Photoluminescence and electroluminescence of a novel green-yellow emitting material-5,6-Bis-[4-(naphthalene-1-yl-phenyl-amino)-phenyl]-pyrazine-2,3-dicarbonitrile

A new compound with intramolecular charge transfer (ICT) property—5,6-Bis-[4-(naphthalene-1-yl-phenyl-amino)-phenyl]-pyrazine-2,3-dicarbonitrile(BNPPDC) was synthesized. The new compound was strongly fluorescent in non-polar and moderately polar solvents, as well as in thin solid film. The absorption and emission maxima shifted to longer wavelength with increasing solvent polarity. The fluorescence quantum yield also increased with increasing solvent polarity from non-polar to moderately polar solvents, then decreased with further increase of solvent polarity. This indicates both “positive” and “negative” solvatokinetic effects co-existed. Using this material as hole-transporting emitter and host emitter, we fabricated two electroluminescent (EL) devices with structures of A (ITO/BNPPDC (45 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) (45 nm)/Mg:Ag (200 nm) and B (ITO/N,N′-diphenyl-N,N′-bis-(3-methylphenyl) (1,1′-diphenyl)4,4′-diamine (TPD) (50 nm)/BNPPDC (20 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) (45 nm)/Mg:Ag (200 nm). The devices showed green-yellow EL emission with good efficiency and high brightness. For example, the device A exhibited a high brightness of 17400 cd/m² at a driving voltage of 11 V and a very low turn-on voltage (2.9 V), as well as a maximum luminous efficiency 3.61 cd/A. The device B showed a similar performance with a high brightness of 12650 cd/m² at a driving voltage of 13 V and a maximum luminous efficiency 3.62 cd/A. In addition, the EL devices using BNPPDC as a host and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as a dopant (configuration: ITO/TPD (60 nm)/BNPPDC:DCJTB (2%) (30 nm)/TPBI (35 nm)/Mg:Ag (200 nm)) showed a good performance with a brightness of 150 cd/m² at 4.5 V, a maximum brightness of 12600 cd/m² at 11.5 V, and a maximum luminous efficiency of 3.30 cd/A.     

Luminescence intensity and dopant concentration in AlN:Tb

The luminescence intensity of Tb³⁺ embedded in sputter deposited and subsequently annealed AlN shows strong concentration dependence. The intensity of the characteristic green luminescence of Tb³⁺ rises at low concentrations with rising concentration. At higher concentrations, strong concentration quenching occurs. The optimum concentration range of Tb³⁺ in AlN was observed to range between 2.5 at% Tb and 3.6 at% Tb. The experimentally evaluated concentration dependence results can be described by a rate equation approach based on available models of luminescence quenching.     

ZnS:TbF3,ErF3,HoF3ACTFEL的亮度与跃迁几率的关系

测量了低浓度下ZnS:TbF3,ErF3和ZnS:TbF3,HoF3交流电致发光薄膜(ACTFEL)中的Tb3+,Er3+,HO3+主要发射峰的相对积分强度,并与Tb3+,Er3+,Ho3+主要发射能级的辐射跃迁几率及高浓度时的发光特性进行了比较,发现稀土氟化物掺杂的ZnS薄膜电致发光(ACTFEL)的最大亮度,不完全取决于稀土离予本身的辐射跃迁几率,而主要取决于稀土离子主要发射能级激发态的运动。本文首次给出了Tb3+离子在ZnS中的强度参数Ω2=3.2×10-19cm2,Ω4=1.6×10-19cm2,Ω6=2.6×10-19cm2,并应用这组Ωλ参数估算了Tb3+离子的交叉弛豫(cross-relaxation)几率。     

Effect of oxygen content and deposition temperature on the characteristics of thin silver films deposited by magnetron sputtering

Thin silver films were prepared by direct current magnetron sputtering in a single-ended in-line sputter system at various substrate temperatures and in O₂ contents in sputter gas, and their electrical, optical, structural and morphological properties together with the compositional properties were investigated. When deposited at room temperature, the electrical and optical properties of Ag films deteriorated with addition of O₂ to sputter gas. Deposition of Ag films in O₂ added sputter gas promoted the formation of Ag crystallites with (2 0 0) plane parallel to the substrate surface. The electrical resistivity and optical reflection of Ag films deposited above 100 °C were not affected by the sputtering plasma containing oxygen. X-ray photoelectron spectroscopic analysis showed that Ag films deposited above 100 °C in O₂ added sputter gas did not possess surplus oxygen in the film, and that the oxidation states of these films were almost identical to that of Ag films deposited in pure Ar gas.     

