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3 Sep 2012

Volume 101, Issue 10, Articles (10xxxx)

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Appl. Phys. Lett. 101, 103101 (2012); http://dx.doi.org/10.1063/1.4748099 (5 pages)

Massimo Cuscunà, Annalisa Convertino, Emiliano Zampetti, Antonella Macagnano, Alessandro Pecora, Guglielmo Fortunato, Laura Felisari, Giuseppe Nicotra, Corrado Spinella, and Faustino Martelli
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Degradation induced decrease of the radiative quantum efficiency in organic light-emitting diodes

Tobias D. Schmidt, Daniel S. Setz, Michael Flämmich, Bert J. Scholz, Arndt Jaeger, Carola Diez, Dirk Michaelis, Norbert Danz, and Wolfgang Brütting

Appl. Phys. Lett. 101, 103301 (2012); http://dx.doi.org/10.1063/1.4749815 (4 pages)

Online Publication Date: 4 September 2012

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The efficiency decrease during electrical operation of organic light-emitting diodes is a crucial issue for both applied and fundamental research. In order to investigate degradation processes, we have performed an efficiency analysis for phosphorescent state-of-the-art devices in the pristine state and after an accelerated aging process at high current density resulting in a luminance drop to less than 60% of the initial value. This loss in efficiency can be explained by a decrease of the radiative quantum efficiency of the light-emitting guest/host system from 70% to 40%, while other factors determining the efficiency are not affected.
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85.60.Jb Light-emitting devices
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Single crystal field-effect transistors containing a pentacene analogue and their application in ethanol vapor detection

Guangyao Zhao, Huanli Dong, Lang Jiang, Huaping Zhao, Xiang Qin, and Wenping Hu

Appl. Phys. Lett. 101, 103302 (2012); http://dx.doi.org/10.1063/1.4750063 (3 pages)

Online Publication Date: 5 September 2012

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Organic field-effect transistors (OFETs) have attracted attention in the field of organic electronics. The conducting channel of OFETs is highly sensitive to out stimulus, so OFETs can be used as ultrasensitive sensors and actuators. Here, a new kind of OFET containing micro- and nanocrystals of a pentacene analogue is reported. The OFET exhibited a mobility of 0.8 cm2 V−1 s−1 and an on/off ratio of 1.7 × 107. Moreover, it displayed high sensitivity, selectivity, and reproducibility in the detection of ethanol gas, indicating its great potential as a sensor for ethanol gas.
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85.30.Tv Field effect devices
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
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Carrier doping to the organic Mott insulator by conjugating with tetrathiafulvalene

Y. Takahashi, Y. Nakagawa, K. Hayakawa, T. Inabe, and T. Naito

Appl. Phys. Lett. 101, 103303 (2012); http://dx.doi.org/10.1063/1.4750066 (4 pages) | Cited 1 time

Online Publication Date: 5 September 2012

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The electrical conductivity of the organic Mott insulator ET-F2TCNQ (ET = bis(ethylenedithio)tetrathiafulvalene, F2TCNQ = 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane) crystal was found to be enhanced by conjugation with a tetrathiafulvalene (TTF) single crystal on its surface; surface sheet resistance decreased from 5 × l05 to 2 × 103 Ω/sq. The mechanism of this decrement was investigated through optical and atomic force microscopy measurements at the interface. When TTF was conjugated to the ET-F2TCNQ crystal, electron injection from TTF and complex formation between TTF and F2TCNQ occurred. Neutral ET molecules were consequently generated at the interface, and this charge doping broke the Mott insulating state.
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73.25.+i Surface conductivity and carrier phenomena
73.20.Fz Weak or Anderson localization
61.72.up Other materials
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
72.80.Le Polymers; organic compounds (including organic semiconductors)
73.20.At Surface states, band structure, electron density of states
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Inverted top-emitting blue electrophosphorescent organic light-emitting diodes with high current efficacy

K. A. Knauer, E. Najafabadi, W. Haske, and B. Kippelen

Appl. Phys. Lett. 101, 103304 (2012); http://dx.doi.org/10.1063/1.4750141 (4 pages) | Cited 1 time

Online Publication Date: 5 September 2012

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Two different types of inverted top-emitting blue electrophosphorescent organic light-emitting diodes (OLEDs) are demonstrated that differ only in the choice of high electron mobility transport layers. The electron transport layer consists of either 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TpPyPB) or 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene) (TmPyPB). Devices with TpPyPB exhibit a current efficacy of 5.1 cd/A at 1259 cd/m2. OLEDs with TmPyPB show higher performance with a current efficacy of 33.6 cd/A at 1126 cd/m2. The difference in performance of OLEDs with TmPyPB is due to a combination of TmPyPB's higher triplet energy that decreases exciton transfer to the ETL and altered charge balance.
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85.60.Jb Light-emitting devices
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Electrical stabilities and carrier transport mechanisms of flexible organic bistable devices based on CdSe-InP core-shell nanoparticle/polystyrene nanocomposites

