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Vasilopoulou M.,Institute of Microelectronics, Greece | Kennou S.,University of Patras | Ladas S.,University of Patras | Georga S.N.,University of Patras | And 6 more authors.
Organic Electronics: physics, materials, applications | Year: 2013

In this work, we demonstrate efficient polyfluorene-based light emitting diodes on which conformal, thin ZrO2 layers, formed by atomic layer deposition at a relatively low temperature (175 °C), in order to avoid introducing any damage in the organic under layer, efficiently inject electrons from their high lying conduction band to the polymer's LUMO. An optimal thickness of 2 nm for ZrO2 results in a threefold improvement in luminous current efficiency compared to the reference device. The relationship between the thickness of the ZrO2 layer and the device operational characteristics is further investigated and the possible reasons for the improved device performance are discussed based on the experimental results obtained by a combination of photoemission spectroscopy and electrical/optical measurements. © 2012 Elsevier B.V. All rights reserved.


Vasilopoulou M.,Institute of Microelectronics, Greece | Palilis L.C.,Institute of Microelectronics, Greece | Georgiadou D.G.,Institute of Microelectronics, Greece | Argitis P.,Institute of Microelectronics, Greece | And 10 more authors.
Thin Solid Films | Year: 2011

Tungsten oxide (WO3) films with thicknesses ranging from 30 to 100 nm were grown by Hot Filament Vapor Deposition (HFVD). Films were studied by X-Ray Photoemission Spectroscopy (XPS) and were found to be stoichiometric. The surface morphology of the films was characterized by Atomic Force Microscopy (AFM). Samples had a granular form with grains in the order of 100 nm. The surface roughness was found to increase with film thickness. HFVD WO3 films were used as conducting interfacial layers in advanced hybrid organic-inorganic optoelectronic devices. Hybrid-Organic Light Emitting Diodes (Hy-OLEDs) and Organic Photovoltaics (Hy-OPVs) were fabricated with these films as anode and/or as cathode interfacial conducting layers. The Hy-OLEDs showed significantly higher current density and a lower turn-on voltage when a thin WO3 layer was inserted at the anode/polymer interface, while when inserted at the cathode/polymer interface the device performance was found to deteriorate. The improvement was attributed to a more efficient hole injection and transport from the Fermi level of the anode to the Highest Occupied Molecular Orbital (HOMO) of a yellow emitting copolymer (YEP). On the other hand, the insertion of a thin WO3 layer at the cathode/polymer interface of Hy-OPV devices based on a polythiophene-fullerene bulk-heterojunction blend photoactive layer resulted in an increase of the produced photogenerated current, more likely due to improved electron extraction at the Al cathode. © 2010 Elsevier B.V. All rights reserved.


Vasilopoulou M.,Institute of Microelectronics, Greece | Douvas A.M.,Institute of Microelectronics, Greece | Georgiadou D.G.,Institute of Microelectronics, Greece | Palilis L.C.,University of Patras | And 8 more authors.
Journal of the American Chemical Society | Year: 2012

Molybdenum oxide is used as a low-resistance anode interfacial layer in applications such as organic light emitting diodes and organic photovoltaics. However, little is known about the correlation between its stoichiometry and electronic properties, such as work function and occupied gap states. In addition, despite the fact that the knowledge of the exact oxide stoichiometry is of paramount importance, few studies have appeared in the literature discussing how this stoichiometry can be controlled to permit the desirable modification of the oxide's electronic structure. This work aims to investigate the beneficial role of hydrogenation (the incorporation of hydrogen within the oxide lattice) versus oxygen vacancy formation in tuning the electronic structure of molybdenum oxides while maintaining their high work function. A large improvement in the operational characteristics of both polymer light emitting devices and bulk heterojunction solar cells incorporating hydrogenated Mo oxides as hole injection/extraction layers was achieved as a result of favorable energy level alignment at the metal oxide/organic interface and enhanced charge transport through the formation of a large density of gap states near the Fermi level. © 2012 American Chemical Society.


Vasilopoulou M.,Institute of Microelectronics, Greece | Palilis L.C.,Institute of Microelectronics, Greece | Palilis L.C.,University of Patras | Georgiadou D.G.,Institute of Microelectronics, Greece | And 10 more authors.
Applied Physics Letters | Year: 2011

We report a significant improvement in the performance of single layer polymer light-emitting diodes (PLEDs), based on the green emitting copolymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2, 1′,3}-thiadiazole)] , upon inserting a very thin layer of partially reduced molybdenum oxide (MoOx, where x=2.7) at the polymer/Al cathode interface. Both fully oxidized (x=3) and partially reduced (x=2.7) thin molybdenum oxide layers were investigated as electron injection layers and their influence on PLED device performance was examined. Improved current density, luminance, and efficiency was achieved only in the case of devices with a thin partially reduced MoO2.7 film as electron injection layer, as a result of improved electron injection and more facile transfer at the modified polymer/Al interface. © 2011 American Institute of Physics.


