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1 Aug 2011

Volume 99, Issue 5, Articles (05xxxx)

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Appl. Phys. Lett. 99, 051112 (2011); http://dx.doi.org/10.1063/1.3617421 (3 pages)

Wei Li, Jun Chen, Gerard Nouet, Liang-yao Chen, and Xunya Jiang
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An organic p-i-n homojunction as ultra violet light emitting diode and visible-blind photodiode in one

S. Hamwi, T. Riedl, and W. Kowalsky

Appl. Phys. Lett. 99, 053301 (2011); http://dx.doi.org/10.1063/1.3617427 (3 pages) | Cited 11 times

Online Publication Date: 1 August 2011

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Organic p-i-n homojunctions that function both as ultra violet light emitting diode (peak wavelengths around 375 nm and 415 nm) and visible-blind photodiode are reported. They are considered as the organic counterpart to what has exclusively been known from inorganic semiconductors, as yet. The diodes are based on the ambipolar material 4,4′-Bis(carbazol-9-yl)-biphenyl (CBP) which is p- and n-type doped adjacent to the electrodes. We study the emission characteristics of the p-i-n homojunction for varied doping concentrations and subsequently focus on its characteristics as photodiode. A superlinear relation of photocurrent and incident light intensity is found and attributed to an intensity-dependent photoconductivity.
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85.60.Dw Photodiodes; phototransistors; photoresistors
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Role of oxygen-bonds in the degradation process of phosphorescent organic light emitting diodes

Ines Rabelo de Moraes, Sebastian Scholz, Björn Lüssem, and Karl Leo

Appl. Phys. Lett. 99, 053302 (2011); http://dx.doi.org/10.1063/1.3617459 (3 pages) | Cited 6 times

Online Publication Date: 1 August 2011

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For improving the lifetime of organic light emitting diodes (OLEDs), it is essential to understand the chemical reaction pathways involved in the degradation process of these devices. Using the laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) technique, an OLED based on the phosphorescent emitter tris(phenylpyridine)iridium (Ir(ppy)3) was investigated. We show that the chemical dissociation reaction of the Ir(ppy)3 molecule and an additional complexation of its fragments with the used hole blocking material 4,7-diphenyl-1,10-phenanthroline is the main intrinsic degradation mechanism. This finding underlines the exciton induced dissociation mechanism of phosphorescent emitters, even at emitters without any metal-oxygen-bonds in the ligand system.
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85.60.Jb Light-emitting devices
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Two-dimensional orientation control of organic semiconducting amorphous films by mechanical brushing

Toshiharu Arai, Kenichi Goushi, Hiroko Nomura, Tomohiko Edura, and Chihaya Adachi

Appl. Phys. Lett. 99, 053303 (2011); http://dx.doi.org/10.1063/1.3608315 (3 pages) | Cited 1 time

Online Publication Date: 2 August 2011

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We demonstrate that horizontally oriented organic semiconducting molecules in amorphous films can be oriented uniaxially by mechanical brushing. The absorption of polarized light in the direction parallel to the brushing direction was higher than the absorption in the perpendicular direction. Thus, simultaneous uniaxial and in-plane molecular orientation in amorphous films was realized by brushing. In addition, we demonstrate a polarized electroluminescence from the films treated by mechanical brushing.
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81.05.Fb Organic semiconductors
81.05.Gc Amorphous semiconductors
78.66.Qn Polymers; organic compounds
78.66.Jg Amorphous semiconductors; glasses
78.60.Fi Electroluminescence
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Effect of light scattering on the transmission spectra of organic nanocrystals

Heisuke Ishino, Selvakumar V. Nair, Kazuya Nakagawa, Takayoshi Kobayashi, and Eiji Tokunaga

Appl. Phys. Lett. 99, 053304 (2011); http://dx.doi.org/10.1063/1.3623434 (3 pages) | Cited 1 time

Online Publication Date: 3 August 2011

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The effect of light scattering on the extinction spectra of organic nanocrystals was investigated by absorption and extinction measurement and by theoretical calculation for typical organic nanocrystals of α-perylene. In the extinction spectra, the exciton band shows a size-dependent redshift and tail toward longer wavelengths due to scattering as the crystal size is increased beyond ∼50 nm. In the absorption spectra, the exciton band blueshifts from that in the bulk crystal for crystal sizes less than ∼200 nm due to the anisotropic exciton structure in perylene. The calculation qualitatively supports these results. The effect of scattering always makes the extinction band redshift from the absorption band.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
71.35.-y Excitons and related phenomena
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.40.-q Absorption and reflection spectra: visible and ultraviolet
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Organic photovoltaic power conversion efficiency improved by AC electric field alignment during fabrication

