White Organic Light-emitting Diodes Research Articles

Versatile Molecular Structure Strategy Toward Highly Efficient Single‐Component White Organic Light–Emitting Diodes

AbstractLuminophores' dual emission (DE) properties hold great potential for realizing single‐component white organic light–emitting diodes (WOLEDs). This study illustrates that the unique and vibrant DE phenomena with different luminous mechanisms can be formed through simple modulation of molecular structures. Four target luminophores, namely 2‐TPE‐PPI, 2‐TPE‐PI, 2‐TPE‐An‐PPI, and 2‐TPE‐An‐PI, capable of DE under different conditions, are intentionally designed and successfully synthesized. Owing to the inherent flexibility of the minor molecular backbone and minor steric hindrance, 2‐TPE‐PPI and 2‐TPE‐PI exhibit DE spectra in dilute solutions with different solvent polarities. The intrinsic cause of the DE phenomenon in 2‐TPE‐An‐PPI and 2‐TPE‐An‐PI arises from the localized distribution of frontier molecular orbits resulting from the presence of an anthracene unit and the formation of an exciter group through intermolecular interactions involving anthracene. Remarkably, single‐emissive‐layer WOLEDs based on 2‐TPE‐An‐PPI and 2‐TPE‐An‐PI demonstrate stable white emission with CIE coordinates at (0.33, 0.39) and (0.30, 0.39), respectively, closely approaching the CIE coordinates of standard white light. Moreover, they maintain stable EL spectra from 4 to 10 V, an exceptional attribute rarely observed in many white light devices.

Pure white emission full thermally activated delayed fluorescence organic light emitting diode with a supplementary emission layer

In addition to efficiency and lifetime, spectral stability, high color rendering index (CRI), and the Commission Internationale de l’Eclairag (CIE) coordinate close to the equi-energy white point (0.33, 0.33) are also important parameters for the development of white organic light emitting diodes (WOLEDs). However, how to realize these advantages at the same time is a challenge. In this paper, a series of white devices based on complementary color full thermally activated delayed fluorescence (full-TADF) emitters were fabricated with the aim to obtain WOLEDs with high color rendering index and CIE coordinate near the equi-energy white point. Through optimizing the doping concentration of the emitters, adding a supplementary emission layer, modifying the thickness of the main emission layer and the supplementary emission layer, the best device displays the CIE coordinates of (0.34, 0.35) and the CRI of 86, exhibiting good white emission. The results in this paper may provide a feasible method for realizing complementary color full-TADF WOLEDs with pure white emission.

High‐Efficiency Hybrid White Organic Light‐Emitting Diodes with Extremely Low Efficiency Roll‐Off and Superior Color Stability Enabled by an Emitting System with Imidazole‐Biphenyl Derivatives

AbstractHybride white organic light‐emitting diodes (WOLEDs) have experienced great success as a prospective technology for the new generation lighting source. Nevertheless, WOLEDs exhibiting excellent effciency and good color stability at high operation brightness are still rare. Herein, a simple but efficient blue molecule, pyrenoimidazole‐biphenyl (PPyIM), which exhibits promising external quantum efficiency (EQE) of 7.6% and 7.2% at the luminescence of 1000 −and 5000 cd m−2 in nondoped blue OLED, is first developed. And then, phenanthroimidazole‐biphenyl (PPIM) with analogous molecular skeleton of PPyIM is chosen as the host of phosphorescent dye PO‐01, and the resulting doped device achieves excellent EQE of 26.5% at the high luminescence of 5000 cd m−2. Furthermore, adopting PPyIM as nondoped blue emissive layer, the two‐color hybrid WOLED exhibites stable warm white emission with CIE coordinates of (0.454, 0.439), color rendering index (CRI) of 45, and correlated color temperature (CCT) of 2996 K. The maximum EQE reaches 23.5%, and the EQE still maintains 21.2% even at the luminance of 5000 cd m−2. Notably, this device exhibits exceptionally stable warm white emission, the CIE coordinates variation is only (0.004, 0.003) in the wide luminance range from 400 to 10000 cd m−2, representing the best outcome for hybrid WOLEDs to date.

