Application In Organic Light-emitting Diodes Research Articles

The Effect of Molecular Structure on the Properties of Fluorene Derivatives for OLED Applications.

A new family of symmetrical fluorene derivatives with different types of substituents attached to the C-2 and C-7 positions of the fluorene core synthesized by the Sonogashira coupling reactions is reported. The electronic structures and the properties of the compounds investigated by means of photoelectron emission spectroscopy, UV-Vis absorption and photoluminescent spectroscopy as well as by DFT and TD-DFT theoretical calculations are discussed. It is shown that the nature of substituents influences the π-conjugation of the molecules. No intermolecular charge transfer within the investigated wavelength range is observed. The applicability of the synthesized compounds in organic light-emitting diodes (OLEDs) based on exciplex emission is demonstrated. The advanced co-deposition technique with the tuned OLED architecture was applied and resulted in improved OLED parameters.

Lateral Diffusion of Holes in Anodic Buffer Layers and Its Application in Organic Light-Emitting Diodes

Lateral Diffusion of Holes in Anodic Buffer Layers and Its Application in Organic Light-Emitting Diodes

A 2-D coordination polymer based on 3-(2-pyridyl)-5-(4-chlorophenyl)-1,2,4-triazine ligands and lead(II) bromide: synthesis, structure, luminescent properties, and application in organic light-emitting diodes

The combination of PbBr2 with 3-(2-pyridyl)-5-(4-chlorophenyl)-1,2,4-triazine (PCPT) results in a 2-D Pb(II) coordination polymer, denoted as [Pb9(PCPT)4Br18] n . This compound has been identified and studied through CHN analysis, FT-IR, and 1H NMR spectroscopy, in addition to detailed analysis using single-crystal X-ray diffraction techniques. Considering the primary and secondary Pb–N and Pb–Br bonds, Pb1 and Pb2 exhibit hemidirected pentagonal bipyramidal geometries, whereas Pb3, Pb4, and Pb5 demonstrate holodirected octahedral geometries. The 2-D structure transitions into a 3-D arrangement due to the presence of non-covalent C–H…Br and π…π stacking interactions. The interactions highlighted above enhance the structural stability and support the transfer of charge and energy between metal centers, positioning the polymer as a potential candidate for luminescence applications. Analysis of the electro-optical characteristics of this compound indicates its suitability as a luminescent material for incorporating into organic light-emitting diodes.

Optoelectronic properties study of arylamine-functionalized benzothiophenes and benzothiophene S,S-Dioxides. Application in solution-processed organic light-emitting diodes

Conjugated organic molecules based on core benzothiophene (BT) and benzothiophene S,S-dioxide (BTO), substituted with triphenylamines at positions 3,5,6 and 2,4,7, were designed via Suzuki couplings. Optoelectronic properties were studied, and theoretical calculations were carried out to understand the structured absorbance spectra of the various compounds. All compounds exhibited bright emission within the visible region, ranging from blue for BT to yellow-orange for BTO. Organic light-emitting diode (OLED) devices were fabricated through an air-processed spin-coating method where all layers, except the top cathode, were solution-processed. Device luminance exceeded 1000 cd/m2 using 2,4,7-triphenylaminebenzothiophene S,S-dioxide, which in turn was successfully translated to a large-area module slot-die coated on a plastic substrate.

Asymmetric structural strategy enables efficient MR-TADF emitters with low efficiency roll-off and doping-concentration insensitivity

Multiple resonance (MR) thermally activated delayed fluorescence (TADF) materials containing B/N hold great promise for application in efficient narrowband organic light-emitting diodes (OLEDs). However, overcoming the challenges of reducing efficiency roll-off while minimizing aggregation-caused quenching to achieve highly efficient, low efficiency roll-off and doping concentration-insensitive MR-TADF emitters remains a formidable task. To this end, we designed two asymmetric B/N MR-TADF emitters by integrating a twisted heptagonal tribenzo[b,d,f]azepine (TBA) unit with a bulky substituted carbazole unit. The new compounds inherit the excellent properties of both parent skeletons. BN-TPCzTBA achieves a high photoluminescence quantum yield of up to 98.6 % and attains a maximum external quantum efficiency (EQE) of up to 36.8 % without the use of a sensitizer. The EQE remains at 22.8 % at a high brightness of 1000 cd m−2, demonstrating a low efficiency roll-off (37 %). Importantly, the twisted, bulky, and asymmetrical structure of these molecules allows them to sustain over 30 % of the EQEmax with a consistent full width at half maximum (FWHM) even at doping concentrations of up to 30 %. This thereby presents an innovative molecular design strategy that addresses critical issues in MR-TADF material engineering, combining low efficiency roll-off with doping concentration insensitivity.

