Fabrication of red, green, and blue organic light-emitting diodes using m-MTDATA as a common hole-injection layer

Organic light-emitting diodes (OLEDs) of metal-semiconductor-metal (MSM) structure have been fabricated by using m-MTDATA [4,4′,4′’-tris (3-methylphenylphenylamino) triphenylamine] as a hole-injection layer (HIL). The m-MTDATA is shown to be an effective hole-injecting material for the OLED, in that the insertion of m-MTDATA greatly reduces the roughness of anode surface, lowers the turn-on voltage, and increases the luminous efficiency. Red, green and blue OLEDs were fabricated, and their color coordinates in CIE chromaticity were found to be (0.600, 0.389), (0.240, 0.525) and (0.171, 0.171), respectively. The luminous efficiencies of the fabricated OLEDs were 1.4 lm/W at 106 cd/m² for red, 1.4 lm/W at 100 cd/m² for green, and 2.0 lm/W at 104 cd/m² for blue.     

Bipolar Hosts Based on a Rigid 9,10-Dihydroanthracene Scaffold for Full-Color Electrophosphorescent Devices

Two bipolar hosts, CzDPO and CzDOxa , comprising a hole-transporting carbazole and an electron-transporting diphenyl phosphine oxide or oxadiazole with a saturated linker (9,10-dihydroanthracene) have been synthesized and characterized. The saturated linker limited the effective extension of π conjugation, leading to high triplet energies (E_T=2.97–2.98 eV). The diastereomers with a rigid configuration provided an amorphous thin film with high thermal (T_d=415–444 °C) and morphological stability (T_g=188–224 °C). The high triplet energies and bipolar carrier transport characteristics (ambipolariy) of CzDPO and CzDOxa can facilitate exothermic energy transfer to the dopants and balance carrier injection/transport in the emission layers. As a result, they were utilized as universal hosts for various phosphorescent OLEDs (from blue to red), showing average external quantum efficiencies (η_ext=8.2–13.1 %) and low efficiency roll-off. In addition, we also fabricated dual-emitter white organic light-emitting diodes with a co-doped single emissive layer. WOLED hosted by CzDOxa exhibited satisfactory device efficiencies (14.4 %, 25.7 cd A⁻¹, 27 lm W⁻¹) with highly stable chromaticity (CIE_x=0.26–0.28 and CIE_y=0.38) at brightnesses from 560 to 15200 cd m⁻².     

A New Multifunctional Triazine?Carbazole Compound with High Triplet Energy for High-Performance Blue Fluorescence,Green and Red Phosphorescent Host,and Hybrid White Organic Light-Emitting Diodes

A blue fluorescent compound, 9-[4-(4,6-diphenoxy-1,3,5-triazin-2-yl)phenyl]-9H-carbazole (POTC), the triplet energy level of which reaches 2.76 eV, has been designed and synthesized. POTC is an excellent blue emitter as well as host for green and red phosphors, and therefore, matches the requirements of the host for single-emitting-layer fluorescence and phosphorescence hybrid white organic light-emitting diodes (OLEDs). The blue, green, red, and white devices based on POTC show maximum external quantum efficiencies (EQEs) of 2.4, 22.4, 13.0, and 8.1 %, respectively. Even at a high brightness of 1000 cd m⁻², these values maintain EQEs of 2.3, 22.1, 11.1, and 7.0 %, respectively, indicating less than 15 % roll-offs from the maxima.     

Improving the image of an organic light‐emitting diode using a dye‐polariser

The present work investigated the image improvement of an organic light‐emitting diode (OLED) by using a dye‐polariser on the panel of an OLED. There are many key vision indexes that can be used to indicate the image performances of flat panel displays (FPDs), such as pixel solution, brightness, view angle, visual reflective sensitivity, contrast ratio, colour saturation and response time. In this study, a dye‐polariser was applied on the panel of an OLED and experiments were conducted to examine the image performances using some relative key vision indexes. The results clearly show the effectiveness of the dye‐polariser used. The OLED showed a reduction in visual reflective sensitivity by 86.6%, improved the contrast ratio of the image to 2.4 and 2.7 times in an indoor (or office) ambience (490 cd/m²) and an outdoor ambience (1375 cd/m²), respectively, increased colour saturation from 59.4% to 66.7%, and reinforced the weaknesses of the red and blue organic fluorescent materials. © 2011 Canadian Society for Chemical Engineering     

Construction of Efficient Solid Emitters with Tetraphenylethene Trimers for Non-doped Blue OLEDs

Three TPE trimers with meta or para linkage modes have been successfully synthesized. When fabricated as emissive layers in non-doped OLEDs, they all exhibit blue or deep-blue emissions with maximum current efficiency up to 4.03 cd A⁻¹, further verifying the facile but ingenious approach by utilizing meta-linkage mode in longer conjugated systems.     

