White Organic Light-emitting Diodes Research Articles

Multiple emission mechanism based four-peak tuning strategy to achieve ultra-high color rendering index and chromatic-stable white organic light emitting diodes

Abstract We successfully fabricated white organic light-emitting diodes(WOLEDs) based on double light-emitting layers(EMLs) with ultra-high color rendering index (CRI)and stable chromaticity by multiple mechanism four peaks tuning. The phosphorescent blue monomer and red excimer emission from Platinum [1,3-difluoro-4,6-di (2-pyridinyl) benzene]chloride(Pt-4) together with the deep blue fluorescence and yellow phosphorescence emission from a co-emission light-emitting layer realized excellent four-peak tuning WOLEDs. Ultra-high CRI of 94 and extremely stable Commission Internationale deL'Eclairage (CIE) coordinates of (0.420 ± 0.003,0.360 ± 0.002) over a wide voltage range were presented. The high CRI is ascribed to broad and continuous four peaks emission covering the fully visible spectrum. The mixed bipolar interlayer effectively control the distribution of excitons in the two regions of emission, which makes the chromaticity of WOLEDs extremely stable. Also the stable emission mechanism of two EMLs contribute to chromatic stability. Significantly, the devices showed high efficiency under the premise of high CRI and stable chroma by effective strategic of structural design.

Blue Molecular Emitter-Free and Doping-Free White Organic Light-Emitting Diodes With High Color Rendering

White organic light-emitting diodes (WOLEDs) with ultrahigh color rendering index (CRI ≥90) are ideal for lighting. However, ultrahigh CRIs are usually obtained via complicated architectures and blue molecular emitters are required. Herein, with the combination of blue exciplex/electroplex and green/red phosphors, blue molecular emitter-free and doping-free WOLEDs have been developed. An ultrahigh CRI of 95 is achieved, which is the highest for blue molecular emitter-free and doping-free WOLEDs. Additionally, a maximum total external quantum efficiency of 7.5% and power efficiency of 21.3 lm W−1 are obtained, which are the highest for blue molecular emitter-free WOLEDs with CRI ≥95. The origin of ultrahigh CRIs is unveiled by studying effects of the phosphor thickness, blue molecular emitter-free system, phosphor location, and pick of phosphors. Such results not only give an insightful understanding of simplified but high-performance WOLEDs, but also provide an alternative strategy to achieve ultrahigh CRIs.

Multifunctional Bipolar Materials Serving as Emitters for Efficient Deep‐Blue Fluorescent OLEDs and as Hosts for Phosphorescent and White OLEDs

AbstractBlue luminescent materials are always highly desired for organic light‐emitting diodes (OLEDs) but the electroluminescence (EL) performances for most blue emitters still need to be improved in both efficiency and stability. In this work, four bipolar molecules with twisted donor‐acceptor structures (PIPD‐MP‐DPA, PIPD‐MP‐IMDB, PIPD‐MP‐DMAC, and PIPD‐MP‐DPAC) are designed and synthesized. These molecules possess good thermal and electrochemical stabilities, and exhibit strong deep‐blue photoluminescence. Their non‐doped OLEDs show deep‐blue EL emissions at 418–438 nm, and high maximum external quantum efficiencies (ηext,max) of up to 4.4% with a CIEy value of only 0.078. Furthermore, the phosphorescent OLEDs (PhOLEDs) based on PIPD‐MP‐DPA host achieve excellent ηext,max of 25.9% and 28.0% for green and orange, respectively, with small efficiency roll‐offs. In addition, PIPD‐MP‐DPA is also employed to construct hybrid white OLEDs (WOLEDs), giving high ηext,max of 21.6% for two‐color WOLEDs, and 16.3% for four‐color WOLEDs. In brief, these bipolar materials reveal outstanding EL performances in deep‐blue fluorescent, phosphorescent, and hybrid white OLEDs, indicating huge potential applications in display and lighting techniques.

