Doping-free hybrid white organic light-emitting diodes with fluorescent blue,phosphorescent green and red emission layers

Industrialized white organic light-emitting diodes (OLEDs) currently require host-guest doping, a complicated process necessitating precise control of the guest concentration to get high efficiency and stability. Two doping-free, hybrid white OLEDs with fluorescent blue, and phosphorescent green and red emissive layers (EMLs) are reported in this work. An ultra-thin red phosphorescent EML was situated in a blue-emitting electron transport layer (ETL), while the ultra-thin green phosphorescent EML was placed either in the ETL (Device 1), or the hole transport layer (HTL) (Device 2). Device 2 exhibits higher efficiency and more stable spectrum due to the enhanced utilization of excitons by ultra-thin green EML at the exciton generation zone within the HTL. Values of current efficiency (CE), power efficiency (PE), and CRI obtained for the optimized hybrid white OLEDs fabricated through a doping-free process were of 23.2 cd/A, 20.5 lm/W and 82 at 1000 cd/m², respectively.     

Cadmium-free quantum dots based violet light-emitting diodes: High-efficiency and brightness via optimization of organic hole transport layers

Bright and efficient violet quantum dot (QD) based light-emitting diodes (QD-LEDs) with heavy-metal-free ZnSe/ZnS have been demonstrated by choosing different hole transport layers, including poly(4-butyl-phenyl-diphenyl-amine) (poly-TPD), poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB), and poly-N-vinylcarbazole (PVK). Violet QD-LEDs with maximum luminance of about 930 cd/m², the maximum current efficiency of 0.18 cd/A, and the peak EQE of 1.02% when poly-TPD was used as HTL. Higher brightness and low turn-on voltage (3.8 V) violet QD-LEDs could be fabricated when TFB was used as hole transport material. Although the maximum luminance could reach up to 2691 cd/m², the devices exhibited only low current efficiency (∼0.51 cd/A) and EQE (∼2.88%). If PVK is used as hole transport material, highly efficient violet QD-LEDs can be fabricated with lower maximum luminance and higher turn-on voltages compared with counterpart using TFB. Therefore, TFB and PVK mixture in a certain proportion has been used as HTL, turn-on voltage, brightness, and efficiency all have been improved greatly. The QD-LEDs is fabricated with 7.39% of EQE and 2856 cd/m² of maximum brightness with narrow FWHM less than 21 nm. These results represent significant improvements in the performance of heavy-metal-free violet QD-LEDs in terms of efficiency, brightness, and color purity.     

A high thermal stability terpyridine derivative as the electron transporter for long-lived green phosphorescent OLED

A new terpyridine-based compound of 2,2′,7,7′-tetra([2,2':6′,2″-terpyridin]-4′-yl)-9,9′-spirobi[fluorene] (4oTPSF) was designed and synthesized as the electron transporter in organic light-emitting diodes (OLEDs). 4oTPSF exhibited excellent thermal stability with high glass transition temperature (T_g) of 250 °C and melting temperature (T_m) of 460 °C during the thermal measurement. The excellent thermal stability is attributed to the molecular structure, that the steric effect of rigid twisted spirobiflourene and the connected terpyridine (TPY) resulted in a decrease of the intermolecular π-stacking interaction. The studies on electrical characteristics of electron-only devices revealed that 4oTPSF showed high electron-transporting capability, as good as the conventional electron-transporting material (ETM) 1,3,5-tris(N-phenylbenzimid-azol-2-yl-benzene (TPBi). A series of green phosphorescent OLEDs (PhOLEDs) based on bis(2-phenylpyridine)iridium(III)(2,2,6,6-tetramethylheptane-3,5-diketonate) (Ir(ppy)₂tmd) or tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)₃) as emitter and 4oTPSF as ETM displayed a turn-on voltage of 2.23 V and a maximum power efficiency of 97.8 l m/W and a half-life (T₅₀) of 101, 5680 and 319 390 h at an initial luminance of 10 000, 1000 and 100 cd/m², respectively. The lifetime of 4oTPSF-based device was twice more than the lifetime of TPBi-based device.     

