Emitters In Organic Light-emitting Diodes Research Articles

Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis.

Organic compounds that show Thermally Activated Delayed Fluorescence (TADF) have become wildly popular as next-generation emitters in organic light emitting diodes (OLEDs). Since 2016, a subset of these have found increasing use as photocatalysts. This review comprehensively highlights their potential by documenting the diversity of the reactions where an organic TADF photocatalyst can be used in lieu of a noble metal complex photocatalyst. Beyond the small number of TADF photocatalysts that have been used to date, the analysis conducted within this review reveals the wider potential of organic donor-acceptor TADF compounds as photocatalysts. A discussion of the benefits of compounds showing TADF for photocatalysis is presented, which paints a picture of a very promising future for organic photocatalyst development.

Ab-initio search for efficient red thermally activated delayed fluorescence molecules for organic light emitting diodes.

In this work, we present a computational study on 105 selected organic molecules in order to find suitable candidates for using as thermally activated delayed fluorescent (TADF) emitters in organic light emitting diodes (OLEDs), in the emission range of red light. Based on time-dependent density functional theory (TD-DFT) computations, three promising candidates were found, predicted to have low singlet-triplet splittings, lower than 0.06eV, and TADF rates of 0.124, 0.154 and 0.231 1/μs. Then, using an experimental-theory calibration approach, the emission wavelength of the molecules were estimated to be 570, 476, and 623nm, respectively. For the molecule whose emission wavelength (623nm) is predicted to be in our desired range, we measured the photoluminescence (PL) spectrum and find out that its emission peak is within the predicted accuracy of the employed method. Moreover, we benchmarked the performance of density functional based tight-binding (DFTB) method for future screening works and find out that, this method is an efficient pre-screening tool, useful in searching for molecules with desired emission wavelengths.

Structural, Optical and Decay Properties of Zinc(II) 8-Hydroxyquinoline and Its Thin Film

Zinc(II) 8-hydroxyquinoline (Znq2) green luminescent material and its blended thin film in a poly(methyl methacrylate) (PMMA) matrix have been prepared. XRD analysis confirms the formation of the compound and its presence in the PMMA blended thin film. The orbital molecular geometry, quantum chemical calculations and band gap energy of Znq2 phosphor have been confirmed from the DFT study. The morphology of the phosphor was shown by the SEM images. UV–Vis absorption, photoluminescence and decay analysis of zinc(II) 8-hydroxyquinoline phosphor confirms its suitability as a green emitter for organic light-emitting diodes. Uniform distribution of Znq2 powder in the PMMA matrix and its roughness was estimated by AFM analysis. Luminescence decay study was utilized to demonstrate its lifetime analysis by the time-correlated single-photon counting (TCSPC) system.

Fast spin-flip enables efficient and stable organic electroluminescence from charge-transfer states

A spin-flip from a triplet to a singlet excited state, that is, reverse intersystem crossing (RISC), is an attractive route for improving light emission in organic light-emitting diodes, as shown by devices using thermally activated delayed fluorescence (TADF). However, device stability and efficiency roll-off remain challenging issues that originate from a slow RISC rate (kRISC). Here, we report a TADF molecule with multiple donor units that form charge-resonance-type hybrid triplet states leading to a small singlet–triplet energy splitting, large spin–orbit couplings, and a dense manifold of triplet states energetically close to the singlets. The kRISC in our TADF molecule is as fast as 1.5 × 107 s−1, a value some two orders of magnitude higher than typical TADF emitters. Organic light-emitting diodes based on this molecule exhibit good stability (estimated T90 about 600 h for 1,000 cd m−2), high maximum external quantum efficiency (>29.3%) and low efficiency roll-off (<2.3% at 1,000 cd m−2). An organic molecule, 5Cz-TRZ, with multiple donor units supports fast reverse intersystem crossing, allowing fabrication of high-performance organic light-emitting diodes.

