Investigation of an efficient YbF3/Al cathode for tris-(8-hydroxyquinoline)aluminum-based small molecular organic light-emitting diodes

Efficient tris-(8-hydroxyquinoline)aluminum (Alq₃)-based organic light-emitting diodes (OLEDs) using YbF₃ as the electron injection layer have been investigated. With an YbF₃ (3.0 nm)/Al cathode, the device with Alq₃ as the emitting layer achieved a better performance than the control device with a LiF (0.5 nm)/Al cathode. The release of the low-work-function metal Yb is responsible for the performance enhancement. From the analysis by atomic force spectroscopy and X-ray photoemission spectroscopy, it is observed that the Alq₃-cathode interface could be well covered by YbF₃ at an optimum thickness of 3.0 nm, which helps to prevent the contact between Alq₃ and Al, and to reduce the destruction of Alq₃ by Al.     

Optical properties of white organic light-emitting devices fabricated with three primary-color emitters by using organic molecular-beam deposition

White organic light-emitting devices (WOLEDs) with Mg:Ag/Alq₃/Alq₃:DCJTB/Alq₃/DPVBi/α-NPD/ITO and Mg:Ag/Alq₃/DPVBi:DCJTB/Alq₃/DPVBi/α-NPD/ITO structures were fabricated with three primary-color emitters of red, green, and blue by using organic molecular-beam deposition. Electroluminescence spectra showed that the dominant white peak for the WOLEDs fabricated with host red-luminescence Alq₃ and DPVBi layers did not change regardless of variations in the current. The Commission Inernationale de l'Eclairage (CIE) chromaticity coordinates for the two WOLEDs were stable, and the WOLEDs at 40 mA/cm² with luminances of 690 and 710 cd/cm² showed an optimum white CIE chromaticity of (0.33, 0.33). While the luminance yield of the WOLED fabricated with a host red-luminescent Alq₃ emitting layer below 30 mA/cm³ was larger than that of the WOLED fabricated with a DPVBi layer, above 30 mA/cm², the luminance yield of the WOLED fabricated with the DPVBi layer was higher than that of the WOLED with the Alq₃ layer and became more stable with increasing current density. These results indicate that WOLEDs fabricated with a host red-luminescence DPVBi layer without any quenching behavior hold promise for potential applications in backlight sources in full-color displays.     

Characteristics of sandwich-structured Al2O3/HfO2/Al2O3 gate dielectric films on ultra-thin silicon-on-insulator substrates

Sandwich-structure Al₂O₃/HfO₂/Al₂O₃ gate dielectric films were grown on ultra-thin silicon-on-insulator (SOI) substrates by vacuum electron beam evaporation (EB-PVD) method. AFM and TEM observations showed that the films remained amorphous even after post-annealing treatment at 950 °C with smooth surface and clean silicon interface. EDX- and XPS-analysis results revealed no silicate or silicide at the silicon interface. The equivalent oxide thickness was 3 nm and the dielectric constant was around 7.2, as determined by electrical measurements. A fixed charge density of 3 × 10¹⁰ cm⁻² and a leakage current of 5 × 10⁻⁷A/cm² at 2 V gate bias were achieved for Au/gate stack /Si/SiO₂/Si/Au MIS capacitors. Post-annealing treatment was found to effectively reduce trap density, but increase in annealing temperature did not made any significant difference in the electrical performance.     

Blue organic light-emitting diodes with low driving voltage and maximum enhanced power efficiency based on buffer layer MoO3

Blue organic light-emitting devices based on wide bandgap host material, 2-(t-butyl)-9, 10-di-(2-naphthyl) anthracene (TBADN), blue fluorescent styrylamine dopant, p-bis(p-N,N-diphenyl-amino-styryl)benzene (DSA-Ph) have been realized by using molybdenum oxide (MoO₃) as a buffer layer and 4,7-diphenyl-1,10-phenanthroline (BPhen) as the ETL. The typical device structure used was glass substrate/ITO/MoO₃ (5 nm)/NPB (30 nm)/[TBADN: DSA-Ph (3 wt%)](35 nm)/BPhen (12 nm)/LiF (0.8 nm)/Al (100 nm). It was found that the MoO₃∥BPhen-based device shows the lowest driving voltage and highest power efficiency among the referenced devices. At the current density of 20 mA/cm², its driving voltage and power efficiency are 5.4 V and 4.7 Lm/W, respectively, which is independently reduced 46%, and improved 74% compared with those the m-MTDATA∥Alq₃ is based on, respectively. The J-V curves of ‘hole-only’ devices reveal that a small hole injection barrier between MoO₃∥NPB leads to a strong hole injection, resulting low driving voltage and high power efficiency. The results strongly indicate that carrier injection ability and balance shows a key significance in OLED performance.     

