Materials For Organic Light-emitting Diodes Research Articles

Discs to a 'Bright' Future: Exploring Discotic Liquid Crystals in Organic Light Emitting Diodes in the Era of New-Age Smart Materials.

With the advent of a new decade and the paradigm shift of every sphere of urban life to virtual platforms, it has become imperative for the global researcher community to channelize efforts into upgradation of the existing display-technology. In this context, discotic liquid crystals (DLCs) are a class of self-assembling organic materials that are recently being explored in fabricating the emissive layers of organic light emitting diodes (OLEDs). With their unique inherent structural and functional properties, they have the potential to challenge the currently prevailing OLED-emitters. Yet the applications of this promising class of materials in OLEDs have not been comprehensively reviewed in literature till now. In this account, we present an overview of the developments in the field of luminescent DLC-based emitters, supported by their associated photophysical phenomena and their performance parameters as emitters in fabricated OLED devices.

Indolo[3,2,1-jk]carbazole-Derived Narrowband Violet-Blue Fluorophores: Tuning the Optical and Electroluminescence Properties by Chromophore Juggling.

The development of rigid polyaromatic building blocks for narrowband violet fluorophores has received tremendous attention. Herein, we designed and synthesized two new triangle-shaped rigid building blocks, namely, 2,5-di-tert-butylindolo[3,2,1-jk]carbazole (tBuICz) and 2,11-di-tert-butylindolo[3,2,1-jk]carbazole-4-carbonitrile (tBuICzCN), and tethered them with different chromophores to yield a series of violet-blue fluorophores, viz., ICzTPA-ICzPICN, and studied their structure-function relationship. The appended chromophores and cyano unit played a vital role in controlling the optical and electrical properties of the compounds. Except triphenylamine-substituted derivatives, the compounds showed pure violet emission (λem ≤ 403 nm). Intriguingly, the compounds exhibited narrow-band emission with a full-width at half-maximum ≤ 40 nm, attributed to the rigidity of the ICz core. The emission of the compounds displayed positive solvatochromism, which is ascribed to the photoinduced intramolecular charge transfer in the excited state. The compounds revealed excellent thermal robustness with T5d ≥ 363 °C. The triphenylamine-featuring derivatives displayed a high-lying HOMO compared to their congeners due to their electron-rich nature. When we applied these materials in organic light-emitting diodes, ICzPI outperformed in the series with an EQEmax of 3.07% and a current efficiency of 1.04 cd/A. Notably, its CIEy ∼ 0.046 precisely matched with the Rec.2020 standard of deep-blue color (CIEy ∼ 0.046).

Comparative study of thermally activated delayed fluorescent properties of donor–acceptor and donor–acceptor–donor architectures based on phenoxazine and dibenzo[a,j]phenazine

A new thermally activated delayed fluorescence (TADF) compound based on a donor–acceptor (D–A) architecture (D = phenoxazine; A = dibenzo[a,j]phenazine) has been developed, and its photophysical properties were characterized. The D–A compound is applicable as an emitting material for efficient organic light-emitting diodes (OLEDs), and its external quantum efficiency (EQE) exceeds the theoretical maximum of those with prompt fluorescent emitters. Most importantly, comparative study of the D–A molecule and its D–A–D counterpart from the viewpoints of the experiments and theoretical calculations revealed the effect of the number of the electron donor on the thermally activated delayed fluorescent behavior.

A compact polymer\u2013inorganic hybrid gas barrier nanolayer for flexible organic light-emitting diode displays

Atomic layer infiltration technology allows the formation of a nanometer-thick polymer-inorganic hybrid barrier layer in polymer material for flexible organic light-emitting diode (OLED) displays. In this study, according to transmission electron microscopy and secondary-ion mass spectrometry analysis results under various process conditions, a compact polymer-inorganic hybrid nanolayer was successfully formed in a polymer and good barrier performance was revealed with a low water vapor transmission rate under optimal process conditions. Additionally, through gas chromatography-mass spectrometry measurements after ultra-violet radiation testing, polymer out-gassing decreased compared to bare polymers. Based on barrier properties, the polymer with a polymer-inorganic hybrid barrier nanolayer was applied to a flexible OLED display as a substrate. During storage tests and folding tests, the flexible OLED display exhibits good reliability and better flexibility compared to those with an inorganic barrier layer. These results confirm that the polymer-inorganic hybrid nanolayer is suitable for barrier layer formation in flexible OLED displays.

