Blue Organic Light-emitting Diodes Research Articles

Asymmetric anthracene host decorated with benzothienocarbazole for high efficiency blue organic light-emitting diodes

In this work, we describe an asymmetric anthracene-based 5-(4-(10-phenylanthracen-9-yl)phenyl)-5H-benzo [4,5]thieno[3,2-c]carbazole (ATDBT) for high efficiency doped and non-doped organic light-emitting diodes (OLEDs). Anthracene triggered triplet-triplet annihilation and structurally induced bipolar characteristics of the ATDBT significantly accelerated the triplet to singlet up-conversion process. The fabricated devices eventually achieved high efficiency with a small efficiency roll-off at high luminance. The doped device with ATDBT as the host has a maximum external quantum efficiency (EQE) of more than 8%, pure-blue emission, and maximum luminance of more than 40,000 cd/m2. The non-doped ATDBT device also showed a high EQE of 4.9%. As a result, ATDBT-type asymmetric molecular structures are vital for the development of high-efficiency doped and non-doped blue OLEDs.

Improved Thermal Stability and Operational Lifetime of Blue Fluorescent Organic Light-Emitting Diodes by Using a Mixed-Electron Transporting Layer

Improved Thermal Stability and Operational Lifetime of Blue Fluorescent Organic Light-Emitting Diodes by Using a Mixed-Electron Transporting Layer

Perovskite Quantum Dots for Emerging Displays: Recent Progress and Perspectives.

The excellent luminescence properties of perovskite quantum dots (PQDs), including wide excitation wavelength range, adjustable emission wavelength, narrow full width at half maximum (FWHM), and high photoluminescence quantum yield (PLQY), highly match the application requirements in emerging displays. Starting from the fundamental structure and the related optical properties, this paper first introduces the existing synthesis approaches of PQDs that have been and will potentially be used for display devices, and then summarizes the stability improving approaches with high retention of PQDs’ optical performance. Based on the above, the recent research progress of PQDs in displays is further elaborated. For photoluminescent display applications, the PQDs can be embedded in the backlighting device or color filter for liquid crystal displays (LCD), or they may function as the color conversion layer for blue organic light-emitting diodes (OLED) and blue micro-scale light-emitting diodes (μLED). In terms of next-generation electroluminescent displays, notable progress in perovskite quantum-dot light emitting diodes (PeQLED) has been achieved within the past decade, especially the maximum external quantum efficiency (EQE). To conclude, the key directions for future PQD development are summarized for promising prospects and widespread applications in display fields.

Quantum Dot-Based White Organic Light-Emitting Diodes Excited by a Blue OLED

In this study, white organic light-emitting diodes (OLEDs) consisting of red quantum dots (RQD) and green quantum dots (GQD) were investigated. These are the most exciting new lighting technologies that have grown rapidly in recent years. The white OLED development processes used consisted of the following methods: (a) fabrication of a blue single-emitting layer OLED, (b) nanoimprinting into QD photoresists, and (c) green and red QD photoresists as color conversion layers (CCL) excited by blue OLEDs. To fabricate the blue OLED, the HATCN/TAPC pair was selected for the hole injection/transport layer on ITO and TPBi for the electron transport layer. For blue-emitting material, we used a novel polycyclic framework of thermally activated delayed fluorescence (TADF) material, ν-DABNA, which does not utilize any heavy metals and has a sharp and narrow (FWHM 28 nm) electroluminescence spectrum. The device structure was ITO/HATCN (20 nm)/TAPC (30 nm)/MADN: ν-DABNA (40 nm)/TPBi (30 nm)/LiF (0.8 nm)/Al (150 nm) with an emitting area of 1 cm × 1 cm. The current density, luminance, and efficiency of blue OLEDs at 8 V are 87.68 mA/cm2, 963.9 cd/m2, and 1.10 cd/A, respectively. Next, the bottom emission side of the blue OLED was attached to nanoimprinted RQD and GQD photoresists, which were excited by the blue OLED in order to generate an orange and a green color, respectively, and combined with blue light to achieve a nearly white light. In this study, two different excitation architectures were tested: BOLED→GQD→RQD and BOLED→RQD→GQD. The EL spectra showed that the BOLED→GQD→RQD architecture had stronger green emissions than BOLED→RQD→GQD because the blue OLED excited the GQD PR first then RQD PR. Due to the energy gap architectures in BOLED-GQD-RQD, the green QD absorbed part of the blue light emitted from the BOLED, and the remaining blue light penetrated the GQD to reach the RQD. These excited spectra were very close to the white light, which resulted in three peaks emitting at 460, 530, and 620 nm. The original blue CIE coordinates were (0.15, 0.07). After the excitation combination, the CIE coordinates were (0.42, 0.33), which was close to the white light position.