Kostis I.,Institute of Microelectronics, Greece | Kostis I.,Technological and Educational Institute of Pireaus | Kostis I.,University of Aegean | Vourdas N.,Institute of Microelectronics, Greece | And 6 more authors.
Thin Solid Films | Year: 2013

Tungsten oxide films with various stoichiometries, undoped and hydrogen doped were deposited by heating a W wire at 660 C and at a base pressure of 13 Pa set by various gasses (O2, N2, H2) or gas mixtures (N2-O2 10% in O2, forming gas, FG) and pulsed injection of O2 or H2. Using this method and dependent on the deposition conditions four classes of hot-wire (hw) tungsten oxide films were synthesized: i) stoichiometric (hwWO3), ii) oxygen deficient (sub-stoichiometric, hwWOx with x < 3), iii) stoichiometric and hydrogen doped (hwWO3:H), and iv) sub-stoichiometric and hydrogen doped (hwWOx:H). Due to the pulsed injection of O2 the W wire re-oxidizes during deposition thus creating WO3 vapors continuously, so films deposited by this method do not suffer by thickness limitations and, moreover, exhibit high porosity. The optical properties of these classes of films, studied with spectroscopic ellipsometry (SE) measurements, differ substantially between them indicating corresponding differences in their electronic structures. So, hwWO3 films were semiconducting exhibiting a band gap near 3 eV. Sub-stoichiometric hwWOx deposited with up to 2 injections of O2 were semi-metallic preserving some features of the electronic structure of the pure metal, while further injection of O2 leads to stoichiometric films. Fourier transform infrared spectroscopy and SE measurements indicated that hwWOx films contain H bonded with the O ions while in hwWO 3:H and hwWOx:H the H is incorporated in films by direct bonding with the W ions. © 2013 Elsevier B.V.


Papadimitropoulos G.,Greek National Center For Scientific Research | Kostis I.,Greek National Center For Scientific Research | Kostis I.,Technological and Educational Institute of Pireaus | Trantalidis S.,Greek National Center For Scientific Research | And 3 more authors.
Physica Status Solidi (C) Current Topics in Solid State Physics | Year: 2015

Vanadium oxide films were chemically vapor deposited (CVD) on oxidized Si substrates covered with CVD tungsten (W) thin films and on glass substrates covered with indium tin oxide (ITO) films, using vanadium(V) oxy-tri-isopropoxide (C9H21O4V) vapors. X-ray diffraction (XRD) measurements showed that the deposited films were composed of a mixture of vanadium oxides; the composition was determined mainly by the deposition temperature and less by the precursor temperature. At temperatures up to 450 °C the films were mostly composed by monoclinic VO2. Other peaks corresponding to various vanadium oxides were also observed. X-ray microanalysis confirmed the composition of the films. The surface morphology was studied with atomic force microscopy (AFM) and scanning electron microscopy (SEM). These measurements revealed that the morphology strongly depends on the used substrate and the deposition conditions. The well-known metal-insulator transition was observed near 75 °C for films mostly composed by monoclinic VO2. Films deposited at 450 °C exhibited two transitions one near 50 °C and the other near 60 °C possibly related to the presence of other vanadium phases or of important stresses in them. Finally, the vanadium oxide thin films exhibited significant sensory capabilities decreasing their resistance in the presence of hydrogen gas with response times in the order of a few seconds and working temperature at 40 °C. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Kostis I.,Technological and Educational Institute of Pireaus | Vourdas N.,Advanced Materials and Processes | Papadimitropoulos G.,Advanced Materials and Processes | Douvas A.,Advanced Materials and Processes | And 3 more authors.
Journal of Physical Chemistry C | Year: 2013

The electronic structure of disordered-amorphous molybdenum oxide films was investigated near the band gap by optical absorption and photoluminescence (PL) measurements. It was found that, in nearly stoichiometric films, the 3.2 eV wide gap is free of states and a PL band at 3.7 eV was attributed to electronic transitions between the Mo 4d bonding-antibonding orbitals. In sub-stoichiometric films, electronic states appear within the gap giving rise to additional PL emission within the range 3-3.25 eV. The band gap remains at 3.2 eV, and intermediate bands (IB) are formed within the gap for oxygen-deficient hydrogenated samples. A substantial increase of the PL intensity within the range 2.75-3.75 eV was observed, attributed to the IBs and the increase of the density of Mo 4d antibonding states within the conduction band (CB). The band gap decreases to 2.7 eV in sub-stoichiometric and hydrogenated samples, while the rest of the elctronic structure remains unchanged as for hydrogenated samples also giving enhanced PL intensity within the same range. © 2013 American Chemical Society.


