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.     

Fabrication of highly transparent CsPbBr3 quantum-dot thin film via bath coating for light emission applications

Metal halide perovskite is not easy to form a film due to its high crystallinity, which makes it unfavourable for its application in self-luminous LED. Herein, we propose a film synthesis process for perovskite quantum dots (PeQDs) based on bath coating. The proposed method is low in cost, can be performed in an atmospheric environment at room temperature, and is suitable for the rapid mass production of closely stacked PeQDs on the deposition substrate with excellent luminous characteristics. By controlling the deposit time and rotation speed, the monolayer QD film can be achieved. The deposited monolayer QD film shows a high transmittance of up to 86% and a narrow FWHM of 18 nm. The device fabricated by monolayer QD film shows a low threshold voltage of 3 V, the maximum luminance of 150 cd/m², the maximum current efficiency of 0.085 cd/A, the maximum EQE of 0.027%, and the high color purity of 87%. Due to the above properties, monolayer QD film elements have potential in smart home appliances and wearable devices, especially AR glasses.     

Electroluminescent Characteristics of DBPPV–ZnO Nanocomposite Polymer Light Emitting Devices

We have demonstrated that fabrication and characterization of nanocomposite polymer light emitting devices with metal Zinc Oxide (ZnO) nanoparticles and 2,3-dibutoxy-1,4-poly(phenylenevinylene) (DBPPV). The current and luminance characteristics of devices with ZnO nanoparticles are much better than those of device with pure DBPPV. Optimized maximum luminance efficiencies of DBPPV–ZnO (3:1 wt%) before annealing (1.78 cd/A) and after annealing (2.45 cd/A) having a brightness 643 and 776 cd/m² at a current density of 36.16 and 31.67 mA/cm² are observed, respectively. Current density–voltage and brightness–voltage characteristics indicate that addition of ZnO nanoparticles can facilitate electrical injection and charge transport. The thermal annealing is thought to result in the formation of an interfacial layer between emissive polymer film and cathode.     

Green electroluminescent polyfluorenes containing 1,8-naphthalimide moieties as color tuner

A series of copolymers (CNPFs) containing low-band-gap 1,8-naphthalimide moieties as color tuner was prepared by a Yamamoto coupling reaction of 2,7-dibromo-9,9-dioctylfluorene (DBF) and different amount of 4-(3,6-dibromocarbazol-9-yl)-N-(4′-tert-butyl-phenyl)-1,8-naphthalimide (Br-CN) (0.05-1 mol% feed ratio). The light emitting properties of the resulting copolymers showed a heavy dependence on the feed ratio. In photoluminescence (PL) studies, an efficient color tuning through the Förster energy transfer mechanism was revealed from blue to green as the increase of Br-CN content, while in electroluminescence (EL) studies, the color tuning was found to go through a charge trapping mechanism. It was found that by introduction of a very small amount of Br-CN (0.1-0.5 mol%) into polyfluorene, the emission color can be tuned from blue to pure green with Commission International de l'Echairage (CIE) coordinates being (0.21, 0.42) and (0.21, 0.48). A green emitting EL single-layer device based on CNPF containing 0.1 mol% of Br-CN showed good performances with a low turn-on voltage of 4.2 V, a brightness of 9104 cd/m², the maximum luminous efficiency of 2.74 cd/A and the maximum power efficiency of 1.51 lm/W. To further improve the EL performances through balancing the charge trapping process, a copolymer (BCNPF05) derived from 0.5 mol% of a triarylamine-containing 4-{3,6-bis-[4″-(4?-bromophenyl-p-tolyl-amino)-phenyl]-carbazol-9-yl}-N-(4′-tert-butyl-phenyl)-1,8-naphthalimide (Br-BCN) and 99.5 mol% of 2,7-dibromo-9,9-dioctylfluorene was also prepared. As expected, a single layer EL device based on BCNPF05 exhibited better performances with a brightness of 14228 cd/m², the maximum luminous efficiency of 4.53 cd/A and the maximum power efficiency of 1.57 lm/W.     

