Low voltage organic light emitting diode using p–i–n structure

Efficient n-type doping has been achieved by doping Liq in electron transport material Alq₃. Detailed investigation of current density–voltage characteristics of electron only devices with different doping concentrations of Liq in Alq₃ has been performed. An increase in current density by two orders of magnitude has been achieved with 33 wt% of Liq doped in Alq₃. Organic light emitting diode with p–i–n structure was fabricated using F₄-TCNQ doped α-NPD as hole transport layer, Ir(ppy)₃ doped CBP as emitting layer and 33 wt% Liq doped Alq₃ as electron transport layer. Comparison of OLEDs fabricated using undoped Alq₃ and 33 wt% Liq doped Alq₃ as electron transport layer shows reduction in turn on voltage from 5 to 2.5 V and enhancement of power efficiency from 5.8 to 10.6 lm/W at 5 V.     

Multicolor emitting from a single component emitter: New iridium(III) complexes with ancillary ligand 2-(2-hydroxyphenyl) benzothiazole

Four new types of Ir(III) complexes (ppy)₂Ir(LX) (ppy = 2-phenylpyridine, LX = BTZ, 3-MeBTZ, 4-MeBTZ, and 4-TfmBTZ) were synthesized and investigated by optical spectroscopy, electrochemistry as well as density functional theory (DFT). These complexes all exhibit multi-emission bands under ultraviolet light excitation at room temperature because there are efficient energy transfers between different excited states (¹LC, ¹MLCT, ¹ILCT, ³MLCT, ³ILCT), especially for complexes I–III. The blue emitting relates to the fluorescence of Ir(R-BTZ) and the green emission originates in ³MLCT emission of Ir(ppy)₂ fragment. The red emission corresponds to ³ILCT transition (R-BTZ → ppy) for complexes I–III, but ³MLCT transition of Ir(4-TfmBTZ)₂ fragment for (ppy)₂Ir(4-TfmBTZ). The red emission peak wavelength can be fine-tuned from 583 to 626 nm by the electron withdrawing or donating substituent at 3- or 4-position of BTZ ligand. For these single component complexes with multicolor emitting, a very promising application is the generation of white light. Although these complexes span non-uniformly the spectral range of the visible region and consequently are insufficient to produce satisfactory white light characteristics, the luminescence mechanism of them provides a new strategy for designing organic white light-emitting material.     

Synthesis and characterization of novel 2,5-diphenyl-1,3,4-oxadiazole derivatives of anthracene and its application as electron transporting blue emitters in OLEDs

With a general aim to make anthracene derivatives multifunctional (n-type emitter) and also study their suitability as electron transport layers for organic light emitting diodes (OLED), we report the synthesis and characterization of five novel molecules in which the 9 and 10 positions of anthracene have been directly substituted by 2,5-diphenyl-1,3,4-oxadiazole groups. We have carried out detailed characterization of these molecules which include photophysical, electrochemical, thermal, electroluminescent and computational studies. The electron affinity is very high, around 3.7 eV, and the ionization potential is around 6.7–6.8 eV, which is relatively higher than the most commonly used electron transport electroluminescent layer Alq₃. The studies reveal that the new molecules being reported by us, in addition to the high thermal stability, are quite efficient in a two layer unoptimized nondoped device with the device structure ITO/α-NPD/10a–11b/LiF/Al and have an emission in pure blue. They also show very high efficiency as electron transport layer in device structure ITO(120 nm)/α-NPD(30 nm)/Ir(ppy)₃ doped CBP(35 nm)/BCP(6 nm)/10a(28 nm)/LiF(1 nm)/Al(150 nm). From these studies we conclude that these anthracene derivatives also have considerable potential as multifunctional layers and as electron transport layers in OLED.     

Thermal aging of single-layer polymer light-emitting diodes composed of a carbazole and oxadiazole containing copolymer doped with singlet or triplet emitters

An electron-transporting monomer was synthesized that was structurally and energetically similar to the small molecule 2-biphenyl-4-yl-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (tBu-PBD). The monomer was copolymerized with 2-(9H-carbazol-9-yl)ethyl 2-methylacrylate and the resulting copolymer was utilized in organic light emitting devices which employed fluorescent coumarin 6 (C6) or phosphorescent tris(2-phenylpyridine)iridium(III) [Ir(ppy)₃] emitters. The copolymer devices exhibited a mean luminance of ca. 400 and 3552 cd/m² with the C6 and Ir(ppy)₃ emitters, respectively, that was stable with thermal aging at temperatures ranging from 23 °C to 130 °C. Comparable poly(9-vinyl-9H-carbazole)/tBu-PBD blend devices exhibited more pronounced variations in performance with thermal aging.     

