Deep-Blue Organic Light-Emitting Diodes for Ultrahigh-Definition Displays

doi:10.21203/rs.3.rs-3489134/v1

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Deep-Blue Organic Light-Emitting Diodes for Ultrahigh-Definition Displays

https://doi.org/10.21203/rs.3.rs-3489134/v1

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published 19 Aug, 2024

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Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials have emerged as promising candidates for next-generation ultra-high definition displays due to their narrowband emission and triplet-harvesting capability. However, achieving optimal color purity and device efficiency for blue MR-TADF emitters has presented challenges. Here we demonstrate an effective approach to attain superior deep-blue molecules by constructing twisted boron/nitrogen/oxygen embedded higher-order fused-ring frameworks with fully resonating structures. The optimized emitter exhibits high rigidity and minimized bonding/anti-bonding character for ultra-sharp emission, along with near-degenerate singlet and triplet states and large spin-orbit couplings for rapid spin-flip. This combination of features allows our emitter to produce deep-blue emission at 458 nm with an exceptionally narrow full-width at half-maximum (FWHM) of 12 nm in solution, and a reverse intersystem crossing rate constant (k_RISC) of 2.60 × 10⁶ s⁻¹, on par with those of heavy-atom-based MR-TADF molecules. The related single unit organic light-emitting diode (OLED) achieves an external quantum efficiency (EQE) of 31.5% at color coordinates of (0.130, 0.050), and sets a new benchmark with its 13 nm FWHM, outperforming conventional light-emitting diodes, perovskite, and quantum-dot devices. Furthermore, the two-unit stacked tandem hyperfluorescence OLED realizes an ultra-high EQE of 74.5% and demonstrates low efficiency roll-off at high luminance. This exceptional performance represents a significant advancement in the quest to balance efficiency and color purity in the deep-blue region, marking an important step toward power-efficient ultrawide color gamut displays.

Physical sciences/Optics and photonics/Lasers, LEDs and light sources/Organic LEDs

Physical sciences/Materials science/Materials for optics/Lasers, LEDs and light sources/Organic LEDs

The development of deep-blue emitters with high efficiency and ultrapure luminescence is a pivotal challenge that directly impacts the current state and future prospects of organic light-emitting diodes (OLEDs) technique¹. This issue becomes particularly salient as ultrahigh-definition displays (UHD) increasingly necessitate narrow-band organic emitters to meet the growing demand for highly saturated colors. Looking forward, large-area OLED displays may adopt a hybrid approach, combining tandem blue OLEDs with down-conversion red/green quantum dot films. In such configurations, blue OLEDs serve dual roles as both the light source and pumping source, which further emphasizes the importance of high-performance blue emitters.

To date, blue phosphorescent²^-4 and donor-acceptor-type thermally activated delayed fluorescence (TADF) emitters⁵^-7 can attain 100% internal quantum efficiency, but face challenges in maintaining color purity due to structural relaxation and vibrational coupling between the first singlet excited state (S₁) and the ground state (S₀). To design emitters that align intrinsically with the recent BT.2020 blue standard—characterized by Commission Internationale de l'Éclairage (CIE) coordinates of (0.131, 0.046)—it is crucial to concurrently optimize the emission peak, narrow the full width at half maximum (FWHM), and limit spectral tailing⁸. The significance of the latter two criteria arises from the observation that, within the blue spectral region, narrower Gaussian-shaped spectra consistently exhibit lower emission energies relative to broader spectra with equivalent CIE coordinates. This characteristic offers two distinct benefits: first, the shift toward lower emission energies enhances luminous efficiency, aligning with the human eye's sensitivity function⁹; second, it allows for the selection of more stable host materials and sensitizers, thus improving device stability¹⁰. Consequently, the pursuit of BT.2020-compliant deep-blue emitters with exceptionally sharp emission profiles is imperative.

In recent years, the concept of multi-resonance (MR) has emerged as a promising avenue to tackle the challenge of color purity by engineering polycyclic aromatic hydrocarbons (PAHs) with ortho-linked nitrogen/boron atoms¹¹^-13. The rigid fused-ring structure and non-bonding molecular orbitals act to minimize the vibronic progression in emission spectra, typically resulting in a narrow FWHM in the range of 20 to 40 nm¹⁴^-16. Another advantage is the reduction in exchange energy, which leads to a diminished singlet-triplet energy gap (ΔE_ST), thus facilitating TADF. Owing to these notable attributes and the inherently large optical bandgap of MR frameworks, there has been a pronounced focus on exploring blue MR-TADF materials with even narrower emissions that can effectively rival their inorganic counterparts, such as quantum dots¹⁷^-19 and perovskites^20,21. Recent research endeavors have centered on extending the π-conjugation of MR frameworks by introducing multi-boron fused PAHs, a strategy proven effective in generating narrower linewidth, alongside reduced ΔE_ST with enhanced radiative decay rate²²^-26. However, this approach often causes an increase in molecular weight and a red-shift in the emission spectrum, which can impede the vapor deposition device fabrication process and obstruct the realization of deep-blue OLEDs with desirable color purity. At present, the strategy for concurrently realizing deep-blue emitters with FWHMs below 15 nm, and CIE_y less than 0.05 in device remains elusive.

