Organic Light-emitting Diodes Research Articles

Optical, Photophysical, and Electroemission Characterization of Blue Emissive Polymers as Active Layer for OLEDs.

Polymers containing π-conjugated segments are a diverse group of large molecules with semiconducting and emissive properties, with strong potential for use as active layers in Organic Light-Emitting Diodes (OLEDs). Stable blue-emitting materials, which are utilized as emissive layers in solution-processed OLED devices, are essential for their commercialization. Achieving balanced charge injection is challenging due to the wide bandgap between the HOMO and LUMO energy levels. This study examines the optical and photophysical characteristics of blue-emitting polymers to contribute to the understanding of the fundamental mechanisms of color purity and its stability during the operation of OLED devices. The investigated materials are a novel synthesized lab scale polymer, namely poly[(2,7-di(p-acetoxystyryl)-9-(2-ethylhexyl)-9H-carbazole-4,4'-diphenylsulfone)-co-poly(2,6-diphenylpyrydine-4,4'-diphenylsulfone] (CzCop), as well as three commercially supplied materials, namely Poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO), poly[9,9-bis(2'-ethylhexyl) fluorene-2,7-diyl] (PBEHF), and poly (9,9-n-dihexyl-2,7-fluorene-alt-9-phenyl-3,6-carbazole) (F6PC). The materials were compared to evaluate their properties using Spectroscopic Ellipsometry, Photoluminescence, and Atomic Force Microscopy (AFM). Additionally, the electrical characteristics of the OLED devices were investigated, as well as the stability of the electroluminescence emission spectrum during the device's operation. Finally, the determined optical properties, combined with their photo- and electro-emission characteristics, provided significant insights into the color stability and selectivity of each material.

Suppression of Initial Degradation via an Interfacial Charge-Induced Overshooting Effect in Solution-Processed Organic Light-Emitting Diodes.

As the chemical stability of organic materials in organic light-emitting diodes (OLEDs) greatly impacts devices' lifetime, a thoughtful and advanced design of materials and device structures is necessary. In our work, we have achieved lifetime enhancement at its initial stage for solution-processed OLEDs. This improvement was realized through the implementation of a double electron transporting layer (dETL) composed of 2-[4-(9,10-dinaphthalen-2-yl-anthracen-2-yl)-phenyl]-1-phenyl-1H-benzoimidazole (ET) and hydroxyquinolinolato-lithium (Liq). A giant surface potential was generated at the surface of a constituent electron transport layer (ETL) that contained a higher concentration of Liq with high polarity. This giant surface potential simultaneously promoted the injection of trapped/accumulated electrons through the interface within dETL and the injection of holes from the anode, generating more exciton recombination events and ultimately enhancing efficiency by 133.0% and lifetime LT95 (luminance dropped by 5%) by 300% with an overshooting effect. Additionally, the degradation at the emitting layer was mitigated by shifting the degradation zone to the dETL, which was evidenced by laser desorption/ionization-time-of-flight (LDI-TOF) mass spectroscopy.

Improved Charge Recombination Efficiency in Organic Light‐Emitting Transistors via Luminescent Radicals

AbstractLuminescent radicals are attracting attention as emitters in electroluminescent devices thanks to the exploitation of doublet excitons. Recent studies reveal that exciton formation in radical organic light‐emitting diodes (OLEDs) primarily occurs through a charge trapping mechanism. Although typically detrimental for OLEDs, this might be a key process to elucidate light emission in organic light‐emitting transistors (OLETs). Here, a unipolar n‐type architecture suitable for the implementation of radical emitters is introduced, designed based on computational calculations. The operation of the as‐realized devices incorporating the newly synthesized [2,6‐dichloro‐4‐(2,6‐dimethoxyphenyl)phenyl](3,5‐dichloro‐4‐pyridyl) (2,4,6‐trichlorophenyl)methyl radical is investigated via transient electroluminescence measurements to demonstrate the occurrence of long‐living emission ascribed to the charge trapping mechanisms. Moreover, a comprehensive understanding of the processes governing radical‐OLET is obtained by recording complete 2D maps of both optical and electrical response of the device as a function of applied voltages. Notably, the trapping of electrons by radical moieties is demonstrated to generate a negative charge density in the emissive layer that facilitates holes to be injected: increasing the balance of opposite charge carriers, a tenfold enhancement of the external quantum efficiency (EQE) at the proper source‐drain and source‐gate voltage conditions is reported to reach a maximum EQE value of 0.2%.

