Above 30% external quantum efficiency in green delayed fluorescent organic light-emitting diodes

Theoretical simulation of TADF character of 3,9'-bicarbazole-modified 2,4,6-triphenyl-1,3,5-triazine

CONTEXT

Three donor (D)-acceptor (A)-type temperature-activated delayed fluorescent (TADF) molecules of 9-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-3,9'-bicarbazole (o-TrzDCz), 9-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-3,9'-bicarbazole (m-TrzDCz), and 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-3,9'-bicarbazole (p-TrzDCz) were designed in this paper, and the photophysical properties, including the intersystem crossing rate, the reorganization energies (λ), and the intersystem crossing/reverse intersystem crossing (ISC/RISC) rate, were simulated to explore the effect of substitution sites on their TADF character. The values of the twist angle between the D and A moieties in ground state and the molecular root-mean-square deviation (RMSD) of the S1 and T1 states referenced to the S0 state indicate that o-TrzDCz possess bigger steric hindrance and stabler molecular configuration. The λ values of the ISC/RISC process should be 0.06/0.04 eV for o-TrzDCz, which are much smaller than those of m-TrzDCz (0.51/0.41 eV) and p-TrzDCz (1.93/1.06 eV). At the same time, o-TrzDCz possess the biggest kRISC (7.28 × 106 s-1) and kr (3.12 × 106 s-1) values and the smallest kp (0.10 s-1) value among the three titled molecules. These data indicate that o-TrzDCz should have more excellent TADF character than m-TrzDCz and p-TrzDCz. In a word, this research presents that adjusting the molecular linking manner should be a charming way to explore novel high-efficient TADF molecules.

METHODS

Quantum chemical calculations were performed at PBE0/6-31G* level by Gaussian 09 and ORCA 4.1.0 software packages, and reorganization energies and Huang-Rhys were performed by the DUSHIN program and MOMAP 2019B software package based on the Gaussian 09 output files, while the phosphorescence rates were performed at B3LYP/6-31G* level by Dalton 2021.

Theoretical Insights into the Photophysical Properties of 4CzIPN Doped in Different Hosts: A Multiscale Study

As a typical thermally activated delayed fluorescence (TADF) emitter with green emission, 4CzIPN has attracted much attention recently. Most studies indicated that 4CzIPN doped in different hosts presented different performances; thus, the hosts should have an obvious influence on its photophysical properties. Herein, the influence of four kinds of hosts, including m-CzPym, m-CzTrz, p-CzPym, and p-CzTrz, on the photophysical properties of 4CzIPN is investigated. Molecular dynamics simulations were performed to simulate the host-guest conformations, and the photophysical properties were studied using the combined quantum mechanics/molecular mechanics method coupled with the thermal-vibration correlation function method. It is found that 4CzIPN in doped films has larger transition dipole moments and spin-orbital coupling constants compared to that in nondoped films. Faster radiative decay, intersystem crossing rates, and higher fluorescence efficiency could be obtained in doped films. Our work helps to better understand the photophysical properties of 4CzIPN in doped films and may favor the design of new hosts.

Derivatives of 2-Pyridone Exhibiting Hot-Exciton TADF for Sky-Blue and White OLEDs

Development of emissive materials for utilization in organic light-emitting diodes (OLEDs) remains a highly relevant research field. One of the most important aspects in the development of efficient emitters for OLEDs is the efficiency of triplet-to-singlet exciton conversion. There are many concepts proposed for the transformation of triplet excitons to singlet excitons, among which thermally activated delayed fluorescence (TADF) is the most efficient and widespread. One of the variations of the TADF concept is the hot exciton approach according to which the process of exciton relaxation into the lowest energy electronic state (internal conversion as usual) is slower than intersystem crossing between high-lying singlets and triplets. In this paper, we present the donor-acceptor materials based on 2-pyridone acceptor coupled to the different donor moieties through the phenyl linker demonstrating good performance as components of sky-blue, green-yellow, and white OLEDs. Despite relatively low photoluminescence quantum yields, the compound containing 9,9-dimethyl-9,10-dihydroacridine donor demonstrated very good efficiency in sky-blue OLED with the single emissive layer, which showed an external quantum efficiency (EQE) of 3.7%. It also forms a green-yellow-emitting exciplex with 4,4',4″-tris[phenyl(m-tolyl)amino]triphenylamine. The corresponding OLED showed an EQE of 6.9%. The white OLED combining both exciplex and single emitter layers demonstrated an EQE of 9.8% together with excellent current and power efficiencies of 16.1 cd A-1 and 6.9 lm W-1, respectively. Quantum-chemical calculations together with the analysis of photoluminescence decay curves confirm the ability of all of the studied compounds to exhibit TADF through the hot exciton pathway, but the limiting factor reducing the efficiency of OLEDs is the low photoluminescence quantum yields caused mainly by nonradiative intersystem crossing dominating over the radiative fluorescence pathway.

Conformational isomeric thermally activated delayed fluorescence (TADF) emitters: mechanism, applications, and perspectives

Thermally activated delayed fluorescence (TADF) materials have received enormous attention and the mechanism behind them has been investigated in depth. It has been found that some donor-acceptor (D-A) type TADF emitters could obviously exhibit dual stable conformations in the ground states and their distributions significantly affect the physical properties and device performances. Therefore, professional analysis and a summary of the relationship between molecular structures and performances are very important. In this review, we first summarize the mechanism and properties of TADF emitters with conformational isomerism. We also classify their recent progress according to their different applications, and provide an outlook on their perspectives.

