Materials For Organic Light-emitting Diodes Research Articles

Recent progress in circularly polarized multi-resonance thermally activated delayed fluorescence materials for organic light-emitting diodes

Recent progress in circularly polarized multi-resonance thermally activated delayed fluorescence materials for organic light-emitting diodes

Dimesitylborane as Electron Accepting Unit in High Performance Yellow Single-Layer Phosphorescent Organic Light Emitting Diode.

A new host material for Single-Layer Phosphorescent Organic Light-Emitting Diodes (SL-PhOLED) is reported, namely SPA-2-FDMB, using the dimesitylborane (DMB) fragment as an acceptor unit. The molecular design is constructed on the general donor-spiro-acceptor architecture, which consists of connecting, via a spiro bridge, a donor and an acceptor units in order to avoid strong interaction between them. The DMB fragment is known for many electronic applications (notably Aggregation-Induced Emission) but has not been used yet for SL-PhOLED applications. This appears particularly interesting, as the development of this simplified technology has shown that only a few electron-accepting fragments such as diphenylphosphine oxide can provide high-performance devices. Herein, the yellow-emitting SL-PhOLED using SPA-2-FDMB as host presents an External Quantum Efficiency of 8.1% (Current Efficiency of 24.9cd.A-1) with a low threshold voltage of 2.6V. As SPA-2-FDMB presents a sharp HOMO/LUMO difference, the good matching of HOMO and LUMO energy levels with the Fermi level of the electrodes is responsible for these performances. The low LUMO level of -2.61eV also appears particularly important. These performances are, to date, the highest reported for a yellow/orange-emitting SL-PhOLED and show the potential of DMB unit in the single-layer technology.

Suppression of Initial Degradation via an Interfacial Charge-Induced Overshooting Effect in Solution-Processed Organic Light-Emitting Diodes.

As the chemical stability of organic materials in organic light-emitting diodes (OLEDs) greatly impacts devices' lifetime, a thoughtful and advanced design of materials and device structures is necessary. In our work, we have achieved lifetime enhancement at its initial stage for solution-processed OLEDs. This improvement was realized through the implementation of a double electron transporting layer (dETL) composed of 2-[4-(9,10-dinaphthalen-2-yl-anthracen-2-yl)-phenyl]-1-phenyl-1H-benzoimidazole (ET) and hydroxyquinolinolato-lithium (Liq). A giant surface potential was generated at the surface of a constituent electron transport layer (ETL) that contained a higher concentration of Liq with high polarity. This giant surface potential simultaneously promoted the injection of trapped/accumulated electrons through the interface within dETL and the injection of holes from the anode, generating more exciton recombination events and ultimately enhancing efficiency by 133.0% and lifetime LT95 (luminance dropped by 5%) by 300% with an overshooting effect. Additionally, the degradation at the emitting layer was mitigated by shifting the degradation zone to the dETL, which was evidenced by laser desorption/ionization-time-of-flight (LDI-TOF) mass spectroscopy.

Characteristics Study of OLED Materials

Abstract Organic Light Emitting Diodes (OLEDs) are emerging as promising alternatives to conventional lighting and display technologies due to their unique properties and potential energy efficiency. This study conducts a comprehensive investigation into OLED materials, focusing on their optical, electrical, and structural characteristics. Through experimental analysis and theoretical modeling, the study delves into the performance and behaviour of various OLED materials, including organic semiconductors, charge transport layers, and emissive materials. Critical aspects such as charge carrier mobility, exciton formation, and device stability are examined to understand the mechanisms governing OLED operation. The findings offer valuable insights into optimizing OLED devices for applications ranging from consumer electronics to lighting and signage. This research contributes fundamental knowledge and practical guidelines for material selection, device fabrication, and performance assessment, thereby advancing OLED technology. The insights provided pave the way for future innovations in OLED materials and device design, potentially leading to improvements in efficiency, brightness, and longevity.