White Organic Light-Emitting Diodes with External Quantum Efficiency of 20.77% at 10000cdm-2 and Ultra-High Color Stability by Controlling Exciton Distribution.

Although brightness and efficiency have been continually improved, the inability to achieve superior efficiency, color stability, and low-efficiency roll-off simultaneously in white organic light-emitting diodes (OLEDs) remains a knotty problem restricting the commercial application. In this paper, emission balance for two different horizontal orientation emitting molecules is maintained by using hole transport materials and bipolar host materials to control carriers' recombination and exciton diffusion. Impressively, the obtained devices exhibit extremely stable white emission with small chromaticity coordinates variation of (0.0023, 0.0078) over a wide brightness range from 1000 to 50000cdm-2. Meanwhile, the optimal white OLED realizes the power efficiency, current efficiency, and external quantum efficiency up to 70.68lmW-1, 85.53cdA-1, and 24.33%, respectively at the practical brightness of 1000cdm-2. Owing to reduced heterogeneous interfaces and broadening recombination region, this device exhibits a high EQE over 20% under high luminance of 10000cdm-2, demonstrating slight efficiency roll-off. The operating mechanism of the device is analyzed by versatile experimental and theoretical evidences, which concludes precise manipulation of charges and excitons is the key points to achieve these excellent performances. This work provides an effective strategy for the design of high-performance white OLEDs.

Highly Efficient Hybrid White Organic Light‐Emitting Diodes with External Quantum Efficiency Exceeding 30% by Integrating Electron‐Trapping Sensitized Structure and Gradient‐Doping System

AbstractIn this work, efficient two‐color (blue and yellow) hybrid white organic light‐emitting diodes (WOLEDs) with high efficiencies are constructed by introducing iridium(III) bis(4,6‐(difluorophenyl) pyridinato‐N,C2’)picolinate (FIrpic) as electron sensitizer. Experimental results demonstrate that FIrpic with the deep lowest unoccupied molecular orbital (LUMO) level enables preferential electron trapping to collect excessive electrons, thus realizing carrier's balance and high recombination probability. The obtained cool WOLED exhibits the maximum external quantum efficiency (EQE), current efficiency (ηc), and power efficiency (ηp) of 31.05%, 76.36 cd A−1 and 69.92 lm W−1, respectively. Moreover, by gradient doping FIrpic into yellow EML, the generated forward built‐in electric field originating from gradient electrons distribution reduces carriers transport resistance and improves the transport of holes; the diffusion effect of gradient electrons distribution promotes electrons’ transport, thus enhancing the utilization probabilities of carriers and excitons. Besides, FIrpic acts also as an energy transfer ladder to facilitate the energy transfer from host to yellow emitter. Consequently, warm WOLED with enhanced yellow emission achieves the maximum EQE, ηc, and ηp of 33.27%, 75.66 cd A−1 and 88.04 lm W−1, respectively.

Exploration of OLED structures for high-resolution microdisplays with enhanced efficiency and color gamut

The subpixel size of high-resolution organic light-emitting diode (OLED) microdisplays measures a few micrometers. To achieve a full-color microdisplay, the current preference is to use photolithography technology on a white OLED instead of using a fine metal mask for patterning. The application of photolithography techniques allows for the creation of microcavity structures or color filters (CFs) to enhance color purity in each emitted color. This paper focuses on investigating specialized OLED device structures for high-resolution microdisplays. The characteristics of each structure, including efficiency and color gamut, are thoroughly discussed. The structure, combining microcavity and CF, achieves the widest color gamut, while microcavity structures show higher efficiency compared to CF-only structures. Tandem structures are also considered for improved efficiency and stability.

