Thermally Activated Delayed Fluorescence Devices Research Articles

A Low‐Cost Facile Synthesis of Solution‐Processable N‐Phenylcarbazole/Triazine Hybrid Host for Orange Thermally‐Activated Delayed Fluorescent Organic Light‐Emitting Diodes

An N‐phenylcarbazole/triazine hybrid of 2,4,6‐tris(9‐phenyl‐9H‐carbazol‐3‐yl)‐1,3,5‐triazine (TPhCz‐Trz) is successfully synthesized using cheap raw materials via facile synthesis method with a high yield of 60.9%. Good thermal (Td, 284 °C) and morphological stability are realized due to its rigid structure and three‐dimensional steric hindrance. The high triplet energy level of 2.88 eV for the host is very important to inhibit the energy reverse from guest to host. Surprisingly, orange thermally‐activated delayed fluorescent (TADF) organic light‐emitting diodes (OLEDs) are achieved by using excellent green emitter 9,10‐bis(4‐(3,6‐ditertbutyl‐9H‐carbazol‐9‐yl)‐2,6‐dimethyl‐phenyl)‐9,10‐diboraanth‐racene (tBuCzDBA) with the maximum external quantum efficiency of 14.0%. The large bathochromic‐shift is probably induced by the polarity of the host compared to the emission peak of 542 nm of tBuCzDBA‐based green TADF device. This indicates high performance of devices with different colors would be achieved through optimizing molecular structure of the host and device framework using tBuCzDBA as emitter.

Near-infrared OLEDs with high radiance of ∼105 mW Sr−1 m−2 for single-wavelength oxygen saturation detection

Real-time in-home monitoring oxygen saturation (SpO2) is of great significance for early diagnosis and treatment of severe COVID-19, however, which should be based on multi-wavelength technology and complicated equipment. Herein, as a proof of concept, 740 nm is chosen as singlet light source for single-wavelength SpO2 detection, since absorbance differences between oxyhemoglobin (HbO2) and other interferences are the most rational. To controllably optimize excited-state characteristics of a near-infrared (NIR) thermally activated delayed fluorescence (TADF) emitters, an “extra conjugation” method is demonstrated by CNQP-TPA, whose 2,3-dicyanoquinoxaline phenanthroline (CNQP) acceptor forms strong intramolecular hydrogen bonds with two phenyls of triphenylamine (TPA) donors, which not only reduce excited-state relaxation-induced energy lose, but also provide more space for accurate modulation of intermolecular charge transfer (CT) interactions. As consequence, at the suitable doping concentration, CNQP-TPA endowed its device with a high radiance reaching ∼105 mW Sr−1 m−2 and desired electroluminescence peak at 744 nm. This device is further used as light source to detect SpO2 variation under single-wavelength mode. These results indicate the potential of NIR TADF materials and devices for in-home SpO2 monitoring and pre-diagnosis and prevention of severe COVID-19.

Deuterium Renders Fast Risc and Low Non‐Radiation Decays of TADF Emitters

AbstractA simple strategy to simultaneously achieve fast reverse intersystem crossing (RISC) and low non‐radiative decay rates in thermally activated delayed fluorescence (TADF) emitters is still lacking. Here, the green TADF emitters containing aromatic ketones and prepared D‐Ph and D‐PhCz containing different amounts of deuterium are deuterated to study the mechanism of deuterium on the performance improvement of TADF devices. Due to the heavy atom effect of deuterium, the non‐radiative transition rate constant (knr) of D‐Ph and D‐PhCz decreases. Most importantly, deuterium reduces the reorganization energy (λ) of (T1‐S1), which greatly improves the reverse intersystem crossing rate constant (kRISC) and the exciton utilization efficiency. The results show that the external quantum efficiency of D‐Ph and D‐PhCz increases to 25.69% and 24.92%, respectively. This study provides a simple and efficient strategy for reducing non‐radiative energy loss and increasing kRISC to achieve high‐performance TADF emitters.

