Multifunctional ZnO:V thin films deposited by rf-magnetron sputtering from aerogel nanopowder target material

ZnO:V thin films have been grown onto suprasil substrates by rf-magnetron sputtering at room temperature using nanocrystalline powder synthesized by modified sol-gel method. In our approach the water for hydrolysis used in the synthesis of nanopowder was slowly released by esterification reaction followed by a thermal drying in ethyl alcohol at 250 °C. The effects of V concentration on structural, electrical, morphological and optical properties were studied. The as-deposited films with a thickness of about 0.4 μm were polycrystalline with a hexagonal wurtzite structure and preferentially oriented in the (002) crystallographic direction. The films present high optical transmittance in the visible range of approximately 90%, carrier concentration of about 10²⁰ cm^− 3 and electrical resistivity of 10^− 3 Ω cm at room temperature. In the as-prepared state the films also present ferromagnetic properties attributed to the presence of vanadium based secondary phases.     

Photo- and electroluminescence properties of fluorene-based copolymers containing electron- or hole-transporting unit

We report the photophysical and electroluminescence (EL) properties of two fluorene-based copolymers, poly{[9,9-bis(2-ethylhexyl)fluorene-2,7-diyl]-alt-[6,6′- bis(3-phenylquinoxaline)-2,2′-diyl]} (Qx-PF) and poly{[9,9-bis(2- ethylhexyl)fluorene-2,7-diyl]-alt-[N,N′-diphenyl-N,N′-bis(4-phenyl)-1,1′-biphenyl-4,4′-diamine]} (TPD-PF). The two copolymers in thin films show blue emission approximately 429-452 nm with relatively narrow bandwidth upon photoexcitation. Electroluminescence has been demonstrated using TPD-PF as the active polymer in the light-emitting electrochemical cell (LEC) with a turn-on voltage at 2.8 V and an EL efficiency of 0.002 cd/A. Due to the improved electron-transporting property, the Qx-PF-based LEC achieves the EL efficiency of 0.07 cd/A, 35 times higher than that of the TPD-PF-based device. Compared to the photoluminescence spectra, EL spectra show enhanced excimer emission, which is primarily related to self-heating of the devices during operation. The main process involved in the decrease of the light intensity during device operation is the electrochemical degradation of the polymer blend.     

Study on the structural, electrical, optical, adhesive properties and stability of Ga-doped ZnO transparent conductive films deposited on polymer substrates at room temperature

Flexible optoelectronic devices are attractive because of light weight, small volume, flexibility and easy transport. Transparent conductive oxide thin films deposited on polymer substrates could satisfy the flexibility for optoelectronic devices. Ga-doped ZnO (GZO) films have been prepared on polycarbonate substrates by radio frequency magnetron sputtering at room temperature. The dependence of the structural, electrical, optical and adhesive properties for films on the sputtering powers was investigated. We also investigated the stability of the electrical property through doing Hall-effect measurements 18 months later. The lowest sheet resistance was 5.8 Ω/sq. After 18 months, the lowest sheet resistance was 6.5 Ω/sq. The stability of the electrical property is excellent. The average transmittance in the visible region of all the films was as high as 85 %, using air as reference. The good transparency-conducting property, excellent stability and room-temperature deposition on polymeric substrates enable GZO films to be widely used in optoelectronic devices.     

Preparation and properties of thin ZnS:Cu films phosphors

Thin films of ZnS and ZnS:Cu were prepared by an original metalorganic chemical vapour deposition (MOCVD) method under atmospheric pressure onto a glass substrate heated up to 230–250 °C. The film thickness varied from 0.6 to 1 μm. The thin films were doped with Cu and Cl by the thermal treatment during 1 h at 600 °C at atmospheric pressure in the blend composed of a ZnS powder with Cu and Cl compounds. These films were used for fabrication of the thin film electroluminescent (TFEL) devices with a conventional double insulating structure. The structural properties were investigated by use of X-ray diffraction (XRD) techniques and atomic force microscopy (AFM). Electroluminescent (EL) spectra, electrical and EL characteristics were investigated. The EL spectra and characteristics as well as structural parameters depend on the growth conditions and significantly modified after the annealing. Blue color emission with brightness of 10 cd m^− 2 under a sine wave excitation at 60 V and 5 kHz was obtained. The degradation behavior of the TFEL devices with ZnS:[Cu, Cl] films fabricated using an original non-vacuum methods of deposition and annealing is the same as that of commercial thin film phosphor.     

Morphological changes in polycrystalline films via vapour phase transport

Polycrystalline films of anthracene prepared by vapour deposition show dramatic morphological and electrical changes over a period of a few months. The process appears to be caused by vapour phase transport at room temperature, and a simple model can be used to predict its rate. Although anthracene films are unstable at room temperature, other organic materials such as perylene are adequately stable to this type of degradation, and the model may be used as an aid to the selection of materials for device applications.     