Exciton Up-Conversion by Well-Distributed Carbon Quantum Dots in Luminescent Materials for an Efficient Organic Light-Emitting Diode.

In this work, we proposed an efficient and straightforward up-conversion process to enhance the external quantum efficiency in a red-emission organic light-emitting diode (OLED). The carbon quantum dots in the luminescent materials initiated the up-conversion by doping the (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) in an amount of 0.001 wt. %, and the external quantum efficiency (EQE) increased from approximately 80% to 9.27% without spectrum change. The time-resolved photoluminescence was applied to understand the mechanism of EQE enhancement in the PCBM-doped OLED. Two decay-time constants fit the TRPL. After PCBM doping, the extended PL intensity indicated increased time constants. The time constants increased from 1.06 and 4.02 ns of the reference sample to 3.48 and 11.29 ns of the PCBM-doped material, respectively. The nonradiative energy transfer (NRET) mechanism was proposed responsible for the decay-time enhancement. The excitons in the PCBM, either by excitation or injection, will transfer to the phosphorescent material nonradiatively. As the PCBM has lower energy levels than the luminous material for electrons, the backward exciton transfer is a kind of up-conversion. With the increased amounts of excitons in the luminescent material, the luminescent external quantum efficiency and the decay-time increased. This up-conversion method is not limited to the red-emission OLED; it could also be applied to blue or green emission.

TADF molecules with π-extended acceptors for simplified high-efficiency blue and white organic light-emitting diodes

<h2>Summary</h2> Carbazole-benzonitrile derivatives (CzBNs) are promising high-efficiency and stable blue thermally activated delayed fluorescence (TADF) materials in organic light-emitting diodes (OLEDs), which, however, strongly rely on multiple-donor structures. Here, we demonstrate a strategic implementation of extending the acceptor π-system with parapositioned π-conjugation tails in fewer donor structures, producing a simple and versatile approach to enhance performances of CzBNs. This molecular structure not only enhances TADF properties by delocalizing the frontier molecular orbital distribution and facilitating states mixing but also improves outcoupling efficiency from the preferential horizontal emitting dipole orientation. The proof-of-concept blue emitter affords monochromic devices peaking at 469 nm with a remarkable maximum external quantum efficiency (EQE_max) of 35.7%. Furthermore, simplified bicolor and tricolor white OLEDs using the same compound as emitting host achieve impressive EQE_maxs of 29.3% and 21.1%, respectively. The state-of-the-art performances of both blue and white fluorescent devices validate the superiority of this molecular design strategy

Ytterbium oxide electron injection interface in organic light-emitting diode

The ytterbium oxide (Yb3+) is found to have an extremely low work function of 2.42 eV, which is even lower than that of its metallic form Yb0 (2.64 eV). The stability of oxides makes Yb3+ an ideal electron injection material for both top-emitting and bottom-emitting organic light-emitting diodes (TOLED and BOLED). The device test data indeed show that at 1000 nit luminance, the TOLED has a 94 cd/A current efficiency and 70 lm/W power efficiency, and BOLED has a 76 cd/A and 60 lm/W efficiency, respectively. X-ray and ultraviolet photoemission spectroscopical studies indicate that the Fermi level of the metal oxide is pinned to the lowest unoccupied molecular orbital of the electron transport layer, leading to the formation of a cathode interface with an ultra-low electron injection barrier.