High-Performance Narrowband Pure-Red OLEDs with External Quantum Efficiencies up to 36.1% and Ultralow Efficiency Roll-Off.

High-color-purity blue and green organic light-emitting diodes (OLEDs) have been resolved thanks to the development of B/N-based polycyclic multiple resonance (MR) emitters. However, due to the derivatization limit of B/N polycyclic structures, the design of red MR emitters remains challenging. Herein, a series of novel red MR emitters is reported by para-positioning N-π-N, O-π-O, B-π-B pairs onto a benzene ring to construct an MR central core. These emitters can be facilely and modularly synthesized, allowing for easy fine-tuning of emission spectra by peripheral groups. Moreover, these red MR emitters display excellent photophysical properties such as near-unity photoluminescence quantum yield (PLQY), fast radiative decay rate (kr ) up to 7.4 × 107 s-1 , and most importantly, narrowband emission with full-width at half-maximum (FWHM) of 32nm. Incorporating these MR emitters, pure red OLEDs sensitized by phosphor realize state-of-the-art device performances with external quantum efficiency (EQE) exceeding 36%, ultralow efficiency roll-off (EQE remains as high as 25.1% at the brightness of 50000cdm-2 ), ultrahigh brightness over 130000cdm-2 , together with good device lifetime.

Reducing Spontaneous Orientational Polarization via Semiconductor Dilution Improves OLED Efficiency and Lifetime

The spontaneous alignment of molecular dipoles that occurs in many organic light-emitting diodes (OLEDs) is known to blunt their performance, but is not easy to control within a given device stack. This study shows that coevaporating a small amount of polyethylene in the electron-transport layer of an OLED dramatically reduces the spontaneous orientational polarization, leading to increased device efficiency and lifetime due to decreased exciton-polaron annihilation in the emissive layer. This result highlights the potential of semiconductor dilution to improve OLED performance, and provides a new means to understand exciton-polaron-driven degradation in blue OLEDs.

Structurally modified [1,2,4]triazolo[1,5‑a]pyridine derivatives as promising materials for highly efficient blue fluorescent organic light-emitting diodes

Here, we designed and synthesized three donor–acceptor (D-A) emitters, 1-phenyl-2-(4-(2-phenyl-[1,2,4]triazolo[1,5-a]pyridin-6-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (m-PPI-TP), 1-phenyl-2-(4-(2-phenyl-[1,2,4]triazolo[1,5-a]pyridin-7-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (p-PPI-TP) and 1-phenyl-2-(4-(2-phenyl-[1,2,4]triazolo[1,5-a]pyridin-8-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (o'-PPI-TP), based on [1,2,4]triazolo[1,5-a]pyridine (TP) as electron-transporting unit and 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole (PPI) as hole-transporting moiety serving as emitting block concurrently. All three compounds were demonstrated to be thermally stable with decomposition temperatures (Td) in the range of 454–476 °C and high photoluminescence quantum yields over 86%. Besides, high exciton utilization efficiency (EUE) via “hot exciton” process was proved according to photophysical experiments and theoretical calculations analysis. What’s more, the structure–property relationships between different substituted sites of the TP building block and PPI unit were also unveiled. Finally, the non-doped blue and doped deep-blue OLEDs were fabricated with high external quantum efficiency (EQE) approaching 8%. The yellow and red phosphorescent OLEDs (PHOLEDs) based on m-PPI-TP as the host material achieved the best EL performance with EQE of 26.55% and 20.35%, respectively.