Machine Learning-Driven precise design of stable OLED Materials: Predicting and enhancing Multi-State C-N bond dissociation energies

The intrinsic stability of organic light-emitting diode (OLED) materials critically hinges on the bond dissociation energies (BDEs) of fragile bonds, particularly C-N bonds. Despite the necessity of considering BDEs in multiple electronic states due to the complex working conditions of organic materials in OLED devices, comprehensive exploration remains challenging. Herein, we constructed a high-quality dataset, CNBDE-24, comprising nearly 1500 data points for BDEs across multiple electronic states, obtained through an active learning-guided density functional theory (DFT) calculation strategy. Machine learning models trained on the CNBDE-24 dataset exhibit excellent accuracy in predicting BDEs for OLED materials in neutral, triplet excited (T1), and anionic states, achieving an adjusted coefficient of determination exceeding 0.90 on the test set and bypassing the need for high-cost calculation. Moreover, utilizing neutral state BDEs as pre-training data significantly reduces model error and provides deeper insights into the relationships between different BDEs. We find that BDEs in neutral are influenced by the electronic properties of the nitrogen atom, as indicated by descriptors such as Gasteiger charges. Incorporating C-N bonds into aromatic systems and slightly dispersing N atom charges can inherently increase multi-state BDEs without altering luminescent properties. Additionally, anionic BDEs are determined by the extent of charge transfer during bond cleavage and can be regulated by modifying the C-side fragment. High-throughput virtual screening reveals a new stable purple emitter with a dominant 0–0 transition at 360 nm, which was validated through DFT and excited-state property evaluations. Strategies for enhancing BDEs in multiple resonance thermally activated delayed fluorescence materials without altering excited-state properties are also demonstrated.

Synergistic Intramolecular Non-Covalent Interactions Enable Robust Pure-Blue TADF Emitters.

Stability-issues of organic light-emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) require further advancements, especially in pure-blue range of CIEy < 0.20, existing a dilemma between color purity and device lifetime. Though improving bond-dissociation-energy (BDE) can effectively improve material intrinsic stability, strategies to simultaneously improve BDE and photophysical performances are still lacking. Herein, it is disclosed that synergistic intramolecular non-covalent interactions (Intra-NI) can achieve not only the highest C─N BDE among blue TADF materials, but enhanced molecular-rigidity, near-unity photoluminescent quantum yields and short delayed lifetime. Pure-blue TADF-OLEDs based on proof-of-concept TADF material realize high external-quantum-efficiency and record-high LT80@500cdm-2 of 109h with CIEy = 0.16. Furthermore, deep-blue TADF-sensitized devices exhibit high LT80@500cdm-2 of 81h with CIEy = 0.10. The findings provide new insight into the critical role of Intra-NI in OLED materials and open the way to tackling vexing stability issues for developing robust pure-blue organic emitters and other functional materials.

Theoretical insight on electronic and optical properties of some phosphorescent iridium(III) complexes for organic light-emitting diode devices

Organic light-emitting diode (OLED) structures have been extensively investigated because of their potential applications in various fields. It is known that organic or metal-mediated organic compounds are used in the layers of OLEDs. Within OLED materials, iridium(III) compounds have attract much attention owing to their many advantages. Therefore, we predicted the OLED behaviors of twenty-five phosphorescent iridium(III) complexes using theoretical chemical methods. All theoretical calculations were performed with the B3LYP hybrid functional using Gaussian 16 and Amsterdam Modeling Suite 2023 programs. While the 6–31G(d) basis set for non-metal atoms and LANL2DZ basis set for iridium metal was preferred in the computations carried out by using Gaussian program, whereas in the calculations performed with the Amsterdam Modelling Suite program, the TZP basis set was used for all atoms. From the theoretically obtained results, it is seen that Ir6, Ir7, Ir22-Ir25 complexes can be proposed as good candidate for hole injection layer materials in OLED based on indium tin oxide as an anode. Within complexes investigated, it has been determined that Ir16 and Ir17 complexes are the most suitable candidates for electron transport layer materials, whereas Ir16 complex can be used as both a hole transfer layer and ambipolar materials. Furthermore, it has been predicted that Ir4, Ir7, Ir8, and Ir23 complexes could be preferred as hole blocking layer compounds. From the detailed analysis of the singlet-triplet transitions of the complexes studied, it could be said that all iridium(III) complexes investigated could be used as highly efficient phosphorescent OLED molecules.

