Thermally Activated Delayed Fluorescence Material as Host with Novel Spiro‐Based Skeleton for High Power Efficiency and Low Roll‐Off Blue and White Phosphorescent Devices

Efficiency roll‐off in blue organic light‐emitting diodes especially at high brightness still remains a vital issue for which the excitons density‐dependent mechanism of host materials takes most responsibility. Additionally, the efficiency roll‐off leads to high power consumption and reduces the operating lifetime because higher driving voltage and current are required. Here, by subtly modifying the triphenylamine to oxygen‐bridged quasi‐planar structure, a novel thermally activated delayed fluorescence type blue host Tri‐o‐2PO is successfully developed. Efficiency roll‐off based on Tri‐o‐2PO is ultralow with external quantum efficiency (EQE) just dropping by around 2% in the high luminance range from 1000 cd m^?2 to 10 000 cd m^?2. As expected, low turn‐on voltage (≈2.9 V) of device is also achieved, which is close to the theory limit value (≈2.62 V). Super‐high power efficiency (≈60 lm W^?1) and EQE (>22%) are also achieved when utilizing Tri‐o‐2PO as host. Furthermore, two‐color warm‐white light with CIE of (0.45, 0.43) and correlated color temperature of 2921 K is also fabricated and a champion EQE of 21% is delivered. These excellent performances prove the strategy of bridging the triphenylamine to reduce ΔE_st is validated and suggest the great potential of this novel skeleton.     

Achieving High‐Performance Nondoped OLEDs with Extremely Small Efficiency Roll‐Off by Combining Aggregation‐Induced Emission and Thermally Activated Delayed Fluorescence

Luminescent materials with thermally activated delayed fluorescence (TADF) can harvest singlet and triplet excitons to afford high electroluminescence (EL) efficiencies for organic light‐emitting diodes (OLEDs). However, TADF emitters generally have to be dispersed into host matrices to suppress emission quenching and/or exciton annihilation, and most doped OLEDs of TADF emitters encounter a thorny problem of swift efficiency roll‐off as luminance increases. To address this issue, in this study, a new tailor‐made luminogen (dibenzothiophene‐benzoyl‐9,9‐dimethyl‐9,10‐dihydroacridine, DBT‐BZ‐DMAC) with an unsymmetrical structure is synthesized and investigated by crystallography, theoretical calculation, spectroscopies, etc. It shows aggregation‐induced emission, prominent TADF, and interesting mechanoluminescence property. Doped OLEDs of DBT‐BZ‐DMAC show high peak current and external quantum efficiencies of up to 51.7 cd A^?1 and 17.9%, respectively, but the efficiency roll‐off is large at high luminance. High‐performance nondoped OLED is also achieved with neat film of DBT‐BZ‐DMAC, providing excellent maxima EL efficiencies of 43.3 cd A^?1 and 14.2%, negligible current efficiency roll‐off of 0.46%, and external quantum efficiency roll‐off approaching null from peak values to those at 1000 cd m^?2. To the best of the authors' knowledge, this is one of the most efficient nondoped TADF OLEDs with small efficiency roll‐off reported so far.     

Utilizing a Spiro TADF Moiety as a Functional Electron Donor in TADF Molecular Design toward Efficient “Multichannel” Reverse Intersystem Crossing

Designing thermally activated delayed fluorescence (TADF) materials with an efficient reverse intersystem crossing (RISC) process is regarded as the key to actualize efficient organic light‐emitting diodes (OLEDs) with low efficiency roll‐off. Herein, a novel molecular design strategy is reported where a typical TADF material 10‐phenyl‐10H, 10′H‐spiro[acridine‐9, 9′‐anthracen]‐10′‐one (ACRSA) is utilized as a functional electron donor to design TADF materials of 2,4,6‐triphenyl‐1,3,5‐triazine(TRZ)‐p‐ACRSA and TRZ‐m‐ACRSA. It is unique that the intramolecular charge transfer of the ACRSA moiety and the intramolecular and through‐space intermolecular charge transfer between the TRZ and ACRSA moieties, provide a “multichannel” effect to enhance the rate of the reverse intersystem crossing process (k_risc) exceeding 10^?6 s^?1. TADF OLEDs based on TRZ‐p‐ACRSA as an emitter show a maximum external quantum efficiency (EQE) of 28% with reduced efficiency roll‐off (EQEs of 27.5% and 22.1% at 100 and 1000 cd m^?2, respectively). Yellow phosphorescent OLEDs utilizing TRZ‐p‐ACRSA as a host material show record‐high EQE of 25.5% and power efficiency of 115 lm W^?1, while phosphorescent OLEDs based on TRZ‐m‐ACRSA show further lower efficiency roll‐off with EQEs of 25.2%, 24.3%, and 21.5% at 100, 1000, and 10 000 cd m^?2, respectively.     