Designing a solution-processable electron transport layer for transparent organic light-emitting diode

In this work, the extensively used opaque metal cathodes of the conventionally structured OLEDs were replaced with the widely used transparent electrode indium tin oxide (ITO) for solution-processed transparent organic light-emitting diode (T-OLED). A new solution-processable electron transport layer (ETL), aside from facilitating the efficient injection of electrons into the T-OLED, protected the organic emission layer (EML) of the T-OLED against the plasma damage during top ITO cathode sputter deposition. The newly designed solution-processed ETL was the composite of the zinc oxide nanoparticles (ZnO-NPs), and cesium carbonate-doped ethoxylated polyethyleneimine (d-PEIE) with the semi-hydrophilic poly (methyl methacrylate) (PMMA) interlayer coated on the EML insured the good wettability and contact of the hydrophilic ETL with the hydrophobic EML. The solution-processed T-OLED emitted the total maximum luminance of about 2417 cd/m² (bottom side emission at 1455 cd/m² and top emission at 962 cd/m²), total maximum current efficiency at 3.12 cd/A (bottom and top emissions at about 1.78 and 1.34 cd/A, respectively), and total maximum power efficiency at 1.64 l m/W (bottom and top emissions at about 0.95 and 0.69 l m/W, respectively) while having a very high optical transmittance of around 85% at 550 nm light wavelength.     

Stacked inverted top-emitting green electrophosphorescent organic light-emitting diodes on glass and flexible glass substrates

Stacked inverted top-emitting green electrophosphorescent organic light-emitting diodes (OLEDs) are demonstrated on glass and flexible glass substrates. A single-unit OLED is shown to have a current efficacy of 46.8 cd/A at a luminance of 1215 cd/m². When two of these OLEDs are stacked, the double-unit OLED exhibits a current efficacy more than twice that of the single-unit OLED, with a current efficacy of 97.8 cd/A at a luminance of 1119 cd/m². With the addition of an optical outcoupling layer of N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (α-NPD) on top of the semitransparent gold anode, the double-unit stacked OLED achieves a maximum current efficacy of 205 cd/A at a luminance of 103 cd/m², maintaining a high current efficacy of 200 cd/A at a luminance of 1011 cd/m². These stacked inverted OLED combine the advantages of inverted OLEDs with the benefits of having a stacked architecture.     

High-performance azure blue quantum dot light-emitting diodes via doping PVK in emitting layer

Highly bright and efficient azure blue quantum dot-based light-emitting diodes (QD-LEDs) have been demonstrated by employing ZnCdSe core/multishell QDs as emitters and the crucial development we report here is the ability to dramatically enhance the efficiency and brightness through doping poly vinyl(N-carbazole) (PVK) in the emissive layer to balance the charge injection. The best device displays remarkable features like maximum luminance of 13,800 cd/m², luminous efficiency of 6.41 cd/A, and external quantum efficiency (EQE) of 8.76%, without detectable red-shift and broadening in electroluminescence (EL) spectra with increasing voltage as well as good spectral matching between photoluminescence (PL) and EL. Such azure blue quantum-dot LEDs show a 140% increase in external quantum efficiency compared with QD-LEDs without PVK. More important, the peak efficiency of the QD-LEDs with PVK dopant is achieved at luminance of about 1000 cd/m², and high efficiency (EQE > 8%) can be maintained with brightness ranging from 200 to 2400 cd/m². There are two main aspects of the role of PVK in the proposed system. Firstly, the lower HOMO of PVK than (poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) can reduce the potential barrier for 0.4 eV at the interface of QDs and hole transport layer which could result in higher hole injection efficiency along with good EQE as compared to TFB-only HTLs. Secondly, with PVK acting as buffer layer of TFB and QDs, the exciton energy transfer from the organic host to the QDs can be effectively improved.     