Aside from the spectral challenges, another significant hurdle faced by MR-TADF emitters is their slow reverse intersystem crossing (RISC) process, which leads to severe efficiency roll-offs in devices²⁷. According to Fermi's golden rule, both reducing ΔE_ST and increasing spin-orbit couplings (SOC) between associated singlet and triplet excited states are effective in accelerating RISC²⁸^-30. While the aforementioned linear extension of the MR-scaffold can result in small ΔE_ST values (<0.1 eV), the S₁ and T₁ states often share a similar short-range charge-transfer (SR-CT) exciton character, resulting in an extremely small SOC (<0.1 cm⁻¹) that limits the RISC rate constants (k_RISC) to the order of 10⁴-10⁵ s⁻¹. Although recent demonstrations suggest that incorporating heavy-atom effect³¹^-34 or adopting twisted structures^35,36 can enhance the SOC of MR-TADF systems, the large atomic radius of heavy elements and the increased helical curvature inevitably induce structural changes between ground and excited states, leading to a broader emission spectrum. Consequently, achieving a harmonious balance between color purity and device efficiency remains a formidable challenge in the ongoing development of deep-blue MR-TADF OLEDs.

In this context, we aimed to delineate a systematic approach for achieving superior deep-blue molecules that exhibit both ultrasharp emission and fast exciton dynamics. Distinct from reported emitters that predominantly relied on fully planarized structures, our design integrated a highly distorted, double-boron-based MR-TADF motif into a linearly extended π-skeleton. In this configuration, both a substantial SOC inherited from the twisted core structure and minimized ΔE_ST from the higher-order fused-ring frameworks could be realized, which collectively resulted in a rapid k_RISC of up to 2.60 × 10⁶ s⁻¹. Moreover, through a judicious selection of elements, the emission maximum was finely positioned at 458 nm for the quadruple-borylated emitter, with an exceptionally narrow FWHM as low as 12 nm in its solution state. Our theoretical and experimental findings underscored the critical importance of employing a resonant structure with fully isolated phenyl rings to minimize bonding/anti-bonding character and, consequently, to suppress vibration modes and obtain ultrasharp emission. As a control experiment, a structurally similar emitter was designed with a more flattened geometry by covalently linking two twisted terminal phenyl units, which resulted in a significantly lower k_RISC and broadened emission spectra despite its higher rigidity. Based on our champion material, we achieved a maximum external quantum efficiency (EQE) of 31.6% and a small FWHM of 13 nm at CIE coordinates of (0.140, 0.050) in the corresponding OLED, which satisfied the BT.2020 standard. The utilization of a TADF sensitizer afforded a more efficient deep-blue device with an ultrahigh EQE of 44.7% and a FWHM of 16 nm. Moreover, our two-unit tandem OLED demonstrated a further improvement in EQE, reaching 74.5%, with very low efficiency roll-off at high luminance levels. These results substantiate that our design strategy offers a viable solution to overcome the limitations inherent in previous deep-blue MR-TADF emitters.

Molecular design and theoretical calculation

Our molecular design strategy and associated theoretical prediction results at the B3LYP-B3(DJ)/6-31G(d,p) level are illustrated in Fig. 1. The DPA-B2, bearing the same double-borylated π-skeleton as B2 reported in ref³⁷, was selected as the parent emitter. With reference to the renowned emitter ν-DABNA ²³, DPA-B2 featuring one less nitrogen atom displayed an enlarged energy gap and hypsochromically shifted emission, which is advantageous for preserving the emission peak in the deep-blue region after π-extension. Additionally, the more distorted geometry with the presence of a highly twisted B,N,B-doped [4]helicene, caused by mismatched bond lengths between C-N and C-B bonds, had a strongly positive effect on SOC. This was evident from the substantial improvements in the SOC matrix elements as assessed using PySOC (< S₁|Ĥ_SOC|T₁ > = 0.15 and 0.01 cm^− 1, <S₁|Ĥ_SOC|T₂ > = 1.12 and 0.21 cm^− 1, <S₁|Ĥ_SOC|T₃ > = 0.28 and 0.19 cm^− 1 for DPA-B2 and ν-DABNA, respectively). The elevated SOC values in DPA-B2 arose from the out-of-plane transitions due to the associated changes in the angular momentum. This observation was consistent with state-of-the-art theoretical and experimental studies that underscored the significance of twists/non-planarity in augmenting SOC in graphenoid structures^38–42. The above characteristics render DPA-B2 an ideal candidate for further structural modifications.