Dong Yeol Yun, Woo Seung Song, Tae Whan Kim, Sung Woo Kim, and Sang Wook Kim

Appl. Phys. Lett. 101, 103305 (2012); http://dx.doi.org/10.1063/1.4748873 (4 pages) | Cited 1 time

Online Publication Date: 5 September 2012

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Flexible organic bistable devices (OBDs) for the memory characteristics utilizing CdSe-InP core-shell nanoparticle/polystyrene nanocomposites were fabricated on indium-tin-oxide-coated polyethylene terephthalate substrates. Current-voltage measurements on Al/CdSe-InP nanoparticles embedded in polystyrene layer/indium-tin-oxide/polyethylene terephthalate devices without and with bending exhibit wide-range current hysteresis behaviours with ON/OFF ratios of 1 × 107 and 1 × 105, respectively. The endurance number of the ON/OFF switchings without bending was 1 × 105 cycles. The switching characteristics of the OBDs after bending were stable enough to distinguish the ON and OFF. The carrier transport mechanisms of the OBDs are described on the basis of the current-voltage results.
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84.30.Sk Pulse and digital circuits
85.35.-p Nanoelectronic devices
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Correlation between density of paramagnetic centers and photovoltaic degradation in polythiophene-fullerene bulk heterojunction solar cells

Reg Bauld, Leesa M. Fleury, Marima Van Walsh, and Giovanni Fanchini

Appl. Phys. Lett. 101, 103306 (2012); http://dx.doi.org/10.1063/1.4749813 (4 pages)

Online Publication Date: 6 September 2012

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We investigated the physical processes underlying the degradation of poly(3-hexyl-thiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) photovoltaics under harsh environmental conditions during a 70-70-70 test (70% humidity at 70 °C from 0 to 70 h) using a variety of analytical techniques aimed at monitoring moisture incorporation. While the total oxygen content did not significantly increase during the test, a limited fraction of oxygen forms paramagnetic centers in P3HT, PCBM and, more limitedly, P3HT:PCBM heterojunctions. A strong correlation exists between the density of paramagnetic centers and the decay in the AM 1.5 photoconversion efficiency of the devices.
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88.40.jp Multijunction solar cells
88.40.hj Efficiency and performance of solar cells
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Hopping photoconductivity in an exponential density of states

A. Iacchetti, D. Natali, M. Binda, L. Beverina, and M. Sampietro

Appl. Phys. Lett. 101, 103307 (2012); http://dx.doi.org/10.1063/1.4750239 (4 pages) | Cited 2 times

Online Publication Date: 6 September 2012

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Photoconductivity in organic semiconductors is modeled taking into account the hopping nature of charge transport in an exponential density of states and exploiting the transport energy concept. Explicit and analytical expressions describing device behavior are obtained, and a robust procedure is outlined to extract from experimental measurements relevant material parameters, including the charge density dependent bulk mobility, as we show on a prototypical all-organic photoconductor. This qualifies the photoconductor not only as a technologically relevant device but also as a test bed for the optoelectronic characterization of disordered materials.
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72.40.+w Photoconduction and photovoltaic effects
72.80.Le Polymers; organic compounds (including organic semiconductors)
71.20.Rv Polymers and organic compounds
72.20.Ee Mobility edges; hopping transport
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Highly sensitive tactile sensors integrated with organic transistors

Jiseok Kim, Tse Nga Ng, and Woo Soo Kim

Appl. Phys. Lett. 101, 103308 (2012); http://dx.doi.org/10.1063/1.4751354 (5 pages) | Cited 1 time

Online Publication Date: 7 September 2012

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This paper presents a highly sensitive capacitive pressure sensor composed of a polymer dielectric film with a nano-needle structure. The nano-needle polymer films were prepared by facile fabrication methods including breath figures formation followed by stamping. The pressure sensitivity of the sensor reached 1.76 kPa−1 in the low pressure range (<1 kPa), which is comparable to the sensitivity of human skin. Analysis of the geometries and densities effect was shown, and the nano-needle film showed better sensitivity in comparison to films with hemispherical or conical structures. The pressure sensors were integrated with printed organic thin film transistors to enable flexible, large-area tactile sensing applications.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
85.40.-e Microelectronics: LSI, VLSI, ULSI; integrated circuit fabrication technology
85.30.Tv Field effect devices
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