Efficient solution-processed green and white phosphorescence organic light-emitting diodes based on bipolar host materials

Four carbazole-based bipolar host materials are utilized for solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). These bipolar materials consist of an electron-donor unit (carbazole) linking to a fluorene unit bearing various electron-acceptor units (oxadiazole, cyano, and benzimidazole) via a saturated carbon, giving sufficiently high triplet energies due to the lack of direct electronic coupling between the donor and acceptor(s). The resulting physical properties and bipolar characteristics render the realization of efficient solution-processed green and white OLEDs feasible. The best green light-emitting device based on bipolar host CzFCBI incorporating a stepwise hole-injection/transporting system exhibit a low drive voltage, a maximum external quantum efficiency of 14.0%, a current efficiency of 49.0 cd/A, and a power efficacy of 55.0 lm/W. Moreover, the CzFOXa-based two-component (blue–orange) white light-emitting device shows a warmish-white emission with a maximum external quantum efficiency of 6.9% and stable chromaticity coordinates at different luminance levels and yield a high color rendering index (CRI) reaching 76 at a luminance of 1000 cd/m².     

Fluorene bilayer for polymer organic light-emitting diode using efficient ionization method for atomized droplet

We present a solution-processed planar fluorene bilayer by an ultrasonic atomized deposition method in combination with a needle electrode as an ionization part for an atomized droplet. An important advantage of our method is that the atomized droplet is efficiently charged using a needle electrode, which speeds up the deposition rate of the polymer thin film. The deposition rate increases 2 to 3 times compared to a that obtained with a conventional technique without using the ionization method, and real-time monitoring of landed droplets indicates that the number of droplets increased as the voltage applied to the needle electrode was increased, owing to the highly charged atomized droplets. Furthermore, the TFB/F8BT bilayer was achieved by optimizing the substrate temperature, and the polymer organic light-emitting diode exhibits a luminance value exceeding 12,000 cd/m² by insertion of the TFB as an electron blocking layer. The maximum current efficiency of the fluorene bilayer device was 6.64 cd/A, which was a 3.2-fold increase compared to that obtained with the reference device without the TFB electron blocking layer.     

A hybrid white organic light-emitting diode with stable color and reduced efficiency roll-off by using a bipolar charge carrier switch

A hybrid white organic light-emitting diode (WOLED) with an emission layer (EML) structure composed of red phosphorescent EML/green phosphorescent EML/spacer/blue fluorescent EML was demonstrated. This hybrid WOLED shows high efficiency, stable spectral emission and low efficiency roll-off at high luminance. We have attributed the significant improvement to the wide distribution of excitons and the effective control of charge carriers in EMLs by using mixed 4,4′,4″-tri(9-carbazoyl) triphenylamine (TCTA) and bis[2-(2-hydroxyphenyl)-pyridine] beryllium (Bepp₂) as the host of phosphorescent EMLs as well as the spacer. The bipolar mixed TCTA:Bepp_2, which was proved to be a charge carrier switch by regulating the distribution of charge carriers and then the exciton recombination zone, plays an important role in improving the efficiency, stabilizing the spectrum and reducing the efficiency roll-off at high luminous. The hybrid WOLED exhibits a current efficiency of 30.2 cd/A, a power efficiency of 32.0 lm/W and an external quantum efficiency of 13.4% at a luminance of 100 cd/m², and keeps a current efficiency of 30.8 cd/A, a power efficiency of 27.1 lm/W and an external quantum efficiency of 13.7% at a 1000 cd/m². The Commission Internationale de l’Eclairage (CIE) coordinates of (0.43, 0.43) and the color rendering index (CRI) of 89 remain nearly unchanged in the whole range of luminance.     

