Light-emitting Layer Research Articles

Synthesis, aggregation-induced emission, and electroluminescence of a new compound based on tetraphenylethene, carbazole, and dimesitylboron moieties

A novel molecule, 3,6-bis(dimesitylboryl)-9-(4-(1,2,2-triphenylvinyl)phenyl)-9H-carbazole (DTPC), was synthesized by introducing two dimesitylboron groups and one tetraphenylethene group to the 3-position, 6-position, and 9-position of carbazole, respectively. The structure of DTPC was fully characterized by proton nuclear magnetic resonance spectroscopy, mass spectrometry, and elemental analysis. The thermal, electrochemical, and photophysical properties of DTPC were studied by thermogravimetry in conjunction with electrochemistry, UV–vis absorption spectroscopy and fluorescence spectroscopy. The results reveal that DTPC has excellent aggregation-induced emission properties. The multilayer electroluminescent device fabricated by using DTPC as light-emitting layer emits the blue light (λmax 489 nm) with CIE coordinates (0.17, 0.29) and shows good electroluminescent performances with the turn-on voltage of 5.7 V, maximum luminance of 5709 cd/m2 (at 15 V) and maximum luminance efficiency of 4.31 cd/A (at 8.2 V).

Color control of electrochemiluminescence based on duty ratio of applied waveform voltage

In this research, we succeeded in controlling the light-emitting color obtained from the light-emitting surface of an organic electrochemiluminescence (ECL) device with a duty ratio of the applied voltage. The color change was based on the absorption of light by the light-emitting layer. Because we were able to control the light-emitting relative quantities in each electrode by changing the duty ratio. Long light-emitting lifetime will be expected in the future, because the light-emitting material used was poly(3-hexylthiophene-2,5-diyl) (P3HT), a conductive polymer. We fabricated the ECL device with a light-emitting layer thickness of 25.0 µm. In this device, the ECL color was changed from orange to red by changing the duty ratio from 50 to 10%. More ECL colors are expected to be realized by selecting the light-emitting material with appropriate absorption and ECL spectra.

Electroluminescent layers based on ZnS:Cu deposited into matrices of porous anodic Al2O3

It is suggested to use a new nanocomposite material—nanostructures of copper-doped zinc sulfide in a matrix of porous aluminum oxide—as a light-emitting layer of electroluminescent sources of light. The material was deposited by thermal evaporation in a vacuum. The microstructure of the layers, impurity distribution in the electroluminescent-phosphor layer, and electroluminescence spectra at various copper concentrations in ZnS:Cu were studied.

Metal-free phthalocyanine single crystal: Solvothermal synthesis and near-infrared electroluminescence

Abstract A metal-free purple H2Pc single crystal was synthesized by a facile solvothermal method, and its solubility and near-infrared (NIR) optical properties were also investigated due to its potential applications as a light-emitting layer for OLEDs. The H2Pc single crystal is insoluble in 1-chlorine naphthalene and other organic solvents. It gives a wide absorption in the range from 620 nm to 679 nm and a wide emission in near 922 nm. As an active light-emitting layer, H2Pc was employed to fabricate electroluminescent (EL) devices with a structure of ITO/NPB (30 nm)/Alq3:H2Pc (30 nm)/BCP (20 nm)/Alq3 (20 nm)/Al. The emission center is at 936 nm when the H2Pc doping concentration is 20 wt%. The doping concentration strongly governs the emission intensity. When doping concentration decreases from 10 wt% to 1 wt%, the emission intensity remarkably fades, and simultaneously the emission center undergoes a blue shift.

Highly efficient pure red organic light-emitting devices based on tris(1-phenyl-isoquinoline) iridium(III) with another wide gap iridium(III) complex as sensitizer

In this work, we report the realization of highly efficient pure red organic co-doped electroluminescent device with tris(1-phenyl-isoquinoline) iridium(III) (Ir(piq)3) and iridium(III) bis(4,6-(difluorophenyl)pyridinato-N,C2′)picolinate (FIrpic) as emitter and sensitizer, respectively. By selecting 4,4′,4″-Tri(9-carbazoyl)triphenylamine and 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine as host materials, a series of devices with single or double light-emitting layer(s) were fabricated and investigated. The well-known wide gap blue emitter FIrpic, which possesses low-lying energy levels, was co-doped minutely into electron dominant light-emitting layer. Compared with reference devices, co-doped devices displayed significant improvement of device performances attributed to improved carriers' balance and broadening recombination zone. Finally, the 0.3 wt% co-doped double light-emitting layers device obtained the maximum brightness, current efficiency, power efficiency and external quantum efficiency up to 28,031 cd/m2, 14.89 cd/A, 12.99 lm/W and 9.0%, respectively. Even at the certain brightness of 1000 cd/m2, EL efficiency as high as 12.06 cd/A can be retained by the same device.

