Organic Light-emitting Diode Structure Research Articles

Theoretical insight on electronic and optical properties of some phosphorescent iridium(III) complexes for organic light-emitting diode devices

Organic light-emitting diode (OLED) structures have been extensively investigated because of their potential applications in various fields. It is known that organic or metal-mediated organic compounds are used in the layers of OLEDs. Within OLED materials, iridium(III) compounds have attract much attention owing to their many advantages. Therefore, we predicted the OLED behaviors of twenty-five phosphorescent iridium(III) complexes using theoretical chemical methods. All theoretical calculations were performed with the B3LYP hybrid functional using Gaussian 16 and Amsterdam Modeling Suite 2023 programs. While the 6–31G(d) basis set for non-metal atoms and LANL2DZ basis set for iridium metal was preferred in the computations carried out by using Gaussian program, whereas in the calculations performed with the Amsterdam Modelling Suite program, the TZP basis set was used for all atoms. From the theoretically obtained results, it is seen that Ir6, Ir7, Ir22-Ir25 complexes can be proposed as good candidate for hole injection layer materials in OLED based on indium tin oxide as an anode. Within complexes investigated, it has been determined that Ir16 and Ir17 complexes are the most suitable candidates for electron transport layer materials, whereas Ir16 complex can be used as both a hole transfer layer and ambipolar materials. Furthermore, it has been predicted that Ir4, Ir7, Ir8, and Ir23 complexes could be preferred as hole blocking layer compounds. From the detailed analysis of the singlet-triplet transitions of the complexes studied, it could be said that all iridium(III) complexes investigated could be used as highly efficient phosphorescent OLED molecules.

Spatially Resolved Functional Group Analysis of OLED Materials Using EELS and ToF-SIMS.

Electron energy-loss spectroscopy (EELS) is widely used in analyzing the electronic structure of inorganic materials at high spatial resolution. In this study, we use a monochromator to improve the energy resolution, allowing us to analyze the electronic structure of organic light-emitting diode (OLED) materials with greater precision. This study demonstrates the use of the energy-loss near-edge structure to map the nitrogen content of organic molecules and identify the distinct bonding characteristics of aromatic carbon and pyridinic nitrogen. Furthermore, we integrate EELS with time-of-flight secondary ion mass spectrometry for molecular mapping of three different bilayers composed of OLED materials. This approach allows us to successfully map functional groups in the by-layer OLED and measure the thickness of two OLED layers. This study introduces spatially resolved functional group analysis using electron beam spectroscopy and contributes to the development of methods for complete nanoscale analysis of organic multilayer architectures.

Hollow Microcavity Electrode for Enhancing Light Extraction.

Luminous efficiency is a pivotal factor for assessing the performance of optoelectronic devices, wherein light loss caused by diverse factors is harvested and converted into the radiative mode. In this study, we demonstrate a nanoscale vacuum photonic crystal layer (nVPCL) for light extraction enhancement. A corrugated semi-transparent electrode incorporating a periodic hollow-structure array was designed through a simulation that utilizes finite-difference time-domain computational analysis. The corrugated profile, stemming from the periodic hollow structure, was fabricated using laser interference lithography, which allows the precise engineering of various geometrical parameters by controlling the process conditions. The semi-transparent electrode consisted of a 15 nm thick Ag film, which acted as the exit mirror and induced microcavity resonance. When applied to a conventional green organic light-emitting diode (OLED) structure, the optimized nVPCL-integrated device demonstrated a 21.5% enhancement in external quantum efficiency compared to the reference device. Further, the full width at half maximum exhibited a 27.5% reduction compared to that of the reference device, demonstrating improved color purity. This study presents a novel approach by applying a hybrid thin film electrode design to optoelectronic devices to enhance optical efficiency and color purity.

Lithographic convex pattern as a wavelength-independent light extraction structure for efficient organic light-emitting diodes

Micrometer-scale ITO convex matrixes are used to improve the light extraction efficiency of OLEDs. Through optical manipulation, this scheme enhances the EQEs by up to 34.5% for blue, 22.4% for green, 28.6% for red, and 31.3% for NIR OLEDs.