Overcoming power efficiency limitation of white fluorescence light‐emitting diodes via multilevel‐hydrogen‐bond matrix

AbstractHigh power efficiency and low efficiency roll‐off at practical luminance are two requirements for new‐generation energy‐saving lighting technologies, which are still bottlenecks of thermally activated delayed fluorescence (TADF) white organic light‐emitting diodes (WOLED), despite the advantages of TADF materials and devices in low cost and high sustainability. Herein, we developed a spiro phosphine oxide host named SSOXSPO, which can form multiple and multidirectional intermolecular hydrogen bonds (IHB). The resulted multilevel IHB network integrates long‐range ordered and short‐range disordered alignments for suppressing triplet‐polaron quenching (TPQ) and triplet‐triplet annihilation (TTA). Electronic characteristics of SSOXSPO matrix are further regulated, leading to the optimal exciton allocation through balancing energy and charge transfer. As consequence, using SSOXSPO as host, the single‐emissive‐layer TADF WOLEDs realized the record performance, including ultralow operation voltage as ∼4.0 V, power efficiency beyond fluorescent tube (70.1 lm W−1) and negligible external quantum efficiency roll‐off (3%) at 1000 nits for indoor lighting. This work demonstrates that multiple interplays supported by host matrixes in TADF WOLEDs can facilitate the synergistic effects of TADF emitters on 100% exciton utilization.

Longevity gene responsible for robust blue organic materials employing thermally activated delayed fluorescence

The 3rd-Gen OLED materials employing thermally-activated delayed fluorescence (TADF) combine advantages of first two for high-efficiency and low-cost devices. Though urgently needed, blue TADF emitters have not met stability requirement for applications. It is essential to elucidate the degradation mechanism and identify the tailored descriptor for material stability and device lifetime. Here, via in-material chemistry, we demonstrate chemical degradation of TADF materials involves critical role of bond cleavage at triplet state rather than singlet, and disclose the difference between bond dissociation energy of fragile bonds and first triplet state energy (BDE-ET1) is linearly correlated with logarithm of reported device lifetime for various blue TADF emitters. This significant quantitative correlation strongly reveals the degradation mechanism of TADF materials have general characteristic in essence and BDE-ET1 could be the shared “longevity gene”. Our findings provide a critical molecular descriptor for high-throughput-virtual-screening and rational design to unlock the full potential of TADF materials and devices.

Thermally Enhanced and Long Lifetime Red TADF Carbon Dots via Multi-Confinement and Phosphorescence Assisted Energy Transfer.

Thermally activated delayed fluorescence (TADF) materials, which can harvest both singlet and triplet excitons for high-efficiency emission, have attracted widespread concern for their enormous applications. Nevertheless, luminescence thermal quenching severely limits the efficiency and operating stability in TADF materials and devices at high temperature. Herein, a surface engineering strategy is adopted to obtain unique carbon dots (CDs)-based thermally enhanced TADF materials with ≈250% enhancement from 273 to 343K via incorporating seed CDs into ionic crystal network. The rigid crystal network can simultaneously boost reverse intersystem crossing process via enhancing spin-orbit coupling between singlet and triplet states and suppressing non-radiative transition rate, contributing to the thermally enhanced TADF character. Benefiting from efficient energy transfer from triplet states of phosphorescence center to singlet states of CDs, TADF emission at ≈600nm in CDs displays a long lifetime up to 109.6ms, outperforming other red organic TADF materials. Thanks to variable decay rates of the delayed emission centers, time and temperature-dependent delayed emission color has been first realized in CDs-based delayed emission materials. The CDs with thermally enhanced and time-/temperature-dependent emission in one material system can offer new opportunities in information protection and processing.

Multiple‐Resonance‐Type TADF Emitter as Sensitizer Improving the Performance of Blue Fluorescent Organic Light‐Emitting Diodes

AbstractDue to the limitation of donor and acceptor group selection, the efficient thermally activated delayed fluorescence (TADF) type sensitizer used for blue organic light‐emitting diodes (OLEDs) is rare. Multiple resonance (MR) type TADF emitters can easily achieve efficient blue emission. And the compounds exhibit small Stokes shift and lower absorption energy under the same emission color compared with traditional TADF, mitigating the damage of high‐energy absorption of sensitizer on material stability. However, their characteristics as sensitizers have not been explored. In this work, a deep‐blue MR‐TADF compound (3tPAB) is selected as a sensitizer for both blue traditional fluorescence and MR‐TADF OLED. Given the improved photoluminescence quantum yield and the utilization of triplet excitons, the TADF‐sensitized fluorescent device using 3tPAB as sensitizer presents a maximum external quantum efficiency (EQEmax) of 14.4%, showing ≈2.4 times increase compared with the device without sensitizer. More impressively, TADF‐sensitized TADF (TST) device using 3tPAB as sensitizer and MR‐TADF compound PhDMAC‐BN as emitter exhibits EQEmax of 33.9%. And in TST device, efficient Förster energy transfer is demonstrated, thus, the device can maintain high color purity. The work first demonstrates the feasibility of MR‐TADF as a sensitizer and provides a new strategy for developing high‐performance blue OLED with high color purity.

Design of highly stable thermally activated delayed fluorescence emitters via the overlap degree of HOMO-LUMO distributions

Two questions including the judgement of thermally activated delayed fluorescence (TADF) property and high stability are crucial for designing TADF emitters. In this work, based on the experimental TADF emitters, we investigated the essential reason of improving the stability of TADF device by introducing tert-butyl and phenyl into the peripheral position of carbazole, and suggested to judge the TADF property via the concrete HOMO-LUMO overlap calculation. To design highly stable TADF emitters, several pairs of positional isomers combining tert-Butyl-substituted carbazole as donor with different acceptors are designed by connecting the donor and acceptor groups through the orth-, meta- and para-position. Via the HOMO-LUMO overlap calculation, the potential TADF emitters have been point out. Our results find that the HOMO-LUMO overlap values are dependent on the linkage styles and acceptor species. For example, the designed molecules m-Cz-BF/BF-Cz/NOBF2-DTCz/CPS, o-CNImPy-Cz/PQM-Cz, and m-, p-Cz-NAI have the potential to perform better TADF performance than other isomers. These results indicate that the HOMO-LUMO overlap could be used to identify synthetic motifs for promising TADF candidates among structural isomers.

Synergistic Control of Hot Exciton Relaxation and Exciton–Polaron Dispersion Using a Polaron Retarder for Long Lifetime in Thermally Activated Delayed Fluorescence Organic Light‐Emitting Diodes

AbstractThe operational stability of thermally activated delayed fluorescence (TADF) organic light‐emitting diodes (OLEDs) is an essential property to utilize their efficient triplet harvesting ability and reduce the usage of expensive metal complexes in commercial applications. In this study, a device strategy for extending the device lifetime using the synergistic effects of the hot exciton relaxation and polaron dispersion is demonstrated. By doping a polaron retarder to a conventional TADF‐emitting layer, the lifetime of the blue TADF device is enhanced by 98% at an initial luminance of 1000 cd m−2. Kinetic modeling describing the device degradation indicates that the concurrent contribution via the suppression of exciton and polaron interactions and hot exciton‐related degradation are the origin of the lifetime enhancement. It is demonstrated that the introduction of the polaron retarder finely fits to the ground and that the excited energetic relationship of the host–dopant structure simultaneously improves the operation lifetime and device efficiency without sacrificing other device characteristics.

Ultrapure Blue Thermally Activated Delayed Fluorescence (TADF) Emitters Based on Rigid Sulfur/Oxygen-Bridged Triarylboron Acceptor: MR TADF and D-A TADF.

Organic light-emitting diodes (OLEDs) still face a significant challenge in finding blue thermally activated delayed fluorescence (TADF) emitters that can achieve narrowband emission and high efficiency. In this work, we successfully design and synthesize a novel kind of TADF emitters based on rigid sulfur/oxygen-bridged triarylboron acceptor for ultrapure blue with narrowband electroluminescence. Time-dependent density functional theory (TD-DFT) calculations and photophysical results indicate the different intramolecular charge-transfer (ICT) character of two emitters. Benefiting from the rigid aromatic framework, both emitters exhibited deep-blue emission at 444 and 447 nm with a small full-width at half-maximum (fwhm) of about 33 nm, and a small singlet (S1)-triplet (T1) energy gap (ΔEST) of 0.23 and 0.36 eV. Consequently, OLEDs based on PhCz-TOSBA and TPA-TOSBA exhibit deep blue electroluminescence at 456 nm with fwhm of about 55 nm, affording high external quantum efficiencies (EQEs) of 16.69% with CIE coordinates of (0.14, 0.15) and 16.65% with CIE coordinates of (0.14, 0.12), respectively. These findings show that PhCz-TOSBA and TPA-TOSBA are superior emitters in ultrapure blue TADF devices.

The Effect of Carrier Transport Layer on the Electroluminescent Properties of Solution‐Processed Thermally Activated Delayed Fluorescent Device based on 4CzPN

The performance of solution‐processed organic light‐emitting diodes (OLEDs) based on thermally activated delayed fluorescent (TADF) emitter of 3,4,5,6‐tetrakis (carbazol‐9‐yl)‐1, 2‐dicyanobenzene (4CzPN) is optimized via adjusting the carrier transport layer. The device with 1,3,5‐Tris(1‐phenyl‐1Hbenzimidazol‐2‐yl) benzene (TPBi) as electron transport layer (ETL) demonstrates much higher efficiency compared to the device with 1, 3, 5‐Tri [(3‐pyridyl)‐phen‐3‐yl] benzene (TmPyPB) as ETL. The proposed mechanism is that triplet‐charge annihilation is alleviated since fewer holes are accumulated at the interface between the emission layer (EML) and ETL due to lower lying highest occupied molecular orbital (HOMO) of TPBi. In addition, the hole transport material of (N,N′‐bis (3‐methylphenyl)‐N,N′‐bis (phenyl)‐benzidine) (TPD) inserted at the PEDOT:PSS/EML interface could block electron leakage and suppress the exciton quenching by PEDOT:PSS. And thus, the device efficiency is improved. The current efficiency of 18.46 cd A−1 and external quantum efficiency of 5.99% are achieved for the solution‐processed 4CzPN‐based device with TPBi as ETL and TPD as hole transport layer (HTL). It is illuminated that suppressing exciton quenching via manipulating carrier behaviors in the EML is an efficient approach for fulfilling high‐performance solution‐processed TADF devices.

From para to ortho: Incarnating conventional TADF molecules into AIE-TADF molecules for highly-efficient non-doped OLEDs

Conventional thermally activated delayed fluorescence (TADF) molecules are generally constructed by planar moieties with a linear donor–acceptor (D-A) configuration but suffer from aggregation-caused quenching (ACQ). Herein, we provide a strategy to develop aggregation-induced emission TADF (AIE-TADF) emitters that are consisted of ACQ moieties through the substitute change from para-, meta- to ortho-position, which means incarnating conventional TADF molecules into AIE-TADF molecules. The ortho-substituted molecule exhibits a highly twisted conformation to hinder π-π stacking and thus avoids ACQ, while a large D-A twisted angle contributes to fast reversible intersystem crossing and low non-radiative decay, thus facilitating the AIE-TADF feature in neat film. The ortho-substituted molecule renders the non-doped devices with a high external quantum efficiency of 22.14%. These results certify that adjusting substitute position is a simple and practical strategy for the development of high-efficiency TADF materials and devices.

Analysis of efficiency variations in ν-DABNA based thermally activated delayed fluorescence OLED devices

Recently, diboron based blue thermally activated delayed fluorescence (TADF) material, N7,N7,N13,N13,5,9,11,15-octaphenyl-5,9,11,15-tetrahydro-5,9,11,15-tetraaza-19b, 20b-diboradinaphtho[3,2,1-de:1′,2′,3′-jk] pentacene-7,13-diamine (ν-DABNA) has drawn attention from the researchers due to its good device performances as well as narrow full-width at half maximum (FWHM) in OLED devices. In this work, we report a detailed analysis to understand the exact reason behind the efficiency variations in ν-DABNA based TADF devices using different types of host materials. Interestingly, we found that triplet exciton density of ν-DABNA molecule is mainly located at the central region, so the triplet exciton energy transfer from the host material is hindered due to the presence of peripheral diphenylamine extensions. Hence, our analysis suggests that utilizing wide band gap host materials in ν-DABNA based TADF devices exhibit excellent device efficiencies with the support of trap assisted emission mechanism. This study brings an in-depth understanding of efficiency variations in the ν-DABNA based OLED devices.

Dipole Moment- and Molecular Orbital-Engineered Phosphine Oxide-Free Host Materials for Efficient and Stable Blue Thermally Activated Delayed Fluorescence.

To utilize thermally activated delayed fluorescence (TADF) technology for future displays, it is necessary to develop host materials which harness the full potential of blue TADF emitters. However, no publication has reported such hosts yet. Although the most popular host for blue TADF, bis[2‐(diphenylphosphino)phenyl]ether oxide (DPEPO) guarantees high‐maximum external quantum efficiency (EQEmax) TADF devices, they exhibit very short operational lifetimes. In contrast, long‐lifespan blue TADF devices employing stable hosts such as 3′,5‐di(9H‐carbazol‐9‐yl)‐[1,1′‐biphenyl]‐3‐carbonitrile (mCBP‐CN) exhibit much lower EQEmax than the DPEPO‐employed devices. Here, an elaborative approach for designing host molecules is suggested to achieve simultaneously stable and efficient blue TADF devices. The approach is based on engineering the molecular geometry, ground‐ and excited‐state dipole moments of host molecules. The engineered hosts significantly enhance delayed fluorescence quantum yields of TADF emitters, as stabilizing the charge‐transfer excited states of the TADF emitters and suppressing exciton quenching, and improve the charge balance. Moreover, they exhibit both photochemical and electrochemical stabilities. The best device employing one of the engineered hosts exhibits 79% increase in EQEmax compared to the mCBP‐CN‐employed device, together with 140% and 92‐fold increases in operational lifetime compared to the respective mCBP‐CN‐ and the DPEPO‐based devices.

Approaching Nearly 40% External Quantum Efficiency in Organic Light Emitting Diodes Utilizing a Green Thermally Activated Delayed Fluorescence Emitter with an Extended Linear Donor-Acceptor-Donor Structure.

Thermally activated delayed fluorescence (TADF) emitters featuring preferential horizontal emitting dipole orientation (EDO) are in urgent demand for enhanced optical outcoupling efficiency in organic light-emitting diodes (OLEDs). However, simultaneously manipulating EDO and optoelectronic properties remains a formidable challenge. Here, an extended linear D-A-D structure with both enlarged donor (D) and acceptor (A) π-systems is established, not only elaborately manipulating parallel horizontal molecular orientation and EDO along its long axis by multi-driving-forces for a high horizontal dipole ratio (Θ// ), but also delocalizing distribution of frontier energy levels for optimized electronic properties. The proof-of-the-concept emitter simultaneously affords a high Θ// of 92%, a high photoluminescence quantum yield of 95%, and a fast reverse intersystem crossing rate of 1.16 × 106 s-1 . The corresponding OLED achieves a champion maximum external quantum efficiency of 39.1% among all green TADF devices without any external light-extraction techniques, together with a maximum power efficiency of 112.0lm W-1 and alleviated efficiency roll-off. These findings may inspire even better full-color TADF emitters that push the device efficiency toward the theoretical limits.