The influence of type-I and type-II triplet multiple quantum well structure on white organic light-emitting diodes

We demonstrate high-efficient white organic light-emitting diodes (WOLEDs) based on triplet multiple quantum well (MQW) structure and focus on the influence on WOLEDs through employing different potential barrier materials to form type-I and type-II MQWs, respectively. It is found that type-I MQW structure WOLEDs based on 1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene as potential barrier layer (PBL) offers high electroluminescent (EL) performance. That is to say, maximum current efficiency and power efficiency are achieved at about 1,000 cd/m² with 16.4 cd/A and 8.3 lm/W, which increase by 53.3% and 50.9% over traditional three-layer structure WOLEDs, respectively, and a maximum luminance of 17,700 cd/m² is earned simultaneously. The achievement of high EL performance would be attributed to uniform distribution and better confinement of carriers within the emitting layer (EML). However, when 4,7-diphenyl-1,10-phenanthroline or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline is used as PBL to form type-II MQW structure, poor EL performance is obtained. We attribute that to improper energy level alignment between the interface of EML/PBL, which leads to incomplete confinement and low recombination efficiency of carriers, a more detailed mechanism was argued.     

On the luminance saturation of phosphor-in-glass (PiG) films for blue-laser-driven white lighting: Effects of the phosphor content and the film thickness

To investigate the luminance saturation in high-power blue-laser-driven solid state lighting, the Y₃Al₅O₁₂: Ce (YAG)-based PiG films was co-fired with a high thermally conductive sapphire substrate. When the PtG ratio or the thickness of the PiG film increases, the luminance saturation becomes worse and both of the luminous flux and luminous efficacy decrease. With an additional sapphire substrate coated with an anti-reflection layer on one side and a blue-pass filter on another side attached to the PiG film, the film shows an improvement in luminous flux and efficacy, and produces the white light with a luminous flux of 1709 lm, a luminous efficacy of 211 lm W^?1 and a correlated color temperature of 6602 K under the maximum (10.3 W mm^?2) blue laser light excitation. It indicates that the PiG film could be applied in high power laser-driven solid state lighting when its microstructure is carefully tailored.     

Polylactide‐perylene derivative for blue biodegradable organic light‐emitting diodes

In this work we demonstrate, for the first time, the use of polylactic acid (PLA) as a biodegradable host matrix for the construction of the active emissive layer of organic light‐emitting diode (OLED) devices for potential use in bioelectronics. In this preliminary study, we report a robust synthesis of two fluorescent PLA derivatives, pyrene‐PLA ( AH10 ) and perylene‐PLA ( AH11 ). These materials were prepared by the ring opening polymerisation of l ‐lactide with hydroxyalkyl‐pyrene and hydroxyalkyl‐perylene derivatives using 1,8‐diazabicyclo[5.4.0]undec‐7‐ene as catalyst. OLEDs were fabricated from these materials using a simple device architecture involving a solution‐processed single‐emitting layer in the configuration ITO/PEDOT:PSS/PVK:OXD‐7 (35%): AH10 or AH11 (20%)/TPBi/LiF/Al (ITO, indium tin oxide; PEDOT:PSS, poly(3,4‐ethylenedioxythiophene) doped with poly(styrenesulfonic acid); PVK, poly(vinylcarbazole); OXD‐7, (1,3‐phenylene)‐bis‐[5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole]; TPBi, 2,2′,2″‐(1,3,5‐benzenetriyl)tris(1‐phenyl‐1H‐benzimidazole)). The turn‐on voltage for the perylene OLED at 10 cd m^–2 was around 6 V with a maximum brightness of 1200 cd m^–2 at 13 V. The corresponding external quantum efficiency and device current efficiency were 1.5% and 2.8 cd A^–1 respectively. In summary, this study provides proof of principle that OLEDs can be constructed from PLA, a readily available and renewable bio‐source. © 2020 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry.     

