Vertical nanowire array-based light emitting diodes

Electroluminescence from a nanowire array-based light emitting diode is reported. The junction consists of a p-type GaN thin film grown by metal organic chemical vapor deposition (MOCVD) and a vertical n-type ZnO nanowire array grown epitaxially from the thin film through a simple low temperature solution method. The fabricated devices exhibit diode like current voltage behavior. Electroluminescence is visible to the human eye at a forward bias of 10 V and spectroscopy reveals that emission is dominated by acceptor to band transitions in the p-GaN thin film. It is suggested that the vertical nanowire architecture of the device leads to waveguided emission from the thin film through the nanowire array.     

Room temperature electroluminescence from arsenic doped p-type ZnO nanowires/n-ZnO thin film homojunction light-emitting diode

The highly arrayed arsenic doped p-ZnO nanowires/n-ZnO thin film homojunction light-emitting diode was fabricated on semi-insulated Si substrate. The homojunction was consisted of high-quality n-ZnO thin film grown by metal–organic chemical vapor deposition technology following arsenic doped ZnO nanowires grown by chemical vapor deposition. The device shows good rectification characteristic with a turn-on voltage of ~4.8 V and reverse breakdown voltage of ~18 V. Moreover, two distinct electroluminescence bands centered at 2.35 and 3.18 eV are detected from this device under forward bias at room temperature.     

Electrohydrodynamic atomization approach to graphene/zinc oxide film fabrication for application in electronic devices

Graphene-based composites represent a new class of materials with potential for many applications. Metal, semiconductor, or any polymer properties can be tuned by attaching it to graphene. Here, a new route for fabrication of graphene based composites thin films has been explored. Graphene flakes (<4 layers) and a well-known semiconductor zinc oxide (ZnO) (<50 nm particle size) have been dispersed in N-methylpyrrolidone and ethanol, respectively. Thin film of graphene flakes is deposited and decorated with ZnO nanoparticles to fabricate graphene/ZnO composite thin film on silicon substrate by electro hydrodynamic atomization technique. Graphene/ZnO composite thin film has been characterized morphologically, structurally and chemically. To investigate electronic behavior of the composite thin film, it is deployed as cathode in a diode device i.e. indium tin oxide/poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate)/polydioctylfluorene-benzothiadiazole/(graphene/ZnO). The J–V analysis of diode device has shown that at voltage of 1 V, the current density in organic structure is at low value of 4.69 × 10^?3 A/cm² and when voltage applied voltage is further increased; the device current density has increased by the order of 200 that is 1.034 A/cm² at voltage of 12 V.     

Carrier transport mechanisms and photovoltaic characteristics of Au/toluidine blue/n-Si/Al heterojunction solar cell

Toluidine blue (TB)/n-silicon heterojunction solar cell was fabricated by depositing TB film on n-silicon wafer using thermal deposition technique. X-ray diffraction patterns of the TB film show presence of crystals with size 30 nm dispersed in amorphous matrix. The current–voltage–temperature performance of Au/TB/n-Si/Al device was studied in dark and under illumination conditions. The device showed diode behavior. The diode parameters such as ideality factor, barrier height, series and shunt resistance were determined using a conventional I–V–T characteristics. The analysis of the diode characteristics in forward bias direction confirmed that the transport mechanisms of the Au/TB/n-Si/Al solar cell at applied potential?<?0.1 V is thermionic emission and at high electric field?>?0.1 V is Ohmic conduction. The operating conduction mechanisms in reverse bias direction are Pool–Frenkel effect followed by Schootky field lowering mechanism. The small value of activation energy in reverse bias direction indicates that the conduction process is expected to be by tunneling of electrons between nearest-neighbor sites and it is temperature independent. The photo conduction characteristics of the diode suggests its application as a solar cell.     