Cindy X. Zhao, Xizu Wang, Wenjin Zeng, Zhi K. Chen, Beng S. Ong, Kewei Wang, Lulu Deng, and Gu Xu

Appl. Phys. Lett. 99, 053305 (2011); http://dx.doi.org/10.1063/1.3623477 (3 pages) | Cited 2 times

Online Publication Date: 4 August 2011

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Ultra-low frequency AC field was employed to align p/n polymers during organic photovoltaic device fabrication. The resulting devices show 15% increase in power conversion efficiency and four-fold increase in parallel resistance. Supported by the transmission electron microscopy and atomic force microscopy images, the performance enhancement is attributed to the optimized morphology and enlarged p/n interface by AC field, which is more effective than DC, possibly explained by the argument of better mixing via back-and-forth shaking than a single swing.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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Conductance switching in organic ferroelectric field-effect transistors

Kamal Asadi, Paul W. M. Blom, and Dago M. de Leeuw

Appl. Phys. Lett. 99, 053306 (2011); http://dx.doi.org/10.1063/1.3621857 (3 pages) | Cited 3 times

Online Publication Date: 4 August 2011

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Staggered bottom-contact top-gate organic ferroelectric field-effect transistors are fabricated with poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) as ferroelectric gate and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] as semiconductor. Polarization reversal of the ferroelectric gate is monitored by displacement transients in the gate current. By varying both the source and drain biases and by using fully and partially polarized transistors, we show that conductance switching only requires polarization of P(VDF-TrFE) at the source electrode. Polarization at the drain is irrelevant and does not impede charge extraction.
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85.30.Tv Field effect devices
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
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Electrical properties and non-volatile memory effect of the [Fe(HB(pz)3)2] spin crossover complex integrated in a microelectrode device

Tarik Mahfoud, Gábor Molnár, Saioa Cobo, Lionel Salmon, Christophe Thibault, Christophe Vieu, Philippe Demont, and Azzedine Bousseksou

Appl. Phys. Lett. 99, 053307 (2011); http://dx.doi.org/10.1063/1.3616147 (3 pages) | Cited 11 times

Online Publication Date: 4 August 2011

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We report on the deposition of thin films of the [Fe(HB(pz)3)2] (pz = pyrazolyl) molecular spin crossover complex by thermal evaporation. By means of impedance measurements and Raman microspectroscopy, we show that the films maintain the structure and properties of the bulk material. The conductivity of the films decreases by ca. 2 orders of magnitude when the freshly deposited compound goes through a first (irreversible) thermal phase change above ca. 380 K. This property can be exploited as a non-volatile (read-only) memory effect.
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75.70.Ak Magnetic properties of monolayers and thin films
78.30.Jw Organic compounds, polymers
84.30.Sk Pulse and digital circuits
81.15.Dj E-beam and hot filament evaporation deposition
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Orientation of organic molecules in a monolayer vis-à-vis their molecular orbitals and transport gap

Sudipto Chakrabarti, Sukumar Dey, and Amlan J. Pal

Appl. Phys. Lett. 99, 053308 (2011); http://dx.doi.org/10.1063/1.3623441 (3 pages) | Cited 4 times

Online Publication Date: 5 August 2011

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We form a monolayer of magnetic organic molecules with its plane parallel or perpendicular to the substrate. The molecules in a monolayer are oriented with an external magnetic field followed by immobilization though an electrostatic binding. In this work, from scanning tunneling microscopy (STM) measurements, we show that conductivity, molecular orbitals, and transport gap of the molecules in a monolayer depend on its orientation. From measurements carried out with different tip-to-molecule distances, we observe that the STM tip also influences molecular orbitals and transport-gap of molecules.
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73.61.Ph Polymers; organic compounds
75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Xx Molecular magnets
75.70.Ak Magnetic properties of monolayers and thin films
68.37.Ma Scanning transmission electron microscopy (STEM)
73.50.-h Electronic transport phenomena in thin films
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