Multiple roles of bathocuproine employed as a buffer-layer in organic light-emitting diodes

Efficiency and brightness and carriers injection have been obviously improved by using bathocuproine (BCP) as a buffer-layer in organic light-emitting diodes. Compared with the bufferless device, the quantum efficiency of device ITO/NPB (10 nm)/Alq₃ (10 nm)/BCP (2.4 nm)/Al has increased four times at the same current density (32 mA/cm²). Moreover, the buffer layer has changed the current-voltage properties and the turn-on voltage has obviously decreased. Considering BCP and Al³⁺ can react conveniently under room temperature, we suggest that a complex cathode structure of BCP/[(Al)_x(BCP)_y]^3x+/Al has formed under electric field and the new cation [(Al)_x(BCP)_y]^3x+ at the BCP/Al interface has improved the internal electric field and then enhanced the electrons injection. we conclude that: for a very thin (<1 nm) BCP buffer layer, improving electron injection will principally responsible to the improvement of the performance of the OLEDs; for a thicker BCP layer, there will be a synthetic function of BCP: improving electron injection, hole-blocking and electron-transporting.     

Enhanced brightness of organic light-emitting diodes based on Mg:Ag cathode using alkali metal chlorides as an electron injection layer

Different thicknesses of cesium chloride (CsCl) and various alkali metal chlorides were inserted into organic light-emitting diodes (OLEDs) as electron injection layers (EILs). The basic structure of OLED is indium tin oxide (ITO)/N,N′-diphenyl-N,N′-bis(1-napthyl-phenyl)-1.1′-biphenyl-4.4′-diamine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq₃)/Mg:Ag/Ag. The electroluminescent (EL) performance curves show that both the brightness and efficiency of the OLEDs can be obviously enhanced by using a thin alkali metal chloride layer as an EIL. The electron injection barrier height between the Alq₃ layer and Mg:Ag cathode is reduced by inserting a thin alkali metal chloride as an EIL, which results in enhanced electron injection and electron current. Therefore, a better balance of hole and electron currents at the emissive interface is achieved and consequently the brightness and efficiency of OLEDs are improved.     

Effect of SiO2/Si3N4 dielectric distributed Bragg reflectors (DDBRs) for Alq3/NPB thin-film resonant cavity organic light emitting diodes

In this article, we report on the effect of SiO₂/Si₃N₄ dielectric distributed Bragg reflectors (DDBRs) for Alq₃/NPB thin-film resonant cavity organic light emitting diode (RCOLED) in increasing the light output intensity and reducing the linewidth of spontaneous emission spectrum. The optimum DDBR number is found as 3 pairs. The device performance will be bad by further increasing or decreasing the number of DDBR. As compared to the conventional Alq₃/NPB thin-film organic light emitting diode (OLED), the Alq₃/NPB thin-film RCOLED with 3-pair DDBRs has the superior electrical and optical characteristics including a forward voltage of 6 V, a current efficiency of 3.4 cd/A, a luminance of 2715 cd/m² under the injection current density of 1000 A/m², and a full width at half maximum (FWHM) of 12 nm for emission spectrum over the 5-9 V bias range. These results represent that the Alq₃/NPB thin-film OLED with DDBRs shows a potential as the light source for plastic optical fiber (POF) communication system.     

The effect of alkaline metal chlorides on the properties of organic light-emitting diodes

The multilayer organic light-emitting diodes (OLEDs) have been fabricated with a thin alkaline metal chloride layer inserted inside an electron transport layer (ETL), tris (8-hydroxyquinoline) aluminum (Alq₃). The alkaline metal chloride layer was inserted inside 60 nm Alq₃ at d=0, 10, 20 and 30 nm positions (d is the distance of the interlayer away from the Al cathode). The devices, with alkaline metal chlorides inserted at the Alq₃/Al interface, showed electron injection and electroluminescence (EL) intensity improvements. When the alkaline metal chlorides were inserted inside the Alq₃ layer at 10, 20 or 30 nm position apart from the Al cathode, both EL intensity and efficiency were enhanced for the devices with a thin potassium chloride (KCl) or rubidium chloride (RbCl) layer. On the contrary, the improvements were not observed for the OLEDs with a thin sodium chloride (NaCl) layer. A proposed insulator buffer layer model is employed to explain these characteristics of the devices.     