The electron inductive effect of dual non-conjugated trifluoromethyl acceptors for highly efficient thermally activated delayed fluorescence OLEDs

Recently, pure organic twisted donor-acceptor (D-A) structured thermally activated delayed fluorescence (TADF) materials have been extensively demonstrated as a new generation emitter in organic light-emitting diodes (OLEDs) due to their low-cost and high-efficiency. Herein, we designed and synthesized a highly twisted TADF emitter 2,3,5,6-tetra(3,6-di-tert-butyl-9H-carbazol-9-yl)bis(trifluoromethyl)benzene (4tCzDCF3Ph) by a facile catalyst-free C-N coupling reaction. The two electron-withdrawing non-conjugated trifluoromethyl (CF3) moieties bearing strong electron inductive effects instead of other widely reported conjugation effects contributes for the necessary D-A interactions for TADF emission. 4tCzDCF3Ph exhibited an extremely small singlet-triplet energy gap (ΔEST) of 0.02 eV, fast kRISC rate of 7.02 × 105 s−1 and modest photoluminescence quantum yield (PLQY) of ~70%. A low turn-on voltage of 2.6 V, along with maximum current efficiency of 50.6 cd A−1, power efficiency of 40.7 lm W−1 and external quantum efficiency (EQE) of 16.2% could be facilely achieved for 4tCzDCF3Ph based green TADF OLEDs, which is significantly higher than <1% of 4CzCF3Ph and 9.5% of 5CzCF3Ph TADF emitters based on single trifluoromethyl acceptor.

Structural Design of Blue‐to‐Red Thermally‐Activated Delayed Fluorescence Molecules by Adjusting the Strength between Donor and Acceptor

AbstractOwing to the faster response time, wider viewing angle and thinner thickness of organic light emitting diodes (OLEDs) than the conventional displays, the development of high‐efficiency light‐emitting materials has become the focus of attention. In recent years, the TADF materials have aroused widespread attention and experienced rapid development as an emitter in OLEDs, since the maximum internal quantum efficiency can reach 100% theoretically. TADF materials have exhibited the potential to be a low‐cost alternative to expensive phosphorescent luminophors. Whereas, the high resolution displays must endow the characteristics of stable and efficient panchromatic emission. The chemical structure of TADF materials are generally composed of electrical donor and acceptor, the emission colors and efficiency can be adjusted by the charge transfer between them. In this paper, TADF emitters with different emission colors are reviewed, accompanied with the investigation on their molecular design strategies and photophysical properties. In addition, the development direction and application of TADF materials are prospected.

Mechanochromic Delayed Fluorescence Switching in Propeller-Shaped Carbazole-Isophthalonitrile Luminogens with Stimuli-Responsive Intramolecular Charge-Transfer Excited States.

Herein, the universal design of high-efficiency stimuli-responsive luminous materials endowed with mechanochromic luminescence (MCL) and thermally activated delayed fluorescence (TADF) functions is reported. The origin of the unique stimuli-triggered TADF switching for a series of carbazole-isophthalonitrile-based donor-acceptor (D-A) luminogens is demonstrated based on systematic photophysical and X-ray analysis, coupled with theoretical calculations. It was revealed that a tiny alteration of the intramolecular D-A twisting in the excited-state structures governed by the solid morphologies is responsible for this dynamic TADF switching behavior. This concept is applicable to the fabrication of bicolor emissive organic light-emitting diodes using a single TADF emitter.

Aggregate-State Effects in the Atomistic Modeling of Organic Materials for Electrochemical Energy Conversion and Storage Devices: A Perspective.

Development of new functional materials for novel energy conversion and storage technologies is often assisted by ab initio modeling. Specifically, for organic materials, such as electron and hole transport materials for perovskite solar cells, LED (light emitting diodes) emitters for organic LEDs (OLEDs), and active electrode materials for organic batteries, such modeling is often done at the molecular level. Modeling of aggregate-state effects is onerous, as packing may not be known or large simulation cells may be required for amorphous materials. Yet aggregate-state effects are essential to estimate charge transport rates, and they may also have substantial effects on redox potentials (voltages) and optical properties. This paper summarizes recent studies by the author’s group of aggregation effects on the electronic properties of organic materials used in optoelectronic devices and in organic batteries. We show that in some cases it is possible to understand the mechanism and predict specific performance characteristics based on simple molecular models, while in other cases the inclusion of effects of aggregation is essential. For example, it is possible to understand the mechanism and predict the overall shape of the voltage-capacity curve for insertion-type organic battery materials, but not the absolute voltage. On the other hand, oligomeric models of p-type organic electrode materials can allow for relatively reliable estimates of voltages. Inclusion of aggregate state modeling is critically important for estimating charge transport rates in materials and interfaces used in optoelectronic devices or when intermolecular charge transfer bands are important. We highlight the use of the semi-empirical DFTB (density functional tight binding) method to simplify such calculations.