Bipolar 1,8-naphthalimides showing high electron mobility and red AIE-active TADF for OLED applications

Aiming to design bipolar organic semiconductors with high electron mobility and efficient red thermally activated delayed fluorescence (TADF), three donor-acceptor compounds were designed and synthesized selecting 1,8-naphthalimide as an acceptor and phenoxazine, 3,7-di-tert-butylphenothiazine or 2,7-di-tert-butyldimethyl-9,10-dihydroacridine as donor moieties. Aggregation induced emission enhancement was detected for the compounds causing efficient TADF in the solid-state. Photoluminescence quantum yields up to 77% were observed for the films of the compounds doped in a host. The compounds exhibited small singlet-triplet splitting (0.03-0.05 eV), and high reverse intersystem crossing rates of 2.08 × 10⁵-1.13 × 10⁶ s^-1. The compounds were characterized by satisfactory hole and electron-injecting properties with ionization potentials of 5.72-5.83 eV and electron affinities of 2.79-2.91 eV. Bipolar charge transport was revealed by time of flight measurements. Electron transport with low dispersity and mobilities exceeding 2 × 10^-3 cm² V^-1 s^-1 was observed at an electric field of 4.6 × 10⁵ V cm^-1. The compounds were used as emitters in red electroluminescent devices, which showed maximum external quantum efficiencies up to 8.2%. Utilization of host-guest systems as light-emitting materials with hosts preferably transporting holes and TADF guests which preferably transport electrons allowed maximum efficiencies to be achieved at a practical brightness of 700-2200 cd m^-2. DFT calculations of the geometry, electronic structure, absorption and photoluminescence spectra of all compounds were carried out to prove the conclusions drawn from the experiment. The results of the calculations clearly show that the first excited state for all compounds is the intramolecular charge transfer state. Quantitative analysis of the separation degree of electronic density during excitation allows the observed dependence of the blue shift value in the absorption and emission spectra on the increasing polarity of the solvent to be explained.

Intramolecular charge transfer dynamics in the excited states of diphenylamine substituted 1,3,4-oxadiazole derivatives

The excited state dynamics processes of two diphenylamine substituted symmetric 1,3,4-oxadiazole derivatives in different solvents were studied through femtosecond transient absorption spectroscopy. It was revealed that in cyclohexane, the locally excited (LE) state relaxes to the intramolecular charge transfer (ICT) state within 2 ps timescale, then decays to the ground state. In tetrahydrofuran and acetonitrile, the ICT state can be stabilized via solvation, so besides the LE → ICT conversion within 1 ps, another ICT → solvent stabilized ICT (SSICT) conversion in the 1-200 ps time scale could also be observed, and eventually decay to the ground state. Whereas, ICT → SSICT process in acetonitrile is slower than that in tetrahydrofuran, which leads to radiationless deactivation dominates the ICT state deactivation process and low fluorescence quantum efficiency. These results offer a guidance to understand the relationship of low fluorescence quantum efficiency and excited state deactivation mechanism of organic π-conjugated molecules, which would be very helpful for designing new advanced opto-electronic materials.

Efficiency of Thermally Activated Delayed Fluorescence Sensitized Triplet Upconversion Doubled in Three-Component System

As in many fields, the most exciting endeavors in photon upconversion research focus on increasing the efficiency (upconversion quantum yield) and performance (anti-Stokes shift) while diminishing the cost of production. In this vein, studies employing metal-free thermally activated delayed fluorescence (TADF) sensitizers have garnered increased interest. Here, for the first time, the strategy of ternary photon upconversion is utilized with the TADF sensitizer 2,4,5,6-tetrakis(carbazol-9-yl)isophthalonitrile (4CzIPN), resulting in a doubling of the upconversion quantum yield in comparison to the binary system employing p-terphenyl as the emitter. In this ternary blend, the sensitizer 4CzIPN is paired with an intermediate acceptor, 1-methylnaphthalene, in addition to the emitter molecule, p-terphenyl, yielding a normalized upconversion quantum yield of 7.6% while maintaining the 0.83 eV anti-Stokes shift. These results illustrate the potential benefits of utilizing this strategy of energy-funneling, previously used only with heavy-metal based sensitizers, to increase the performance of these photon upconversion systems.

Recent progress in thermally activated delayed fluorescence emitters for nondoped organic light-emitting diodes

Nondoped organic light-emitting diodes (OLEDs) have drawn immense attention due to their merits of process simplicity, reduced fabrication cost, etc. To realize high-performance nondoped OLEDs, all electrogenerated excitons should be fully utilized. The thermally activated delayed fluorescence (TADF) mechanism can theoretically realize 100% internal quantum efficiency (IQE) through an effective upconversion process from nonradiative triplet excitons to radiative singlet ones. Nevertheless, exciton quenching, especially related to triplet excitons, is generally very serious in TADF-based nondoped OLEDs, significantly hindering the pace of development. Enormous efforts have been devoted to alleviating the annoying exciton quenching process, and a number of TADF materials for highly efficient nondoped devices have been reported. In this review, we mainly discuss the mechanism, exciton leaking channels, and reported molecular design strategies of TADF emitters for nondoped devices. We further classify their molecular structures depending on the functional A groups and offer an outlook on their future prospects. It is anticipated that this review can entice researchers to recognize the importance of TADF-based nondoped OLEDs and provide a possible guide for their future development.

