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May 2012

Volume 5, Issue 5

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Electrical stabilities and memory mechanisms of organic bistable devices fabricated utilizing a poly(3,4-ethylene-dioxythiophene): Poly(styrenesulfonate) layer with a poly(methyl methacrylate) buffer layer

Jung Min Son, Woo Seung Song, Chan Ho Yoo, Dong Yeol Yun, and Tae Whan Kim

Appl. Phys. Lett. 100, 183303 (2012); http://dx.doi.org/10.1063/1.4709399 (3 pages)

Online Publication Date: 1 May 2012

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Organic bistable devices (OBDs) based on a poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer with a poly(methyl methacrylate) (PMMA) buffer layer were fabricated on indium-tin-oxide (ITO)-coated polyethylene terephthalate (PET) flexible substrates. Current-voltage curves for the Al/PEDOT:PSS/PMMA/ITO/PET device showed current bistabilities with an ON/OFF current ratio of 1 × 103, indicative of a significant enhancement of memory storage. The endurance number of the ON/OFF switchings for the OBDs was above 1 × 105 cycles showing high potential applications in read only memory devices. The memory mechanisms for the OBDs on the basis of oxidation and reduction operations were attributed to the filament processes.
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84.30.Sk Pulse and digital circuits
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Performance enhancement of organic light-emitting diodes by chlorine plasma treatment of indium tin oxide

X. A. Cao and Y. Q. Zhang

Appl. Phys. Lett. 100, 183304 (2012); http://dx.doi.org/10.1063/1.4709426 (4 pages)

Online Publication Date: 1 May 2012

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The characteristics of green phosphorescent organic light-emitting diodes (OLEDs) fabricated on ITO/glass substrates pretreated with low-energy O2 and Cl2 plasma were compared. At 20 mA/cm2, the OLEDs with O2 and Cl2 plasma-treated indium tin oxide (ITO) had voltages of 9.6 and 7.6 eV, and brightness of 9580 and 12380 cd/m2, respectively. At ∼104 cd/m2, the latter had a 30% higher external quantum efficiency and a 74% higher power efficiency. Photoelectron spectroscopies revealed that Cl2 plasma treatment created stable In-Cl bonds and raised the work function of ITO by up to 0.9 eV. These results suggest that the better energy level alignment at the chlorinated ITO/organic interface enhances hole injection, leading to more efficient and more reliable operation of the OLEDs. The developed plasma chlorination process is very effective for surface modification of ITO and compatible with the fabrication of various organic electronics.
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85.60.Jb Light-emitting devices
52.77.-j Plasma applications
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High efficiency warm-white organic light emitting diodes from a single emitter in graded-doping device architecture

Unnat S. Bhansali, Huiping Jia, Iain W. H. Oswald, Mohammad A. Omary, and Bruce E. Gnade

Appl. Phys. Lett. 100, 183305 (2012); http://dx.doi.org/10.1063/1.3702452 (4 pages)

Online Publication Date: 2 May 2012

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We report high efficiency single-emitter white organic light emitting diodes (SWOLEDs) from a phosphorescent platinum-pyridyltriazolate complex. We have demonstrated good control of the recombination zone and its impact on device performance by adjusting the number of emissive layers, their thicknesses and relative positions in the device stack. Device optimization has resulted in SWOLEDs with a peak power efficiency (PE) = 30.4 ± 1.3 lm/W, external quantum efficiency = 17.0% ± 0.1%, and correlated color temperature = 3450 K, within acceptable warm-white range despite the color rendering index being only 62. The devices exhibit negligible performance roll-off at 500–1000 cd/m2.
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85.60.Jb Light-emitting devices
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Identification of device degradation positions in multi-layered phosphorescent organic light emitting devices using water probes

Hitoshi Yamamoto, Chihaya Adachi, Michael S. Weaver, and Julie J. Brown

Appl. Phys. Lett. 100, 183306 (2012); http://dx.doi.org/10.1063/1.4711129 (4 pages)

