White OLEDs based on novel emissive materials such as Zn(HPB)2 and Zn(HPB)q

Organic light emitting diodes (OLEDs) show a lot of advantages for display purposes. Because OLEDs provide white light emission with high efficiency and stability, it is desirable to apply OLEDs as an illumination light source and backlight in LCD displays. We synthesized new emissive materials, namely [2-(2-hydroxyphenyl)benzoxazole] (Zn(HPB)₂) and [(2-(2-hydroxyphenyl)benzoxazole)(8-hydoxyquinoline)] (Zn(HPB)q), which have a low molecular compound and thermal stability. We studied white OLEDs using Zn(HPB)₂ and Zn(HPB)q. The fundamental structures of the white OLEDs were ITO/PEDOT:PSS (23 nm)/NPB (40 nm)/Zn(HPB)₂ (40 nm)/Zn(HPB)q (20, 30 or 40 nm)/Alq₃ (10 nm)/LiAl (120 nm). As a result, when the thickness of the Zn(HPB)q layer was 20 nm, white emission is achieved. We obtained a maximum luminance of 15325 cd/m² at a current density of 997 mA/cm². The CIE (Commission International de l'Eclairage) coordinates are (0.28, 0.35) at an applied voltage of 9.75 V.     

Highly efficient blue OLEDs based on diphenylaminofluorenylstyrenes end-capped with heterocyclic aromatics

In this paper, we have designed four diphenylaminofluorenylstyrene derivatives end-capped with heterocyclic aromatic groups, such as 9-phenylcabazole, 4-dibenzofuran, 2-benzoxazole, 2-quinoxaline, respectively. These materials showed blue to red fluorescence with maximum emission wavelengths of 476–611 nm, respectively, which were dependent on the structural and electronic nature of end-capping groups. To explore the electroluminescent properties of these materials, multilayer OLEDs were fabricated in the following sequence: ITO/DNTPD (40 nm)/NPB (20 nm)/2% doped in MADN (20 nm)/Alq₃ (40 nm)/Liq. (1 nm)/Al. Among those, a device exhibited a highly efficient blue emission with the maximum luminance of 14,480 cd/m² at 9 V, the luminous efficiency of 5.38 cd/A at 20 mA/cm², power efficiency of 2.77 lm/W at 20 mA/cm², and CIE_x,y coordinates of (0.147, 0.152) at 8 V, respectively.     

Electronic and thermal properties of compounds bearing diimide,azomethine and triphenylamine units

New triphenylamine containing azomethine diimides and two kinds of poly(azomethine imide)s, i.e., linear and branched were synthesized. These compounds were prepared from two diamines, that is, N,N′-bis(4-amino-2,3,5,6-tetramethylphenyl)phtalene-1,2,4,5-dicarboximide (DAPhDI), N,N′-bis(5-aminonaphtalen)naphthalene-1,4,5,8-dicarboxyimide (DANDI-2) and 4-formyltriphenylamine, 4,4′-diformyltriphenylamine and 4,4′,4″-triformyltriphenylamine. The structures of the compounds were characterized by means of FTIR, ¹H NMR spectroscopy and elemental analysis; the results show an agreement with the proposed structure. Thermal properties of prepared azomethine diimides and polymers were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Obtained compounds exhibited high thermal stability with 5% weight-loss temperatures above 390 °C. Azomethine diimides exhibited glass-forming properties with high glass-transition temperatures 216 and 308 °C. Optical properties of the prepared compounds were investigated by UV–vis and photoluminescence (PL) measurements. All compounds emitted blue light in NMP solution and in solid state as blend with PMMA. The electrochemical properties, that is, orbital energies and resulting energy gap were estimated based on cyclic voltammetry (CV). All synthesized material showed reversible reduction process, furthermore AzPhDI and AzNDI showed partially reversible oxidation process. Electrochemical band gap was found in the range 1.23–1.70 eV. Low molecular weight model compounds were tested as bipolar host materials in blue phosphorescent organic light emitting diodes (OLEDs). The devices exhibited turn-on voltages of about 5.5 V and maximum brightness of 40–220 cd/m².     