The mechanism, exciton leaking channels, and reported molecular design strategies of TADF emitters for high-performance nondoped OLEDs are summarized. Their molecular structures depending on the functional A groups are further classified.

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 112.0 lm 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.

Thermally Activated Delayed Fluorescence Amorphous Molecular Materials for High-Performance Organic Light-Emitting Diodes

Small-molecule thermally activated delayed fluorescence (TADF) materials have been extensively developed to actualize efficient organic LEDs (OLEDs). However, organic small molecules generally compromise thin film quality and stability due to the tendency of crystallization, aggregation, and phase separation, which hence degrade the efficiency and long-term stability of the OLEDs. Here, for the first time, we exploit the unique molecular configuration of the bimesitylene scaffold to design two highly efficient TADF amorphous molecular materials with excellent thermal and morphological stabilities. The twisted and rigid bimesitylene scaffold thwarts regular molecular packing and crystallization, thereby guaranteeing homogeneous and stable amorphous thin films. Meanwhile, the highly twisted geometry of the bimesitylene scaffold efficiently breaks the molecular conjugation and thus conserves the high energies of the lowest locally excited triplet states (³LE) above the lowest charge transfer states (¹CT and ³CT), leading to small singlet-triplet energy splitting and fast reverse intersystem crossing. These TADF emitters exhibit high photoluminescence quantum yields of 0.90 and 0.69 and short TADF lifetimes of 4.94 and 1.44 μs in doped films, based on which the greenish-blue and greenish-yellow OLEDs achieve external quantum efficiencies of 23.2 and 16.2%, respectively, with small efficiency roll-off rates and perfect color stability.

Rational Utilization of Intramolecular Hydrogen Bonds to Achieve Blue TADF with EQEs of Nearly 30% and Single Emissive Layer All-TADF WOLED

A series of highly efficient blue thermally activated delayed fluorescence (TADF) compounds, SON-Cz, SON-tBuCz, and SON-PhCz, were developed. Pyridinyl was introduced as the bridging unit between carbazole donors and sulfone acceptor. Intramolecular hydrogen bonds between the pyridine N atom and carbazole H atoms were detected in single crystals, which suppressed the twisting of carbazole rings and dramatically increased the molecular rigidity. At the same time, tert-butyl or phenyl were incorporated at the 3,6-sites of carbazole ring to tune electron donating ability or enlarge HOMO delocalization. All these hydrogen bonds featured TADF compounds exhibited much improved photoluminescence quantum yields (PLQYs) and excellent efficiencies in their doped blue organic light-emitting diodes. In particular, SON-tBuCz and SON-PhCz exhibited the maximum external quantum efficiencies (EQEs) of 29.59% and 28.22% with CIE coordinates of (0.17, 0.22) and (0.21, 0.36), respectively. The excellent performance benefits from the carbazole structure modification and the intramolecular hydrogen bonds, which bring more rigid structures and eliminate nonradiative transitions. Furthermore, a single emissive layer all-TADF white OLED was fabricated using SON-tBuCz as the blue emitter and 4CzTPN-Ph as the orange emitter to give an EQE of 23.51% with a high CRI of 71, which is among the top efficiencies ever reported for all-TADF WOLEDs so far.

Triarylamine-Pyridine-Carbonitriles for Organic Light-Emitting Devices with EQE Nearly 40

Highly efficient thermally activated delayed fluorescence (TADF) molecules are in urgent demand for solid-state lighting and full-color displays. Here, the design and synthesis of three triarylamine-pyridine-carbonitrile-based TADF compounds, TPAPPC, TPAmPPC, and tTPAmPPC, are shown. They exhibit excellent photoluminescence quantum yields of 79-100% with small ΔE_ST values, fast reverse intersystem crossing (RISC), and high horizontal dipole ratios (Θ_//  = 86-88%) in the thin films leading to the enhancement of device light outcoupling. Consequently, a green organic light-emitting diode (OLED) based on TPAmPPC shows a high average external quantum efficiency of 38.8 ± 0.6%, a current efficiency of 130.1 ± 2.1 cd A^-1 , and a power efficiency of 136.3 ± 2.2 lm W^-1 . The highest device efficiency of 39.8% appears to be record-breaking among TADF-based OLEDs to date. In addition, the TPAmPPC-based device shows superior operation lifetime and high-temperature resistance. It is worth noting that the TPA-PPC-based materials have excellent optical properties and the potential for making them strong candidates for TADF practical application.

Red to orange thermally activated delayed fluorescence polymers based on 2-(4-(diphenylamino)-phenyl)-9H-thioxanthen-9-one-10,10-dioxide for efficient solution-processed OLEDs