Multiresonant Thermally Activated Delayed Fluorescence Emitters Based on Heteroatom‐Doped Nanographenes: Recent Advances and Prospects for Organic Light‐Emitting Diodes

Since the first report in 2015, multiresonant thermally activated delayed fluorescent (MR‐TADF) compounds, a subclass of TADF emitters based on a heteroatom‐doped nanographene material, have come to the fore as attractive hosts as well as emitters for organic light‐emitting diodes (OLEDs). MR‐TADF compounds typically show very narrow‐band emission, high photoluminescence quantum yields, and small ΔE_ST values, typically around 200 meV, coupled with high chemical and thermal stabilities. These materials properties have translated into some of the best reported deep‐blue TADF OLEDs. Here, a detailed review of MR‐TADF compounds and their derivatives reported so far is presented. This review comprehensively documents all MR‐TADF compounds, with a focus on the synthesis, optoelectronic behavior, and OLED performance. In addition, computational approaches are surveyed to accurately model the excited state properties of these compounds.     

High‐Performance Hybrid White Organic Light‐Emitting Diodes with Superior Efficiency/Color Rendering Index/Color Stability and Low Efficiency Roll‐Off Based on a Blue Thermally Activated Delayed Fluorescent Emitter

Thermally activated delayed fluorescence (TADF)‐based white organic light‐emitting diodes (WOLEDs) are highly attractive because the TADF emitters provide a promising alternative route to harvest triplet excitons. One of the major challenges is to achieve superior efficiency/color rendering index/color stability and low efficiency roll‐off simultaneously. In this paper, high‐performance hybrid WOLEDs are demonstrated by employing an efficient blue TADF emitter combined with red and green phosphorescent emitters. The resulting WOLED shows the maximum external quantum efficiency, current efficiency, and power efficiency of 23.0%, 51.0 cd A^?1, and 51.7 lm W^?1, respectively. Moreover, the device exhibits extremely stable electroluminescence spectra with a high color rendering index of 89 and Commission Internationale de L'Eclairage coordinates of (0.438, 0.438) at the practical brightness of 1000 cd m^?2. The achievement of these excellent performances is systematically investigated by versatile experimental and theoretical evidences, from which it is concluded that the utilization of a blue‐green‐red cascade energy transfer structure and the precise manipulation of charges and excitons are the key points. It can be anticipated that this work might be a starting point for further research towards high‐performance hybrid WOLEDs.     

Efficiency Enhancement of Organic Light‐Emitting Diodes Incorporating a Highly Oriented Thermally Activated Delayed Fluorescence Emitter

An organic light‐emitting diode (OLED) with the blue emitter CC2TA showing thermally activated delayed fluorescence (TADF) is presented exhibiting an external quantum efficiency () of 11% ± 1%, which clearly exceeds the classical limit for fluorescent OLEDs. The analysis of the emission layer by angular dependent photoluminescence (PL) measurements shows a very high degree of 92% horizontally oriented transition dipole moments. Excited states lifetime measurements of the prompt fluorescent component under PL excitation yield a radiative quantum efficiency of 55% of the emitting species. Thus, the radiative exciton fraction has to be significantly higher than 25% due to TADF. Performing a simulation based efficiency analysis for the OLED under investigation allows for a quantification of individual contributions to the efficiency increase originating from horizontal emitter orientation and TADF. Remarkably, the strong horizontal emitter orientation leads to a light‐outcoupling efficiency of more than 30%.     

