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27 Oct 2008

Volume 93, Issue 17, Articles (17xxxx)

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Appl. Phys. Lett. 93, 172501 (2008); http://dx.doi.org/10.1063/1.3005561 (3 pages)

Yan Wang, X. F. Han, and X.-G. Zhang
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Three-dimensional organic field-effect transistors: Charge accumulation in the vertical semiconductor channels

Mayumi Uno, Y. Tominari, and J. Takeya

Appl. Phys. Lett. 93, 173301 (2008); http://dx.doi.org/10.1063/1.3007984 (3 pages) | Cited 12 times

Online Publication Date: 29 October 2008

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A three-dimensional organic field-effect transistor is developed to accumulate charge in its vertical semiconductor channel so that space availability for the field-induced carriers is essentially multiplied. A multicolumnar structure is built incorporating a vertical layer of soluble benzothieno-benzothiophene derivative. Pronounced field-effect performance is realized with the well-defined saturation in the output, where extremely large value of channel width divided by length enables current amplification up to ∼ 2 μA in a square pixel even with relatively low carrier mobility of 10−4 cm2/V s. The result demonstrates usefulness of the three-dimensional structure in achieving sufficient current for matrix-controlling devices in flat-panel displays.
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85.30.Tv Field effect devices
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Horizontal molecular orientation in vacuum-deposited organic amorphous films of hole and electron transport materials

Daisuke Yokoyama, Akio Sakaguchi, Michio Suzuki, and Chihaya Adachi

Appl. Phys. Lett. 93, 173302 (2008); http://dx.doi.org/10.1063/1.2996258 (3 pages) | Cited 33 times

Online Publication Date: 29 October 2008

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Using wide-range variable angle spectroscopic ellipsometry, the authors demonstrate large optical uniaxial anisotropy of vacuum-deposited organic amorphous films of hole and electron transport materials having long or planar molecular structures. The ordinary refractive indices and extinction coefficients were higher than the extraordinary ones, revealing that the molecules in the amorphous films are horizontally oriented. The horizontal orientation requires significant modifications in the understanding of both the electrical and optical characteristics of amorphous films when we use materials having long or planar molecular structures.
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78.66.Qn Polymers; organic compounds
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
73.61.Ph Polymers; organic compounds
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Electrical instability in vanadyl-phthalocyanine thin-film transistors

Lijuan Wang, Guojun Liu, Feng Zhu, Feng Pan, and Donghang Yan

Appl. Phys. Lett. 93, 173303 (2008); http://dx.doi.org/10.1063/1.3009208 (3 pages) | Cited 5 times

Online Publication Date: 29 October 2008

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We investigated the electrical instability of vanadyl-phthalocyanine (VOPc) thin-film transistors (TFTs) at various temperatures. The results demonstrate a slow threshold voltage shift in the bias stress process and a rapid recovery after the removal of bias stress, which indicates that a slower degradation process occurs in the on state while a faster removal in the off state of VOPc TFTs. The shift of threshold voltage comes from traps generated at the organic/dielectrics interface. Additionally, a relaxation time of 107 s was obtained at room temperature according to the stretched exponential model, which is comparable to a-Si:H TFTs. Therefore, VOPc TFTs are suitable to be applied in flat panel displays.
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85.30.Tv Field effect devices
85.40.-e Microelectronics: LSI, VLSI, ULSI; integrated circuit fabrication technology
42.79.Kr Display devices, liquid-crystal devices
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Inverted small molecule organic photovoltaic cells on reflective substrates

Xiaoran Tong, Rhonda F. Bailey-Salzman, Guodan Wei, and Stephen R. Forrest

Appl. Phys. Lett. 93, 173304 (2008); http://dx.doi.org/10.1063/1.3005173 (3 pages) | Cited 8 times

Online Publication Date: 29 October 2008

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We demonstrate top-illuminated, inverted, small molecule photovoltaic cells grown on reflective substrates employing copper phthalocyanine as the donor and 3,4,9,10-perylenetetracarboxylic bis-benzimidazole as the acceptor, with a sputter-deposited transparent indium tin oxide top cathode and a metal anode, thereby reversing the conventional charge extraction properties of these contacts. The best device achieved a peak power conversion efficiency of 0.74±0.03%, reasonably consistent with the optical simulations under 1 sun AM1.5G illumination giving 0.83±0.02%. This work suggests that inverted organic solar cells grown on reflective substrates have potential uses such as for power-generating coatings on opaque surfaces.
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84.60.Jt Photoelectric conversion
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Direct observation of the electronic states of single crystalline rubrene under ambient condition by photoelectron yield spectroscopy

Yasuo Nakayama, Shinichi Machida, Takeo Minari, Kazuhito Tsukagishi, Yutaka Noguchi, and Hisao Ishii

Appl. Phys. Lett. 93, 173305 (2008); http://dx.doi.org/10.1063/1.2998650 (3 pages) | Cited 17 times

Online Publication Date: 30 October 2008

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The electronic states of single crystalline (SC) rubrene were experimentally observed by photoelectron yield spectroscopy without the sample charging problem. The ionization energy (Is) in the SC phase was determined to be 4.85(±0.05) eV, which is reduced by 0.45 eV compared to that of the amorphous film. The changes in the electronic states during photo-oxidation reaction and under ambient air were also observed directly to reveal the further reduction in Is, which can be attributed to the generation of polar oxide molecules and reversible physisorption of H2O, respectively.
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73.20.At Surface states, band structure, electron density of states
79.60.Fr Polymers; organic compounds
82.30.-b Specific chemical reactions; reaction mechanisms
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