MgF2 as an interlayer to enhance the stability of thermally activated delayed fluorescence based organic electroluminescence devices

Stability is one of the major challenges in organic semiconductor based optoelectronic devices. A comparative study of thermally activated delayed fluorescence (TADF) based organic light emitting diodes (OLEDs) with alkali-halide lithium fluoride (LiF) vs alkaline halide magnesium fluoride (MgF2) inorganic electron injection interlayers is presented. A TADF emitter 4CzIPN doped in CBP is used as an active layer (thickness = 15 nm @6wt. % doping) in an OLED structure: Glass/ITO/PEDOT:PSS/NPD/CBP/CBP:4CzIPN/TPBi/interlayer/Al. Prior to this comparative study, a separate exercise is carried out to obtain an optimal thickness of an MgF2 interlayer on the basis of leakage current and efficiency in the TADF-OLEDs. OLEDs with an LiF interlayer showed an external quantum efficiency (EQE) of 19.7% in comparison with an MgF2 interlayer-based OLED showed slightly lower average EQE ∼19.1% at a luminance level of 100 cd/m2; these efficiency numbers are averaged over ∼60 OLEDs. These slight changes in EQE are supported by the relative photoluminescence quantum yield measurements with a whole device stack. However, alkaline halide MgF2 based TADF-OLEDs showed approximately seven-fold enhancement in the stability (LT60) under identical operating conditions. In situ photoluminescence monitoring of operational TADF-OLEDs confirmed that the probable cause of reduced lifetime is degradation of an LiF/TPBi interface.

P‐120: Accelerated Lifetime Testing and Degradation Mechanisms of a Blue TADF OLED

Organic light‐emitting diodes (OLEDs) are a widespread commercial display technology but they are still subject of ongoing research with the aim to expand their application ranges through stack and material design. Operational stability is one of the most important key properties that are steadily being improved. The long lifetimes of several thousands of hours of state‐of‐the‐art OLED structures at standard operating conditions demand for accelerated lifetime tests (ALTs) and the use of appropriate scaling laws for elevated temperatures and current densities. Here we present ALTs for a blue thermally activated delayed fluorescent (TADF) OLED and demonstrate the successful determination of the scaling parameters. Moreover, we study the microscopic degradation mechanisms using in‐situ electrical characterization methods.

Study on Working Principle, Structure, Enhancement Technology, and Applications of Organic Light-emitting Diodes

Nowadays OLEDs outperform normal LEDs in terms of ease-processing, flexibility, skinniness, lightweight, and manufacturing cost. However, there is still much room to improve in terms of materials, efficiency, and longevity. Improving the performance of OLEDs has become the most popular research area. In order to make full use of excitons after recombination of carriers, phosphorescent OLED has been proposed. Recently, there is a gradual trend for phosphorescent OLEDs to be replaced by TADF OLEDs, as these TADF OLEDs can not only exhibit 100% internal quantum efficiency and are cheap to produce because they do not contain precious metals. At the same time, TADF OLEDs are considered to have more room for development especially in the aspect of longevity and color. Therefore, more researches are still needed to solve the blue OLED problem because compared to other colors, such as red and green OLEDs, blue OLEDs still have a big gap in stability and efficiency. In this work, the history of OLEDs, their working principle, the technologies that have been improved, and the cross-border derivative applications were studied.

Effects of Thermal Treatment on DC Voltage-Driven Color Conversion in Organic Light-Emitting Diode.

A DC voltage-dependent color-tunable organic light-emitting diode (CTOLED) was proposed for lighting applications. The CTOLED consists of six consecutive organic layers: the hole injection layer, the hole transport layer (HTL), two emission layers (EMLs), a hole blocking layer (HBL), and an electron transport layer (ETL). Only one metal-free phthalocyanine (H2Pc) layer with a thickness of 5 nm was employed as the EML in the CTOLED on a green organic light-emitting diode (OLED) structure using tris (8-hydroxyquinoline) aluminum (III) (Alq3). The current density-voltage-luminance characteristics of the CTOLEDs before and after thermal treatment were characterized and analyzed. Several Gaussian peaks were also extracted by multipeak fitting analysis of the electroluminescent spectra. In the CTOLED before thermal treatment, green emission was dominant in the entire voltage range from low to high voltages, and blue and infrared were emitted simultaneously and at relatively low intensities at low and high voltages, respectively. In the CTOLED after thermal treatment, the dominant color conversion from blue to green was observed as the applied voltage increased, and the infrared emission was relatively low over the entire voltage range. By simulating the CTOLED with and without traps at the H2Pc interface using a technology computer-aided design simulator, we observed the following: 1. After thermal treatment, the CTOLED emitted blue light by exciton generation at the H2Pc-HBL interface because of the small electron transport through the H2Pc thin film due to the dramatic reduction of traps in the low-voltage regime. 2. In the high-voltage regime, electrons reaching the HBL were transferred to Alq3 by resonant tunneling in two quantum wells; thus, green light was emitted by exciton generation at the HTL-Alq3 interface.

