter with thermally activated delayed fluorescence (TADF) and its application to NIR-OLEDs. The NIR-TADF emitter, TPA-PZTCN, has a high photoluminescence quantum yield of over 40 % with a peak wavelength at 729 nm even in a highly doped codeposited film. The EL peak wavelength of the NIR-OLED is 734 nm with an EQE of 13.4 %, unprecedented among raremetal-free NIR-OLEDs in this spectral range. TPA-PZTCN can sensitize a deeper NIR fluorophore to achieve a peak wavelength of approximately 900 nm, resulting in an EQE of over 1 % in a TADF-sensitized NIR-OLED with high operational device durability (LT 95 > 600 h.).

By simple modification of the functional groups on the boron–nitrogen‐containing skeleton, the energy level of the highest occupied molecular orbital (EHOMO) of emitters can be easily adjusted. Blue‐emission derivatives are developed, which are capable of showing small full width at half maximums and high photoluminescence quantum yields. Blue thermally activated delayed fluorescence (TADF)‐assisted fluorescence organic light‐emitting diodes (TAF‐OLEDs) based on two new emitters as the terminal emitter are fabricated, resulting in high external quantum efficiency (EQE) of up to 21.9%, high color purity, and high brightness (Lmax = 63 777 cd m−2). By analyzing the transient electroluminescence spectra of the TAF‐OLEDs, it is found that a smaller EHOMO difference between TADF‐assistant dopant (TADF‐AD) and terminal emitter efficiently helps to decrease hole trapping inside the emitting layer, hence resulting in a lower efficiency rolloff and a longer operational device lifetime. TAF‐OLEDs based on CzBNCz as a terminal emitter having the closest EHOMO to that of TADF‐AD show a maximum EQE of 21.9% together with a reduced efficiency rolloff (EQEs of 21.2% and 19.8% at 100 and 1000 cd m−2, respectively). This research provides a designing principle for a terminal emitter in TAF‐OLEDs with well‐matched energy levels towards reaching the requirements of commercial displays.

Three thermally activated delayed fluorescence (TADF) molecules, namely PQ1, PQ2, and PQ3, are composed of electron‐accepting (A) tetrabenzo[a,c]phenazine (TBPZ) and electron‐donating (D) phenoxazine (PXZ) units are designed and characterized. The combined effects of planar acceptor manipulation and high steric hindrance between D and A units endow high molecular rigidity that suppresses nonradiative decay of the excitons with improved photoluminescence quantum yields (PLQYs). Particularly, the well‐aligned excited states involving a singlet and a triplet charge‐transfer excited states and a localized excited triplet state in PQ3 enhances the reverse intersystem crossing rate constant (kRISC) with a short delay lifetime (τd). The orange–red OLED based on PQ3 displays a maximum external EL quantum efficiency (EQE) of 27.4 % with a well‐suppressed EL efficiency roll‐off owing to a completely horizontal orientation of the transition dipole moment in the film state.

Large external quantum efficiency rolloff at high current densities in organic light-emitting diodes (OLEDs) is frequently caused by the quenching of radiative singlet excitons by long-lived triplet excitons [singlet–triplet annihilation (STA)]. In this study, we adopted a triplet scavenging strategy to overcome the aforementioned STA issue. To construct a model system for the triplet scavenging, we selected 2,6-dicyano-1,1-diphenyl-λ5σ4-phosphinine (DCNP) as the emitter and 4,4′-bis[(N-carbazole)styryl]biphenyl (BSBCz) as the host material by considering their singlet and triplet energy levels. In this system, the DCNP’s triplets are effectively scavenged by BSBCz while the DCNP’s singlets are intact, resulting in the suppressed STA under electrical excitation. Therefore, OLEDs with a 1 wt.%-DCNP-doped BSBCz emitting layer demonstrated the greatly suppressed efficiency rolloff even at higher current densities. This finding favourably provides the advanced light-emitting performance for OLEDs and organic semiconductor laser diodes from the aspect of the suppressed efficiency rolloff.

Quenching of singlets by long-lived triplets is a serious issue for lasing from organic laser dyes, especially under long pulse excitation duration. As a strategy to scavenge or manage unnecessary triplets, an organic laser dye is dispersed into a host material having high singlet and low triplet energy levels [a large singlet-triplet energy gap (ΔE ST )]. However, finding such a host material having a triplet scavenging capability is limited. In this study, we synthesized an organic laser dye, 2,6-dicyano-1,1-diphenyl-λ 5 σ 4 -phosphinine (DCNP), having a small ΔE ST of ~0.44 eV, and thus we were able to employ 4-4΄-bis[(N-carbazole)styryl] biphenyl (BSBCz) as a triplet scavenging host, i.e., the triplets formed on DCNP are easily transferred to BSBCz. A 1 wt.%-DCNP-doped BSBCz film was formed on a mixed-order distributed feedback grating, showed lasing with a low threshold value of ~0.86 µJ cm −2 and a FWHM of ~0.5 nm. Because of the suppressed singlet-triplet annihilation, we demonstrated DCNP-based laser devices operating under continuous-wave operation, with a low threshold of 72 W cm −2 and a long laser half-lifetime of ~3 min. Our results This article is protected by copyright. All rights reserved.3 demonstrated a possibility of the wider selection of host materials, easing a material design strategy of fabricating high-performance laser devices in future.Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff))