Rational Molecular Design Overcoming the Long Delayed Fluorescence Lifetime and Serious Efficiency Roll-Off in Blue Thermally Activated Delayed Fluorescent Devices

Blue thermally activated delayed fluorescent (TADF) devices with short excited-state lifetime, high reverse intersystem crossing rate, and low-efficiency roll-off were developed by managing the molecular structure of donor–acceptor-type blue emitters. Three isomers of blue TADF emitters with a diphenyltriazine acceptor and three carbazole donors were synthesized. The position of the donor moieties in the phenyl linker connecting the donor and acceptor moieties was controlled to devise compounds with a short delayed fluorescence lifetime. A blue TADF emitter with three carbazole donors at 2-, 3-, and 4- positions of a phenyl linker shortened the excited state lifetime to 4.1 μs, showed a high external quantum efficiency of 20.4 %, and low efficiency roll-off of less than 10 % at 1000 cd m⁻². Therefore, a molecular design distorting the donors by aligning them in a consecutive way is useful to resolve the issues of long delayed fluorescence lifetime and efficiency roll-off of blue TADF devices.     

Aggregation-Enhanced Emission and Thermally Activated Delayed Fluorescence of Derivatives of 9-Phenyl-9H-Carbazole: Effects of Methoxy and tert-Butyl Substituents

Derivatives of 9-phenyl-9H-carbazole were synthesized as efficient emitters exhibiting both thermally activated delayed fluorescence and aggregation-induced emission enhancement. Effects of methoxy and tert-butyl substituents at the different positions of carbazolyl groups on the properties of the emitters were studied. Depending on the substitutions, photoluminescence quantum yields (PLQY) of non-doped solid films of the compounds ranged from 17 % to 53 % which were much higher than those observed for the solutions in low-polarity solvent toluene. Compounds substituted at C-3 and C-6 positions of carbazole moiety by methoxy- and tert-butyl- groups showed the highest solid-state PLQY. Ionization potentials of the studied derivatives in solid-state were found to be in the short range of 5.75–5.89 eV. Well-balanced hole and electron mobilities were detected for tert-butyl-substituted compound. They exceeded 10⁻⁴ cm² (V×s)⁻¹ at electric fields higher than 3×10⁵ V cm⁻¹. Two compounds with the highest solid-state PLQYs showed higher efficiencies in non-doped organic light-emitting diodes than in the doped devices. Maximum external quantum efficiency of 7.2 % and brightness of 15000 cd m⁻² were observed for the best device.     

Quinoxaline and Pyrido[x,y-b]pyrazine-Based Emitters: Tuning Normal Fluorescence to Thermally Activated Delayed Fluorescence and Emitting Color over the Entire Visible-Light Range

Quinoxaline (Q), pyrido[2,3-b]pyrazine (PP) and pyrido[3,4-b]pyrazine (iPP) are used as electron acceptors (A) to design a series of D–π–A-type light-emitting materials with different donor (D) groups. By adjusting the molecular torsion angles through changing D from carbazole (Cz) to 10-dimethylacridine (DMAC) or 10H-phenoxazine (PXZ) for a fixed A, the luminescence is tuned from normal fluorescence to thermally activated delayed fluorescence (TADF). By gradually enhancing the intramolecular charge-transfer extent through combining different D and A, the emission color is continuously and regularly tuned from pure blue to orange–red. Organic light-emitting diodes (OLEDs) containing these compounds as doped emitters exhibit bright electroluminescence with emission colors covering the entire visible-light range. An external quantum efficiency (η_ext) of 1.2 % with excellent color coordinates of (0.16, 0.07) is obtained for the pure-blue OLED of Q-Cz. High η_ext values of 12.9 (35.9) to 16.7 % (51.9 cd A⁻¹) are realized in the green, yellow, and orange–red TADF OLEDs. All PP- and iPP-based TADF emitters exhibit superior efficiency stabilities to that of analogues of Q. This provides a practical strategy to tune the emission color of Q, PP, and iPP derivatives with the same molecular skeletons over the entire visible-light range.     

