Organic Light-emitting Diodes Research Articles

Modulating Efficiency and Color of Thermally Activated Delayed Fluorescence by Rationalizing the Substitution Effect.

Thermally activated delayed fluorescence (TADF) constitutes the process by which third-generation organic light-emitting diodes (OLEDs) are being designed and produced. Despite several years of trial-and-error attempts, mainly driven by chemical intuition about how to improve a certain aspect of the process, few studies focused on the in-depth description of its two key properties: efficiency of the T1 → S1 intersystem crossing and further S1 → S0 emission. Here, by means of a newly developed theoretical formalism, we propose a systematic rationalization of the substituent effect in a paradigmatic class of OLED compounds, based on phenothiazine-dibenzothiophene-S,S-dioxide, known as PTZ-DBTO2. Our methodology allows to discern among geometrical and electronic effects induced by the substituent, deeply understanding the relationships existing between charge transfer, spin density, geometrical deformations, and energy modulations between electronic states. By our results, we can finally elucidate, depending on the substituent, the fate of the overall TADF process, quantitatively assessing its efficiency and predicting the color emission. Moreover, the general terms by which this methodology was developed allow its application to any chromophore of interest.

Large‐Area and Uniform Blade‐Coated Deep‐Blue Polymer Light‐Emitting Diodes Based on Triphenylethylene‐Encapsulated π‐Conjugated Polymer

AbstractEmerging large‐area flexible polymer light‐emitting diodes (PLEDs) are crucial for flat‐panel displays. The development of uniform, large‐area, flexible, and ultra‐deep‐blue PLEDs remains challenging owing to the lack of efficient and stable deep‐blue light‐emitting conjugated polymers with robust deep‐blue emission, excellent large‐scale solution‐processing capacity, and stable electrical properties. In this study, a multidimensional steric triphenylethylene (TPE) segment is introduced into the substitution side chain of polydiarylfluorenes (P‐F) for deep‐blue large‐area blade‐coated and flexible PLEDs. The encapsulation of backbone chains with extremely weak interchain π–π interactions facilitated the preparation of large‐area PLEDs via blade coating. Large‐area uniform P‐F films (16 cm2) are fabricated. Flexible PLEDs are fabricated with efficient deep‐blue emission (CIE: 0.15, 0.07) and a maximum brightness of 2314 cd m−2. Notably, the large‐area PLEDs exhibited a narrow‐band deep‐blue emission with a CIE of (0.15, 0.08) at ≈1000 mA cm−2 and an emitting area of 3.6 cm2. Furthermore, a preliminary patterned PLED array display with a pixel size of 800 × 1000 µm2 is successfully created, which exhibited stable and uniform deep‐blue light‐emitting pixel performance (CIE: 0.15 0.08). These results demonstrated the effectiveness of the TPE encapsulation strategy for large‐area PLEDs.

Maximization of cytochrome C oxidase enzyme activity by optimizing color conversion from red organic light-emitting diodes

Photobiomodulation (PBM) therapy using red and near-infrared (NIR) light can effectively activate cytochrome C oxidase (CCO) enzyme to promote wound healing in living organisms. To maximize this therapeutic effect, the use of a light source with a spectrum absorbed by the CCO enzyme is necessary. A red/NIR light source optimized for CCO enzyme was obtained by partially color-converting a red organic light-emitting diode with copper indium sulfide/zinc sulfide quantum dots (QDs). To increase color conversion efficiency, the dispersion of QDs was improved by using a fibrous color conversion layer (FCCL). By optimizing the QD content in the FCCL, a red/NIR light source similar to the absorption spectrum of CCO enzyme was obtained with a spectral matching ratio of 81.7 %. When this light source was used in PBM therapy, it was experimentally verified that it showed a much faster wound closure effect compared to general red light, and it was proven that this was due to increased activity of the CCO enzyme. This study is a method of obtaining a red/NIR light source for PBM suitable for a specific enzyme performing a specific function, and is particularly useful due to its excellent form factor of being large, thin, and flexible.

