Low Efficiency Roll-off Research Articles

Regulating excited states by varying different acceptors of D-π-A emitters for efficient non-doped blue electroluminescence with high luminance and low efficiency roll-off.

Chromophores with hybridized local and charge-transfer (HLCT) excited state are promising for the realization of high performance blue organic light-emitting diodes (OLEDs). The rational manipulation of HLCT excited state for efficient emitters remains challenging. Herein, we present three donor-π-acceptor (D-π-A) molecules (mPAN, mPANPH, and mPNAPH) with phenanthro[9,10-d]imidazole (PI) and pyridinyl as donor and π-bridge respectively. Changes in various kinds of polycyclic aromatic derivative acceptors (anthracene, 9-phenylanthracene, and 1-phenylnaphthalene) could manipulate the excited states and optoelectronic properties. Theoretical calculations reveal that the S1 state of mPNAPH exhibits HLCT nature while the other two molecules show local excited (LE) state dominated feature. The photophysical properties also demonstrate this characteristic. Therefore, compared with mPAN and mPANPH, mPNAPH has higher photoluminescence quantum yield (PLQY) whether in solutions or neat films.Ultimately, the non-doped devices based on these emitters show high luminance larger than 35000 cd m-2, and high maximum external quantum efficiencies (EQEmaxs) larger than 5% with low efficiency roll-off. In particular, the mPNAPH-based device displays an excellent performance of pure blue emission at 456 nm with Commission Internationale de L'Eclairage coordinate of (0.15, 0.16) and EQEmax of 6.13% that benefited from the HLCT state and high-lying reverse intersystem crossing process.

Constructing solution-processable heavy metal platinum (II) complex with narrowband emission by integrating multiple resonance molecular system

The narrowband emission required by wide color gamut display is an extremely important research topic for any luminescence mechanism, which has made significant progress in traditional fluorescence and thermally activated delayed fluorescence (TADF) based on purely organic compounds, but is far from mature in organometallic complexes. Herein, we propose a feasible molecular design paradigm for constructing the desirable organic electroluminescence (EL) emitter with narrowband emission by integrating an original multiple resonance TADF (MR-TADF) fragment into the classical platinum (II) complex. The solution-processed organic light-emitting diode (OLED) by employing the target model platinum (II) complex BNCPPt as the dopant emitter shows light green emission with a peak of 507 nm and narrow full-width at half-maximum (FWHM) of 35 nm, which achieves a maximum external quantum efficiency (EQE) of 13.5% with very low efficiency roll-off of 4.4% at 1000 cd m−2.

High efficiency double light-emitting layer sky blue phosphor organic light-emitting diode with slow emission efficiency attenuation

In this study, by doping the iridium (III) double ((4,6-difluorobenzene) -pyridine-N, C2’Pyridinformate (FIrpic) into 4,4′, 4″ three (carbazole) trioxilamine (TcTa) and 9 (4 tert-butyl-benzene) −3,6 double (triphenylsilyl) -9H carbazole (CzSi) layers as light emitting layers (EMLs) to achieve sky blue phosphor organic electroluminescence (EL) devices with very low efficiency roll-off. The dependence of the EL performance on the doping concentration of the light-emitting molecules and the different EML thicknesses are studied in detail. Experimental data show that the thin-film-doped TcTa layer is very important for the efficiency of the device. Finally, the optimal device achieves extremely high power efficiency, current efficiency, and external quantum efficiency (EQE), reaching 74.82 lm/W, 66.68 cd/A, and 31.9 %, respectively. At a certain luminance of 2000 cd/m2 (3.4 V), the same device retained power efficiency with current efficiency and external quantum efficiency of 57.71 lm/W, 62.46 cd/A and 29.3 %, respectively, based on theoretical discussion, the improved EL efficiency was attributed to the energy levels of the host and luminescent molecules, which helps to balance the charge carrier in the emitter molecule distribution and expand the recombination region.

A Breakthrough in Solution-Processed Ultra-Deep-Blue HLCT OLEDs: A Record External Quantum Efficiency Exceeding 10% Based on Novel V-Shaped Emitters.