Most highly efficient thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs) are multi-layer devices fabricated by thermal vacuum evaporation techniques, which are unfavorable for real applications. However, there are only a few reported examples of efficient solution-processed TADF OLEDs, in particular TADF polymer OLEDs. Herein, a series of solution-processable TADF conjugated polymers (PCTXO/PCTXO-Fx (x = 25, 50 and 75)) were designed and synthesized by copolymerization of 2-(4-(diphenylamino)-phenyl)-9H-thioxanthen-9-one-10,10-dioxide (TXO-TPA) as a red/orange emissive TADF unit, 9,9′-((fluorene-9,9-diyl)-bis(octane-8,1-diyl))-bis(3,6-di-tert-butylcarbazole) as host/hole-transporting unit and 2,7-N-(heptadecan-9-yl)carbazole as a conjugated linker and solubilizing group. They possessed a conjugated backbone with donor TPA-carbazole/fluorene moieties and a pendent acceptor 9H-thioxanthen-9-one-10,10-dioxide (TXO) forming a twisted donor–acceptor structure. These polymers in neat films displayed red/orange color emissions (601–655 nm) with TADF properties, proved by theory calculations and transient PL decay measurements. Their hole-transporting capability was improved when the content of 9,9′-((fluorene-9,9-diyl)-bis(octane-8,1-diyl))-bis(3,6-di-tert-butylcarbazole) within the polymers increased. All polymers were successfully employed as emitters in solution-processed OLEDs. In particular, the doped OLED fabricated with PCTXO exhibited an intense deep orange emission at 603 nm with the best electroluminescence performance (a maximum external quantum efficiency 10.44%, a maximum current efficiency of 14.97 cd A⁻¹ and a turn-on voltage of 4.2 V).

TADF conjugated polymers having 2-(4-(diphenylamino)-phenyl)-9H-thioxanthen-9-one-10,10-dioxide as a TADF unit showed red/orange color emissions and enabled OLED devices with a maximum external quantum efficiency of 10.44% and a maximum current efficiency of 14.97 cd A⁻¹

Lifetime-Extending 3-(4-Phenylbenzo[4,5]thieno[3,2-d]pyrimidin-2-yl)benzonitrile Acceptor for Thermally Activated Delayed Fluorescence Emitters

Highly efficient and long-living green thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) were developed using benzothienopyrimidine-4-benzonitrile acceptor-derived compounds as the TADF emitters. A molecular design merging the benzothienopyrimidine-4-benzonitrile acceptor with either indolocarbazole or diindolocarbazole was employed to prepare two TADF emitters, 5-(2-phenylbenzo[4,5]thieno[3,2-d]pyrimidin-4-yl)-2-(5-phenylindolo[3,2-a]carbazol-12(5H)-yl)benzonitrile and 2-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)-5-(2-phenylbenzo[4,5]thieno[3,2-d]pyrimidin-4-yl)benzonitrile (BTPDIDCz), as the green and greenish-yellow emitters. Among the two emitters, BTPDIDCz with the diindolocarbazole donor combined with the benzothienopyrimidine-4-benzonitrile acceptor demonstrated a high external quantum efficiency of 24.5% and 3 times longer device lifetime than the state-of-the-art green emitter. This work proposed the potential of benzothienopyrimidine-4-benzonitrile as the acceptor for long lifetime in TADF emitters.

Thermally Activated Delayed Fluorescence from d10 -Metal Carbene Complexes through Intermolecular Charge Transfer and Multicolor Emission with a Monomer-Dimer Equilibrium

A series of two-coordinate AuI and CuI complexes (3 a, 3 b and 5 a, 5 b) are reported as new organometallic thermally activated delayed fluorescence (TADF) emitters, which are based on the carbene-metal-carbazole model with a pyridine-fused 1,2,3-triazolylidene (PyTz) ligand. PyTz features low steric hindrance and a low-energy LUMO (LUMO=-1.47 eV) located over the π* orbitals of the whole ligand, which facilitates intermolecular charge transfer between a donor (carbazole) and an accepter (PyTz). These compounds exhibit efficient TADF with microsecond lifetimes. Temperature-dependent photoluminescence kinetics of 3 a supports a rather small energy gap between S1 and T1 (ΔES 1 - T 1 =60 meV). Further experiments reveal that there are dual-emission properties from a monomer-dimer equilibrium in solution, exhibiting single-component multicolor emission from blue to orange, including white-light emission.

Photoluminescent Behaviors of Thermally Activated Delayed Fluorescence Polymeric Emitters in Nanofibers

Anisotropic 1D nanostructures with high surface-area-to-volume ratio display the enhanced optoelectronic properties of light-emitting compounds compared to bulk or 2D systems. To study the effect of nanometer-constrained space on photoluminescent behavior of thermally activated delayed fluorescence (TADF) polymeric emitters, electrospinning technique is used to produce nanofibers of TADF emitters. Herein, two TADF polymer (P1 and P3) nanofibers with 90% polyacrylonitrile (PAN) are fabricated and their photophysical properties are studied and compared with their spin-coated film counterparts. The distinguishing polarized photoluminescencent properties of P1/PAN or P3/PAN electrospun nanofibers are obtained due to high orientation degree and superior molecular arrangement. Moreover, the better TADF properties in nanofibers can be observed comparing with their spin-coated films, including longer-lived excited states, higher photoluminescence quantum efficiency, lower internal conversion decay rate, and higher reverse intersystem crossing rate constant.

Acceptor plane expansion enhances horizontal orientation of thermally activated delayed fluorescence emitters

Manipulating orientation of organic emitters remains a formidable challenge in organic light-emitting diodes (OLEDs). Here, expansion of the acceptor plane of thermally activated delayed fluorescence (TADF) emitters was demonstrated to selectively modulate emitting dipole orientation. Two proof-of-the-concept molecules, PXZPyPM and PXZTAZPM, were prepared by introducing a planar 2-phenylpyridine or 2,4,6-triphenyl-1,3,5-triazine substituent into a prototypical molecule (PXZPM) bearing a pyrimidine core and two phenoxazine donors. This design approach suppressed the influence of substituents on electronic structures and associated optoelectronic properties. Accordingly, PXZPyPM and PXZTAZPM preserved almost the same excited states and similar emission characteristics as PXZPM. The expanded acceptor plane of PXZPyPM and PXZTAZPM resulted in a 15 to 18% increase in horizontal ratios of emitting dipole orientation. PXZPyPM supported its green device exhibiting an external quantum efficiency of 33.9% and a power efficiency of 118.9 lumen per watt, competitive with the most efficient green TADF OLEDs reported so far.