Building on DPA-B2, we designed triple- and quadruple-borylated emitters, DPA-B3 and DPA-B4 by incorporating one and two rigid boron/nitrogen/oxygen-embedded triangulene units into the peripheral benzenes, where the boron atoms were linearly aligned with nitrogen/oxygen at their ortho-/para-positions. The inclusion of oxygen aimed to minimize the red-shift, given its lesser contribution to π-conjugation extension compared to nitrogen, due to its lower atomic orbital energy^43–46. Time-dependent density functional theory (TD-DFT) predictions revealed only a marginal decrease in S₁ energy from DPA-B2 (3.12 eV) to DPA-B3 (2.97 eV) and DPA-B4 (2.88 eV), despite the gradually enlarged π-systems. This indicated the success of our molecular design in preserving deep-blue emission. Notably, while linear π-extension enhances the overall structural planarity, the highly distorted geometry of DPA-B2 center remains intact, and the number of twisted regions also rises (Supplementary Fig. 30). With this unique structure, we observed both reduced ΔE_ST due to promoted SR-CT within the conjugation plane (0.32 eV and 0.26 eV for DPA-B3 and DPA-B4, respectively, compared with 0.41 eV for DPA-B2), and retained large SOC induced by the embedded highly twisted helicene subunits (for example, <S₁|Ĥ_SOC|T₁ > = 0.02 cm^− 1, <S₁|Ĥ_SOC|T₂ > = 0.63 cm^− 1 and < S₁|Ĥ_SOC|T₃ > = 0.66 cm^− 1 for DPA-B4, where both T₂ and T₃ lie between S₁ and T₁). To account for electron correlation in double excitations, we also conducted higher-level RI-SCS-CC2 calculations for excited-state energy estimations^22,47. The ΔE_ST values of these compounds decreased progressively with the extension of the π-skeleton, from 0.10 eV for DPA-B2 to 0.04 eV for DPA-B3 and 0.02 eV for DPA-B4, aligning well with TD-DFT calculations and photophysical outcomes (vide infra). The close-to-resonant S₁ and T₁ states and considerable SOC matrix elements in DPA-B4 are expected to translate into a high k_RISC value based on the following relationship: k_RISC ∝ <S₁|Ĥ_SOC|T_n>²/ΔE_ST, which was crucial for triplet exciton utilization in the EL device.

To discern the relationship between structure and emission properties, we calculated and analyzed the reorganization energy and Huang-Rhys factors (S) using the Molecular Materials Property Prediction Package (MOMAP) software. Our analysis revealed that the DPA-B2 parent molecule underwent substantial structural relaxation during the excitation-emission process, owing to its distorted geometry. This was reflected by the apparent increase in theoretically predicted total reorganization energy when compared to that of ν-DABNA (Fig. 1d and Supplementary Fig. 31). However, this adverse effect was mitigated in DPA-B3 and DPA-B4, which exhibited progressively decreasing total reorganization energy due to their enlarged fused-ring systems. The relationships of reorganization energy and S with normal vibration modes for the S₁→S₀ transition are further illustrated in Fig. 1e and Supplementary Figs. 33-S34, respectively. Obviously, the net effect of linear π-extension and non-bonding frontier molecular orbitals diminished the vibronic coupling strength of both low-frequency modes (< 500 cm^− 1, dominated by the rotational vibrations of trimethylbenzene and the twisting vibration of the π-skeleton) and high-frequency modes (> 500 cm^− 1, the swing of C-H at the peripheral benzene rings and the stretching vibration of C-C bonds in the backbone). Specifically, DPA-B4 demonstrated very tiny S values below 0.25 for all vibration modes, which benefitted the formation of both a narrowed FWHM and weakened shoulder peak in the spectrum.

From the above discussion, it was clear that i) linearly π-extended MR scaffold, ii) reasonable element combination, iii) fully non-bonding character, and iv) partial twisted geometry were four key elements in our molecular design principle towards high-performance deep-blue emitters. To further elucidate the importance of the latter two factors, we designed a control emitter, Cz-B4, through intramolecular cyclization of the terminal phenyl units in the B,N,B-doped [4]helicene from DPA-B4. This minor structural difference did not induce notable changes in S₁ energy, but it triggered considerably larger ΔE_ST (0.32 eV from TD-DFT, and 0.12 eV from RI-SCS-CC2 calculations) due to the introduction of π-bonding/antibonding character at the biphenyl site, as well as an overall decrease in SOC values (similar < S₁|Ĥ_SOC|T₁ > = 0.05 cm^− 1, <S₁|Ĥ_SOC|T₂ > = 0.61 cm^− 1 but significantly reduced < S₁|Ĥ_SOC|T₃ > = 0.08 cm^− 1) due to the more planarized geometry. On the other hand, although the intramolecular ring fusion induced higher rigidity in Cz-B4, as evidenced by smaller root-mean-square displacement (RMSD) values between S₀ and S₁ geometries (0.026 Å for Cz-B4 compared to 0.042 Å for DPA-B4, Supplementary Fig. 32), we observed a 33% increase in its total reorganization energy. This increase is attributed to intensified high-frequency modes with relatively larger S. Of note, the dominant stretching mode at 1654.9 cm^− 1 is closely associated with the stretching vibration of the central biphenyl unit, highlighting the adverse effects of π-bonding/antibonding molecular orbitals on emission broadening.

