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A simple-structure TADF sensitizer enables high-performance red hyperfluorescent OLEDs with an external quantum efficiency of 40%

Ruihan Zhong, Zhengqi Xiao, Zhanxiang Chen, Ying Gao, Jingsheng Miao, Yang Zou* and Chuluo Yang
Shenzhen Key Laboratory of New Information Display and Storage Materials, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China. E-mail: yangzou@szu.edu.cn

First published on 19th August 2025

Therefore, ideal TADF emitters for cutting-edge displays must exhibit high photoluminescence quantum yield (PLQY), fast k_RISC and narrowband emission spectra. However, to date, only a handful of TADF materials have successfully achieved these key parameters simultaneously, leaving a performance gap for full-color OLED displays that require high efficiency, low efficiency roll-off and color purity.

Aside from the molecular design for emitters, from the perspective of device engineering, hyperfluorescence (HF) technology enables a versatile and effective approach to simultaneously realize high efficiency, color purity, device stability, and low efficiency roll-off.^5,6 In HF OLEDs, the energy from both singlet and triplet excitons harvested by the sensitizer transfers non-radiatively to a terminal narrowband emitter via Förster resonance energy transfer (FRET). So far, by taking advantage of the high PLQY and narrowband emission inherited from the MR emitters, as well as the exceptional exciton harvested by the sensitizer, a few high-performance HF OLEDs have been reported.^7–10 Nevertheless, most of the efficient sensitizers reported so far are noble-metal-based phosphorescent materials, and efficient pure-organic TADF sensitizers are still rare. An effective TADF sensitizer should not only exhibit fast k_RISC to ensure efficient triplet exciton utilization but also exhibit high PLQY, a rapid radiative decay rate (k_r), and a large spectral overlap with the absorption profile of the terminal emitter. Till now, despite the development of several high-performance TADF sensitizers,^5,6,11–14 most of them have exhibited emission confined to the blue or green regions, limiting their compatibility with red-emitting MR terminal emitters. And designing purely organic TADF sensitizers suitable for red emission remains challenging because of the intrinsic trade-off k_r and k_RISC, as well as the energy band gap law that might lead to non-radiative decay for emitters with long-wavelength emission.¹⁵

The 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (BO) moiety is a well-established building block that serves as an electron acceptor in the construction of efficient sensitizers. Since the boron atom in the MR-type BO moiety could locate the LUMO on the atomic scale, BO-based TADF emitters usually displayed fast k_RISC.^16–18 More importantly, the BO moiety could also endow the emitter with fast k_r due to its inherent SR-CT character.^16–19 As a result, a few BO-based TADF sensitizers targeting deep-blue to green terminal emitters have been documented in the literature (Scheme 1). Recently, Duan et al. reported a TADF emitter, namely, 2BOICz, by connecting a trifluoromethyl group decorated MR moiety (CFBO) to the indolocarbazole donor.¹⁶ The emitters demonstrated both a rapid k_RISC of 1.2 × 10⁶ s⁻¹ and a k_r of 5.9 × 10⁷ s⁻¹, along with a well-controlled k_ISC of 0.3 × 10⁷ s⁻¹, enabling high performance yellow HF devices with simultaneous high stability, high device efficiency, and low efficiency roll-off. In this context, aiming to design TADF sensitizers targeting red emitters, herein we designed a TADF sensitizer, namely, CFBOPXZ, by connecting CFBO with the well-known phenoxazine (PXZ) donor. It is anticipated that aside from the high PLQY, rapid k_r and k_RISC inherited from the CFBO subunit by its hybridized SR-LR-CT character, the replacement of the indolocarbazole fragment with PXZ in 2BOICz could induce red-shifted emission (presumably to the orange-red region) due to its stronger electron donating ability, allowing for spectral compatibility with the red terminal emitter in the HF system. As expected, the designed TADF sensitizer exhibited high PLQY and fast k_r and k_RISC in the ideal spectral region. This high-performance TADF sensitizer was then paired with a narrowband red MR emitter in HF devices, in which a high external quantum efficiency (EQE) of 40.0% was achieved, along with excellent device stability and very low efficiency roll-off.

