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13 Dec 2010

Volume 97, Issue 24, Articles (24xxxx)

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Appl. Phys. Lett. 97, 241101 (2010); http://dx.doi.org/10.1063/1.3525583 (3 pages)

Kanghee Lee and Jaewook Ahn
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Charge accumulation dynamics in organic thin film transistors

X. Y. Chen, H. Zhu, and S. D. Wang

Appl. Phys. Lett. 97, 243301 (2010); http://dx.doi.org/10.1063/1.3526374 (3 pages) | Cited 4 times

Online Publication Date: 14 December 2010

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We have developed a transient current model to quantitatively describe the charge accumulation dynamics in organic thin film transistors. This model indicates that the charge accumulation process is faster at higher gate bias and/or higher field-effect mobility, and the experimental results are consistent with the theoretical expectations. A strong gate bias dependence of the field-effect mobility is observed, which suggests that the charge traps in the charge accumulation layer may limit the device performance not only at steady state but also at transient state.
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85.30.Tv Field effect devices
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Solution processed polymer light-emitting diodes utilizing a ZnO/organic ionic interlayer with Al cathode

Hongseok Youn and Minyang Yang

Appl. Phys. Lett. 97, 243302 (2010); http://dx.doi.org/10.1063/1.3526308 (3 pages)

Online Publication Date: 14 December 2010

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This letter reports polymer light-emitting diodes that employ a soluble zinc oxide (ZnO) nanoparticle (NP) and organic ionic interlayer as an electron-injection layer exhibits remarkable enhancement of device performance despite aluminum cathode. The ionic solution infiltrated into ZnO NP layer, which contains poly(ethylene oxide) and tetra-n-butylammonium tetrafluoborate, significantly lowers the large electron-injection barrier by forming a permanent interfacial dipole. The polymer, phenyl substituted poly(para-phenylene vinylene) known as “Super Yellow,” yellow light-emitting diodes employing the ZnO NP and ionic interlayer show a maximum efficiency of 6.3 cd/A at a 1209 cd/m2 and 5.4 V. The maximum brightness of the device reached 24 000 cd/m2 at 9 V.
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85.60.Jb Light-emitting devices
82.45.Fk Electrodes
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High-performance organic field-effect transistors based on copper/copper sulphide bilayer source-drain electrodes

Wen Gu, Weipeng Jin, Bin Wei, Jianhua Zhang, and Jun Wang

Appl. Phys. Lett. 97, 243303 (2010); http://dx.doi.org/10.1063/1.3526737 (3 pages)

Online Publication Date: 14 December 2010

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We demonstrate an organic field-effect transistor (OFET) using copper (Cu) and copper sulfide (CuxS) as the source-drain electrodes. The OFET using Cu/CuxS results in a fivefold higher field-effect mobility and a 12 V reduction in threshold voltage as compared with the conventional devices with gold electrodes. The improvements of device performances are attributed to the enhancement of charge-injection. X-ray photoelectron spectroscopy revealed that sulfate ions were formed during deposition, which is considered to be responsible for the improvements.
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85.30.Tv Field effect devices
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Causes of efficiency roll-off in phosphorescent organic light emitting devices: Triplet-triplet annihilation versus triplet-polaron quenching

Dandan Song, Suling Zhao, Yichun Luo, and Hany Aziz

Appl. Phys. Lett. 97, 243304 (2010); http://dx.doi.org/10.1063/1.3527085 (3 pages) | Cited 4 times

Online Publication Date: 16 December 2010

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Delayed electroluminescence measurements are used to probe and differentiate between triplet-triplet-annihilation (TTA) and triplet-polaron-quenching (TPQ) processes and their correlation with efficiency roll-off in fac-tris(2-phenylpyridine) iridium-based phosphorescent organic light emitting devices. Investigations on devices employing 4,4′-bis(9-carbazolyl)-1,1′-biphenyl (CBP) and 4,4′,4″-tris(N-carbazolyl) triphenylamine, two widely used host materials, show that the efficiency roll-off is primarily due to TPQ processes. Guest-guest TTA, on the other hand, is found to play no major role, contrary to speculations, especially at low guest concentrations. Evidence of host-host TTA in certain cases, and its possible contribution to exciton quenching in the case of devices with CBP host, is also reported.
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85.60.Jb Light-emitting devices
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Molecular n-type doping for air-stable electron transport in vacuum-processed n-channel organic transistors

Joon Hak Oh, Peng Wei, and Zhenan Bao

Appl. Phys. Lett. 97, 243305 (2010); http://dx.doi.org/10.1063/1.3527972 (3 pages) | Cited 2 times

Online Publication Date: 17 December 2010

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The effects of n-type doping on the air-stability of vacuum-processed n-channel organic transistors have been investigated using perylene diimides and pyronin B as the active layer and dopant, respectively. Systematic studies on the influence of doping location revealed the n-type doping of bulk active layer or channel region significantly improves air-stability by compensating for the trapped electrons with the donated mobile electrons. Although n-type doping at the electrode contact could readily turn on the devices, it could not confer air-stable electron transport. The described approach would open up opportunities to enable and improve the stability of n-channel organic transistors in air.
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85.30.Tv Field effect devices
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Effects of alignment layer thickness on the pretilt angle of liquid crystals

Jong-Ho Son and Wang-Cheol Zin

Appl. Phys. Lett. 97, 243306 (2010); http://dx.doi.org/10.1063/1.3528209 (3 pages)

Online Publication Date: 17 December 2010

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Mixture solutions of vertical- and planar-aligning polyimide precursors were coated on bare glass. The concentrations of the solutions were varied to control the thicknesses of the films. The resulting blend films were baked to induce imidization and then rubbed. The thicknesses (t) of the blend film and of the pure vertical-alignment film affected their surface energies; the pretilt angle can be fully controlled in the range 5.5° ≤ Θ0 ≤ 87° by adjusting t. The surface energy of pure planar-alignment layers was independent of t.
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61.30.Vx Polymer liquid crystals
61.25.H- Macromolecular and polymers solutions; polymer melts
61.41.+e Polymers, elastomers, and plastics
81.65.-b Surface treatments
65.40.gp Surface energy
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