Synthesis, characterization and photophysical properties

With the established design strategy in place, we conducted the synthesis of the emitters using a one-shot multiple borylation process with moderate yields (the detailed synthetic procedure and characterization are given in the Supplementary Information) ³⁷. These four compounds exhibited remarkable thermal stability, as evidenced by high decomposition temperatures (T_d, corresponding to a 5% weight loss) of 376°C, 424°C, 497°C, and 513°C for DPA-B2, DPA-B3, DPA-B4, and Cz-B4, respectively (Supplementary Fig. 36), underscoring their potential suitability for OLED fabrication via vacuum thermal deposition. Single crystals of DPA-B3 and DPA-B4 were successfully cultivated through the slow evaporation of an ethanol/dichloromethane mixture at room temperature (Supplementary Fig. 38, Supplementary Tables 1–2). These crystals revealed quasi-planar skeletons with length-to-width ratios of approximately 2.40:1.87 and 2.93:1.77 times their height, respectively. Moreover, the molecular edges exhibited significant twisting due to the incorporation of multiple helicene moieties and perpendicularly aligned phenyl and mesitylene units. This quasi-planar architecture with twisted molecular edges offers several additional advantages, including a high horizontal ratio of emitting dipole orientation (Θ_//)^48,49 and the mitigation of aggregation-induced quenching^50,51. Consequently, these structural characteristics were expected to significantly enhance the efficiency of OLEDs.

The steady-state photophysical properties of the four emitters were next investigated in dilute toluene solution (1 × 10^− 5 M) at 300 K, with the results provided in Fig. 1a and Table 1. The photoluminescence (PL) spectra of the four compounds revealed intense emission bands in the deep-blue area, with emission maxima at 440, 448, 458, 455 nm, Stokes shift values of 16, 10, 10, 14 nm, FWHM values of 32, 14, 12, 24 nm and CIE coordinates of (0.150, 0.037), (0.150, 0.035), (0.140, 0.050), (0.139, 0.072) for DPA-B2, DPA-B3, DPA-B4 and Cz-B4, respectively. The observed trends in the Stokes shift and FWHM values in the DPA-Bn series indicated a positive effect of linear π-extension in suppressing S₁-S₀ structural shifts, thereby compensating for the negative spectral effects caused by the twisted geometry. In contrast, the increase in Stokes shift and FWHM values in Cz-B4 was in agreement with its enlarged reorganization energy, emphasizing the importance of minimizing π-bonding/antibonding interactions in designing ultranarrow-band deep-blue emitters. Notably, the FWHM value of DPA-B4 sets a record for organic blue emitters, and despite its relatively low energy emission onset/maxima, it exhibits excellent CIE_y of 0.050, closely approaching the BT.2020 blue standards. This impressive performance can be attributed to its narrow, near-Gaussian shaped spectrum and minimal vibrational shoulder peak, as a result of the well suppressed vibration modes in DPA-B4. The PL spectra of these emitters in different solvents revealed a positive solvatochromic effect with slight bathochromic shifts and marginally increased FWHMs from hexane to acetonitrile (Supplementary Fig. 39), confirming the SR-CT nature of the emission and maintained high color purity upon raising the environmental polarities.

Table 1

The physical data of DPA-B2, DPA-B3, DPA-B4 and Cz-B4.
Emitter	λ_abs^a [nm]	λ_em^a [nm]	FWHM^a [nm]	E_HOMO^b [eV]	E_LUMO^c [eV]	E_S1^d [eV]	E_T1^d [eV]	∆E_ST^e [eV]	τ_p^f [ns]	τ_d^f [µs]
DPA-B3	424	440	32	-5.21	-2.38	2.92	2.72	0.20	2.4	45.6
DPA-B3	438	448	14	-5.14	-2.38	2.83	2.69	0.14	1.9	12.5
DPA-B4	448	458	12	-5.14	-2.43	2.78	2.76	0.02	1.8	2.5
Cz-B4	441	455	24	-5.19	-2.47	2.79	2.64	0.15	3.8	17.4

^aPeak of absorption (λ_abs), fluorescence (FL) spectra, full-width at half-maximum (FWHM) of fluorescence measured in toluene (1 × 10^− 5 M). ^bHOMO energy determined in dichloromethane. ^cEstimated from the HOMO and E_g. ^dS₁, and T₁ energy value measured in diluted toluene solution at 77 K. ^eS₁-T₁ energy gap. ^fPrompt fluorescence lifetime (τ_PF), delayed fluorescence lifetime (τ_DF).