High-power-efficiency thermally activated delayed fluorescence white organic light-emitting diodes based on asymmetrical host engineering

Developing thermally activated delayed fluorescence (TADF) white organic light emitting diodes (WOLED) featuring high power efficiencies faces a challenge in balancing carrier transport, energy transfer and quenching suppression. Here, we demonstrate that host matrix is crucial for the modification of optoelectronic process in emissive layers (EML) and the achievement of high power efficiency. Through asymmetrically introducing two or three diphenylphosphine oxide (DPPO) groups, three phosphine oxide (PO) hosts 24DBFDPO , 26DBFDPO and 248DBFTPO were constructed, in which dibenzofuran (DBF) chromophore is embedded by rationally spatial distribution of DPPO groups in 26DBFDPO and 248DBFTPO . Direct steric effect of DPPOs suppresses the host-dopant and dopant-dopant interaction induced quenching and mitigates excessive dopant-dopant energy transfer. Dually-doped white TADF films of 248DBFTPO reveal the improved white color purity with a high quantum yield of ~ 90%. Simultaneously, remote steric and electron-withdrawing effects of DPPOs remedy carrier transport in 248DBFTPO films. Based on a simple trilayer single-EML device structure, 248DBFTPO endowed its full-TADF WOLEDs with the state-of-the-art performance, especially the top-rank power efficiency beyond 70 lm W −1 . This work indicates host engineering can provide a feasible approach to realize the practical application of TADF daily lighting. • Host engineering based Asymmetric phosphine oxide hosts were implemented for white TADF diodes. • Intermolecular interactions in host matrixes are accurately adjusted with spatial asymmetry of phosphine oxide groups. • Carrier mobility of 10 −6 cm 2 V −1 s −1 for neat films and ~ 90% PLQYs of blue and white TADF doped films are achieved. • High power efficiency beyond 70 lm W −1 was realized by a simple trilayer single-emissive-layer TADF WOLED.

Bee-shaped host with ideal polarity and energy levels for high-efficiency blue and white fluorescent organic light-emitting diodes

An ideal host for blue emitters with thermally activated delayed fluorescence (TADF) should bear a moderate polarity to relax charge transfer (CT) states of dopant for a reduced singlet-triplet energy gap (ΔEST) while utter-mostly keeping blue emission, a high triplet energy to confine exciton, and a small ΔEST for feasible charge injection. However, it is rather challenging to simultaneously fulfill all those requirements as the strong CT characteristic necessary for a small ΔEST usually enlarge polarity and lower energy levels. Here, a symmetrical ortho-linked diphenylphosphine oxide (DPO)/carbazole derivative is developed, of which the configuration resembles the bee with carbazoles as front wings, phenyl rings on DPO as back wings, hooked together by π-π interaction. This motif features a short distance of charges for a moderate polarity of 3.45 D, a twisted non-conjugation structure for a small ΔEST of 0.24 eV with a high triplet of 2.98 eV and multiple intermolecular interactions for enhanced charge transport. The corresponding blue TADF device realizes a compromise between efficiency and colors with a maximum external quantum efficiency (EQEmax) of 35.7% and a CIEy of 0.20. Furthermore, a full-TADF white device with this host realizes a EQEmax of 26.1%, representing the state-of-the-art performance.

Optical and emission properties of terpolymer active materials for white OLEDs (WOLEDs)

White Organic Light-Emitting Diodes (WOLEDs) have attracted an enormous interest because they can be implemented in numerous lighting applications as next-generation light sources. In this work, terpolymers bearing blue, yellow and red chromophores are used for the fabrication of WOLED devices. These systems provide easy tuning of white color by changing weight ratios between the chromophores. The optical and opto-electronic characterization is carried out by near infrared–visible-far ultraviolet (NIR–Vis–FUV) Spectroscopic Ellipsometry (SE), Photoluminescence (PL), and Electroluminescence (EL), whereas the structural characterization is achieved by Atomic Force Microscopy (AFM), to provide valuable information toward the optimization and functionalization of these WOLED devices.