Efficient thermally activated delayed fluorescence materials from symmetric anthraquinone derivatives for high-performance red OLEDs

Constructing efficient red thermally activated delayed-fluorescence (TADF) materials for high-performance organic light-emitting diodes (OLEDs) remains challenging due to the formidable barrier of energy gap law. In this work, a design strategy of connecting two donor units to the adjacent positions of electron acceptor is proposed for creating red luminescent materials, and four Y-shaped TADF molecules consisting of strong electron-withdrawing anthraquinone (AQ) acceptor and triphenylamine or acridine-based donors are designed and synthesized. They exhibit strong red emissions (604−618 nm) in toluene solutions and orange/red emissions (566−608 nm) with good photoluminescence quantum yields (43−68%) in doped films, and enjoy small singlet-triplet energy gaps (0.02−0.10 eV) and fast reverse intersystem crossing processes (1.5–7.3 × 105 s−1), which are attributed to the unique Y-shape structure. A maximum external quantum efficiency of 19.5% with an electroluminescence peak at 616 nm is achieved for AQ-PTPA-based red doped device, representing the highest level for red TADF-OLEDs based on AQ acceptor in the literature. This work can provide guidance for the design of efficient red delayed-fluorescence molecules for the application in OLEDs.

(Invited) Quantitative Prediction of Exciton Dynamics

Thermally activated delayed fluorescence (TADF) is now an established way to convert inhelently dark triplet excitons into light through reverse intersystem crossing (RISC).1-4 To realize the RISC process, minimizing the energy gap (ΔE ST) between S1 and T1 is considered to be effective theoretically, and this has been realized experimentally. At the beginning of the research, RISC had been the rate-limiting process in TADF, although RISC was possible. In such situation, we have shown that acceleration of RISC has now become possible through a variety of methods.5-7 Although excellent TADF materials and highly efficient TADF-based organic light-emitting diodes (OLEDs) have been realized as described above, the fundamental understanding is still insufficient. In the presentation, we first show our recently-proposed method for quantitatively predicting rate constants of all electronic transitions relevant to light emission.8-10 The method proposed here has successfully predicted all rate constants quantitatively and therefore can predict TADF performance precisely. The method also allows quantitative predictions of dynamics (time evolution) of experimentally-obtained rate constants for radiative, non-radiative, intersystem crossing, and RISC as well as exciton population dynamics.11 Such an approach will contribute significantly to the fundamental science of exciton dynamics.We express sincere thanks to my group members. This work was supported by JSPS KAKENHI Grant Numbers JP20H05840 (Grant-in-Aid for Transformative Research Areas, “Dynamic Exciton”). Computation time and NMR measurements were supported by the international Joint Usage/Research Centre at the Institute for Chemical Research, Kyoto University, Japan. Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C., Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 2012, 492, 234. Kaji, H., et al., Purely organic electroluminescent material realizing 100% conversion from electricity to light. Nat. Commun. 2015, 6, 8476. Yang, Z., et al., Recent advances in organic thermally activated delayed fluorescence materials. Chem. Soc. Rev. 2017, 46, 915. Wong, M. Y.; Zysman-Colman, E., Purely organic thermally activated delayed fluorescence materials for organic light-emitting diodes. Adv. Mater. 2017, 29, 1605444. Wada, Y.; Nakagawa, H.; Matsumoto, S.; Wakisaka, Y.; Kaji, H., Organic light emitters exhibiting very fast reverse intersystem crossing. Nat. Photon. 2020, 14, 643. Ren, Y.; Wada, Y.; Suzuki, K.; Kusakabe, Y.; Geldsetzer, J.; Kaji, H., Efficient blue thermally activated delayed fluorescence emitters showingvery fast reverse intersystem crossing. Appl. Phys. Express 2021, 14, 071003. Wada, Y.; Wakisaka, Y.; Kaji, H., Efficient direct reverse intersystem crossing between charge transfer‐type singlet and triplet states in a purely organic molecule. ChemPhysChem 2021, 22, 625. Shizu, K.; Kaji, H., Theoretical determination of rate constants from excited states: Application to benzophenone. J. Phys. Chem. A 2021, 125, 9000. Shizu, K.; Kaji, H., Comprehensive understanding of multiple resonance thermally activated delayed fluorescence through quantum chemistry calculations. Commun. Chem. 2022, 5. Shizu, K.; Ren, Y. X.; Kaji, H., Promoting reverse intersystem crossing in thermally activated delayed fluorescence via the heavy-atom effect. J. Phys. Chem. A 2023, 127, 439.Shizu, K.; Kaji, H., submitted.