Constructing a Novel Dendron for a Self‐Host Blue Emitter with Thermally Activated Delayed Fluorescence: Solution‐Processed Nondoped Organic Light‐Emitting Diodes with Bipolar Charge Transfer and Stable Color Purity

Self‐host thermally activated delayed fluorescence (TADF) materials have recently been identified as effective emitters for solution‐processed nondoped organic light‐emitting diodes (OLEDs). However, except for the carbazole unit, few novel dendrons have been developed to build self‐host TADF emitters. This study reports two self‐host blue materials, tbCz‐SO and poCz‐SO, with the same TADF emissive core and different dendrons. The influence of the peripheral dendrons on the photophysical properties and electroluminescent performances of the self‐host materials were systematically investigated. The transient fluorescence and electroluminescence spectra indicated that the diphenylphosphoryl carbazole units could effectively encapsulate the emissive core to reduce the concentration quenching effect and to enhance reverse intersystem crossing. By using tbCz‐SO and poCz‐SO as host‐free blue emitters, the performance of the solution‐processed nondoped OLED device demonstrated that a more balanced charge transfer from the bipolar dendrons would offer a better current efficiency of 10.5 cd A⁻¹ and stable color purity with Commission Internationale de L'Eclairage units of (0.18, 0.27).     

Four‐Coordinate Boron Emitters with Tridentate Chelating Ligand for Efficient and Stable Thermally Activated Delayed Fluorescence Organic Light‐Emitting Devices

A new class of four‐coordinate donor‐acceptor fluoroboron‐containing thermally activated delayed fluorescence (TADF) compounds bearing a tridentate 2,2′‐(pyridine‐2,6‐diyl)diphenolate (dppy) ligand has been successfully designed and synthesized. Upon varying the donor moieties from carbazole to 10H‐spiro[acridine‐9,9′‐fluorene] to 9,9‐dimethyl‐9,10‐dihydroacridine, these boron derivatives exhibit a wide range of emission colors spanning from blue to yellow with a large spectral shift of 2746 cm^?1, with high PLQYs of up to 96 % in the doped thin film. Notably, vacuum‐deposited organic light‐emitting devices (OLEDs) made with these boron compounds demonstrate high performances with the best current efficiencies of 55.7 cd A^?1, power efficiencies of 58.4 lm W^?1 and external quantum efficiencies of 18.0 %. More importantly, long operational stabilities of the green‐emitting OLEDs based on 2 with half‐lifetimes of up to 12 733 hours at an initial luminance of 100 cd m^?2 have been realized. This work represents for the first time the design and synthesis of tridentate dppy‐chelating four‐coordinate boron TADF compounds for long operational stabilities, suggesting great promises for the development of stable boron‐containing TADF emitters.     

Judicious Choice of N-Heterocycles for the Realization of Sky-Blue- to Green-Emitting Carbazolylgold(III) C^C^N Complexes and Their Applications for Organic Light-Emitting Devices

A new class of sky-blue- to green-emitting carbazolylgold(III) C^C^N complexes containing pyrazole or benzimidazole moieties has been successfully designed and synthesized. Through the judicious choice of the N-heterocycles in the cyclometalating ligand and the tailor-made carbazole moieties, maximum photoluminescence quantum yields of 0.52 and 0.39 have been realized in the green- and sky-blue-emitting complexes, respectively. Solution-processed and vacuum-deposited organic light-emitting devices (OLEDs) based on the benzimidazole-containing complexes have been prepared. The sky-blue-emitting device shows an emission peaking at 484 nm with a narrow full-width at half-maximum of 57 nm (2244 cm⁻¹), demonstrating the potential of this class of complexes in the application of OLEDs with high color purity. In addition, high maximum external quantum efficiencies of 12.3 % and a long operational half-lifetime of over 5300 h at 100 cd m⁻² have been achieved in the vacuum-deposited green-emitting devices.     

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.     

RGB Small Molecules Based on a Bipolar Molecular Design for Highly Efficient Solution‐Processed Single‐layer OLEDs

In this paper, we describe a bipolar molecular design for small molecule solution‐processed organic light emitting diodes (OLEDs). Combining the rigidity of the conjugated emissive cores and the flexibility of the peripheral alkyl‐linked carbazole groups, two series of highly efficient bipolar RGB (red, green, blue) emitters have been synthesized and characterized. The emissive cores are composed of electron‐withdrawing groups; the carbazole groups endow the materials electron‐donating units. Such bipolar structures are advantageous for the carrier injection and balance. Four peripheral carbazole groups are introduced in T‐series materials (TCDqC, TCSoC, TCBzC, TCNzC), and another four in O‐series materials (OCDqC, OCSoC, OCBzC, OCNzC). With the single‐layer device configuration of ITO/PEDOT:PSS/emitting layer/CsF/Al, two green devices exhibited excellent performance with a maximum luminescence efficiency of over 6.4 cd A^?1, and a high maximum luminance of more than 6700 cd m^?2. In addition, compared with the T‐series, the luminescence efficiency of blue and red devices based on O‐series materials increased from 1.6 to 2.8 cd A^?1 and 0.2 to 1.3 cd A^?1, respectively. To our knowledge, the performance of the blue device based on OCSoC is among the best of the blue small‐molecule solution‐processed single‐layer devices reported so far.     