Synthesis,characterization and electroluminescence properties of iridium complexes based on pyridazine and phthalazine derivatives with C^NN structure

We reported the synthesis and characterization of three novel tris-cyclometalated iridium(III) complexes with pyridazine and phthalazine derivatives as ligands bearing C^NN structure, namely, Ir(PYA)₃, Ir(PHB)₃, and Ir(PHC)₃. Reactions of the ligands with IrCl₃·3H₂O directly afforded tris-cyclometalated iridium(III) complexes in 31–39% yields. X-ray analysis revealed that the complexes exhibited the facial configuration, and the Ir–C bond lengths of the complexes are 2.009(3), 2.013(3), 2.019(3) Å for Ir(PYA)₃, and 1.993(4), 2.002(4), 2.004(4) Å for Ir(PHB)₃, respectively. The iridium complexes-based OLEDs fabricated by spin-coating technique exhibited promising performance. At 4 wt% doping level and a practical luminance of 100 cd m^?2, the external quantum efficiencies of the devices using Ir(PYA)₃, Ir(PHB)₃ and Ir(PHC)₃ as dopants reached 9.0, 6.9 and 9.3% photons/electron, respectively.     

Synthesis,photoluminescent and electroluminescent properties of a novel europium(III) complex involving both hole- and electron-transporting functional groups

A novel europium(III) complex involving a carbazole fragment as hole-transporting group and an oxadiazole fragment as electron-transporting group was synthesized and used as red light-emitting material in organic light-emitting diodes (OLEDs). The complex is amorphous, and exhibits high glass transition temperature (T_g = 157 °C) and high thermal stability with a 5% weight loss temperature of 367 °C. Two devices, device 1: ITO/NPB (40 nm)/Eu(III) complex (30 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm) and device 2: ITO/NPB (40 nm)/3% Eu(III) complex: CBP (30 nm)/BCP (10 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm), were fabricated, where NPB is N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine, Alq₃ is tris(8-hydroxyquinoline) Al(III), CBP is 4,4′-bis(carbazole-9-yl)-biphenyl, and BCP is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, respectively. In contrast with device 1, owing to less self-quenching and better charge confinement, device 2 shows improved performances: the maximum luminance of device 2 was dramatically increased from 199 to 1845 cd/m², the maximum current efficiency was increased from 0.69 to 2.62 cd/A, the turn-on voltage was decreased from 9.5 to 5.5 V, and higher color purity was attained.     

Structural evolution and grain growth kinetics of the Fe–28Al elemental powder during mechanical alloying and annealing

The structural evolution and grain growth kinetics of the Fe–28Al (28 at.%) elemental powder during mechanical alloying and annealing were studied. Moreover, the alloying mechanism during milling the powder was also discussed. During mechanical alloying the Fe–28Al elemental powder, the solid state solution named Fe(Al) was formed. The lattice parameter of Fe(Al) increases and the grain size of Fe(Al) decreases with increasing milling time. The Fe and Al particles were first deformed, and then, the composite particles of the concentric circle-like layers were generated. Finally, the composite particles were substituted by the homogeneous Fe(Al) particles. The continuous diffusion mixing mechanism is followed, mainly by the diffusion of Al atoms into Fe. During annealing the milled Fe–28Al powder, the order transformation from Fe(Al) to DO₃-Fe₃Al and the grain growth of DO₃-Fe₃Al occurred. The grain growth kinetic constant, K = 1.58 × 10⁻⁹ exp(−540.48 × 10³/RT) m²/s.     

Synthesis and luminescence of red,fluorinated iridium (III) complexes containing alkenyl benzothiazole ligand

Two novel iridium(III) complexes (2-FSBT)₂Ir(acac) and (4-FSBT)₂Ir(acac) (2-FSBT, (E)-2-(2-fluorostyryl)benzo[d]thiazole; 4-FSBT, (E)-2-(4-fluorostyryl)benzo[d]thiazole; acac, acetylacetone) were synthesized and characterized by ¹H NMR and mass spectrometry. The organic light emitting diodes based on these complexes with the structure of ITO/m-MTDATA(10 nm)/NPB(20 nm)/CBP:Ir-complex(X %, 30 nm)/BCP(10 nm)/Alq₃(30 nm)/LiF(1 nm)/Al(100 nm) were fabricated. The device based on (2-FSBT)₂Ir(acac) exhibited a maximum efficiency of 9.32 cd/A, a luminance of 8800 cd/cm²; and the device based on (4-FSBT)₂Ir(acac) showed a maximum efficiency of 8.5 cd/A, a luminance of 6986 cd/cm². The Commission International de L’Eclairage (CIE) coordinates (1931) of these complexes were (0.619, 0.381) and (0.621, 0.378), respectively.     