(Invited) Precise Determination of Exciton Binding Energy of Organic Semiconductors

Strongly bound excitons in organic semiconductors are crucial for the operation of organic optoelectronic devices. However, accurate experimental data on the exciton binding energy of organic semiconductors are lacking. In this study, we determine the exciton binding energy as the difference between the optical and transport bandgaps with a precision of 0.1 eV. From the systematic study of the exciton binding energy of 42 organic semiconductors (15 non-fullerene acceptors, 4 fullerene acceptors, 13 low-bandgap polymers, 7 organic light-emitting diode materials, and 3 crystalline materials), we found that the exciton binding energy is a quarter of the transport gap regardless of the material. We interpret this unexpected relationship in terms of a hydrogen atom model.

Understanding Spin-Triplet Excited States in Carbene-Metal-Amides.

Carbene-metal-amides (CMAs) are emerging delayed fluorescence materials for organic light-emitting diode (OLED) applications. CMAs possess fast, efficient emission owing to rapid forward and reverse intersystem crossing (ISC) rates. The resulting dynamic equilibrium between singlet and triplet spin manifolds distinguishes CMAs from most purely organic thermally activated delayed fluorescence emitters. However, direct experimental triplet characterization in CMAs is underutilized, limiting our detailed understanding of the ISC mechanism. In this work, we combine time-resolved spectroscopy with tuning of state energies through environmental polarity and metal substitution, focusing on the interplay between charge-transfer (3CT) and local exciton (3LE) triplets. Unlike previous photophysical work, we investigate evaporated host : guest films of CMAs and small-molecule hosts for increased device relevance. Transient absorption reveals an evolution in the triplet excited-state absorption (ESA) consistent with a change in orbital character between hosts with differing dielectric constants. Using quantum chemical calculations, we simulate ESAs of the lowest triplet states, highlighting the contribution of only 3CT and donor-moiety 3LE states to spectral features, with no strong evidence for a low-lying acceptor-centered 3LE. Thus, our work provides a blueprint for understanding the role of triplet excited states in CMAs which will enable further intelligent optimization of this promising class of materials.

Spatially Resolved Functional Group Analysis of OLED Materials Using EELS and ToF-SIMS.

Electron energy-loss spectroscopy (EELS) is widely used in analyzing the electronic structure of inorganic materials at high spatial resolution. In this study, we use a monochromator to improve the energy resolution, allowing us to analyze the electronic structure of organic light-emitting diode (OLED) materials with greater precision. This study demonstrates the use of the energy-loss near-edge structure to map the nitrogen content of organic molecules and identify the distinct bonding characteristics of aromatic carbon and pyridinic nitrogen. Furthermore, we integrate EELS with time-of-flight secondary ion mass spectrometry for molecular mapping of three different bilayers composed of OLED materials. This approach allows us to successfully map functional groups in the by-layer OLED and measure the thickness of two OLED layers. This study introduces spatially resolved functional group analysis using electron beam spectroscopy and contributes to the development of methods for complete nanoscale analysis of organic multilayer architectures.

