Green Thermally Activated Delayed Fluorescence Research Articles

Exploring the potential of QSAR in the discovery of novel green TADF materials: an experimental and theoretical study

Thermally activated delayed fluorescence (TADF) materials have garnered significant attention in developing high-efficiency organic light-emitting diodes (OLEDs) for next-generation displays. Despite the progress in TADF research, the increasing complexity of molecular structures poses challenges in material design and synthesis. This study explores the potential of quantitative structure–activity relationship (QSAR) calculations, particularly AutoQSAR, to streamline TADF OLED material selection and design. By leveraging the predictive capabilities of QSAR, we aimed to enhance the accuracy and efficiency of material development. We employed computational modeling, synthesis, and device fabrication to evaluate the performance of a newly developed green TADF dopant material. Our findings indicate that QSAR-guided design predicts material properties effectively and optimizes OLED performance. The synthesized TADF material demonstrated promising efficiency, highlighting the advantages of integrating QSAR calculations into the material discovery process. This study underscores the feasibility of QSAR methodologies in the context of OLED materials, suggesting a pathway for faster and more accurate development. Future improvements in QSAR techniques and collaborative efforts between computational and experimental research will be essential in driving further advancements in OLED technology.

Effect of tert‐Butylation on the Photophysics of Thermally Activated Delayed Fluorescence Emitters

Thermally activated delayed fluorescence (TADF) emitters potentially can provide organic light‐emitting diodes with 100% internal quantum efficiency by harvesting triplet excitons. Generally, TADF emitters are small molecules that are not applicable for solution processability. The addition of tert‐butyl groups to the periphery of TADF emitters has proven to improve their solubility in various organic solvents, reduce aggregation‐induced quenching, and enhance the photoluminescence quantum yield (PLQY). This article studies the photophysical influence of the tert‐butyl group attached to an emitter with a carbazole acceptor and a triazine donor. The resulting t3CzTrz‐F is a blue–green TADF emitter, in which the addition of a tert‐butyl group increases the rate of reverse intersystem crossing (rISC), while simultaneously decreasing the nonradiative decay rate substantially. In addition, dilution of t3CzTrz‐F in a host matrix in film results in an enhanced PLQY, which is associated with a decrease in the nonradiative decay constant, while there is no change in the rISC rate. Through a solid‐state NMR study, the change in rISC and nonradiative rate upon tert‐butylation by enlarged intermolecular spacing and reduced vibrational and rotational freedom is rationalized, resulting in improved photophysical performance.

Tailored Fabrication of Full-Color Ultrastable Room-Temperature Phosphorescence Carbon Dots Composites with Unexpected Thermally Activated Delayed Fluorescence.

The development of single-system materials that exhibit both multicolor room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) with tunable after glow colors and channels is challenging. In this study, four metal-free carbon dots (CDs) are developed through structural tailoring, and panchromatic high-brightness RTP is achieved via strong chemical encapsulation in urea. The maximum lifetime and quantum yield reaches 2141 ms and 56.55%, respectively. Moreover, CDs-IV@urea, prepared via coreshell interaction engineering, exhibits a dual afterglow of red RTP and green TADF. The degree of conjugation and functional groups of precursors affects the binding interactions of the nitrogen cladding on CDs, which in turn stabilizes triplet energy levels and affects the energy gap between S1 and T1 (ΔEST) to induce multicolor RTP. The enhanced wrapping interaction lowers the ΔEST, promoting reverse intersystem crossing, which leads to phosphorescence and TADF. This strong coreshell interaction fully stabilizes the triplet state, thus stabilizing the material in water, even in extreme environments such as strong acids and oxidants. These afterglow materials are tested in multicolor, time, and temperature multiencryption as well as in multicolor in vivo bioimaging. Hence, these materials have promising practical applications in information security as well as biomedical diagnosis and treatment.

Circularly Polarized Organic Light-Emitting Diodes Based on Chiral Hole Transport Enantiomers.