Correlation between optimized thicknesses of capping layer and thin metal electrode for efficient top‐emitting blue organic light‐emitting diodes

AbstractThe optical properties of the materials composing organic light‐emitting diodes (OLEDs) are considered when designing the optical structure of OLEDs. Optical design is related to the optical properties, such as the efficiency, emission spectra, and color coordinates of OLED devices because of the microcavity effect in top‐emitting OLEDs. In this study, the properties of top‐emitting blue OLEDs were optimized by adjusting the thicknesses of the thin metal layer and capping layer (CPL). Deep blue emission was achieved in an OLED structure with a second cavity length, even when the transmittance of the thin metal layer was high. The thin metal film thickness ranges applicable to OLEDs with a second microcavity structure are wide. Instead, the thickness of the thin metal layer determines the optimized thickness of the CPL for high efficiency. A thinner metal layer means that higher efficiency can be obtained in OLED devices with a second microcavity structure. In addition, OLEDs with a thinner metal layer showed less color change as a function of the viewing angle.

Approaches for Fabricating Tri‐ and Tetraphenylethene‐Based Blue Organic Light‐Emitting Diodes Using Donor–Acceptor and Non‐Donor–Acceptor Molecular Architectures

Light-Emitting Diodes Article number 2200206 by Samuthira Nagarajan and co-workers focuses on the design methodologies using triphenylethene- and tetraphenylethene-based non-donor–acceptor (non-D-A) and donor–acceptor (D-A) structures for non-doped blue organic light-emitting diodes (OLEDs). In the non-D-A system, triphenylamine and carbazole-based compounds demonstrate better performance, with the highest efficiency of 58300 and 5200 cd m–2. In the D-A system, imidazole-based compounds display the highest efficiency of 65150 cd m–2.

Hole Transport Layer Microroughening for Waveguide Mode Extraction in Organic Light‐Emitting Diodes

Herein, simple and scalable approach to enhance the out‐coupling efficiency in organic light‐emitting diodes (OLEDs) is presented. By exposing the 4,4′‐bis[N‐(1‐naphthyl‐1‐)‐N‐phenylamino]‐biphenyl (NPB) hole transport layer to hydrofluoroether (HFE) solvent for several minutes, a controllable microroughening of the film is achieved, which results in the formation of random scattering centers in the OLED structure. Optimized NPB microroughening for phosphorescent OLED with Ir(ppy)3 emitter leads to luminous efficacy improvement of 23% (from 64 to 79 lm W−1 at 1000 cd m−2) without additional external out‐coupling. Angular resolved spectroscopy of the OLED with an internal scattering layer reveals constant color almost independent of the viewing angle and Lambertian intensity distribution.

Cavity-Suppressing Electrode Integrated with Multi-Quantum Well Emitter: A Universal Approach Toward High-Performance Blue TADF Top Emission OLED

HighlightsA blue thermally activated delayed fluorescence top-emission organic light-emitting diode (TEOLED) structure that combines a cavity-suppressing transparent electrode and a multi-quantum-well emissive layer is proposed.The TEOLED exhibits high external quantum efficiency (18.05%), high color purity (full width at half maximum ~ 59 nm), reduced efficiency roll-off (~ 46% at 1000 cd m−2), and low angular dependence

Organic light-emitting diode structure for high color gamut in high-resolution microdisplay: Over 90% color gamut based on BT.2020

To implement primary colors in high-resolution organic light-emitting diode (OLED) microdisplays, color filters or cavity structures are applied to white OLEDs (WOLEDs). In terms of the high luminance and high color gamut required by microdisplays, two methods were experimentally compared in this study. In addition, a method for applying a color filter to WOLEDs with high-order cavity structures is proposed. The luminance of the proposed structure is lower than that of the cavity-only structure, but is similar to that obtained by applying a color filter to normal WOLEDs. Although previous methods do not satisfy the BT.2020 standard, the proposed structure achieves 90.8% of the BT.2020 standard.