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.

Design and performance of sulfur and selenium-substituted triarylboron D3-A TADF emitters for OLED applications

This study presents the design, synthesis, and comprehensive theoretical and photophysical analysis of two new D3-A type thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diode (OLED) applications. Utilizing a triarylboron core as the electron-accepting group and phenothiazine (PTZ) or phenoselenazine (PSZ) as electron-donating units, the molecules BTP-S and BTP-Se were developed. The D3-A structure supports the separation of frontier molecular orbitals (FMOs), leading to minimized singlet-triplet energy gaps (ΔEST), which are crucial for the TADF mechanism. Density functional theory (DFT) calculations presented that BTP-S and BTP-Se exhibit band gaps (Eg) of 2.52 and 3.23 eV, respectively, with BTP-S showing an ΔEST value as low as 0.007 eV for the S1-T1 transition at the lowest energy conformation. Photophysical studies revealed high photoluminescence quantum yields (PLQYs) for both compounds, with BTP-S achieving up to 85 % in mCP films and BTP-Se up to 59 %. In vacuum-processed OLEDs, BTP-S achieved a maximum external quantum efficiency (EQE) of 25.3 %, a current efficiency (ηc) of 195.8 cd/A, and a maximum luminance (Lmax) of 17356 cd/m2, while BTP-Se reached an EQE of 7.5 %, an ηc of 132.19 cd/A, and an Lmax of 16826 cd/m2 likely limited by the contributions of a folded-donor conformer enabled by the Se substitution. These findings underscore the impact of donor unit selection and conformation on the TADF characteristics, and provide valuable insights for designing high-performance OLED materials.

The experimental and DFT approaches on electronic, thermal and conductivity properties of non-linear optical bearing fused aromatic chalcones towards prospective OLEDs

New derivatives of push-pull chalcone was successfully synthesized to demonstrate as NLO active on the impact of organic light emitting diode (OLED) materials. A total of three fused-aromatic chalcones (AA, AB, AC) with different substituents have been characterized through TGA, Z-scan analysis, 1H & 13C NMR, IR and UV–Vis. All the targeted compounds are nonlinear refraction (NLR) active with the range of χ(3) (∼ × 10–6 esu). AC reveals the highest value of first order hyperpolarizability (βtot) at 528.08 × 10–30 esu upon the substitution of strong electron acceptor. Theoretical investigation was performed by employing the theory of B3LYP/6–31 G (d,p) level via density functional theory (DFT) to optimize the geometries, molecular electrostatic potential (MEP) and frontier molecular orbitals (FMO). The predicted electronic properties of the molecule are supported by the experimental results, confirming the theoretical conclusions. The examination of its potential as emissive layer in OLED application was constructed on ITO substrate for electroluminescence behaviour evaluation under various direct current (DC) voltages supply. The preliminary outcomes reveal that these simple chalcone derivatives are capable of being utilised in any optoelectronic application.

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.

1,2,4-triazine derived binuclear lead(ii) complexes: synthesis, spectroscopic and structural investigations and application in organic light-emitting diodes (OLEDs).

Two novel binuclear complexes of Pb(ii) were synthesized by reacting a 3-(2-pyridyl)-5-(4-methoxyphenyl)-1,2,4-triazine (PMPT) ligand with different anionic co-ligands (1: bromide, 2: acetate and isothiocyanate) in a 1 : 1 molar ratio of PMPT ligands to lead(ii) salts. The complexes, [Pb2(μ-PMPT)2Br4] (1) and [Pb2(μ-PMPT)2((μ-CH3COO)2(NCS)2] (2), were characterized using various physicochemical techniques such as CHN analysis, FT-IR spectroscopy, and 1H NMR spectroscopy. Additionally, their structures were determined using single-crystal X-ray diffraction. Based on the obtained structural parameters, complex 1 exhibited a PbN3Br2 environment, while complex 2 displayed a PbN4O3 environment, with holodirected and hemidirected coordination spheres, respectively. Within the crystal network of the complexes, there were interactions involving C-H⋯X (X: O, S, N) as well as π-π stacking. The Pb(ii) complexes were further investigated for their potential use as the emitting layer in organic light-emitting devices (OLEDs). The current-voltage and luminescence-voltage characteristics, as well as the electroluminescence (EL) properties of the complexes, were studied.