	Scheme 1 Chemical structures and device performance of the BO-based TADF sensitizers.

The chemical structure of CFBOPXZ is shown in Scheme 1. The acceptor moiety could be readily synthesized via nucleophilic substitution followed by the borylation reaction, and it was further connected with PXZ via the Buchwald–Hartwig C–N coupling reaction to obtain the desired product in good yield (Scheme S1 in the SI). The final product was clearly identified by ¹H and ¹³C NMR spectroscopy and high-resolution mass spectrometry (HR-MS) (Fig. S1–S5, SI). CFBOPXZ showed excellent thermal stability as investigated by thermogravimetric analysis (TGA, Fig. S6, SI), as well as a simple structure with a low molecular weight and good thermostability allowing refining and processing by vacuum sublimation.

Computational simulations based on the density functional theory (DFT) calculations were performed to determine the geometrical and electronical properties of the designed emitters at the molecular level. As shown in Fig. 1, the simulated conformation of the molecule adopted a highly twisted conformation with a dihedral angle of 78.5° between the CFBO moiety and the PXZ donor. And the frontier molecular orbitals also showed good spatial separation because of this twisted conformation: the HOMO was mainly distributed on the PXZ moiety, while the LUMO showed MR-like boron-atomically localized nature, which further led to a small singlet–triplet splitting energy (ΔE_ST) of 0.02 eV. According to natural transition orbital (NTO) distributions, both the S₁ and T₁ states revealed dominant LR-CT character, the small hole–electron overlap on the central benzene ring led to a moderate oscillation strength (f = 0.0163), and the corresponding SOCME between S₁ and T₁ (〈S₁|Ĥ_SOC|T₁〉) was 0.04 cm⁻¹. The predicted small ΔE_ST of 0.02 eV of CFBOPXZ indicated good TADF behaviour, which was beneficial for triplet exciton utilization in the EL device.

	Fig. 1 Theoretical calculations. Optimized molecular structure, FMO distribution, related state energy energies, NTOs and SOCMEs of CFBOPXZ acquired from the theoretical calculation.

In dilute toluene solution, CFBOPXZ exhibited strong absorption bands in the UV-vis spectrum (Fig. 2), with features observed at wavelengths ≤280 nm and in the range of 280–350 nm, corresponding primarily to π–π* and n–π* transitions, respectively. Additionally, a broad CT band was detected between 400 and 550 nm. Notably, a sharp absorption peak appeared around 380 nm. Comparative analysis of the absorption spectra of the individual structural fragments of CFBOPXZ (Fig. S7, SI) attributed this distinct peak to the BO acceptor units. This characteristic absorption band originated from the MR-type acceptor moiety and is attributed to its intrinsic SR–CT transition with high oscillator strength. The presence of this feature highlights the contribution of the MR-type SR–CT component to the overall electronic transition from the ground state to the excited states. And because of the stronger electron donating nature of PXZ than that of indolocarbazole, CFBOPXZ showed orange-red emission in toluene solution with the emission maximum at 602 nm, which was red-shifted by 75 nm compared to its indolocarbazole analogue. In addition, CFBOPXZ showed bright yellowish-orange emission in the 1,3-dihydro-1,1-dimethyl-3-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)indeno-[2,1-b]carbazole (DMIC-TRZ) host with a PLQY of 84%. Interestingly, a shoulder peak around 475 nm was observed in the phosphorescence spectra at 77 K, resulting in an apparent negative ΔE_ST of −0.10 eV. This shoulder peak has been previously attributed to the MR-type acceptor fragment and has been observed in certain TADF emitters with the MR fragment.²⁰ The emergence of such higher-lying triplet states can be explained by the substantial energy gap between the T₁ and T₂ states, which suppresses internal conversion at low temperature (77 K). Additionally, the rigid MR fragment possessing SR–CT character likely reduces non-radiative decay pathways, thereby promoting observable phosphorescence.