Improving efficiency in RGB phosphorescent and white organic light emitting diodes via donor-spiro-boron acceptor host materials

The universal host material with boron acceptor core structure is exceptionally sparse to be designed for red, green, and blue in efficient phosphorescent organic light emitting diodes (PhOLEDs) accompanying white OLEDs, possessing indistinguishable device feature. Herein, two boron acceptors spiro-host materials namely 1'-(4-(dimesitylboranyl)phenyl)-10-phenyl-10H-spiro[acridine-9,9′-fluorene] (TPA-PBM) and 1-(4-(dimesitylboranyl)phenyl)-3′,6′-dimethylspiro[fluorene-9,8′-indolo[3,2,1-de]acridine] (2MeCz-PBM) were designed and synthesized. The designed rigid donor strategy exhibited good device performance among the reported boron donor-spiro-acceptor (D-spiro-A) skeleton for organic light emitting diodes (OLEDs). In particular, we fabricate RGB three-color phosphorescent OLEDs based on TPA-PBM and 2MeCz-PBM. The red devices exhibit a maximum external quantum efficiency (EQE) of nearly 28 % (26.8 % for TPA-PBM and 27.5 % for 2MeCz-PBM), with an electroluminescence spectrum at 608 nm. The green/blue devices obtain maximum EQEs of 21.2 %/15.3 % and 21.8 % 17.3 %, for TPA-PBM and 2MeCz-PBM, respectively. Furthermore, green devices display an extremely low efficiency roll-off with decreasing 1 % and 3 % at luminance of 5000 cd m−2. Additionally, the two-color white OLED using 2MeCz-PBM exhibits EQEmax and PEmax are 24.5 % and 62.9 lm W−1 respectively, the EQE remain 23.8 % at commercial lighting brightness of 1000 cd m−2. The WOLED also shows a stable white spectrum with CIE varying range of (0.41, 0.47) to (0.39, 0.46) at 100 cd m−2 to 15000 cd m−2. All obtained results confirmed that our targeted molecules have advantage of carrier balance and efficient charge recombination, which might replace many other commercial host materials.

Highly Horizontal Oriented Tricomponent Exciplex Host with Multiple Reverse Intersystem Crossing Channels for High-Performance Narrowband Electroluminescence and Eye-Protection White Organic Light-Emitting Diodes.

Despite multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters with small full-width at half maximum are attractive for wide color-gamut display and eye-protection lighting applications, their inefficient reverse intersystem crossing (RISC) process and long exciton lifetime induce serious efficiency roll-off, which significantly limits their development. Herein, a novel device concept of building highly efficient tricomponent exciplex with multiple RISC channels is proposed to realize reduced exciton quenching and enhanced upconversion of nonradiative triplet excitons, and subsequently used as a host for high-performance MR-TADF organic light-emitting diodes (OLEDs). Compared with traditional binary exciplex, the tricomponent exciplex exhibits obviously improved photoluminescence quantum yield, emitting dipole orientation and RISC rate constant, and a record-breaking external quantum efficiency (EQE) of 30.4% is achieved for tricomponent exciplex p-PhBCzPh: PO-T2T: DspiroAc-TRZ (50: 20: 30) based OLED. Remarkably, maximum EQEs of 36.2% and 40.3% and ultralow efficiency roll-off with EQEs of 26.1% and 30.0% at 1000cd m-2 are respectively achieved for its sky-blue and pure-green MR-TADF doped OLEDs. Additionally, the blue emission unit hosted by tricomponent exciplex is combined with an orange-red TADF emission unit to achieve a double-emission-layer blue-hazard-free warm white OLED with an EQEmax of 30.3% and stable electroluminescence spectra over a wide brightness range.

67‐1: High‐performance Tandem White OLED Microdisplays for Virtual Reality and Mixed Reality

To achieve wide‐gamut and high‐efficiency tandem white OLED (WOLED) microdisplays, we propose a new structure leveraging from the high‐order antinodes and patterned microcavities. The color gamut coverages of 95% Rec. 2020 and 92% Rec. 2020 can be achieved in B/G/R WOLED with a moderate microcavity and B/YG WOLED with a strong microcavity, respectively. We have also boosted the optical efficiency by 62% by optimizing the B/YG WOLED structure. Such a WOLED microdisplay helps to reduce the power consumption of virtual reality (VR) and mixed reality (MR) displays while keeping a wide color gamut.

Advances in Host-Free White Organic Light-Emitting Diodes Utilizing Thermally Activated Delayed Fluorescence: A Comprehensive Review.