High efficiency and high color purity deep‐blue organic light‐emitting diodes with blue index >500

AbstractThermally activated delayed fluorescent (TADF) and phosphorescent blue organic light‐emitting diodes (OLEDs) have been developed to overcome the relatively low triplet exciton utilization of traditional fluorescent OLEDs. However, broad emission spectra originating from charge transfer process limit the commercial application of such blue OLEDs. Herein, an effective phosphor‐sensitized fluorescent (PSF) OLED device structure was designed. PSF OLED exhibited a maximum blue index (BI) of 508 cd/A/CIEy with CIEy of 0.060, which has been the highest efficiency result reported for PSF deep‐blue OLEDs. A nearly 100% improvement in operational lifetime LT95 (L/L0 = 95%) was achieved successfully by using lower triplet energy (T1) and deuterated host materials. Impressively, the sensitized OLEDs maintained BI of 407 cd/A/CIEy, narrow emission spectra (FWHM = 17 nm), and high color purity (CIEy = 0.046), revealing the attractive technical advantages and commercial application potential.

Advances in display technology: augmented reality, virtual reality, quantum dot-based light-emitting diodes, and organic light-emitting diodes

Virtual reality, augmented reality, quantum dot light-emitting diodes, and organic light-emitting diodes have progressed over the last two years. Key achievements in these displays are discussed in terms of device performance.

Deep-Blue and Fast Delayed Fluorescence from Carbene-Metal-Amides for Highly Efficient and Stable Organic Light-Emitting Diodes.

Linear gold complexes of the "carbene-metal-amide" (CMA) type are prepared with a rigid benzoguanidine amide donor and various carbene ligands. These complexes emit in the deep-blue range at 424 and 466nm with 100% quantum yields in all media. The deep-blue thermally activated delayed fluorescence (TADF) originates from a charge transfer state (CT) with an excited state lifetime as low as 213ns, resulting in fast radiative rates of 4.7 × 106 s-1. The high thermal and photo-stability of these CMA materials enabled us to fabricate highly energy efficient organic light-emitting diodes (OLED) in host-guest architectures. We report deep-blue OLED devices with electroluminescence at 416nm and 457nm with practical external quantum efficiencies of up to 23% at 100cd m-2 with excellent colour coordinates CIE (x; y) = 0.16; 0.07 and 0.17; 0.18. The operating stability of these OLEDs is the longest reported to date (LT50 = 1 hour) for deep-blue CMA emitters, indicating a high promise for further development of blue OLED devices. Our findings inform the molecular design strategy and correlation between delayed luminescence with high radiative rates and CMA OLED device operating stability. This article is protected by copyright. All rights reserved.

Thickness‐Dependent Formation Capacity and Mechanism of Planar (β)‐Conformation in Polydiarylfluorenes for Stable Deep‐Blue Polymer Light‐Emitting Diodes

AbstractConjugated polymers with p‐orbital overlap show intramolecular‐conformation‐dependent photophysical and electrical properties. Compared to the non‐planar segments, the planar conformation segments have a relatively long effective conjugated length and rigid backbone structure, which are beneficial for the enhancement of optoelectronic properties. Herein, the effect of film thickness on the formation capacity of planar (β) conformations in polyfluorene (poly[4‐(octyloxy)‐9,9‐diphenylfluorene‐2,7‐diyl]‐co‐[5‐(octyloxy)‐9,9‐diphenylfluorene‐2,7‐diyl], PODPF) for the fabrication of stable deep‐blue polymer light‐emitting diodes (PLEDs) is explored. Thickness‐tunable films with thicknesses ranging from 15 to 270 nm by controlling the concentration of precursor solution are prepared. The main observations are as follows. The PODPF chains in thin films are less likely to adopt the β conformations associated with the loose interchain packing in nano‐aggregates than those in thick films. Red‐shifted and well‐resolved emission spectra are observed for the annealed film with a high thickness, indicating the greater presence of β‐conformation segments. More interestingly, interchain aggregation is a necessary but not sufficient condition for the formation of β conformations. Finally, PLEDs based on β‐conformation segments have outstanding brightness, low turn‐on voltage, and stable deep‐blue electroluminescent behavior, confirming the unique advantage of planar conformation for optoelectronic applications.

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

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

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.

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.