High efficiency in blue organic light-emitting diodes using an anthracene-based emitting material

A highly efficient deep blue emitting material based on anthracene core structure, 9,10-bis-[4-(2-(4-naphthalene-1-yl-phenyl)-vinyl)-phenyl]anthracene (NSA), was synthesized and the device performances of blue organic light-emitting diodes (OLEDs) with NSA as an emitting material were investigated. High efficiency value of 7.75 Candela (cd)/A was obtained in NSA blue devices compared with 3.6 cd/A of 9,10-bis(4-(2,2-diphenylvinyl)phenyl anthracene devices. The introduction of a phenylanthracene core and a rigid naphthylphenyl side group gave high thermal stability due to non-coplanar structure and limited intermolecular interactions, resulting in high efficiency in blue OLEDs.     

Effect of graphene concentration on performance of MEH:PPV/graphene nanocomposite based devices

Herein, we explore the effect of graphene concentration on performance of poly [2-methoxy-5-2-ethylhexoxy-p-phenylene vinylene] (MEH:PPV)/graphene nanocomposite based devices. The surface morphology of the nanocomposites analyzed through emission scanning electron microscopy suggest that increase in graphene concentration results in the formation of aggregation. Optical and structural properties of the nanocomposites examined through UV–Vis absorption and Raman spectra revealed that the addition of graphene has no affect on the conjugation length and structure of the MEH:PPV. The electrical characteristics of devices have been investigated by I–V measurement under dark and illumination at room temperature. The devices show increase in the current value and reduction on turn-on voltage with increase in the graphene concentration up to 5 wt%. However, increase in graphene concentration above 5 wt% leads to the performance degradation of the devices. Moreover, charge transport mechanisms of the devices have been explained through Ohmic behavior at lower voltages and trap charge limited conduction at higher voltages.     

High-performance transparent inorganic-organic hybrid thin-film n-type transistors

High-performance thin-film transistors (TFTs) that can be fabricated at low temperature and are mechanically flexible, optically transparent and compatible with diverse substrate materials are of great current interest. To function at low biases to minimize power consumption, such devices must also contain a high-mobility semiconductor and/or a high-capacitance gate dielectric. Here we report transparent inorganic-organic hybrid n-type TFTs fabricated at room temperature by combining In2O3 thin films grown by ion-assisted deposition, with nanoscale organic dielectrics self-assembled in a solution-phase process. Such TFTs combine the advantages of a high-mobility transparent inorganic semiconductor with an ultrathin high-capacitance/low-leakage organic gate dielectric. The resulting, completely transparent TFTs exhibit excellent operating characteristics near 1.0 V with large field-effect mobilities of >120 cm2 V(-1) s(-1), drain-source current on/off modulation ratio (I(on)/I(off)) approximately 10(5), near-zero threshold voltages and sub-threshold gate voltage swings of 90 mV per decade. The results suggest new strategies for achieving 'invisible' optoelectronics.     

锗纳米镶嵌薄膜的电致发光及其机制

采和射频磁控溅射技术,在Ge纳米镍嵌薄膜的基础上制备出电致发光器件。器件的结构为半透明Au膜／Ge纳米镍嵌薄膜／p-Si基片。当正向邻居坟大于6V时,用肉眼可以观察到可见的电致发光,但在反向偏压下探测不到光发射。所测电致发光谱中只有一个发光峰,峰位在510nm（2．4eV,绿光）,并且随着正向偏坟的升高,峰位不发生移动;对于不同温度退火的样品,峰位也保持不变。根据分析结果讨论了可能的电致发光机制。     

Room temperature deposition of IZTO transparent anode films for organic light-emitting diodes

This study was to investigate anodic electrode IZTO films deposited by pulsed DC magnetron sputter at room temperature with various oxygen partial pressures onto glass substrate and to analyze the structural, electrical, and optical properties, as well as the relationship between the chemical binding state of the surface and the characteristics of IZTO films. In addition, the prepared IZTO films were used to fabricate the organic light emitting diodes (OLEDs) as an anode layer to study the device performances. The IZTO film deposited at optimal oxygen partial pressure of 2.0% in sputtering process showed the best properties, such as a low electrical resistivity and high optical transmittance of <5.1 × 10^?4 Ω cm and >80% in the visible wavelength of 400–800 nm, respectively. The OLED characteristics with the optimum condition showed good brightness and the lowest turn-on voltage of >10,000 cd/m² and 4.67 V. These results indicate that IZTO films can be a promising candidate as an alternative TCO electrode material for flexible and OLED devices.     