Synthesis and Optoelectronic Properties of a Solution-Processable Anthraquinone/Fluorene Hybrid Bipolar Fluorescent Material

The solution-processable, anthraquinone-based, fluorene bipolar fluorescent material 2-(9,9'-bis (2-ethylhexyl)-9H-fluoren-2-yl)anthracene-9,10-dione (FAA) was synthesized via a Suzuki reaction. The photophysical properties of FAA were subsequently investigated by acquiring absorption and photoluminescence spectra, and its optical properties were studied using computational density functional theory methods. Data obtained from single-carrier devices incorporating FAA demonstrated its well-matched bipolar charge-transport characteristics. The electroluminescence performance of this material was also examined by doping FAA into a 1,3-di(9H-carbazol-9-yl)benzene (mCP) matrix as the light-emitting layer via spin coating to produce an organic light-emitting diode (OLED) with an indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene:poly(styrenesulfonate) (PEDOT:PSS)/mCP:FAA/3,3'-(5'-(3-(pyridin-3-yl)phenyl)-[1,1':3',1 ''-terphen-yl]-3,3 ''-diyl)dipyridine (TmPyPb)/LiF/Al structure. This device exhibited a maximum luminance of 1719 cd.m(-2) with a turn-on voltage of 7.4 V, along with maximum current and power efficiencies of 1.66 cd.A(-1) and 0.56 Im.W-1, respectively. The electroluminescence mechanism of the OLED is discussed based on the energy level diagrams of the functional layers.

Application of CdSe/ZnS/CdS/ZnS Core–multishell Quantum Dots to Modern OLED Technology

Semiconductor quantum dots (QDs) obtained by means of colloidal synthesis are widely used in fabrication of organic light-emitting devices (OLEDs). We have fabricated QD–OLEDs with an active light-emitting layer formed by novel CdSe/ZnS/CdS/ZnS core–multishell QDs that have luminescence quantum yield of more than 90%. We have compared the performance of QD–OLEDs with different thickness of the active layer and demonstrated that the best performance is achieved at 45-55nm, probably due to field redistribution inside the device. Optimization of the thicknesses of transport layers of QD–OLED is likely to further improve the performance of such devices.

A Color-Tunable Polychromatic Organic-Light-Emitting-Diode Device With Low Resistive Intermediate Electrode for Roll-to-Roll Manufacturing

A flexible organic-light-emitting diode (OLED) with capability to show 16 million colors is fabricated on plastic barrier-film substrate, which can produce arbitrary shape with arbitrary colors, suitable for artistic expressions. Independently controlled red, green, and blue light-emitting layers are stacked vertically, so that no visible structure can be observed even with magnifiers from right-in-front measurement. In the past, large voltage drop of intermediate electrode was preventing this approach to be applied to actual electronic devices. However, according to the surface mobility control using Fick’s law analysis, low sheet resistance 7.34 $\Omega $ /□ on plastic film is developed, so that 7.17-cm2 area emission is successfully achieved. With optical length optimization for each color stack, more than 100% color reproduction in National Television Committee Standard is achieved by stack design. The device can be used for colored illumination, as well as for organic-light-emitting display pixels for three times emission than the conventional pixel design. The device is fabricated on plastic substrate, so that the polychromatic OLED device is manufacturable with roll-to-roll production line.

Toward highly efficient blue organic light-emitting diodes: fabricating a good-quality emissive layer cast from suitable solvents.

Spin-cast from various solvents, emissive layers show different film morphologies and performances in solution-processed organic light-emitting diodes (OLEDs). Here we fabricated and demonstrated highly efficient blue OLEDs based on bis[3,5-difluoro-2-(2-pyridyl)phenyl]-(2-carboxypyridy)iridium(iii) by choosing several kinds of solvents for spin-coating. Experiments indicate that the single-layer device with an emissive film cast from chlorobenzene shows its best performance with a highest current efficiency of 18.99 cd A(-1), a maximum luminance of 20.5 × 10(3) cd m(-2) and an emission band centered at 474 nm. The efficiency achieved is the highest reported for solution-processed simple-manufactured OLEDs doped with transition metal phosphors emitting in the blue region.