The photophysical properties of four emitters in film states, specifically doped in 7-((2'-methyl-[1,1'-biphenyl]-4-yl)oxy)-3,11-di-o-tolyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (DOBNA-OAr)^23,52, were subjected to further analysis. The doping concentration was set at 1% to prevent interchromophore quenching. As shown in Supplementary Fig. 42, the emission spectra bore a resemblance to those observed in the solution state. The slightly broadened FWHMs of 40, 20, 16, and 29 nm could be attributed to the enhanced polarity of the surrounding components and intermolecular interactions. Subsequently, the absolute Ф_PLs of these emitters in the corresponding deposited film were evaluated, yielding values of 82%, 93%, 99%, and 93% for DPA-B2, DPA-B3, DPA-B4 and Cz-B4, respectively. The Ф_PL value of the DPA-B4-doped film approached unity, owing to suppressed nonradiative transitions stemming from the extended rigid skeleton with minimized bonding/anti-bonding character. Furthermore, the energy levels of S₁/T₁ were calculated to be 2.92/2.72 eV, 2.83/2.69 eV, 2.78/2.76 eV, and 2.79/2.64 eV from the onset of fluorescence and phosphorescence spectra at 77 K (Supplementary Fig. 40), resulting in ΔE_STs of 0.20, 0.14, 0.02 eV, and 0.15 eV, respectively. These values exhibited very good consistency with the above theoretical results.

The TADF properties were subsequently evidenced by the transient decay curves of the doped films (Fig. 2b). At 300 K, all the emitters exhibited bi-exponential decays with nanosecond-scale and microsecond-scale components, corresponding to prompt fluorescence and TADF, respectively. DPA-B4 displayed a shortened prompt lifetime (τ_PF = 1.8 ns) and delayed lifetime (τ_DF = 2.5 µs), compared to DPA-B2 (2.4 ns / 45.6 µs), DPA-B3 (1.9 ns / 12.5 µs), and Cz-B4 (3.8 ns / 17.4 µs). To provide a quantitative understanding of the kinetic processes, key rate constants such as singlet radiative decay (k_r), non-radiative decay (k_nr), and k_RISC were determined (Supplementary Table 3, refer to Supplementary Information for calculation details). Consistent with its highest Ф_PL, the DPA-B4-doped film exhibited the highest k_r of 8.47 × 10⁷ s^-1 and the lowest k_nr of 0.86 × 10⁷ s^-1. The k_RISCs were determined to be 0.10 × 10⁶, 0.44 × 10⁶, 2.60 × 10⁶ and 0.11 × 10⁶ s^-1 for DPA-B2, DPA-B3, DPA-B4 and Cz-B4, respectively. Notably, DPA-B4 displayed the fastest RISC process, which was 26-fold higher than that of DPA-B2. These values surpass those reported for any previously known heavy-element free MR-TADF molecules, even with more extended MR frameworks²⁵, and are on par with those of selenium MR-TADF molecules³¹. This is a clear consequence of its near-zero ΔE_ST and large SOC values, origined from the twisted molecular skeleton. The superiority of DPA-B4 in both singlet and triplet exciton harvesting would significantly enhance EL efficiency and mitigate roll-off issue at high brightness in devices.

The distinctive molecular configurations of the four compounds imparted a noteworthy degree of anisotropy in the doped films, resulting in horizontal molecular orientation ratios (Θ_//) of 91%, 94%, 97%, and 97% for DPA-B2, DPA-B3, DPA-B4, and Cz-B4, respectively, as elucidated by the angle-dependent p-polarized photoluminescence spectra (Supplementary Fig. 44). The general upward trend in these ratios demonstrated that the presence of larger fused ring systems promoted a horizontal dipole orientation due to the expansion of the molecular plane, wherein the transition dipole moment resides. These high Θ_// values were expected to positively impact the optical out-coupling factor (η_out) when compared to common molecules with randomly oriented dipoles, consequently enhancing the EQEs of the devices.