Improving the Efficiency of AIEgen-Based Nondoped Blue Organic Light-Emitting Diode by Rational Isomer Engineering

Improving the Efficiency of AIEgen-Based Nondoped Blue Organic Light-Emitting Diode by Rational Isomer Engineering

Zero-Zero Energy-Dominated Degradation in Blue Organic Light-Emitting Diodes Employing Thermally Activated Delayed Fluorescence.

Blue-emitting organic light-emitting diodes (OLEDs) fall significantly behind other OLEDs in operational stability. To better understand the key factors governing the stability of blue OLEDs employing thermally activated delayed fluorescence (TADF), nine efficient sky-blue to green TADF emitters with different frontier orbital energy levels and different TADF lifetimes have been designed and synthesized on the basis of charge-transfer (CT) acridine/phenyltriazine derivatives. Among them, ToDMAC-TRZ, a molecule composed of a 9,9-dimethyl-2,7-di-o-tolyl-9,10-dihydroacridine donor and a 2,4,6-triphenyl-1,3,5-triazine acceptor, shows a quantum yield of nearly 1 and a TADF lifetime as short as 0.59 μs in thin film. However, the stability of OLEDs is independent of the frontier orbital energy levels and TADF lifetime of the emitter. In contrast, the device half-life is found to decrease by five-sixths as the 0-0 energy of the singlet excitons increases by about 0.06 eV, which can be well-explained by the Arrhenius equation employing a photoreaction model. Whether in photoluminescence or electroluminescence, the contribution of long-lifetime triplet excitons to degradation is much lower than expected, which can be accounted for by how the solid-state solvation effect reduces the energy of the 3CT state and how most molecules have a low-lying locally excited triplet state.

Boron‐Based Multi‐Resonance TADF Emitter with Suppressed Intermolecular Interaction and Isomer Formation for Efficient Pure Blue OLEDs (Small 19/2022)

Pure Blue Organic Light-Emitting Diodes In article number 2107574, Jeong-Hwan Lee, Yun-Hi Kim, and co-workers use a strategic substitution of the bulky groups to effectively suppress the intermolecular interaction and the formation of isomers in mBP-DABNA-Me. Therefore, the mBP-DABNA-Me–based organic light-emitting diode exhibits an efficiency of 24.3% with a pure blue emission, color-coordinate of (0.124, 0.140). Notably, the substituents hinder the bathochromic shift, keeping the pure blue emission even at a high concentration of 25%.

High Efficiency of over 25% and Long Device Lifetime of over 500 h at 1000 nit in Blue Fluorescent Organic Light-Emitting Diodes.

In this study, a multiple resonance (MR) type blue emitter is synthesized, characterized, and evaluated for highly efficient and stable blue fluorescent organic light-emitting diodes (OLEDs). The MR blue fluorescent emitter has a di-tert-butyl benzene substituent in the MR core structure to minimize quenching mechanisms by intermolecular interaction. The emitter shows a high photoluminescence quantum yield and small full width at half maximum of 22 nm, which realize high external quantum efficiency (EQE) of 11.4% in the single unit OLED and device lifetime up to 95% of the initial luminance (LT95 ) of 208 h at 1000 cd m-2 and over 10000 h at 100 cd m-2 . The optimized tandem device of the new blue emitter achieves high EQE over 25% and extremely long LT95 of over 500h at 1000 cd m-2 and 30000 h at 100 cd m-2 . The lifetime of this work is one of the best data of blue OLED lifetime reported in the literature.