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.

Identification of host-guest systems in green TADF-based OLEDs with energy level matching based on a machine-learning study

Booming progress has been made in both the molecular design concept and the fundamental electroluminescence (EL) mechanism of thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs) in recent years. One of the requirements for TADF-based OLEDs having high external quantum efficiency (EQE) is the favorable energy level alignment between the host and the guest to promote the energy transfer and improve the carrier balance. However, strategies to optimize the TADF-based OLED performance by selecting suitable host-guest systems in the light-emitting layer are far from enough. In this work, we investigated guest-host systems through the use of two machine-learning approaches (feature-based and similarity-based algorithms) from our recent effort for the optimization of TADF-based OLEDs. The Random Forest (RF) algorithm based on the features of electronic and photo-physical properties can accurately predict the EQE of green TADF-based OLEDs with average correlation coefficients of R2 = 0.85 for the training set and R2 = 0.74 for the testing set. Also, the Support Vector Regression (SVR) algorithm based on similarity metrics between pairs of materials (e.g., host and guest) in terms of electronic parameters can provide reasonable device performance prediction (R2 = 0.72) through the optimization procedure of the parameters. These results show that the predictive capability and model applicability of both machine-learning models can be used to identify suitable host-guest systems and explore complex relationships in green TADF-based OLEDs.

Diboron-Based Delayed Fluorescent Emitters with Orange-to-Red Emission and Superior Organic Light-Emitting Diode Efficiency

For the application of organic light-emitting diodes (OLEDs) in lighting and panels, the basic requirement is to include a full spectrum color range. Compared with the development of blue and green luminophores in thermally activated delayed fluorescence (TADF) technology, the progress of orange-to-red materials is slow and needs further investigation. In this study, three diboron compound-based materials, dPhADBA, dmAcDBA, and SpAcDBA, were designed and synthesized by nucleophilic arylation of three amine donors on 9,10-diboraanthracene (DBA) in a two-step reaction. With increasing electron-donating ability, they show orange-to-red emission with TADF characteristics. The electroluminescence of these diboron compounds exhibits emissions λ_max at 613, 583, and 567 nm for dPhADBA, dmAcDBA, and SpAcDBA, respectively. It is noteworthy that the rod-like D-A-D structures can achieve high horizontal ratios (84-86%) and outstanding device performance for orange-to-red TADF OLEDs: the highest external quantum efficiencies for dPhADBA, dmAcDBA, and SpAcDBA are 11.1 ± 0.5, 24.9 ± 0.5, and 30.0 ± 0.8%, respectively. Therefore, these diboron-based molecules offer a promising avenue for the design of orange-to-red TADF emitters and the development of highly efficient orange-to-red OLEDs.

Rational Molecular Design of Highly Efficient Yellow-Red Thermally Activated Delayed Fluorescent Emitters: A Combined Effect of Auxiliary Fluorine and Rigidified Acceptor Unit

Molecular design strategies are crucial to develop highly efficient and long-wavelength thermally activated delayed fluorescent (TADF) emitters because the inherent limitation of the energy gap law degrades the efficiency of the red or orange TADF emitters. To resolve the low efficiency issue, we designed and synthesized two TADF emitters, 4,4'-(6-(9,9-dimethylacridin-10(9H)-yl)-7-fluoroquinoxaline-2,3-diyl)dibenzonitrile (FDQCNAc) and 11-(9,9-dimethylacridin-10(9H)-yl)-12-fluorodibenzo[a,c]phenazine-3,6-dicarbonitrile (FBPCNAc), by utilizing fluorine and peripheral cyano-substituted quinoxaline and phenazine acceptors of 4,4'-(6-fluoroquinoxaline-2,3-diyl)dibenzonitrile (FDQCN) and 11-fluorodibenzo[a,c]phenazine-3,6-dicarbonitrile (FBPCN), respectively. A fluorine atom at the ortho position of the acridine donor acts as an auxiliary acceptor to minimize the singlet-triplet energy gap (ΔE_ST) below 0.1 eV and promotes the reverse intersystem crossing (RISC) process. Organic light-emitting diodes (OLEDs) fabricated with FDQCNAc and FBPCNAc emitters demonstrated high external quantum efficiencies (EQEs) of 27.6 and 23.8% in the yellow-red TADF OLEDs, respectively. In particular, the combination of the F auxiliary acceptor unit and the rigidified FBPCN acceptor unit enabled red-shifted emission by about 58 nm without much sacrifice of the EQE in the red region.