Electroluminescent performance

Motivated by the excellent photophysical parameters demonstrated by these deep-blue emitters, their potential electroluminescence (EL) performance was further assessed in devices. The OLEDs were fabricated with the following optimized configuration: indium tin oxide ITO/dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN, 5 nm)/1,1-bis((di-4-tolylamino)phenyl)-cyclohexane (TAPC, 30 nm)/tris(4-carbazolyl-9-ylphenyl)amine (TCTA, 15 nm)/1,3-di(9H-carbazol-9-yl)benzene (mCBP, 10 nm)/99 wt% DOBNA-OAr: 1 wt% emitter (20 nm)/2,8-bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF, 15 nm)/1-(4-(10-([1,1′-biphenyl]-4-yl)anthracen-9-yl)phenyl)-2-ethyl-1H-benzo[d]-imidazole (ANT-BIZ, 30 nm)/8-hydroxyquinolinolato-lithium (Liq, 2 nm)/aluminum (Al, 100 nm). HAT-CN was used as a hole injection layer, TAPC and TCTA functioned as the hole transporting layers, and ANT-BIZ were employed as electron-transport layers, respectively. mCBP and PPF served as exciton-blocking layers, and Liq and Al were acted as the electron injection layer and cathode, respectively. The chemical structures of the organic materials, the energy levels diagram and the EL performance are depicted in Fig. 3, Supplementary Fig. 45 and detailed parameters were provided in Table 2.

All the fabricated devices displayed nearly identical turn-on voltages (V_on) of 3.6 V and high maximum luminescence (L_max) over 10000 cd m^-2. The EL spectra maintained stable at different voltages (from the turn-on voltage to 9 V) for all devices measured, indicating that the exciton recombination zone was almost consistent at different voltages. The maximum emission peaks were recorded at 442, 448, 457 and 456 nm for DPA-B2, DPA-B3, DPA-B4 and Cz-B4, with the associated FWHM values of 30, 15, 13, and 26 nm, respectively, which were consistent with the trend observed in toluene solution and doped film. The CIE coordinates of the four devices were (0.150, 0.052), (0.150, 0.050), (0.140, 0.050) and (0.139, 0.077), respectively, with the DPA-Bn series approaching the BT.2020 standard for primary blue emission. Of particular note, the device based on DPA-B4 exhibited the narrowest FWHM value of 13 nm, which is not only the smallest value reported for OLEDs to date (Supplementary Fig. 46), but also smaller than that of quantum-dot and perovskite light-emitting diodes⁵³.

Table 2

The EL data of the blue OLEDs.
Device	λ_EL^a [nm]	L_max^b [cd m^-2]	CE^c [cd A^-1] Max (Average)	PE^d [lm W^-1] Max (Average)	EQE^e [%] Max (Average)	EQE^f [%] 100/1000	FWHM^g [nm]	CIE^h [x, y]
DPA-B2	442	11790	10.5 (9.6 ± 0.9)	8.5 (7.4 ± 1.0)	21.9 (20.2 ± 1.8)	15.6/10.0	30	(0.150, 0.052)
DPA-B3	448	10960	15.8 (15.2 ± 0.5)	12.7 (12.2 ± 0.5)	30.6 (29.6 ± 0.9)	25.8/17.2	15	(0.150, 0.050)
Cz-B4	456	11843	15.3 (14.7 ± 0.8)	10.9 (10.6 ± 0.9)	21.3 (20.4 ± 1.1)	19.1/14.3	26	(0.139, 0.077)
DPA-B4	457	10081	17.8 (16.2 ± 0.7)	12.9 (12.7 ± 0.6)	31.5 (30.6 ± 0.5)	28.7/23.6	13	(0.140, 0.050)
DPA-B4 (HF)	460	39329	43.3 (41.4 ± 1.6)	37.7 (34.8 ± 2.7)	44.7 (42.6 ± 1.9)	43.9/38.5	16	(0.150, 0.110)
DPA-B4 (Tandem)	458	56246	77.3 (73.9 ± 3.1)	33.7 (31.7 ± 1.8)	74.5 (70.4 ± 2.9)	73.3/65.3	17	(0.142, 0.116)

^aElectroluminescence peak wavelength. ^bMaximum luminance. ^cMaximum current efficiency. ^dMaximum power efficiency. ^eMaximum external quantum efficiency. ^fExternal quantum efficiency at 100, 1000 cd m^-2. ^gFull-width at half-maximum. ^hCommission Internationale de l’Eclairage coordinates. HF represents hyperfluorescent device. Tandem represents the two-unit tandem OLED.