	Fig. 2 (a) UV-vis absorption (Abs) and fluorescence spectra in toluene (1.0 × 10−5 M) at room temperature; (b) fluorescence (Fl.) spectra at 300 K and phosphorescence (Phos.) spectra at 77 K for the 10 wt% emitter in DMIC-TRZ.

Transient PL decay spectra were measured to investigate the TADF behavior of CFBOPXZ. As depicted in Fig. 3a, CFBOPXZ displayed distinct bi-exponential decay with a prompt fluorescence as well as a delayed fluorescence in degassed toluene solution, and the DF component was almost quenched by oxygen under aerated conditions, which obviously indicated TADF character. Moreover, CFBOPXZ also displayed characteristic double-exponential decay curves in the doped film (10 wt% in DMIC-TRZ, Fig. 3b), and the fitting prompt and delayed fluorescence lifetimes were 36 ns/1.35 μs, together with a delayed ratio of 45%. According to these values, CFBOPXZ not only showed high PLQY but also exhibited a fast k_r of 1.3 × 10⁷ s⁻¹ and a k_RISC of 1.1 × 10⁶ s⁻¹. These parameters were difficult to achieve simultaneously for materials in this spectral region. Of particularly note, CFBOPXZ demonstrated a positive dependence on temperature in the temperature-dependent transient PL decay curves (Fig. 3c), with delayed lifetimes decreased as the temperature increased. Based on the kinetic parameters acquired from temperature-dependent transient PL decay curves (Table S1, SI), the activation energy (ΔE^TADF_a) was determined to be only 18 meV by fitting the data with eqn (S1), meaning that it was a typical D–A-type TADF emitter, rather than an emitter with negative ΔE_ST.

	Fig. 3 (a) Transient PL decay curves in toluene (1.0 × 10−5 M) at room temperature; (b) transient PL decay curves in the DMIC-TRZ host; (c) temperature-dependent transient PL decay curves in the DMIC-TRZ host; (d) ln kRISC as a function of 1000 T−1 at different temperatures.

The high PLQY and obvious TADF character along with broad emission spectra in the orange-red emission region of CFBOPXZ indicated that it was an ideal sensitizer for a red HF device. The HF device was fabricated with the following device configuration: ITO/HAT-CN (5 nm)/TAPC (30 nm)/TCTA (15 nm)/mCBP (10 nm)/emission layer (1 wt% FSBN and 10 wt% CFBOPXZ in DMIC-TRZ, 40 nm)/ANT-BIZ (30 nm)/Liq (2 nm)/Al (100 nm). In the device, dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN) was used as a hole injection layer; 1,1-bis((di-4-tolylamino)phenyl)-cyclohexane (TAPC) and (1-(4-(10-([1,1′-biphenyl]-4-yl)anthracen-9-yl)phenyl)-2-ethyl-1H-benzo[d]-imidazole) (ANT-BIZ) were used as hole- and electron-transport layers, respectively; 1,3-di(9H-carbazol-9-yl)benzene (mCBP) was used as an exciton-blocking layer. In the emission layer, a red MR emitter, namely, FSBN, with varying doping concentrations (0.5–1.2 wt%) was chosen as the terminal emitter due to its good spectral compatibility with CFBOPXZ (Fig. S8, SI).²¹ The control EL device without the FSBN terminal emitter and a device without the CFBOPXZ sensitizer were also fabricated for comparison. The chemical structures of the materials used in the EL devices are shown in Fig. S9, SI.