2,5-Dicarbazole-functioned siloles with aggregation-enhanced emission for application in organic light-emitting diodes

2,3,4,5-Tetraarylsiloles are efficient solid-state luminescent materials with good electron-transporting ability. Substitution at the 2,5-positions of silole rings produce various outstanding functional materials that can be used as active layers in organic light-emitting diodes (OLEDs). In this work, two 2,5-dicarbazole-substitued siloles, (2-Cz)2MTPS and (3-Cz)2MTPS, are facilely synthesized and fully characterized. Their thermal, photophysical, electrochemical, and electroluminescent properties are investigated systematically. The results show that these 2,5-dicarbazole-functined siloles are thermally stable and feature aggregation-enhanced emission characteristics with high solid-state photoluminescence efficiencies. Nondoped OLEDs [ITO/N,N'-di(1-naphthyl)-N,N'-diphenyl-benzidine (NPB) (60 nm)/emitter (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)] fabricated by adopting (2-Cz)2MTPS and (3-Cz)2MTPS as light-emitting layers exhibit good performances, with high luminance of 28240 cd m−2 and electroluminescence efficiency of 4.5 cd A−1.

Polyphenylbenzene as a platform to fabricated blue-emitting polymers: The effect of aggregation inhibition and application in PLED

The modification of polyfluorene is of significance. Here according to the two factors that influence the blue emission, we design and synthesize two copolymers which contain polyphenylbenzene and fluorene, polyphenylbenzene and carbazole, respectively. Thermogravimetric analysis (TGA) results conforms that the introducing of polyphenylbenzene unit can improve thermal stability of the polymer. Through the research of photophysical data and comparing with copolymers contain benzene unit, we get that the polyphenylbenzene unit can not only largely improve the solubility, but also effectively inhibit the contaction between polymer chains. This can guarantee the original blue emission even in solid state. Theoretical calculation results show that in the optimized structures, the dihedral angle between polyphenylbenzene and fluorene unit is 61.1°, bigger than that in the polyphenylbenzene and benzene copolymer (36.9°). This further conform that the introducing of polyphenylbenzene unit can make polymers twisted and reduce conjugation. Also, the formation of excimers can be inhibited which can guarantee blue emission. Based on their good emission efficiency and stability, we fabricated polymer light emission diodes using them as light emission layers. Devices results show that polyphenylbenzene and carbazole copolymer can exhibit better deep-blue emission, the CIE can stay at (0.19, 0.13). All these results show that the polyphenylbenzene unit can be used as an effective building block to fabricate polymer with deep-blue emission.

All-Quantum-Dot Infrared Light-Emitting Diodes.

Colloidal quantum dots (CQDs) are promising candidates for infrared electroluminescent devices. To date, CQD-based light-emitting diodes (LEDs) have employed a CQD emission layer sandwiched between carrier transport layers built using organic materials and inorganic oxides. Herein, we report the infrared LEDs that use quantum-tuned materials for each of the hole-transporting, the electron-transporting, and the light-emitting layers. We successfully tailor the bandgap and band position of each CQD-based component to produce electroluminescent devices that exhibit emission that we tune from 1220 to 1622 nm. Devices emitting at 1350 nm achieve peak external quantum efficiency up to 1.6% with a low turn-on voltage of 1.2 V, surpassing previously reported all-inorganic CQD LEDs.

Microfluidic White Organic Light-Emitting Diode Based on Integrated Patterns of Greenish-Blue and Yellow Solvent-Free Liquid Emitters.

We demonstrated a novel microfluidic white organic light-emitting diode (microfluidic WOLED) based on integrated sub-100-μm-wide microchannels. Single-μm-thick SU-8-based microchannels, which were sandwiched between indium tin oxide (ITO) anode and cathode pairs, were fabricated by photolithography and heterogeneous bonding technologies. 1-Pyrenebutyric acid 2-ethylhexyl ester (PLQ) was used as a solvent-free greenish-blue liquid emitter, while 2,8-di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene (TBRb)-doped PLQ was applied as a yellow liquid emitter. In order to form the liquid white light-emitting layer, the greenish-blue and yellow liquid emitters were alternately injected into the integrated microchannels. The fabricated electro-microfluidic device successfully exhibited white electroluminescence (EL) emission via simultaneous greenish-blue and yellow emissions under an applied voltage of 100 V. A white emission with Commission Internationale de l’Declairage (CIE) color coordinates of (0.40, 0.42) was also obtained; the emission corresponds to warm-white light. The proposed device has potential applications in subpixels of liquid-based microdisplays and for lighting.