Characterization of Y2O3 gate dielectric on n-GaAs substrates

Physical and electrical properties of sputtered deposited Y₂O₃ films on NH₄OH treated n-GaAs substrate are investigated. The as-deposited films and interfacial layer formation have been analyzed by using X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). It is found that directly deposited Y₂O₃ on n-GaAs exhibits excellent electrical properties with low frequency dispersion (<5%), hysteresis voltage (0.24 V), and interface trap density (3 × 10¹² eV⁻¹ cm⁻²). The results show that the deposition of Y₂O₃ on n-GaAs can be an effective way to improve the interface quality by the suppression on native oxides formation, especially arsenic oxide which causes Fermi level pinning at high-k/GaAs interface. The Al/Y₂O₃/n-GaAs stack with an equivalent oxide thickness (EOT) of 2.1 nm shows a leakage current density of 3.6 × 10⁻⁶ A cm⁻² at a V_FB of 1 V. While the low-field leakage current conduction mechanism has been found to be dominated by the Schottky emission, Poole-Frenkel emission takes over at high electric fields. The energy band alignment of Y₂O₃ films on n-GaAs substrate is extracted from detailed XPS measurements. The valence and conduction band offsets at Y₂O₃/n-GaAs interfaces are found to be 2.14 and 2.21 eV, respectively.     

High-k Al2O3 MOS structures with Si interface control layer formed on air-exposed GaAs and InGaAs wafers

This paper attempts to realize unpinned high-k insulator-semiconductor interfaces on air-exposed GaAs and In_0.53Ga_0.47As by using the Si interface control layer (Si ICL). Al₂O₃ was deposited by ex situ atomic layer deposition (ALD) as the high-k insulator. By applying an optimal chemical treatment using HF acid combined with subsequent thermal cleaning below 500 °C in UHV, interface bonding configurations similar to those by in situ UHV process were achieved both for GaAs and InGaAs after MBE growth of the Si ICL with no trace of residual native oxide components. As compared with the MIS structures without Si ICL, insertion of Si ICL improved the electrical interface quality, a great deal both for GaAs and InGaAs, reducing frequency dispersion of capacitance, hysteresis effects and interface state density (D_it). A minimum value of D_it of 2 × 10¹¹ eV⁻¹ cm⁻² was achieved both for GaAs and InGaAs. However, the range of bias-induced surface potential excursion within the band gap was different, making formation of electron layer by surface inversion possible in InGaAs, but not possible in GaAs. The difference was explained by the disorder induced gap state (DIGS) model.     

Enhanced ferroelectric property of (Pb0.95Ca0.05)(Nb0.02Zr0.80Ti0.20)O3 thin films prepared by RF magnetron sputtering

(Pb_0.95Ca_0.05)(Nb_0.02Zr_0.80Ti_0.20)O₃ [PCNZT] thin films were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering with and without a LaNiO₃ [LNO] buffer layer. Ca and Nb elements in PZT films enhance the ferroelectric property, LaNiO₃ buffer layer improves the crystal quality of the PCNZT thin films. PCNZT thin films possess better ferroelectric property than that of PZT films for Ca and Nb ion substitution, moreover, PCNZT thin films with a LNO buffer layer possess (1 0 0) orientation and good ferroelectric properties with high remnant polarization (P_r = 38.1 μC/cm²), and low coercive field (E_c = 65 kV/cm), which is also better than that of PCNZT thin films without a LNO buffer layer (P_r = 27.9 μC/cm², E_c = 74 kV/cm). The result shows that enhanced ferroelectric property of PZT films can be obtained by ion substitution and buffer layer.     

Emission properties of dopants rubrene and coumarin 6 in Alq3 films

We have investigated the emission properties of dopants 5,6,11,12-tetraphenylnapthacene (rubrene) and 3-(2′-benzothiazolyl)-7-diethylaminocoumarin (coumarin 6) as well as co-doping of these two dopants in tris (8-hydroxyquinolinato) aluminum (Alq₃) films in double-layer organic light emitting diodes (OLEDs). We varied the rubrene (Rb) doping concentration in Alq₃:Rb films up to 10 wt%. The maximum luminescence efficiency of ∼6.5 cd/A was observed for Rb doping concentration of ∼0.7 wt% in Alq₃:Rb film, which was nearly double efficiency compared to pure Alq₃ device. The co-doping of dopants of C-6 and Rb in the ratio of 1:1 and 1:2 in Alq₃ films reduced the bias voltage compared to pure Alq₃ and Alq₃:C-6 devices for the same current density. The maximum luminescence efficiency was improved to ∼7 cd/A in Alq₃:{C-6:Rb(1:2)} film OLED. The direct recombination of holes and electrons in the dopant molecules may be responsible for the improvement of the luminescence efficiency. We also observed the shifting of photoluminescence (PL) and electroluminescence (EL) peaks position from ∼515 to ∼562 nm by co-doping of Rb and C-6 in Alq₃.     