Quasi-solid-state dye-sensitized solar cells from hydrophobic poly(hydroxyethyl methacrylate/glycerin)/polyaniline gel electrolyte

Hydrophobic poly(hydroxyethyl methacrylate/glycerin) [poly(HEMA/GR)] gel with a three-dimensional (3D) framework was successfully fabricated and employed to integrate with polyaniline (PANi). The resultant poly(HEMA/GR)/PANi gel electrolyte exhibited interconnective porous structure for holding I⁻/I₃⁻, giving a similar conduction mechanism and ionic conductivity to that of liquid system but a much enhanced retention of I⁻/I₃⁻ redox couple. Fourier transform infrared spectroscopy, X-ray diffraction patterns, cyclic voltammograms as well as electrochemical impedance spectroscopy were employed to evaluate the molecular structure, crystallinity, and the electrochemical behaviors, showing that the combination of PANi with poly(HEMA/GR) caused a lower charge-transfer resistance and higher electrocatalytic activity for the I₃⁻/I⁻ redox reaction in the gel electrolyte. An efficiency of 6.63% was recorded from the quasi-solid-state DSSC assembled with the poly(HEMA/GR)/PANi gel electrolyte at 100 mW cm⁻².     

Tunable red emission by incorporation of a rubrene derivative in p-type and n-type hosts in organic light emitting devices

A red-emitting rubrene derivative, 2-formyl-5,6,11,12-tetraphenylnaphthacene (2FRb) was separately doped into 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) and tris-(8-hydroxyquinoline) aluminum (Alq₃) as the emitting layer. The emission can be tuned from 580 nm to 607 nm in NPB host and 560 nm to 622 nm in Alq₃ host. The Alq₃-hosted devices show better performances: more saturated pure red emission peaking at 622 nm with CIE_x,y = [x = 0.65, y = 0.35], a maximum luminance efficiency of 2.42 cd/A and a maximum luminance of 3100 cd/m² by doping 3.2 wt.% 2FRb in Alq₃ host. It indicates that the derivative of rubrene is a promising bipolar dopant for red light-emitting device.     

Correlation of recombination zone and device performances of phosphorescent white organic light-emitting diodes

We demonstrate a general method for tuning the color performance of white organic light-emitting diodes (WOLEDs) by inserting 0.5 nm thick red emitting layer in different location of blue phosphorescent emitting layer. The Commission Internationale de L'Eclairage (CIE) coordinates of WOLEDs were dependent on the position of red emitting layer and they were correlated with recombination zone of the blue phosphorescent emitting layer. Red shift of white CIE was observed as the location of red emitting layer get close to recombination zone of blue emitting layer. In addition, CIE of WOLEDs was kept stable between 100 cd/m² and 10,000 cd/m².     

Synthesis and electroluminescent properties of blue-emitting t-butylated bis(diarylaminoaryl)anthracenes for OLEDs

A new series of blue fluorescent emitters based on t-butylated bis(diarylaminoaryl) anthracenes were synthesized and their electroluminescent properties investigated. Into these blue materials, t-butyl groups were introduced to both prevent molecular aggregation between the blue emitters through steric hindrance and reduce self-quenching. As such, this would contribute to overall improvement in OLED efficiency. To explore the electroluminescent properties of these materials, multilayered OLEDs were fabricated into a device structure of: ITO/NPB(50 nm)/blue emitters doped in ADN(30 nm)/Alq₃(20 nm)/Liq(2 nm)/Al(100 nm). All devices showed efficient blue emissions. In particular, one device exhibited highly efficient sky blue emissions with a maximum luminance of 11,060 cd/m² at 12.0 V and respective luminous and power efficiencies of 6.59 cd/A and 2.58 lm/W at 20 mA/cm². The peak wavelength of the electroluminescence was 468 nm with CIEx,y coordinates of (0.159, 0.198) at 12.0 V. In addition, a deep blue device with CIEx,y coordinates of (0.159, 0.151) at 12.0 V showed a luminous efficiency of 4.2 cd/A and power efficiency of 1.66 lm/W at 20 mA/cm².     

High efficiency green phosphorescent organic light emitting device with (TCTA/TCTA0.5TPBi0.5/TPBi): Ir(ppy)3 emission layer