Multicolor and near-infrared electroluminescence from the light-emitting devices with rare-earth doped TiO2 films

We report on multicolor and near-infrared electroluminescence (EL) from the devices using rare-earth doped TiO2 (TiO2:RE) films as light-emitting layers, which are ascribed to the impact excitation of RE3+ ions, with the EL onset voltages below 10 V. The devices are in the structure of ITO/TiO2:RE/SiO2/Si, in which the SiO2 layer is ∼10 nm thick and RE includes Eu, Er, Tm, Nd, and so on. With sufficiently high positive voltage applied on the ITO electrode, the conduction electrons in Si can tunnel into the conduction band of SiO2 layer via the trap-assisted tunneling mechanism, gaining the potential energy ∼4 eV higher than the conduction band edge of TiO2. Therefore, as the electrons in the SiO2 layer drift into the TiO2:RE layer, they become hot electrons. Such hot electrons impact-excite the RE3+ ions incorporated into the TiO2 host, leading to the characteristic emissions.

Electronic structure of the polymer-cathode interface of an organic electroluminescent device investigated using operando hard x-ray photoelectron spectroscopy

The electronic structure of a polymer-cathode interface of an operating organic light-emitting diode (OLED) was directly investigated using hard X-ray photoelectron spectroscopy (HAXPES). The potential distribution profile of the light-emitting copolymer layer as a function of the depth under the Al/Ba cathode layer in the OLED depended on the bias voltage. We found that band bending occurred in the copolymer of 9,9-dioctylfluorene (50%) and N-(4-(2-butyl)-phenyl)diphenylamine (F8-PFB) layer near the cathode at 0 V bias, while a linear potential distribution formed in the F8-PFB when a bias voltage was applied to the OLED. Direct observation of the built-in potential and that band bending formed in the F8-PFB layer in the operating OLED suggested that charges moved in the F8-PFB layer before electron injection from the cathode.

Efficient single light-emitting layer pure blue phosphorescent organic light-emitting devices with wide gap host and matched interlayer

In this work, we report the highly efficient pure blue electroluminescent (EL) device based on bis[(3,5-difluoro-4-cyanophenyl)pyridine]picolinate iridium(III) (FCNIrpic) doped 9-(4-tert-Butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) film. The matched energy levels of FCNIrpic and CzSi are helpful in facilitating the trapping of carriers, while the high triplet energy of CzSi can well avoid the undesired reverse energy transfer. More importantly, the injection of holes was further accelerated by inserting 5nm 4,4′,4″-Tri(9-carbazoyl)triphenylamine (TcTa) film between hole transport layer and lighting-emitting layer (EML) as interlayer. Consequently, EL performances were significantly enhanced attributed to wider recombination zone and better balance of holes and electrons. Interestingly, single-EML device displayed higher performances than those of double-EMLs device. Finally, pure blue EL device with the structure of ITO/MoO3 (3nm)/TAPC (40nm)/TcTa (5nm)/FCNIrpic (20%): CzSi (30nm)/TmPyPB (40nm)/LiF (1nm)/Al (100nm) realized the maximum brightness, current efficiency, power efficiency and external quantum efficiency up to 12,505cd/m2, 36.20cd/A, 28.42lm/W and 16.9%, respectively. Even at the high brightness of 1000cd/m2, current efficiency and external quantum efficiency up to 17.40cd/A and 8.1%, respectively, can be retained by the same device.

Synthesis and optoelectronic properties of a solution-processed red-emitting tetra(arylvinyl)anthracene cruciform

In this paper, we report the synthesis and optoelectronic properties of a new anthracene-centered cruciform, 2,6-bis(9,9-di(2-ethylhexyl)fluorene-2-yl-vinyl-2)- 9,10-bis(9-(2-ethylhexyl)carbazole-3-yl-vinyl-2)anthracene (FCA). FCA was prepared by the Heck coupling 2,6-bis(diethoxylphosphorylmethyl)-9,10- dibromoanthracene and 9-(2-ethylhexyl)-3-vinylcabazole, followed by the Wittig–Horner reaction with 9,9-di(2-ethylhexyl)-2-formylfluorene. The existence of strongly twisted 9,10-branch made FCA exhibit moderate aggregation-enhanced emission and piezofluorochromism, and the introduction of numerous branched alkyl chains render FCA with good solubility and film-forming ability. FCA could be used as solution-processed light-emitting layer to afford red emission with peak wavelength of 612 nm and maximal luminous efficacy of 1.3 cd/A.