Combined Shewhart–EWMA and Shewhart–CUSUM monitoring schemes for time between events

AbstractTo improve the detecting abilities of upward and downward parameter changes in high‐quality process, two combined schemes by integrating memory‐type, that is, exponentially weighted moving average (EWMA) or cumulative sum (CUSUM), and memoryless, that is, Shewhart, are proposed for monitoring the time between events (TBE), which is modeled as an exponential distributed variable. A Monte Carlo simulation method is employed to obtain the Run Length () properties, that is average run length (), of the proposed schemes for different parameter settings. The nearly optimal monitoring parameter combinations under different change size are obtained by minimizing the out‐of‐control with the satisfied in‐control . Using the designed parameters, the performance of the proposed monitoring schemes is compared with the existing EWMA and CUSUM TBE. The results show that the proposed combined Shewhart–EWMA or Shewhart–CUSUM TBE generally perform better than the corresponding EWMA or CUSUM TBE for large changes and they also show better performance than the Shewhart TBE for small changes. Finally, a real dataset of organic light‐emitting diode (OLED) failure time from Sumsung company is employed to indicate the usage and implementation of combined TBE schemes.

Third-Generation Anticancer Photodynamic Therapy Systems Based on Star-like Anionic Polyacrylamide Polymer, Gold Nanoparticles, and Temoporfin Photosensitizer

Photodynamic therapy (PDT) is a non-invasive anticancer treatment that uses special photosensitizer molecules (PS) to generate singlet oxygen and other reactive oxygen species (ROS) in a tissue under excitation with red or infrared light. Though the method has been known for decades, it has become more popular recently with the development of new efficient organic dyes and LED light sources. Here we introduce a ternary nanocomposite: water-soluble star-like polymer/gold nanoparticles (AuNP)/temoporfin PS, which can be considered as a third-generation PDT system. AuNPs were synthesized in situ inside the polymer molecules, and the latter were then loaded with PS molecules in an aqueous solution. The applied method of synthesis allows precise control of the size and architecture of polymer nanoparticles as well as the concentration of the components. Dynamic light scattering confirmed the formation of isolated particles (120 nm diameter) with AuNPs and PS molecules incorporated inside the polymer shell. Absorption and photoluminescence spectroscopies revealed optimal concentrations of the components that can simultaneously reduce the side effects of dark toxicity and enhance singlet oxygen generation to increase cancer cell mortality. Here, we report on the optical properties of the system and detailed mechanisms of the observed enhancement of the phototherapeutic effect. Combinations of organic dyes with gold nanoparticles allow significant enhancement of the effect of ROS generation due to surface plasmonic resonance in the latter, while the application of a biocompatible star-like polymer vehicle with a dextran core and anionic polyacrylamide arms allows better local integration of the components and targeted delivery of the PS molecules to cancer cells. In this study, we demonstrate, as proof of concept, a successful application of the developed PDT system for in vitro treatment of triple-negative breast cancer cells under irradiation with a low-power LED lamp (660 nm). We consider the developed nanocomposite to be a promising PDT system for application to other types of cancer.

Optical and electrical properties and applications of two-dimensional carbon-based nanomaterials

As an emerging material, two-dimensional (2D) carbon-based nanomaterials have unique optical and electrical properties due to their unique atomic layer structure. On the one hand, due to their high transmittance, high carrier mobility, controllable Fermi level, and wide spectral light saturation absorption characteristics, they can be applied in new-generation solar cells, organic light-emitting diodes, touch screens, and fiber optic devices. On the other hand, due to their unique nanostructure, they have high specific surface area, low diffusion distance, high conductivity, and ion conductivity. They can serve as substrates and work together with other materials as electrode materials for future fuel cells and lithium-ion batteries and have significant advantages in fields such as sensors. This article mainly summarizes the impact of two-dimensional carbon-based nanomaterials represented by graphene, Mxene, and 2D covalent organic frameworks (COFs) on their optical and electrical properties, and summarizes the latest practical applications and research progress of these nanomaterials in the field of optics and electronics. In addition, a corresponding summary and outlook have been made on the problems that need to be solved in future applications of 2D carbon-based nanomaterials.

