Efficient selenium-integrated TADF OLEDs with reduced roll-off

Organic light emitters based on multiresonance-induced thermally activated delayed fluorescent materials have great potential for realizing efficient, narrowband organic light-emitting diodes (OLEDs). However, at high brightness operation, efficiency roll-off attributed to the slow reverse intersystem crossing (RISC) process hinders the use of multiresonance-induced thermally activated delayed fluorescent materials in practical applications. Here we report a heavy-atom incorporating emitter, BNSeSe, which is based on a selenium-integrated boron–nitrogen skeleton and exhibits 100% photoluminescence quantum yield and a high RISC rate (kRISC) of 2.0 × 106 s−1. The corresponding green OLEDs exhibit excellent external quantum efficiencies of up to 36.8% and ultra-low roll-off character at high brightnesses (with very small roll-off values of 2.8% and 14.9% at 1,000 cd m−2 and 10,000 cd m−2, respectively). Furthermore, the outstanding capability to harvest triplet excitons also enables BNSeSe to be a superior sensitizer for a hyperfluorescence OLED, which shows state-of-the-art performance with a high excellent external quantum efficiency of 40.5%, power efficiency beyond 200 lm W−1, and luminance close to 20,0000 cd m−2. Green OLEDs based on BNSeSe offer high operational efficiency and reduced efficiency roll-off.

Organic light-emitting diodes (OLEDs) with simultaneously high efficiency and narrowband emission become increasingly important to the demand for energy-saving and high-quality displays. Thanks to the pioneering work by Hatakeyama and colleagues 1,2 , multiresonance thermally activated delayed fluorescent (TADF) emitters have emerged with a narrowband emission that could fulfill such requirements. A high external quantum efficiency (EQE) of up to 34% and an electroluminescence with a full-width at half-maximum (FWHM) of 18 nm demonstrated their great potential towards practical applications 3 . However, multiresonance-induced thermally activated delayed fluorescene (MR-TADF) emitters usually possess long delay lifetimes of several tens of microseconds, which usually lead to large efficiency roll-off at high brightnesses, impeding their commercialization [4][5][6][7][8][9][10] .
To reduce efficiency roll-off, both singlet and triplet excitons should be converted expeditiously. The intrinsic large Frank-Condon overlap integral ensures MR-TADF emitters with a fast radiative rate of singlet excitons. Although the recycle of triplet excitons depends on the reverse intersystem crossing (RISC) channel, and thus the rate of reversed intersystem crossing (k RISC ) becomes the key factor to realize low efficiency roll-off [11][12][13][14] . According to Fermi's golden rule, k RISC between two states is described as [15][16][17][18] where <S|Ĥ SOC |T> is the spin-orbit coupling (SOC) matrix element, ΔE ST is the energy difference between the corresponding states, k B is the Boltzmann constant and T is the temperature. To enhance k RISC , one of the practicable strategies is narrowing ΔE ST to provide a smaller energy gap, which benefits the up-conversion process from the low-energy Article https://doi.org/10.1038/s41566-022-01083-y that the introduction of selenium may immensely promote SOC and in turn accelerate the RISC processes.