Electroluminescence of fluorescent–phosphorescent organic light-emitting diodes with regular,inverted, and symmetrical structures

The influence of the device structure on the electroluminescence (EL) properties of fluorescent–phosphorescent organic light emitting diodes (OLEDs) was demonstrated. Four devices with regular-, inverted-, compensated- and symmetrical-emission layers (EMLs) were prepared. In regular-EML device, DCJTB emission increased when the phosphorescent sensitized EML was thickened. In inverted-EML device, low electron energy barrier at the Bphen/BCzVB interface resulted in weakened blue emission. The compensated-EML device, prepared with a red color-compensated layer, showed a color-tunable broadband white emission. Conversely, device with a quantum-like symmetrical-EML showed a narrow color-temperature range. Stable EL efficiency was obtained from regular, compensated, and symmetrical-EML devices. In contrast, EL efficiency of inverted-EML device rolled off significantly, though it had the highest EL efficiency of 11.4 cd/A.     

Photoluminescent and electroluminescent characteristics of various Bi-activated niobate-based oxide phosphor thin films

The photoluminescent (PL) and electroluminescent (EL) characteristics in the thin films of various Bi-activated niobate-based oxide phosphors have been investigated. LaNbO₄:Bi, GdNbO₄:Bi and YNbO₄:Bi phosphor thin films were prepared on thick BaTiO₃ ceramic sheet substrates by either a conventional or a combinatorial radio frequency magnetron sputtering deposition followed by postannealing at a high temperature. Blue PL emissions were observed from Bi-activated niobate-based oxide phosphor thin films postannealed at a temperature above approximately 800 °C in air or a pure Ar gas atmosphere. The excitation spectra of the blue PL emissions as well as the blue emission spectra observed from LaNbO₄:Bi, GdNbO₄:Bi and YNbO₄:Bi phosphor thin films exhibited an increasing red-shift in that order. In addition, blue, whitish blue-green or blue-green emissions were observed from thin-film electroluminescent devices fabricated with a LaNbO₄:Bi, a GdNbO₄:Bi or a YNbO₄:Bi thin-film emitting layer, respectively. However, the EL emission spectra observed from these TFEL devices exhibited a red-shift relative to the PL emission spectra observed from the respective phosphor thin films. The resulting difference between the observed PL and EL emission spectra may be explained by the 6 s²-6 s6p transitions in Bi³⁺ ions. In particular, the long-wavelength bands observed in the EL emissions may be attributable to the selection rule resolved by a high electric field applied to the phosphor thin films.     

AgBr nanocrystal-dispersed silsesquioxane-titania hybrid films for holographic materials

AgBr-doped silsesquioxane-titania films with a thickness of approximately 5 μm were prepared using a sol-gel technique. The dopant AgBr crystals were converted into very small Ag nanoparticles (mainly several nanometers) upon blue laser irradiation, leading to an increase in the absorption of the film in the visible region. An excess of Ag⁺ ions in the film was required for the conversion of AgBr into Ag upon blue laser irradiation. A maximum diffraction efficiency of 0.03% was achieved after two-beam interference exposure with a blue laser for 1300 s. These results imply that the films are suitable for use as holographic materials.     

Impact of thickness on vacuum deposited PbSe thin films

Lead Selenide thin films were prepared by vacuum evaporation technique with different thickness ranges from 50 to 200 nm on glass substrates. The structural studies revealed that the prepared films are strongly oriented on (2 0 0) plane with rock-salt crystal structure. The various structural parameters such as grain size (D), lattice constant (a), micro strain (ε) and dislocation density (δ) were calculated. The surface morphology of the films was also analyzed. The optical absorption of the films starts with visible region and obtained energy gap of the films lies between 1.5 and 1.9 eV. The room temperature Photoluminescence spectrum shows the emission peak at visible region (380-405 nm) and the blue shift was observed with decreasing the film thickness. The electrical mobility, resistivity, carrier concentration and mean free path (L) of the free carriers of the films were studied for all the samples and compared.     

Violet-blue LEDs based on p-GaN/n-ZnO nanorods and their stability

In this paper, we report a fabrication, characterization and stability study of p-GaN/n-ZnO nanorod heterojunction light-emitting devices (LEDs). The LEDs were assembled from arrays of n-ZnO vertical nanorods epitaxially grown on p-GaN. LEDs showed bright electroluminescence in blue (440 nm), although weaker violet (372 nm) and green-yellow (550 nm) spectral components were also observed. The device characteristics are generally stable and reproducible. The LEDs have a low turn-on voltage (～5 V). The electroluminescence (EL) is intense enough to be noticed by the naked eye, at an injection current as low as ～ 40 μA (2.1 × 10(-2) A cm(-2) at 7 V bias). Analysis of the materials, electrical and EL investigations point to the role of a high quality of p-n nano-heterojunction which facilitates a large rectification ratio (320) and a stable reverse current of 2.8 μA (1.4 × 10(-3) A cm(-2) at 5 V). Stability of EL characteristics was investigated in detail. EL intensity showed systematic degradation over a short duration when the LED was bias-stressed at 30 V. At smaller bias (<20 V) LEDs tend to show a stable and repeatable EL characteristic. Thus a simple low temperature solution growth method was successfully exploited to realize nanorod/film heterojunction LED devices with predictable characteristics.     