Development of inverted OLED with top ITO anode by plasma damage-free sputtering

A newly-developed damage-free sputtering system, Hyper-thermal Neutral Beam (HNB) sputtering, was evaluated for the production of organic light emitting diode (OLED) devices. An indium tin oxide (ITO) pixel layer as an anode electrode was deposited directly on the Hole Transport Layer (HIL) of an Inverted Bottom emission OLED (IBOLED) structure. Two types of IBOLED devices were fabricated to assess the level of process damage by the HNB sputtering deposition process. The characteristics of the leakage current at reverse bias and the UV exposure test confirmed that HNB ITO sputtering does not damage the underlying organic layers in the IBOLED device with the top ITO anode deposited by HNB sputtering, which is in comparison with a normal IBOLED with a top Au anode deposited by conventional thermal evaporation. However, the IBOLED device using HNB sputtering process showed some degradation of the turn-on characteristics and current efficiency. This degradation was not induced by damage from the HNB sputtering process but was generated by the permeation of Fe impurities from the stainless steel chamber and the low conductivity of HNB sputtered ITO thin film.     

Single wire radial junction photovoltaic devices fabricated using aluminum catalyzed silicon nanowires

Single nanowire radial junction solar cell devices were fabricated using Si nanowires synthesized by Al-catalyzed vapor-liquid-solid growth of the p(+) core (Al auto-doping) and thin film deposition of the n(+)-shell at temperatures below 650 °C. Short circuit current densities of 11.7 mA cm(-2) were measured under 1-sun AM1.5G illumination, showing enhanced optical absorption. The power conversion efficiencies were limited to < 1% by the low open circuit voltage and fill factor of the devices, which was attributed to junction shunt leakage promoted by the high p(+)/n(+) doping. This demonstration of a radial junction device represents an important advance in the use of Al-catalyzed Si nanowire growth for low cost photovoltaics.     

Growth and temperature dependent characterization of pulsed laser deposited Ag/n-ZnO/p-Si/Al heterojunction

The Ag/n-ZnO/p-Si(100)/Al heterojunction diodes were fabricated by pulsed laser deposition of zinc oxide (ZnO) thin films on p-type silicon. The X-ray diffraction analysis shows the formation of ZnO thin film with hexagonal structure having strong (002) plane as preferred orientation. The energy band gap of ZnO films simultaneously deposited on quartz substrate was calculated from the measured UV–Visible transmittance spectra. High purity vacuum evaporated silver and aluminum thin films were used to make contacts to the n-ZnO and p-silicon, respectively. The current–voltage and capacitance–voltage characteristics of Ag/n-ZnO/p-Si(100)/Al heterostructures were measured over the temperature range of 80–300 K. The Schottky barrier height and ideality factor were determined by fitting of the measured current–voltage data into thermionic emission diffusion equation. It is observed that the barrier height decreases and the ideality factor increases with decrease of temperature and the activation energy plot exhibit non-linear behavior. This decrease in barrier height and increase in ideality factor at low temperature are attributed to the occurrence Gaussian distribution of barrier heights. The capacitance–voltage characteristics of Ag/n-ZnO/p-Si(100)/Al heterojunction diode were also studied over the wide temperature range. Capacitance–voltage data are used to estimate the barrier height and impurity concentration in n-type ZnO.     

In-line deposition of organic light-emitting devices for large area applications

Light-emitting diodes based on organic semiconductors show promising features for display and lighting applications. A vertical in-line deposition technique for organic light-emitting diode (OLED) manufacturing was developed.OLED devices with electrically doped transport layers show low operating voltage, high efficiency and long lifetime. The preparation of p-i-n type devices was performed with the in-line fabrication tool resulting in highly efficient OLED with low operating voltage. The lowest operating voltage was achieved for green diodes with 2.9 V for 100 cd/m². This demonstrates that the p-i-n device concept can be applied under manufacturing conditions. In-line manufactured highly efficient red, green and blue OLED are presented.One important aspect for fabrication cost is the used ground contact, which is commonly made by indium tin oxide (ITO). For low cost fabrication an alternative for ITO has to be used. In this work, ITO was replaced by aluminium doped zinc oxide (ZAO). The results are comparable to OLEDs using ITO as transparent conductive oxide.     