Crystallographic and photophysical properties
As shown in the crystal structures of BNSSe and BNSeSe, molecules exhibit rigid configurations, which are beneficial for suppressing non-radiative transitions (Supplementary Fig. 12 and 13). Due to the folded configurations of phenothiazine and phenoselenazine units, molecules in BNSSe and BNSeSe crystals are twisted and stacked loosely. The photophysical properties of BNSSe and BNSeSe-including absorption, fluorescence at 298 K and phosphorescence spectra at 77 K-were measured in toluene, as collected in Fig. 2a Table 1 , a universal bipolar host with a hole mobility of 1.03 × 10 −4 cm 2 V -1 s -1 and an electron mobility of 7.35 × 10 −4 cm 2 V -1 s -1 at an electric field of 1.2 × 10 6 V cm -1 (ref. 37 ). As depicted in Fig. 2c, all four doped films exhibit green emission with slight bathochromic shifts with respect to their toluene solution. Photoluminescence decay curves of those films are shown in Fig. 2d. Both τ PF and τ DF drop in the sequence: 2PXZ BN > 2PTZBN > BNSSe > BNSeSe. Furthermore, Φ PL values of 2PXZBN, 2PTZBN, BNSSe and BNSeSe were measured to be 71%, 91%, 99% and 100%, respectively, with an obvious escalating trend as τ DF shortened. The improved Φ PL values of the 1-wt%-DMIC-TRZ-doped BNSSe and BNSeSe films are also associated with their large torsional angles to enlarge intermolecular distance and suppress aggregation-caused quenching. The radiative decay rate constants of fluorescence (k r,S ), intersystem crossing (k ISC ) and k RISC are further analysed-using a method provided in the literature-on the basis of these lifetimes and quantum yields. With the increasing number of selenium atoms, k RISC is greatly enhanced from 6.0 × 10 5 s −1 for BNSSe to 2.0 × 10 6 s −1 for BNSeSe, noting that the latter is, to the best of our knowledge, the largest value among all MR-TADF emitters (typically in the order of 10 3 -10 5 s −1 ; see Supplementary Table 10). Such a tremendous enhancement to k RISC is in line with the enhanced SOCs.
In this context we are driven to develop MR-TADF emitters with fast RISC, which not only facilitates their emissive properties as emitters but also ensures that they are excellent sensitizers for highly efficient hyperfluorescence OLEDs.