Ca/Ag bilayer cathode for transparent white organic light-emitting devices

We have demonstrated that the compositional modification of the Ca/Ag films is principally responsible for a high transmittance (over 70% in the visible range) and low sheet resistance (10-12 Ω/sq). X-ray photoelectron spectroscopy (XPS) sputter depth profiling of Ca/Ag structure reveals the presence of Ca(OH)₂ and Ca metal. A chemical model of the Ca/Ag cathode is proposed. Using transparent ITO anode and Ca (10 nm)/Ag (10 nm) cathode, efficient white organic light-emitting devices (WOLEDs) emitting from both sides have been fabricated. Brightness of 3813 cd/m² and Commission Internationale de l’Eclairage (CIE) coordinates (0.36, 0.34) at 10 V through ITO anode and values of 1216 cd/m² and (0.33, 0.30) through Ca/Ag cathode are reported. A low turn-on voltage of 5.5 V is measured.     

Electrophysical properties of zinc-sulfide films obtained by high-frequency magnetron sputtering

The structure and electrophysical properties of zinc-sulfide films obtained by high-frequency magnetron sputtering are investigated. It is shown that the electrical strength of the films is no less than 10⁶ V·cnr^–1, the width of the forbidden band is 3.2–3.3 eV, and the refractive index is 1.8. The films are tested as electroluminescent layers in thin-film electroluminescent emitters. Luminescence with a brightness of 150 cd/m² is obtained.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 29–32, June, 1990.It remains to thank A. A. Miller for perfecting the electron-microscope investigations.     

A comparative study of electrode effects on the electrical and luminescent characteristics of Alq3/TPD OLED: Improvements due to conductive polymer (PEDOT) anode

The performance of organic light emitting device (OLED) structures, based on identically fabricated Alq₃/TPD active regions, with various anode and cathode electrode structures are compared, and performance differences related to the different anode structure. The best performance was achieved with a conductive polymer, 3,4-polyethylenedioxythiopene-polystyrenesultonate (PEDOT), used as an anode layer, yielding a brightness of 1720 cd/m² at 25 V, a turn-on voltage of 3 V, and electroluminescence (EL) efficiency and external quantum efficiency of 8.2 cd/A and 2%, respectively, at a brightness of 100 cd/m² and 5 V. Compared to a baseline device (TPD/Alq₃/Al), PEDOT anodes substantially reduce the turn-on voltage and made current injection almost linear after turn-on, whiles devices incorporating a LiF and CuPc layers significantly improved device efficiency while slightly improving turn-on voltage and maintaining superlinear I-V injection. This is attributed to the reduced barrier at the organic-organic interface in PEDOT, the ‘ladder’ effect of stepping the band offset over several interfaces, and the favorable PEDOT film morphology. The benefit of the PEDOT anode is clearly seen in the improvement in device brightness and the high external quantum efficiency obtained.     

Blue organic electroluminescent devices based on a distyrylarylene derivative as emitting layer and a terbium complex as electron-transporting layer

With a blue distyrylarylene derivative, 4,4′-bis(2,2-di(2-methoxyphenyl)ethenyl)-1,1′-biphenyl (CBS) as emitting material, double-layer and triple-layer electroluminescent (EL) devices were fabricated. For the device using tris-(1-phenyl-3-methyl-4-isobutyryl-5-pyrozolone)-bis(triphenyl phosphine oxide) terbium (Tb(PMIP)₃(TPPO)₂) as the electron-transporting layer, blue EL emission with a maximum luminance of 253 cd/m² was achieved at 19 V. The difference of Tb(PMIP)₃(TPPO)₂ and tris(8-hydroxyquinolinate)aluminum (AlQ) as the electron-transporting materials in these devices were compared and discussed.     