π‐Extended Phenoxazine‐Based Asymmetric MR‐TADF Emitters for Narrow Emission Band Green OLEDs with a Power Efficiency of 150 lm W−1, an EQE of 27%, and an LT95 of Over 3000 h at 1000 cdm−2

AbstractAlthough multi‐resonance thermally activated delayed fluorescent (MR‐TADF) emitters simultaneously achieve high color purity and high efficiency in organic light‐emitting devices (OLEDs), MR‐TADF‐based OLEDs suffer from severe efficiency roll‐off and lifetime issues for practical application. To overcome these issues, a key solution is hyper OLED technology sensitized by TADF or phosphorescent emitters. In this study, a series of π‐extended phenoxazine‐based asymmetric MR‐TADF emitters for long lifetime, low‐power consumption, and narrow emission band green OLEDs is developed. As a sensitizer, a well‐known phosphorescent emitter is used, fac‐tris(2‐phenylpyridinato‐C2,N)iridium(III) [Ir(ppy)3]. Consequently, these MR‐TADF emitters achieve hyper OLEDs with a power efficiency of 150 lm W−1, an external quantum efficiency of 27%, and a long lifetime LT95 of over 3000 h at high brightness of 1000 cdm−2. These performances are among the best in scientific literature.

Diffusion‐Free Intramolecular Triplet–Triplet Annihilation Contributes to the Enhanced Exciton Utilization in OLEDs

AbstractTriplet–triplet annihilation (TTA), or triplet fusion, is a biexcitonic process in which two triplet‐excited molecules can combine their energy to promote one into an excited singlet state. To alleviate the dependence of the TTA rate and yield on triplet diffusion in both solid and solution environments, intramolecular TTA (intra‐TTA) has been recently proposed in conjugated molecular systems able to hold multiple triplet excitons simultaneously. Developing from the previous demonstration of TTA performance enhancement in sensitized upconversion solutions, here similar improvements in triplet harvesting in solid‐state films are reported under electrical excitation in organic light emitting diodes (OLEDs). At low dye concentration and low current densities, the intra‐TTA active OLED shows a +40% improved external quantum efficiency with respect to the reference device, and a TTA spin‐statistical factor f 4DPA of 0.4, close to that determined in fluid solution for the individual chromophore (0.45). These results therefore indicate the utility of this molecular design strategy across a wider range of TTA applications, and with particular utility in the further development of low‐power TTA‐enhanced OLEDs.

Quantitative structure-property relationship of glass transition temperatures for organic compounds

The glass transition temperatures (T gs) of materials used in the manufacture of organic light-emitting diodes (OLEDs) determine their thermal stability. Three Dragon descriptors, TPC, RBF and TDB04s, were adopted to develop quantitative structure-property relationships (QSPR) for the prediction of the T gs of 66 compounds (Data Set I) for OLED application, by applying random forest (RF) and support vector machine (SVM). The RF Model A, based on a training set (44 compounds), was validated with a test set (22 compounds). The RF Model A possesses a coefficient of determination R 2 of 0.942 and a root mean square (rms) error of 10.750 K for the training set and R 2 of 0.909 and rms error of 11.102 K for the test set, which are more accurate than the results from the SVM model. The RF Model A was further validated with 63 OLED molecules (Data Set II). Moreover, the three Dragon descriptors (TPC, RBF and TDB04s) were used to build another T g QSPR model (named RF Model B) for a large dataset of 1934 OLED molecules (Data Set III), which achieved rms errors of 16.79 K for Data Set III, 22.89 K for Data Set I and 20.17 K for Data Set II.