The ever-growing prominence and widespread acceptance of organic light-emitting diodes (OLEDs), particularly those employing thermally activated delayed fluorescence (TADF), have firmly established them as formidable contenders in the field of lighting technology. TADF enables achieving a 100% utilization rate and efficient luminescence through reverse intersystem crossing (RISC). However, the effectiveness of TADF-OLEDs is influenced by their high current density and limited device lifetime, which result in a significant reduction in efficiency. This comprehensive review introduces the TADF mechanism and provides a detailed overview of recent advancements in the development of host-free white OLEDs (WOLEDs) utilizing TADF. This review specifically scrutinizes advancements from three distinct perspectives: TADF fluorescence, TADF phosphorescence and all-TADF materials in host-free WOLEDs. By presenting the latest research findings, this review contributes to the understanding of the current state of host-free WOLEDs, employing TADF and underscoring promising avenues for future investigations. It aims to serve as a valuable resource for newcomers seeking an entry point into the field as well as for established members of the WOLEDs community, offering them insightful perspectives on imminent advancements.

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.

Tailored efficient and reliable double luminescent layer hybrid WOLEDs via doping engineering

Doping engineering has been widely utilized to increase the efficiency of White organic light-emitting diodes (WOLEDs). In this study, a blue phosphor material named DMAC-DPS and an orange phosphor material named PO-01 are integrated into the host materials Bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) and carbazole-based 4,4′-biscarbazole-p-biphenyl (CBP) by incorporating the principle of complementary color luminescence, resulting in a doped double-luminescent layer hybrid WOLED. The developed device structure consists of ITO/MoO3/TCTA/DPEPO:DMAC-DPS/CBP:PO-01 (or CBP:PO-01/DPEPO:DMAC-DPS)/TAZ/Alq3/LiF/Al. The transfer of energy between the host and guest materials is achieved by controlling the thickness and position of the emitting layer, leading to a more balanced emission of blue and yellow light and an overall increase in device efficiency. The developed WOLED exhibits a maximum current efficiency of 26.8 cd A−1, a power efficiency of 16.8 lm W−1, and an external quantum efficiency of 10.95%. The stable color coordinates of the device remains consistent, varying from (0.34, 0.40) to (0.33, 0.39) at brightness levels ranging from 100 to 1000 cd m−2. Technically, the incorporation of blue and orange phosphor materials into the host materials DPEPO and CBP, respectively, resulting in a doped double-luminescent layer hybrid WOLED, has shown a more balanced emission of blue and yellow light and resulted in increased efficiency. The reliable color coordinates corroborate the good color stability, making it a promising candidate for various applications. Furthermore, the controlled transfer of energy between the host and guest materials has led to a more balanced emission of blue and yellow light. Our developed doping engineering methods have shown potential for increased efficiency and good color stability, making the developed WOLED a promising candidate for various applications.

A Breakthrough in Color‐Tunable All‐Fluorescent White OLEDs through the Cooperation Between a New Blue HLCT Fluorophore and a Long‐Wavelength TADF Emitter

AbstractIt is a vitally important and challenging task to achieve high efficiency and wide color‐tunable white organic light emitting diodes (CT‐WOLEDs) meeting the needs of various decoration and lighting applications. Here, a high‐performance single‐cell all‐fluorescent CT‐WOLED with a single‐doped single‐emissive‐layer structure has been developed employing a multi‐functional hybrid local and charge transfer (HLCT) fluorophore 2‐(4‐(9H‐carbazol‐9‐yl)−2′,5′‐difluoro‐[1,1′:4′,1′'‐terphenyl]−4‐yl)−1‐phenyl‐1H‐phenanthro[9,10‐d]imidazole (PICZ2F) as a deep‐blue emitter as well as an effective host for a yellow thermally activated delayed fluorescence emitter. The record‐breaking power efficiency of 89.50 lm W‒1 and correlated color temperature (CCT) span from 3749 to 18 279 K are achieved simultaneously, which is not only one of the state‐of‐the‐art CT‐WOLEDs reported so far, but also comparable to the best values from all‐fluorescent white devices with a single‐emissive‐layer. Moreover, by using PICZ2F as an emitter, an external quantum efficiency (EQE) of 7.10% with the Commission Internationale de lEclairage (CIE) coordinates of (0.155, 0.068) is achieved in the doped device, and a low‐efficiency roll‐off even at a high luminance of 15 000 cd m−2 (EQE15000: 5.80%) is achieved in the non‐doped device. Overall, the findings provide a promising strategy to develop simple but efficient CT‐WOLEDs.