Porous Ce:YAG ceramics with controllable microstructure for high-brightness laser lighting

In order to meet the increasing demand for high-power laser diode lighting and displays, phosphor converters with high-brightness and high-directionality ought to be constructed to enhance the luminance and luminous efficacy. However, the pores formed during the sintering of phosphor ceramics affect the scattering effect and directionality of light. Therefore, porosity optimization and pore size regulation need to be explored. In this work, a series of Ce:YAG ceramics with various porosities and pore sizes were prepared. The influences of porosity and pore size on the microstructure, light confinement ability, and optical properties of Ce:YAG ceramics were studied. The ceramic phosphor with a porosity of 10 vol.% and a pore size of 3 μm exhibits a good spot confinement ability and shows a high luminous flux value of 3430 lm and a central luminance (1669 592 cd/m²) under blue laser excitation. The 10 vol.% Ce:YAG ceramic phosphor with a pore size of 5 μm has the highest emission intensity and gives a maximum luminous efficacy of 268 lm/W and a luminous flux of 4020 lm under 30 W/mm² blue laser excitation. Thus, the porous Ce:YAG ceramics are expected to be a promising candidate for high-brightness laser lighting and projection applications.     

Electro‐active oligomers containing pendent carbazolyl or indolyl groups as host materials for OLEDs

Oligoethers containing electroactive carbazolyl, indolyl or 2‐phenylindolyl fragments were synthesized and characterized by NMR spectroscopy, elemental analysis and gel permeation chromatography. The oligomers represent amorphous materials of high thermal stability with glass transition temperatures of 107–161°C. The electron photoemission spectra of layers of the synthesized oligomers showed ionization potentials of 5.9–5.95 eV. Some of the derivatives were tested as host materials in phosphorescent OLEDs with iridium(III) [bis(4,6‐difluorophenyl)‐pyridinato‐N,C2′]picolinate as the guest. The device based on oligomer containing carbazolyl fragments exhibited the best overall performance with a turn‐on voltage of 3.5 V, maximum power efficiency of 4.1 lm/W and maximum brightness of 937 cd/m². © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Hole-limiting conductive vinyl copolymers for AlQ3-based OLED applications

A series of soluble conductive vinyl copolymers containing a hole-transporting N-(4-methoxyphenyl)-N-phenylnaphthalen-1-amine (MeONPA) moiety and an electron-transporting/hole-blocking 2,5-diphenyl-1,3,4-oxadiazole (OXA) moiety at different composition ratios were synthesized and characterized. The copolymers were applied as the hole-transporting layer (HTL) for a series of heterojunction Organic Light-emitting Diodes (OLEDs) employing the commonly used green emitter tris(8-hydroxyquinolinato)aluminum (AlQ₃) as the electron-transporting layer. AlQ₃ is known to have inferior electron mobility compared to most typical hole-transporting materials. As a result, oxidative degradation of the AlQ₃ emitters caused by the excessive holes accumulated at the interface led to deterioration of the device over time. From the measurement of hole current only devices using electron blocking gold as cathode (ITO/PEDOT:PSS/copolymer/Au), it was found that the hole current for the copolymers reduced as the OXA composition increased. Optimum performance for the AlQ₃-based OLED (ITO/PEDOT:PSS/copolymer/AlQ₃/Ca/Al) was achieved for a 82/18 (molar ratio) (MeONPA/OXA) copolymer. The maximum current efficiency and luminance were 4.2 cd/A and ca 24,000 cd/m² respectively for the charge-balanced copolymer compared to 3.5 cd/A and 6600 cd/m² for similar device employing a homopolymer P(MeONPA) as the HTL.     

