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1 Feb 2010

Volume 96, Issue 5, Articles (05xxxx)

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Appl. Phys. Lett. 96, 053107 (2010); http://dx.doi.org/10.1063/1.3280078 (3 pages)

Desalegne Teweldebrhan, Vivek Goyal, Muhammad Rahman, and Alexander A. Balandin
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Organic light-emitting diodes containing multilayers of organic single crystals

Hajime Nakanotani and Chihaya Adachi

Appl. Phys. Lett. 96, 053301 (2010); http://dx.doi.org/10.1063/1.3298558 (3 pages) | Cited 5 times

Online Publication Date: 1 February 2010

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Double-heterostructure (DH) organic light-emitting diodes (OLEDs) with thick carrier transport layers based on organic single crystals have been fabricated. Although the total thickness of the organic layers ( ∼ 1.4 μm) is one order of magnitude greater than that of conventional thin-film OLEDs, a current density of 100 mA/cm2 was achieved at 19 V. The major advantage of the single crystal-based DH-OLED was that a very low applied electric field ( ∼ 104 V/cm) was required for the onset of carrier injection and transport, because of the high carrier mobilities of the single crystal layers.
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85.60.Jb Light-emitting devices
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Thermal stability of organic thin-film transistors with self-assembled monolayer dielectrics

Kenjiro Fukuda, Tomoyuki Yokota, Kazunori Kuribara, Tsuyoshi Sekitani, Ute Zschieschang, Hagen Klauk, and Takao Someya

Appl. Phys. Lett. 96, 053302 (2010); http://dx.doi.org/10.1063/1.3299017 (3 pages) | Cited 11 times

Online Publication Date: 1 February 2010

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We have investigated the annealing effects on 2 V-operation pentacene organic thin-film transistors (TFTs) with self-assembled monolayer-based gate dielectrics. When pentacene TFTs without passivation layers are annealed at 100 °C, the pentacene crystal structure changes to the bulk phase, resulting in an irreversible degradation of electronic performances. This degradation is suppressed by a 2.5-μm-thick passivation layer. The mobility of the encapsulated TFTs decreases by only 12% upon annealing at 140 °C. We have also investigated the bias-stress effect and found the drain current of low-voltage pentacene TFTs annealed at 70 °C decreases by 1% during continuous bias stress for 1 h.
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85.30.Tv Field effect devices
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Yellow-green light-emitting electrochemical cells with long lifetime and high efficiency

Andreas Sandström, Piotr Matyba, and Ludvig Edman

Appl. Phys. Lett. 96, 053303 (2010); http://dx.doi.org/10.1063/1.3299018 (3 pages) | Cited 12 times

Online Publication Date: 1 February 2010

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We show that the electrochemical stability window of the constituent components in light-emitting electrochemical cells (LECs), e.g., the electrolyte, should be considered in order to minimize undesired side reactions. By designing and operating LECs in accordance with straightforward principles, we demonstrate sandwich cells that turn on fast at room temperature (<2 s), and which emit significant yellow-green light (>100 cd/m2) during 25 days of uninterrupted operation at low voltage (<4 V) and high power conversion efficacy ( ∼ 6 lm/W). We further demonstrate that it is possible to attain balanced p- and n-type doping and a centered p-n junction in such planar LECs based on the conjugated polymer “superyellow.”
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85.60.Jb Light-emitting devices
82.47.-a Applied electrochemistry
82.45.Wx Polymers and organic materials in electrochemistry
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Magnetism in Co-doped tris-8-hydroxyquinoline aluminum studied by first-principles calculations

Fenggong Wang, Zhiyong Pang, Liang Lin, Shaojie Fang, Ying Dai, and Shenghao Han

Appl. Phys. Lett. 96, 053304 (2010); http://dx.doi.org/10.1063/1.3308477 (3 pages) | Cited 4 times

Online Publication Date: 2 February 2010

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The electronic and magnetic properties of Co-doped tris-8-hydroxyquinoline aluminum (Alq3) are studied by first-principles calculations. Our results indicate that the local magnetic moments in doped Alq3 originate from the localized d states of Co atom. Electron transfer takes place from Co atom to Alq3 molecule, which is mainly localized on the quinolate ligand, resulting in formation of bound magnetic polarons. The indirect ferromagnetic exchange interaction between two bound magnetic polarons antialigning with the same magnetic ion promotes the collective magnetism found in recent experiments.
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75.30.Cr Saturation moments and magnetic susceptibilities
71.38.-k Polarons and electron-phonon interactions
75.50.Dd Nonmetallic ferromagnetic materials
61.72.U- Doping and impurity implantation
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Thickness dependent absorption spectra in conjugated polymers: Morphology or interference?