Extremely Efficient White Organic Light‐Emitting Diodes for General Lighting

Highly power‐efficient white organic light‐emitting diodes (OLEDs) are still challenging to make for applications in high‐quality displays and general lighting due to optical confinement and energy loss during electron‐photon conversion. Here, an efficient white OLED structure is shown that combines deterministic aperiodic nanostructures for broadband quasi‐omnidirectional light extraction and a multilayer energy cascade structure for energy‐efficient photon generation. The external quantum efficiency and power efficiency are raised to 54.6% and 123.4 lm W^?1 at 1000 cd m^?2. An extremely small roll‐off in efficiency at high luminance is also obtained, yielding a striking value of 106.5 lm W^?1 at 5000 cd m^?2. In addition to a substantial increase in efficiency, this device structure simultaneously offers the superiority of angular color stability over the visible wavelength range compared to conventional OLEDs. It is anticipated that these findings could open up new opportunities to promote white OLEDs for commercial applications.     

Singlet–Triplet Splitting Energy Management via Acceptor Substitution: Complanation Molecular Design for Deep‐Blue Thermally Activated Delayed Fluorescence Emitters and Organic Light‐Emitting Diodes Application

A barely reached balance between weak intramolecular‐charge‐transfer (ICT) and small singlet–triplet splitting energy (ΔE_ST) for reverse intersystem crossing from non‐emissive triplet state to radiative singlet state impedes the realization of deep‐blue thermally activated delayed fluorescence (TADF) materials. By discarding the twisted‐ICT framework for a flattened molecular backbone and introducing a strong acceptor possessing n–π* transition character, hypsochromic color, a large radiative rate (k_F), and small ΔE_ST are achieved simultaneously. Six molecules with a 9,9‐dimethyl‐10‐phenyl‐9,10‐dihydroacridine (i‐DMAc) donor are synthesized and investigated. Coinciding with time‐dependent density functional theory, the reduced dihedral angles between donor (D) and acceptor (A) weaken ICT from dispersed charge density and enable a large k_F from increased frontier molecular orbitals overlap. Despite the separated highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO) population, the intercalation of phenyl bridges between D–A increases k_F but significantly lowers the local triplet excited state, indicating small HOMO and LUMO overlap is not a sufficient, but necessary condition for reduced ΔE_ST. Integrating short conjugation length and carbonyl or triazine acceptors into the complanation molecules, deep‐blue TADF organic light‐emitting diodes demonstrate maximum external quantum efficiencies of 11.5% and 10.9% with Commission Internationale de l'Eclairage coordinates of (0.16, 0.09) and (0.15, 0.11), respectively, which is quite close to the stringent National Television System Committee blue standard.     

Exciton‐Adjustable Interlayers for High Efficiency,Low Efficiency Roll‐Off,and Lifetime Improved Warm White Organic Light‐Emitting Diodes (WOLEDs) Based on a Delayed Fluorescence Assistant Host

Recently, a new route to achieve 100% internal quantum efficiency white organic light‐emitting diodes (WOLEDs) is proposed by utilizing noble‐metal‐free thermally activated delayed fluorescence (TADF) emitters due to the radiative contributions of triplet excitons by effective reverse intersystem crossing. However, a systematic understanding of their reliability and internal degradation mechanisms is still deficient. Here, it demonstrates high performance and operational stable purely organic fluorescent WOLEDs consisting of a TADF assistant host via a strategic exciton management by multi‐interlayers. By introducing such interlayers, carrier recombination zone could be controlled to suppress the generally unavoidable quenching of long‐range triplet excitons, successfully achieving remarkable external quantum efficiency of 15.1%, maximum power efficiency of 48.9 lm W⁻¹, and extended LT50 lifetime (time to 50% of initial luminance of 1000 cd m⁻²) exceeding 2000 h. To this knowledge, this is the first pioneering work for realizing high efficiency, low efficiency roll‐off, and operational stable WOLEDs based on a TADF assistant host. The current findings also indicate that broadening the carrier recombination region in both interlayers and yellow emitting layer as well as restraining exciplex quenching at carrier blocking interface make significant roles on reduced efficiency roll‐off and enhanced operational lifetime.     

