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25 Jan 2010

Volume 96, Issue 4, Articles (04xxxx)

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Appl. Phys. Lett. 96, 042501 (2010); http://dx.doi.org/10.1063/1.3291942 (3 pages)

Daniel Stickler, Robert Frömter, Holger Stillrich, Christian Menk, Carsten Tieg, Simone Streit-Nierobisch, Michael Sprung, Christian Gutt, Lorenz-M. Stadler, Olaf Leupold, Gerhard Grübel, and Hans Peter Oepen
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Visualization of the cation migration in ionic polymer-metal composite under an electric field

Il-Seok Park, Sang-Mun Kim, Deivid Pugal, Liming Huang, Suk-Wah Tam-Chang, and Kwang J. Kim

Appl. Phys. Lett. 96, 043301 (2010); http://dx.doi.org/10.1063/1.3293290 (3 pages) | Cited 11 times

Online Publication Date: 25 January 2010

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Ionic polymer-metal composites (IPMCs) exhibit a large dynamic bending deformation due to the redistribution of counter-ions inside the polymer. It has not been possible to get the high resolution data of the cation migration. The images obtained so far have only validated the versatile actuation model. The actuation model states that the electrically induced cation movement contributes to the volumetric stress change in the membrane. In this work, a visualization of the cation migration using the fluorescent microscopy is created. The results demonstrated in this letter help to understand the underlying mechanism of the IPMC transduction.
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66.30.hk Polymers
62.20.F- Deformation and plasticity
81.40.Lm Deformation, plasticity, and creep
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Recoverable electroluminescence from a contaminated organic/organic interface in an organic light-emitting diode

L. S. Liao, K. P. Klubek, J. K. Madathil, C. W. Tang, and D. J. Giesen

Appl. Phys. Lett. 96, 043302 (2010); http://dx.doi.org/10.1063/1.3294324 (3 pages) | Cited 1 time

Online Publication Date: 25 January 2010

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An organic/organic interface, like an electrode/organic interface in an organic light-emitting diode (OLED), can be severely affected by ambient contamination. However, we surprisingly found that the contaminated surface or interface can be “cured” by depositing a thin interfacial layer containing a strong reducing agent onto the contaminated surface before finishing the fabrication of the device. For example, in comparison with a regular OLED, an OLED having a 5-min ambient exposure to the light-emitting layer/electron-transporting layer interface drops its initial electroluminescence (EL) intensity by 50%. The decreased EL intensity due to the 5-min ambient exposure can be fully recovered and the improved operational stability can be realized after curing the contaminated interface using a thin Li interfacial layer. The experimental results provide a useful method to cope with the interfacial contamination in OLEDs during a manufacturing process. In addition, our results support the failure mechanism of an Alq-based OLED suggested by [ Papadimitrakopoulos et al., Chem. Mater. 8, 1363 (1996) ].
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85.60.Jb Light-emitting devices
81.65.-b Surface treatments
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Controlling carrier accumulation and exciton formation in organic light emitting diodes

Z. B. Wang, M. G. Helander, Z. W. Liu, M. T. Greiner, J. Qiu, and Z. H. Lu

Appl. Phys. Lett. 96, 043303 (2010); http://dx.doi.org/10.1063/1.3297884 (3 pages) | Cited 11 times

Online Publication Date: 26 January 2010

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It is found that the device performance of organic light emitting diodes (OLEDs) can be significantly improved by separating the carrier accumulation zone from the exciton formation interface. The improvement is explained by suppression of exciton quenching caused by accumulated carriers at the exciton formation interface. It is also found that the position of the exciton formation interface in OLEDs correlates well with the interfacial dipole measured using ultraviolet photoelectron spectroscopy at the interface between a hole transport layer and an electron transport layer. The findings of this work provide useful material selection guidelines in designing high performance OLEDs.
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85.60.Jb Light-emitting devices
71.35.-y Excitons and related phenomena
79.60.-i Photoemission and photoelectron spectra
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Organic light emitting complementary inverters

Ebinazar B. Namdas, Ifor D. W. Samuel, Deepak Shukla, Dianne M. Meyer, Yanming Sun, Ben B. Y. Hsu, Daniel Moses, and Alan J. Heeger

Appl. Phys. Lett. 96, 043304 (2010); http://dx.doi.org/10.1063/1.3293293 (3 pages) | Cited 7 times

Online Publication Date: 26 January 2010

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We show that p- and n-type light emitting field-effect transistors (LEFETs) can be made using “superyellow” as a light-emitting polymer, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) as a p-type material and a naphthalene di-imide as an n-type material. By connecting two of these LEFETs, we have demonstrated a light emitting complementary inverter (LECI). The LECI exhibited electrical and optical characteristics in the first and third quadrant of the transfer characteristics with voltage gain of 6 and 8, respectively.
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85.60.Jb Light-emitting devices
85.30.Tv Field effect devices
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Surface segregation at the aluminum interface of poly(3-hexylthiophene)/fullerene solar cells

Akiko Orimo, Kohji Masuda, Satoshi Honda, Hiroaki Benten, Shinzaburo Ito, Hideo Ohkita, and Hiroshi Tsuji

Appl. Phys. Lett. 96, 043305 (2010); http://dx.doi.org/10.1063/1.3294290 (3 pages) | Cited 28 times