Achieving Narrow FWHM and High EQE Over 38% in Blue OLEDs Using Rigid Heteroatom‐Based Deep Blue TADF Sensitized Host

AbstractSimultaneously obtaining high efficiency and deep blue emission in organic light emitting diodes (OLEDs) remains a challenge. To overcome the demands associated with deep blue thermally activated delayed fluorescence (TADF) emitters, two deep blue TADF materials namely, DBA–BFICz and DBA–BTICz, are designed and synthesized by incorporating oxygen‐bridged boron (DBA) acceptor with heteroatoms, oxygen and sulphur‐based donors, BFICz and BTICz, respectively. Both TADF materials show deep blue photoluminescence emissions below 450 nm by enhancing the optical band gap over 2.8 eV through deeper highest occupied molecular orbital (HOMO) level of heteroatom based donor moieties. At the same time, the photoluminescence quantum yields (PLQYs) of both TADF materials remain over 94%. The TADF device with DBA–BFICz as an emitter exhibits a good external quantum efficiency (EQE) of 33.2%. Since both new TADF materials show deep blue emissions and high efficiencies, hyperfluorescence (HF) OLED devices are fabricated using ν‐DABNA as a fluorescence dopant. DBA–BFICz as a TADF sensitized host in HF–OLED reveals an outstanding EQE of 38.8% along with narrow full width at half maximum of 19 nm in the bottom emission pure blue OLEDs. This study provides an approach to develop deep blue TADF emitters for highly efficient OLEDs.

Novel secondary acceptor based molecular design for superb lifetime in thermally activated delayed fluorescent organic light-emitting diodes through high bond energy and fast up-conversion

The device lifetime of the thermally activated delayed fluorescent (TADF) organic light-emitting diodes (OLEDs) was dramatically improved using a novel secondary acceptor based molecular design, while achieving high quantum efficiency of 18.5%. A series of TADF materials derived from a diphenyltriazine acceptor and a benzonitrile secondary acceptor were developed to study the effect of the secondary acceptor on the TADF parameters and device performances. The secondary acceptor simultaneously increased the bond dissociation energy in triplet excited state and the reverse intersystem crossing rate, which resulted in high external quantum efficiency, reduced efficiency roll-off, and lifetime extension. In particular, the lifetime was extended by more than 20 times, compared to that of the TADF emitter without the secondary acceptor. The correlation of the materials parameters with the lifetime of the green TADF OLEDs revealed that the bond dissociation energy and reverse intersystem crossing rate are well correlated with the device operational lifetime of the TADF OLEDs. Additionally, the secondary acceptor based TADF emitter outperformed the state-of-the-art green TADF emitter in terms of efficiency and lifetime. Optimization of the process condition of the device fabrication provided an extremely long lifetime of 532 h at 1000 cd/m2 up to 90% of initial luminance, which has not been previously achieved in the TADF devices.

20‐2: Invited Paper: High‐Efficiency, Long‐Lifetime, Deep‐Blue Organic Light‐Emitting Diodes