Emitting dipole orientation and molecular orientation of homoleptic Ir(III) complexes

Origin and significance of emitting dipole orientation (EDO) of emitters in organic light-emitting diodes (OLEDs) have been studied extensively in recent years. However, EDO of homoleptic Ir(III) complexes (Homo-IrCs) have received less attention due to their three-fold molecular symmetry and resulting expectation to have random EDO. Herein, we report the EDOs of Homo-IrCs doped in various host materials. Interestingly the EDO of Homo-IrCs varies from preferred horizontal direction to rather vertical direction depending on dopant and host molecules. The variation of EDO of Homo-IrCs is correlated with the direction of the transition dipole moment (TDM) against the molecular C3 axis and the orientation of C3 axis of the dopants in films. The average orientations of the Homo-IrCs are extracted from measured the horizontal dipole ratios (Θs) and calculated TDM directions. Angle between the TDM direction and C3 axis in the Homo-IrCs under investigation are larger than 54.7° corresponding to the mutually orthogonal axes among the three transition dipole vectors so that the EDO of Homo IrCs can deviate from random orientation depending on the orientation of the C3 axis in films. Average molecular orientation of a HOMO-IrC in films varies by 10°–13° depending on host materials. In contrast, smaller variation (<5°) is observed among different Homo IrCs in a same host. The results indicate that host material is one of the dominating factors determining orientation of Homo-IrC molecules. Tris[(3,5-difluoro-4-cyanophenyl)-pyridine]iridium(III) (FCNIr) shows exceptionally high horizontal orientation, which likely originates from the dipole-dipole interaction between the dopant and hosts. FCNIr doped in diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide (TSPO1) shows strong preferred horizontal EDO with the Θ of 0.78, demonstrating the possibility of large Θ value of Homo-IrCs.

Achieving High-Performance Pure-Red Electrophosphorescent Iridium(III) Complexes Based on Optimizing Ancillary Ligands.

Two new iridium(III) complexes were synthesized by introducing two trifluoromethyl groups into an ancillary ligand to develop pure-red emitters for organic light-emitting diodes (OLEDs). The electron-donating ability of the ancillary ligands is suppressed, owing to the electron-withdrawing nature of trifluoromethyl groups, which can reduce the HOMO energy levels compared with those of compounds without trifluoromethyl groups. However, the introduction of trifluoromethyl groups into the ancillary ligand has little impact on the LUMO energy levels. Therefore, a well-tuned, pure-red, excited-state energy was achieved by regulating the relative energy level between the HOMO and LUMO. OLEDs with these complexes as emitters showed high external quantum efficiencies (EQEs) of 26 % and realized high EQEs of about 25 % and fairly low driving voltages of 3.3-3.6 V for practical luminance of 1000 cd m-2 , as well as excellent Commission Internationale de L'Eclairage (CIE) coordinates of (0.66, 0.33) and (0.67, 0.33); thus, this demonstrates the successful molecular design strategy by modifying the electron-donating ability of ancillary ligand.

Top-emission organic light emitting diode with indium tin oxide top-electrode films deposited by a low-damage facing-target type sputtering method

To confirm the effectiveness of the low-damage facing-target sputtering (FTS) method for the deposition of a transparent oxide electrode film on an organic layer, an indium tin oxide (ITO) top electrode was deposited via FTS, and a top emission organic light-emitting diode (OLED) was fabricated. Top and bottom electrode films were deposited using the FTS system; in addition, a top emission a bottom emission OLED were fabricated with an inverse stacking structure. Consequently, both OLEDs exhibited similar operating characteristics, and the top emission OLED had a better luminance efficiency than the bottom emission OLED did. Compared with the OLED fabricated using the evaporation method, the OLEDs fabricated using FTS exhibited similar luminance efficiencies, although they had considerably higher operating voltages. In addition, annealing treatment at 110 °C was observed to lead to a significant reduction in the operating voltages; moreover, the obtained top emission OLEDs exhibited operating characteristics similar to those of the OLED fabricated using the evaporation method. These results indicate that the FTS method is useful for the deposition of the top oxide electrode films of OLEDs.

Mixed-Host Systems with a Simple Device Structure for Efficient Solution-Processed Organic Light-Emitting Diodes of a Red-Orange TADF Emitter.

Charge balance, concentration quenching, and exciton confinement are the most important factors for realizing the use of thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes. Red-orange organic light-emitting diodes of a TADF emitter 2-[4 (diphenylamino)phenyl]-10,10-dioxide-9H-thioxanthen-9-one (TXO-TPA) have been reported by doping in a mixed p-type host system of poly(N-vinylcarbazole) (PVK) and 1,3-bis(N-carbazolyl)benzene (mCP) via solution-processed. We have demonstrated the peak external quantum efficiency of 9.75%, maximum current efficiency of 19.36 cd/A, and power efficiency of 12.17 lm/W along with a CIE coordinate of (0.45, 0.51). The devices were compared with different doping concentrations of TXO-TPA, and a comparative investigation on the effect of the thickness electron transport layer was studied. The results clearly indicated that this solution-processed TXO-TPA device structure is a promising strategy to develop highly efficient but simple OLED structures.