Effects of carbazolyl and diphenylamino substituents bearing methoxy groups on the performance of hole-transporting materials in OLEDs

Newly designed and synthesized derivatives of pentaphenylbenzene with methoxy-substituted carbazolyl or diphenylamino moieties were investigated to estimate their applicability as hole transport materials. Both the compounds exhibit high thermal stability. The intramolecular charge transfer is blocked for the film of the compound containing diphenylamino groups. The intermolecular charge transfer is induced in the film of carbazolyl-containing compound. The derivative of pentaphenylbenzene and diphenylamine exhibits higher hole drift mobility (2.4·10−3 cm2/V·s at the electric field of 5.5·105 V/cm) and by 0.1 eV lower ionization potential than the carbazolyl-containing compound. Both the compounds were utilized as hole-transporting materials in a series of organic light emitting diodes (OLEDs) based on of thermally activated delayed fluorescence. With the maximum values of external quantum efficiency of 25.9 % and power efficiency of 43.4 lm/W, OLEDs containing the layers of the synthesized compounds outperformed the device based on TCTA by 4 %, without the change in spectral properties. Variable angle spectroscopic ellipsometry revealed the moderate average roughness of the films of the compound deposited by the thermal vacuum evaporation technique with an arithmetic mean deviation of not more than 0.8 nm. The prominent hole transport characteristics of the compounds make them good candidates for utilization in optoelectronic devices.

Hexacarbazolylbenzene: An Excellent Host Molecule Causing Strong Guest Molecular Orientation and the High-Performance OLEDs.

Hexacarbazolylbenzene (6CzPh), which is benzene substituted by six carbazole rings, is a simple and attractive compound. Despite the success of a wide variety of carbazole derivatives in organic light-emitting diodes (OLEDs), 6CzPh has not received attention so far. Here, excellent performances of 6CzPh are revealed as a host material in OLEDs regarding conventional host materials. Various strategies are implemented to improve the performance of OLEDs, e.g., triplet utilization by thermally activated delayed fluorescence (TADF) and phosphorescence emitters for maximizing internal quantum efficiency, and molecular orientation control for increasing outcoupling efficiency. The present host material is suited for both criteria. Robustness of the structure and sufficiently high triplet energy enables a high external quantum efficiency with a long device lifetime. Besides, the host material boosts the horizontal molecular orientations of several guest emitters. It is noteworthy that disk-shaped 4CzIPN marks the complete horizontal molecular orientations (Θh = 100%, S = -0.50). These results provide an effective way of improving efficiencies without sacrificing device durability for future OLEDs.

Purely organic room temperature phosphorescent materials toward organic light‐emitting diodes

AbstractPurely organic room temperature phosphorescence (RTP) materials have shown broad application prospects in organic light‐emitting diodes (OLEDs) due to their theoretical 100% exciton utilization, cost‐effectiveness, and flexibility. In recent years, with the deepening of research, various luminescent mechanisms have been proposed, and RTP materials have made significant progress, which have been effectively applied to OLEDs. This article comprehensively reviews the research progress of RTP materials in OLEDs and introduces the development of a series of high‐efficiency RTP materials from the perspective of molecular design strategies and photophysical properties. These conclusions draw a roadmap to address the inherent challenges in utilizing organic RTP materials to specifically advance the investigation of OLEDs.

Uncovering the Aggregation-Induced Emission Mechanisms of Phenoxazine and Phenothiazine Groups.