Modulating the Locally Excited States with a Regulating Substituent for Highly Efficient Red/Near‐Infrared Thermally Activated Delayed Fluorescence Emitters

AbstractDeveloping highly efficient red/near‐infrared (NIR) thermally activated delayed fluorescence (TADF) materials is important for organic light‐emitting diodes (OLEDs). Here, two TADF emitters, APTT and APTI, which have the same D/A backbone but different attaching groups at acenaphtho‐[1,2‐b]pyrazine‐8,9‐dicarbonitrile (APDC) core, are reported. The appended regulating groups can not only suppress the D/A rotation due to the space confinement effects but also modulate the locally excited triplet state (3LE). The improved molecular rigidity suppresses the non‐radiative process, accounting for the improved photoluminescence quantum yields (PLQYs), while the modulated 3LE promotes the reverse intersystem crossing (RISC) process due to the high utilization efficiency of triplets. Consequently, both APTT and APTI demonstrate high PLQY and fast RISC process, thereby enhancing TADF efficiency. The doped devices based on APTT and APTI achieve maximum external quantum efficiency (EQEmax) values of 20.5% and 25.4% with emission peaks at 664 and 670 nm, respectively. The non‐doped devices of APTT and APTI achieve the EQEmax of 2.8% and 2.9% with emission peaks at 788 and 794 nm, respectively. Encouragingly, the non‐doped devices of APTI have set new records for near‐infrared TADF OLEDs based on the APDC core. This study provides an efficient approach to modulating the optoelectronic properties of highly efficient NIR TADF OLEDs.

Nitrogen Effects Endowed by Doping Electron-Withdrawing Nitrogen Atoms into Polycyclic Aromatic Hydrocarbon Fluorescence Emitters.

Unveiling innovative mechanisms to design new highly efficient fluorescent materials and, thereby, fabricate high-performance organic light-emitting diodes (OLEDs) is a concerted endeavor in both academic and industrial circles. Polycyclic aromatic hydrocarbons (PAHs) have been widely used as fluorescent emitters in blue OLEDs, but device performances are far from satisfactory. In response, we propose the concept of "nitrogen effects" endowed by doping electron-withdrawing nitrogen atoms into PAH fluorescence emitters. The presence of the n orbital on the imine nitrogen is conducive to promoting electron coupling, which leads to increased molar absorptivity and an accelerated radiative decay rate of emitters, thereby facilitating the Förster energy transfer (FET) process in the OLEDs. Additionally, electronically withdrawing nitrogen atoms enhances host-guest interactions, thereby positively affecting the FET process and the horizontal orientation factor of the emitting layer. To validate the "nitrogen effects" concept, cobalt-catalyzed multiple C-H annulation has been utilized to incorporate alkynes into the imine-based frameworks, which enables various imine-embedded PAH (IE-PAH) fluorescence emitters. The cyclization demonstrates notable regioselectivity, thereby offering a practical tool to precisely introduce peripheral groups at desired positions with bulky alkyl units positioned adjacent to the nitrogen atoms, which were previously beyond reach through the Friedel-Crafts reaction. Blue OLEDs fabricated with IE-PAHs exhibit outstanding performance with a maximum external quantum efficiency (EQEmax) of 32.7%. This achievement sets a groundbreaking record for conventional blue PAH-based fluorescent emitters, which have an EQEmax of 24.0%.

Bis-Ortho-Donor-Modification of Boracyclic π-Electron Systems beyond Steric Protection to Produce Thermally Activated Delayed Fluorescence Materials.

Polycyclic π-conjugated compounds that contain tricoordinate boron atoms at their periphery represent an attractive class of materials with electron-accepting character. Their molecular design generally requires the introduction of a bulky aryl group onto the boron atom, where it provides predominantly kinetic stabilization. The addition of extra functionality to the aryl group on the boron atom can be expected to further expand the potential utility of this class of materials. Herein, we report the synthesis of a series of boracyclic π-conjugated molecules with firm ortho B∙∙∙N nonbonding interactions by introducing N-containing electron-donors at the ortho-positions of the aryl group on the boron atom. X-ray crystallographic analysis revealed that the combination of a planar boracyclic π-skeleton with only sp2 carbons and a strong electron-donating phenothiazine moiety results in a particularly short B∙∙∙N distance. Theoretical study provided insights into the inherent nature of the B∙∙∙N interaction. Owing to their donor-acceptor (D-A) structures, these molecules exhibit substantially red-shifted fluorescence in solution, albeit that the fluorescence quantum yields (ΦF) are low. In contrast, when incorporated into films, these compounds exhibit thermally activated delayed fluorescence (TADF) with improved ΦF values. Organic light-emitting diodes (OLEDs) fabricated using the ortho-donor-substituted derivatives exhibit orange-red electroluminescence.