It is always a great challenge to achieve high-efficiency solution-processed ultra-deep-blue organic light-emitting diodes (OLEDs) with the Commission Internationale de l'Eclairage (CIE) 1931 chromaticity coordinates matching the blue primary of Rec. International Telecommunication Union-Radiocommunication BT.2100, which specifies high dynamic range television image parameters. Inspired by hybrid local and charge transfer (HLCT) excited state emitters improving exciton utilization through high-lying reverse intersystem crossing, here, a series of high-performance blue emitters by a V-shaped symmetric donor (D)-π-acceptor (A)-π-D design strategy are developed. Here, the large torsions and unstable bonds of D-A structures can be improved through π bridges, and also the conjugation length and donor groups can be easily adjusted. The obtained emitters merit excellent photophysical and electrochemical properties, thermal stability, solution processibility, and HLCT excited state excellence. Results suggest that the OLEDs based on the obtained blue emitters all achieve high maximum external quantum efficiency (EQEmax ) of more than 8% with very low efficiency roll-off. In particular, the device based on 4',5'-bis(4-(9H-carbazol-9-yl)phenyl)spiro[fluorene-9,2'-imidazole] exhibits a satisfactory ultra-deep-blue emission (CIEx , y = 0.1579, 0.0387) and a record-high EQEmax (10.40%) among solution-processed HLCT OLEDs, which is very close to the record EQEmax of devices by vacuum vapor deposition technology.

Alignment of Heptagonal Diimide and Triazine Enables Narrowband Pure-Blue Organic Light-Emitting Diodes with Low Efficiency Roll-Off.

The endeavor to develop high-performance narrowband blue organic light-emitting diodes (OLEDs) with low efficiency roll-off represents an attractive challenge. Herein, we introduce a hetero-acceptor design strategy centered around the heptagonal diimide (BPI) building block to create an efficient thermally activated delayed fluorescence (TADF) sensitizer. The alignment of a twisted BPI unit and a planar diphenyltriazine (TRZ) fragment imparts remarkable exciton dynamic properties to 26tCz-TRZBPI, including a fast radiative decay rate (kR ) of 1.0×107 s-1 and a swift reverse intersystem crossing rate (kRISC ) of 1.8×106 s-1 , complemented by a slow non-radiative decay rate (kNR ) of 6.0×103 s-1 . Consequently, 26tCz-TRZBPI facilitates the fabrication of high-performance narrowband pure-blue TADF-sensitized fluorescence OLEDs (TSF-OLEDs) with a maximum external quantum efficiency (EQEmax ) of 24.3 % and low efficiency roll-off even at a high brightness level of 10000 cd m-2 (EQE10000 : 16.8 %). This showcases a record-breaking external quantum efficiency at a high luminance level of 10000 cd m-2 for narrowband blue TSF-OLEDs.

Efficient and Bright Organic Radical Light-Emitting Diodes with Low Efficiency Roll-Off.

Organic radicals have been of interest due to their potential to replace nonradical-based organic emitters, especially for deep red/near-infrared (NIR) electroluminescence (EL), based on the spin-allowed doublet fluorescence. However, the performance of the radical-based EL devices is limited by low carrier mobility that causes a large efficiency roll-off at high current densities. Here, we report highly efficient and bright doublet EL devices by combining a thermally activated delayed fluorescence (TADF) host that supports both electron and hole transport and a tris(2,4,6-trichlorophenyl)methyl-based radical emitter. Steady-state and transient photophysical studies reveal the optical signatures of doublet luminescence mechanisms arising from both host and guest photoexcitation. The host system presented here allows balanced hole and electron currents, and we report a high maximum external quantum efficiency (EQE) of 17.4% at 707nm peak emission with substantially improved efficiency roll-off: over 70% of the maximum EQE (12.2%) is recorded at 10mA cm-2 , and even at 100mA cm-2 , nearly 50% of the maximum EQE (8.4%) is maintained. Charge recombination is primarily at the radical guest sites, but there is evidence from magneto-EL for host recombination and later energy transfer to the radical guest. We consider this is an important step to the practical application of organic radicals to NIR light-emitting devices. This article is protected by copyright. All rights reserved.