Molecular design and theoretical calculations
The molecules designed in this contribution are presented in Fig. 1a. A conventional boron-nitrogen skeleton was chosen for evaluation due to their high photoluminescence quantum yield (Φ PL ) and narrowband emission. Selenium (Z N = 34) is inserted to enhance SOC in BNSSe and BNSeSe. The asymmetric BNSSe was developed to establish a comparison and comprehend the structure-property relationship. Their oxygen and sulfur counterparts (2PXZBN, 2PTZBN, respectively) were also prepared to fully validate our molecular design. We first used density functional theory (DFT) under the representative B3LYP/6-31G(d,p) set to optimize the ground-state geometries of the four molecules. All four show well-separated HOMO and LUMO distributions. The more twisted structures (see Supplementary  Fig. 11) of BNSSe and BNSeSe (compared with 2PXZBN and 2PTZBN) may induce less dense packing to avoid inter-chromophore interactions. According to the time-dependent DFT analysis, high oscillator strengths beyond 0.2 and relatively small ΔE ST values are acquired in these four emitters, implying a fast radiative decay and potential TADF activity. To more accurately model the excited-state energies by considering electron correlation in the form of double excitations, suitably higher-level SCS-CC2 and ADC(2) calculations were conducted, which provided ∆E ST values of only ~0.12 eV for BNSSe and BNSeSe (Supplementary Table 2). To evaluate the internal heavy atom effect, their SOC matrix elements S 1 -T 1 , S 1 -T 2 and S 1 -T 3 were further evaluated using PySOC 36 . As depicted in Fig. 1a, both 2PXZBN and 2PTZBN show small SOC values; they exhibit very tiny SOCs between S 1 and T 1 (<S 1 |Ĥ SOC |T 1 > = 0.079 and 0.082 cm −1 , respectively). By sharp contrast, substantial enhancements are realized in selenium-containing BNSSe and BNSeSe (<S 1 |Ĥ SOC |T 1 > = 1.580 and 1.431 cm −1 , respectively-nearly 20-times higher than that of 2PXZBN). Spin-orbit couplings between S 1 and T n (n = 2 or 3) for BNSSe and BNSeSe are also much larger than those for 2PXZBN and 2PTZBN (for example, <S 1 |Ĥ SOC |T 2 > of 2.060 cm −1 and <S 1 |Ĥ SOC |T 3 > of 0.905 cm −1 are obtained for BNSSe). Better yet, the values of <S 1 |Ĥ SOC |T 2 > and <S 1 |Ĥ SOC |T 3 > for BNSeSe increase to 2.840 and 2.110 cm −1 , respectively, which are higher than those of its sulfur analogue (2PTZBN, <S 1 |Ĥ SOC |T 2 > = 1.510 cm −1 and <S 1 |Ĥ SOC |T 3 > = 1.112 cm −1 ). These results theoretically support our molecular design, suggesting  Table 2 and Supplementary Table 8, and selectively presented in Fig. 3. Devices A-D all exhibit green electroluminescence peaks at 517, 520, 515 and 512 nm, respectively. Narrowband emission with FWMHs of 50 nm and 48 nm were recorded for devices C and D, respectively. Attributed to the host with dipole charge transport ability, all devices exhibit low turn-on voltages below 2.5 V and extremely high luminances of over 100,000 cd m -2 . All devices exhibit excellent performance, including high maximum EQEs (EQE max ), power efficiencies (PE max ) and current efficiencies (CE max ). As shown in Fig. 3c- 2PXZBN: BNSSe: X 1 = S; X 2 = Se BNSeSe: X 1 = X 2 = Se  To the best of our knowledge, the PEs and CEs of devices based on 2PTZBN, BNSSe and BNSeSe are higher than almost all reported TADF OLEDs. Moreover, devices C and especially D exhibit distinctly reduced efficiency roll-off compared with devices A and B, which suggests that the stronger SOCs between S 1 and T n (n = 1, 2, 3) in BNSSe and BNSeSe suppress exciton annihilation more efficiently under high current densities. It is worth mentioning that the efficiency roll-off of device D is much smaller than those observed in the latest narrowband TADF OLEDs 7,9,[38][39][40][41] ; the EQEs of device D remain 34% at 1,000 cd m -2 , 26.9% at 5,000 cd m -2 and 21.9% at 10,000 cd m -2 . We assume that the very strong SOC and subsequent high k RISC value suppress the triplet-involved annihilation processes in the device (see Supplementary Table 10). The operational lifetimes of devices A−D were preliminarily measured at an initial luminance of 1,000 cd m -2 . Device A shows an LT 50 (the time to reach 50% of the initial luminance) of 158 h, whereas devices B, C and D exhibit shorter LT 50 values of 5.6, 4.8 and 4.1 h, respectively ( Supplementary Fig. 25).
Due to the short delayed fluorescence lifetime, high Φ PL and outstanding electroluminescence performance, we anticipate BNSeSe should be a promising TADF sensitizer candidate for a low-energy emitter. To prove it, one yellow emission MR-TADF emitter BN3 42    The average device parameters in parentheses are based on the measurement of over fifteen independent devices. HF, hyperfluorescence.   BNSeSe-sensitized device E exhibits lower turn-on voltage below 2.4 V, which should be attributed to the better energy alignment between BNSeSe and DMIC-TRZ. As summarized in Table 2 and depicted in Fig. 4, excellent electroluminescence performance was observed for device F, with an EQE max of 38.7%, CE max of 152.2 cd A -1 , PE max of 170.7 lm W -1 , and maximum luminance (L max ) of 154,424 cd m -2 . However, device F shows obvious efficiency roll-off at high luminance-the EQE suddenly drops to 20.4% at 1,000 cd m -2 and 11.9% at 5,000 cd m -2 . By sharp contrast, after introducing BNSeSe as a TADF sensitizer in EML, device E exhibits higher EQEs as well as much smaller efficiency roll-off than device F. An enhanced CE max of 164.5 cd A -1 , PE max of 205.8 lm W -1 and L max of 191,023 cd m -2 were observed for device E. Such high values are never reported in TADF OLEDs. Furthermore, the EQE max of device E is boosted to 40.5%, and the EQEs remain 32.4% at 1,000 cd m -2 and 26.1% at 5,000 cd m -2 . Even at the very high brightness of 10,000 cd m -2 , device E maintains a high EQE of 23.3%. We attributed the improved roll-off to the higher k FRET between BNSeSe and BN3 compared with the k nr + k ISC and k r of BNSeSe in the hyperfluorescence system (Supplementary Fig. 20), which leads to the singlet excitons upconverted from the triplet excitons in BNSeSe being quickly consumed by BN3 through FRET. The efficient energy transfer from BNSeSe to BN3 hinders the ISC process in BNSeSe, avoiding the triplet excitons stack at high luminance. Furthermore, the emission luminance of device E decreased from 1,000 cd m -2 to 500 cd m -2 for 51 h without emission colour change ( Supplementary Fig. 25), and the FWHM remains 40 nm at 1,000 cd m -2 with marginal enlargement compared with device F. To pursue narrower FWHM in hyperfluorescence device, the terminal emitter BN3 was replaced by a green-emissive dopant DtCzB-DPTRZ with narrower FWHM (23 nm in toluene solution with a concentration of 10 -5 mol l -1 ) 7 . The corresponding BNSeSe-sensitized device (G) not only displays a Article https://doi.org/10.1038/s41566-022-01083-y FWHM of only 31 nm, but also manifests very high EQEs of up to 39.6%, which remains at 34.5% at 1,000 cd m -2 and 24.3% at 10,000 cd m -2 , greatly outperforming those of the sensitizer-free control device (H) (Supplementary Fig. 23). The EQE max values are well supported by optical simulations ( Supplementary Fig. 24b,c and Supplementary Table 9). The state-of-the-art performance of BNSeSe-sensitized devices validates our selenium-incorporated MR-TADF material to be ideal sensitizer for MR-TADF emitters with long delayed lifetime. Notably, this is the first example to employ MR-TADF sensitizer for OLEDs, which will greatly contribute to practical applications.