ELECTROLUMINESCENT PROPERTIES OF CERTAIN POLYAROMATIC COMPOUNDS: PART 2–ORGANIC EMITTERS

This review is based on the electroluminescent and optical properties of certain polyaromatics in OLEDs with special attention to their specific function in the emitting layer, performance and emissive color. DAD and TDAD dopants depending on their concentration in the emitting layer exhibit good efficiencies and color purity for blue emission. When copolyethers with diphenylanthracene emitting segments are used in a single layer diode, visible pure blue light (440 nm maximum) around 15-20V can be achieved. J-Aggregates including anthracene moieties contribute to the efficient IR electroluminescence. Blue, white, yellow and orange electroluminescent devices can be obtained by ADN- or ADN-doped with rubrene at ultra low concentrations. Pyrene derivatives (P1, P2) linked to fluorene exhibit high thermal stability, bright blue emission and improved hole injection ability. Aminobenzanthrone derivatives as host emitters emit orange-to-red light with high brightness (25000 cd.m^?2), current efficiency (3.52 cd.A^?1) and power efficiency. Due to its bipolar transport property, when doped in Alq₃ or NPB, tetraphenylnaphtacene shows excellent yellow electroluminescence. Multiple quantum well (MQW) structures including rubrene are helpful in narrower and tunable spectral emission, as well as higher emission efficiency. Perylene derivatives such as copolymer+PVK blend show sharp red emission peaks, photochemical and thermal stability. Pentacene derivatives, DPP, 1-DNP and 2-DNP can be used to obtain highly pure purple color. An increase in the doping concentration of DPP enhances photoluminescent peak intensity. New blue-green emitting dopants like coronene, decacylene and conjugated ladder systems based on phthalocyanines are centers of attraction for electroluminescence in recent years.     

White light emission obtained by direct color mixing in multi-layer organic light-emitting devices

Three different structures of multi-layer organic light-emitting devices, which consisted of two emitting layers separated by a carrier blocking layer, were investigated. Since the emitting layers are constructed to emit different colors, the colors emitted from the structures are mixed. It was found that the colors were directly mixed in the structures of this study due to the carrier blocking layer sandwiched by the two emissive layers. The blocking layer splits the carrier recombination zone, and with the emission color is controlled by balancing the split. For the white light the CIE coordinate of (0.30, 0.33) is obtained at an applied voltage of 14 V. The luminance is measured to be 1,000 cd/m² at 14 V. with the power efficiency of 0.4 lm/W. For a luminance of 100 cd/m² at 11 V., the CIE coordinate is found to be (0.31, 0.34) and the power efficiency was as high as 0.53 lm/W.     

Hybrid light-emitting diodes from anthracene-contained polymer and CdSe/ZnS core/shell quantum dots

In this paper, we added CdSe/ZnS core/shell quantum dots (QDs) into anthracene-contained polymer. The photoluminescent (PL) characteristic of polymer/QD composite film could identify the energy transitions of anthracene-contained polymer and QDs. Furthermore, the electroluminescent (EL) characteristic of hybrid LED also identifies emission peaks of blue polymer and QDs. The maximum luminescence of the device is 970 cd/m² with 9.1 wt.% QD hybrid emitter. The maximum luminous efficiency is 2.08 cd/A for the same device.     

Optical properties of the CaEuAl3O7 phosphor with dual-activator Eu2+/Eu3+ for multifunctional applications

At present, most blue-red composite LED light sources are widely used in the field of plant lighting. However, their full-width at half-maximum of blue light is too small to meet the requirements of plants for photosynthesis. Herein, a dual-emitting single-phase self-luminescent phosphor CaEuAl₃O₇ (CEAO) is reported in this study, which provides broadband blue emission of Eu²⁺ ions and red emissions of Eu³⁺ ions. According to the optical properties of Eu²⁺ and Eu³⁺ ions in the CEAO phosphor, it can be found that all emissions are consistent with the absorption of chlorophylls and pigment carotenoids. In addition, the temperature sensing based on the fluorescence intensity ratio (FIR) in the CEAO phosphor is also studied and the maximum sensitivity (S) can reach as high as 6.90% K⁻¹ at 313 K. The results indicate that the single-component phosphor CEAO with blue and red double-color emission possesses an outstanding potential in plant growth lamps and optical thermometry applications.     