Piezo-phototronic effect on electroluminescence properties of p-type GaN thin films

We present that the electroluminescence (EL) properties of Mg-doped p-type GaN thin films can be tuned by the piezo-phototronic effect via adjusting the minority carrier injection efficiency at the metal-semiconductor (M-S) interface by strain induced polarization charges. The device is a metal-semiconductor-metal structure of indium tin oxide (ITO)-GaN-ITO. Under different straining conditions, the changing trend of the transport properties of GaN films can be divided into two types, corresponding to the different c-axis orientations of the films. An extreme value was observed for the integral EL intensity under certain applied strain due to the adjusted minority carrier injection efficiency by piezoelectric charges introduced at the M-S interface. The external quantum efficiency of the blue EL at 430 nm was changed by 5.84% under different straining conditions, which is 1 order of magnitude larger than the change of the green peak at 540 nm. The results indicate that the piezo-phototronic effect has a larger impact on the shallow acceptor states related EL process than on the one related to the deep acceptor states in p-type GaN films. This study has great significance on the practical applications of GaN in optoelectronic devices under a working environment where mechanical deformation is unavoidable such as for flexible/printable light emitting diodes.     

薄膜厚度和工作压强对室温制备AZO薄膜性能的影响

采用射频磁控溅射法在室温下、普通玻璃基片上制备了AZO透明导电薄膜。用X射线衍射仪、原子力显微镜、紫外-可见分光光度计和四探针测量了不同薄膜厚度和不同工作压强下所得样品的结构、电学和光学性能,结果表明,所制备的AZO薄膜均具有六角纤锌矿结构,沿c轴择优取向生长;在可见光范围内,薄膜平均透过率约为80%;随着薄膜厚度的增加和工作压强的降低,薄膜的电阻率呈下降趋势;得到的薄膜最低方块电阻为7.5Ω/□。     

Leakage current suppression in solution-deposited barium titanate films on copper foils

Processing of high-permittivity ceramic films on free-standing bare copper foil for subsequent organic package integration requires high-temperature crystallization at low oxygen pressures. This frequently can result in incorporation of oxygen vacancies and copper diffusion into the film that enhances leakage current and degrade the reliability characteristics. Leakage current, breakdown voltage and electrical reliability of the devices were improved by incorporating 1% excess barium and manganese dopant. Incorporation of dopants also resulted in enhanced densification and grain refinement. Leakage current analysis indicated Space-Charge-Limited Conduction as the dominant conduction mechanism in both undoped and doped films. The mechanisms by which acceptor dopants suppress oxygen vacancy creation and migration are discussed. Capacitance densities of 1.5–3 μF/cm², with breakdown voltages above 10 V, were demonstrated for 250–500 nm thin barium titanate films.     

Effect of substrate temperature on the physical properties of In2O3:Mo films: Prepared by an activated reactive evaporation

Transparent and high conductive molybdenum doped indium oxide (IMO) thin films have been deposited on glass substrates by activated reactive evaporation. The effect of substrate temperature on the structure, surface morphology, electrical, optical and photoluminescence properties of these films were systematically examined and it was found that crystallinity and growth orientation of the films were significantly influenced by substrate temperature. In addition the films exhibited a low electrical resistivity of 5.2 × 10⁻⁴ Ω cm with an optical transmittance 90% in the visible region of the solar spectrum at a substrate temperature of 573 K. Intensive blue PL bands at 415 and 440 nm were observed in the photoluminescence spectrum at room temperature.     

Plasma exposure inducing crystallization of indium oxide film with improved electrical and mechanical properties at room temperature

A novel plasma exposure technique has been introduced into conventional magnetron sputtering process to enhance the crystallization of indium oxide (In₂O₃) films at room temperature. The effect of plasma exposure technique with different pulsed DC voltages on the electrical and mechanical properties of In₂O₃ films was investigated. It is observed that film crystallization can be significantly enhanced when the pulsed DC voltage (|V _p|) is higher than |?500 V| (|V _p| > |?500 V|). By applying the plasma exposure process, In₂O₃ films prepared at room temperature with thickness of 135 nm shows low resistivity of 4.11 × 10^?4 Ω cm, mobility of 42.1 cm²/Vs, and transmittance over 80 % in the visible range. Compared with the In₂O₃ films without plasma exposure process, the In₂O₃ films with plasma exposure show better crystallization and remarkably higher nanohardness. The plasma exposure technique is a useful candidate technique for enhancing film crystallization at low temperature.