Interfacial charge transfer study of hexacarbonitrile-based intermediate connecter in blue tandem organic light-emitting diodes

The charge generation process of hexaazatriphenylene hexacarbonitrile-based intermediate connectors (ICs) in blue tandem organic light-emitting diodes (OLEDs) is investigated via photoelectron spectroscopic and device studies. Though 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN) is well-known to be a p-type hole-injection layer, our photoelectron spectroscopic results indicate that HATCN becomes the n-type when it is sandwiched between Li-doped 4,7-diphenyl-1,10-phenanthroline (BPhen) and N,N″-diphenyl-N,N″-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) layers, and the charge generation and separation at the Li:BPhen/HATCN/NPB interfaces become energetically favorable, as depicted by their energy level alignments. This explains the superior device performance when Li:BPhen/HATCN/NPB is used as a connecting unit in tandem devices.

Flexible White Lighting Fabricated With Quantum Dots Color Conversion Layers Excited by Blue Organic Light-Emitting Diodes

In this study, HATCN is coated on flexible PET/indium tin oxide (ITO) substrate as a modified layer and a hole injection layer to improve the hole injection from ITO. Then a blue organic light emitting diode (OLED) can successfully be fabricated without a surface treatment procedure (O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma or UV Ozone treatment). The new blue TADF series fluorescent material is employed as emitting layer with a luminescence wavelength of 456 nm. Also, it has a narrow full width at a half maximum (FWHM) of 26 nm featuring excellent color chromaticity. Quantum dot (QD) photoresist is a perfect color conversion material featuring good color adjustability, narrow emission spectrum, high luminous efficiency, and simple spin-coating processes. In this study, the photoresist mixed with green and red quantum dots is used as a color conversion layer (CCL), and a blue OLED is utilized to excite green and red CCL. To test the material of QD photoresist, it is coated on the substrate of another piece of glass first and then the blue OLED is utilized to remotely excite green and red QD CCL. The fluorescence characteristic of QD photoresist is explored to acquire a spectrum of 528 nm and 620 nm. In the device structure of a blue OLED, the electron-and hole-only current density is compared and the hole transport layer TAPC thickness is adjusted to improve the luminance and efficiency. Finally, green and red quantum dot photoresist is mixed using a proper ratio and then directly coated on the back of the PET/ITO substrate. Furthermore, the thickness of the QD photoresist is adjusted to increase the QD excited fluorescence. The blue OLED and QD CCL was integrated to generate three primary colors, i.e., blue, green, and red. Finally, a flexible white OLED lighting panel is successfully fabricated using simple processes. Moreover, it features high-spectrum stability. The CIE coordinates will not drift with bias, thus, it can resist the voltage variation.

High Steric‐Hindrance Windmill‐Type Molecules for Efficient Ultraviolet to Pure‐Blue Organic Light‐Emitting Diodes via Hybridized Local and Charge‐Transfer Excited‐State

AbstractHybridized local and charge‐transfer (HLCT) excited‐state compounds that enable full exciton utilization through a reverse intersystem conversion (RISC) from a high‐lying triplet to a singlet state have attracted attention. Developing high‐performance ultraviolet (UV) and blue organic light‐emitting diodes (OLEDs) is challenging due to difficulties acquiring HLCT molecules with a large energy bandgap and a high photoluminescence. Herein, a new strategy for excellent‐performance UV to blue emitters based on high steric‐hindrance windmill‐type structure is proposed. These emitters exhibit good thermal, morphological, and electrochemical stabilities, as well as HLCT excited‐state characteristics. Results suggest that OLED using CTPPI efficiently emits UV light (396 nm, CIEx,y = 0.16, 0.04) with a maximum external quantum efficiency (EQE) of 7.9% and currently ranks third in UV OLEDs. The lights of these devices are well modulated from UV/deep‐blue to pure‐blue with EQEs greater than 5% by regulating the locally excited (LE) and charge‐transfer (CT) components. Experimental and theoretical investigations indicate that harvesting triplet excitons afford high electroluminescence efficiencies via HLCT excited states in the devices. This study provides an efficient strategy to achieve high‐performance UV and blue OLEDs and offers great flexibility for material design.