Facile Synthesis of 1,7-Phenanthroline Derivatives and Evaluation of Their Properties as Hole-Blocking Materials in Organic Light-Emitting Diodes

Abstract Phenanthroline derivatives are typical simple aza-polycyclic aromatic hydrocarbons that have been used as hole-blocking and electron-transporting materials in organic light-emitting diodes (OLEDs). In contrast to the widely used 1,10-phenanthroline derivatives, other isomers, including 1,7-phenanthrolines, have been explored less, partly due to the lack of effective and selective synthesis methods. This study demonstrates the facile synthesis of 1,7-phenanthroline derivatives bearing various substituents via a multicomponent reaction based on the aza-Diels-Alder reaction. By investigating the correlation between the substituents and their performance as hole-blocking materials in OLEDs, we gained insight into the molecular design of 1,7-phenanthroline derivatives for material applications.

Fused π‐Extended Multiple‐Resonance Induced Thermally Activated Delayed Fluorescence Materials for High‐Efficiency and Narrowband OLEDs with Low Efficiency Roll‐Off

AbstractThe simultaneous achievement of multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) materials with strong narrowband emission and efficient reverse intersystem crossing (RISC) process can further promote the advancement of organic light‐emitting diodes (OLEDs). Herein, a new strategy is proposed to achieve two π‐extended MR‐TADF emitters (NBO and NBNP) peaking at 487 and 500 nm via fusing conjugated high‐triplet‐energy units (carbazole, dibenzofuran) into boron‐nitrogen (B/N) framework, aiming to increase charge transfer delocalization of the B/N skeleton and minimize singlet‐triplet energy gap (∆EST). This strategy endows the two emitters with full width at half maximum of 27 and 29 nm, and high photoluminescence efficiencies above 90% in doped films, respectively. Additionally, considerable rate constants of RISC are obtained due to the small ∆EST (0.12 and 0.09 eV) and large spin‐orbital coupling values. Consequently, the OLEDs based on NBO and NBNP show the maximum external electroluminescence quantum efficiency of up to 26.1% and 28.0%, respectively, accompanied by low‐efficiency roll‐off. These results provide a feasible design strategy to construct efficient MR‐TADF materials for OLEDs with suppressed efficiency roll‐off.

Restriction of intramolecular torsion induces abnormal blue-shifted fluorescence in the aggregate state

The aggregation-induced emission (AIE) effect and corresponding restriction of intramolecular motions strategy are important mechanisms for regulating the performance of novel organic light-emitting diode materials and constituent molecules, such as emission intensity and peak position. Nevertheless, molecules in the aggregate state usually yield a red-shifted fluorescence due to the π-π interaction enhancement. Here, we report a blue-shifted fluorescence mechanism induced by the restriction of intramolecular torsion in the aggregate state for an interesting excited-state intramolecular proton transfer fluorophore. A combined experimental and theoretical method is performed to investigate the excited state dynamics of the molecules in solvent and the aggregate state respectively. Multiple regression analysis reveals that the changes of reorganization energy caused by AIE effect induce the blue-shifted fluorescence. These findings contribute in several ways to our understanding of the AIE effect, and provide a new strategy to regulate the emission performance of AIE materials.

Benchmarking of Density Functionals for the Description of Optical Properties of Newly Synthesized π-Conjugated TADF Blue Emitters.

Computational modeling of the optical characteristics of organic molecules with potential for thermally activated delayed fluorescence (TADF) may assist markedly the development of more efficient emitting materials for organic light-emitting diodes. Recent theoretical studies in this area employ mostly methods from density functional theory (DFT). In order to obtain accurate predictions within this approach, the choice of a proper functional is crucial. In the current study, we focus on testing the performance of a set of DFT functionals for estimation of the excitation and emission energy and the excited singlet-triplet energy gap of three newly synthesized compounds with capacity for TADF. The emitters are designed specifically to enable charge transfer by π-electron conjugation, at the same time possessing high-energy excited triplet states. The functionals chosen for testing are from various groups ranging from gradient-corrected through global hybrids to range-separated ones. The results show that the monitored optical properties are especially sensitive to how the long-range part of the exchange energy is treated within the functional. The accurate functional should also be able to provide well balanced distribution of the π-electrons among the molecular fragments. Global hybrids with moderate (less than 0.4) share of exact exchange (B3LYP, PBE0) and the meta-GGA HSE06 are outlined as the best performing methods for the systems under study. They can predict all important optical parameters correctly, both qualitatively and quantitatively.