Synthesis and electroluminescent performance of thermally activated delayed fluorescence‐conjugated polymers with simple formylphenyl as pendant acceptor

Formylphenyl has been demonstrated to act as an acceptor to construct thermally activated delayed fluorescence (TADF) emitter, and therefore a series of the TADF‐conjugated polymers with formylphenyl as pendant acceptor and carbazole/acridine as backbone donor are designed and synthesized. All polymers involve the twisted donor/acceptor structural moieties with the sufficiently spatial separation between the highest occupied molecular orbital and the lowest unoccupied molecular orbital as well as a small singlet/triplet splitting, and exhibit the legible TADF features confirmed by theoretical calculation and their transient decay spectra. The solution‐processed organic light‐emitting diodes using neat film of the polymers as emissive layer achieve excellent performance with the maximum external quantum efficiency (EQE) of up to 10.6%, the maximum current efficiency of up to 35.3 cd A⁻¹ and the low turn‐on voltage of 2.7 V. Moreover, the EQE still remains 10.3% at the luminance of 1000 cd m⁻² with the low driving voltage of 4.4 V and extremely small efficiency roll‐off. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1989–1996     

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 (k_R) of 1.0×10⁷ s⁻¹ and a swift reverse intersystem crossing rate (k_RISC) of 1.8×10⁶ s⁻¹, complemented by a slow non-radiative decay rate (k_NR) 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 (EQE_max) 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.     

A Tetrahedral Bisacridine Donor Enables Fast Radiative Decay in Thermally Activated Delayed Fluorescence Emitter

Achieving high efficiency and low efficiency roll-off simultaneously is of great significance for further application of thermally activated delayed fluorescent (TADF) emitters. A balance between radiative decay and reversed intersystem crossing must be carefully established. Herein, we propose a qunolino-acridine (QAc) donor composing two acridine with both planar (pAc) and bended (bAc) geometries. Combining with triazine, a TADF emitter QAc-TRZ is assembled. The pAc provides a well interaction with triazine which ensures a decent TADF behavior, while the bAc offers a delocalization of highest occupied molecular orbital (HOMO) which guarantees an enhancement of radiative decay. Remarkably, QAc-TRZ enables a highly efficient organic light emitting diode (OLED) with maximum external quantum efficiency (EQE) of 37.3 %. More importantly, the efficiencies under 100/1000 cd m⁻² stay 36.3 % and 31.7 %, respectively, and remain 21.5 % even under 10 000 cd m⁻².     

Triphenylamino or 9-phenyl carbazolyl-substituted pyrimidine-5-carbonitriles as bipolar emitters and hosts with triplet harvesting abilities

To obtain highly efficient organic semiconductors exhibiting fast emission decays, triplet-harvesting abilities and good bipolar charge-transporting properties for optoelectronic applications, compounds containing triphenylamine or 9-phenylcarbazole as donor moieties and pyrimidine-5-carbonitrile as electron-withdrawing unit were synthesised. Toluene solutions of the compounds demonstrated high photoluminescence quantum yields reaching 98%. As required for electroluminescent device applications, compound containing triphenylamino moiety showed high mobilities of both electrons and holes, which reached 4.4 × 10^?4 cm²/V × s and 7.3 × 10^?3 cm²/V × s, respectively at electric field of 3.6 × 10⁵ V/cm. This triplet-harvesting mechanism was confirmed by the theoretical and experimental studies including a femtosecond transient absorption pump?probe technique and time-resolved electroluminescence spectroscopy. Pure-blue and greenish-blue fluorescent organic light-emitting diodes (OLEDs) with external quantum efficiency (EQE) reaching 7% and 6%, corresposndingly, were obtained using the newly synthesised compounds as emitters. The operation time (T50) of ca. 650 h were observed for blue OLED and of ca. 3800 h for greenish-blue OLED until reaching the half initial brightness (100 cd/m²). EQE of more than 20% and T50 exceeding 20,000 h were observed for electroluminescent devices based on emitter characterised by triplet?triplet annihilation and thermally activated delayed fluorescence which was utilised to test hosting properties of the differently donor-substituted pyrimidine-5-carbonitriles.     