Online Publication Date: 26 January 2010

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The effects of thermal annealing before and after Al deposition on poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blend solar cells were investigated by current density-voltage measurements and x-ray photoelectron spectroscopy (XPS). Compared to the preannealed device, the postannealed device exhibited enhanced open-circuit voltage (VOC), which is ascribed to the decrease in the reverse saturation current density J0. The XPS measurements demonstrated that P3HT is dominant at the Al interface in the preannealed device while PCBM is instead dominant in the postannealed device. This surface-segregated PCBM formed in the postannealed device can serve as a hole-blocking layer at the Al interface to reduce J0, and therefore improve VOC.
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88.40.H- Solar cells (photovoltaics)
68.35.Dv Composition, segregation; defects and impurities
81.40.Gh Other heat and thermomechanical treatments
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
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Understanding the role of tunneling barriers in organic spin valves by hard x-ray photoelectron spectroscopy

F. Borgatti, I. Bergenti, F. Bona, V. Dediu, A. Fondacaro, S. Huotari, G. Monaco, D. A. MacLaren, J. N. Chapman, and G. Panaccione

Appl. Phys. Lett. 96, 043306 (2010); http://dx.doi.org/10.1063/1.3285179 (3 pages) | Cited 11 times

Online Publication Date: 26 January 2010

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We present an ex situ, nondestructive chemical characterization of deeply buried organic-inorganic interfaces using hard x-ray photoelectron spectroscopy. Co/Alq3 and Co/AlOx/Alq3 interfaces were studied in order to determine the role of a thin (1–2 nm) AlOx interdiffusion barrier in organic spin valves. Interfacial Alq3, 15 nm below the surface, exhibits strong sensitivity to the electronic structure of the interfacial region and to the presence of the AlOx. In addition to reducing Co–Alq3 interdiffusion, we find that the barrier prevents charge donation from the Co to the interfacial Alq3, thus preventing the formation of Alq3 anions within the interface region.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
79.60.Jv Interfaces; heterostructures; nanostructures
66.30.Ny Chemical interdiffusion; diffusion barriers
73.20.-r Electron states at surfaces and interfaces
73.40.Gk Tunneling
73.40.Ns Metal-nonmetal contacts
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Enhanced fluorescence by surface plasmon coupling of Au nanoparticles in an organic electroluminescence diode

A. Fujiki, T. Uemura, N. Zettsu, M. Akai-Kasaya, A. Saito, and Y. Kuwahara

Appl. Phys. Lett. 96, 043307 (2010); http://dx.doi.org/10.1063/1.3271773 (3 pages) | Cited 26 times

Online Publication Date: 27 January 2010

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A significant increase in electroluminescence was achieved through coupling with localized surface plasmons in a single layer of Au nanoparticles. We fabricated a thin-film organic electroluminescence diode, which consists of an indium tin oxide (ITO) anode, a Au nanoparticle array, a Cu phthalocyanine hole transport layer, a tris(8-hydroxylquinolianato) aluminum (III) electron transport layer, a LiF electron injection layer, and an Al cathode. The device structure, with size-controlled Au particles embedded on ITO, can be used to realize the optimum distance for exciton-plasmon interactions by simply adjusting the thickness of the hole transport layer. We observed a 20-fold increase in the molecular fluorescence compared with that of a conventional diode structure.
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85.60.Jb Light-emitting devices
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.22.Lp Collective excitations
71.35.-y Excitons and related phenomena
78.55.Hx Other solid inorganic materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
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Light-emitting electrochemical cells using polymeric ionic liquid/polyfluorene blends as luminescent material

Rebeca Marcilla, David Mecerreyes, Gustaf Winroth, Sergio Brovelli, Maria del Mar Rodriguez Yebra, and Franco Cacialli

Appl. Phys. Lett. 96, 043308 (2010); http://dx.doi.org/10.1063/1.3279155 (3 pages) | Cited 19 times

Online Publication Date: 28 January 2010

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We report the use of blends composed of poly(9,9′-dioctylfluorene-alt-benzothiadiazole), F8BT, and a polymeric ionic liquid (PIL), poly(vinyl-ethylimidazolium bistrifluoromethanesulfonimide), as the active layer in light-emitting electrochemical cells (LECs) with the simple indium-tin-oxide/active-layer/Al configuration. The PIL provides both the ionic charge and the transport channel necessary for the devices to operate as LECs resulting in reduction of charge injection barriers at the electrode/active-layer interfaces. We find that the performance of devices using PIL:F8BT blends improved with respect to pure F8BT with maximum luminance increasing from 10–20 cd/m2 for pure F8BT to 2000–4000 cd/m2 for blends. Turn-on voltages were also reduced from above 7 V down to around 3.6–4 V. The maximum external quantum efficiency was increased from 10−3%–10−4% to values higher than 0.1%.
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85.60.Jb Light-emitting devices
82.45.Fk Electrodes
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.40.Ns Metal-nonmetal contacts
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Electrical characteristics of an organic bistable device using an Al/Alq3/nanostructured MoO3/Alq3/p+-Si structure

Tzu-Yueh Chang, You-Wei Cheng, and Po-Tsung Lee

Appl. Phys. Lett. 96, 043309 (2010); http://dx.doi.org/10.1063/1.3299265 (3 pages) | Cited 6 times

Online Publication Date: 29 January 2010

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The electrical properties of a device with an Al/Alq3/nanostructured MoO3/Alq3/p+-Si structure were investigated for organic resistance switching memories. The conductance of the device can be electrically switched to either high conductance or low conductance. The bistable switching of the device is attributed to the MoO3 nanoclusterlike layer interposed between the Alq3 thin films. When the device was switched to high conductance, a space-charge field dominated carrier transportation of the device. The space-charge field was resulted from charges trapped in the MoO3 nanoclusterlike layer. Both retention measurement and write-read-erase-read cycles of the device are also provided.
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85.30.-z Semiconductor devices
84.30.Sk Pulse and digital circuits
81.05.Fb Organic semiconductors
73.22.-f Electronic structure of nanoscale materials and related systems
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