Rare Earth Complex as Electron Trapper and Energy Transfer Ladder for Efficient Red Iridium Complex Based Electroluminescent Devices.

In this work, we experimentally demonstrated the new functions of trivalent rare earth complex in improving the electroluminescent (EL) performances of iridium complex by codoping trace Eu(TTA)3phen (TTA = thenoyltrifluoroacetone, phen = 1,10-phenanthroline) into a light-emitting layer based on PQ2Ir(dpm) (iridium(III)bis(2-phenylquinoly-N,C(2'))dipivaloylmethane). Compared with a reference device, the codoped devices displayed higher efficiencies, slower efficiency roll-off, higher brightness, and even better color purity. Experimental results demonstrated that Eu(TTA)3phen molecules function as electron trappers due to its low-lying energy levels, which are helpful in balancing holes and electrons and in broadening recombination zone. In addition, the matched triplet energy of Eu(TTA)3phen is instrumental in facilitating energy transfer from host to emitter. Finally, highly efficient red EL devices with the highest current efficiency, power efficiency and brightness up to 58.98 cd A(-1) (external quantum efficiency (EQE) of 21%), 61.73 lm W(-1) and 100870 cd m(-2), respectively, were obtained by appropriately decreasing the doping concentration of iridium complex. At certain brightness of 1000 cd m(-2), EL current efficiency up to 51.94 cd A(-1) (EQE = 18.5%) was retained. Our investigation extends the application of rare earth complexes in EL devices and provides a chance to improve the device performances.

Efficient exciplex organic light-emitting diodes with a bipolar acceptor

Efficient exciplex organic light-emitting diodes (OLEDs) have been fabricated with an intermixed light-emitting layer comprising electron-rich m-MTDATA or TAPC as a donor and bipolar material 26DCzPPy as an acceptor. Intermolecular charge–transfer interaction between various donor materials with different highest occupied molecular orbital (HOMO) levels and 26DCzPPy are systematically investigated to explore the formation principle of the exciplex. Various techniques like steady state absorption and photoluminescence (PL) spectra, temperature dependent transient PL decay and time-resolved emission spectra (TRES) using time-correlated single-photon counting (TCSPC) principle were adopted to probe the electronic and optical properties of these exciplex systems. Strong exciplex emission was found from the m-MTDATA:26DCzPPy and TAPC:26DCzPPy mixed systems. The devices comprising these two exciplex systems as the emission layer exhibited a promising external quantum efficiency and a high exciton utilization efficiency due to the triplet exciton up-conversion from triplet to singlet states inspired by their small singlet–triplet exchange energy. Our current work contributes to know more about the unique intermolecular charge–transfer property between electron-rich donors and bipolar acceptors, and provides valuable exploration on developing novel high-performance exciplex OLEDs.

High performance yellow organic electroluminescent devices by doping iridium(III) complex into host materials with stepwise energy levels

In this work, we aim to further improve the electroluminescent (EL) performances of a yellow light-emitting iridium(III) complex by designing double light-emitting layers (EMLs) devices having stepwise energy levels. Compared with single-EML devices, these designed double-EML devices showed improved EL efficiency and brightness attributed to better balance in carriers. In addition, the stepwise distribution in energy levels of host materials is instrumental in broadening the recombination zone, thus delaying the roll-off of EL efficiency. Based on the investigation of carriers' distribution, device structure was further optimized by adjusting the thickness of deposited layers. Finally, yellow EL device (Commission Internationale de l'Eclairage (CIE) coordinates of (0.446, 0.542)) with maximum current efficiency, power efficiency and brightness up to 78.62cd/A (external quantum efficiency (EQE) of 21.1%), 82.28lm/W and 72,713cd/m2, respectively, was obtained. Even at the high brightness of 1000cd/m2, EL efficiency as high as 65.54cd/A (EQE=17.6%) can be retained.