Diode behavior of electrophoretically deposited polyaniline on TiO2 nanoparticulate thin film electrode

An inorganic/organic hetrostructure diode was constructed by the electrophoretic deposition of the p-type polyaniline (PANI) on an n-type titanium oxide (TiO2) nanoparticulate thin film. The bonding and internalization of PANI to TiO2 nanoparticulate thin film were confirmed by the morphological, structural and optical studies of electrophoretically deposited PANI/TIO2 nanoparticulate thin film. The increased size of TiO2 nanoparticles indicated the well penetration of PANI molecules into the pores of mesoporous TiO2 nanoparticulate thin film. The XPS studies of PANI/TiO2 heterostructure exhibited the surface bonding and interaction between PANI molecules and TiO2 nanoparticles. The current-voltage (I-V) characterization of PANI/TiO2 heterostructure was carried out in the forward and the reverse bias at the applied voltage ranges from -1 V to +1 V with a scan rate of 2 mV/s. The constructed Pt/PANI/TiO2 heterostructure device established diodic behavior with non-linear nature of I-V curves.     

Fabrication of ZnO thin film diode using laser annealing

A p-n homojunction was obtained by Nd:YAG laser annealing of Zn₃P₂/n-ZnO thin film. The deposition process of ZnO and Zn₃P₂ thin film was performed by pulsed laser deposition (PLD). A p-n junction was formed by Nd:YAG laser annealing of Zn₃P₂/n-ZnO thin film and showed typical I-V characteristics of a diode. Laser annealing could be a useful technique for the fabrication of an ultraviolet light-emitting diode or an ultraviolet laser diode.     

ZnS:Mn沉积温度对薄膜电致发光器件亮度的影响

采用丝网印刷技术,在Al2O3陶瓷基板上印刷、高温烧结内电极及绝缘层,制备出陶瓷厚膜基板,进而制备了新型厚膜电致发光显示器(TDEL).整个器件结构为陶瓷基板/内电极/厚膜绝缘层/发光层/薄膜绝缘层/ITO透明电极.研究不同基板沉积温度对发光层性能的影响,并对器件的亮度-电压、亮度-频率进行测量.结果显示较高的ZnS:Mn沉积温度明显提高了无机发光器件的发光亮度.其原因主要是由于高的沉积温度提高了ZnS:Mn的成膜质量,提高膜层微晶尺寸大小,从而发光亮度提高.但是我们发现温度继续提高的同时,器件发光亮度趋于饱和,分析原因是由于掺杂Mn浓度过高,影响了发光效果.     

Electrospray deposition of silver nanowire films for transparent electrodes

Silver nanowire (AgNWs) films were fabricated as transparent electrodes by electrostatic spray deposition (ESD) at atmospheric pressure and room temperature. The effects of solution concentration, spray flow rate, applied high voltage, and annealing temperature were characterized to obtain uniform films. AgNWs thin film was produced with ca. 20 Ω/[square] sheet resistance and 83% transparency in the visible range. Morphologies, optical and electrical properties, and stabilities of the films were investigated in this work. A maximum ratio of DC to optical conductivity of 288 was achieved in a 120 nm thick AgNW thin film. Chemical stability was evaluated in various solvents and we found that solvents had little effect on conductivity.     