Lithosphere infiltrometer: A low cost, automated infiltrometer (permeameter) for measurement of soil health, infiltration, and hydraulic conductivity

Accurate measurement of infiltration rate and hydraulic conductivity is important for assessing soil physical health, irrigation management and catchment modelling. Commercially available infiltrometers are often expensive, fragile, and require specialist knowledge to operate and perform complex post-measurement calculations. We developed the Lithosphere Infiltrometer (permeameter) to be usable for both experienced soil researchers, and users without prior technical or scientific expertise in soil science. The Lithosphere Infiltrometer is able measure both surface and subsoil infiltration rate and hydraulic conductivity. It is manufactured using 3D-printed parts and off-the-shelf electronic components. It has a robust, quad-leg design that allows for rapid leveling of the device and accurate setting of water depth via a novel 3D-printed bell chamber. The water level is measured using a U85 JRT laser with ±1 mm accuracy connected to an Adafruit Huzzah 32 microcontroller. Infiltration rate and hydraulic conductivity are calculated at 1 to 10 min time steps, and data is presented in real-time on an OLED display. Data is stored on the device and available for download to a mobile device by Wi-Fi.

Research Progress of Deep-Red to Near-Infrared Electroluminescent Materials Based on Organic Cyclometallated Platinum(II) Complexes.

In recent years, the near-infrared (NIR) light-emitting materials have attracted increasing attention due to the broad application prospects in the fields of military industry, aerospace, lighting, display and wearable devices. As the transition metal complexes, platinum(II) complexes have been shown to emit luminescence efficiently in NIR organic light-emitting diodes because of the unique d8 electron structure. This structure ensures that the platinum(II) complex molecules exhibit a high planarity, variety of excited states, and strong intermolecular interactions. This review summarizes the research progress of deep red to NIR organic light-emitting materials based on platinum(II) complexes in recent years and provides a certain reference for the further design and synthesis of NIR platinum(II) complex luminescent materials with superior performance.

High-Performance Multicolor Organic Light-Emitting Diodes Based on a Pt(II) Carbene Complex Featuring Hemiligand Interaction.

Utilizing a single organic light-emitting diode (OLED) architecture for multicolor emissions can significantly simplify manufacturing progress and broaden applications. Here, we report on a carbene-based Pt(II) complex, designated as Pt(pyiOppy), which exhibits an unusual dimeric packing mode solely by hemiligand π···π stacking. This feature is distinct from the well-known Pt···Pt or Pt···ligand interactions. The dimer persists in new types of orbital combinations, along with its triplet transition state, which are evidenced for the first time. Pt(pyiOppy), under various doping concentrations in a solid matrix, demonstrates multicolor emissions ranging from green to red, all exhibiting high photoluminescent quantum efficiencies (48-97%). The devices incorporating Pt(pyiOppy) can emit green, yellow, orange, and red lights, covering a CIE coordinate range of (0.28-0.65, 0.61-0.34). All the devices also achieve appreciable maximum external quantum efficiencies (9.4-17.2%) and impressive lifetimes of hundreds of hours (LT70 at 1000 cd/m2). These findings showcase a new type of Pt(II) aggregate enabling well-controlled, multicolor high-performance phosphorescent OLEDs.

Switchable Topologically Chiral [2]Catenane as Multiple Resonance Thermally Activated Delayed Fluorescence Emitter for Efficient Circularly Polarized Electroluminescence.

Aiming at the fabrication of circularly polarized organic light-emitting diodes (CP-OLEDs) with high dissymmetry factors (gEL) and color purity through the employment of novel chiral source, topologically chiral [2]catenanes were first utilized as the key chiral skeleton to construct novel multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters. Impressively, the efficient chirality induction and unique switchable feature of topologically chiral [2]catenane not only lead to a high |gPL| value up to 1.6 × 10-2 but also facilitate in situ dynamic switching of the full-width at half-maximum (FWHM) and circularly polarized luminescence (CPL). Furthermore, the solution-processed CP-OLEDs based on the resultant topologically chiral emitters exhibit reveal narrow FWHM of 36 nm, maximum external quantum efficiency of 17.6%, and CPEL with |gEL| of 2.1 × 10-3. This study demonstrates the successful construction of the first CP-MR-TADF emitters based on topological chirality with the highest |gPL| among the reported CP-MR-TADF emitters and excellent device performance to the best of our knowledge. Moreover, it endowed the MR-TADF emitter with distinctive switchable CPL performances, thus providing a novel design strategy as well as a promising platform for developing intelligent CP-OLEDs.