Efficient non-doped sky-blue fluorescent organic light emitting devices based on cyanopyridine-containing anthracene derivatives

Two new cyanopyridine-containing anthracene derivatives with asymmetrical donor-π-acceptor (D-π-A) molecule structure have been successfully synthesized by Suzuki coupling reaction, namely 4-(10-(9-phenyl-9H-carbazol-3-yl)anthracen -9-yl)benzonitrile (3CzAnPyCN) and 4-(10-(4-(9H-carbazol-9-yl)phenyl)anthracen -9-yl)benzonitrile (pCzAnPyCN). The non-doped organic light emitting devices (OLEDs) using 3CzAnPyCN and pCzAnPyCN as emitters simultaneously achieved high efficiency stable sky-blue luminescence, corresponding to the maximum external quantum efficiency (EQE) of 8.07% and 8.13% with the Commission International de L'Eclairage (CIE) coordinates of (0.18, 0.35) and (0.16, 0.24), respectively. In addition, extremely low efficiency roll-offs were also evaluated in this work. The EQE values remained as high as 7.80% and 7.88% at the luminance of 1000 cd/m2, 7.31% and 6.95% at the luminance of 10000 cd/m2 for 3CzAnPyCN and pCzAnPyCN, respectively. It is found, by photophysical experiments and density functional theory analysis, that the excited state property of the luminogens revealed that the “hot excitons” induced reverse intersystem crossing (RISC) played a crucial role in improving efficiency. Such an excellent electroluminescence (EL) performance achieved in these non-doped OLEDs is among the highest values based on sky-blue fluorescent materials, potential for future optical applications.

Enrichment of anchoring sites by introducing supramolecular halogen bonds for the efficient perovskite nanocrystal LEDs

Considering the multi-functionalization of ligands, it is crucial for ligand molecular design to reveal the landscape of anchoring sites. Here, a typical triphenylphosphine (TPP) ligand was employed to explore its effect on the surface of CsPbI3 perovskite nanocrystals (PNCs). Except for the conventionally considered P-Pb coordination, an P-I supramolecular halogen bonding was also found on the NC surface. The coexistence of the above two types of bonding significantly increased the formation energy of iodine vacancy defects and improved the photoluminescence quantum yield of PNCs up to 93%. Meanwhile, the direct interaction of P and I enhanced the stability of the Pb-I octahedra and dramatically inhibited the migration of I ions. Furthermore, the introduction of additional benzene rings (2-(Diphenylphosphino)-biphenyl (DPB)) increased the delocalized properties of the PNC surface and significantly improved the charge transport of the PNCs. As a result, the DPB passivated CsPbI3 NCs based top-emitting LEDs exhibite a peak external quantum efficiency (EQE) of 22.8%, a maximum luminance of 15, 204 cd m−2, and an extremely low-efficiency roll-off of 2.6% at the current density of 500 mA cm−2.

Low-trigger voltage and efficient phosphorescent organic light-emitting diodes with bipolar TADF host

Thermally activated delayed fluorescence (TADF) materials have continued to attract interest due to their rapid improvements in efficiency, non-toxic and potential to be manufactured at energy-saving organic light-emitting diode for full-color display and solid-state lighting. Here, The bipolar and electroluminescence (EL) characteristics of TADF host, 11-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole (DPDDC), were investigated. By incorporating the phosphorescent tris[2-(p-tolyl)pyridine]iridium(III) [Ir(mppy)3] dopant into the matrix, DPDDC showed more excellent EL performance compared with the typical 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host. The turn-on voltage for phosphorescent organic light-emitting diodes (PhOLEDs) was decreased to 2.2 V from 2.5 V by exploiting the TADF host instead of CBP. Crucially, we found that the ratio of Ir(mppy)3 in TADF host plays a significant role in EL process. Remarkable low efficiency roll-off was achieved with above 90% of the current efficiency maintained even at a brightness of 3000 cd m−2, along with high power efficiency of 84.8 lm·W−1 and the external quantum efficiency of 20.6% for DPDDC: 5 wt% Ir(mppy)3-based device.

Peripheral decoration of multi-resonance TADF emitter for narrowband blue OLEDs

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters with high efficiency and narrowband blue emission are highly demanded for high-quality displays. In this study, two MR-TADF emitters, namely CzBN-tDPA and CzBN-mCP, were designed by decorating the well-known emitter of CzBN with different diarylamino moieties. This peripheral decoration strategy successfully results in blue emission with peaks at 465 and 464 nm, narrow full-width at half maximum (FWHM) values of 23 and 21 nm for CzBN-tDPA and CzBN-mCP in toluene solution, respectively. The new emitters exhibit faster rate constants of reverse intersystem crossing and higher photoluminescence quantum yields, compared to the parent molecule. As a result, organic light-emitting diodes based on CzBN-tDPA and CzBN-mCP exhibit narrowband blue emission with maximum external quantum efficiencies (EQEmaxs) of nearly 20% and low efficiency roll-offs. More importantly, decent EQEmax up to 30.6% could be realized in hyperfluorescence devices adopting these two new emitters.

