Application In Organic Light-emitting Diodes Research Articles

Two-Coordinate Thermally Activated Delayed Fluorescence Coinage Metal Complexes: Molecular Design, Photophysical Characters, and Device Application.

Since the emergence of the first green light emission from a fluorescent thin-film organic light emitting diode (OLED) in the mid-1980s, a global consumer market for OLED displays has flourished over the past few decades. This growth can primarily be attributed to the development of noble metal phosphorescent emitters that facilitated remarkable gains in electrical conversion efficiency, a broadened color gamut, and vibrant image quality for OLED displays. Despite these achievements, the limited abundance of noble metals in the Earth's crust has spurred ongoing efforts to discover cost-effective electroluminescent materials. One particularly promising avenue is the exploration of thermally activated delayed fluorescence (TADF), a mechanism with the potential to fully harness excitons in OLEDs. Recently, investigations have unveiled TADF in a series of two-coordinate coinage metal (Cu, Ag, and Au) complexes. These organometallic TADF materials exhibit distinctive behavior in comparison to their organic counterparts. They offer benefits such as tunable emissive colors, short TADF emission lifetimes, high luminescent quantum yields, and reasonable stability. Impressively, both vacuum-deposited and solution-processed OLEDs incorporating these materials have achieved outstanding performance. This review encompasses various facets on two-coordinate TADF coinage metal complexes, including molecular design, photophysical characterizations, elucidation of structure-property relationships, and OLED applications.

Ultrafast spin-flip exciton conversion and narrowband sky-blue luminescence in a fused polycyclic selenaborin emitter.

Thermally activated delayed fluorescence (TADF) materials with high photoluminescence quantum yields and fast reverse intersystem crossing (RISC) capabilities are highly desirable for applications in high-efficiency organic light-emitting diodes. Herein, we report the synthesis as well as structural and photophysical properties of 5,9-diselena-13b-boranaphtho[3,2,1-de]anthracene (SeBSe) as a narrowband-emissive TADF material. The incorporation of two selenium atoms into the boron-fused pentacyclic π-core results in a small singlet-triplet energy gap (ΔE ST) and thereby significant TADF properties. Moreover, theoretical calculations revealed a noticeable spin-orbit coupling enhancement between the singlet and triplet manifolds in SeBSe by virtue of the heavy-atom effect of selenium atoms. Consequently, SeBSe allows ultrafast spin-flip RISC with the rate constant surpassing 108 s-1, which far exceeds the corresponding fluorescence radiative decay rate (∼106 s-1), enabling an ideal singlet-triplet superimposed excited state.

Extended Conjugation Strategy Enabling Red-Shifted and Efficient Emission of Orange-Red Thermally Activated Delayed Fluorescence Materials.

In account of the energy gap law, the development of efficient narrow-band gap thermally activated delayed fluorescence (TADF) materials remains a major challenge for the application of organic light-emitting diodes (OLEDs). The orange-red TADF materials are commonly designed with either large π-conjugated systems or strong intramolecular donor-acceptor (D-A) interactions for red-shift emission and small singlet-triplet energy gap (ΔEST). There are rare reports on the simultaneous incorporation of these two strategies on the same material systems. Herein, two orange-red emitters named 1P2D-BP and 2P2D-DQ have been designed by extending the conjugation degree of the center acceptor DQ and increasing the number distribution of the peripheral donor PXZ units, respectively. The emission peak of 1P2D-BP is red-shifted to 615 nm compared to 580 nm for 2P2D-DQ, revealing the pronounced effect of the conjugation extension on the emission band gap. In addition, the distorted molecular structure yields a small ΔEST of 0.02 eV, favoring the acquisition of a high exciton utilization through an efficient reverse intersystem crossing process. As a result, orange-red OLEDs with both 1P2D-BP and 2P2D-DQ have achieved an external quantum efficiency (EQE) of more than 17%. In addition, the efficient white OLED based on 1P2D-BP is realized through precise exciton assignment and energy transport modulation, showing an EQE of 23.6% and a color rendering index of 82. The present work provides an important reference for the design of high-efficiency narrow-band gap materials in the field of solid-state lighting.