Solution-processed planar white light-emitting diodes based on cadmium-free Cu-In-Zn-S/ZnS quantum dots and polymer

Bright white light-emitting diodes (WLEDs) were fabricated by using a simple solution-processed technique, in which the yellow cadmium-free Cu-In-Zn-S/ZnS core/shell quantum dots (QDs) blending with poly [(9, 9-dioctylfluorenyl-2, 7-diyl)-co-(4, 4′-(N-(p-butylphenyl)) diphenylamine)] (TFB) was used as emissive layers. The color of the electroluminescence (EL) from the device could be tuned from blue-green to white by varying the thermal annealing temperatures, and white EL emission could be obtained under the annealing temperature of 95 °C. A high color rendering index (CRI) of 90 and the Commission Internationale de l'Eclairage (CIE) color coordinates of (0.33, 0.32) were achieved in the WLEDs annealed at 95 °C, respectively. The WLEDs exhibited a low turn-on voltage of 2.5 V and a maximum luminance of 1500 cd/m², which were maintained at 0.1 cd/A over a wide range of luminance from 100 to 1300 cd/m². This work may open up a new way to realize white light in the planar WLEDs based on the cadmium-free QDs.     

An Effective Approach toward Yellow-to-Orange Multi-Resonance TADF Emitters by Integrating Strong Electron Donor into B/N-Based Polycyclic Architecture: High Performance OLEDs with Nearly 40% EQE

B/N-based multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters and the corresponding narrow band emissive organic light-emitting diodes (OLEDs) exhibit great potential for next-generation high-resolution displays. Nonetheless, designing MR-TADF emitters with emission wavelength over 550 nm remains challenging. Herein, an effective approach toward yellow-to-orange MR-TADF emitters by integrating a strong electron-donating indolophenazine building block into the B/N-doped polycyclic aromatic hydrocarbons is proposed. The investigation of photophysical properties reveals that the electron-donating difference between the donor segments of MR framework has a dramatic influence on the luminescent features, including the emission wavelength and full-width at half-maximum (FWHM). These TADF emitters display excellent photophysical characteristics such as near-unity photoluminescence quantum yields and almost 100% horizontal dipole ratio. As a result, yellow and orange OLEDs employing these emitters achieve state-of-the-art device performances with an ultrahigh external quantum efficiency of up to nearly 40%, power efficiency of 163 lm W⁻¹, and luminance close to 120 000 cd m⁻², which set a record among MR-TADF based OLEDs with emission peaks over 550 nm. More impressively, the fabricated device presents outstanding operational stability of LT₉₉ over 110 h at the initial brightness of 3000 cd m⁻².     

Achieving above 30% external quantum efficiency for inverted phosphorescence organic light-emitting diodes based on ultrathin emitting layer

High efficiency inverted phosphorescence organic light-emitting diodes (PhOLEDs) based on ultrathin undoped and doped emitting layer (EML) have been developed. Compared to conventional device, the inverted PhOLED with 0.5 nm undoped EML exhibits significantly larger external quantum efficiency (EQE), due to effective energy transfer from the excited host to the emitter. According to the atomic force microscopy image of EML, the 0.5 nm emitter sandwiched by two hosts can be considered as the emitter doped in two hosts. The inverted device with intentionally doped ultrathin EML (1.5 nm) exhibits the maximum EQE of 31.1%, which is attributed to optimized charge balance and preferred horizontal orientation of emitter. However, such inverted device has large efficiency roll-off at high brightness because of triplet–triplet annihilation process within the ultrathin EML. This can be improved by broadening the doped EML. The inverted device with 10.5 nm doped EML has about EQE of 20 % at 10,000 cd/m². It is expected that our inverted PhOLED will promote development of high efficiency active-matrix organic light-emitting diodes based on the n-type Indium Gallium Zinc Oxide thin film transistor.     

Efficient non-doped blue light emitting diodes based on novel carbazole-substituted anthracene derivatives