Optical and electrical characterization of blended active materials for white OLEDs (WOLEDs)

The purpose of this work is the investigation of white light emission by blending RGB-emitting polymers and the definition of the optimal blending weight ratio, concerning the device operation characteristics and color emission. A comprehensive study of the optical properties and optoelectronic performance of white organic light-emitting diodes (WOLEDs) based on blends of poly(9,9-di-noctylfluorenyl-2,7-diyl) (PFO), poly(9,9-dioctylfluorene-altbaenzothiadiazole) (F8BT) and poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) is reported. It was observed that using relatively low ratios, up to 5 wt% of F8BT and MDMO-PPV into the host polymer PFO, slightly affects the dielectric optical response of the blended films and the characteristic electronic absorptions of PFO. Photoluminescence (PL) studies of the blends reveal that the main mechanism is the energy transfer from PFO to F8BT constituent. However, the electroluminescence (EL) study of blends, demonstrates that, in addition to energy transfer, the individual emission of PFO, F8BT and MDMO-PPV leads to a broad emission of the produced OLED devices that covers the whole visible spectral range. As a result, an almost white emission is achieved proving the potentiality to implement such solution-processable blends for lighting applications.

Exciton engineering based on star-shaped blue thermally activated delayed fluorescence emitters for efficient white organic light-emitting diodes

Accurate exciton allocation in thermally activated delayed fluorescence (TADF) white organic light-emitting diodes (OLEDs) was realized to achieve the efficient efficient white emission.

Weaving host matrices with intermolecular hydrogen bonds for high-efficiency white thermally activated delayed fluorescence.

A thermally activated delayed fluorescence (TADF) white organic light-emitting diode (WOLED) holds great promise for low-cost, large-scale lighting applications. Nevertheless, manipulating exciton allocation in a white TADF single layer is still a challenge. Herein, we demonstrate that the exciton kinetic process of dually doped white TADF films is strongly dependent on the grid regularity of the host matrix. Intermolecular hydrogen bonds (IHBs) are used to weave the matrices of two host molecules DPEQPO and DPSQPO featuring four phosphine oxide (PO) groups and different IHB orientations. The DPSQPO matrix forms regular grids to uniformly disperse and separate dopants, while DPEQPO exhibits chaotic IHBs, in turn inducing a heterogeneous dopant distribution. As a consequence, in both photoluminescence and electroluminescence processes, in contrast to DPEQPO hosted systems with comparable singlet Förster resonance energy transfer and triplet Dexter energy transfer, DPSQPO provides a FRET-predominant exciton allocation between blue and yellow dopants, which markedly suppresses triplet quenching and improves the white color purity, resulting in a state-of-the-art external quantum efficiency up to 24.2% of its single-emissive-layer pure-white TADF diode, in contrast to 16.0% for DPEQPO based analogs. These results indicate the significance of host engineering for exciton kinetics and suggest the feasibility of host grid design for developing high-performance TADF lighting.

Highly efficient, ultralow turn-on voltage red and white organic light-emitting devices based on a novel exciplex host

We have prepared high-performance white OLEDs based on exciplex systems and blue ultra-thin FIrPic layers, exhibiting a low turn-on voltage of 2.2 V with a maximum power efficiency of 34.1 lm W−1, and the CIE coordinate (0.33,0.33) at 1000 cd m−2.

High efficiency and long lifetime fluorescent white organic light-emitting diodes by phosphor sensitization to strategically manage singlet and triplet excitons

The positions of the phosphor-sensitized yellow EML from the exciton recombination zone were controlled to strategically utilize singlet and triplet excitons for blue and yellow emissions to realize high-performance fluorescent WOLEDs.

Manipulation of the size of polystyrene spheres as the templates for internal light out-coupling structures of a white organic light-emitting diode

By adjusting the ratio of nanostructured ITO with the periods of 300 and 350 nm, high-efficiency white OLEDs with controllable EL spectra have been achieved.

Rational arrangement of multiple exciplex hosts for highly efficient white organic light-emitting diodes

Designing a novel exciplex host and arranging rational multiple-exciplex hosts in emissive layers.Two and three color WOLEDs based on multiple exciplex hosts exhibited maximum EQEs of 30.9% and 27.0%, respectively. Remarkedly, their EQEs remained at 22.1% and 21.3% at a high luminance of 5000 cd m−2.