Molecules with both aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) properties are potential organic light-emitting diode materials; however, the AIE and TADF mechanisms are still debatable. In this work, four molecules incorporating carbazole (Cz), phenoxazine (PXZ), and phenothiazine (PTZ) as donor groups to the diphenylsulfone acceptor were investigated. The experiment results indicate that a molecule containing Cz exhibits solely TADF properties, whereas molecules containing PXZ and PTZ demonstrate both TADF and AIE characteristics. As for DPS-PTZ, the result indicates that the thin-film environment restricts molecular twisting, consequently reducing nonradiative decay, thereby attributing to the AIE property by density functional theory and molecular dynamics simulation. As for DPS-PXZ, the result suggests that the restricted access to a conical intersection in a singlet excited via an expansion in the C-S-C angle is the pivotal factor for the AIE characteristic. The C-S-C angle twist of DPS-PXZ is impeded in the aggregate state and resulted in luminescence. Understanding the mechanisms serves as a valuable guide for the development of new AIE systems, enabling their application in various practical domains.

P‐253: Late‐News Poster: The Evaluation on the Stability of Electroluminescent Materials Using Polaron Pair Formation Kinetics

We developed the method for evaluation of the stability of electroluminescent materials by using QEMS (quantum efficiency measurement system). Relative PL (photoluminescence) intensity decay of BDs (blue dopants) with BHs (blue hosts) in solution under UV light irradiation were measured and the rate constants for polaron pair formation by electron transfer were determined through first‐order kinetics analyses. The rate constants of polaron pair formation (6.487E‐04 –1.805E‐03 min‐1 ) were inversely proportional to the lifetime of OLED (organic light‐emitting diode) devices. We expect that this evaluation method can be applied to the development of new OLED materials and longer lifetime OLED devices.

Aggregation-Induced Emission (AIE) in Polymers: Effect of Polymer size on the Fluorescence of Low Molecular Weight PEG and PPG.

Aggregation-induced emission (AIE) is a fascinating phenomenon where specific molecules exhibit enhanced fluorescence upon aggregation. This unique property has revolutionized the design and development of new fluorescent materials for different applications, from biosensors and organic light-emitting diodes (OLEDs) to biomedical imaging and diagnostics. Researchers are creating sensitive and selective sensing platforms, opening new avenues in material science and engineering by harnessing the potential of AIE. To expand the knowledge in this field, this study explored the aggregation-induced emission (AIE) properties of two polymers, namely polyethylene glycol (PEG) and polypropylene glycol (PPG) of low molecular weight (MW) using fluorescence spectroscopy and absorbance (UV). PEG-300 and PPG-725 were the most fluorescent polymers at UV of the ten investigated. Interestingly, AIE did not correlate linearly with molecular weight (MW), and monobutyl ether substitution in PEG with a similar MW substantially altered its AIE. Furthermore, fluorescence precisely quantified low polymer concentrations in water, and non-aqueous solvents suppressed AIE, suggesting potential for AIE manipulation. These findings enhance our understanding of AIE in polymers, fostering the development of novel materials for applications such as biosensors.

Indolo[3,2-a]carbazoles as Engineered Materials for Optoelectronic Applications: Synthesis, Structural Insights, and Computational Screening.

The biological and medicinal importance of indolocarbazoles has been known for the past several decades. However, in recent times, these compounds have been emerging as potential candidates for optoelectronic applications, although several challenges are associated with their synthesis. We report here a Pd(II)-catalyzed process for the synthesis of indolo[3,2-a]carbazoles. The reaction proceeded under neat conditions and in the presence of aqueous nonmetallic oxidant TBHP, and the products were purified directly after the completion of the reaction. Also, the possibility of employing the present method for reaction with gram-scale feed was investigated. A detailed single-crystal analysis of several indolo[3,2-a]carbazoles revealed how the molecular arrangement can be tuned by altering the functionalization. Finally, the developed molecules were screened computationally to assess their potential for possible use as hole transport materials (HTMs) for organic light-emitting diodes (OLEDs).

Exploring Acceptor-Functionalized Perylenes as Hole Transport Materials for Solution Process Organic Light-Emitting Diodes

Exploring Acceptor-Functionalized Perylenes as Hole Transport Materials for Solution Process Organic Light-Emitting Diodes