Highly thin film with aerosol-deposited perovskite quantum dot/metal oxide composite for perfect color conversion and luminance enhancement

Erstwhile, displays using down-converting color changing medium utilized color filters as an essential part of clarifying the trade-off between true colors and transmittance. In this study, we report the development of a novel, smart process for producing thin luminescent films that completely convert colors and enhance light intensity. Highly dense films are prepared by the co-deposition of perovskite quantum dots and metal oxide particles as color conversion layers and scattering agents, respectively. These layers can be directly deposited on blue organic light-emitting diodes using ultrasonic-assisted aerosol deposition (UAD). This method can allow displays using a down-conversion system to streamline the process due to absence of color filters. Using this method, films can be a minimum of 3 µm thick for complete conversion of blue light, a quarter that of inkjet-printed films. We demonstrate 7,353 cd m-2 in green and 411 cd m-2 in red converted emissions with no blue light leakage. Furthermore, the developed technique was shown to be compatible with versatile patterning processes. For example, UAD may be combined with a fine-metal shadow mask to produce a pattern of 30-μm-diameter dots. Even without using a mask, the method still produces a pattern of 13 μm wide lines. This system provides a new approach that may serve as an alternative to inkjet printing.

TADF molecules with π-extended acceptors for simplified high-efficiency blue and white organic light-emitting diodes

<h2>Summary</h2> Carbazole-benzonitrile derivatives (CzBNs) are promising high-efficiency and stable blue thermally activated delayed fluorescence (TADF) materials in organic light-emitting diodes (OLEDs), which, however, strongly rely on multiple-donor structures. Here, we demonstrate a strategic implementation of extending the acceptor π-system with parapositioned π-conjugation tails in fewer donor structures, producing a simple and versatile approach to enhance performances of CzBNs. This molecular structure not only enhances TADF properties by delocalizing the frontier molecular orbital distribution and facilitating states mixing but also improves outcoupling efficiency from the preferential horizontal emitting dipole orientation. The proof-of-concept blue emitter affords monochromic devices peaking at 469 nm with a remarkable maximum external quantum efficiency (EQE_max) of 35.7%. Furthermore, simplified bicolor and tricolor white OLEDs using the same compound as emitting host achieve impressive EQE_maxs of 29.3% and 21.1%, respectively. The state-of-the-art performances of both blue and white fluorescent devices validate the superiority of this molecular design strategy

Rare Earth Complexes with 5d-4f Transition: New Emitters in Organic Light-Emitting Diodes.

Organic light-emitting diodes (OLEDs) are considered as next-generation displays and lighting technologies. During the past three decades, various luminescent materials such as fluorescence, phosphorescence, and thermally activated delayed fluorescence materials have been subsequently investigated as emitters. To date, blue OLEDs are still the bottleneck as compared to red and green ones because of the lack of efficient emitters with simultaneous high exciton utilization efficiency and long-term stability. Recently, d-f transition rare earth complexes have been reported as new emitters in OLEDs with potential high efficiency and stability. In this Perspective, we present a brief introduction to OLEDs and an overview of the previous electroluminescence study on d-f transition rare earth complexes. This is followed by our recent developments in cerium(III) complex- and europium(II) complex-based OLEDs. We finally discuss the challenges and opportunities for OLED study based on d-f transition rare earth complexes.