The Influence of Thickness and Spectral Properties of Green Color-Emitting Polymer Thin Films on Their Implementation in Wearable PLED Applications.

A systematic investigation of optical, electrochemical, photophysical, and electrooptical properties of printable green color-emitting polymer (poly(9,9-dioctylfluorene-alt-bithiophene)) (F8T2) and spiro-copolymer (SPG-01T) was conducted to explore their potentiality as an emissive layer for wearable polymer light-emitting diode (PLED) applications. We compared the two photoactive polymers in terms of their spectral characteristics and color purity, as these are the most critical factors for wearable lighting sources and optical sensors. Low-cost, solution-based methods and facile architecture were applied to produce rigid and flexible light-emitting devices with high luminance efficiencies. Emission bandwidths, color coordinates, operational characteristics, and luminance were also derived to evaluate the device's stability. The tuning of emission's spectral features by layer thickness variation was realized and was correlated with the interplay between H-aggregates and J-aggregates formations for both conjugated polymers. Finally, we applied the functional green light-emitting PLED devices based on the two studied materials for the detection of Rhodamine 6G. It was determined that the optical detection of the R6G photoluminescence is heavily influenced by the emission spectrum characteristics of the PLED and changes in the thickness of the active layer.

Inverted All-Inorganic Nanorod-Based Light-Emitting Diodes via Electrophoretic Deposition.

High performance and high stability in all-inorganic solution processed nanocrystal-based light-emitting diodes (LEDs) are highly attractive for large area devices compared to organic material-based LEDs. In this work, an inverted all-inorganic LED structure is designed to have an easy integration with thin-film transistors. Adopting robust inorganic materials such as Ni1-x O nanoparticle films as a hole transport layer (HTL) is beneficial for the performance of LED. Herein, we have optimized the HTL by introducing Mg into Ni1-x O to bridge the difference in energy offset between the nanorod emissive layer and the HTL, in addition to the advantages of low temperature solubility of Ni1-x O:Mg nanoparticles. Furthermore, CdSe/CdS-based nanorods via electrophoretic deposition (EPD) are amassed in a vertically aligned (VA-NR) fashion as an emissive layer to facilitate the carrier transportation. Fostering these approaches enabled an EQE of 1.2% of the fabricated device, establishing the viability for further development of efficient and highly stable nanocrystal-based LEDs.

Dendritic Oligomers‐Based Exciplex Emitters Realizing Solution‐Processed Organic Light‐Emitting Diodes with Nearly 30% External Quantum Efficiency

AbstractDue to the feature of the strong intermolecular interactions causing aggregation‐caused excitons quenching (ACQ) of solution process, realizing high‐performance solution‐processed organic light‐emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters remains a formidable challenge nowadays. To address this issue, herein, a novel strategy is proposed to construct solution‐processable ternary exciplex emitters by utilizing the dendritic oligomer as the substrate, simultaneously realizing excellent solution‐process adaptability, good dispersibility to restrain harmful ACQ, and improved exciton utilization with multiple reverse intersystem crossing channels. Accordingly, dendritic oligomer‐based exciplex systems are constructed by combining small‐molecule donor and acceptor components with the multi‐carbazole‐substituted tetraphenylsilane‐core analog as substrate. And their film morphologies, photoluminescence quantum yields (ΦPLs), and non‐radiative decay rates of triplet excitons (knrTs) are significantly improved. Among them, DMAC‐DPS:PO‐T2T:SimCP3 exhibits the highest ΦPL of 97.5% and the lowest knrT of 1.0 × 103 s−1, and realizes the best maximum external quantum efficiency of 29.7% in solution‐processed OLED, which is comparable with the highest electroluminescence efficiencies of all solution‐processable emitters. Therefore, this work will provide a prospective pathway to develop efficient solution‐processable exciplex emitters and promote the progress of solution‐processed OLEDs.