Study on low-temperature evaporation of Ag2O-based Ag electrode and electron injection layer and their application in OLEDs

Organic light-emitting diodes (OLEDs) have become one of the mainstream lighting and display technologies. The vacuum thermal evaporation is the most widely adopted method for the preparation of organic and metal materials of OLEDs. The thermal deposition of the commonly used silver (Ag) and aluminum (Al) electrodes requires high temperature and long time, which greatly increases the cost of the fabricating process. Therefore, we selected silver oxide (Ag2O) powder instead of Ag pellets as the precursor for evaporating Ag electrodes. Compared to Ag pellets and Al wires, Ag2O-based Ag electrode need lower evaporation temperature and shorter preheating time. In addition, the agglomeration phenomenon on the surface of the Ag2O-based Ag film is prevented, which also increases the carrier concentration of Ag electrode. Moreover, by doping bathophenanthroline (Bphen) in Ag2O powders, the phenanthroline-metal (Bphen-Ag) complexes with higher electron mobility and stronger electron injecting ability can be achieved. We applied Ag2O-based Ag electrode and 10 wt.% Ag2O-based Ag:Bphen as electron injection layer to achieve high-efficiency red phosphorescent inverted OLEDs, with the maximum current efficiency, external quantum efficiency, and power efficiency of 17.79 cd A−1, 20.71%, and 12.14 lm W−1, respectively. This method provides a new strategy for preparing highly efficient inverted red OLED devices.

Flexible Embedded Metal Meshes by Sputter-Free Crack Lithography for Transparent Electrodes and Electromagnetic Interference Shielding.

A facile and novel fabrication method is demonstrated for creating flexible poly(ethylene terephthalate) (PET)-embedded silver meshes using crack lithography, reactive ion etching (RIE), and reactive silver ink. The crack width and spacing in a waterborne acrylic emulsion polymer are controlled by the thickness of the polymer and the applied stress due to heating and evaporation. Our innovative fabrication technique eliminates the need for sputtering and ensures stronger adhesion of the metal meshes to the PET substrate. Crack trench depths over 5 μm and line widths under 5 μm have been achieved. As a transparent electrode, our flexible embedded Ag meshes exhibit a visible transmission of 91.3% and sheet resistance of 0.54 Ω/sq as well as 93.7% and 1.4 Ω/sq. This performance corresponds to figures of merit (σDC/σOP) of 7500 and 4070, respectively. For transparent electromagnetic interference (EMI) shielding, the metal meshes achieve a shielding efficiency (SE) of 42 dB with 91.3% visible transmission and an EMI SE of 37.4 dB with 93.7% visible transmission. We demonstrate the highest transparent electrode performance of crack lithography approaches in the literature and the highest flexible transparent EMI shielding performance of all fabrication approaches in the literature. These metal meshes may have applications in transparent electrodes, EMI shielding, solar cells, and organic light-emitting diodes.

Efficient deep-red Aggregate-Induced emission material and “Hot-exciton” electroluminescence

Organic deep-red fluorescent materials have a wide range of applications in organic light-emitting diodes (OLEDs), biological imaging, night-vision equipment, and other fields. Here, we designed and synthesized a new deep-red emissive donor–acceptor (D-A) compound TPANZPPICN, whose non-doped OLED achieves stable pure deep-red electroluminescence (EL) at 692 nm with a maximum external quantum efficiency (EQE) of 2.67 % in non-doped OLED and a maximum EQE of 7.96 % at 660 nm in doped OLED. Magneto-electroluminescence (MEL) measurements reveal a negative response in the doped OLED, which is a significant character of reversed intersystem crossing (RISC) on high-lying excited states. Both theoretical calculations and experimental results confirm that the hot-exciton mechanism is responsible for the high efficiency EL of doped OLED, which proves to be a novel approach to realize the efficient deep-red organic fluorescent OLEDs.