A comparative study of two advanced spraying techniques for the deposition of biologically active enzyme coatings onto bone-substituting implants

Surface modification of implant materials with biomolecule coatings is of high importance to improve implant fixation in bone tissue. In the current study, we present two techniques for the deposition of biologically active enzyme coatings onto implant materials. The well-established thin film ElectroSpray Deposition (ESD) technique was compared with the SAW-ED technique that combines high-frequency Surface Acoustic Wave atomization with Electrostatic Deposition. By immobilizing the enzyme alkaline phosphatase (ALP) onto implant surfaces, the influence of both SAW-ED and ESD deposition parameters on ALP deposition efficiency and ALP biological activity was investigated. ALP coatings with preserved enzyme activity were deposited by means of both the SAW-ED and ESD technique. The advantages of SAW-ED over ESD include the possibility to spray highly conductive protein solutions, and the 60-times faster deposition rate. Furthermore, significantly higher deposition efficiencies were observed for the SAW-ED technique compared to ESD. Generally, it was shown that protein inactivation is highly dependent on both droplet dehydration and the applied electrical field strength. The current study shows that SAW-ED is a versatile and flexible technique for the fabrication of functionally active biomolecule coatings.     

Fabrication of flexible polymer dispersed liquid crystal films using conducting polymer thin films as the driving electrodes

Conducting polymers exhibit good mechanical and interfacial compatibility with plastic substrates. We prepared an optimized coating formulation based on poly(3,4-ethylenedioxythiophene) (PEDOT) and 3-(trimethoxysilyl)propyl acrylate and fabricated a transparent electrode on poly(ethylene terephthalate) (PET) substrate. The surface resistances and transmittance of the prepared thin films were 500-600 Ω/□ and 87% at 500 nm, respectively. To evaluate the performance of the conducting polymer electrode, we fabricated a five-layer flexible polymer-dispersed liquid crystal (PDLC) device as a PET-PEDOT-PDLC-PEDOT-PET flexible film. The prepared PDLC device exhibited a low driving voltage (15 VAC), high contrast ratio (60:1), and high transmittance in the ON state (60%), characteristics that are comparable with those of conventional PDLC film based on indium tin oxide electrodes. The fabrication of conducting polymer thin films as the driving electrodes in this study showed that such films can be used as a substitute for an indium tin oxide electrode, which further enhances the flexibility of PDLC film.     

Organometallic hybrid perovskites: structural,optical characteristic and application in Schottky diode

CH₃NH₃PbI₃ thin film was deposited by a dual-source evaporation system under high vacuum (~10^?4 Pa). The crystallographic phase of the thin film was determined by X-ray diffraction and its perovskite structure was confirmed. The crystal of annealed perovskite film was extremely smooth and significantly larger than that of as-deposited. The optical property of the thin film was investigated in the spectral range 300–1800 nm. By analyzing the absorption coefficient (α), the optical band gap (1.58 eV) and Urbach energy (0.082 eV) were revealed. The Al/CH₃NH₃PbI₃/ITO Schottky diode was fabricated in order to explore the potential applications of CH₃NH₃PbI₃. The basic device parameters, barrier height and ideality factor were determined by the current–voltage (I–V) measurement. It can be found that the charge transport was governed by space-charge-limited current mechanism by studying the forward bias characteristic.     

Preparation of polycrystalline CdZnTe thick film Schottky diode for ultraviolet detectors

High resistivity (3 × 10⁹ Ω cm) polycrystalline CdZnTe thick films with thickness of 25 μm–150 μm were grown on SnO₂: F-coated glass substrates by close-spaced sublimation method. The properties of polycrystalline CdZnTe films were studied by XRD, SEM and EDS, respectively. A CdZnTe film Schottky diode detector was also fabricated and investigated using current–voltage and capacitance–voltage methods. The photo-current density of the device was about 1508.69 nA/mm² under light illumination (λ = 260 nm), at an applied negative voltage of 15 V. The results showed that polycrystalline CdZnTe thick film was suitable for application in ultraviolet detectors.     