The circularly polarized organic light-emitting diodes (CP-OLEDs) demonstrate promising application in 3D display due to the direct generation of circularly polarized electroluminescence (CPEL). But the chiral luminescence materials face challenges as intricated synthetic route, enantiomeric separation, etc. Herein, fresh CP-OLEDs are designed based on chiral hole transport material instead of chiral emitters. A pair of hole transport enantiomers (R/S-NPACZ) exhibit intense dissymmetry factors (|gPL|) about 5.0 × 10-3. With R/S-NPACZ as hole transport layers, CP-OLEDs are fabricated employing six achiral phosphorescence and thermally activated delayed fluorescence (TADF) materials with different wavelengths, in consistence with the generated CPEL spectra. The CP-OLEDs based on achiral red, green, and blue iridium(III) complexes exhibit external quantum efficiencies (EQEs) of 14.9%, 30.7%, and 14.1% with |gEL| factors of 8.8 × 10-4, 2.3 × 10-3, and 2.0 × 10-3, respectively. Moreover, the devices using achiral blue, blueish-green, and green TADF materials display EQEs of 24.1%, 17.9%, and 25.4% with |gEL| factors of 1.0 × 10-3, 3.6 × 10-3, and 2.2 × 10-3, respectively. As far as known, it is the first example of CP-OLEDs based on chiral hole transport materials, which act as the organic circularly polarizers and have potential to generate CPEL from achiral luminescence materials.

Color‐Tunable TADF Conjugated Polymers Toward Voltage‐Regulating White OLEDs for Intelligent Lighting

AbstractColor‐tunable luminescent polymers are highly desirable materials for meeting the daily needs of humans in terms of circadian rhythm, yet they are rarely reported. Herein, a feasible approach is developed for color‐tunable thermally activated delayed fluorescence (TADF) conjugated polymers by incorporating green and red TADF units into blue fluorescent polymeric backbone. The solution‐processed OLEDs based on the synthesized polymers exhibit high‐efficiency red emission with a maximum external quantum efficiency (EQEmax) of 26.1% under low voltage (6 V). With a slight increase in voltage, warm white emission with EQEmax of 23.9%, color rendering index (CRI) of 76 and low corelated color temperature (CCT) of 1700 K is obtained. These results represent the highest efficiencies among the red and warm white OLEDs based on TADF polymers, respectively. Intriguingly, under higher voltage (10 V), warm white emission with high CRI of 87, CCT of 3216 K, and luminance of over 5300 cd m−2 is accessible. This work provides a promising approach for intelligent lighting by voltage‐regulating white OLEDs.

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.

Judicious Design of Pyridine‐Based Hosts for Stable, Efficient, and Low‐Driving Thermally Activated Delayed Fluorescence Organic Light‐Emitting Diodes

AbstractAlthough thermally activated delayed fluorescence (TADF) based organic light‐emitting diodes (OLEDs) continue to achieve high efficiencies, their device stability still does not necessarily meet industrial standards. To improve device stability, careful design of a stable host material is crucial. Herein, with reference to a common p‐type host material, 3,3′‐di(9H‐carbazol‐9‐yl)‐1,1′‐biphenyl (mCBP), three novel host materials based on a pyridine unit, mCBP‐1N, CzPyPhCz, and CzPyBF, are systematically designed to improve the stabilities efficiently. The green and blue TADF OLEDs are fabricated with these three new hosts. The maximum external quantum efficiencies (EQEmaxs) of green OLEDs are 20.3 % and 14.0% for CzPyBF and mCBP hosts, respectively. Further, high EQEs of 19.2% and 12.9% are maintained even at 1000 cd m−2. Most importantly, the device lifetime of the CzPyBF host can achieve LT95 of 62.7 h, which is 2.5 times longer than that of mCBP. Also, lower turn‐on voltages are achieved by using CzPyPhCz in blue and green devices. With host engineering, the TADF OLEDs are simultaneously improved with lower turn‐on voltages, higher EQEs, and longer device lifetimes.

Performance improved white full-organic light emitting diodes (WOLED) based on tricolor dual-layer of hyperfluorescence (HF)

Efficiency-improved white-light organic light emitting diodes (WOLED) was designed, which based on thermally activated delayed fluorescence (TADF) tricolor-hyperfluorescent dual-layers. Benefited from the efficient utilization of triplet excitons by effective reverse intersystem crossing (RISC), this WOLED features improved efficiency and long lifetime. In this work, blue OLED based on emitting layer of mCP:BD340 was developed and dual-emissive hyperfluorescent OLEDs based on mono-emitting layer of CBP:4CzIPN (green TADF emitter):DBP (red emissive fluorescent molecule) were investigated. Then, the hyperfluorescent WOLED based on the two emitting layers was designed. The good performances were benefited from the highly efficient hyperfluorescent system. This device architecture design demonstrated here represents a kind of full-organic, low-cost white-light OLED for power-efficient light source.