Comparison of the carrier mobility,unipolar conduction,and light emitting characteristics of phosphorescent host–dopant system

We have investigated the charge carrier transport mobility and hole-only current behavior of phosphorescent host–dopant mixture, and evaluated the efficiency and lifetime behavior of organic light emitting device (OLED) containing the corresponding light emitting host–dopant system. The carrier drift mobilities of the phosphorescent host, 4,4′-N,N′-dicarbazole-biphenyl (CBP), doped with green-emitting fac-tris(2-phenylpyridine) iridium (Ir(ppy)₃) or bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3′) iridium(acetyl-acetonate) (Btp₂Ir(acac)) red-emitting dopants, were directly investigated with time of flight (TOF) photoconductivity method. The resolved electron mobility of phosphorescent host–dopant layer by TOF-PC method showed the significant reduction at CBP:(Btp₂Ir(acac)) layer, which can be explained by the electron trapping. Measured hole-only current data also shows the reduction, which is more significant at CBP:(Btp₂Ir(acac)) as expected from larger energy level offset. The efficiency, spectral emitting properties, and device stability of phosphorescent OLEDs with identical host–dopant layer were evaluated. Compared with CBP:(Btp₂Ir(acac)), device with CBP:Ir(ppy)₃ emitter shows the spectral response and half-lifetime less dependent upon the hole/exciton blocking layer and its thickness. Such device data was well-correlated with probed TOF and hole current behavior.     

Manufacture of brightness enhancement films (BEFs) by ultraviolet (UV) irradiation and their applications for organic light emitting diodes (OLEDs)

By ultraviolet (UV) irradiation, brightness enhancement films (BEFs) have been successfully manufactured with UV-curable polymers and applied for organic light emitting diodes (OLEDs).With BEFs, either green OLEDs (BEF/ITO glass/NPB (30 nm)/Alq₃ (65 nm)/LiF (0.5 nm)/Al (100 nm)) or white OLEDs (BEF/ITO glass/TAPC (40 nm)/mCP:Os:Firpic mixture (weight ratio = 82:17:1; 25 nm)/BCP (15 nm)/Alq₃ (30 nm)/LiF (0.5 nm)/Al(150 nm)) exhibit better electroluminescent performances than those without BEFs. In case of green OLEDs, the luminance and electroluminescent yield with 45° compound BEFs are, respectively, 1.51-fold and 1.42-fold (at 9 V, 60 mA/cm²) larger than those without BEFs. In case of white OLEDs, moreover, the luminance and electroluminescent yield with 45° compound BEFs are, respectively, 1.28-fold and 1.21-fold (at 9 V, 16 mA/cm²) larger than those without BEFs.     

Microstructure and oxidation behaviour of Ir-rich Ir–Al binary alloys

In the current study, alloys of Ir–11Al, Ir–23Al, Ir–30Al, Ir–41Al and Ir–45Al (at.%) were prepared to investigate the microstructure and oxidation behaviour of Ir-rich Ir–Al alloys. Ir(Al)_ss and/or β-IrAl intermetallic phases were found to exist in the prepared alloys. During isothermal oxidation at 1100 °C, the Ir(Al)_ss and β-IrAl individually changed to porous and dense Al₂O₃. The microstructure of the oxide scale formed on Ir–23Al was similar to that of its former alloy which possessed a dendrite-like configuration. It was found that the mass change of Ir–45Al followed a parabolic law, showing the best oxidation resistance among the Ir–Al alloys.     

Phase equilibria between the B2, L12, and fcc phases in the Ir–Ni–Al system

The isotherms of the Ir–Ni–Al in the composition range up to 50 at% Al are presented at 1573 K. The phase constitution and microstructure of the Ir–Ni–Al alloys were examined using X-ray diffractometry (XRD), scanning electron microscopy (SEM) with an electron probe microanalyzer (EPMA), and transmission electron microscopy (TEM) after heat treatment at 1573 K for 168 h. The B2-NiAl and B2-IrAl phases connected with each other at 1573 K. The highest solubility limit of Ir into Ni₃Al was about 3.5 at% in the tested alloys. Then, a wide fcc + B2 and a narrow fcc + L1₂ and B2 + L1₂ two-phase region appeared in the isothermal section. In part of the B2 phase, a martensitic transformation from the B2 to the L1₀ phase was observed.     