Fabrication and characterization of transparent MEH-PPV/n-GaN (0 0 0 1) heterojunction devices

n-GaN/MEH-PPV thin film heterojunction diode was fabricated by depositing MEH-PPV thin film using spin-coating process on n-GaN (0 0 0 1). The junction properties were evaluated by measuring I-V characteristics. I-V characteristics exhibited well defined rectifying behavior with a barrier height of 0.89 eV and ideality factor of 1.7. The optical band gap of the MEH-PPV film using optical absorption method was found to be 2.2 eV and the fundamental absorption edge in the film is formed by the direct allowed transitions. At higher electric fields, the conductivity mechanism of the film shows a trap charge limited current mechanism. The obtained results indicate that the electronic parameters of the heterojunction diode are affected by properties of MEH-PPV organic film.     

Controlled deposition of new organic ultrathin film as a gate dielectric layer for advanced flexible capacitor devices

This letter describes a new organic (1-bromoadamantane) ultrathin film as gate dielectric, which was successfully deposited by sol–gel spin-coating process on a flexible polyimide substrate at room temperature. The metal–insulator-metal (MIM) device with organic (1-bromoadamantane) ultrathin (10 nm) film as gate dielectric layer operated at gate voltage of 5.0 V, showing a low leakage current density (5.63 × 10^?10 A cm^?2 at 5 V) and good capacitance (2.01 fF μm^?2 at 1 MHz). The chemical structure of the 1-bromoadamantane layer was investigated by Fourier transform infrared spectrometer. The excellent leakage current density and better capacitance, probably due to the presence of polar, non-polar, low-polar groups, and bromine atoms in ultrathin film. Practical properties of the film in MIM capacitor such as dielectric constant as well as bending result of leakage current density and breakdown voltage have been better related to such fundamental adhesion nature over flexible substrate. This permits estimation of the properties of new dielectric in thin film form and short lists of the best materials for low loss and good capacitance flexible capacitors could be drawn up in future.     

Improvement of white organic light emitting diode performances by an annealing process

White organic light emitting diode (OLED) devices with the structure ITO/PHF:rubrene/Al, in which PHF (poly(9,9-di-n-hexylfluorenyl-2,7-diyl)) is used as blue light emitting host and rubrene (5,6,11,12-tetraphenylnapthacene) as an orange dye dopant, have been fabricated. Indium tin oxide (ITO) coated-glass and aluminium were used as anode and cathode, respectively. The devices were fabricated with various rubrene-dopant to obtain a white light emission. The OLED device that composed of several concentrations of rubrene-doped PHF film was prepared in this study. It was found that the concentration of rubrene in the PHF-rubrene thin film matrix plays a key role in producing the white color emission. In a typical result, the device composed of 0.06 wt.% rubrene-dopant produced the white light emission with the Commission Internationale de L'Eclairage (CIE) coordinate of (0.30,0.33). The turn-on voltage and the brightness were found to be as low as 14.0 V and as high as 6540 cd/m², respectively. The annealing technique at relatively low temperature (50 °C, 100 °C, and 150 °C) was then used to optimize the performance of the device. In a typical result, the turn-on voltage of the device could be successfully reduced and the brightness could be increased using the annealing technique. At an optimum condition, for example, annealed at 150 °C, the turn-on voltage as low as 8.0 V and the brightness as high as 9040 cd/m² were obtained. The mechanism for the improvement of the device performance upon annealing will be discussed.     

Fabrication of optical gratings by shrinkage of a rubber material

Ordered wavy surface structures generated by deposition of a metal thin film on a pre-stretched PDMS plate were fabricated and its potential application for optical gratings was proposed. The orientation of the generated structures was always perpendicular to the pre-stretched direction and the pitch of the structure could be adjusted ranging from 4.5 μm to 6.8 μm by controlling the strength of the pre-stretched strain and the thickness of the surface metal film. Based on these periodic structures, various optical gratings were demonstrated. With a slight modification of the fabrication scheme, gratings with different orientations can be fabricated on both sides of the PDMS plate, the double-sided gratings, could be fabricated. It is believed the current method has the potential for the fabrication of a large-scale diffractive grating at lower costs.