Exciton Dynamics and Optically Pumped Lasing in 1‐Naphthylmethylamine‐Based Quasi‐2D Perovskite Films

AbstractFilms of the quasi‐2D perovskite based on 1‐naphthylmethylamine (NMA) are promising as the gain medium for optically pumped lasing and future electrically pumped lasing because of its low lasing threshold and small electroluminescence efficiency rolloff. However, reasons for the low threshold and small efficiency rolloff are still unclear. Therefore, exciton dynamics are investigated in NMA‐based quasi‐2D perovskite films. It is found that quenching of bright excitons by other excitons or charge carriers is unlikely in NMA‐based quasi‐2D perovskite films, which is one reason for the low lasing threshold and small efficiency rolloff. Moreover, thermally stimulated current measurements reveal that the defect levels inside the band gap of the NMA‐based quasi‐2D perovskite are shallow, with a depth of ≈0.3 eV, causing a decrease in nonradiative exciton recombination through the defects. Therefore, population inversion can be easily achieved, leading to the low lasing threshold as well. For fabrication of NMA‐based quasi‐2D perovskite laser devices with even lower lasing thresholds, a circular‐shaped optical resonator, and small‐molecule‐based defect passivation are used. Optically pumped lasing can be obtained from these devices, with a threshold of ≈1 µJ cm−2, which is one of the lowest values ever reported in any perovskite lasers.

Realizing High Brightness Quasi-2D Perovskite Light-Emitting Diodes with Reduced Efficiency Roll-Off via Multifunctional Interface Engineering.

Quasi-2D perovskites have recently flourished in the field of luminescence due to the quantum-confinement effect and the efficient energy transfer between different n phases resulting in exceptional optical properties. However, owing to the lower conductivity and poor charge injection, quasi-2D perovskite light-emitting diodes (PeLEDs) typically suffer from low brightness and high-efficiency roll-off at high current densities compared to 3D perovskite-based PeLEDs, which is undoubtedly one of the most critical issues in this field. In this work, quasi-2D PeLEDs with high brightness, reduced trap density, and low-efficiency roll-off are successfully demonstrated by introducing a thin layer of conductive phosphine oxide at the perovskite/electron transport layer interface. The results surprisingly show that this additional layer does not improve the energy transfer between multiple quasi-2D phases in the perovskite film, but purely improves the electronic properties of the perovskite interface. On the one hand, it passivates the surface defects of the perovskite film; on the other hand,it promotes electron injection and prevents hole leakage across this interface. As a result, the modified quasi-2D pure Cs-based device shows a maximum brightness of > 70,000cdm-2 (twice that of the control device), a maximum external quantum efficiency (EQE) of > 10% and a much lower efficiency roll-off at high bias voltages.

Combining Carbazole Building Blocks and ν-DABNA Heteroatom Alignment for a Double Boron-Embedded MR-TADF Emitter with Improved Performance.

Building blocks and heteroatom alignments are two determining factors in designing multiple resonance (MR)-type thermally activated delayed fluorescence (TADF) emitters. Carbazole-fused MR emitters, represented by CzBN derivatives, and the heteroatom alignments of ν-DABNA are two star series of MR-TADF emitters that show impressive performances from the aspects of building blocks and heteroatom alignments, respectively. Herein, a novel CzBN analog, Π-CzBN, featuring ν-DABNA heteroatom alignment is developed via facile one-shot lithium-free borylation. Π-CzBN exhibits superior photophysical properties with a photoluminescence quantum yield close to 100 % and narrowband sky blue emission with a full width at half maximum (FWHM) of 16 nm/85 meV. It also gives efficient TADF properties with a small singlet-triplet energy offset of 40 meV and a fast reverse intersystem crossing rate of 2.9×105 s-1 . The optimized OLED using Π-CzBN as the emitter achieves an exceptional external quantum efficiency of 39.3 % with a low efficiency roll-off of 20 % at 1000 cd m-2 and a narrowband emission at 495 nm with FWHM of 21 nm/106 meV, making it one of the best reported devices based on MR emitters with comprehensive performance.