Inverted with power efficiency over 220 lm W–1

The white organic light-emitting diodes (WOLEDs) have great potential in the fields of solid-state lighting. Continued efforts have been made to catch up with the power efficiency (PE) of inorganic light-emitting diodes. Apart from low operating voltage and near unity internal quantum efficiency, improvement the efficiency of light extraction, especially suppression of surface plasmon (SP) modes is essential for realizing ultimate PE. In contrast to corrugated structure which could suffer from spectral selectivity, higher leakage current and potential degradation issue stemmed from the rough structure, increasing the distance between the emitter and the metallic electrode becomes an effective avenue towards suppressing SP modes while maintaining the planar structure. In this work, an inverted bottom-emitting structure with thick hole transport layer (HTL) composed of multi-period organic heterojunctions is demonstrated to effectively extract the SP modes without any electrical property degradation in the current density range for lighting. Investigation on electrical properties of thickened HTL with increased count of organic heterojunctions indicates no increase of driving voltage at same current density in lighting application condition. By attaching an matched high refractive index lens, a record maximum forward-viewing PE of 228.4 lm W-1 is obtained for the thick WOLED fabricated on high refractive index glass substrate, exhibiting significant enhancements of 57% compared to the counterpart of the conventional thin WOLEDs. An unambiguous picture is given to depict the regularity derived from increase of emitter-to-metal distance by combination of a full optical simulation and experimental measurements excluded the influence of electrical property degradation.

High efficiency white organic light-emitting diodes with co-doped iridium complexes as blue and yellow emitters

In this work, high performance white organic light-emitting diodes (WOLEDs) were designed and manufactured through co-doping two iridium complexes iridium(III)[bis(4,6-difuorophenyl)pyridinato-N,C2’]picolinate (FIrpic) and bis(2-(9,9-diethyl-9Hfluoren-2-yl)−1-phenyl-1H-benzoimidazol-N,C3)iridium(acetylacetonate) ((fbi)2Ir(acac)) into 9-(4-tert-butylphenyl)−3,6-bis(triphenylsilyl)−9H-carzole (CzSi) film as blue and yellow emitters. By inserting TcTa film as ladder layer, reduced exciton energy losses and direct exciton generation on emitters were realized, which caused adjusted carriers’ balance and enhanced emission efficiency. Consequently, the optimal WOLED with structure of ITO/HAT-CN (6 nm)/HAT-CN (0.2 wt%): TAPC (50 nm)/TcTa (7 nm)/(fbi)2Ir(acac) (1.4 wt%): FIrpic (14 wt%): CzSi (6 nm)/Tm3PyP26PyB (66 nm)/LiF (1 nm)/Al (100 nm) obtained the maximal current efficiency of 61.31 cd/A, corresponding to a maximal power efficiency of 66.38 lm/W. As current density increases from 10 to 100 mA/cm2, Commission Internationale de l'Eclairage (CIE) coordinates of this device shifted slightly from (0.306, 0.391) to (0.285, 0.372). At 1000 (3.8 V) and 2000 cd/m2 (4.3 V), the same device retained the high current efficiencies up to 38.74 and 32.67 cd/A, respectively.

High fluorescence efficiency of dual-wavelength white OLED with NPB emission and triplet annihilation

A high-efficiency fluorescent dual-wavelength white organic light-emitting diode (WOLED) was fabricated with a single emission layer (EML) comprising ITO/MoO3/NPB/Spiro-Pye:DCJTB/Bphen/LiF/Al. OLEDs with a 2 nm-thick MoO3 showed a 6% decrease in the operating voltage and a 14% increase in the external quantum efficiency (EQE). The NPB emission that occurred near the NPB/EML interface was investigated with a carrier blocking layer containing Ir(ppy)3 and Ir(piq)3. Excess electrons in the EML affected the emission of NPB, even at a low voltage of 4 V. The maximum luminescence, current efficiency, and power efficiency of the WOLED were 21,070 cd/m2, 18.09 cd/A, and 8.77 lm/W, respectively. A high EQE of 8.16% was achieved, which exceeded the theoretical limit of 5% for all-fluorescent OLEDs.