The effect of annealing on the room temperature ferromagnetism in co-sputtered In2O3: C thin films

In this paper, we report investigation of room temperature (RT) ferromagnetism in In₂O₃ (InO) thin films doped with carbon prepared by the co-sputtering method. InO thin films both undoped and C doped with varied thicknesses in the range of 45 to 80 nm were synthesized on Si substrates with varied C concentrations. The carbon concentration was varied from 1.6 to 9.3 at%. The undoped InO films showed no trace of ferromagnetism. Carbon doped films (InO:C) exhibited ferromagnetism at RT, which was of the orders of 10⁻⁵ emu and varied strongly with C concentrations. It is observed that the magnetization reached a maximum value of 5.7 emu/cm³ at 4 at% C. Annealing of the InO:C films in an oxygen environment resulted in a decrease in the magnetization, indicating the crucial role of oxygen vacancies in the films. It is concluded that the oxygen vacancies were important and compete with C substitution for the RT ferromagnetism.     

Combinatorial thin film sputtering investigation of cerium concentration in Lu2SiO5 scintillators

Lutetium oxyorthosilicate (LSO) thin films with a cerium thickness gradient were sputter deposited to investigate the optimum cerium concentration for emission intensity. Thin film cerium concentration ranged from 0.06 to 0.88 at%. To compare the thin film samples to single crystal LSO, a set of single crystal LSO samples were investigated with cerium concentrations of 0.0015, 0.0095 and 0.078 at%. The thin film samples showed peak photoluminescence emission intensity at a cerium concentration of 0.35 at%; however, the single crystal samples exhibited peak photoluminescence emission intensity at a lower cerium concentration of 0.0095 at%. The photoluminescence excitation and emission spectra as a function of concentration demonstrate the concentration quenching behavior and the mechanisms are speculated to be due to radiative (self-absorption) and non-radiative energy transfer, which may be phonon assisted.     

Wettability of ZnO: A comparison of reactively sputtered; thermally oxidized and vacuum annealed coatings

We have studied the wettability of sputter deposited ZnO, thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings. The X-ray diffraction patterns showed the formation of hexagonal-wurtzite structure of ZnO, which was further confirmed by micro-Raman spectroscopy data. The X-ray photoelectron spectroscopy data indicated that the sputter deposited ZnO coatings were more stoichiometric than thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings. The wettability measurements indicated that water contact angles of 158.5° and 155.2° with sliding angles of 2° and 4° were achieved for thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings, respectively. The superhydrophobicity observed in thermally oxidized Zn-ZnO and vacuum annealed coatings is attributed to the nanorod cluster like morphology along with the presence of high fraction of micron scale air pockets. The water droplet on such surfaces is mostly in contact with air pockets rather than solid surface, leading to high contact angle. Whereas, the sputter deposited ZnO coatings exhibited a maximum water contact angle of 110.3°. This is because the sputter deposited ZnO coatings exhibited a densely packed nanograin-like microstructure without any air pockets. The work of adhesion of water was very low for thermally oxidized Zn-ZnO (5.06 mJ/m²) and vacuum annealed ZnO coatings (6.71 mJ/m²) when compared to reactively sputtered ZnO coatings (90.41 mJ/m²). The apparent surface free energy (SFE) for these coatings was calculated using Neumann method and the SFE values for sputter deposited ZnO, thermally oxidized Zn-ZnO and vacuum annealed ZnO coatings were 32.95, 23.21 and 18.78 mJ/m², respectively.     

Organic electroluminescent devices and their application

Several organic electroluminescent devices have been fabricated by multi-source high vacuum deposition system. For high brightness organic electroluminescent device, the maximum brightness is over 40000 cd/m². For quantum well structures, quantum size effect has been investigated and the high light emission efficiencies of the devices have been obtained. White-light emission from organic multi-quantum well structures is proposed at first. Brightness of the white-light MQW devices reaches 4000 cd/m² at 17 V. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998. The research is supported by the National “863” Project of China [No. 863-307-05-05(02)] and by the National Natural Science Foundation of China [No. 69637010].     