Theoretical study on a series of iridium(III) complexes with low-efficiency roll-off properties for application in OLEDs

To find a series of suitable materials for phosphorescent organic light-emitting diodes (OLEDs), in this paper, we design six phosphorescent Ir(III) complexes that have low-efficiency roll-off properties. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were used to study the properties of (1-methyl-3-phenyl-2H-1λ^4,3λ^4-benzodiazole [dfbmb])2Ir(acetylacetonate [acac]), (dfbmb)2Ir(picolinate [pic]), (dfbmb)2Ir((Z)-5-tert-butyl-N'-(trifluoroethanimidoyl)pyridine-2-carboximidamide [tptz]), (1-(4-fluorophenyl)-3-methyl-2H-1λ^4,3λ^4-benzodiazole [F-dfbmb])2Ir(acac), (F-dfbmb)2Ir(pic), and (F-dfbmb)2Ir(tptz). Here, the main ligands are dfbmb and F-dfbmb, while the auxiliary ligands are acac, pic, and tptz. The advantages of these complexes were analyzed in terms of electronic structure, frontier molecular orbital (FMO) properties, spectral properties, electron affinities, ionization potentials, and OLED display effect. We hope that our research will contribute to the development of the OLED industry.

Exploring color space: an investigation of heteroaryl-substituted benzobis[1,2-d:4,5-d′]oxazoles and their application in organic light-emitting diodes

The color tuning of benzobisoxazole-based OLEDs to achieve white emission with high brightness is reported.

Silicon-based peripheral steric donor modifications for a high-efficiency multi-resonance thermally activated delayed fluorescence emitter

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters have broad applications in organic light-emitting diodes (OLEDs).

Highly efficient platinum(ii) complexes overcoming Pt–Pt interactions and their applications in organic light-emitting diodes

By introducing the methyl substituent acetylacetone and benzyl substituents, Pt(ii) complexes restrain Pt–Pt interactions in pure films and maintain nearly identical PL spectra with dilute solution. The maximum EQE of non-doped OLED reaches 19.6%.

Spiers Memorial Lecture: Challenges and prospects in organic photonics and electronics.

While a substantial amount of research activity has been conducted in fields related to organic photonics and electronics, including the development of devices such as organic field-effect transistors, organic photovoltaics, and organic light-emitting diodes for applications encompassing organic thermoelectrics, organic batteries, excitonic organic materials for photochemical and optoelectronic applications, and organic thermoelectrics, this perspective review will primarily concentrate on the emerging and rapidly expanding domain of organic bioelectronics and neuromorphics. Here we present the most recent research findings on organic transistors capable of sensing biological biomarkers down at the single-molecule level (i.e., oncoproteins, genomes, etc.) for the early diagnosis of pathological states and to mimic biological synapses, paving the way to neuromorphic applications that surpass the limitations of the traditional von Neumann computing architecture. Both organic bioelectronics and neuromorphics exhibit several challenges but will revolutionize human life, considering the development of artificial synapses to counteract neurodegenerative disorders and the development of ultrasensitive biosensors for the early diagnosis of cancer to prevent its development. Moreover, organic bioelectronics for sensing applications have also triggered the development of several wearable, flexible and stretchable biodevices for continuous biomarker monitoring.