Rational design of red iridophosphors based on 1-(6-methoxynaphthalene-2-yl)isoquinoline ligand for solution‐processed OLEDs

Phosphorescent metal complexes find widespread application in organic light-emitting diodes (OLEDs), with iridium(III) complexes being particularly favored for their superior performance. Owing to the flexible modifications of both main and auxiliary ligands, the photophysical properties of iridium(III) complexes can be finely tuned. In this study, two red neutral iridium(III) complexes, designated as Ir1 and Ir2, are successfully synthesized and characterized. Various auxiliary ligands including acetylacetone and 2-picolinic acid are employed in combination with 1-(6-methoxynaphthalene-2-yl)isoquinoline as the cyclometalating ligand. Both complexes exhibit red emission, with Ir1 emitting at 631 nm and Ir2 at 615 nm. Furthermore, they demonstrated good solubility in common organic solvents, facilitating device fabrication via solution methods. Electroluminescent (EL) devices based on complexes Ir1 and Ir2 are further prepared using spin-coating method, achieving maximum external quantum efficiencies (EQEs) of 3.30 % and 4.52 %, respectively, and both devices exhibited bright red EL with CIE coordinates of (0.68, 0.32) and (0.66, 0.34), which were very close to the standard red coordinates of (0.67, 0.33).

Boosting External Quantum Efficiency to 12.0 % of an Ultraviolet OLED by Engineering the Horizontal Dipole Orientation of a Hot Exciton Emitter.

Currently, much research effort has been devoted to improving the exciton utilization efficiency and narrowing the emission spectra of ultraviolet (UV) fluorophores for organic light-emitting diode (OLED) applications, while almost no attention has been paid to optimizing their light out-coupling efficiency. Here, we developed a linear donor-acceptor-donor (D-A-D) triad, namely CDFDB, which possesses high-lying reverse intersystem crossing (hRISC) property. Thanks to its integrated narrowband UV photoluminescence (PL) (λPL: 397 nm; FWHM: 48 nm), moderate PL quantum yield (ϕPL: 72 %, Tol), good triplet hot exciton (HE) conversion capability, and large horizontal dipole ratio (Θ//: 92 %), the OLEDs based on CDFDB not only can emit UV electroluminescence with relatively good color purity (λEL: 398 nm; CIEx,y: 0.161, 0.040), but also show a record maximum external quantum efficiency (EQEmax) of 12.0 %. This study highlights the important role of horizontal dipole orientation engineering in the molecular design of HE UV-OLED fluorophores.

Blue Phosphorescent Pt(II) Compound Based on Tetradentate Carbazole/2,3'-Bipyridine Ligand and Its Application in Organic Light-Emitting Diodes.

The tetradentate ligand, merging a carbazole unit with high triplet energy and dimethoxy bipyridine, renowned for its exceptional quantum efficiency in coordination with metals like Pt, is expected to demonstrate remarkable luminescent properties. However, instances of tetradentate ligands such as bipyridine-based pyridylcarbazole derivatives remain exceptionally scarce in the current literature. In this study, we developed a tetradentate ligand based on carbazole and 2,3'-bipyridine and successfully complexed it with Pt(II) ions. This novel compound (1) serves as a sky-blue phosphorescent material for use in light-emitting diodes. Based on single-crystal X-ray analysis, compound 1 has a distorted square-planar geometry with a 5/6/6 backbone around the Pt(II) core. Bright sky-blue emissions were observed at 488 and 516 nm with photoluminescent quantum yields of 34% and a luminescent lifetime of 2.6 μs. TD-DFT calculations for 1 revealed that the electronic transition was mostly attributed to the ligand-centered (LC) charge transfer transition with a small contribution from the metal-to-ligand charge transfer transition (MLCT, ~14%). A phosphorescent organic light-emitting device was successfully fabricated using this material as a dopant, along with 3'-di(9H-carbazol-9-yl)-1,1'-biphenyl (mCBP) and 9-(3'-carbazol-9-yl-5-cyano-biphenyl-3-yl)-9H-carbazole-3-carbonitrile (CNmCBPCN) as mixed hosts. A maximum quantum efficiency of 5.2% and a current efficiency of 15.5 cd/A were obtained at a doping level of 5%.