Temperature dependent triplet-exciton relay from thermally activated delayed fluorescence (TADF) to self-sensitized triplet–triplet annihilation (TTA) of asymmetrical diphenylsulfone-based blue emitter

The utilization of triplet excitons is greatly concerned in organic luminophores. For thermally activated delayed fluorescence (TADF) emitters, the reverse intersystem crossing (RISC) which activates the triplet excitons at low temperature remains challenging without thermal stimulation. Herein, a temperature-dependent promotion relay of the triplet excitons from TADF to self-sensitized triplet − triplet annihilation up-conversion (TTA-UC) was demonstrated in an asymmetrical TADF luminophore based on 9,9-dimethyl-9,10-dihydroacridine (DMAC) donor and diphenylfonyl (DPS) acceptor. Single-crystal structure analysis combined with theoretical calculations revealed that the alternate strong-and-weak contact of the molecules reduced the singlet–triplet splitting energy (ΔEST), amplified the singlet–triplet coupling (SOC) strength and exciton coupling (EC), leading to highly efficient RISC process over a wide range of temperatures. Moreover, benefiting from the extreme small ΔEST value and large SOC strength, highly efficient blue OLEDs were achieved with the highest CEmax, PEmax and EQEmax of 32.0 cd A-1, 33.5 lm W−1 and 14.7%, and low efficiency roll-off. This work provides a design guidance for high utilization of the triplet excitons through RISC to improve the luminescence efficiency and reduce the device efficiency roll-off.

High-performance single-emitting-layer hybrid white organic light-emitting diodes featuring a stable blue emission component from green thermally activated delayed fluorescence

In this work, we demonstrate single-emitting-layer hybrid white organic light-emitting diodes (WOLEDs) with high-efficiency and low efficiency roll-off. Here, the white emission arises from stable blue emission via electroluminescence (EL) spectral shift from green thermally activated delayed fluorescence (TADF) emitter and yellow phosphorescent emitter. WOLEDs shows a maximum current efficiency of 56.77 cd/A, maximum power efficiency of 50.19 lm/W, and maximum external quantum efficiency of 20.00%. Compared to the devices composed of conventional blue TADF emitters, the device based on EL spectral shift-induced blue emission exhibit higher performances, indicating that efficient white emission can be obtained without a typical blue emitter. Our study provides an alternative way to fabricate high-performance WOLEDs based on EL spectral shift technology.

Highly Efficient Solution-Processed Bluish-Green Thermally Activated Delayed Fluorescence Compounds Using Di(pyridin-3-yl)methanone as Acceptor

Solution-processed devices with thermally activated delayed fluorescence (TADF) compounds have gained great attention due to their low cost and high performance. Here, two solution-processable TADF emitters named ACCz-DPyM and POxCz-DPyM were synthesized by coupled 9,10-dihydro-9,9-dimethylacridine or phenoxazine modified carbazole as donor with di(pyridin-3-yl)methanone as acceptor. Both TADF compounds show same small ΔΕST of 0.04 eV and high PLQY of 66.2% and 58.2%. The devices fabricated by ACCz-DPyM and POxCz-DPyM as emitters show excellent performance as solution-processed with low turn-on voltage of 4.0 and 3.4 V, high luminance of 6209 and 3248 cd m−2 at 8 V, the maximum current efficiency of 9.9 and 15.9 cd A−1, the maximum external quantum efficiency of 6.6% and 6.5% and low efficiency roll-off. The solution-processed device based on ACCz-DPyM shows bluish-green emission. These results show that ACCz-DPyM and POxCz-DPyM are suitable for solution processing devices.

Design, synthesis and photophysical properties of a (quinolin-3-yl)methanone-based thermally activated delayed fluorescence emitter

A new (quinolin-3-yl)methanone-based emitter, PQLM-DMAC, is designed with a highly twisted donor-acceptor conformation. A simple synthetic approach is used to prepare PQLM-DMAC, and its thermal, theoretical, photophysical, and electroluminescence are systematically studied to highlight the applicability in organic light-emitting diodes (OLEDs). PQLM-DMAC exhibits separated frontier molecular orbital distribution and thus can emit prominent thermally activated delayed fluorescence (TADF) suitable for triplet harvesting in OLEDs. Furthermore, PQLM-DMAC exhibits short excited-state lifetimes and decent photoluminescence quantum yield, enabling high efficiencies and low efficiency roll-offs in OLEDs.

Efficient Cyan Delayed Fluorescence Luminogen as Sensitizing Host for OLEDs with High Efficiencies and Extremely Low Roll-Offs.