Design and Synthesis of a Y-Type Thioxanthone-Carbazole for the Application in Blue and Yellow Organic Light-Emitting Diodes

Design and Synthesis of a Y-Type Thioxanthone-Carbazole for the Application in Blue and Yellow Organic Light-Emitting Diodes

Synthesis and characterization of novel fluorene–based green copolymers and their potential application in organic light-emitting diodes

Fluorene-benzothiadiazole (PFBT), commercially known as F8BT, and fluorene-thiophene (PFT) copolymers were synthesized by the Suzuki-Miyaura coupling polycondensation technique in the presence of a phase transfer catalyst (Aliquat 336). These novel copolymers showed higher molar masses, lower dispersivity, and thermal stability above 400 °C when synthesized under the modified (M) action of Aliquat 336 (F8BT-M). Polymer Light-emitting diodes (PLEDs) were assembled with the novel synthesized copolymers as the emitter layer. For F8BT-M, a maximum luminance of 327 cd/m2 was achieved, while a diode containing just the commercial F8BT resulted in only 210 cd/m2. As for the PFT copolymers, it was observed that for the Aliquat 336 sample synthesized in a 60h extended (E) (PFT-ME), the luminance of the PLED was much higher than that where the reaction time was 48 h (PFT-M), with a luminance of 168 cd/m2 and 54 cd/m2, respectively. Without Aliquat 336, the PFT copolymer did not show an electroluminescent signal. The improved electroluminescence in the copolymers with Aliquat 336 was attributed to the improved synthesis conditions, i.e., reaction time and catalytic system employed. The results suggest that copolymers with luminescent and morphological properties, more suitable for light-emitting devices, might be obtained with simple modifications during synthetic polycondensation reactions.

Cost-Effective Approach for Modeling of Multiresonant Thermally Activated Delayed Fluorescence Emitters.

Multiresonant thermally activated delayed fluorescence (MR-TADF) emitters have recently attracted great interest for application in organic light-emitting diodes due to their remarkable electroluminescent efficiency and narrow emission spectra. It is therefore essential to establish computational methodologies that can accurately model the excited states of these materials at manageable computational costs. With regard to MR-TADF design and their associated photophysics, previous works have highlighted the importance of wave function-based methods, at much higher computational costs, over the traditional time-dependent density functional theory approach. Herein, we employ two independent techniques built on different quantum mechanical frameworks, highly correlated wave function-based STEOM-DLPNO-CCSD and range-separated double hybrid density functional, TD-B2PLYP, to investigate their performance in predicting the excited state energies in MR-TADF emitters. We demonstrate a remarkable mean absolute deviation (MAD) of ∼0.06 eV in predicting ΔEST compared to experimental measurements across a large pool of chemically diverse MR-TADF molecules. Furthermore, both methods yield superior MAD in estimating S1 and T1 energies over earlier reported SCS-CC2 computed values [J. Chem. Theory Comput. 2022, 18, 4903]. The short-range charge-transfer nature of low-lying excited states and narrow fwhm values, hallmarks of this class of emitters, are precisely captured by both approaches. Finally, we show the transferability and robustness of these methods in estimating rates of radiative and nonradiative events with adequate agreement against experimental measurements. Implementing these cost-effective computational approaches is poised to streamline the identification and evaluation of potential MR-TADF emitters, significantly reducing the reliance on costly laboratory synthesis and characterization processes.

Thienothiophene-based organic light-emitting diode: synthesis, photophysical properties and application.

A donor-π-acceptor (D-π-A)-type pull-push compound, DMB-TT-TPA (8), comprising triphenylamine as donor and dimesitylboron as acceptor linked through a thieno[3,2-b]thiophene (TT) π-conjugated linker bearing a 4-MeOPh group, was designed, synthesized, and fabricated as an emitter via a solution process for an organic light-emitting diode (OLED) application. DMB-TT-TPA (8) exhibited absorption and emission maxima of 411 and 520 nm, respectively, with a mega Stokes shift of 109 nm and fluorescence quantum yields both in the solid state (41%) and in solution (86%). The optical properties were supported by computational chemistry using density functional theory for optimized geometry and absorption. A solution-processed OLED was fabricated using low turn-on voltage, which had performances with maximum power, current, and external quantum efficiencies of 6.70 lm/W, 10.6 cd/A, and 4.61%, respectively.