Low Roll‐Off and High Efficiency Orange Organic Light Emitting Diodes with Controlled Co‐Doping of Green and Red Phosphorescent Dopants in an Exciplex Forming Co‐Host

An exciplex forming co‐host is introduced in order to fabricate orange organic light‐emitting diodes (OLEDs) with high efficiency, low driving voltage and an extremely low efficiency roll‐off, by the co‐doping of green and red emitting phosphorescence dyes in the host. The orange OLEDs achieves a low turn‐on voltage of 2.4 V, which is equivalent to the triplet energy gap of the phosphorescent‐green emitting dopant, and a very high external quantum efficiency (EQE) of 25.0%. Moreover, the OLEDs show low efficiency roll‐off with an EQE of over 21% at 10 000 cdm^?2. The device displays a very good orange color (CIE of (0.501, 0.478) at 1000 cdm^?2) with very little color shift with increasing luminance. The transient electroluminescence of the OLEDs indicate that both energy transfer and direct charge trapping takes place in the devices.     

Over 10% EQE Near‐Infrared Electroluminescence Based on a Thermally Activated Delayed Fluorescence Emitter

Significant effort has been made to develop novel material systems to improve the efficiency of near‐infrared organic light‐emitting diodes (NIR OLEDs). Of those, fluorescent chromophores are mostly studied because of their advantages in cost and tunability. However, it is still rare for fluorescent NIR emitters to present good color purities in the NIR range and to have high external quantum efficiency (EQE). Here, a wedge‐shaped D‐π‐A‐π‐D emitter APDC‐DTPA with thermally activated delayed fluorescence property and a small single‐triplet splitting (ΔE_st) of 0.14 eV is presented. The non‐doped NIR device exhibits excellent performance with a maximum EQE of 2.19% and a peak wavelength of 777 nm. Remarkably, when 10 wt% of APDC‐DTPA is doped in 1,3,5‐tris(1‐phenyl‐1H‐benzimidazol‐2‐yl)benzene host, an extremely high EQE of 10.19% with an emission peak of 693 nm is achieved. All these values represent the best result for NIR OLEDs based on a pure organic fluorescent emitter with similar device structure and color gamut.     

Improving the Efficiency of Red Thermally Activated Delayed Fluorescence Organic Light‐Emitting Diode by Rational Isomer Engineering

The development of efficient red thermally activated delayed fluorescence (TADF) emitters with an emission wavelength beyond 600 nm remains a great challenge for organic light‐emitting diodes (OLEDs). Herein, two pairs of isomers are designed and synthesized by attaching electron‐donor 9,9‐diphenyl‐9,10‐dihydroacridine (DPAC) moiety to the different positions of two kinds of highly rigid planar acceptor cores (PDCN and PPDCN). Their TADF efficiencies and emission maxima (599–726 nm) are regulated by molecular isomer manipulation. Interestingly, the photoluminescence quantum yields (Φ_PLs) of trans‐isomers T‐DA‐1 and T‐DA‐2 (78% and 89%) are remarkably higher than those of their corresponding cis‐isomers C‐DA‐1 and C‐DA‐2 (12% and 14%). Significantly increased Φ_PL values can be explained by single crystal structures and theoretical simulation. As a result, a deep red TADF‐OLED based on T‐DA‐2 displays a maximum external quantum efficiency (EQE) of 26.26% at 640 nm. Notably, at a brightness of 100 cd m^?2, the EQE value of T‐DA‐2‐based device still remains at an extremely high level of 23.95%, representing the highest value for reported red TADF‐OLEDs at the same brightness. These results provide a reasonable pathway to optimize optoelectronic properties and thereby construct efficient red TADF emitters through rational isomer engineering.     