Synthesis and optoelectronic properties of luminescent copolyfluorenes slightly doped with thiophene chromophore

This paper describes the synthesis of new copolyfluorenes (P05-P5) slightly doped with 2,5-bis(2-phenyl-2-cyanovinyl)thiophene (GM, <3.4 mol%) and their application in electroluminescent (EL) devices. In film state, EL spectra of the copolyfluorenes are very different from photoluminescence (PL) spectra, which have been ascribed to charge trapping in GM and energy transfer from fluorene segments to GM chromophores. The maximum brightness and current efficiency of EL device from P05 (5230 cd/m², 0.65 cd/A) are significantly enhanced when compared with those from poly(9,9-dihexylfluorene) (PF) (1310 cd/m², 0.18 cd/A). The EL device using blend of P5 and PF (w/w = 10/1) as emitting layer exhibits near-white emission with CIE coordinate being (0.26, 0.32). The results demonstrate that the copolyfluorenes slightly doped with GM chromophore are promising emitting materials for optoelectronic devices.     

Study on the wet processable antimony tin oxide (ATO) transparent electrode for PLEDs

A polymer light emitting diodes (PLEDs) was fabricated using the wet processable antimony tin oxide (ATO) as the transparent electrode by spin coating method. PLED were fabricated with ATO (or ITO)/PEDOT:PSS/polymer/BaF₂/Ba/Al configurations. Electrical and optical properties of ATO transparent electrode were measured. Transmittance of ATO thin film was more than 90% in the visible region, sheet resistance was 30 Ω/□ and had a strong solvent resistance. The maximum brightness and maximum efficiency of PLED device using an ATO transparent electrode was 3637 cd/m² and 1.03 cd/A, respectively.     

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.     

Eu3+‐doped glass as a color rendering index enhancer in phosphor‐in‐glass

A new method for improving color rendering index (CRI) and low correlated color temperature (CCT) in high‐power white‐light‐emitting diodes (WLEDs) is proposed. We used a configuration of phosphor‐in‐glass (PIG) and studied light output changes with the increment in concentration of yellow‐emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) phosphor. The PIG was coupled on the top of blue‐light‐emitting diodes (LED) chip (465 nm). To compensate the lack of red emission in the phosphor, Eu³⁺‐doped tellurium glass with different europium content was employed as a red emitter. The suitable contents of YAG:Ce³⁺ and Eu³⁺ were 7.5 weight percent (wt%) and 3 mol percent (mol%), respectively. The CRI value went from 72 to 82, whereas the CCT was reduced from 24 933 to 6434 K. The proposed structure can improve CCT as well as CRI of WLEDs just by placing a glass on top.     

Self-assembled GaInNAs/GaAsN quantum dot lasers: solid source molecular beam epitaxy growth and high-temperature operation

Self-assembled GaInNAs quantum dots (QDs) were grown on GaAs (001) substrate using solid-source molecular-beam epitaxy (SSMBE) equipped with a radio-frequency nitrogen plasma source. The GaInNAs QD growth characteristics were extensively investigated using atomic-force microscopy (AFM), photoluminescence (PL), and transmission electron microscopy (TEM) measurements. Self-assembled GaInNAs/GaAsN single layer QD lasers grown using SSMBE have been fabricated and characterized. The laser worked under continuous wave (CW) operation at room temperature (RT) with emission wavelength of 1175.86 nm. Temperature-dependent measurements have been carried out on the GaInNAs QD lasers. The lowest obtained threshold current density in this work is ∼1.05 kA/cm² from a GaInNAs QD laser (50 × 1,700 μm²) at 10 °C. High-temperature operation up to 65 °C was demonstrated from an unbonded GaInNAs QD laser (50 × 1,060 μm²), with high characteristic temperature of 79.4 K in the temperature range of 10–60 °C.     

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.     

Green polymer-light-emitting-diodes based on polyfluorenes containing N-aryl-1,8-naphthalimide and 1,8-naphthoilene-arylimidazole derivatives as color tuner

A novel series of green light emitting single polymers were prepared by end-capping of N-aryl-1,8-naphthalimide and 1,8-naphthoilenearylimidazole derivatives into polyfluorene. The electroluminescence (EL) spectra of polymers (P1 ∼ P5) exhibit greenish-blue, bluish-green, pure green, and yellowish-green emission (λ_max = 465 nm, 490 nm, 500 nm, and 545 nm, respectively) from compounds (M1 ∼ M5). It was found that by the introduction of a small amount of compounds (M1 ∼ M5) (5 mol-%) into polyfluorene, the emission color can be tuned from the blue to green region. The color tuning was found to have gone through charge trapping and Förster energy transfer. The device of P4 emits pure green light with Commission Internationale de l'Eclairage (CIE) coordinates of (0.20, 0.41), and exhibits a maximum brightness of 11500 cd/m² at 12 V with a structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) [PEDOT:PSS]/PVK/emission layer/Ca/Ag. The device of P5 emits yellowish green light with Commission Internationale de l'Eclairage (CIE) coordinates of (0.36, 0.56), and exhibits a maximum brightness of 6534 cd/m² at 17 V.     