Inverse design of organic light-emitting diode structure based on deep neural networks

Abstract The optical properties of thin-film light emitting diodes (LEDs) are strongly dependent on their structures due to light interference inside the devices. However, the complexity of the design space grows exponentially with the number of design parameters, making it challenging to optimize the optical properties of multilayer LEDs with rigorous electromagnetic simulations. In this work, we demonstrate an artificial neural network that can predict the light extraction efficiency of an organic LED structure in 30 ms, which is ∼103 times faster than the rigorous simulation in a single-treaded execution with root-mean-squared error of 1.86 × 10−3. The effective inference time per structure is brought down to ∼0.6 μs with unaltered error rate with parallelization. We also show that our neural networks can efficiently solve the inverse problem – finding a device design that exhibits the desired light extraction spectrum – within the similar time scale. We investigate the one-to-many mapping issue of the inverse problem and find that the degeneracy can be lifted by incorporating additional emission spectra at different observing angles. Furthermore, the forward neural network is combined with a conventional genetic algorithm to address additional large-scale optimization problems including maximization of light extraction efficiency and minimization of angle dependent color shift. Our approach establishes a platform for tackling computation-heavy optimization tasks with one-time computational cost.

Novel scattering and color converting substrates for simple-structured white organic light-emitting diodes

Organic light-emitting diodes (OLEDs) have shown great success in the display applications recently. However, the applications of OLEDs in lighting are still limited due to their complex device structures. Here, we developed a novel phosphor doped glass substrate with both high scattering and excellent color conversion capability to greatly simplify the device structures of white organic light-emitting diodes (WOLEDs). A simple-structured WOLED comprising a blue OLED and the scattering fluorescent substrate was demonstrated to realize high quality white light for lighting applications. The WOLED exhibits a turn-on voltage of 2.7 V, a maximum power efficiency of 29.8 lm/W, an external quantum efficiency (EQE) of 14.2%, a color rendering index (CRI) of 86, and a correlated color-temperature (CCT) of 3900 K. The low turn-on voltage can be attributed to the single emissive layer structure used in the WOLED. The high power efficiency as well as the high EQE are due to both the high color conversion efficiency and the high scattering capability of the fluorescent substrate. In addition, the WOLED is favorable for high-quality solid-state lighting in our daily life due to its high color rendering ability along with an adequate CCT CC. • A fluorescent substrate was proposed for simple structured WOLED with promising device efficiency and color rendering capability. • The function of the proposed fluorescent substrates was analyzed and discussed. • The WOLEDs utilizing the fluorescent substrates exhibited an EQE of 14.2% and CRI of 86.

Parameter optimization of light outcoupling structures for high-efficiency organic light-emitting diodes

Organic light-emitting diodes (OLEDs) have successfully entered the display market and continue to be attractive for many other applications. As state-of-the-art OLEDs can reach an internal quantum efficiency of almost 100%, light outcoupling remains one of the major screws left to be turned. The fact that no superior outcoupling structure has been found underlines that further investigations are needed to understand their prospect. In this paper, we use two-dimensional titanium dioxide block arrays as a model of an internal light outcoupling structure and investigate the influence of its geometrical parameters on achieving the highest external quantum efficiency (EQE) for OLEDs. The multivariable problem is evaluated with the visual assistance of scatterplots, which enables us to propose an optimal period range and the block width-to-distance ratio. The highest EQE achieved is 45.2% with internal and external structures. This work contributes to the highly desired prediction of ideal light outcoupling structures in the future.