High‐Efficiency Near‐Infrared Fluorescent Organic Light‐Emitting Diodes with Small Efficiency Roll‐Off: A Combined Design from Emitters to Devices

The simultaneous realization of high quantum yield and exciton utilizing efficiency (η_r) is still a formidable challenge in near‐infrared (NIR) fluorescent organic light‐emitting diodes (FOLEDs). Here, to achieve a high quantum yield, a novel NIR dye, 4,9‐bis(4‐(diphenylamino)phenyl)‐naphtho[2,3‐c ][1,2,5]selenadiazole, is designed and synthesized with a large highest occupied molecular orbital/lowest unoccupied molecular orbital overlap and an aggregation‐induced emission property, which demonstrates a high photoluminescence quantum yield of 27% at 743 nm in toluene and 29% at 723 nm in a blend film. For a high η_r, an orange‐emitting thermally activated delayed fluorescent material, 1,2‐bis(9,9‐dimethyl‐9,10‐dihydroacridine)‐4,5‐dicyanobenzene, is chosen as the sensitizing host to harvest triplet excitons in devices. The optimized devices achieve a good η_r of 45.7% and a high external quantum efficiency up to 2.65% at 730 nm, with a very small efficiency roll‐off of 2.41% at 200 mA cm^?2, which are among the most efficient values for NIR‐FOLEDs over 700 nm. The effective utilization of triplet excitons via the thermally activated delayed fluorescence‐sensitizing host will pave a way to realize high‐efficiency NIR‐FOLEDs with small efficiency roll‐off.     

Light Out‐Coupling Efficiencies of Organic Light‐Emitting Diode Structures and the Effect of Photoluminescence Quantum Yield

Results obtained from modeling the light out‐coupling efficiency of an organic light‐emitting diode (OLED) structure containing the recently developed first‐generation fac‐tris(2‐phenylpyridine) iridium‐cored dendrimer (Ir‐G1) as the emissive organic layer are reported. Comparison of the results obtained for this material with those of corresponding structures based upon small‐molecule and polymer emissive materials is made. The calculations of out‐coupling efficiency performed here take account of many factors, including the photoluminescence quantum yield (PLQY) of the emissive materials. Further, how each material system might perform with regard to out‐coupling efficiency when a range of possible PLQYs are considered is shown. The calculations show that the very high efficiency of dendrimer‐based OLEDs can be attributed primarily to their high PLQY.     

A Multifunctional Bipolar Luminogen with Delayed Fluorescence for High‐Performance Monochromatic and Color‐Stable Warm‐White OLEDs

Increasing exciton utilization and reducing exciton annihilation are crucial to achieve high performance of organic light‐emitting diodes (OLEDs), which greatly depend on molecular engineering of emitters and hosts. A novel luminogen (SBF‐BP‐DMAC) is synthesized and characterized. Its crystal and electronic structures, thermal stability, electrochemical behavior, carrier transport, photoluminescence, and electroluminescence are investigated. SBF‐BP‐DMAC exhibits enhanced photoluminescence and promotes delayed fluorescence in solid state and bipolar carrier transport ability, and thus holds multifunctionality of emitter and host for OLEDs. Using SBF‐BP‐DMAC as an emitter, the nondoped OLEDs exhibit maximum electroluminescence (EL) efficiencies of 67.2 cd A^?1, 65.9 lm W^?1, and 20.1%, and the doped OLEDs provide maximum EL efficiencies of 79.1 cd A^?1, 70.7 lm W^?1, and 24.5%. A representative orange phosphor, Ir(tptpy)₂acac, is doped into SBF‐BP‐DMAC for OLED fabrication, giving rise to superior EL efficiencies of 88.0 cd A^?1, 108.0 lm W^?1, and 26.8% for orange phosphorescent OLEDs, and forward‐viewing EL efficiencies of 69.3 cd A^?1, 45.8 lm W^?1, and 21.0% for two‐color hybrid warm‐white OLEDs. All of these OLEDs can retain high EL efficiencies at high luminance, with very small efficiency roll‐offs. The outstanding EL performance demonstrates the great potentials of SBF‐BP‐DMAC in practical display and lighting devices.