Simultaneously achieving a high efficiency and long lifetime in deep‐blue organic light‐emitting diodes (OLEDs) is challenging because most high‐efficiency blue OLEDs suffer from a short device lifetime. We describe high‐efficiency, long‐lifetime deep‐blue thermally activated delayed fluorescence (TADF) OLEDs by designing new blue TADF materials and developing a triplet‐exciton‐recycling device protocol. A new deep‐blue TADF emitter that harvests the singlet excitons of the fluorescent emitter was embedded in the device structure of the triplet‐exciton‐distributed (TED) TADF devices built on a new device protocol. Two TADF materials, one distributing and one emitting, were doped into a host with a triplet energy between those of these two materials. Codoping the TADF materials to realize TED‐TADF in the low‐triplet‐energy host achieved a high external quantum efficiency of 28.3% and almost quadrupled the device lifetime relative to that of the reference device with only the emitting TADF material. Recycling the triplet excitons of the distributing TADF material that were quenched by the host and the singlet‐exciton‐harvested TADF emission of the emitting TADF material were the main mechanisms realizing simultaneous high efficiency and extended lifetime. The TADF devices based on this new approach combined with an optically optimized device stack demonstrated >30% efficiency, a lifetime of >6,000 h, and a y color coordinate <0.10, which are unprecedented device metrics for deep‐blue OLEDs. This is the first work reporting such a device lifetime for the deep‐blue OLEDs with a y color coordinate below 0.09.

Over 30 000 h Device Lifetime in Deep Blue Organic Light‐Emitting Diodes with y Color Coordinate of 0.086 and Current Efficiency of 37.0 cd A−1

AbstractThe development of highly efficient and long‐life deep blue devices has been an ongoing challenge in the field of organic light‐emitting diodes (OLEDs) because of the short lifetime inherent to highly efficient blue devices. The efficiency and lifetime issues of the deep blue OLEDs are resolved in this work by engineering the device architecture by managing the triplet excitons of the phosphorescence and thermally activated delayed fluorescence (TADF) emitters. The CN‐modified imidazole type Ir emitter and boron‐based TADF emitter with similar emission energy are combined to develop an energy exchanging phosphorescence and TADF (EPHTADF) device. An emission mechanism inducing both forward and backward energy transfer concurrently between the Ir phosphor and TADF emitter shortens the triplet exciton lifetime of the two emitters, which dramatically enhances the device lifetime of the blue devices. Deep blue OLEDs triggering EPHTADF emission ultimately yields device performances including high current efficiency (CE) of 37.0 cd A−1, color‐coordinate corrected CE of 430.9 cd A−1, extrapolated device lifetime of more than 30 000 h, and deep blue color coordinates of (0.122, 0.086). This work demonstrates the potential of EPHTADF devices to serve as the solution to the problematic issues of the blue OLEDs.

Acceptor-Donor-Acceptor-Type Orange-Red Thermally Activated Delayed Fluorescence Materials Realizing External Quantum Efficiency Over 30% with Low Efficiency Roll-Off.

Two new orange-red thermally activated delayed fluorescence (TADF) materials, PzTDBA and PzDBA, are reported. These materials are designed based on the acceptor-donor-acceptor (A-D-A) configuration, containing rigid boron acceptors and dihydrophenazine donor moieties. These materials exhibit a small ΔEST of 0.05-0.06eV, photoluminescence quantum yield (PLQY) as high as near unity, and short delayed exciton lifetime (τd ) of less than 2.63 µs in 5 wt% doped film. Further, these materials show a high reverse intersystem crossing rate (krisc ) on the order of 106 s-1 . The TADF devices fabricated with 5 wt% PzTDBA and PzDBA as emitting dopants show maximum EQE of 30.3% and 21.8% with extremely low roll-off of 3.6% and 3.2% at 1000 cd m-2 and electroluminescence (EL) maxima at 576 nm and 595nm, respectively. The low roll-off character of these materials is analyzed by using a roll-off model and the exciton annihilation quenching rates are found to be suppressed by the fast krisc and short delayed exciton lifetime. These devices show operating device lifetimes (LT50 ) of 159 and 193 h at 1000 cd m-2 for PzTDBA and PzDBA, respectively. The high efficiency and low roll-off of these materials are attributed to the good electronic properties originatng from the A-D-A molecular configuration.