Development of white organic light emitting diodes based on carbazole-derived compounds

The object of research is the thermal, photophysical, and electrophysical properties of newly synthesized carbazole-derived compounds and organic light-emitting structures based on them. The problem consists in the comprehensive solution of scientific and technical problems of improving the characteristics of white organic light-emitting diodes (OLED), expanding their emission spectrum, improving color and energy characteristics. The results of the thermal, electrophysical and photophysical properties of the investigated carbazole compounds were obtained. They demonstrated good thermal stability. Absorption spectra in solid films were recorded in the range of 300–350 nm. Photoluminescence spectra were observed at a wavelength of 407 nm for the first and second compounds and 430 nm for the third. The quantum yield of photoluminescence in films for compounds 1, 2, and 3 was 16 %, 7 %, and 7 %, respectively. Organic light-emitting structures of white emission color with color coordinates (0.31, 0.35), (0.32, 0.34) and (0.38, 0.34) close to natural white light (0.33, 0.33) were formed using the thermovacuum sputtering method. The turn-on voltage of the white OLED is 6 V, the maximum brightness of the light-emitting structures was 10,000 cd/m2. The devices demonstrated a sufficiently high external quantum efficiency of 5 % to 7 %. The obtained results are explained by the mixing of different types of electroluminescence, namely excitonic and electromeric. Electromeric radiation is obtained due to transport layers. This approach improves such an important parameter of white light as its quality, which includes color coordinates and color rendering index. Due to their color characteristics, white light-emitting diodes based on carbazole-derived compounds are promising candidates for use in modern lighting systems. A separate advantage of these light-emitting structures is the dependence of the color gamut of their radiation on the applied voltage. In addition, organic LEDs based on carbazole-derived compounds have low energy consumption and are environmentally friendly due to the absence of toxic substances in their design, which creates prerequisites for both global energy savings and a reduction of the industrial burden on the environment.

High-efficiency crystalline white organic light-emitting diodes

Crystalline white organic light-emitting diodes (C-WOLEDs) are promising candidates for lighting and display applications. It is urgently necessary, however, to develop energy-saving and high-efficiency C-WOLEDs that have stable and powerful emission to meet commercial demands. Here, we report a crystalline host matrix (CHM) with embedded nanoaggregates (NA) structure for developing high-performance C-WOLEDs by employing a thermally activated delayed fluorescence (TADF) material and orange phosphorescent dopants (Phos.-D). The CHM-TADFNA-D WOLED exhibit a remarkable EQE of 12.8%, which is the highest performance WOLEDs based on crystalline materials. The device has a quick formation of excitons and a well-designed energy transfer process, and possesses a fast ramping of luminance and current density. Compared to recently reported high-performance WOLEDs based on amorphous material route, the C-WOLED achieves a low series-resistance Joule-heat loss ratio and an enhanced photon output, demonstrating its significant potential in developing the next-generation WOLEDs.

Highly efficient fluorescent and hybrid white organic light-emitting diodes based on a bimolecular excited system: publisher's note.

This publisher's note contains a correction to Opt. Lett.48, 5771 (2023)10.1364/OL.506371.

High-Performance Tandem White Micro-OLEDs for Virtual Reality and Mixed Reality Displays

To achieve wide-gamut and high-efficiency tandem white OLED (WOLED) microdisplays, we propose a new structure leveraging high-order antinodes and patterned microcavities. The color gamut coverages of 95% Rec. 2020 and 92% Rec. 2020 can be achieved in B/G/R tandem WOLED with a moderate microcavity and B/YG tandem WOLED with a strong microcavity, respectively. We have also boosted the optical efficiency by 62% for the tandem B/YG WOLED using the high-order antinodes at optimal conditions. Such a WOLED microdisplay helps reduce the power consumption of virtual reality (VR) and mixed reality (MR) displays while keeping a wide color gamut.