A white emitting poly(phenylenevinylene)

A white emitting copolymer with the polyphenylenevinylene (PPV) structure is obtained via the Stille cross-coupling reaction. Substitution of hydrogen atoms with fluorine atoms on the vinylene units of poly(1,4-dialkoxyphenylenevinylene) shifts the emission from orange-red to blue. White emission is obtained by combining dialkoxyphenylenedifluorovinylene and dialkoxyphenylenevinylene units in proper ratio. The two complementary emitters are obtained separately by Stille polymerization reaction. Then, the two reaction mixtures are combined without purification in different ratios and further reacted in similar experimental conditions. A white luminescent material is obtained using 99/1 mixing ratio. OLED devices fabricated with this copolymer shows near-white emission with CIE (0.30, 0.40) and excellent stability in the range 10–200 cd/m².     

Carbon nanotube modified electrodes for enhanced brightness in organic light emitting devices

Acid oxidised multiwall carbon nanotubes (COOH-MWCNT) have been introduced as a hole injection buffer layer in an organic light emitting diodes (OLED). We show that the OLED with COOH-MWCNT as a buffer layer provides higher brightness with lower operating voltages. The addition of a COOH-MWCNT buffer layer has enabled a further increase in the brightness of our OLED devices operating in excess of 20,000 cd/m² due to enhanced hole injection by several orders of magnitude. The increase in current injection and brightness does not alter the optical emission spectrum at different operating voltages in these devices. A model is proposed to explain this increase in current injection and brightness based on the modified electron energy band alignment.     

Color stability and suppressed efficiency roll-off in white organic light-emitting diodes through management of interlayer and host properties

Color stability and efficiency roll-off of white light-emitting diodes (WOLEDs) with blue fluorescent and red phosphorescent emitting materials were manipulated by controlling the charge transport properties of interlayer and triplet host materials. A pure white emission was observed in WOLEDs with a bipolar interlayer and a hole transport type triplet host material. A white color coordinate of (0.31, 0.35) and a current efficiency of 14.4 cd/A were obtained. In addition, color index of WOLEDs could be kept stable up to a high luminance of 10,000 cd/m² and an efficiency roll-off was also suppressed.     

High color rendering index composite phosphor-in-glass for high-power white laser lighting

The aim of this study is to investigate a thermally robust white color converter for high-power solid-state lighting, especially laser lighting. Lu₃Al₅O₁₂:Ce³⁺/CaAlSiN₃:Eu²⁺ phosphor-in-glass samples (LuAG&CASN-PiGs) were synthesized via a low temperature co-sintering technique. The prepared LuAG&CASN-PiGs exhibited remarkably high internal quantum efficiency of 87 %. Tunable warm white light-emitting diodes (wLED) were acquired by tailoring the sample thicknesses and phosphor contents. The optimized sample showed a high luminous flux of 183.68 lm under a blue laser diodes (LDs). In addition, the chromaticity of white LDs based on the LuAG&CASN-PiGs shifted from cool to warm white by changing the sample thicknesses. High quality white light in wLDs was achieved (Ra＝95). More importantly, the constructed LuAG&CASN-PiG converted LDs with a heat sink exhibited the luminous efficiency of 216.79 lm W^?1. The results revealed that the prepared LuAG&CASN-PiG had great potential for application in solid-state laser light sources.     

Synthesis of copolyfluorenes with high fluorenone contents and its application in electroluminescent device by simple blending

Poly(fluorene‐co‐fluorenone)s containing high contents of fluorenone chromophore were synthesized by the Suzuki coupling reaction to study their electroluminescent (EL) properties. The copolymers are thermally stable below 430°C (T_d) in nitrogen atmosphere. In film state, their absorption and photoluminescence spectra (peaked at 373–382 nm and 562–564 nm, respectively) are mainly originated from fluorenone units, because of its efficient energy transfer. Both LUMO and HOMO energy levels, estimated from their cyclic voltammograms, are lowered slightly (−3.17→−3.23 eV, −5.84→−5.89 eV) with increasing contents of electron‐withdrawing fluorenone units. Double‐layer EL devices, using the copolyfluorenes or their blends with poly(9,9‐dihexylfluorene) ( PF ) as emitting layer, show exclusive emission originated from fluorenone chromophore (565 nm) when its content is high. Blending 0.02–5 wt % of PF‐33 (fluorenone fraction: 0.37) with PF greatly enhances device performance (610 cd m⁻² → 4400 cd m⁻² and 0.45 cd A⁻¹ → 1.52 cd A⁻¹). Our results demonstrate that the copolyfluorenes are promising emitting materials for EL devices by simple blending. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011