Machine learning-enabled discovery of multi-resonance TADF molecules: Unraveling PLQY predictions from molecular structures

Unlocking the potential of multi-resonance thermally activated delayed fluorescence (MR-TADF) molecules for advanced organic light-emitting diode applications requires an insightful understanding of the relationship between molecular structures and photoluminescence quantum yield (PLQY). Utilizing molecular descriptors as inputs for machine learning (ML) algorithms, further illuminated by SHapley Additive exPlanations (SHAP) to interpret the ML model outcomes, this method effectively connects molecular structures to PLQY, providing targeted guidance for molecular design. A vast molecular library is generated via variational autoencoders, allowing for a comprehensive exploration of molecular space beyond conventional chemical intuition. High-throughput virtual screening, combined with our PLQY-focused model and a secondary model for emission peak wavelength prediction, efficiently identify promising candidates with blue-emitting properties. The robustness of our predictions is substantiated through quantum chemistry calculations. The integrative methodology proposed in this work not only streamlines the discovery of MR-TADF molecules but also provides a replicable framework for the intelligent design of other optoelectronic materials.

Excited-State Engineering Enables Efficient Deep-Blue Light-Emitting Diodes Exhibiting BT.2020 Color Gamut.

Organic luminescent materials that exhibit thermally activated delayed fluorescence (TADF) can convert non-emissive triplet excitons into emissive singlet states through a reverse intersystem crossing (RISC) process. Therefore, they have tremendous potential for applications in organic light-emitting diodes (OLEDs). However, with the development of ultra-high definition 4K/8K display technologies, designing efficient deep-blue TADF materials to achieve the Commission Internationale de l'Éclairage (CIE) coordinates fulfilling BT.2020 remains a significant challenge. Here, an effective approach is proposed to design deep-blue TADF molecules based on hybrid long- and short-range charge-transfer by incorporation of multiple donor moieties into organoboron multiple resonance acceptors. The resulting TADF molecule exhibits deep-blue emission at 414nm with a full width at half maximum (FWHM) of 29nm, together with a thousand-fold increase in RISC rate. OLEDs based on the champion material achieve a record maximum external quantum efficiency (EQE) of 22.8% with CIE coordinates of (0.163, 0.046), approaching the coordinates of the BT.2020 blue standard. Moreover, TADF-assisted fluorescence devices employing the designed material as a sensitizer exhibit an exceptional EQE of 33.1%. This work thus provides a blueprint for future development of efficient deep-blue TADF emitters, representing an important milestone towards meeting the blue color gamut standard of BT.2020.

Multifunctional applications of organic light-emitting diodes and room temperature phosphorescence based on benzoyl-fluorene hybrid molecules

Organic light-emitting diodes (OLEDs) and room temperature phosphorescence (RTP) materials have attracted great interests for their wide application in lighting and other fields. How to efficiently utilize triplet excitons to achieve the multifunctionality of OLED and RTP simultaneously is a major challenge. Herein, two blue molecules, 3PCZFB and 3PCZBF were reported. Theoretical calculations indicate that both molecules possess hybrid localized charge transfer (HLCT) characteristic. More importantly, both can simultaneously obtain electro-fluorescence and RTP properties by multiple harvesting of triplet excitons with high spin-orbit coupling matrix element (SOCME). The introduction of benzoyl fluorene increased the contribution of the n-orbital, resulting in a high SOCME, which is beneficial to both high exciton utilization efficiency (EUE) and RTP. The non-doped OLED of 3PCZFB achieved maximum external quantum efficiency (EQEmax) of 4.9 % and EUE of 58.6 % at 459 nm. Besides, 0.1 % 3PCZFB and 3PCZBF doped in PMMA sheets achieve RTP efficiencies of 6.6 % at 524 nm and 25.9 % at 506 nm, as well as lifetimes of 379.8 ms and 204.8 ms. This is the first time Benzoyl fluorene-based molecules achieved dual-function applications in OLED and RTP by utilizing and recycling triplet excitons of T1.

From monomer to polymer: Controlled synthesis and comprehensive analysis of poly(p-phenylene vinylene) via ROMP

This study focuses on synthesis and evaluation of [2.2] paracyclophane-1,9-diene to produce a soluble poly(p-phenylenevinylene) (PPV) derivative homopolymer using ring-opening metathesis polymerization (ROMP). The resulting homopolymer displayed a narrow polydispersity index (PDI) of 1.22, indicating precise control over polymerization. The PPV derivative exhibited excellent solubility in various organic solvents. Photophysical properties, including optical absorption and fluorescence emission spectra, were investigated to evaluate utilization in optoelectronic devices. Optical band gaps ranged from 2.21 to 2.25 eV for thin films and 2.07 to 2.19 eV for solutions, while the electrochemical band gap, determined by cyclic voltammetry, was 2.37 eV. These materials demonstrated promising fluorescence activity in various solvents and thin films, suggesting potential applications in organic light-emitting diodes (OLEDs) and related optoelectronic devices.

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.