Scintillation and anomalous emission in elpasolite Cs2LiLuCl6:Ce

The luminescence and scintillation properties of Cs₂LiLuCl₆:0.5%Ce³⁺ are presented. Special attention is devoted to a 9.4 ns fast emission at 275 nm that can only be excited via the highest cubic field 5d_e state of Ce. Contrary to Cs₃LuCl₆ and Cs₂LiYCl₆, where the same type of fast emission was observed, the emission in Cs₂LiLuCl₆ is still observed at room temperature. Assuming that the 5d_e state is located inside the host conduction band (CB), we propose that the emission originates from a mixed state at or just below the bottom of the CB and ends at the 4f ground state of Ce³⁺. To proof this model we studied the thermal quenching of the anomalous luminescence and performed X-ray photoelectron spectroscopy. A model for a temperature-activated energy transfer from the anomalous state to the lowest 5d_t excited state of Ce³⁺ explains most of the results. Besides the 275 nm emission, the material shows 5d_t-4f Ce³⁺ emission at 370 and 406 nm and 2 ns fast core-valence luminescence when excited with 16-22 eV photons. The scintillation properties of Cs₂LiLuCl₆:Ce are briefly discussed.     

Study of photophysical properties AlBaq5

Pure and Ba²⁺ doped Alq₃ complexes were synthesized by simple precipitation method at room temperature, maintaining stoichiometric ratio. These complexes were characterized by XRD, UV–vis and FT-IR and photoluminescence (PL) spectra. XRD analysis reveals the polycrystalline nature of the synthesized complexes, while UV and FTIR confirm the molecular structure and the completion of quinoline ring formation and presence of quinoline structure in the metal complex. PL spectra of Alq₃ compared with barium doped complexes exhibit highest intensity in comparison to Alq₃ phosphor, which proves that barium enhances PL emission intensity of Alq₃ phosphor. The excitation spectra of the synthesized complexes are in the range of 300–480 nm with a broad peak in the range of 429–440 nm and shoulder at 380 nm, but with varying intensity. The emission wavelength lies in the range of 501–506 nm. Among all the synthesized complexes, AlBa₂q₅ exhibits maximum emission intensity. These remarkable properties of AlBaq₅ could be considered as promising materials as optoelectronic materials as well as green light emissive materials for OLEDs, PLLCD and solid state lighting applications.     

Investigation of charge carrier mobility in 5,6,11,12-tetraphenylnapthacene (rubrene) and coumarin 6 doped Alq3 films

The doping effect on charge carrier mobility in tris (8-hydroxyquinolinato) aluminum (Alq₃) was studied by time-of-flight (TOF) measurements. The polar dopant, coumarin 6 (C-6) and extensive π conjugated dopant, 5,6,11,12-tetraphenylnaphthacene (rubrene) were used for this study. The co-doped of rubrene (Rb) with C-6 into Alq₃ improved the carrier mobility compared to the single doped Alq₃:C-6 film. The carrier mobility in single doped Alq₃:C-6 film did not follow the linear relationship of Poole-Frenkel (PF) model with applied electric field. The mobility was in agreement with the PF model at two different ranges of electric fields (F) separated by a critical field . The mobility in co-doped Alq₃:(Rb:C-6) film followed the linear relationship with the PF model. The energetic disorder was found as ∼0.32 eV in co-doped films. It was ∼0.55 and ∼0.27 eV before and after the critical field in Alq₃:C-6 film. The values of positional disorders in co-doped films were estimated as ∼1.8 and it was ∼2 in Alq₃:C-6 film at . The organic light emitting diode performance of the co-doped film was improved compared to single doped film. The luminescence efficiency was improved tremendously to ∼6  Cd/A in co-doped device at 45 mA/cm² current compared to Alq₃:C-6 film device of ∼1  Cd/A.     

UV exposure and photon degradation of Alq3 powders

Tris-(8-hydroxyquinoline)aluminum (Alq₃) is a widely used light emitting material. It is also used as an electron transporting layer in organic light emitting devices (OLEDs). Degradation is, however, a major problem in these devices. The device performance is affected by parameters such as air, moisture and light exposure [1,2]. In this work the effect of photon degradation of Alq₃ in air is investigated. Alq₃ phosphor powder was synthesized using a co-precipitation method and recrystalized in acetone. The structure of the sample was determined by using x-ray diffraction (XRD). The averaging particle size estimated from the broadened XRD peaks using Scherrer's equation was 40±4 nm in diameter. The excitation photoluminescence data that was collected correspond well to the absorption data. To study the photon degradation, the sample was irradiated with an UV lamp for ∼330h. The emission data was collected and the change in photoluminescence intensity with time was monitored.     