Multiple charge transfer disk-like emitters with fast fluorescence radiation rate and high horizontal dipole orientation for pure blue organic light-emitting diodes

• Multiple CT channels and 3 LE states contribute to the speedy k r and RISC processes. • High Θ and small FWHM are realized due to the rigid disk-like structure. • η r and η out above 30% are achieved owing to triplet exciton utilization and high Θ . • EQE above 10% was realized for the standard blue device based on BO3N. The efficiency of pure blue Organic light-emitting diodes (OLEDs) still needs to be improved, which is limited by the utilization of the forbidden transition triplet excitons and low light-out-coupling efficiency in devices. Herein, two blue emitters were developed through combining multiple diphenylamine (DPA) donors with an oxygen-bridged boron (DBA) or triphenyl triazine (TRZ) acceptor. Notably, a narrow-band deep blue emission with a peak of 423 nm and a Full-width at half-maximum (FWHM) of 34 nm is realized for the boron-based emitter in toluene solution. Thanks to their multiple Charge transfer (CT) channels and high orientated disk-like structure, high radiative transition rates ( k r > 10 8 s −1 ) and high horizontal emitting dipole ratios ( Θ > 80%) can be realized simultaneously. Meanwhile, the introduction of the peripheral DPA units can also provide multiple localized triplet ( 3 LE) states for spin up-conversion of forbidden transition triplet excitons. Owing to the effective up-conversion of triplet excitons and high light-out-coupling efficiency, maximum external quantum efficiency of more than 10% is achieved for the boron emitter-based pure blue OLEDs with a Commission Internationale de l’Eclairage (CIE) coordinates of (0.14, 0.08).

Hybrid Device of Blue GaN Light-Emitting Diodes and Organic Light-Emitting Diodes with Color Tunability for Smart Lighting Sources.

A lighting device with a wide color-tunable range is still a challenge for lighting based on either organic light-emitting diodes (OLEDs) or inorganic LEDs. In this work, we first proposed a novel hybrid device of organic LEDs and inorganic blue GaN LEDs to achieve full white and other colors. Organic LEDs were stacked with green and red emissive layers and connected with blue GaN LEDs in parallel but in opposite polarity voltage. Under the alternate-current (AC) driving, the hybrid structure can be controlled independently by applying timing variable opposite voltages to emit the light from either blue LEDs or the stacked OLEDs for forming mixed colors. The hybrid device can generate white light, varying in a wide range by changing the amplitude and duty ratio (DR) of AC-driving signals, from cold white to standard white and to warm white (3668–11 833 K). When an AC voltage of (4.80 V, −2.45 V) was applied, the device has a high color gamut of 95.24% National Television System Committee (NTSC) and a high color rendering index (Ra) of 92.4%. The novel hybrid device with the blue LED and OLED in opposite polarity exhibits potential applications in smart solid-state lighting, display, and light communication.

Investigation of 4,4′-bis[(N- carbazole) styryl] biphenyl (BSB4) for a pure blue fluorescent OLED with enhanced efficiency nearing the theoretical limit

4,4′-bis[(N-carbazole) styryl] biphenyl (BSB4 or BSBCz) is one of the widely studied organic fluorescent materials for blue organic electroluminescent devices in the recent times. In this work, BSB4 is used as a guest material to construct the host-guest matrix for the emissive layer (EML) of a pure blue fluorescent organic light-emitting diode (OLED). A pure blue emission suitable for display application with a Commission Internationale de l’Eclairage coordinate of (0.147,0.070) is achieved by the blue-shift of the emission spectrum of the host-guest matrix from that of the pristine guest (BSB4) molecules. The optimization of OLED structures is carried out by considering (a) charge balance in the EML for high exciton density, and (b) optical interference of generated light in the organic layers for increased light outcoupling. A thorough comparative study on the use of different combinations of widely used hole and electron transport layers to obtain charge balance in the EML of the OLED, thereby enhancing the external quantum efficiency (EQE) is shown. Optical interference effects in the fabricated OLEDs are analyzed by optical simulation of each device structure by transfer matrix method. With the optimized device structures, we are able to overcome the 2% EQE limit that has been reported so far for blue fluorescent OLEDs with BSB4 as light emitting material and achieve a maximum EQE of 4.08%, which is near to the theoretical limit of EQE for fluorescent OLEDs.