Narrowband and Pure Violet Organic Emitter with a Full Width at Half Maximum of 14 nm and y Color Coordinate of Below 0.02

Violet organic light-emitting diodes (OLEDs) with a very narrow emission spectrum with a full width at half maximum of 14 nm and y color coordinate of 0.02 are developed using a indolo[3,2,1-jk]carbazole-derived pure violet emitter. The violet emitter, 2,5,13,16-tetra-tert-butylindolo[3,2,1-jk]-indolo[1',2',3':1,7]indolo[2,3-b]carbazole (tDIDCz), is designed to have a very rigid molecular structure driven by the multiresonance-type core structure through the alternating carbon and nitrogen atoms. The violet emitter is decorated with t-butyl groups to prevent intermolecular aggregation and packing, which allow pure violet emission without excimer emission. The violet OLEDs derived from the tDIDCz emitter show a violet color coordinate of (0.164, 0.018) with a narrow emission spectrum and a full width at half maximum of 14 nm (105 meV). The external quantum efficiency of the pure violet OLEDs is 3.3%. This is the first work reporting pure violet emission without any ultraviolet emission below 380 nm and blue emission above 450 nm by showing a very narrow emission spectrum.

Harnessing a New Co-Host System and Low Concentration of New TADF Emitters Equipped with Trifluoromethyl- and Cyano-Substituted Benzene as Core for High-Efficiency Blue OLEDs

A strategic approach combining a new co-host system and low concentration of new thermally activated delayed fluorescence (TADF) emitters to make efficient blue TADF organic light-emitting diode (OLED) was developed. The benchmark TADF molecule, 4CzIPN, was adopted as a probe to examine the feasibility of a co-host composing of a hole transporter SimCP and an electron transporter oCF₃-T2T. As a result, a sky blue device with 1 wt % 4CzIPN doped in SimCP:oCF₃-T2T co-host exhibited 100% energy transfer and achieved a high external quantum efficiency (EQE) up to 26.1%. Importantly, this device showed a limited efficiency rolloff with an EQE of 24% at 1000 cd m^-2. To further shift the emission toward blue, three new TADF molecules, 4CzIPN-CF₃, 3CzIPN-H-CF₃, and 3CzIPN-CF₃, modified either by lowering the electron-withdrawing ability of the acceptor group or by reducing the number of carbazole donors of 4CzIPN, have been synthesized and characterized. Among them, 4CzIPN-CF₃ and 3CzIPN-H-CF₃ display hypsochromic shift emissions compared to that of 4CzIPN. These new compounds were then explored for their potential applications as TADF emitters. Blue TADF OLEDs with 1 wt % of 4CzIPN-CF₃ and 3CzIPN-H-CF₃ dispersed in SimCP:oCF₃-T2T co-host achieved EQEs of 23.1 and 16.5% and retained high EQEs of 20.9 and 14.7% at 1000 cd m^-2, respectively.

High-Efficiency Red Organic Light-Emitting Diodes with External Quantum Efficiency Close to 30% Based on a Novel Thermally Activated Delayed Fluorescence Emitter

Researchers have spared no effort to design new thermally activated delayed fluorescence (TADF) emitters for high-efficiency organic light-emitting diodes (OLEDs). However, efficient long-wavelength TADF emitters are rarely reported. Herein, a red TADF emitter, TPA-PZCN, is reported, which possesses a high photoluminescence quantum yield (Φ_PL ) of 97% and a small singlet-triplet splitting (ΔE_ST ) of 0.13 eV. Based on the superior properties of TPA-PZCN, red, deep-red, and near-infrared (NIR) OLEDs are fabricated by utilizing different device structure strategies. The red devices obtain a remarkable maximum external quantum efficiency (EQE) of 27.4% and an electroluminescence (EL) peak at 628 nm with Commission Internationale de L'Eclairage (CIE) coordinates of (0.65, 0.35), which represents the best result with a peak wavelength longer than 600 nm among those of the reported red TADF devices. Furthermore, an exciplex-forming cohost strategy is adopted. The devices achieve a record EQE of 28.1% and a deep-red EL peak at 648 nm with the CIE coordinates of (0.66, 0.34). Last, nondoped devices exhibit 5.3% EQE and an NIR EL peak at 680 nm with the CIE coordinates of (0.69, 0.30).

Triazine-Acceptor-Based Green Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes

High-efficiency thermally activated delayed fluorescence (TADF) is leading the third-generation technology of organic light-emitting diodes (OLEDs). TADF emitters are designed and synthesized using inexpensive organic donor and acceptor derivatives. TADF emitters are a potential candidate for next-generation display technology when compared with metal-complex-based phosphorescent dopants. Many studies are being conducted to enhance the external quantum efficiencies (EQEs) and photoluminescent quantum yield of green TADF devices. Blue TADF reached an EQE of over 35% with the support of suitable donor and acceptor moieties based on a suitable molecular design. The efficiencies of green TADF emitters can be improved when an appropriate molecular design is applied with an efficient device structure. The triazine acceptor has been identified as a worthy building block for green TADF emitters. Hence, we present here a review of triazine with various donor molecules and their device performances. This will help to design more suitable and efficient green TADF emitters for OLEDs.

Recent Progress of Singlet-Exciton-Harvesting Fluorescent Organic Light-Emitting Diodes by Energy Transfer Processes

The external quantum efficiency (EQE) of organic light-emitting diodes (OLEDs) has been dramatically improved by developing highly efficient organic emitters such as phosphorescent emitters and thermally activated delayed fluorescent (TADF) emitters. However, high-EQE OLED technologies suffer from relatively poor device lifetimes in spite of their high EQEs. In particular, the short lifetimes of blue phosphorescent and TADF OLEDs remain a big hurdle to overcome. Therefore, the high-EQE approach harvesting singlet excitons of fluorescent emitters by energy transfer processes from the host or sensitizer has been explored as an alternative for high-EQE OLED strategies. Recently, there has been a big jump in the EQE and device lifetime of singlet-exciton-harvesting fluorescent OLEDs. Recent progress on the materials and device structure is discussed herein.