Tetradentate Gold(III) Complexes as Thermally Activated Delayed Fluorescence (TADF) Emitters: Microwave-Assisted Synthesis and High-Performance OLEDs with Long Operational Lifetime

Structurally robust tetradentate gold(III)-emitters have potent material applications but are rare and unprecedented for those displaying thermally activated delayed fluorescence (TADF). Herein, a novel synthetic route leading to the preparation of highly emissive, charge-neutral tetradentate [C^C^N^C] gold(III) complexes with 5-5-6-membered chelate rings has been developed through microwave-assisted C−H bond activation. These complexes show high thermal stability and with emission origin (³IL, ³ILCT, and TADF) tuned by varying the substituents of the C^C^N^C ligand. With phenoxazine/diphenylamine substituent, we prepared the first tetradentate gold(III) complexes that are TADF emitters with emission quantum yields of up to 94 % and emission lifetimes of down to 0.62 μs in deoxygenated toluene. These tetradentate Au^III TADF emitters showed good performance in vacuum-deposited OLEDs with maximum EQEs of up to 25 % and LT₉₅ of up to 5280 h at 100 cd m⁻².     

π-Extended Carbazole Derivatives as Host Materials for Highly Efficient and Long-Life Green Phosphorescent Organic Light-Emitting Diodes

High-performance organic light-emitting diodes (OLEDs) that use phosphorescent and/or thermally activated delayed fluorescence emitters are capable of realizing 100 % electron-to-photon conversion. The host materials in these OLEDs play crucial roles in determining OLED performance. Carbazole derivatives are frequently used as host materials, among which 3,3-bis(9H-carbazol-9-yl)biphenyl ( mCBP ) is often used for lifetime testing in scientific studies. In this study, the π conjugation of the carbazole unit was expanded to enhance OLED lifetime by designing and developing two benzothienocarbazole (BTCz)-based host materials, namely m1BTCBP and m4BTCBP . Among these host materials, m1BTCBP formed a highly efficient [Ir(ppy)₃]-based OLED with an operational luminescence half-life (LT₅₀) of over 300 h at an initial luminance of approximately 12000 cd m⁻² (current density: 25 mA cm⁻²). The LT₅₀ value at 1000 cd cm⁻² was estimated to be about 23 000 h. This performance is clearly higher than that of mCBP -based OLEDs (LT₅₀≈8500 h).     

Electron-Rich Twistacene-Modified Arylboron Donor–Acceptor Systems: Synthesis,Photophysics, and Electroluminescence with Hot Exciton Response

A series of twistacene-functionalized donor (D)-π-acceptor (A) derivatives ( 2 – 5 ) have been designed and synthesized, in which twistacene can be regarded as a promising platform for electron-rich systems for fluorescence emitters. The connecting modes and various acceptors are also examined to investigate the effect of structural changes on the photophysical, electrochemical, and thermal properties. The strong electron-withdrawing capability of the arylboron-modified benzonitrile unit can effectively separate the HOMO and LUMO energy levels of 4 / 5 , which is beneficial for the formation of thermally activated delayed fluorescence (TADF) molecules. Cyan and orange organic light-emitting diodes based on 4 and 5 exhibit promising electroluminescence with a maximum brightness of 7643 cd m⁻² for device-4 and 14871 cd m⁻² for device-5.     

Luminescent Chiral Exciplexes with Sky-Blue and Green Circularly Polarized-Thermally Activated Delayed Fluorescence

Luminescent exciplexes based on a chiral electron donor and achiral acceptors are reported as a new approach to design circularly polarized (CP) and thermally activated delayed fluorescence (TADF) emitters. This strategy results in rather high CP luminescence (CPL) values with g_lum up to 7×10⁻³, one order of magnitude higher in comparison to the CPL signal recorded for the chiral donor alone (g_lum ∼7×10⁻⁴). This increase occurs concomitantly with a CPL sign inversion, as a result of the strong charge-transfer emission character, as experimentally and theoretically rationalized by using a covalent chiral donor-acceptor model. Interestingly, blue, green-yellow and red chiral luminescent exciplexes can be obtained by modifying with the electron accepting character of the achiral unit while keeping the same chiral donor unit. These results bring new (inter)molecular guidelines to obtain simply and efficiently multi-color CP-TADF emitters.     