A new high efficiency green light emitting phosphorescent device with an emission layer consisting of {4,4',4'-tris(N-carbazolyl)-triphenylamine[TCTA]/TCTA_0.5TPBi_0.5/1,3,5-tris(N-phenylbenzimiazole-2-yl)benzene[TPBi]}:tris(2-phenylpyridine)iridium(III)[Ir(ppy)₃] was fabricated and its electroluminescence characteristics were evaluated in comparison with those of devices with emission layers made of (TCTA_0.5TPBi_0.5):Ir(ppy)₃ and (TCTA/ TPBi):Ir(ppy)₃.The device with the emission layer consisting of (TCTA/TCTA_0.5TPBi_0.5/TPBi):Ir(ppy)₃ showed a luminance of 11,000 cd/m² at an applied voltage of 8 V and maximum current efficiency of 63 cd/A under a luminance of 500 cd/m². The peak wavelength in the electroluminescent spectral and color coordinate on the Commission Internationale de I'Eclairage(CIE) chart were 513 nm and (0.31, 0.62) in this device, respectively. Under a luminance of 10000 cd/m², the current efficiency of this device was 55 cd/A, which is 1.4 and 1.1 times better than those of the devices with the emission layers made of (TCTA_0.5TPBi_0.5):Ir(ppy)₃ and (TCTA/TPBi):Ir(ppy)₃, respectively.     

Synthesis and electroluminescent properties of blue emitting materials based on arylamine-substituted diphenylvinylbiphenyl derivatives for organic light-emitting diodes

This paper reports the synthesis and electroluminescent properties of a series of blue emitting materials with arylamine and diphenylvinylbiphenyl groups for applications to efficient blue organic light-emitting diodes (OLEDs). All devices exhibited blue electroluminescence with electroluminescent properties that were quite sensitive to the structural features of the dopants in the emitting layers. In particular, the device using dopant 4 exhibited sky-blue emission with a maximum luminance, luminance efficiency, power efficiency, external quantum efficiency and CIE coordinates of 39,000 cd/m², 12.3 cd/A, 7.45 lm/W, 7.71% at 20 mA/cm² and (x = 0.17, y = 0.31) at 8 V, respectively. In addition, a blue OLED using dopant 2 with CIE coordinates (x = 0.16, y = 0.18) at 8 V exhibited a luminous efficiency, power efficiency and external quantum efficiency of 4.39 cd/A, 2.46 lm/W and 2.97% at 20 mA/cm², respectively.     

Synthesis and optoelectronic properties of oxadiazole-functionalized iridium complexes in the poly(vinylcarbazole)-hosted devices

A class of oxadiazole-functionalized iridium complexes was used as phosphor emitters in poly (vinylcarbazole)-hosted devices. Efficient green electrophosphorescences were achieved in the devices with a maximum luminance efficiency of 9.3 cd/A at 10.6 mA/cm² and brightness of 3882 cd/m² at 92.1 mA/cm². More importantly, the iridium complexes-doped devices exhibited a low turn-on voltage of 7.0 V and an applied voltage of 9.2 V at 500 cd/m². The good optoelectronic properties of the complexes were attributed to the enhanced electron-injection and transport properties resulting from the effect of oxadiazole ligands in the complexes.     

Cathodoluminescence of CaWO4 and SrWO4 thin films prepared by spray pyrolysis

Thin films of CaWO₄ and SrWO₄ were prepared on glass substrates by spray pyrolysis. The effects of preparation conditions and monovalent, bivalent and trivalent cation doping on cathodoluminescence (CL) properties of the films were studied. Polycrystalline CaWO₄ and SrWO₄ films formed a scheelite structure after being annealed above 300°C. They exhibited analogous cathodoluminescence consisting of a blue emission band at 447 nm and a blue-green emission band at 487 nm. The blue and blue-green emission intensities increased with substrate and annealing temperature. Annealing atmosphere and doping with Ag⁺, Pb²⁺ and La³⁺ did not influence the characteristics of the blue and blue-green emissions, whereas Eu³⁺ did. The results indicated both the blue and blue-green emissions originated from the WO₄²⁻ molecular complex. The luminance and efficiency for CaWO₄ film were 150 cd/m² and 0.7 lm/W at 5 kV and 57 μA/cm².     

Visible to near-infrared organic light-emitting diodes using phosphorescent materials by solution process

The characteristics of visible to near-infrared OLEDs with co-doping three phosphorescent dyes, iridium (III) bis(2-(4,6-difluorephenyl)pyridinato-N,C²′) (FIrpic), tris(1-phenylisoquinoline)iridium(III) (Ir(piq)₃), Pt-tetraphenyltetrabenzoporphyrin (Pt(tpbp)) in poly(N-vinylcarbazole) host as blue, red and near-infrared emitters are investigated. Visible to near-infrared OLEDs covering the wavelength range from 450 to 850 nm were achieved. The device with 11.7 wt.% FIrpic, 0.3 wt.% Ir(piq)₃ and 0.1 wt.% Pt(tpbp) showed white light emission of CIE (0.34, 0.39). The co-doping results in efficient cascade energy transfer from host through Ir complexes. For 0.1 wt.% Pt(tpbp), the optimal device exhibited the maximum output power of 3 mW/cm², maximum luminance of 2900 cd/m² and the maximum efficiency of 7 cd/A.     

