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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.

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.

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.

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.

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