Ion-induced secondary electron emission from ZnS thin films deposited by closed-spaced sublimation

ZnS thin films were deposited on soda lime glass and aluminum substrates by close-spaced sublimation technique. The change in composition, structural and optical properties of the films was investigated as a function of the substrate temperature. The deposited films were stoichiometric and crystalline in nature having cubic structure oriented only along (1 1 1) plane. The energy band gap of the films deposited at the substrate temperature of 150, 250 and 350 °C was 3.52, 3.58 and 3.63 eV respectively. These films were then bombarded with 2-10 keV energy pulsed Ar⁺ beam and their electron yield was determined from impinging ion and emitted electron currents. The electron yield of ZnS films was much high as compared to the metals. The electron yield of ZnS films increased with energy of the incident ion and got saturated at about 8 keV. The most important result of this study was that the electron yield of ZnS films having same composition was different. Monte Carlo simulations performed to interpret these experimental findings showed that the dissimilar electron yields of ZnS films is due to the combined effect of energy band gap, surface barrier potential and density of the films.     

Efficient small molecular and polymer organic devices using bis[2-(4-tert-butylphenyl)benzothiazolato-N,C′] iridium (III) (acetylacetonate) dye as emitter

Small molecular organic light-emitting diodes (MOLED) and polymer organic light-emitting diodes (POLED) were fabricated with yellow light emission phosphorescent dye bis[2-(4-tert-butylphenyl)benzothiazolato-N,C²′] iridium (III) (acetylacetonate) doped in different hosts. The electroluminescent (EL) spectra of both devices shown two peaks generated from iridium dye but the position of main peak changed and became broader for POLED. The maximum luminance of 10,500 cd/m² achieved at 12.5 V for MOLED is higher than maximum luminance of 9996 cd/m² at 20 V for POLED. The maximum power efficiency of small molecular device is 6.4 lm/W, which is higher than 2.3 lm/W of polymer device, but the efficiency of both devices will roll off at large current density.     

Surface modification of γ-TiAl alloys by acetylene plasma deposition

Surfaces of two γ-TiAl alloys, Ti-47 at% Al-2at% Nb-2 at% Cr (MJ12) and Ti-47 at% Al-2 at% Nb-2 at% Mn + 0.8 at% TiB₂ (MJ47), have been modified by acetylene plasma deposition at bias voltages of −4, −5 and −6 kV for 3.6 × 10³ s (1 h) and 1.44 × 10⁴ s (4 h). Knoop hardness (HK) of the alloys is increased with the increase of bias voltage and prolonged time for the deposition. HK of MJ12 and MJ47 deposited at −6 kV for 1.44 × 10⁴ s is, respectively, 3.36 and 3.32 times as hard as the untreated alloys. SEM and AFM analyses show that the deposited alloys compose of a number of nano-dots which reflect their surface properties. The phases analyzed by XRD are in accord with the elements analyzed by EDX.     

Highly efficient flexible organic light-emitting devices utilizing F4-TCNQ/m-MTDATA multiple quantum well structures

Flexible organic light-emitting devices (FOLEDs) based on multiple quantum well (MQW) structures, which consist of alternate layers of 2,3,5,6-Tetrafluoro-7,7,8,8,-tetracyano-quinodimethane (F4-TCNQ) and 4,4′,4″-tris-(3-methylphenylphe-nylamino)tripheny-lamine (m-MTDATA) have been fabricated. The Alq₃-based device with double quantum well (DQW) structure exhibits the remarkable electroluminescent (EL) performances for the brightness of 23,500 cd/m² at 14 V and the maximum current efficiency of 7.0 cd/A at 300.3 mA/cm², respectively, which are greatly improved by 114% and 56% compared with the brightness of 10,958 cd/m² at 14 V and the maximum current efficiency of 4.5 cd/A at 174.0 mA/cm² for the conventional device without MQW structures. These results demonstrate that the EL performances of FOLEDs could be greatly improved by utilizing the novel MQW structures, and the reason for this improvement has also been explained by the effect of interfacial dipole and interfacial doping between F4-TCNQ and m-MTDATA in this article.     