Intramolecular Borylation via Sequential B-Mes Bond Cleavage for the Divergent Synthesis of B,N,B-Doped Benzo[4]helicenes.

New symmetric and unsymmetric B,N,B-doped benzo[4]helicenes 3-6 a/b have been achieved in good yields, using a three-step process, starting from N(tolyl)3 in a highly divergent manner (7 examples). A borinic acid functionalized 1,4-B,N-anthracene 1 was found to display unprecedented reactivity, acting as a convenient and highly effective precursor for selective formation of bromo-substituted B,N,B-benzo[4]helicenes 2 a/2 b via intramolecular borylation and sequential B-Mes bond cleavage in the presence of BBr3 . Subsequent reaction of 2 a/2 b with Ar-Li provided a highly effective toolbox for the preparation of symmetrically/unsymmetrically functionalized B,N,B-helicenes. Their high photoluminescence quantum yields along with the small ΔEST suggest their potential as thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs).

Aromatic imide/amide-based organic small-molecule emitters for organic light-emitting diodes

Aromatic imide/amide-based organic small molecules as emitters in organic light-emitting diodes have caught increasing attention. This study summarized their advances in terms of device performance and molecular design rules over the past 20 years.

Efficient near ultraviolet emissive (CIEy &lt; 0.06) organic light-emitting diodes based on phenanthroimidazole–alkyl spacer–carbazole fluorophores: experimental and theoretical investigation

Efficient near ultraviolet light-emitting materials are of particular importance in organic light emitting devices (OLEDs) because of their potential for application in high-quality flat panel displays and white OLEDs.

Ab Initio Many-Body Perturbation Theory Calculations of the Electronic and Optical Properties of Cyclometalated Ir(III) Complexes

Cyclometalated Ir(III) compounds are the preferred choice as organic emitters in organic light-emitting diodes. In practice, the presence of the transition metal surrounded by carefully designed ligands allows fine-tuning of the emission frequency as well as good efficiency of the device. To support the development of new compounds, experimental measurements are generally compared with absorption and emission spectra obtained from ab initio calculations. The standard approach for these calculations is time-dependent density functional theory (TDDFT) with a hybrid exchange-correlation functional like B3LYP. Because of the size of these compounds, the application of more complex quantum chemistry approaches can be challenging. In this work, we used many-body perturbation theory approaches, in particular the GW approximation with the Bethe-Salpeter equation (BSE) implemented in Gaussian basis sets, to calculate the quasiparticle properties and the absorption spectra of six cyclometalated Ir(III) complexes, going beyond TDDFT. In the presented results, we compared standard TDDFT simulations with BSE calculations performed on top of perturbative G0W0 and accounting for eigenvalue self-consistency. Moreover, in order to investigate in detail the effect of the DFT starting point, we concentrated on Ir(ppy)3 and performed GW-BSE simulations starting from different DFT exchange-correlation potentials.

Exploration of High Efficiency AIE‐Active Deep/Near‐Infrared Red Emitters in OLEDs with High‐Radiance

AbstractLimiting by classic donor–acceptor (D–A) strategy based on charge transfer (CT) process dominated emission, the high‐efficiency organic deep/near infrared red (DR/NIR) emitters with desirable photoluminescence quantum yields (PLQYs) and satisfactory excitons utilization efficiencies (EUEs) are still a challenge. Herein, three new DR/NIR luminogens (TNZPPI, TNZtPPI and TNZ2tPPI) based on naphtho[2,3‐c][1,2,5]thiadiazole (NZ) group are synthesized. Their interesting characterization of hybrid excited states containing tuned local excited (LE) and CT components are confirmed, and the effective high‐lying reverse intersystem crossing (RISC) channel might be activated because of their larger T2–T1 energy gap and smaller T4–S2 energy splitting. Thanks for their higher fluorescence quantum yields in film (24–38%), the TNZPs‐based non‐doped devices exhibit bright NIR emission with higher maximum radiance of 21447–36027 mW Sr−1 m−2, whose performance are better than most reported pure organic NIR devices. Enjoying deep analysis of their solvation effect and aggregation‐induced emission (AIE)‐activity, the doped organic light emitting diodes (OLEDs) are fabricated, whose performances are very good with identical National Television System Committee saturated red‐emitting behaviors. The results in TNZPs show that the electronic effect of molecule structure and intermolecular interactions all are relative to their performance, and which is very important for the design high‐efficiency NZ‐based OLED materials.