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.

Peripherally Heavy-Atom-Decorated Strategy Towards High-Performance Pure Green Electroluminescence with External Quantum Efficiency over 40 .

Heavy-atom integration into thermally activated delayed fluorescence (TADF) molecule could significantly promote the reverse intersystem crossing (RISC) process. However, simultaneously achieving high efficiency, small roll-off, narrowband emission and good operational lifetime remains a big challenge for the corresponding organic light-emitting diodes (OLEDs). Herein, we report a pure green multi-resonance TADF molecule BN-STO by introducing a peripheral heavy atom selenium onto the parent BN-Cz molecule. The organic light-emitting diode device based on BN-STO exhibited state-of-the-art performance with a maximum external quantum efficiency (EQE) of 40.1 %, power efficiency (PE) of 176.9 lm W-1 , well-suppressed efficiency roll-off and pure green gamut. This work reveals a feasible strategy to reach a balance between fast RISC process and narrow full width at half maximum (FWHM) of MR-TADF by heavy atom effect.

Power Efficiency Enhancement of Organic Light-Emitting Diodes Due to the Favorable Horizontal Orientation of a Naphthyridine-Based Thermally Activated Delayed Fluorescence Luminophore.

Four emitters based on the naphthyridine acceptor moiety and various donor units exhibiting thermally activated delayed fluorescence (TADF) were designed and synthesized. The emitters exhibited excellent TADF properties with a small ΔE ST and a high photoluminescence quantum yield. A green TADF organic light-emitting diode based on 10-(4-(1,8-naphthyridin-2-yl)phenyl)-10H-phenothiazine exhibited a maximum external quantum efficiency of 16.4% with Commission Internationale de L'éclairage coordinates of (0.368, 0.569) as well as a high current and power efficiency of 58.6 cd/A and 57.1 lm/W, respectively. The supreme power efficiency is a record-high value among the reported values of devices with naphthyridine-based emitters. This results from its high photoluminescence quantum yield, efficient TADF, and horizontal molecular orientation. The molecular orientations of the films of the host and the host doped with the naphthyridine emitter were explored by angle-dependent photoluminescence and grazing-incidence small-angle X-ray scattering (GIWAXS). The orientation order parameters (ΘADPL) were found to be 0.37, 0.45, 0.62, and 0.74 for the naphthyridine dopants with dimethylacridan, carbazole, phenoxazine, and phenothiazine donor moieties, respectively. These results were also proven by GIWAXS measurement. The derivative of naphthyridine and phenothiazine was shown to be more flexible to align with the host and to show the favorable horizontal molecular orientation and crystalline domain size, benefiting the outcoupling efficiency and contributing to the device efficiency.

Solution processable carbazole-benzophenone derivatives as bipolar hosts enabling high-efficiency stable green TADF organic LEDs

Solution-processable bipolar materials acting as hosts, soluble in common organic solvents, are key materials for thermally activated delayed fluorescence (TADF) emitters.

Novel carbazole host materials for solution processed TADF Organic Light Emitting Diodes

Several functional and internal layers have been used to improve the performance of solution-processed Organic Light Emitting Diodes (OLED) devices. In the case of thermally activated delayed fluorescence (TADF) OLEDs, a large number of emitter materials have already been reported, but the design and development of potential host materials are overlooked. Efficient solution processable host materials for TADF OLEDs can reduce the complexity in the device architecture and production cost. In this work, we utilized a series of carbazole-based solution-processable host materials (ME4, ME5, ME9, ME10, and ME13) to fabricate the 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) based green TADF OLEDs. Among them, the ME9 composing device showed the best performance. The resultant device exhibited a maximum power efficacy of 40.0 lm/W, current efficacy of 44.7 cd/A, and external quantum efficiency of 14.1% with a maximum luminance of 16,220 cd/m2. The excellent performance may attribute to the low singlet-triplet energy gap (ΔEST), high photoluminescence quantum yield (PLQY), high thermal stability, and unique porous morphology of the ME9.