As depicted in Fig. 4 and Fig. S10, SI, and summarized in Table S2, SI, the CFBOPXZ-based device showed a broad EL spectrum (FWHM = 98 nm) similar to its PL spectra with a λ_EL of 570 nm. Despite a moderate EQE_max of 26.3% because of the low horizontal orientation ratio of CFBOPXZ (Fig. S11a, SI), thanks to the rapid k_RISC of CFBOPXZ, the device exhibited very small efficiency roll-off, maintaining EQEs of 24.5% and 15.9% at brightness of 1000 cd m⁻² and 10000 cd m⁻², respectively. As the terminal emitter was added, the HF device showed characteristic narrowband red emission with a λ_EL of 621 nm and a FWHM of 59 nm from FSBN, and the shoulder peak at ∼570 nm from CFOPXZ was gradually suppressed as the doping concentration increased from 0.5 wt% to 1.2 wt%, implying almost complete energy transfer from the sensitizer to the terminal emitter. The energy transfer from CFBOPXZ to FSBN could be evidenced by their PL spectra, in which the characteristic emission from CFBOPXZ at 570 nm was almostly quenched upon adding 1 wt% FSBN into the HF system, showing narrowband red emission from FSBN (Fig. S12, SI). In addition, upon the addition of 1 wt% of FSBN, the PL lifetimes were shortened (Fig. S13, SI). By employing eqn (S2), the rate of FRET (k_FRET) in the EMLs containing 1 wt% of FSBN and 10 wt% sensitizer was calculated to be as high as 3.4 × 10⁷ s⁻¹.

	Fig. 4 (a) Device configuration and the energy level diagrams; (b) EL spectra; (c) EQE versus luminance curves; (d) luminance decay measured at the initial brightness of 1000 cd m−2 of the EL devices.

Benefiting from the excellent exciton utilization of the sensitizer, the HF device realized the highest EQE_max of 40.0% with 1 wt% of FSBN, which was not only higher than that of a sensitizer-free device but also almost twice that of the previously reported pure-organic red HF device. The high EQE closely matched theoretical efficiencies predicted by optical simulations (Fig. S11b, SI), which were attributed to near-unity PLQY of the EML, the high horizontal dipole orientation ratio up to 99% of FSBN (Fig. S14, SI), as well as the advanced device structure. A further increase in the doping concentration to 1.2 wt% led to a decrease in device performance probably due to aggregation caused quenching and exciton trapping effect. Moreover, compared to the sensitizer-free device with the EQE declined to only 6.5% at the brightness of 10000 cd m⁻², the HF device demonstrated remarkably reduced efficiency roll-off: maintaining the EQE of 29.3% and 17.0% at the brightness of 1000 cd m⁻² and 10000 cd m⁻², respectively. Importantly, because of the heavy-atom-free chemical structure of CFBOPXZ, these EL devices exhibited good device operational stability. Specifically, at the initial brightness of 1000 cd m⁻², the LT₅₀ value (the time required for the luminance to drop to 50% of the initial value) was as large as 1447 hours for CFBOPXZ-based devices (Fig. 4d), which was much more stable than that of the previously reported sensitizer (an LT₅₀ of only 45 h).¹⁶ Even more, the HF device demonstrated further improved device stablity with an LT₉₀ of 90 h. The improved stability of the HF device could be attributed to the efficient exciton transfer from the sensitizer to the highly stable FSBN terminal emitter, which avoided the undesired high-energy triplet exciton accumulation on the CFBOPXZ.

In conclusion, by connecting a strong BO-based acceptor with the phenoxazine donor, a simple-structure pure-organic TADF sensitizer targeting red emission was successfully designed and synthesized. The sensitizer exhibited excellent optical properties including high PLQY, rapid radiative decay and reverse intersystem crossing rates in the ideal orange-red spectral region. Consequently, pure-organic red HF OLEDs employing the developed sensitizer achieved remarkable device performance, with an EQE_max of 40.0% and low efficiency roll-off, maintaining an EQE of 17.0% even at a very high brightness of 10000 cd m⁻², along with excellent operational stability. This study provides a novel and impactful strategy to design high-performance red sensitizers for full-color OLED display technologies.

We gratefully acknowledge financial support from the National Natural Science Foundation of China (52130308), the Guangdong Basic and Applied Basic Research Foundation (2025A1515010777), the Shenzhen Science and Technology Program (ZDSYS20210623091813040 and JCYJ20220818095816036), and the Research Team Cultivation Program of Shenzhen University (2023DFT004). We also thank the Instrumental Analysis Center of Shenzhen University for analytical support.

Conflicts of interest

Data availability

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