Inkjet Printing of Mixed-Host Emitting Layer for Electrophosphorescent Organic Light-Emitting Diodes

We have investigated the impact of the ink formulation on the properties of an inkjet-printed small molecular mixed host in a phosphorescent organic light-emitting diode (PhOLED). Host solubility, film roughness, and device efficiency improved by blending tris(4-carbazoyl-9-ylphenyl)amine (TCTA) with pyrido[3',2':4,5]furo[2,3- b]pyridine (3CzPFP). At a host ratio of 60:40 (TCTA/3CzPFP), the brightness increased by 33%, the efficiency roll-off at 1000 cd/m² dropped to well below 10%, and the luminance half-lifetime (LT₅₀) improved by 80% in comparison to the device with a single host (100% TCTA). When the optimized ink was deposited by inkjet printing, a maximum external quantum efficiency of 8.9% and a current efficiency of 28.8 cd/A were achieved at 1000 cd/m² brightness. This amounted to around 84% of the efficiency of a spin-cast reference device. The obtained results provide a blueprint for designing enhanced PhOLEDs with inkjet-printed mixed hosts.

Exciplex-Based Electroluminescence: Over 21% External Quantum Efficiency and Approaching 100 lm/W Power Efficiency

Benzimidazole-triazine-based electron acceptor PIM-TRZ with high triplet exited-state energy and strong electron-transport ability was newly developed. A series of highly efficient exciplex emitters have been fabricated. The TAPC:PIM-TRZ (TAPC: di-[4-( N, N-ditoly amino)-phenyl]cyclohexane) film shows a high photoluminescence (PL) quantum yields (PLQY, Φ_f) of 93.4%, and the device based on TAPC:PIM-TRZ exhibits a low turn-on voltage of 2.3 V, high maximum efficiency of 71.2 cd A^-1 (current efficiency, CE), 97.3 lm W^-1 (power efficiency, PE), and 21.7% (external quantum efficiency, EQE), as well as a high EQE of 16.2% at a luminance of 5000 cd m^-2. The device displays the highest efficiency among reported organic light-emitting devices with an exciplex film as the emitting layer. Furthermore, a green device is also fabricated with a TAPC:PIM-TRZ cohost using C545T (C545T: (10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1 H,5 H,11 H-benzopyrano[6,7-8- I, j]quinolizin-11-one)) as the dopant, and the highest CE, PE, and EQE are 68.3 cd A^-1, 86.6 lm W^-1, and 20.2%, respectively.

Isomeric Quinoxalinedicarbonitrile as Color-Managing Acceptors of Thermally Activated Delayed Fluorescent Emitters

Highly efficient yellow and red thermally activated delayed fluorescent (TADF) organic light-emitting diodes were developed using two quinoxalinedicarbonitrile units, quinoxaline-6,7-dicarbonitrile (6,7-DCQx) and quinoxaline-5,8-dicarbonitrile (5,8-DCQx) as electron acceptors, and t-butylcarbazole or 9,9-dimethylacridan as electron donors. The strong electron-withdrawing capability of the 5,8-DCQx and 6,7-DCQx acceptors enabled from yellow to red emission in the TADF emitters. Comparing 5,8-DCQx and 6,7-DCQx, a red-shifted emission by about 40 nm was observed in the 5,8-DCQx acceptor-based emitters, but the external quantum efficiency of the TADF devices was over 20% irrespective of the acceptor, proving the usefulness of the two acceptor moieties as the building blocks of the TADF emitters. Therefore, the quinoxalinedicarbonitrile acceptors are promising as the acceptor moiety for designing yellow or red TADF emitters.

Controlling Horizontal Dipole Orientation and Emission Spectrum of Ir Complexes by Chemical Design of Ancillary Ligands for Efficient Deep-Blue Organic Light-Emitting Diodes

Deep-blue emitting Iridium (Ir) complexes with horizontally oriented emitting dipoles are newly designed and synthesized through engineering of the ancillary ligand, where 2',6'-difluoro-4-(trimethylsilyl)-2,3'-bipyridine (dfpysipy) is used as the main ligand. Introduction of a trimethylsilyl group at the pyridine and a nitrogen at the difluoropyrido group increases the bandgap of the emitter, resulting in deep-blue emission. Addition of a methyl group (mpic) to a picolinate (pic) ancillary ligand or replacement of an acetate structure of pic with a perfluoromethyl-triazole structure (fptz) increases the horizontal component of the emitting dipoles in sequence of mpic (86%) > fptz (77%) > pic (74%). The organic light-emitting diode (OLED) using the Ir complex with the mpic ancillary ligand shows the highest external quantum efficiency (31.9%) among the reported blue OLEDs with a y-coordinate value lower than 0.2 in the 1931 Commission Internationale de L'Eclairage (CIE) chromaticity diagram.