Modeling of phase stability of the fcc phases in the Ni–Ir–Al system using the cluster/site approximation method coupling with first-principles calculations

A combined CSA (cluster/site approximation)/FP (first-principles) calculation approach was employed to investigate the phase stability and thermodynamic properties of the face-centered cubic (fcc) phases of the Ni–Ir–Al system. For the constituent binaries of the Ni–Ir–Al system, enthalpies of formation of the Ni_xIr_1−x, Ni_xAl_1−x and Ir_xAl_1−x fcc compounds were calculated by first-principles approach at x = 0.75, 0.5 and 0.25 at 0 K, respectively. The pair exchange energies of the Ni–Ir and Al–Ir systems in the CSA model were obtained from FP calculated enthalpies of formation, while those for the Ni–Al binary were adopted from previous work. Thermodynamic model parameters of the fcc phases for the Ni–Ir–Al ternary system were then obtained from the constituent binaries via extrapolation. The calculated isothermal section at 1573 K is in good agreement with the experimental data within the uncertainties of the calculations and experiments.     

Synthesis,photophysical and electrophosphorescent properties of a novel fluorinated rhenium(I) complex

A novel fluorinated rhenium complex, i.e., Re-BFPP (BFPP, 2, 3-bis(4-fluorophenyl)pyrazino[2,3-f][1,10]phenanthroline) was designed, synthesized and characterized by ¹H NMR and mass spectroscopy. The light-emitting and electrochemical properties of this complex were studied. The organic light-emitting diodes (OLEDs) employing Re-BFPP as a dopant emitter with the structures of ITO/m-MTDATA (10 nm)/NPB (20 nm)/CBP: X wt.% Re-BFPP (30 nm)/Bphen (10 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm) were successfully fabricated and a broad electroluminescent peak at 553 nm was observed. The 10 wt.% Re-BFPP doped device exhibited the maximum luminance of 6342 cd/m² and a peak current efficiency of 17.9 cd/A, corresponding to the power efficiency of 8.1 lm/W.     

Effect of methanol treatment on performance of phosphorescent dye doped polymer light-emitting diodes

Methanol treatment was performed on the emissive layer (EML) of conjugated polymer hosted phosphorescent dye doped polymer light-emitting diode (PLED) with a device configuration of indium tin oxide (ITO)/poly(ethylene dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS)/poly(N-vinylcarbazole) (PVK)/poly(9,9-dioctylfluorene) (PFO):30 wt% 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD):2 wt% fac-tris(2-phenylpyridine) iridium (III) (Ir(ppy)₃)/Ba/Al. Improvement of the external quantum efficiency and luminous efficiency were observed. After methanol treatment, partial electron-transportant dopant PBD was selectively removed from the EML. Though PBD was doped in the EML to achieve more balanced charge transport, its partially removal resulted in further enhancement of PLED performances. The surface topography of EML shows the reduction of PBD aggregation which is detrimental to luminescence.     

Oxidation of Fe–Si,Fe–Al and Fe–Si–Al alloys in CO2–H2O gas at 800 °C

Binary Fe–(1, 2, 3)Si and Fe–(2, 4, 6)Al, and ternary Fe–(2, 3)Si–(4, 6)Al alloys (all in wt%) were oxidised in Ar–20% CO₂, with and without H₂O, at 800 °C. All binary alloys except Fe–6Al, in all gases, formed a thin outer layer of Fe₃O₄, an intermediate Fe₃O₄ + FeO layer, an inner FeO + Fe₂SiO₄ (or FeAl₂O₄) layer and internally precipitated SiO₂ (or FeAl₂O₄). Ternary alloys and Fe–6Al developed a protective Al₂O₃ layer beneath Fe₂O₃ in Ar–20% CO₂. Water vapour affected ternary alloy oxidation only slightly, but Fe–6Al oxidized internally in high H₂O-content gas, and its scale was non-protective.     