A three-dimensional nanostructured PANI/MnOx porous microsphere and its capacitive performance

Three-dimensional nanostructured polyaniline (PANI) and manganese oxide (MnO _x ) composite porous microspheres were prepared by oxidizing aniline with KMnO₄ under interfacial chemical synthesis with 4-amino-thiophenol (4-ATP) as the structure-directing agent on the Au substrate. Surface morphology and chemical composition of PANI/MnO _x microsphere were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, thermo gravimetric-differential thermal analysis, and Fourier transform infrared spectrum. The result displayed that concentration of KMnO₄ played a key role in forming the 3D nanostructured porous microspheres. To obtain the regular shapes and uniform sizes of the porous microspheres, the optimal concentration of oxidant was 0.15 mol L⁻¹. The electrochemistry performances of PANI/MnO _x microsphere were determined by cyclic voltammograms, electrochemical impedance spectroscopy, and galvanostatic charge–discharge. The specific capacitance of the 3D nanostructured PANI–MnO _x porous microspheres exhibited a maximum value of 828 F g⁻¹ at current density of 2 mA cm⁻² over a potential range of 0.0–0.9 V versus SCE. It has improved 365 and 88 % comparing with that of PANI (178 F g⁻¹) and MnO _x (440 F g⁻¹) obtained at the similar condition. The charge–discharge tests showed the PANI/MnO _x microsphere possessed a good cycling stability. It maintained about 84.2 % of the initial capacitance after 1000 cycles at a current density of 2.0 mA cm⁻².     

Transition metal ferrocenyl dithiocarbamates functionalized dye-sensitized solar cells with hydroxy as an anchoring group

Three new transition-metal dithiocarbamates involving ferrocene (Fc), namely [Co(FcCH₂EtOHdtc)₃] (Co), [M(FcCH₂EtOHdtc)₂] M = Ni (Ni), Cu (Cu) (EtOHdtc = N-ethanol dithiocarbamate), have been synthesized and characterized by microanalyses, FTIR, ¹H and ¹³C NMR spectroscopies and single crystal X-ray diffraction technique. The peak broadening in the ¹H spectrum of the copper complex indicates the paramagnetic behavior of this compound. The observed single quasi-reversible cyclic voltammograms for the complexes indicate the stabilization of a metal center (except copper) other than Fe in their characteristic oxidation state. These complexes have been used as photo-sensitizer in dye-sensitized solar cells which indicates that Co displays the best photosensitization property with an overall conversion efficiency of 3.25 ± 0.04%. The low cell efficiency of Ni and Cu complexes may be due to slow regeneration of the dye by iodine/iodide redox couple followed by charge injection into TiO₂.     

High thermal stability fluorene-based hole-injecting material for organic light-emitting devices

Novel N¹,N³,N⁵-tris(9,9-diphenyl-9H-fluroen-2-yl)-N¹,N³,N⁵-triphenylbenzene-1,3,5-triamine (TFADB) was synthesized and characterized as a hole-injecting material (HIM) for organic light-emitting devices (OLEDs). By incorporating fluorene group TFADB shows a high glass-transition temperature T_g > 168 °C, indicative of excellent thermal stability. TFADB-based devices exhibited the highest performance in terms of the maximum current efficiency (6.0 cd/A), maximum power efficiency (4.0 lm/W), which is improved than that of the standard device based on 4-4′-4″Tris(N-(naphthalene-2-yl)-N-phenyl-amino)triphenylamine (2T-NATA) (5.2 cd/A, 3.6 lm/W). This material could be a promising hole-injecting material, especially for the high temperature applications of OLEDs and other organic electronic devices.     

Preparation of Cu2ZnSnS4 thin films by sulfurization of stacked metallic layers

Stacked precursors of Cu, Sn, and Zn were fabricated on glass/Mo substrates by electron beam evaporation. Six kinds of precursors with different stacking sequences were prepared by sequential evaporation of Cu, Sn, and Zn with substrate heating. The precursors were sulfurized at temperatures of 560 °C for 2 h in an atmosphere of N₂ + sulfur vapor to fabricate Cu₂ZnSnS₄ (CZTS) thin films for solar cells. The sulfurized films exhibited X-ray diffraction peaks attributable to CZTS. Solar cells using CZTS thin films prepared from six kinds of precursors were fabricated. As a result, the solar cell using a CZTS thin film produced by sulfurization of the Mo/Zn/Cu/Sn precursor exhibited an open-circuit voltage of 478 mV, a short-circuit current of 9.78 mA/cm², a fill factor of 0.38, and a conversion efficiency of 1.79%.     