Design of Efficient Exciplex Emitters by Decreasing the Energy Gap Between the Local Excited Triplet (3LE) State of the Acceptor and the Charge Transfer (CT) States of the Exciplex

A series of thermally activated delayed fluorescence (TADF) exciplex based on the TX-TerPy were constructed. The electronic coupling between the triplet local excited states (³LE) of the donors and acceptor and the charge transfer states had a great influence on the triplet exciton harvesting and Φ_PL. Herein, based on this strategy, three donor molecules TAPC, TCTA, and m-MTDATA were selected. The local triplet excited state (³LE) of the three donors are 2.93, 2.72 and 2.52 eV in pure films. And the ³LE of TX-TerPy is 2.69 eV in polystyrene film. The energy gap between the singlet charge transfer (¹CT) states of TAPC:TX-TerPy (7:1), TCTA:TX-TerPy (7:1) and the ³LE of TX-TerPy are 0.30 eV and 0.20 eV. Finally, the Φ_PL of TAPC:TX-TerPy (7:1) and TCTA:TX-TerPy (7:1) are 65.2 and 69.6%. When we changed the doping concentration of the exciplex from 15% to 50%, the ratio of the triplet decreased, and Φ_PL decreased by half, perhaps due to the increased energy gap between ¹CT and ³LE. Therefore, optimizing the ¹CT, ³CT, and ³LE facilitated the efficient exciplex TADF molecules.

New Aggregation-Induced Delayed Fluorescence Luminogens With Through-Space Charge Transfer for Efficient Non-doped OLEDs

In this work, two tailor-made luminogens comprising of electron donors (acridine and phenoxazine) and acceptor (triazine) bridged by the through-space conjugated hexaphenylbenzene (HPB) are synthesized and characterized. Their thermal stability, electrochemical behaviors, crystal, and electronic structures, and photophysical properties are systematically investigated. The crystal and electronic structures reveal that the peripheral phenyls in HPB are closely aligned in a propeller-like fashion, rendering efficient through-space charge transfer between donor and electron moieties. These molecules display weak fluorescence with negligible delayed component in solutions but strong fluorescence with greatly increased delayed component upon aggregate formation, namely aggregation-induced delayed fluorescence (AIDF). Their neat films exhibit high photoluminescence quantum yields (PLQY), and prominent delayed fluorescence. The non-doped organic light-emitting diodes (OLEDs) based on these new luminogens exhibit excellent performance with maximum external quantum efficiency of 12.7% and very small efficiency roll-off of 2.7% at 1,000 cd m⁻². Designing AIDF molecules with through-space charge transfer could be a promising strategy to explore robust luminescent materials for efficient non-doped OLEDs.

Highly efficient green TADF organic light-emitting diodes by simultaneously manipulating hole and electron transport

This work demonstrates effective performance improvement by simultaneous manipulating of the hole injection and electron transport layers for (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) based green thermally activated delayed fluorescent (TADF) organic light-emitting diodes (OLEDs). A 3 wt% sorbitol doped PEDOT:PSS layer results in the highest maximum current efficiency (CE_max) of 28.28 cd A^-1 and external quantum efficiency (EQE) of 17.04%. Single carrier devices denote that hole mobility gradually rises with the sorbitol ratio. The electroluminescence mainly originates from the emission of 4CzIPN. Atomic force microscopy images imply that 3 wt% sorbitol doped PEDOT:PSS film includes the largest PEDOT aggregate, which contributes to a higher electric conductivity thus the better performance of 3 wt% sorbitol doped device. Also the 4CzIPN ratio in the emissive layer was optimized, and 4 wt%-4CzIPN in CBP achieves the highest EQE of 20.99% and CE_max of 34.99 cd A^-1. The EL spectrum is independent of the luminous angle at a low 4CzIPN ratio but becomes more sensitive to the luminous angle at a high 4CzIPN ratio. Finally, we find out that the TADF OLED performance is very sensitive to TPBi thickness ranging from 20 nm to 65 nm, and 40 nm of TPBi achieves a CE_max up to 64.10 cd A^-1 and an excellent EQE of 25.14%, ascribing from its more balanced carrier transport.

Theoretical study on photophysical properties of a series of functional pyrimidine-based organic light-emitting diodes emitters presenting thermally activated delayed fluorescence

Issue concerning accurate prediction of the reverse intersystem crossing rate (k_RISC ) is critical for developing novel efficient thermally activated delayed fluorescence (TADF) materials. In this contribution, the k_RISC rates from the lowest excited triplet T₁ state to the lowest excited singlet S₁ state were evaluated for five donor-π-acceptor-type pyrimidine-based TADF emitters using the semiclassical Marcus theory. Both the singlet-triplet energy difference (ΔE_ST ) and spin-orbit coupling (V) between the S₁ and T₁ states were investigated by performing the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. In addition, their fluorescence emission wavelengths (λ_em ) were also calculated at the TD-DFT level. The predicted k_RISC and λ_em values are found to reproduce well the available experimental findings. The present results reveal that the k_RISC rates of molecules possessing the unsymmetrical diphenyl pyrimidine acceptor core are calculated to be slightly larger than those of their analogues with the symmetrical diphenyl pyrimidine. In addition, introducing two tert-butyl groups into the 2,7-positions of the donor moiety of the latter is also an effective method for increasing k_RISC when designing TADF emitters. Such a difference is related to the nature of the T₁ excited state. A more remarkable charge-transfer (CT) contribution to the state can achieve a smaller ΔE_ST , leading to a more efficient RISC process, and consequently a shorter delayed fluorescence lifetime as observed experimentally. © 2019 Wiley Periodicals, Inc.

Novel yellow thermally activated delayed fluorescence emitters for highly efficient full-TADF WOLEDs with low driving voltages and remarkable color stability

Novel yellow thermally activated delayed fluorescence (TADF) emitters were developed for full-TADF white organic light-emitting diodes with a stable CRI of 72 and efficiencies of 48.22 cd A⁻¹ and 20.16%.