Online Publication Date: 3 May 2012

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We exposed regions of green phosphorescent organic light emitting devices (PHOLEDTMs) consisting of a fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) as the phosphorescent emitter to a partial pressure of water of 3 × 10−4 Pa during device fabrication to induce degradation in a specific region of the multi-layered devices. We identified the interface between the hole transport layer and the emissive layer as the most susceptive region to degradation. We discuss the luminance loss mechanism and estimate an operational lifetime of 10 000 h, after 20% loss of the initial luminance from 1000 cd/m2, is attainable from an Ir(ppy)3 PHOLED fabricated under ultra-high vacuum conditions.
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85.60.Jb Light-emitting devices
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Nonvolatile floating gate organic memory device based on pentacene/CdSe quantum dot heterojuction

Ik-Soo Shin, Jung-Min Kim, Jun-Ho Jeun, Seok-Hyun Yoo, Ziyi Ge, Jong-In Hong, Jin Ho Bang, and Yong-Sang Kim

Appl. Phys. Lett. 100, 183307 (2012); http://dx.doi.org/10.1063/1.4711209 (4 pages)

Online Publication Date: 3 May 2012

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An organic floating-gate memory device using CdSe quantum dots (QDs) as a charge-trapping element was fabricated. CdSe QDs were localized beneath a pentacene without any tunneling insulator, and the QD layer played a role as hole-trapping sites. The band bending formed at the junction between pentacene and QD layers inhibited back-injection of holes trapped in CdSe into pentacene, which appeared as a hysteretic capacitance-voltage response during the operation of the device. Nearly, 60% of trapped charge was sustained even after 104 s in programmed state, and this long retention time can be potentially useful in practical applications of non-volatile memory.
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84.30.Sk Pulse and digital circuits
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
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Temperature dependence of frequency response characteristics in organic field-effect transistors

Xubing Lu, Takeo Minari, Chuan Liu, Akichika Kumatani, J.-M. Liu, and Kazuhito Tsukagoshi

Appl. Phys. Lett. 100, 183308 (2012); http://dx.doi.org/10.1063/1.4711211 (4 pages)

Online Publication Date: 4 May 2012

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The frequency response characteristics of semiconductor devices play an essential role in the high-speed operation of electronic devices. We investigated the temperature dependence of dynamic characteristics in pentacene-based organic field-effect transistors and metal-insulator-semiconductor capacitors. As the temperature decreased, the capacitance-voltage characteristics showed large frequency dispersion and a negative shift in the flat-band voltage at high frequencies. The cutoff frequency shows Arrhenius-type temperature dependence with different activation energy values for various gate voltages. These phenomena demonstrate the effects of charge trapping on the frequency response characteristics, since decreased mobility prevents a fast charge response for alternating current signals at low temperatures.
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85.30.Tv Field effect devices
84.32.Tt Capacitors
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Non-volatile organic memory devices comprising SiO2 and C60 showing 104 switching cycles

P. Siebeneicher, H. Kleemann, K. Leo, and B. Lüssem

Appl. Phys. Lett. 100, 193301 (2012); http://dx.doi.org/10.1063/1.4712057 (4 pages)

Online Publication Date: 8 May 2012

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We present a non-volatile organic memory device comprising a thin SiO2 layer, the organic semiconductor C60, and an organic n-type doped layer between two metallic electrodes. The memory device shows a stable hysteresis in the current-voltage characteristics with an ON/OFF ratio in the range of three or higher and reasonable switching behavior with 104 write-read-erase-read cycles. The data retention time reaches from several hours up to several days and depends on the read out frequency. We exclude a filamentary conduction mechanism as cause of the memory effect and propose that the presence of charge carrier traps at the interface of the C60 layer with the oxide causes the hysteresis of this organic non-volatile memory device.
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84.30.Sk Pulse and digital circuits
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Direct and charge transfer state mediated photogeneration in polymer–fullerene bulk heterojunction solar cells

M. Mingebach, S. Walter, V. Dyakonov, and C. Deibel

Appl. Phys. Lett. 100, 193302 (2012); http://dx.doi.org/10.1063/1.4711849 (4 pages)

Online Publication Date: 8 May 2012

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We investigated photogeneration yield and recombination dynamics in blends of poly(3-hexyl thiophene) (P3HT) and poly[2-methoxy-5 -(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) with [6,6]-phenyl-C61butyric acid methyl ester (PC61BM) by means of temperature dependent time delayed collection field measurements. In MDMO-PPV:PC61BM, we find a strongly field dependent polaron pair dissociation which can be attributed to geminate recombination in the device. Our findings are in good agreement with field dependent photoluminescence measurements published before, supporting a scenario of polaron pair dissociation via an intermediate charge transfer state. In contrast, polaron pair dissociation in P3HT:PC61BM shows only a very weak field dependence, indicating an almost field independent polaron pair dissociation or a direct photogeneration. Furthermore, we found Langevin recombination for MDMO-PPV:PC61BM and strongly reduced Langevin recombination for P3HT:PC61BM.
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88.40.jp Multijunction solar cells
88.40.H- Solar cells (photovoltaics)
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Topological asymmetry induced electronic transport in three terminal graphene nanoribbon structure