Solid-State Properties and Spectroscopic Analysis of Thin-Film TPBi

We characterized the prominent electron transport layer 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) via single-crystal X-ray diffraction, grazing incidence X-ray diffraction (GIXRD), infrared reflection absorption spectroscopy (IRRAS), and quantum mechanical calculations. The crystals generated via vapor diffusion are of the orthorhombic space group Pbca, with a unit cell [a = 19.3935(2) b = 12.81750(10) c = 28.5610(3) Å] containing eight TPBi molecules, and screw axes and glide planes along all three crystallographic axes. Thin-film analysis becomes viable with unit cell and symmetry data, and GIXRD measurements, which demonstrate that when the amorphous TPBi thin films are annealed, the molecules preferentially orient with the a–b crystallographic face exposed at the surface and with the central benzene rings oriented 29° from the surface normal. Changes in vibrational modes at the surface, studied via infrared reflection absorption spectroscopy (IRRAS), concur with the X-ray based assignments. A minor conformer of TPBi with C3 symmetry was also identified via computational methods, appearing 0.95 kcal/mol higher in energy at the MP2/6-31G*//B3LYP/6-31G* level of theory. The combined structural insight allows fine-tuning of a device structure for organic light-emitting diodes (OLEDs) and organic photovoltaic applications.

Insights into Charge Transport in High-Efficiency Green Solution-Processed Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes with a Single Emitting Layer

Simple solution-processed structures of organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) have been demonstrated, but their efficiency and roll-off are ...

Ultra High-efficiency Integrated Mid Infrared to Visible Up-conversion System

In this paper, we have introduced and investigated an integrated optoelectronic chip for the up-conversion of mid-infrared to visible light. A thin layer of the nanocrystalline photoconductive PbSe is put on the Base of the NPN bipolar junction transistor and a doped phosphorescence organic light-emitting diode is placed on the Collector contacts. The incoming mid-infrared light is converted into an electric current by quantum dot photodetector, then amplified by the NPN bipolar junction transistor, and finally, the amplified current is driven through the Collector in the organic light-emitting diode. The organic light-emitting diode is designed to emit a green color. Our findings indicated that the proposed devices provide an up-conversion process from mid-infrared to visible light with a high-efficiency rate. The quantum dot photodetector is designed to detect 3 μm and also the organic light-emitting diode works at 523 nm. It is easy to tune the 3 ~ 5 μm incoming light by tuning the PbSe quantum dots, and the output light is tuned by tuning the organic light-emitting diode structure. Thus, the proposed structure is highly flexible regarding receiving mid-infrared and generating visible light. It is concluded that the external quantum efficiency for the proposed structure for 3 μm to 523 nm is 600. Also, the enhancement of the transistor current gain (β) can further increase the conversion efficiency of the proposed device. Moreover, different structures such as Darlington can be used instead of the bipolar junction transistor to enhance conversion efficiency.

Impact of pixel surface topography onto thin-film encapsulated top-emitting organic light-emitting diodes performances

Two different designs of top-emitting green OLEDs (Organic Light-Emitting Diodes) have been studied. The first one presents a planar OLED architecture. The second one presents an OLED having a topographic surface, so as to simulate a pixel partitioning of a display using an electrically insulating, 200 nm-thick, resist. It has been observed that the topography has a large impact on OLED performances. Studying devices using an ALD (Atomic Layer Deposition)-deposited Al2O3 barrier film shows that topographic OLEDs have a lower stability under storage in 65 °C/85% RH conditions compared to planar ones, with a difference in ageing models between the two devices. As the ALD deposition technology has a high conformity, which implies that ALD-deposited Al2O3 barrier films should be as good on topographic devices as on planar ones, we inferred that the topographic OLED Achilles’ heel lies rather in the OLED structure rather than in the Al2O3 encapsulation itself. Thus, topographic and planar unencapsulated OLEDs (without Al2O3) were studied: interestingly, it has been observed that planar OLEDs can live several weeks, while topographic OLEDs show a very short shelf lifetime (in laboratory atmosphere, at 21 °C/50% RH), of only a couple of hours. It will be shown that the topographic OLED surface tends to reduce the thickness of the PVD (Physical vapour Deposition)-deposited layers in the OLED, as this is expected for a non-conformal deposition PVD technique, on tapered angle regions of the resist (pixel edges). While this thickness variation would not be critical for thick electrodes, as for instance for bottom-emitting devices made on glass substrates, this thickness reduction turns out to be a critical point for the ultrathin, 15 nm, silver cathode, used as semi-transparent electrode in this top-emitting architecture and will therefore be discussed in the framework of using OLED top-emitting architectures in (micro)display technology.