In this study, we synthesized three anthracene derivatives featuring carbazole moieties as side groups - 2-tert-butyl-9,10-bis[4-(9-carbazolyl)phenyl]anthracene (Cz⁹PhAnt), 2-tert-butyl-9,10-bis{4-[3,6-di-tert-butyl-(9-carbazolyl)]phenyl}anthracene (tCz⁹PhAnt), and 2-tert-butyl-9,10-bis{4′-[3,6-di-tert-butyl-(9-carbazolyl)]biphenyl-4-yl}anthracene (tCz⁹Ph₂Ant) - for use in blue organic light emitting devices (OLEDs). The anthracene derivatives presenting rigid and bulky tert-butyl-substituted carbazole units possessed high glass-transition temperatures (220 °C). Moreover, the three anthracene derivatives exhibited strong blue emissions in solution, with high quantum efficiencies (91%). We studied the electroluminescence (EL) properties of non-doped OLEDs incorporating these anthracene derivatives, with and without a hole-transporting layer (HTL). OLEDs incorporating an HTL provided superior EL performance than did those lacking the HTL. The highest brightness (6821 cd/m²) was that for the tCz⁹PhAnt-based device; the greatest current efficiency (2.1 cd/A) was that for the tCz⁹Ph₂Ant-based device. The devices based on these carbazole-substituted anthracene derivatives also exhibited high color purity.     

Organic light-emitting diodes on shape memory polymer substrates for wearable electronics

Green electrophosphorescent organic light-emitting diodes (OLEDs) with inverted top-emitting structures are demonstrated on bio-compatible shape memory polymer (SMP) substrates for wearable electronic applications. The combination of the unique properties of SMP substrates with the light-emitting properties of OLEDs pave to the way for new applications, including conformable smart skin devices, minimally invasive biomedical devices, and flexible lighting/display technologies. In this work, SMPs were designed to exhibit a considerable drop in modulus when a thermal stimulus is applied, allowing the devices to bend and conform to new shapes when its glass transition temperature is reached. These SMP substrates were synthesized using 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO), trimethylolpropane tris(3-mercaptopropionate) (TMTMP), and tricyclo[5.2.1.0^2,6]decanedimethanol diacrylate (TCMDA), and show a low glass transition temperature of 43 °C, as measured using dynamic mechanical analysis (DMA). The OLEDs fabricated on these substrates exhibit high performance with a maximum efficacy of 33 cd/A measured at a luminance of 1000 cd/m², and a peak luminance of over 30,000 cd/m².     

CBP超薄层对有机电致磷光器件效率的影响

在空穴传输层(HTL)和发光层(EML)间插入4,4-N,N′-二咔唑基联苯(CBP)超薄层,制备了结构为ITO/NPB/CBP(xnm)/CBP:Ir(ppy)3/BCP/Alq3/LiF/Al有机电致磷光器件。与未插入CBP超薄层的器件相比,CBP超薄层的引入可以有效阻挡Ir(ppy)3的三线态能量通过Dexter能量转移到HTL的NPB中,减少无辐射能量损失,提高了器件发光效率。调整CBP薄层的厚度,当x为3nm时,器件的效率提高幅度最大,从x为0nm时的9.0cd/A提高到16.9cd/A。     

Enhanced light-outcoupling in organic light-emitting diodes through a coated scattering layer based on porous polymer films

We report on the development and detailed investigation of green phosphorescent organic light-emitting diodes (OLEDs) with porous light-scattering layers prepared by combining a simple coating of polyimide-precursor and the immersion precipitation method. With the proper choice of solvent/non-solvent system in the pore-generation process, an external quantum efficiency (EQE) of 30.8% and a power efficiency (PE) of 82.2 lm/W can be realized at a luminance of 1000 cd/m². By analyzing the angular characteristics in terms of the luminance- and color-variation of OLEDs fabricated with various porous films, we find that the device performance is closely linked to both the light scattering from the porosity and the ray-path change, which depends on the pore shape. Raytracing optical simulation was also used to confirm that the shape-control of the pores in a porous polymer film modulates the light-outcoupling efficiency in OLEDs.     

High efficiency green phosphorescent top-emitting organic light-emitting diode with ultrathin non-doped emissive layer

Ultrathin non-doped emissive layer (EML) has been employed in green phosphorescent top-emitting organic light-emitting diodes (TOLEDs) to take full advantages of the cavity standing wave condition in a microcavity structure. Much higher out-coupling efficiency has been observed compared to conventional doped EML with relatively wide emission zone. A further investigation on dual ultrathin non-doped EMLs separated by a special bi-layer structure demonstrates better charge carrier balance and improved efficiency. The resulting device exhibits a high efficiency of 125.0 cd/A at a luminance of 1000 cd/m² and maintains to 110.9 cd/A at 10,000 cd/m².     