Organic light-emitting diodes (OLEDs) using thermally activated delayed fluorescence (TADF) materials to sensitizing conventional fluorescence dopants (CFDs) have attracted intensive research interest due to the extraordinary device performances. Herein, a cyan luminogen (TCz-BP-SFAC) with TADF and aggregation-enhanced emission (AEE) character is developed as a sensitizing host for CFDs. TCz-BP-SFAC owns excellent thermal and electrochemical stabilities, prefers horizontal dipole orientation and demonstrates violent delayed fluorescence with high photoluminescence quantum yields in neat and doped films. It exhibits preeminent electroluminescence (EL) performances with maximum external quantum efficiencies (ηext s) of 25.1% and 30.0% and very low efficiency roll-offs. TCz-BP-SFAC also performs well as a sensitizing host for CFDs of different colors. When TCz-BP-SFAC sensitizing green emitter TTPA and orange emitter TBRB, the devices achieve high ηext s of 16.9% and 17.1% as well as very low efficiency roll-offs of 2.4% and 1.2%, respectively. Moreover, TCz-BP-SFAC can serve as a sensitizing host for two-color all-fluorescence white OLEDs, resulting in high ηext s of ∼18% and very low efficiency roll-offs of ∼5%. The outstanding EL performances predict the great potential of TCz-BP-SFAC as emitter and host in practical display and lighting devices.

TADF Invariant of Host Polarity and Ultralong Fluorescence Lifetimes in a Donor-Acceptor Emitter Featuring a Hybrid Sulfone-Triarylboron Acceptor.

10H-Dibenzo[b,e][1,4]thiaborinine 5,5-dioxide (SO2B)-a high triplet (T1 =3.05 eV) strongly electron-accepting boracycle was successfully utilised in thermally activated delayed fluorescence (TADF) emitters PXZ-Dipp-SO2B and CZ-Dipp-SO2B. We demonstrate the near-complete separation of highest occupied and lowest unoccupied molecular orbitals leading to a low oscillator strength of the S1 →S0 CT transition, resulting in very long ca. 83 ns and 400 ns prompt fluorescence lifetimes for CZ-Dipp-SO2B and PXZ-Dipp-SO2B, respectively, but retaining near unity photoluminescence quantum yield. OLEDs using CZ-Dipp-SO2B as the luminescent dopant display high external quantum efficiency (EQE) of 23.3 % and maximum luminance of 18600 cd m-2 with low efficiency roll off at high brightness. For CZ-Dipp-SO2B, reverse intersystem crossing (rISC) is mediated through the vibronic coupling of two charge transfer (CT) states, without involving the triplet local excited state (3 LE), resulting in remarkable rISC rate invariance to environmental polarity and polarisability whilst giving high organic light-emitting diode (OLED) efficiency. This new form of rISC allows stable OLED performance to be achieved in different host environments.

TADF Invariant of Host Polarity and Ultralong Fluorescence Lifetimes in a Donor‐Acceptor Emitter Featuring a Hybrid Sulfone‐Triarylboron Acceptor**

Abstract10H‐Dibenzo[b,e][1,4]thiaborinine 5,5‐dioxide (SO2B)—a high triplet (T1=3.05 eV) strongly electron‐accepting boracycle was successfully utilised in thermally activated delayed fluorescence (TADF) emitters PXZ‐Dipp‐SO2B and CZ‐Dipp‐SO2B. We demonstrate the near‐complete separation of highest occupied and lowest unoccupied molecular orbitals leading to a low oscillator strength of the S1→S0 CT transition, resulting in very long ca. 83 ns and 400 ns prompt fluorescence lifetimes for CZ‐Dipp‐SO2B and PXZ‐Dipp‐SO2B, respectively, but retaining near unity photoluminescence quantum yield. OLEDs using CZ‐Dipp‐SO2B as the luminescent dopant display high external quantum efficiency (EQE) of 23.3 % and maximum luminance of 18600 cd m−2 with low efficiency roll off at high brightness. For CZ‐Dipp‐SO2B, reverse intersystem crossing (rISC) is mediated through the vibronic coupling of two charge transfer (CT) states, without involving the triplet local excited state (3LE), resulting in remarkable rISC rate invariance to environmental polarity and polarisability whilst giving high organic light‐emitting diode (OLED) efficiency. This new form of rISC allows stable OLED performance to be achieved in different host environments.