Except for the superb color purity, the devices based on DPA-B4 also exhibited considerable maximum EQE and current efficiency (EQE_max and CE_max) of 31.5% and 17.8 cd A^-1, respectively, outstripping those based on DPA-B2 (21.9%, 10.5 cd A^-1), DPA-B3 (30.6%, 15.8 cd A^-1) and Cz-B4 (21.3%, 15.3 cd A^-1), which was a clear consequence of the superior Ф_PL and Θ_// values of DPA-B4. We highlight that the efficiency of the DPA-4BNO-based device was the highest among the reported MR-TADF based OLEDs with CIE_y ≤0.05 (Fig. 3f, Supplementary Table 5). Of note, because the CE also depends on the emission profile, the reduction of high-energy component would lead to a further increase in CE as the brightness is corrected by the human eye sensitivity function. This explains a more drastic increase of CE_max (70%) than EQE_max (44%) of DPA-B4 compared with DPA-B2 due to redshifted emission onset in DPA-B4, reflecting the merit of ultranarrow emission band of DPA-B4 in promoting device efficiency. Moreover, the efficiency roll-off of the DPA-B4-based device (28.7% at 100 cd m^-2 and 23.6% at 1000 cd m^-2) was found to be smaller than that of DPA-B2 (15.6% at 100 cd m^-2 and 10.0% at 1000 cd m^-2), DPA-B3 (25.8% at 100 cd m^-2 and 17.2% at 1000 cd m^-2) and Cz-B4 (19.1%, 14.3 cd A^-1). This can be attributed to the very short triplet state lifetime (2.5 µs) and fast RISC process (k_RISC ~ 2.60 × 10⁶ s^-1) for DPA-B4, which reduce the triplet exciton density and inhibit bimolecular exciton deactivations through triplet-triplet annihilation (TTA) and singlet-triplet annihilation (STA) processes.

To further improve the device performance and suppress the efficiency roll-off, a sensitization strategy was employed for the champion emitter DPA-B4 considering its short prompt emission lifetime. A well-known TADF emitter reported in ref.⁵⁴, 5-(3,11-dimethyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (mMDBA-DI), was selected as a sensitizer by taking advantage of its well-matched PL spectra with the absorption spectra of DPA-B4 (Supplementary Fig. 47), high Φ_PL and fast RISC process. As illustrated in Fig. 4, despite of slight bathochromic shifted CIE of (0.150, 0.110) due to increased host polarity, the DPA-4BNO-based device maintained a very small FWHM of 16 nm. Meanwhile, this device displayed an ultrahigh EQE_max of 44.7% and CE_max of 43.3 cd A^-1, with mitigated efficiency decline at high brightness: EQE values of 43.9% at 100 cd m^-2, and 38.5% at 1000 cd m^-2, corresponding to EQE drops of merely 1.8% and 13.8%, respectively. This remarkable improvement in efficiency and reduction in efficiency roll-off can be attributed to more efficient triplet harvesting facilitated by the sensitizer and rapid exciton consumption through a long-range Förster resonance energy transfer (FRET) pathway. While DPA-B4 primarily served as a fluorescence emitter in this hyperfluorescence device, and the triplet upconversion of the sensitizer was typically the rate-determining step in exciton dynamics, triplet population on the terminal emitter remained possible^10,55,56. This suggests that an accelerated RISC process for the terminal emitter should be advantageous even in the presence of a sensitizer. For comparison purposes, devices DPA-B3 and Cz-B4, based on the same device structure, exhibited not only inferior EQE_max but also more pronounced roll-off, underscoring the advantage of the higher k_RISC of DPA-B4 (Supplementary Figs. 48–49 and Supplementary Table 4).

Inspired by the above results, a two-unit tandem OLED was constructed to fully leverage the narrow FWHM and high efficiency of DPA-B4 (Fig. 4d, 4f)⁵⁷. The resulting EL spectrum closely mirrored that of the single-unit device, featuring a FWHM of 17 nm. Nonetheless, the maximum brightness was increased by 43%, and the required current density was also reduced at the same brightness. The tandem OLED achieved EQE_max and CE_max values as high as 74.5% and 77.3 cd A^-1, respectively, representing an increase of approximately 1.7 and 1.5 times compared to the single EL unit device. Impressively, this tandem device maintained EQEs exceeding 40% across a wide luminance range of 0–15,000 cd m^-2, adequately covering the luminance requirements for flat-panel displays and general lighting applications. This exceptional performance can be attributed to the efficient charge generation layer and highly effective TADF processes within the tandem OLED architecture.