Theoretical study on the influence of substitution position on the luminescence properties and charge transfer characteristics of thermally activated delayed fluorescent molecules

Thermally active delayed fluorescence (TADF) molecules have potentially applications in organic light-emitting diodes (OLEDs) and biomedical sensing. Although TADF emitters have witnessed a rapid development, it remains challenging to study the relationship between molecular structures and luminescence properties as well as carrier mobility transfer properties in theory. In this work, the photophysical properties and luminescence mechanisms of isomers TPA-APQDCN-C (donors at para-position) and TPA-APQDCN-Y (donors at ortho-position) were studied based on density functional theory (DFT) and thermal vibration correlation function (TVCF) method. The results showed that both TPA-APQDCN-C with para-substituted donor and TPA-APQDCN-Y with ortho-substituted donors exhibit red emission in toluene and crystal state. Furthermore, compared to ortho-substituted donors, para-substituted donors promote a redshift in emission wavelength. In addition, the fluorescence efficiencies of TPA-APQDCN-C is obviously higher than that of TPA-APQDCN-Y due to its larger radiative rate and less non-radiative decay rate. Besides, para-substitution (TPA-APQDCN-C) leads to the smaller energy gap between S1 and T1 and the larger spin–orbit coupling (SOC) constant, which is beneficial for increasing the reverse crossing intersystem (RISC) rates. In addition, the carrier mobilities are studied based on the kinetic Monte Carlo simulations. The calculations show that TPA-APQDCN-C are more beneficial for the transfer of holes compared to TPA-APQDCN-Y. This study reveals TPA-APQDCN-C with donors at para-position has a better TADF properties and hole transfer ability, which holds guiding significance for the design of TADF devices with high luminescence efficiency and rapid hole transfer.

Cascading Energy Transfer for Highly Efficient Deep-Red OLED Emission with Cyclometalated [3+2+1] Iridium Complexes.

The promising cyclometalated iridium (III) complexes have been proved to possess great potential in vacuum-deposited organic light-emitting diodes (OLEDs) applications for full-color displays and white solid-state lighting sources. Herein, based on the unique bidentate ligand of dibenzo[a,c]phenazine (dbpz) group with strong conjugated effect of aromatic rings for red emission, four novel [3+2+1] coordinated iridium (III) emissive materials have been rationally designed and synthesized. The monodentate ligands of -CN and -OCN have been effectively employed to tune the deep-red emission of 628-675nm with high photoluminescence quantum yields up to 98%. Moreover, all devices displayed deep-red color coordinates ranging from (0.675, 0.325) to (0.716, 0.284), which is close to the standard-red color coordinates of (0.708, 0.292), as recommended by International Telecommunication Union Radiocommunication (ITU-R) BT.2020. The device based on nBuIr(dbpz)CN with an exciplex cohost has exhibited maximum external quantum efficiencies of 20.7% and good stability. With nBuIr(dbpz)CN as an effective sensitizer, the nBuIr(dbpz)OCN based phosphorescent OLED devices have successfully demonstrated cascading energy transfer processes, contributing to pure red emission with maximum luminance as high as 6471cd m-2. Therefore, this work has been successfully demonstrated rational molecular design strategy of [3+2+1] iridium complexes to obtain highly efficient deep-red electrophosphorescent emission.

Facile synthesis of anthracene-based derivatives via a magnetically retrievable Fe3O4@SiO2 immobilized selenoether functionalised NHC-Pd(II) heterogenous catalyst: Photophysical, electrochemical and DFT studies of novel 9,10-anthracene based derivatives