Olivier P. M. Gaudin, Ifor D. W. Samuel, Samia Amriou, and Paul L. Burn

Appl. Phys. Lett. 96, 053305 (2010); http://dx.doi.org/10.1063/1.3294636 (3 pages) | Cited 4 times

Online Publication Date: 2 February 2010

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The thickness dependence of the absorption spectrum of spin-coated films of poly[2-(2′-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] has been studied using reflectivity and variable angle spectroscopic ellipsometry measurements. It is found that, for films with thicknesses in the range of 18–178 nm, a single set of optical constants is sufficient to simulate accurately all the experimental data used, including the absorption spectra, independently of the film thickness or the processing conditions. Thus, the observed changes in the absorption spectrum with thickness can be fully accounted for by reflectivity and interference effects alone without the need to invoke morphology differences between films.
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78.66.Qn Polymers; organic compounds
78.40.-q Absorption and reflection spectra: visible and ultraviolet
78.30.-j Infrared and Raman spectra
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
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Emission zone control in blue organic electrophosphorescent devices through chemical modification of host materials

Evgueni Polikarpov, James S. Swensen, Lelia Cosimbescu, Phillip K. Koech, James E. Rainbolt, and Asanga B. Padmaperuma

Appl. Phys. Lett. 96, 053306 (2010); http://dx.doi.org/10.1063/1.3298556 (3 pages) | Cited 8 times

Online Publication Date: 4 February 2010

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We report blue organic light-emitting devices with iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′] picolinate as an emitter doped into a series of phosphine oxide-based host materials that have significantly different charge transport properties: 4-(diphenylphosphoryl)-N,N-diphenylaniline (HM-A1), N-(4-diphenylphosphoryl phenyl) carbazole (PO12), 9-[6-(diphenylphosphoryl)pyridin-3-yl]-9H-carbazole (HM-A5), and 6-(diphenylphosphoryl)-N,N-diphenylpyridin-3-amine (HM-A6). Depending on the nature of the host material, the location of the emission zone can be moved within the emissive layer from the hole transport layer interface to the electron-transport layer interface. The charge transport properties of the materials were evaluated using single carrier devices.
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85.60.Jb Light-emitting devices
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Reduced initial degradation of bulk heterojunction organic solar cells by incorporation of stacked fullerene and lithium fluoride interlayers

Kenji Kawano and Chihaya Adachi

Appl. Phys. Lett. 96, 053307 (2010); http://dx.doi.org/10.1063/1.3297876 (3 pages) | Cited 13 times

Online Publication Date: 5 February 2010

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Reduced initial degradation of bulk heterojunction organic solar cells (OSCs) was achieved by inclusion of stacked fullerene (C60) and lithium fluoride interlayers. By inserting a 3 nm C60 layer and a 0.5 nm LiF layer between the photoactive layer and Al cathode in an OSC, the device lifetime calculated after an initial 8 h continuous illumination period was dramatically improved, with a decrease of only 5% in the power conversion efficiency. The 3 nm C60 layer forms a unique surface pattern of microscopic domes, and the combination of this layer with a 0.5 nm LiF layer formed an Ohmic-like contact between the photoactive layer and the Al cathode, resulting in a significant reduction in the degradation of the OSC.
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88.40.jr Organic photovoltaics
88.40.H- Solar cells (photovoltaics)
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Transport in organic semiconductors in large electric fields: From thermal activation to field emission

J. H. Worne, J. E. Anthony, and D. Natelson

Appl. Phys. Lett. 96, 053308 (2010); http://dx.doi.org/10.1063/1.3309704 (3 pages) | Cited 12 times

Online Publication Date: 5 February 2010

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Understanding charge transport in organic semiconductors in large electric fields is relevant to many applications. We present transport measurements in organic field-effect transistors based on poly(3-hexylthiophene) and 6,13-bis(triisopropyl-silylethynyl) (TIPS) pentacene with short channels, from room temperature down to 4.2 K. Near 300 K transport in both systems is well described by thermally assisted hopping with Poole–Frenkel-type enhancement of the mobility. At low temperatures and large gate voltages, transport in both materials becomes nearly temperature independent, crossing over into field-driven tunneling. These data, particularly in TIPS-pentacene, show that great caution must be exercised when considering more exotic (e.g., Tomonaga–Luttinger liquid) interpretations of transport.
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85.30.Tv Field effect devices
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