In summary, our proposed molecular design strategy, centered on the construction of twisted multi-boron-based frameworks with fully resonating structures, presents a compelling approach for the development of high-performance deep-blue MR-TADF emitters. The optimized emitter demonstrated remarkable photophysical properties, including: i) an ultranarrow emission band (FWHM: 12 nm) and a close-to-unity Ф_PL within the deep-blue region, attributed to the expanded core with high structural rigidity, non-bonding character, and reasonable element combination; ii) a remarkably rapid triplet up-conversion process (k_RISC value on the order of 10⁶ s^− 1), owing to the large SOC matrix elements induced by the twisted structure, as well as the near-zero ∆E_ST derived from the linearly extended MR skeleton. Furthermore, we attained a preferentially horizontal emitting dipole orientation with Θ_// reaching 97%, thereby enhancing light outcoupling efficiency. Utilizing these merits, the OLED device employing a binary emitting layer not only met the BT.2020 blue standard, but also demonstrated leading performance with an EQE_max/1000 of 31.5%/23.6% and a small FWHM of 13 nm. The integration of a TADF sensitizer further boosted the device performance, resulting in an EQE _max/1000 of 44.7%/38.5% while preserving a narrow emission spectrum. Building upon this foundation, the incorporation of a tandem device architecture achieved state-of-the-art efficiency, yielding an EQE_max/1000 of 74.5%/66.7% in the deep-blue region. Consequently, our innovative design strategy represents a significant advancement toward achieving both high color purity and rapid spin-flip process in deep-blue MR-TADF molecules, thereby paving the way for highly efficient OLEDs suitable for UHD applications.

Acknowledgments

This work was supported by the National Natural Science Foundation of China 52130308 (C.Y.), 22375130 (X.C.), 52373192 (J.M.); Shenzhen Science and Technology Program JCYJ20220818095816036 (C.Y.); Shenzhen Science and Technology Program ZDSYS20210623091813040 (C.Y.); Guangdong Basic and Applied Basic Research Foundation 2022A1515110874 (T.H.). We thank the Instrumental Analysis Center of Shenzhen University for the analytical support.

Author contributions

C.Y. supervised the projects; C.Y. and T.H. designed the emitters; T.H. synthesized the emitters; Z.H. provided the sensitizer; T.H. characterized the emitters and measured the photophysical and electrochemical properties; J.M. fabricated and tested the OLEDs; X.C., X.Y and Z.C. performed theoretical calculations; T.H., X.C. and J.M. contributed to the data analysis; T.H. and X.C. contributed to the manuscript writing; C.Y. review & editing the manuscript. All authors discussed the progress of the research and reviewed the manuscript.

Competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data and materials availability

All data are available in the main text or the supplementary materials. The structural data for the DPA-B3 and DPA-B4 reported here are freely available from the Cambridge Crystallographic Data Centre (CCDC No. 2299738 for DPA-B3 and No. 2299737 for DPA-B4)

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Quantum Chemical Calculations:Theoretical chemical calculations were performed by the Gaussian 16 program package. Density functional theory (DFT) and Time-dependent DFT (TD-DFT) calculations were performed at the B3LYP-B3(DJ)/6-31G(d,p) level to attain the optimized molecular geometries. The HOMO and LUMO were obtained on the basis of the optimized geometric configurations. The SOC matrix elements was assessed using PySOC. The singlet and triplet energy levels were obtained based on SCS-CC2/cc-pVDZ level using the MRCC program. Huang-Rhys factor and reorganization energy analysis were obtained by the MOMAP (Molecular Materials Property Prediction Package).

Photophysical Characterization: Shimadzu UV-2700 spectrophotometer (Shimadzu, Japan) was applied to record UV-vis spectra at 25 ºC. Photoluminescence (PL) spectra were determined on a Hitachi F-7100 fluorescence spectrophotometer (Hitachi, Japan) at 25 ºC. Phosphorescence spectra (Phos) were recorded on the Hitachi F-7100 fluorescence spectrophotometer at 77 K. The transient PL decay curves were obtained by FluoTime 300 (PicoQuant GmbH) with a Picosecond Pulsed UV-LASTER (LASTER375) as the excitation source. The photoluminescence quantum yields (Φ_PLs) were achieved by a Hamamatsu UV-NIR absolute PL 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 Measurement: To evaluate the electroluminescent performance of proposed emitters, we fabricated multilayered TADF OLEDs using these emitters as the luminescent layer materials. All of the small molecular organic materials were purchased (except for the TADF emitters) and purification by temperature-gradient sublimation under vacuum condition. ITO glass substrates were washed sequentially with acetone, deionized water and isopropyl alcohol in an ultrasonic cleaner, then dried with N₂ flow and finally transferred into a vacuum chamber for deposition. The organic layers of 8-hydroxyquinolinolato-lithium (Liq) and aluminum (Al) were deposited by thermal evaporation at 5×10^-5 Pa with rates of 0.1 and 3 Å/s, respectively. The other organic layers were deposited at the rates of 0.2-3 Å/s. The emitting area of the device is about 0.09 cm². The current density-voltage-luminance (J-V-L), EQE-L curves and electroluminescence (EL) spectra were measured by using a Keithley 2400 source meter and an absolute EQE measurement system (C9920-12, Hamamatsu Photonics, Japan).

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Deep-Blue Organic Light-Emitting Diodes for Ultrahigh-Definition Displays

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