Activating Energy Transfer Tunnels by Tuning Local Electronegativity of Conjugated Polymeric Backbone for High‐Efficiency OLEDs with Low Efficiency Roll‐Off

AbstractThermally activated delayed fluorescence (TADF) conjugated polymers are attractive for display and illumination applications owing to their excellent device performance and convenient device fabrication. However, conjugated polymers frequently encounter insufficient energy transfer from hosts to TADF units, lowering device performance. Herein, a strategy for improving light‐emitting performances through adjusting the local electronegativity of the polymeric backbones by inserting electron‐withdrawing atoms and activating energy transmission channels is proposed. Meanwhile, strongly electronegative atoms also affect the charge‐transfer natures (CT) of TADF polymers and minimize the energy difference between the lowest singlet and triplet states, leading to a rapid reverse intersystem crossing process through the vibronic coupling between 1CT and 3CT with extremely close energy levels. The produced TADF polymer, pBP‐PXZ, can achieve an external quantum efficiency (EQE) of 23.11%, exhibiting no roll‐off when the luminance is less than 200 cd m−2 whereas only a 3% EQE decrease at 500 cd m−2. The EQE can even maintain above 19% under 1000 cd m−2, which is the highest efficiency among TADF polymer‐based organic light‐emitting diodes (OLEDs) under high luminance. The study provides a new perspective for designing high‐performance OLEDs materials.

Do any types of double-hybrid models render the correct order of excited state energies in inverted singlet-triplet emitters?

Organic emissive materials with the inverted singlet-triplet energy gaps, where in violation of Hund's multiplicity rule the lowest triplet excited-state is higher in energy than the lowest singlet excited-state, have recently come into the limelight. This unique feature is of important relevance, where the emitters meeting the singlet-triplet inversion have potential to usher in the next generation of organic light emitting diodes (OLEDs). Since experimental data in this context are currently sparse, necessity of the cost-effective theoretical tools able to provide reliable results seems to be evident. Following our recent endeavors on the spin-component-scaled (SCS), spin-opposite-scaled (SOS), and SOS-range separated exchange (SOS-RSX) double-hybrids (DHs) as well as other efforts revealing the superior performances of such models for time-dependent computations, in the present work, we develop and validate several models based on the SOS-configuration interaction singles with perturbative doubles correction [SOS-CIS(D)] devoid of any fitting procedure for describing the singlet-triplet inversion. Taking a series of emitters with the available reference values for the inverted singlet-triplet energy gaps as working models, it is unveiled that the extremes of the same-spin and opposite-spin parameters included in the direct and indirect terms of the SOS-CIS(D) correlation energy as well as the nonlocal exchange and correlation contributions do not necessarily work well for the inverted gaps, but particular proportions among them are needed to achieve a reliable accuracy. Perusing the results of our developed methods, the best one based on the Perdew-Burke-Ernzerhof (PBE) exchange and correlation terms and the quadratic integrand model, denominated as SOS0-CIS(D)-PBE-QIDH, is shown to be highly efficient and robust for computations of the inverted singlet-triplet energy gaps. Furthermore, through detailed comparisons, we have also evaluated the performances of a variety of the recently presented DHs, including parameterized, parameter-free, RSX, as well as spin-component and spin-opposite scaling models for the purpose. Dissecting all the findings, it is disclosed that the results of any type of the DHs cannot be reliable, leading to positive energy gaps in most cases. Nonetheless, there are still some approximations, including SCS-PBE-QIDH, dispersion corrected spin-component scaled double-hybrids (DSD) of DSD-PBEP86 and DSD-BLYP, SOS-PBE-QIDH, SOS-ωPBEPP86, and SOS-RSX-QIDH, that can predict the negative singlet-triplet energy gaps for all the considered emitters and provide comparable performances with respect to our proposed model. To wrap up, among the large panel of different families of DHs on the market, the newly proposed model herein alongside these latter functionals can be recommended as the currently best affordable methods for subsequent applications on the inverted singlet-triplet emitters in OLED materials.