Light Extraction Efficiency Enhancement of White Organic Light-Emitting Diodes (OLEDs) by Micro/Nano-Patterning the Substrate Layer

We demonstrate the enhancement of light extraction efficiency of surface-emitting white Organic Light Emitting Diodes (OLEDs) by incorporating micro/nano-metric structures on the outer surface of the 3-layer substrate (SiO2-Si3N4-SiO2). To enhance light extraction efficiency of the OLEDs, various light scattering structures including plano-convex & plano-concave micro-lens array, flat-top & round-top nano-pillars array, and wavy structures were engraved on the outer surface of the substrate layer. For optimization, we varied the thickness of the internal layers of the OLEDs, and height, width, period, and radius of the micro/nano-scale structures. The performance of the micro/nano-structured OLEDs was simulated and analyzed using Lumerical FDTD and GPVDM simulators. We examined the far field light intensity, transmitted power, angular distribution of light, photon escape probability, photon density, internal & external quantum efficiency, and current-voltage curve of the designed OLEDs. We investigated the results in different locations, especially after the substrate layer: Far Field-1 (0 μm), and Far Field-2 (2.5 μm). Compared to conventional OLEDs, the micro/nano-structured OLEDs showed higher external quantum efficiency. The highest external quantum efficiency of 67.304% (Far Field-1) was detected in the round-top nano-pillars array engraved white OLED having structure period of 1.2 μm. We strongly believe that, the proposed micro/nano-structured white OLEDs are suitable for lighting applications.

Highly efficient white organic Light-Emitting diodes utilizing intermolecular interaction based on molecular geometry between host and deep blue Pt(Ⅱ) complex

Previous studies on white organic light-emitting diodes (WOLEDs) using a single Pt dopant relied on square-planar Pt complexes, exhibiting various emission characteristics due to ligand-centered π-π interactions. However, this approach limits the range of the materials selections to those with excimer or aggregation-induced emission (AIE) properties.In this study, a new method for creating WOLEDs with a single Pt dopant is proposed. We observed the emergence of an intermolecular interaction, specifically the formation of an exciplex due to π-π interaction between the n-host (ET) and the Pt complex, leading to a low-energy yellow emission. By varying the mixing ratio of ET and Pt complex, a spectrum of emissions ranging from pure blue to yellow was achievable. Consequently, we constructed 3-stack tandem devices, leveraging color variations based on the ET:Pt ratio. As a result, we successfully achieved a cool white emission with a Color Rendering Index (CRI) of 66.7, International Commission on Illumination (CIE) coordinates of (0.294, 0.380). Additionally, through optimization of the device structure, external quantum efficiency (EQE) was enhanced from 18.3% to 21.9%. This novel and simple device architecture adjusting intermolecular interaction based on molecular geometry between ET and Pt(II) complex provides a solution to materials limitations in WOLEDs.

Ternary donor/acceptor system for simple single-emitting-layer white organic light emitting diodes based pure exciplexes emission

Herein, we designed pure exciplex emission, white organic light emitting diodes (WOLED) with simple single-emitting-layer (S-EML) structure through a ternary donor/acceptor system. Three kinds of donor/acceptor materials in one emitting layer of mCP:PO-T2T(1:1): x% m-MTDATA or TAPC:TPBi (1:1): x% PO-T2T could produce the white emission with high energy blue and low energy orange exciplex emission. As a result, the pure exciplex WOLED based on S-EML structure were achieved successfully with the optimized maximum luminance of 4047 cd m−2, current efficiency of 10.26 cd A−1, power efficiency of 10.57 lm W−1 and EQE of 3.4% with the two complementary color exciplex emissions. To our knowledge, this is the first time to achieve a S-EML structure pure exciplex emission WOLED. The efficient energy transfer from high energy exciplex to low energy exciplex could be also observed in this ternary system.