Photophysics of 9,9-Dimethylacridan-Substituted Phenylstyrylpyrimidines Exhibiting Long-Lived Intramolecular Charge-Transfer Fluorescence and Aggregation-Induced Emission Characteristics.

Six pyrimidine-based push-pull systems substituted at positions C2 and C4/6 with phenylacridan and styryl moieties, employing methoxy or N,N-diphenylamino donors, have been designed and synthesized through cross-coupling and Knoevenagel reactions. X-ray analysis confirmed that the molecular structure featured the acridan moiety arranged perpendicularly to the residual π system. Photophysical studies revealed significant differences between the methoxy and N,N-diphenylamino chromophores. Solvatochromic studies revealed that the methoxy derivatives showed dual emission in polar solvents. Time-resolved spectroscopy revealed that the higher energy band involved very fast (<80 ps) fluorescence, whereas the lower energy one included long components (≈30 ns) due to long-lived intramolecular charge-transfer fluorescence. In contrast to N,N-diphenylamino chromophores, the methoxy derivatives also showed aggregation-induced emission in mixtures of THF/water, as well as dual emission in thin films, covering almost the whole visible spectrum with corresponding chromaticity coordinates not far from that of pure white light. These properties render the methoxy derivatives as very promising organic materials for white organic light-emitting diodes.

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.

Stable, High‐Efficiency Voltage‐Dependent Color‐Tunable Organic Light‐Emitting Diodes with a Single Tetradentate Platinum(II) Emitter Having Long Operational Lifetime

Voltage-dependent, color-tunable organic light-emitting diodes (OLEDs) are appealing tools that can be used for the visualization of electronic output signal of sensors. Nonetheless, the literature-reported color-tunable OLEDs that have a simple single-cell device structure suffer from relatively low efficiency, pronounced efficiency roll-off, color-aging, and short operation lifetime, all of which limit their practical applications. Here, a novel co-host-in-double-emissive-layer (CHIDEL) device, designed to enhance the performance of color-tunable OLEDs with the use of a single tetradentate Pt[O^ N^ C^ N] emitter, is described. When Pt-X-2 is used as a single emitter in an optimized CHIDEL device, a white OLED with tunable Commission International de I'Eclairage (CIE) coordinates from (0.47, 0.44) at 3V to (0.36, 0.48) at 11V, a high color rendering index of 82, and high external quantum efficiency (EQE) of up to 20.75% can be achieved. By using Pt-X-4 as a single emitter, the voltage-dependent color-tunable CHIDEL device, with CIE coordinates shifted from (0.56, 0.43) at 3V to (0.42, 0.55) at 11V, demonstrates a high luminance of beyond 90 000cd m-2 and a high EQE of 23.23% at a luminance of 1300cd m-2 . A long-lifetime time to 90% of the initial luminance (LT90 ) of almost 20000 h is demonstrated for the color-tunable OLED with Pt-X-4 emitting dopant.

Dual Förster resonance energy transfer in ternary PFO/MEH-PPV/F7GA hybrid thin films for white organic light-emitting diodes

The ternary hybrid thin films of poly [9,9′-di-n-octylfluorenyl-2,7-diyl] (PFO)/poly [2methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV)/2-butyl-6- (butylamino) benzo [de] isoquinoline-1,3-dione (F7GA) were successfully prepared with various ratios of F7GA. Dual Förster resonance energy transfer (FRET) from PFO to both F7GA and MEH-PPV was confirmed in the ternary thin films. The distance between PFO and F7GA molecules was smaller than the distance between PFO and MEH-PPV. Besides, white light emission was achieved with cascade energy transfer in this ternary blend system. The lifetime decay components of the acceptor emission were attributed to the presence of an exciplex between the molecules in the acceptor excited state and the donor ground state. In addition to the dual FRET, carrier trapping took place in the device, successfully realizing white organic light-emitting diodes (WOLEDs). The maximum luminance of 2295 cd/m2 and a current density of 12.98 mA/cm2 at CIE coordinates of (0.31, 0.24) was achieved with the device at 1.0 wt% F7GA.