The one-pot synthesis of homoleptic phenylphthalazine iridium(III) complexes and their application in high efficiency OLEDs

A series of homoleptic cyclometalated iridium (III) complexes bearing a 1-phenylphthalazine framework have been synthesized with a facile one-pot method and characterized by X-ray single crystal diffraction. All complexes exhibited orange or red emission peaks at 583–623 nm with short lifetimes of 1.3–2.6 μs and high photoluminescence quantum yields of 41%–82%. These complexes exhibit good thermal stability with high Td of 275–394 °C. The electronic states and orbital distributions of these iridium (III) complexes were studied by density functional theory. These complexes were used as emissive materials in Organic light-emitting diodes (OLEDs). The device based on 2 gave a peak current efficiency and external quantum efficiency (EQE) of 35.1 cd A−1 and 21.2%, respectively. The color coordinates of device based on 3 fully meet that of standard red of the National Television Standards Committee (NTSC). These results suggest that homoleptic phenylphthalazine iridium (III) complexes have potential application as efficient emitters in OLEDs.

Thioxanthen-Based Blue Thermally Activated Delayed Fluorescence Emitters for Organic Light-Emitting Diodes

Novel thermally activated delayed fluorescence (TADF) materials with 9,9-dimethyl-9H-thioxanthene 10,10-dioxide (SB) as an electron acceptor and andspiro[acridine-9,9'-fluorene] (D1) and spiro[acridine-9,9'-xanthene] (D2) as electron donors (2D1-SB and 2D2-SB) and their characteristics were compared with those of a reference material using 9,9-dimethylacridin (DMAC) as an electron donor (2DMAC-SB) for blue organic light-emitting diodes (OLEDs). We obtain the energies of the first singlet (S1) and first triplet (T1) excited states of TADF materials by performing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the ground state using the dependence on charge transfer amounts for the optimal Hartree-Fock percentage in the exchange-correlation of TD-DFT. We show that 2D2-SB would be a suitable blue OLED emitter because it has sufficiently small value (0.064 eV) of energy difference (ΔEST) between the S1 and T1 states, which is favorable for a reverse inter system crossing process from the T1 to S1 states and an emission wavelength of 439.5 nm with sufficiently large oscillator strength (F) value.

Theoretical Study of the Electronic Structure and Properties of Alternating Donor-Acceptor Couples of Carbazole-Based Compounds for Advanced Organic Light-Emitting Diodes (OLED)

Generally, it is difficult to generate a high-performance pure blue emission organic light-emitting diode (OLED). That is because the intrinsically wide band-gap makes it hard to inject charges into the emitting layer in such devices. To solve the problem, carbazole derivatives have been widely used because they have more thermal stability, a good hole transporting property, more electron rich (p-type) material, and higher photoconductivity. In the present work, novel copolymers containing donor-acceptor-acceptor-donor (D-A-A-D) blue compounds used for OLEDs were investigated. The theory of the geometrical and electronic properties of N-ethylcarbazole (ECz) as donor molecule (D) coupled to a series of 6 acceptor molecules (A) for advanced OLEDs were investigated. The acceptors were thiazole (TZ), thiadiazole (TD), thienopyrazine (TPZ), thienothiadiazole (TTD), benzothiadiazole (BTD), and thiadiazolothienopyrazine (TDTP). The ground state structure of the copolymers were studied using Density Functional Theory (DFT) at B3LYP/6-31G(d) level. Molecular orbital analysis study indicated 3 investigated copolymers (ECz-diTZ-ECz, ECz-diTD-ECz, ECz-diBTD-ECz) have efficient bipolar charge transport properties for both electron and hole injection to the TiO2 conduction band (4.8 eV). In addition, the excited states electronic properties were calculated using Time-Dependent Density Functional Theory (TD-DFT) at the same level. Among these investigated copolymer ECz-diTZ-ECz and ECz-diTD-ECz showed the maximum absorption wavelengths (λabs) with blue emitting at 429 and 431 nm, respectively. The results suggested that selected D-A-A-D copolymers can improve the electron- and hole- transporting abilities of the devices. Therefore, the designed copolymers would be a promising material for future development of light-emitting diodes, electrochromic windows, photovoltaic cells, and photorefractive materials.