White Electroluminescence from a Single-Polymer System with Simultaneous Three-color Emission

A series of polymers were synthesized by incorporating low contents of fluorenone (FO) and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (DBT) into the main chain of poly(9,9-dioctylfluorene). White-light emission was obtained from a single polymer by adjusting the FO and DBT contents. All polymers showed good thermal stability with 5% weight loss up to 410 °C and good solubility in common organic solvents. Electroluminescence devices with indium tin oxide/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)/emission layer/Ca/Al structure were found to emit white light with Commission Internationale de l’Eclairage coordinate of (0.37, 0.34). These devices exhibited a maxium brightness of 3414 cd/m² and a maximum current efficiency of 2.79 cd/A.     

Synthesis and optoelectronic properties of thermally cross-linkable fluorene derivative containing hole-transporting triphenylamine terminals

This paper describes the synthesis of a solution-processable and thermally cross-linkable 2,7-bis-[4-bis(4-vinylphenyl)aminophenyl]-9,9-dihexylfluorene (VTF) and its application as hole-transporting layer in multilayer polymer light-emitting diodes (PLEDs). The thermal, photophysical, and electrochemical properties of VTF were investigated by differential scanning calorimetry, thermogravimetric analysis, optical spectroscopy, and cyclic voltammetry. The VTF is readily cross-linked via vinyl groups by heating at 180 °C for 30 min to obtain homogeneous film with excellent solvent resistance. Multilayer PLEDs (ITO/PEDOT:PSS/cured-VTF/MEH-PPV/Ca/Al) were readily fabricated by spin-coating process using cross-linked VTF as hole-transporting layer (HTL). The maximum brightness (13,640 cd/m²) and current efficiency (0.69 cd/A) were superior to those without HTL (ITO/PEDOT:PSS/MEH-PPV/Ca/Al: 7810 cd/m², 0.28 cd/A). In addition, the cured-VTF could replace conventional hole-injection layer (PEDOT:PSS) to reveal comparable performance (8240 cd/m², 0.44 cd/A). Current results indicate that the VTF with four thermally cross-linkable terminal vinyl groups is a promising optoelectronic material, which is readily processed by wet processes.     

Synthesis and characterization of a new iridium(III) complex with bulky trimethylsilylxylene and applications for efficient yellow-green emitting phosphorescent organic light emitting diodes

We designed and synthesized a new iridium(III) complex with phenylpyridine ligands containing a bulky trimethylsilylxylene, 5-(2,5-dimethyl-4-(trimethylsilyl)phenyl)-2-phenylpyridine, by Suzuki coupling reaction and characterized using various spectroscopic studies. Nuclear magnetic resonance studies show its structure to be that of a facial isomer. Thermogravimetric analysis and differential scanning calorimetry studies show its higher thermal stability (?T_5%) of 638 K with a glass transition temperature of 425 K. It shows the photoluminescence emission at 532 nm in solution with the band gap energy of 2.56 eV. The new iridium(III) complex as dopant in phosphorescent organic light emitting diodes exhibits the yellow-green emission at 532 nm as it effectively hinders aggregation formation in the solid state at a dopant concentration of 6%, resulting in higher device efficiencies of 12.7% and 45.7 cd/A. The results show that the new iridium complex could be useful in white organic light emitting diodes for the lighting applications.     

White-light-emitting diodes from single polymer systems based on polyfluorene copolymers end-capped with a dye

New polymer white-light-emitting diodes from single polymer systems have been developed. The polymer systems were based on poly(fluorene-co-benzothiadiazole) backbones end-capped with a green-emission dye, N-phenyl-1,8-naphthalimide. By changing the molar ratio of these three units, the electroluminescence (EL) spectra can be adjusted to white-light emission with a structure of indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid)/emission layer/Ca/Ag. The highest brightness in such a device configuration is 251 cd/m² at a current density of 400 A/m² with Commission Internationale de l'Eclairage (CIE) coordinates of (0.31,0.39). The EL spectra show color stability over different operating voltages.     