Conclusion
In

Theoretical calculations
Quantum chemical calculations were performed by the Gaussian 09 program package 43 . Density functional theory was performed at the B3LYP/6-31G(d,p) level to attain the optimized molecular geometries. The HOMO and the LUMO were obtained logically on the basis of the optimized geometric configurations. Time-dependent DFT calculations were performed at the B3LYP/6-31G(d,p) level to obtain the vertical transitions, as well as the natural transition orbitals of the singlet and triplet states on the basis of the corresponding S 0 geometries. The SDD pseudopotential was used for selenium atoms; SCS-CC2 and ADC(2) calculations were performed using the MRCC program with the cc-pVDZ basis set 44,45 . The SOC calculations were further evaluated using PySOC. The optical simulation of OLED devices was performed using the SETFOS 5.1 (Fluxim) program. The input parameters include refractive index value, extinction coefficient, thickness of each layer values (all measured by ellipsometry), as well as photoluminescence spectrum of the emitting layer.

Photophysical measurements
Ultraviolet-visible absorption and photoluminescence spectra were measured using a Shimadzu UV-2700 spectrophotometer (Shimadzu) and Hitachi F-7100 fluorescence spectrophotometer (Hitachi), respectively. Phosphorescence spectra were recorded on the Hitachi F-7100 fluorescence spectrophotometer at 77 K. The transient photoluminescence decay curves were obtained by FluoTime 300 (PicoQuant GmbH) with a Picosecond Pulsed UV-LASER (LASER375) as the excitation source. The values of Φ PL were measured with a Hamamatsu UV-NIR absolute photoluminescence quantum yield spectrometer (C13534, Hamamatsu Photonics) equipped with a calibrated integrating sphere, the integrating sphere was purged with dry argon to maintain an inert atmosphere.

Device fabrication and performance measurements
The ITO-coated glass substrates with a sheet resistance of 15 Ω square -1 were consecutively ultrasonicated with acetone/ethanol and dried with nitrogen gas flow, follwed by 20 min ultraviolet light-ozone treatment in an ultraviolet-ozone surface processor (PL16 series, Sen Lights Corporation). The sample was then transferred to the deposition system. Both 8-hydroxyquinolinolato-lithium (Liq) and alumina as electron injection and cathode layers, respectively, were deposited by thermal evaporation at 5 × 10 -5 Pa. Furthermore, the organic layers were deposited at rates of 0.2-3 Å s -1 . After the organic film deposition, the Liq and alumina layers were deposited at rates of 0.1 and 3 Å s -1 , respectively. The emitting area of the device is about 0.09 cm 2 . The current densityvoltage-luminance (J-V-L), L-EQE curves and electroluminescence spectra were measured using a Keithley 2400 source meter and an absolute EQE measurement system (C9920-12, Hamamatsu Photonics).

Data availability
The data that support the plots within this paper and other findings of this study are available from the corresponding author on reasonable request. Source Data are provided with this paper.