Bis‐Ortho‐Donor‐Modification of Boracyclic π‐Electron Systems beyond Steric Protection to Produce Thermally Activated Delayed Fluorescence Materials

Polycyclic π‐conjugated compounds that contain tricoordinate boron atoms at their periphery represent an attractive class of materials with electron‐accepting character. Their molecular design generally requires the introduction of a bulky aryl group onto the boron atom, where it provides predominantly kinetic stabilization. The addition of extra functionality to the aryl group on the boron atom can be expected to further expand the potential utility of this class of materials. Herein, we report the synthesis of a series of boracyclic π‐conjugated molecules with firm ortho B∙∙∙N nonbonding interactions by introducing N‐containing electron‐donors at the ortho‐positions of the aryl group on the boron atom. X‐ray crystallographic analysis revealed that the combination of a planar boracyclic π‐skeleton with only sp2 carbons and a strong electron‐donating phenothiazine moiety results in a particularly short B∙∙∙N distance. Theoretical study provided insights into the inherent nature of the B∙∙∙N interaction. Owing to their donor–acceptor (D–A) structures, these molecules exhibit substantially red‐shifted fluorescence in solution, albeit that the fluorescence quantum yields (ΦF) are low. In contrast, when incorporated into films, these compounds exhibit thermally activated delayed fluorescence (TADF) with improved ΦF values. Organic light‐emitting diodes (OLEDs) fabricated using the ortho‐donor‐substituted derivatives exhibit orange‐red electroluminescence.

Progress in Research on White Organic Light-Emitting Diodes Based on Ultrathin Emitting Layers

White organic light-emitting diodes (WOLEDs) hold vast prospects in the fields of next-generation displays and solid-state lighting. Ultrathin emitting layers (UEMLs) have become a research hotspot because of their unique advantage. On the basis of simplifying the device structure and preparation process, they can achieve electroluminescent performance comparable to that of doped devices. In this review, we first discuss the working principles and advantages of WOLEDs based on UEML architecture, which can achieve low cost and more flexibility by simplifying the device structure and preparation process. Subsequently, the successful applications of doping and non-doping technologies in fluorescent, phosphorescent, and hybrid WOLEDs combined with UEMLs are discussed, and the operation mechanisms of these WOLEDs are emphasized briefly. We firmly believe that this article will bring new hope for the development of UEML-based WOLEDs in the future.

High‐luminance, large‐size 4K OLED microdisplays for VR/MR applications

AbstractThis work introduces LG Display's (LGD's) 1.3″ 4K full‐color organic light‐emitting diode (OLED) microdisplay panels with a peak brightness of more than 10,000 nits, which have been newly developed for high‐quality virtual reality/mixed reality (VR/MR) head‐mounted displays. To achieve these advanced high‐luminance OLED microdisplay panels, we have developed a well‐controlled optical microcavity and outcoupling method, along with optimized multi‐stack white OLED devices and CF‐on‐encapsulation technologies tailored to the high pixel density of the panels.

Excitons in confined molecular aggregates

AbstractOrganic optoelectronic materials have attracted extensive attention in the past decades due to their wide applications in organic light‐emitting diodes (OLEDs), organic photovoltaics (OPVs), photocatalysis, etc. Significant advancements have been obtained in the material designs based on the insight into the fundamental physics of exciton related to molecular stacking patterns in solid/condensed states. The exciton characteristics and behaviors are not only a starting point for studying photophysical and photochemical processes on a microscopic level, but also a crucial point in determining the optoelectronic properties of macroscopic aggregates. This review summarizes the historic development of exciton models, accompanied by the discoveries of special molecular stacking patterns (H‐/J‐/X‐/M‐aggregates), and the competitive de‐excitation pathways of excitons including fluorescence, energy transfer, singlet fission, excimer formation and symmetry‐breaking charge separation in the confined aggregate structures. Additionally, it highlights the capabilities of a correlation between molecular stacking modes and exciton behaviors, which provides new insights and perspectives for optimizing exciton character and behavior through the modulation of molecular arrangement in aggregate states, thereby enhancing the performance of optoelectronic materials.