En-Jia Ye, Wen-Quan Sui, and Xuean Zhao

Appl. Phys. Lett. 100, 193303 (2012); http://dx.doi.org/10.1063/1.4714506 (4 pages)

Online Publication Date: 10 May 2012

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Topological configurations play an important role in graphene nanoribbon based devices. In this work, the Büttiker’s ac transport theory is used to study an asymmetric three terminal graphene nanoribbon structure. With the help of Green’s function and related parameters, we show that the topological asymmetries can form capacitive and inductive junctions in this three terminal structure. The transport properties are sensitive to the geometric features of the branches of the junctions and the coupling positions. It is believed that this kind of structure can be useful for the future nanoelectronic devices.
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73.63.Bd Nanocrystalline materials
81.07.Bc Nanocrystalline materials
72.80.Vp Electronic transport in graphene
81.05.ue Graphene
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Direct evidence of n-type doping in organic light-emitting devices: N free Cs doping from CsN3

Jeihyun Lee, Hyunbok Lee, Pyungeun Jeon, Kwangho Jeong, Tae Gun Kim, Jeong Won Kim, and Yeonjin Yi

Appl. Phys. Lett. 100, 203301 (2012); http://dx.doi.org/10.1063/1.4718017 (4 pages)

Online Publication Date: 14 May 2012

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Cesium azide (CsN3) is confirmed to be decomposed during thermal evaporation. Only Cs could be deposited on tris(8-hydroxyquinolinato)aluminum (Alq3) and n-type doping is easily achieved. Organic light-emitting devices with CsN3 show highly improved current density-luminance-voltage characteristics compared to the control device without CsN3. To understand the origin of the improvements, in situ x-ray and UV photoemission spectroscopy measurements were carried out and a remarkable reduction in electron injection barrier is verified with successive deposition of Al on CsN3 on Alq3. CsN3 has a potential as alternative to doping the electron transport layer by replacing the direct deposition of alkali metals.
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85.60.Jb Light-emitting devices
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Modeling charge transfer at organic donor-acceptor semiconductor interfaces

Deniz Çakir, Menno Bokdam, Michel P. de Jong, Mats Fahlman, and Geert Brocks

Appl. Phys. Lett. 100, 203302 (2012); http://dx.doi.org/10.1063/1.4717985 (4 pages)

Online Publication Date: 15 May 2012

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We develop an integer charge transfer model for the potential steps observed at interfaces between donor and acceptor molecular semiconductors. The potential step can be expressed as the difference between the Fermi energy pinning levels of electrons on the acceptor material and holes on the donor material, as determined from metal-organic semiconductor contacts. These pinning levels can be obtained from simple density functional theory calculations.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.40.Ns Metal-nonmetal contacts
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Red-green-blue polymer light-emitting diode pixels printed by optimized laser-induced forward transfer

James Shaw Stewart, Thomas Lippert, Matthias Nagel, Frank Nüesch, and Alexander Wokaun

Appl. Phys. Lett. 100, 203303 (2012); http://dx.doi.org/10.1063/1.4717463 (4 pages)

Online Publication Date: 15 May 2012

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An optimized laser-induced forward transfer (LIFT) technique has been used to fabricate tri-color organic light-emitting diode (OLED) pixels. At reduced pressures, and with a defined donor-receiver gap, patterned depositions of polyfluorene-based OLED pixels have been achieved. OLED pixel functionality has been demonstrated and compared with devices made using conventional deposition techniques. In addition, improved functionality has been obtained by coating the cathode with an electron-injecting layer, a process not possible using conventional OLED fabrication techniques. The OLED pixels fabricated by LIFT reach efficiencies on the range of conventionally fabricated devices and even surpass them in the case of blue pixels.
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85.60.Jb Light-emitting devices
82.45.Fk Electrodes
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