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18 Apr 2005

Volume 86, Issue 16, Articles (16xxxx)

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Numerical simulations of layered and blended organic photovoltaic cells

J. O. Haerter, S. V. Chasteen, S. A. Carter, and J. C. Scott

Appl. Phys. Lett. 86, 164101 (2005); http://dx.doi.org/10.1063/1.1901812 (3 pages) | Cited 11 times

Online Publication Date: 11 April 2005

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We present results obtained from numerical simulations of organic photovaltaic cells as the donor–acceptor morphology evolves from sharply defined layers, to partial blends and finally homogeneous blends. As the mixing percentage increases, the exciton dissociation increases and the diffusion counter-current decreases, resulting in substantially greater short circuit currents but reduced open circuit voltages. Blended structures are more sensitive to mobility than layers due to recombination throughout the bulk. Our model indicates that solar power efficiencies greater than 10% can be achieved when the zero-field charge mobilities approach 10−3 cm2/Vs for partially blended structures.
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84.60.Jt Photoelectric conversion
71.35.Ee Electron-hole drops and electron-hole plasma
73.50.Dn Low-field transport and mobility; piezoresistance
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
85.60.Bt Optoelectronic device characterization, design, and modeling
66.30.-h Diffusion in solids

Manifestation of the connection between dielectric breakdown strength and contact angle saturation in electrowetting

A. G. Papathanasiou and A. G. Boudouvis

Appl. Phys. Lett. 86, 164102 (2005); http://dx.doi.org/10.1063/1.1905809 (3 pages) | Cited 18 times

Online Publication Date: 12 April 2005

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Limiting phenomena on the electrostatically assisted wetting of dielectric solids by conducting liquids are illuminated by means of computer-aided analysis. The importance of the electrostatic edge effects and their influence on the dielectric properties of the solid is raised to demonstrate that contact angle saturation sets in when the electric field strength locally exceeds the breakdown strength of the dielectric solid where the liquid sits. The proposed argument along with the computed predictions is tested against published experimental measurements showing remarkable agreement.
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77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
77.22.Jp Dielectric breakdown and space-charge effects
68.08.Bc Wetting
68.03.Cd Surface tension and related phenomena

GaN-based anion selective sensor: Probing the origin of the induced electrochemical potential

Nikos A. Chaniotakis, Yiannis Alifragis, Alexandros Georgakilas, and Giorgos Konstantinidis

Appl. Phys. Lett. 86, 164103 (2005); http://dx.doi.org/10.1063/1.1896449 (3 pages) | Cited 13 times

Online Publication Date: 12 April 2005

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The gallium nitride (GaN) semiconductor has been used as the sensing element in a chemical sensor for the measurement of charged species in solution. The sensor shows remarkable selectivity for anions, such as sulphate (SO42−) and hydroxide (OH−). It is shown that the GaN surface interacts selectively with Lewis bases as shown by impedance spectra. In addition, both the impedance spectra and the surface induced potential of the sensor element correlate very well with the activity of both the negatively charged hydroxide and the sulphate anions used. These results indicate that there is a direct interaction of the electron deficient gallium in the GaN surface with the Lewis base anionic ligands. A band model for the investigated GaN/KOH-solution system has been deduced.
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82.47.Rs Electrochemical sensors
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
85.30.De Semiconductor-device characterization, design, and modeling
82.45.Rr Electroanalytical chemistry
82.80.Fk Electrochemical methods

Room-temperature phosphonium ionic liquids for supercapacitor application

Elzbieta Frackowiak, Grzegorz Lota, and Juliusz Pernak

Appl. Phys. Lett. 86, 164104 (2005); http://dx.doi.org/10.1063/1.1906320 (3 pages) | Cited 31 times

Online Publication Date: 13 April 2005

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Ionic liquids (ILs) have been used as electrolytes for supercapacitors. Two phosphonium salts such as trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide (IL1) and trihexyl(tetradecyl) phosphonium dicyanamide (IL2) have been selected for this target. To decrease the viscosity of ILs, a small amount of acetonitrile (from 5 to 25 wt %) was added. Supercapacitor based on activated carbon (AC) as electrodes and IL1 with 25 wt % of acetonitrile supplied capacitance values of 100 F/g at a high operating voltage of 3.4 V. Such a supercapacitor reached a high energy of ∼ 40 Wh/kg and a good cyclability.
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82.47.Uv Electrochemical capacitors; supercapacitors
66.10.Ed Ionic conduction
66.20.-d Viscosity of liquids; diffusive momentum transport

Magnetic properties of MnN: Influence of strain and crystal structure

M. Marques, L. K. Teles, L. M. R. Scolfaro, J. Furthmüller, F. Bechstedt, and L. G. Ferreira

Appl. Phys. Lett. 86, 164105 (2005); http://dx.doi.org/10.1063/1.1905787 (3 pages) | Cited 18 times

Online Publication Date: 15 April 2005

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For manganese mononitride (MnN), the total energy versus lattice constant is obtained using the spin density functional theory. Instead of the tetragonally distorted NaCl structure, we study the zinc blende and wurtzite structures in which AlN, GaN, and InN crystallize. The ground state with nonmagnetic, antiferromagnetic (AFM), or ferromagnetic (FM) arrangement of spins depends on the polymorph of MnN and on the lattice constant. At equilibrium lattice constants, in zinc blende it is AFM in [100] direction, and in wurtzite it is FM. The zinc blende polytype of MnN under hydrostatic pressure at the InN lattice constant presents FM ground state. For the wurtzite polytype at the GaN and AlN lattice constants, the AFM is the ground state, but goes back to a FM ground state for the InN lattice constants. For both structures, the system presents a half-metallic state at InN lattice constants (with a total magnetic moment of 4 μB per Mn atom) instead of the metallic state obtained for smaller lattice constants. Results indicate that the FM or the AFM state of Ga1−xMnxN and In1−xMnxN may be related to, relaxed, or strained, MnN incorporations or Mn-rich composition fluctuations.
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71.15.Nc Total energy and cohesive energy calculations
75.50.−y
85.75.−d

Eliminating the hysteresis effect for reactive sputtering processes

T. Nyberg, S. Berg, U. Helmersson, and K. Hartig

Appl. Phys. Lett. 86, 164106 (2005); http://dx.doi.org/10.1063/1.1906333 (3 pages) | Cited 12 times

Online Publication Date: 15 April 2005

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Reactive sputter processes frequently exhibit stability problems. The cause of this is that these processes normally exhibit hysteresis effects in the processing curves. Eliminating the hysteresis would significantly simplify the use of reactive sputtering processes. So far the only known way of eliminating the hysteresis is to increase the pumping speed to unrealistically high values. By an increased understanding of the process we have realized a fully realistic technique to eliminate the hysteresis for reactive sputtering processes. By simply reducing the size of the target sputter erosion zone below a critical value, simulations predicted that hysteresis should be eliminated. This has been experimentally verified for reactive sputtering of Al in an Ar/O2 atmosphere. The fundamental explanation to this behavior as well as the experimental verification are presented.
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79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
81.40.Pq Friction, lubrication, and wear
62.20.Qp Friction, tribology, and hardness
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