Design of Organic Electronic Materials With a Goal-Directed Generative Model Powered by Deep Neural Networks and High-Throughput Molecular Simulations.

In recent years, generative machine learning approaches have attracted significant attention as an enabling approach for designing novel molecular materials with minimal design bias and thereby realizing more directed design for a specific materials property space. Further, data-driven approaches have emerged as a new tool to accelerate the development of novel organic electronic materials for organic light-emitting diode (OLED) applications. We demonstrate and validate a goal-directed generative machine learning framework based on a recurrent neural network (RNN) deep reinforcement learning approach for the design of hole transporting OLED materials. These large-scale molecular simulations also demonstrate a rapid, cost-effective method to identify new materials in OLEDs while also enabling expansion into many other verticals such as catalyst design, aerospace, life science, and petrochemicals.

Vibronic Coupling Effect on the Vibrationally Resolved Electronic Spectra and Intersystem Crossing Rates of a TADF Emitter: 7-PhQAD.

Assessing and improving the performance of organic light-emitting diode (OLED) materials require quantitative prediction of rate coefficients for the intersystem crossing (ISC) and reverse ISC (RISC) processes, which are determined not only by the energy gap and the direct spin-orbit coupling (SOC) between the first singlet and triplet excited-states at a thermal equilibrium position of the initial electronic state but also by the non-Condon effects such as the Herzberg-Teller-like vibronic coupling (HTVC) and the spin-vibronic coupling (SVC). Here we apply the time-dependent correlation function approaches to quantitatively calculate the vibrationally resolved absorption and fluorescence spectra and ISC/RISC rates of a newly synthesized multiple-resonance-type (MR-type) thermally activated delayed fluorescence (TADF) emitter, 7-phenylquinolino[3,2,1-de]acridine-5,9-dione (7-PhQAD), with the inclusion of the Franck-Condon (FC), HTVC, and Duschinsky rotation (DR) effects. The SVC effect on the rates has also been approximately evaluated. We find that the experimentally measured ISC rates of 7-PhQAD originate predominantly from the vibronic coupling, consistent with the previous reports on other MR-type TADF emitters. The SVC effect on ISC rates is about 10 times larger than the HTVC effect, and the latter increases the ISC rates by more than 1 order of magnitude while it slightly affects the vibrationally resolved absorption and fluorescence spectra. The discrepancy between the theoretical and experimental results is attributed to inaccurately describing excited-states calculated by the time-dependent density functional theory as well as to not fully accounting for the complex experimental conditions. This work provides a demonstration of what proportion of ISC and RISC rate coefficients of a MR-type TADF emitter can be covered by the HTVC effect, and it opens design routes that go beyond the FC approximation for the future development of high-performance OLED devices.

Optical and photonic properties dependence on HNMB solvents: An emitter molecule for OLEDs

The development of organic light-emitting diodes (OLEDs) is very important due to their use in new generation lighting and displays, their color resolution and their unique optical properties. Herein, the change of the electronic and optical properties of a new molecule, 4-[(E)-[(2-hydroxy-1-naphthalenyl)methylene]amino]-3-methyl benzoic acid (HNMB) in different solvents are reported by experimental and advanced theoretical calculations. Optical properties are investigated by calculating the optical absorption wavelength, optical refractive index, optical band gap, optical conductance, angle of incidence, angle of refraction, contrast for different solvents. Finally, due to well‐suited photonic and optoelectronic properties, the HNMB can be used as a phosphorescent and OLED material.