Understanding the influence of doping in efficient phosphorescent organic light-emitting diodes with an organic p–i–n homojunction

We report on the development and detailed investigation of highly efficient p–i–n phosphorescent organic light-emitting diodes (PhOLEDs) using 4,4′-bis(carbazol-9-yl)-biphenyl (CBP) as a single organic semiconductor matrix. Following optimization of doping concentration of both the phosphorescent emitter molecule and of the p- and n-type dopants, an external quantum efficiency (EQE) of 15% and a power efficiency (PE) of 28 lm/W are realized at a luminance of 1000 cd/m². These values are comparable to the state-of-the-art for conventional complex multilayered PhOLEDs. By analyzing the device characteristics (i.e. electroluminescence spectra, the current density–voltage behavior of single carrier devices, the transient electroluminescent decay, and the impedance spectroscopy response), we find that the device performance is closely linked to the charge carrier balance in the device, which in turn is governed by the interplay of the conductivities of the doped layers and the transport of each charge carrier species within the emitting layer.     

Triphenyl phosphine oxide-bridged bipolar host materials for green and red phosphorescent organic light-emitting diodes

Two wide band gap functional compounds of phenylbis(4-(spiro [fluorene-9,9'-xanthen]-2-yl)phenyl)phosphine oxide (2SFOPO) and (4-(9-ethyl-9H- carbazol-3-yl)phenyl)(phenyl)(4-(spiro[fluorene-9,9′-xanthen]-2-yl)phenyl)phosphine oxide (SFOPO-CZ) were designed, synthesized and characterized. Their thermal, photophysical, electrochemical properties and device applications were further investigated to correlate the chemical structure of bipolar host materials with the electroluminescent performance for phosphorescent organic light-emitting diodes (PhOLEDs). Both of them show high thermal stability with glass transition temperatures in a range of 105–122 °C and thermal decomposition temperatures at 5% weight loss in a range of 406–494 °C. The optical band gaps of compound 2SFOPO and SFOPO-CZ in CH₂Cl₂ solution are 3.46 and 3.35 eV, and their triplet energy levels are 2.51 eV and 2.52 eV, respectively. The high photoluminescent quantum efficiency of emissive layer of doped green device up to 50% is obtained. Employing the developed materials, efficient green and red PhOLED in simple device configurations have been demonstrated. As a result, the green PhOLEDs of compound SFOPO-CZ doped with tris(2-phenylpyridine) iridium shows electroluminescent performance with a maximum current efficiency (CE_max) of 52.83 cd A⁻¹, maximum luminance of 34,604 cd/m², maximum power efficiency (PE_max) of 39.50 lm W⁻¹ and maximum external quantum efficiency (EQE_max) of 14.1%. The red PhOLED hosted by compound 2SFOPO with bis(2-phenylpyridine)(acetylacetonato) iridium(III) as the guest exhibits a CE_max of 20.99 cd A⁻¹, maximum luminance of 33,032 cd/m², PE_max of 20.72 lm W⁻¹ and EQE_max of 14.0%. Compound SFOPO-CZ exhibits better green device performance, while compound 2SFOPO shows better red device performance in PhOLEDs.     

Efficient fluorescent white organic light-emitting devices based on a ultrathin 5,6,11,12-tetraphenylnaphthacene layer

White organic light-emitting devices (OLEDs) were fabricated using a ultrathin layer 5,6,11,12-tetraphenylnaphthacene as the yellow light-emitting layer and p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-ph) doped in 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) matrix as the blue light-emitting layer. The thickness of rubrene ultrathin layer will seriously affect the device performance, and the device with 1 nm rubrene achieves the best performance, with the maximum luminance of 33,152 cd/m² at 11 V and the maximum current efficiency of 8.69 cd/A at 7 V.