Engineering Intramolecular π‐Stacking Interactions of Through‐Space Charge‐Transfer TADF Emitters for Highly Efficient OLEDs with Improved Color Purity

AbstractPurely organic compounds with thermally activated delayed fluorescence (TADF) are promising sustainable emitters for organic light‐emitting diodes. But the simultaneous realization of a high efficiency, short lifetime, and good color purity in single TADF molecule remains challenging. Herein it is reported that the confinement of rigid and planar N‐ and B‐centered donor and acceptor in sandwich‐type structures, named BNB‐m and BNB‐p, can lead to green TADF emissions with up to unity efficiency and improved color purity from through‐space charge‐transfer excited states. The full width at half maximum of 58 nm (0.28 eV) for BNB‐m is significantly smaller than those for most of the TADF emitters having a twisted donor‐acceptor structure. Single‐crystal structures, NMR spectroscopy, and theoretical simulations manifest the presence of strong noncovalent π‐stacking interactions in BNB‐m which account for the reduced intramolecular motions and enhanced through‐space electronic coupling. With a relatively short delayed fluorescence lifetime of 11 µs, the emitter BNB‐m demonstrates attractive green electroluminescence with a maximum external quantum efficiency (EQE) of 34.9% and an EQE of 27.4% at 1000 cd m−2. This work validates a promising molecular design to TADF emitters which can realize a balanced emission property in terms of color purity and emission lifetime.

Exploiting new feasibility of a phenylquinoline unit for establishing efficient green thermally activated delayed fluorescent emitter with short delayed fluorescent lifetime

Realizing the confluence of high photoluminescence quantum yields (PLQYs) and short delayed fluorescent lifetimes remains a challenge for thermally activated delayed fluorescent (TADF) emitters. A green TADF emitter, 10-(2,4-diphenylquinolin-6-yl)-10 H -phenoxazine (PhQLPXZ), is exploited by directly linking an 2,4-diphenylquinoline unit with an electron-donating (D) 10 H -phenoxazine moiety. For the first time, the phenylquinoline (PhQL) unit is revealed as an electron-accepting unit for fabricating efficient TADF emitters. The dihedral angle between the donor and the acceptor is nearly 90°, which induces a totally separation of frontier orbitals. As a result, the emitter reveals a small singlet-triplet splitting energy of 0.09 eV and a short delayed fluorescence lifetime of 1.3 μs. The green organic light-emitting diodes (OLEDs) employing PhQLPXZ as the emitter exhibit a maximum external quantum efficiency of 15.7%. This study demonstrates for the first time that phenylquinoline derivatives can be employed for constructing TADF emitters for high-efficiency fluorescent OLEDs. • Phenylquinoline derivatives were employed to construct a green TADF emitter with a short delayed lifetime of 1.3 μs. • The molecule endows a vertical dihedral angle (97°) between the acceptor and the donor and high photoluminescence quantum yield over 70% in film. • The device with a doping concentration of 6% exhibited a high EQE of 15.7%, which is one of the best among the devices based on quinolone cored emitters.

Zero-Zero Energy-Dominated Degradation in Blue Organic Light-Emitting Diodes Employing Thermally Activated Delayed Fluorescence.

Blue-emitting organic light-emitting diodes (OLEDs) fall significantly behind other OLEDs in operational stability. To better understand the key factors governing the stability of blue OLEDs employing thermally activated delayed fluorescence (TADF), nine efficient sky-blue to green TADF emitters with different frontier orbital energy levels and different TADF lifetimes have been designed and synthesized on the basis of charge-transfer (CT) acridine/phenyltriazine derivatives. Among them, ToDMAC-TRZ, a molecule composed of a 9,9-dimethyl-2,7-di-o-tolyl-9,10-dihydroacridine donor and a 2,4,6-triphenyl-1,3,5-triazine acceptor, shows a quantum yield of nearly 1 and a TADF lifetime as short as 0.59 μs in thin film. However, the stability of OLEDs is independent of the frontier orbital energy levels and TADF lifetime of the emitter. In contrast, the device half-life is found to decrease by five-sixths as the 0-0 energy of the singlet excitons increases by about 0.06 eV, which can be well-explained by the Arrhenius equation employing a photoreaction model. Whether in photoluminescence or electroluminescence, the contribution of long-lifetime triplet excitons to degradation is much lower than expected, which can be accounted for by how the solid-state solvation effect reduces the energy of the 3CT state and how most molecules have a low-lying locally excited triplet state.