Enhanced power efficiency for white OLED with MoO3 as hole injection layer and optimized charge balance

White OLEDs with a different hole injection layer (MoO₃ or m-MTDATA), and a different electron transport layer (Alq₃ or Bphen) have been investigated. With 9,10-bis (2-naphthyl)-2-t-butylanthracene (TBADN) doped with 3% P-bis (P-N, N-diphenyl-aminostyryl) benzene (DSA-ph) and 0.05% 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7,-tetramethyl-julolidy-9-enyl)-4H-pyran (DCJTB) as white emitting layer, the MoO₃/ /Bphen based device shows the lowest driving voltage and highest power efficiency among the referenced devices. At the current density of 20 mA/cm², its driving voltage and power efficiency are 5.43 V and 4.54 lm/W respectively, which is independently reduced 40% and improved 57% compared with those of the m-MTDATA/ / Alq₃ based one, respectively. The energy level diagram of the devices and single-carrier devices are studied to explain the reasons for the improvement. The results strongly indicate that carrier injection ability and balance shows a key significance in OLED performance.     

Reduced dielectric loss and leakage current in CaCu3Ti4O12/SiO2/CaCu3Ti4O12 multilayered films

The CaCu₃Ti₄O₁₂/SiO₂/CaCu₃Ti₄O₁₂ (CCTO/SiO₂/CCTO) multilayered films were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition method. It has been demonstrated that the dielectric loss and the leakage current density were significantly reduced with the increase of the SiO₂ layer thickness, accompanied with a decrease of the dielectric constant. The CCTO film with a 20 nm SiO₂ layer showed a dielectric loss of 0.065 at 100 kHz and the leakage current density of 6×10⁻⁷ A/cm² at 100 kV/cm, which were much lower than those of the single layer CCTO films. The improvement of the electric properties is ascribed to two reasons: one is the improved crystallinity; the other is the reduced free carriers in the multilayered films.     

Effect of Al2O3 on the electrical properties of ZnO-Pr6O11-based varistor ceramics

The effect of Al₂O₃ on the electrical properties of ZnO-Pr₆O₁₁-based ceramics is investigated in this work. The average grain size of ZnO increased as the Al₂O₃ content increased from 10.3 to 13.5 μm. It was found that a sample doped with Al₂O₃ of 0.005 mol% showed the highest nonlinear current-voltage characteristics with a nonlinear exponent of 43.8 and a leakage current of 0.66 μA. When the Al₂O₃ content was increased, the donor concentration was increased from 0.51×10¹⁸/cm³ to 1.59×10¹⁸/cm³, but the barrier height was decreased from 1.01 to 0.87 eV. The best electrical stability against aging stress was obtained by doping Al₂O₃ of 0.001 mol%.     

Synthesis, physical and photo electrochemical characterization of La-doped SrSnO3

The transport properties of Sr_0.98La_0.02SnO_3−δ in the system Sr_1−xLa_xSnO_3−δ, after which the pyrochlore La₂Sn₂O₇ appears, were investigated over the temperature range 4.2-300 K. The oxide was found to be n-type semiconductor with concomitant reduction of Sn⁴⁺ into Sn²⁺. The magnetic susceptibility was measured down to 4.2 K and is less than 3×10⁻⁵ emu cgs mol⁻¹ consistent with itinerant electron behavior. The electron is believed to travel in a narrow band of Sn:5s character with an effective mass ∼4 m_o. The highest band gap is 4.32 eV and the optical transition is directly allowed. A further indirect transition occurs at 4.04 eV. The electrical conductivity follows an Arrhenius-type law with a thermal activation of 40 meV and occurs by small polaron hopping between nominal states Sn^4+/2+. The linear increase of thermo-power with temperature yields an electron mobility μ_300 K (2×10⁻⁴ cm² V⁻¹ s⁻¹) thermally activated. The insulating-metal transition seems to be of Anderson type resulting from random positions of lanthanum sites and oxygen vacancies. At low temperatures, the conduction mechanism changes to a variable range hopping with a linear plot Ln ρ⁻¹ vs. T⁻⁴. The photo electrochemical (PEC) measurements confirm the n-type conductivity and give an onset potential of −0.46 V_SCE in KOH (1 M). The Mott-Schottky plot C⁻²-V shows a linear behavior from which the flat band potential V_fb=+0.01 V_SCE at pH 7 and the doping density N_D=1.04×10²¹ cm⁻³ were determined.