High performance yellow light-emitting organic electrophosphorescent devices based on Re(I) complex

Yellow organic light-emitting devices (OLEDs) with (2,3-diphenylpyrazino [2,3-f] [1, 10] phenanthroline) Re(CO)₃Br (Re-Dppp) doped CBP as emissive layer were studied. It is found experimentally that Re-Dppp based yellow OLEDs possess high electroluminescent efficiency and brightness: a maximum brightness of 6249 cd/m² at 15 V and a highest current efficiency of 14.2 cd/A with a brightness of 180 cd/m² occurring at 1.3 mA/cm² are achieved, which indicate that Re(I) complexes have a potential application in commercialized PMOLEDs. PACS 85.60.Jb; 33.50.Dq; 33.50.-j; 42.79.Kr; 85.30.De     

Optical and electroluminescent properties of 3,4,6-triphenyl-α-pyrone

The optical and electroluminescent properties of 3,4,6-triphenyl-α-pyrone (α-pyrone), a new blue fluoresce dye, are investigated using films prepared by wet and dry process and organic light-emitting diodes (OLEDs) fabricated with an α-pyrone-emitting layer. The optical properties of α-pyrone are found to be affected by its crystallinity. In the fabrication of OLEDs, wet processing (spin coating) is shown to be more suitable for preparation of the α-pyrone layer than dry processing (thermal evaporation). The best device performance is obtained for a device prepared using poly (n-vinylcarbazole) as the dye host, and a bathocuproine/tris-(8-hydroxyquinoline)aluminum bilayer as a hole-blocking and carrier-injection layer. The maximum luminance of this device is 3000 cd/m² at a current density of 0.2 A/cm², with a current efficiency of 1.8 cd/A at 0.02 A/cm².     

Gas barrier properties of titanium oxynitride films deposited on polyethylene terephthalate substrates by reactive magnetron sputtering

Titanium oxynitride (TiN_xO_y) films were deposited on polyethylene terephthalate (PET) substrates by means of a reactive radio frequency (RF) magnetron sputtering system in which the power density and substrate bias were the varied parameters. Experimental results show that the deposited TiN_xO_y films exhibited an amorphous or a columnar structure with fine crystalline dependent on power density. The deposition rate increases significantly in conjunction as the power density increases from 2 W/cm² to 7 W/cm². The maximum deposition rate occurs, as the substrate bias is −40 V at a certain power densities chosen in this study. The film's roughness slightly decreases with increasing substrate bias. The TiN_xO_y films deposited at power densities above 4 W/cm² show a steady Ti:N:O ratio of about 1:1:0.8. The water vapor and oxygen transmission rates of the TiN_xO_y films reach values as low as 0.98 g/m²-day-atm and 0.60 cm³/m²-day-atm which are about 6 and 47 times lower than those of the uncoated PET substrate, respectively. These transmission rates are comparable to those of DLC, carbon-based and Al₂O₃ barrier films. Therefore, TiN_xO_y films are potential candidates to be used as a gas permeation barrier for PET substrate.     

A terbium (III) complex with triphenylamine-functionalized ligand for organic electroluminescent device

A novel ligand, 4-diphenylamino-benzoic acid (HDPAB), and the corresponding Tb (III) complex, Tb (DPAB)₃ which can be dissolved easily in organic solvents were synthesized and characterized. Organic electroluminescent (EL) device with a structure of indium tin oxide (ITO)/poly(N-vinylcarbazole) (PVK): Tb (DPAB)₃ (50 wt%, 80 nm)/1,3,5-tris-(N-phenylbenzimidazol-2-yl)benzene (TPBI) (30 nm)/tri(8-hydroxyquinoline)aluminum (AlQ) (20 nm)/LiF (1 nm)/Al (150 nm) in which Tb (DPAB)₃ acted as an emitter were fabricated. The maximum luminance of 230 cd m⁻² at 20 V and the maximum efficiency of 0.62 cd A⁻¹ were obtained due to the introduction of hole-transporting group, representing the best result to date among Tb (III) carboxylate complexes based EL devices. These results indicate that modifications of rare earth complexes are a promising way to improve the properties of EL devices.