Confinement of holes and electrons in blue organic light-emitting diodes with additional red emissive layers

We used various emissive layer (EML) structures with ultrathin red EMLs to enhance the charge carrier balance and carrier recombination rate in blue PHOLED devices. These EML materials have different energy gaps between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The ultrathin red EMLs, which were inserted in between the blue EMLs, effectively confined the charge carriers in EML, and increased the carrier recombination rate. The thickness of the individual EML was optimized, under 30 nm of the total thickness of EML. The blue PHOLEDs with ultrathin red EMLs achieved a luminous efficiency of 19.24 cd/A, which was 28.7% higher than those without ultrathin red EMLs, and the maximum external quantum efficiency was 11.81% at 500 cd/m².     

Improvement of color purity in white OLED based on Zn(HPB)2 as blue emitting layer

We synthesized zinc (II) [2-(2-hydroxyphenyl)benzoxazole] (Zn(HPB)₂) as blue emitting materials and evaluated in the organic light emitting diodes (OLEDs). The layer of Zn(HPB)₂ doped with 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) (Zn(HPB)₂:DCJTB) as emitters has been demonstrated. The structure of the device is indium-tin-oxide (ITO)/N,N′-bis-(1-naphthl)-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB, 40 nm)/Zn(HPB)₂/Zn(HPB)₂:DCJTB/Alq₃ (20 nm)/LiF/Al. The thickness of Zn(HPB)₂ layer was 0, 10, 20, 30 nm at the same time the thickness of Zn(HPB)₂:DCJTB layer were 40, 30, 20, 10 nm. When thickness of Zn(HPB)₂ layer was 30 nm and the thickness of Zn(HPB)₂:DCJTB layer was 10 nm, white emission is achieved. The Commission Internationale de l'Eclairage (CIE) coordinates of the white emission are (0.304, 0.332) at an applied voltage of 10.5 V.     

High-efficiency white organic light-emitting devices with a non-doped yellow phosphorescent emissive layer

Highly efficient phosphorescent white organic light-emitting devices (PHWOLEDs) with a simple structure of ITO/TAPC (40 nm)/mCP:FIrpic (20 nm, x wt.%)/bis[2-(4-tertbutylphenyl)benzothiazolato-N,C²′] iridium (acetylacetonate) (tbt)₂Ir(acac) (y nm)/Bphen (30 nm)/Mg:Ag (200 nm) have been developed, by inserting a thin layer of non-doped yellow phosphorescent (tbt)₂Ir(acac) between doped blue emitting layer (EML) and electron transporting layer. By changing the doping concentration of the blue EML and the thickness of the non-doped yellow EML, a PHWOLED comprised of higher blue doping concentration and thinner yellow EML achieves a high current efficiency of 31.7 cd/A and Commission Internationale de l'Eclairage coordinates of (0.33, 0.41) at a luminance of 3000 cd/m² could be observed.     

Synthesis and electro-optical properties of carbazole derivatives with high band gap energy

Two materials containing carbazole moieties and exhibiting a high band gap energy, 3,8-di(9H-carbazol-9-yl)-6-phenylphenanthridine (DCzP) and 3,6-di(naphthalene-2-yl)-9-phenyl-9H-carbazole (DNaC), were synthesized via CN coupling and Suzuki coupling reactions, respectively. The compound DCzP exhibited blue emission with the CIE coordinates of x = 0.165 and y = 0.136 from the OLED device, ITO(indium–tin oxide)/NPB(N,N′-bis(naphthalene-1-yl)-N,N′-bis(phenyl)benzidine)/DCzP/LiF/Al. The doped device, ITO/2-TNATA(4,4′,4″-Tris(2-naphtylphenyl-phenylamino) triphenyl amine)/NPB/DCzP + Ir(ppy)₃/BCP(2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline)/Alq₃(tris(8-hydroxyquinoline)aluminum/LiF/Al, showed bright yellowish-green emission with a maximum luminance of 23,000 cd/m² when the synthesized DCzP was applied as a host material for the phosphorescent green dopant. From the double layer device, ITO/DNaC/Alq₃/LiF/Al, in which DNaC was used as the hole transporting material, the yellowish-green color arising from the Alq₃ was also observed.