A series of anthracene-based derivatives, which are promising candidates for Organic light-emitting diode (OLEDs) applications, have been synthesized through the Suzuki Miyuara cross-coupling reaction of dibromo anthracene and phenylboronic acids derivatives. A magnetically separable heterogeneous catalyst Fe3O4@SiO2@NHC^SePh-Pd(II) was used, and the coupling reaction was investigated under aerobic conditions using water as a green solvent. A very mild approach for the arylation of the 9,10-dibromo anthracene molecules was adopted to harvest a series of highly conjugated anthracene derivatives. The catalyst was reused for up to seven catalytic cycles without much loss in catalytic efficiency. The 55% transformation in the seventh catalytic cycle indicates the catalyst's robustness and high catalytic potential. Necessary photophysical studies were conducted for selected synthesized anthracene-based derivatives, which includes the absorption, emission and cyclic voltammetric studies. Moreover, the thermogravimetric analysis (TGA) was also conducted to check the thermal stability of the compounds. Finally, a Density Functional Theory (DFT) study was conducted, and it revealed that comparing two anthracene derivatives, the Highest occupied molecular orbital (HOMO) and Least unoccupied molecular orbital (LUMO) energy levels, as well as the HOMO-LUMO gap (HLG), differ, with 9-([1,1′-biphenyl]-4-yl)-10-(4-fluorophenyl) anthracene exhibiting a relatively large HLG. The HOMO is localised along the molecule's main axis, while the LUMO is more delocalised on atoms.

Structural optimization of slot-die heads with T-shaped cavity for fabrication of large-area OLEDs

With an attempt to achieve highly uniform large-area organic thin films for potential applications in solution-processable organic light-emitting diodes (OLEDs), we have optimized the structure of slot-die heads with T-shaped cavity (manifold) by computational fluid dynamics (CFD) simulation. Depending on the viscosity of coating solutions, the pressure distribution inside the cavity and the velocity distribution at the outlet are analyzed by varying the structure parameters such as the shim thickness, cavity volume (cavity depth, cavity height, cavity width), and land length. From CFD simulation results, we have derived the maximum shim thickness at which the pressure non-uniformity becomes less than 5 %, which can provide a simple and intuitive design guideline. It is addressed that the maximum shim thickness is most affected by the cavity width, followed by the cross-sectional area of the cavity and then the land length. Using the structurally optimized slot-die head, we have carried out slot-die coating of low-viscosity (80 cPs) poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and high-viscosity (4,800 cPs) polydimethylsiloxane (PDMS) and achieved thin films with the thickness non-uniformity less than 10 %. Furthermore, we have successfully fabricated 120 mm × 20 mm OLED devices with the slot-coated conductive PEDOT:PSS layer, exhibiting the luminance of 801 cd/m2 at 6 V.

Electroactive Dyes Based on 1,8-Naphthalimide with Acetylene Linkers as Promising OLED Materials - the Relationship Between Structure and Photophysical Properties.

Four A-π-D-π-A type small organic molecules with 1,8-naphthalimide motifs were successfully synthesised. The designed compounds are built of two 1,8-naphthalimide units linked via ethynyl π-linkages with selected functionalised donor motifs i. e. 2,2'-bithiophene, fluorene, phenothiazine and carbazole derivative. The synthesis based on Sonogashira cross-coupling allowed us to obtain the presented dyes with good yields. The resulting symmetrical small molecules' optical, electrochemical and thermal properties were thoroughly investigated, and their potential applicability for the OLED devices was demonstrated. In addition, the relationship between molecular structure and properties was considered by employing experimental and theoretical studies. As a result of using various donor groups, it was possible to achieve efficient electroluminescence in the range from green (DEV4) to orange-red light (DEV3) with a maximum luminance of 3 820 cd/m2 for DEV4. Upon the insertion of an acetylene linker to the designed molecules, the free rotation of D and A fragments, and hence the effective π-electron communication within the entire molecule, is possible, which was confirmed by DFT studies. The obtained dyes are characterised by high thermal stability, reversible oxidation-reduction process, satisfactory optoelectronic properties and good solubility in organic solvents, which is advisable for the application in small molecular organic light-emitting diodes (SM-OLEDs) technology.