Iridium(III) complexes with cyclometalated styrylbenzoimidazole ligands: Synthesis,electrochemistry and as highly efficient emitters for organic light-emitting diodes

Two phosphorescent iridium complexes (psbi)₂Ir(acac) and (ppbi)₂Ir(acac) (psbi = 1-phenyl-2-styryl-1H-benzo[d]imidazole, ppbi = 1-phenyl-2-(1-phenylprop-1-en-2-yl)-1H-benzo[d]imidazole, acac = acetylacetonate) were synthesized, and their photophysical, electrochemical and electroluminescent properties were also studied. Organic light-emitting devices with these two complexes as dopant emitters having the structure ITO/NPB (10 nm)/TCTA (20 nm)/x%Ir:CBP (y nm)/BCP (10 nm)/LiF (1 nm)/Al (100 nm) were fabricated. The device based on (psbi)₂Ir(acac) exhibited a maximum brightness of 56,162 cd m^?2, while the device based on (ppbi)₂Ir(acac) gave a maximum brightness of 31,232 cd m^?2. At high brightness of 1000 cd m^?2 and 10,000 cd m^?2, high current efficiencies of 25.7 cd A^?1 and 20.7 cd A^?1 were achieved, respectively, for the (psbi)₂Ir(acac)-based EL device. For the EL device based on (ppbi)₂Ir(acac), current efficiencies of 20.1 cd A^?1 at 1000 cd m^?2 and 14.2 cd A^?1 at 10,000 cd m^?2 were observed.     

Highly efficient solution-processed red organic light-emitting diodes with long-side-chained triplet emitter

Newly synthesized red Ir complexes tris[2-(4-n-hexyl-phenyl)quinoline]iridium(III) and tris[(4-n-hexylphenyl)isoquinoline)]iridium(III) with long alkyl side chains are utilized to demonstrate the high efficiency multi-layer solution-processed red organic light-emitting diodes. Solubilities of these triplet emitters are high which enable them to be uniformly dispersed in the polymer host. Blade coating method is utilized to prepare organic multi-layers without mutual dissolution between different layers. 17 cd/A current efficiency, 10 lm/W power efficiency, and 8.8% external quantum efficiency can be achieved for the device with CsF/Al cathode. 10,000 cd/m² is reached at 10 V. Similar quantum efficiency is also achieved with an electron-transport layer and LiF/Al cathode.     

Laves phases in the ternary systems Ti–{Pd,Pt}–Al

Formation and crystal structure of Laves phases in the systems Ti–{Pd,Pt}–Al were investigated employing XPD (X-ray powder diffraction), XSCD (X-ray single crystal diffraction) and EPMA (electron probe microanalysis) techniques. Laves phases with MgZn₂ type (space group: P6₃/mmc) and its variant with the Nb(Ir,Al)₂-type (a√3 × a√3 × c supercell of MgZn₂-type, space group: P6₃/mcm) were found in both systems. Formation of a particular structure type is dependent on temperature and composition. Laves phases with the Nb(Ir,Al)₂-type form around 25 at.% of Pd,Pt at 950 °C. The MgZn₂-type Laves phase Ti(Pt,Al)₂ was not observed at 950 °C, but it forms in as-cast alloys at a slightly lower Pt content, Ti_37.8Pt_19.0Al_43.2. In the Ti–Pd–Al system at 950 °C the MgZn₂-type phase exists at the Pd-poor side of the homogeneity region whilst the Nb(Ir,Al)₂-type phase is slightly richer in Pd. Phase relations associated with the Ti–Pt–Al Laves phase were established at 950 °C and reveal a new compound TiPtAl that derives from hexagonal ZrBeSi-type (ordered Ni₂In-type, a = 0.43925(4) nm, c = 0.54844(5); space group P6₃/mmc; R_F2 = 0.015 from single crystal data). Atom distribution in the compound shows a slight deviation from full atom order Ti(Pt_0.97Al_0.03)(Al_0.98Pt_0.02).     

Polysilane based ultraviolet light-emitting diodes with improved turn-on voltage,stability and color purity

We demonstrate ultraviolet organic light-emitting diodes (OLEDs) with improved stability, low turn-on voltage and color purity by changing the cathode and annealing temperature of the polymer film. The electron injection process, which limits the electroluminescent performance of organic devices, has been enhanced tremendously by inserting a layer of LiF with appropriate thickness between the cathode and a poly(n-butylphenylsilane) (PS-4) layer, whose device structure is ITO/PEDOT:PSS/polysilane (PS-4)/LiF/Al. Devices with a LiF (6 Å) have the turn-on voltage of 4 V, which is lower than 7 V of devices made with Ca/Al layer. By inserting LiF as the anode interfacial layer, there is increase in the injection of electrons from Al (cathode) side due to tunneling effect and also act as hole blocking layer which enhance the recombination of electron and hole in the emissive layer. PS-4 is spin coated and annealed in vacuum for 1 h at different temperatures (90–120 °C). EL Spectra from these devices consists of white emission along with the UV peak. White emission is significantly suppressed when PS-4 is annealed at higher temperature and threshold voltage is lowest at 110 °C annealing temperature.