Narrowband Fluorescent Emitters Based on BN-Doped Polycyclic Aromatic Hydrocarbons for Efficient and Stable Organic Light-Emitting Diodes

Organic light-emitting diodes (OLEDs) using conventional fluorescent emitters are currently attracting considerable interests due to outstanding stability and abundant raw materials. To construct high-performance narrowband fluorophores to satisfy requirements of ultra-high-definition displays, a strategy fusing multi-resonance BN-doped moieties to naphthalene is proposed to construct two novel narrowband fluorophores. Green Na−sBN and red Na−dBN, manifest narrow full-width at half-maxima of 31 nm, near-unity photoluminescence quantum yields and molecular horizontal dipole ratios above 90 %. Their OLEDs exhibit the state-of-the-art performances including high external quantum efficiencies (EQE), ultra-low efficiency roll-off and long operational lifetimes. The Na−sBN-based device achieves EQE as high as 28.8 % and remains 19.8 % even at luminance of 100,000 cd m⁻², and Na−dBN-based device acquires a record-high EQE of 25.2 % among all red OLEDs using pure fluorescent emitters.     

Configurationally Stable Platinahelicene Enantiomers for Efficient Circularly Polarized Phosphorescent Organic Light-Emitting Diodes

Chiral materials with circularly polarized luminescence (CPL) are potentially applicable for 3D displays. In this study, by decorating the pyridinyl-helicene ligands with -CF₃ and -F groups, the platinahelicene enantiomers featured superior configurational stability, as well as high sublimation yield (>90 %) and clear CPPL properties, with dissymmetry factors (|g_PL|) of approximately 3.7×10⁻³ in solution and about 4.1×10⁻³ in doped film. The evaporated circularly polarized phosphorescent organic light-emitting diodes (CP-PhOLEDs) with two enantiomers as emitters exhibited symmetric CPEL signals with |g_EL| of (1.1–1.6)×10⁻³ and decent device performances, achieving a maximum brightness of 11 590 cd m⁻², a maximum external quantum efficiency up to 18.81 %, which are the highest values among the reported devices based on chiral phosphorescent Pt^II complexes. To suppress the effect of reverse CPEL signal from the cathode reflection, the further implementation of semitransparent aluminum/silver cathode successfully boosts up the |g_EL| by over three times to 5.1×10⁻³.     

Solution-Processable Pure-Red Multiple Resonance-induced Thermally Activated Delayed Fluorescence Emitter for Organic Light-Emitting Diode with External Quantum Efficiency over 20 %

Developing solution-processable red organic light-emitting diodes (OLEDs) with high color purity and efficiency based on multiple resonance thermally activated delayed fluorescence (MR-TADF) is a formidable challenge. Herein, by introducing auxiliary electron donor and acceptor moieties into the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distributed positions of multiple resonance skeleton simultaneously, an effective strategy to obtain red MR-TADF emitters was represented. The proof-of-the-concept molecule BN-R exhibits a narrowband pure-red emission at 624 nm, with a high luminous efficiency of 94 % and a narrow bandwidth of 46 nm. Notably, the fabricated solution-processable pure-red OLED based on BN-R exhibits a state-of-the-art external quantum efficiency over 20 % with the Commission Internationale de I’Éclairage coordinates of (0.663, 0.337) and a long operational lifetime (LT₅₀) of 1088 hours at an initial luminance of 1000 cd m⁻².     

A Simple and Strong Electron‐Deficient 5,6‐Dicyano[2,1,3]benzothiadiazole‐Cored Donor‐Acceptor‐Donor Compound for Efficient Near Infrared Thermally Activated Delayed Fluorescence

Despite the success of thermally activated delayed fluorescent (TADF) materials in steering the next generation of organic light‐emitting diodes (OLEDs), effective near infrared (NIR) TADF emitters are still very rare. Here, we present a simple and extremely high electron‐deficient compound, 5,6‐dicyano[2,1,3]benzothiadiazole (CNBz), as a strong electron‐accepting unit to develop a sufficiently strong donor‐acceptor (D?A) interaction for NIR emission. End‐capping with the electron‐donating triphenylamine (TPA) unit created an effective D?A?D type system, giving rise to an efficient NIR TADF emissive molecule (λ_em=750 nm) with a very small ΔE_ST of 0.06 eV. The electroluminescent device using this NIR TADF emitter exhibited an excellent performance with a high maximum radiance of 10020 mW Sr^?1 m^?2, a maximum EQE of 6.57% and a peak wavelength of 712 nm.