New bipolar host materials for high power efficiency green thermally activated delayed fluorescence OLEDs

Four bipolar molecules, namely m-CzPym, p-CzPym, m-CzTrz, and p-CzTrz, with carbazole (Cz) donor and a benzonitrile-substituted pyrimidine (Pym) or triazine (Trz) acceptor core were synthesized and characterized. The electron deficiency of heteroaryl cores together with the substitution pattern of benzonitrile were employed to tune the energy levels as well as the thermally activated delayed fluorescence (TADF) characteristics. The four molecules exhibited TADF behavior with inferior photoluminescent quantum yields (PLQYs) that limit their applications as emitters. These bipolar molecules were employed as TADF host materials for the benchmark TADF emitter 4CzIPN to achieve high-performing green TADF organic light-emitting diodes (OLEDs). Among the molecules, m-CzPym-hosted TADF-OLEDs achieved a maximum external quantum efficiency (EQEmax) of 31.5%, maximum power efficiency (PEmax) of 95.6 lm/W, and maximum current efficiency (CEmax) of 100.2 cd/A. Notably, p-CzPym-hosted TADF-OLEDs also achieved a PEmax of 116.5 lm/W, turn-on voltage of 2.5 V, and impressive low efficiency roll-off performance (>89% of EQEmax at 5000 cd/m2), representing one of the highest efficiencies ever reported in 4CzIPN-doped devices. The high device efficiency can be ascribed to the balanced ambipolar carrier-transporting character of the host materials and high PLQY as well as the outstanding light outcoupling efficiency of the emitting layer.

Donor Arylmethylation toward Horizontally Oriented TADF Emitters for Efficient Electroluminescence with 37% External Quantum Efficiency

AbstractDesigning thermally activated delayed fluorescence (TADF) emitters with high horizontal orientation dipole ratios (Θ//) is regarded as one of the key measures to achieve high light outcoupling efficiency and thus efficient organic light‐emitting diodes (OLEDs). Herein, donor arylmethylation is presented to construct horizontally oriented TADF emitters. Two compounds PFDMAC‐TRZ and DPFDMAC‐TRZ are designed and synthesized through Friedel‐Crafts arylmethylation of their parent TADF molecule DMAC‐TRZ with 9‐phenyl‐9‐fluorenol. The 9‐phenyl‐9‐fluorene substituents endow PFDMAC‐TRZ and DPFDMAC‐TRZ with similar excited states and emission characteristics as DMAC‐TRZ due to the unique σ–π conjugation. Impressively, with an arylmethylated donor, PFDMAC‐TRZ and DPFDMAC‐TRZ show an enhanced horizontal orientation dipole ratio of 78% and 81%, respectively. Green TADF OLED based on DPFDMAC‐TRZ reaches a maximum external quantum efficiency of 37.0% and power efficiency of 124.5 lm W−1, which represents one of the most efficient green OLEDs reported so far. This work provides a facile yet powerful strategy to design and construct horizontally oriented luminogens for high‐performance optical devices.

Benzonitrile-based AIE polymer host with a simple synthesis process for high-efficiency solution-processable green and blue TADF organic light emitting diodes

Polymer host materials have great potential for enabling small molecular thermally activated delayed fluorescence (TADF) emitters to construct solution-processed organic light-emitting diodes (OLEDs).

Intrinsically Ionic, Thermally Activated Delayed Fluorescent Materials for Efficient, Bright, and Stable Light‐Emitting Electrochemical Cells

AbstractSolid‐state light‐emitting electrochemical cells (LECs) using sustainable and eco‐friendly materials and affording high brightness, efficiency, and stability are highly desired. Here, intrinsically ionic, thermally activated delayed fluorescence (TADF) materials 1–3 for efficient, bright, and stable LECs are reported. 1–3 feature carbazole‐type donors and cationic triazine‐type acceptors, which are located ortho to each other on the phenyl linkers. Through‐space charge‐transfer (CT) dominates the CT transitions in 1–3. In doped and neat films, 1–3 show blue and green TADF emission, respectively, with reverse intersystem crossing rates at around 7.0 × 105 s−1. 1–3 possess excellent electrochemical stability (except for the oxidation of 1) and film‐forming abilities. LECs using neat films of 1–3 as the single active layers afford green electroluminescence with peak brightness/peak external quantum efficiency (EQE) of up to 572 cd m−2/6.8% under 4.0 V and peak brightness/peak EQE/half‐lifetime of up to 860 cd m−2/5.4%/48 h under 50 A m−2. A longer half‐lifetime of 218 h has further been achieved at 162 cd m−2 under 10 A m−2. The work reveals the bright prospect for the development of efficient, bright, and stable LECs with intrinsically‐ionic TADF materials.