The Rise of Tandem Perovskite Light-Emitting Diode.

In 2024, tandem perovskite light-emitting diodes (tandem-PLEDs) achieved a breakthrough external quantum efficiency of 43.42%, with an organic electroluminescence (EL) unit stacked atop a perovskite EL unit, surpassing the previous single-junction perovskite LEDs. This innovative design enables a higher brightness at lower currents, enhancing the longevity and efficiency of the tandem-PLEDs. Additionally, the tandem-PLEDs can also be fabricated by combining a perovskite EL unit with a perovskite quantum dot unit. In this perspective, the key advancements in tandem-PLEDs are highlighted, focusing on the development of perovskite-organic materials, perovskite-perovskite quantum dots, and the design principles for obtaining efficient and stable charge generation layers. But more importantly, the challenges and solutions are discussed in fabricating all-perovskite tandem LEDs using strongly polar solvents that have yet to be reported nowadays. This comprehensive guide aims to support researchers in advancing the practical deployment of tandem-PLED technology.

Defect Engineering in ZnO and Ni-Doped ZnO Nanostructures for Efficient Organic Light Emitting Diode (OLED) Application

Defect Engineering in ZnO and Ni-Doped ZnO Nanostructures for Efficient Organic Light Emitting Diode (OLED) Application

Characteristics Study of OLED Materials

Abstract Organic Light Emitting Diodes (OLEDs) are emerging as promising alternatives to conventional lighting and display technologies due to their unique properties and potential energy efficiency. This study conducts a comprehensive investigation into OLED materials, focusing on their optical, electrical, and structural characteristics. Through experimental analysis and theoretical modeling, the study delves into the performance and behaviour of various OLED materials, including organic semiconductors, charge transport layers, and emissive materials. Critical aspects such as charge carrier mobility, exciton formation, and device stability are examined to understand the mechanisms governing OLED operation. The findings offer valuable insights into optimizing OLED devices for applications ranging from consumer electronics to lighting and signage. This research contributes fundamental knowledge and practical guidelines for material selection, device fabrication, and performance assessment, thereby advancing OLED technology. The insights provided pave the way for future innovations in OLED materials and device design, potentially leading to improvements in efficiency, brightness, and longevity.

Enhanced Emitting Dipole Orientation Based on Asymmetric Iridium(III) Complexes for Efficient Saturated-Blue Phosphorescent OLEDs.

Three novel asymmetric Ir(III) complexes have been rationally designed to optimize their emitting dipole orientations (EDO) and enhance light outcoupling in blue phosphorescent organic light-emitting diodes (OLEDs), thereby boosting their external quantum efficiency (EQE). Bulky electron-donating groups (EDGs), namely: carbazole (Cz), di-tert-butyl carbazole (tBuCz), and phenoxazine (Pxz) are incorporated into the tridentate dicarbene pincer chelate to induce high degree of packing anisotropy, simultaneously enhancing their photophysical properties. Angle-dependent photoluminescence (ADPL) measurements indicate increased horizontal transition dipole ratios of 0.89 and 0.90 for the Ir(III) complexes Cz-dfppy-CN and tBuCz-dfppy-CN, respectively. Analysis of the single crystal structure and density functional theory (DFT) calculation results revealed an inherent correlation between molecular aspect ratio and EDO. Utilizing the newly obtained emitters, the blue OLED devices demonstrated exceptional performance, achieving a maximum EQE of 30.7% at a Commission International de l'Eclairage (CIE) coordinate of (0.140, 0.148). Optical transfer matrix-based simulations confirmed a maximum outcoupling efficiency of 35% due to improved EDO. Finally, the tandem OLED and hyper-OLED devices exhibited a maximumEQE of 44.2% and 31.6%, respectively, together with good device stability. This rational molecular design provides straightforward guidelines to reach highly efficient and stable saturated blue emission.