Warm with high color rendering index white light from hybridization of Ca2BO3Cl:Eu2+ yellow phosphor and CdSe/ZnS nanocrystals

Yellow emitting Ca₂BO₃Cl:Eu²⁺ phosphor was prepared by solid state reaction at 900 °C. The particle was monoclinic crystal structure, and showed broad band emission at around 540–590 nm due to the 5d–4f transition. Single Ca₂BO₃Cl:Eu²⁺ phosphor converted white LED exhibited the CIE coordinates of (0.3441, 0.2675) with low CRI of 67.4. Hybridization of Ca₂BO₃Cl:Eu²⁺ with 535 and 610 nm emitting CdSe/ZnS nanocrystals contributed to increasing white spectrum and generated the warm color temperature (4055 K) with high CRI (83.9) of white light. The acceptable color stability was also observed from (0.3687, 0.3051) at 20 mA to (0.3645, 0.3101) at 80 mA.     

Electrophosphorescence from iridium complex-doped mesogen-jacketed polymers

A mesogen-jacketed polymer P-Ct {poly{2,5-bis[(5-tert-butylphenyl)-1,3,4-oxadiazole]styrene}} has been investigated as a host material for IrMDPP [Ir(III)bis(5-methyl-2,3-diphenylpyrazine) (acetyl acetonate)] doped polymer electrophosphorescent device. It was found that the device with P-Ct was more efficient than that with PVK. The maximum luminance and external luminous efficiency can reach 3702 cd/m² and 0.83 cd/A, respectively, which were higher than those of device with PVK (1999 cd/m² and 0.68 cd/A), which can be partly explained by the more balanced carrier injection and transportation and longer lifetime of excitons in P-Ct-TPD-IrMDPP. It was also found that as the IrMDPP content increased in P-Ct-TPD, the EL spectra color shifts from green-yellow to yellow-orange and were different from PL spectra, which was partly due to the dominating role of direct charge-trapping and recombination in the EL process over the energy-transfer routes.     

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.     

2D photonic crystal layer assisted thiosilicate ceramic plate with red‐emitting film for high quality w‐LEDs

In this work, we have succeeded in obtaining high quality warm w‐light‐emitting‐diodes (LEDs) by adopting hybrid two‐dimensional (2D) structure of SiN_x photonic crystal layer (PCL) assisted cyan‐emitting ceramic‐plate thiosilicate SrLa₂Si₂S₈:Ce³⁺ with red‐emitting film SrLiAl₃N₄:Eu²⁺ phosphor on a 430 nm blue LED chip at 350 mA. 2D SiN_x PCL was capped with thiosilicate is because it can enhance the luminous efficacy and maintain the low correlated color temperature (CCT) and high color‐rendering index (CRI). High luminous efficacy (82.3 lm/W), high special CRI (R₉=75) as well as the low CCT (5431 K) of the optimal w‐LED was obtained due to the assistances of 2D SiN_x PCL and narrow‐band red‐emitting phosphor with the doping percentage at 10 wt%. The synthesis processes, structural analysis, optical properties and LED device performances were detailed investigated to find out the relationship between the optimum composition and good optical properties. Based on intriguing luminescence properties by the 2D SiN_x PCL and red‐emitting film phosphor introducing, we proclaim this method could also have high potential application in other phosphor‐converted w‐LEDs.     

Properties of polymer light‐emitting diodes coated on surface‐treated ITO/glass substrates

We fabricated blue polymer light‐emitting diodes (PLEDs) with indium tin oxide (ITO)/PEDOT : PSS/PVK/PFO‐poss/LiF/Al structures. All of the organic film layers were prepared by the spin‐coating method on plasma and heat‐treated ITO/glass substrates. The dependences of the optical and electrical properties of the PLEDs on the plasma and heat treatment of the ITO film and the introduction of poly(N‐vinylcarbazole) (PVK) layer were investigated. The AFM measurements indicated that the surface roughness of the ITO transparent film was improved by the plasma and heat treatment. In the emission spectra, the intensity of the excimer peaks of the PFO‐poss [polyhedral oligomeric silsesquioxane‐terminated poly(9,9‐dioctylfluorene)] emission layer were decreased for the PLED device with the PVK film layer compared with the one without the PVK layer. The maximum current density, luminance and current efficiency of the PLEDs were found to be about 470 mA/cm², 486 cd/m² at an input voltage of 12 V and 0.55 cd/A at 100 cd/m² in luminance, respectively. The color coordinates (CIE chart) of the blue PLEDs were in the range of x = 0.17 ～ 0.20, y = 0.13 ～ 0.16, and the peak emission spectrum was about 430 nm, showing a good blue color. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008