Vapor deposition rate modifies anisotropic glassy structure of an anthracene-based organic semiconductor.

We control the anisotropic molecular packing of vapor-deposited glasses of ABH113, a deuterated anthracene derivative with promise for future organic light emitting diode materials, by changing the deposition rate and substrate temperature at which they are prepared. We find that at substrate temperatures from 0.65Tg to 0.92Tg, the deposition rate significantly modifies the orientational order in the vapor-deposited glasses as characterized by x-ray scattering and birefringence. Both measures of anisotropic order can be described by a single deposition rate-substrate temperature superposition (RTS). This supports the applicability of the surface equilibration mechanism and generalizes the RTS principle from previous model systems with liquid crystalline order to non-mesogenic organic semiconductors. We find that vapor-deposited glasses of ABH113 have significantly enhanced density and thermal stability compared to their counterparts prepared by liquid-cooling. For organic semiconductors, the results of this study provide an efficient guide for using the deposition rate to prepare stable glasses with controlled molecular packing.

Recent development of pyridine based charge transporting materials for organic light-emitting diodes and perovskite solar cells

In recent years, donor-acceptor based pyridine derivatives have significantly been used in organic solar cells (OSCs), thermally activated delayed fluorescence (TADF), organic light emitting diodes (OLEDs), perovskite solar cells (PSCs) etc.

Steady-state and transient optoelectronic characteristics of styrene-and quinoline-based derivative

Styrene and quinoline groups are commonly incorporated into the organic fluorescent materials for organic light-emitting diodes (OLEDs). In this work, a type of small molecule derived from styrene and quinoline, with a chemical structure of 2,2'-(2,5-dimethoxy-1,4-phylenedivinylene)bis-8- acetoxyquinoline (MPV-AQ), is employed as the emitter and electron transporting material in the OLEDs, and its optoelectronic characteristics such as charge-carrier injection, transporting and recombination are investigated by the steady-state and transient technologies. It is found that the electron injection from the cathode into the MPV-AQ layer shows the Fowler-Nordheim (FN) tunneling characteristic in the N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB)/MPV-AQ bilayer OLED, which is different from the Richardson-Schottky (RS) thermionic emission in the electron-only device based on the MPV-AQ single-layer. The difference in electron injection is attributed to the bend of energy bands of MPV-AQ in the NPB/MPV-AQ device, which can be caused by the charge accumulation at the NPB/MPV-AQ interface. The accumulated charges should mainly be the holes on the side of NPB layer because the electron mobility of MPV-AQ is much lower than the hole mobility of NPB. Owing to the bending of lowest unoccupied molecular orbital (LUMO) of MPV-AQ, the tunneling distance for electrons is significantly reduced, which is favorable for the FN tunneling. The barrier height for electron injection is calculated to be 0.23 eV by fitting the current-voltage curve of the NPB/MPV-AQ bilayer OLED. And the electron mobility of MPV-AQ is determined by the delay time of transient electroluminescence (EL) and shows field-dependence with the value on the order of 10&lt;sup&gt;–6&lt;/sup&gt; cm&lt;sup&gt;2&lt;/sup&gt;/(V·s). In addition, the electron-hole recombination coefficient is obtained from the long time component of the temporal decay of the EL intensity, and the coefficient is found to decrease with the applied voltage increasing, which is consistent with the efficiency roll-off in this bilayer OLED. This study may provide a foundation for understanding the electronic processes of carrier injection, transport and recombination in the OLEDs, which is helpful in improving the device performance.

High-performance three-coordinated organoboron emitters for organic light-emitting diodes

Three-coordinated organoboron compounds of diversified structures and multifunctionality are highly attractive materials for high-performance organic light-emitting diodes.