Kostis I.,Advanced Materials and Processes | Kostis I.,Technological and Educational Institute of Pireaus | Vasilopoulou M.,Advanced Materials and Processes | Papadimitropoulos G.,Advanced Materials and Processes | And 3 more authors.
Surface and Coatings Technology | Year: 2013

A hot-wire atomic layer deposition system is used for the synthesis of tungsten oxide films with various stoichiometries, undoped and hydrogen doped. For this synthesis, no precursors are used and only a W wire is heated at 660°C at a base pressure of 0,1Torr set by the flow of various gases (O2, N2, H2) or gas mixtures (N2-O2 10% in H2, forming gas, FG) and the pulsed injection of O2 or H2. Four classes of hot-wire tungsten oxide films were synthesized: i) stoichiometric (hwWO3), ii) oxygen deficient (sub-stoichiometric, hwWOx with x<3), iii) stoichiometric and hydrogen doped (hwWO3:H) and iv) sub-stoichiometric and hydrogen doped (hwWOx:H). During deposition, due to the pulsed injection of O2, the W wire re-oxidizes creating continuously WO3 vapors and so films deposited do not suffer by thickness limitations. Moreover, they are highly porous. Each one of the four classes of the deposited tungsten oxides exhibits its own optical properties, which indicates that each class has its own electronic structure. So, hwWO3 films were semiconducting, exhibiting a band gap near 3eV. Sub-stoichiometric hwWOx deposited with up to 2 injections of O2 were semi-metalic, exhibiting some features of the electronic structure of the pure metal, while further injection of O2 leads to stoichiometric films. The injection of H2 during deposition leads to the formation of atomic H, which dopes the films. Depositions carried out with base pressure set by O2 or N2 and with injection of H2, lead to the formation of hwWO3:H films that exhibit optical properties similar to Na lightly-doped WO3 films. In base pressure set by H2 or FG and with H2 injection, substoichiometric, hydrogen-doped films were obtained, exhibiting optical properties corresponding to a metal substantially different than W. Fourier transform infrared spectroscopy and spectroscopic ellipsometry measurements indicated that the hwWOx films grown in the presence of hydrogen in the deposition ambience contain H bonded with the O ions. When these films are doped with hydrogen, protons are bonded not only with the oxygen but also with the tungsten ions. © 2013 Elsevier B.V.


Papadimitropoulos G.,Institute of Microelectronics, Greece | Kostis I.,Technological and Educational Institute of Pireaus | Triantafyllopoulou R.,Institute of Microelectronics, Greece | Tsouti V.,Institute of Microelectronics, Greece | And 2 more authors.
Microelectronic Engineering | Year: 2012

In this work, the gas sensing properties of porous hot-wire WO3 (hwWO3) thin films have been investigated. These films were deposited on oxidized silicon substrates by heating a tungsten filament in a vacuum chamber. The resistivity variations of these configurations caused by changes in their environment were monitored. Reversible changes of resistivity, of the order of several kΩ cm, were observed in hwWO3 films caused by the presence or upon removal of H2 and without superficial doping of samples with noble metals. The magnitude of these changes, related to the sensitivity, was found to depend on hydrogen concentration and temperature of measurement. The time needed (response time) for the resistivity to drop after H2 exposure was found comparable to that needed to recover to its initial value after H2 removal. Response times of the order of a few seconds were measured on hwWO3 films much shorter than those measured on WO3 samples deposited by chemical vapor deposition. © 2011 Elsevier B.V. All rights reserved.


Vourdas N.,Institute of Microelectronics, Greece | Papadimitropoulos G.,Institute of Microelectronics, Greece | Kostis I.,Technological and Educational Institute of Pireaus | Vasilopoulou M.,Institute of Microelectronics, Greece | Davazoglou D.,Institute of Microelectronics, Greece
Thin Solid Films | Year: 2012

Substoichiometric tungsten oxide films were deposited on Si substrates by heating metallic filaments at temperatures of 920, 1020 and 1070 K at a total pressure of 133 Pa and H 2 partial pressure of 13.3 Pa at or near room temperature. Due to their substoichiometry and deposition method samples were named hot-wire WO x (hwWO x) films (x < 3). No optical gap was identified by optical (spectroscopic ellipsometry) measurements made on hwWO x films and resistivity was found to increase with temperature. This metallic character of hwWO x films was attributed to substoichiometry, which causes the creation of electronic states within the band gap and shifts the Fermi level in the conduction band. The growth of hwWO x films occurred from substoichiometric WO x vapors produced in two steps: a) by the H 2 reduction of the superficial (native) oxide of the heated tungsten filament to WO x and b) evaporation of the last. Vapors were composed of WO x particles with dimensions of 5-6 nm or clusters of such particles and condensing on the cold substrate form hwWO x films. The substoichiometry of hwWO x samples was related to the presence of many unsaturated bonds in them, which re-constructed easily upon thermal annealing above 420 K or even after exposure to the electron beam of the transmission electron microscope leading to the formation of crystallites with dimensions of 5 nm within samples. © 2011 Elsevier B.V. All rights reserved.

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