Pyrene-encapsulated light-emitting π-conjugated polymer with excellent ozone tolerance capacity for large-area and flexible ultra-deep-blue PLEDs with CIEy = 0.08

Emerging flexible polymer light-emitting diodes (PLEDs) are crucial for the development of information display and solid lighting. However, achieving ultrastable light-emitting polymers with robust emissions and outstanding chem-physic stabilities, remains a serious challenge due to the intrinsic oxidative and degrading properties of aromatic units. Herein, we present a universal and convenient pyrene-encapsulation strategy to enhance the ozone tolerance capacity of deep-blue light-emitting conjugated polymers (PPyDPF) for the narrowband flexible PLEDs. More intestingly, compared to the comparison polymers, PPyDPF showed a lower free volume ratio of 71.78 %, facilitating dense interchain packing, which contributes to its excellent ozone tolerance capacity. Large-area PLEDs were fabricated, which exhibit ultra-deep-blue emission with a FWHM of 40 nm and Commission internationale de l’éclairage (CIE) coordinates of (0.16, 0.08). The demonstrated potential for application in light-emitting devices underscores the significance of this approach, providing an effective strategy for developing aging-resistant light-emitting conjugated polymers for optoelectronic applications.

Machine Learning-Driven precise design of stable OLED Materials: Predicting and enhancing Multi-State C-N bond dissociation energies

The intrinsic stability of organic light-emitting diode (OLED) materials critically hinges on the bond dissociation energies (BDEs) of fragile bonds, particularly C-N bonds. Despite the necessity of considering BDEs in multiple electronic states due to the complex working conditions of organic materials in OLED devices, comprehensive exploration remains challenging. Herein, we constructed a high-quality dataset, CNBDE-24, comprising nearly 1500 data points for BDEs across multiple electronic states, obtained through an active learning-guided density functional theory (DFT) calculation strategy. Machine learning models trained on the CNBDE-24 dataset exhibit excellent accuracy in predicting BDEs for OLED materials in neutral, triplet excited (T1), and anionic states, achieving an adjusted coefficient of determination exceeding 0.90 on the test set and bypassing the need for high-cost calculation. Moreover, utilizing neutral state BDEs as pre-training data significantly reduces model error and provides deeper insights into the relationships between different BDEs. We find that BDEs in neutral are influenced by the electronic properties of the nitrogen atom, as indicated by descriptors such as Gasteiger charges. Incorporating C-N bonds into aromatic systems and slightly dispersing N atom charges can inherently increase multi-state BDEs without altering luminescent properties. Additionally, anionic BDEs are determined by the extent of charge transfer during bond cleavage and can be regulated by modifying the C-side fragment. High-throughput virtual screening reveals a new stable purple emitter with a dominant 0–0 transition at 360 nm, which was validated through DFT and excited-state property evaluations. Strategies for enhancing BDEs in multiple resonance thermally activated delayed fluorescence materials without altering excited-state properties are also demonstrated.

An efficient thermally activated delayed fluorophore featuring an oxygen-locked azaryl ketone acceptor

Thermally activated delayed fluorescence (TADF) dyes are promising for applications like bioimaging, sensing, and optoelectronic devices due to their efficient exciton utilization without noble metals, making them ideal for organic light-emitting diodes (OLEDs). In this contribution, we introduce a new TADF emitter, 3-(9,9-dimethylacridin-10(9H)-yl)-12H-chromeno[2,3-b]quinolin-12-one (CQLO-DMAC), which incorporates an intramolecular oxygen-locked azaryl ketone acceptor, CQLO. This design enhances molecular rigidity and electron-accepting strength. The CQLO-DMAC compound, featuring a bulky donor, shows significant intramolecular charge transfer and TADF emission. Comprehensive analyses, including spectroscopic studies and theoretical calculations, reveal high luminescence efficiency and reduced non-radiative losses. Utilizing CQLO-DMAC in OLEDs, we achieved a green-yellow emission peak at 548 nm and a maximum external quantum efficiency of 17.4%. The findings highlight the potential of CQLQ as a highly effective acceptor in designing of TADF electroluminescent dyes.