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19 Aug 2002

Volume 81, Issue 8, pp. 1369-1534

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Anode modification of polyfluorene-based polymer light-emitting devices

M. K. Fung, S. L. Lai, S. W. Tong, M. Y. Chan, C. S. Lee, S. T. Lee, W. W. Wu, M. Inbasekaran, and J. J. O’Brien

Appl. Phys. Lett. 81, 1497 (2002); http://dx.doi.org/10.1063/1.1499519 (3 pages) | Cited 17 times

Online Publication Date: 9 August 2002

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A glycerol-modified poly(3,4-ethylene dioxythiophene) (PEDOT): poly(styrene sulfonate) (PSS) layer was used as an anode buffer layer in polymer light-emitting devices using poly(9,9-dioctylfluorene) (F8) as the emitter. Devices with a configuration of indium tin oxide/PEDOT:PSS (with or without glycerol)/F8/CsF/Al were fabricated. It was found that the glycerol-modified device showed a much larger current density than the unmodified device. At an operating voltage of 6 V, the glycerol-modified device showed a luminance of 1300 Cd/m2 and a current efficiency of 1.7 Cd/A compared to the corresponding values of 500 Cd/m2 and 1.3 Cd/A in the unmodified device. Analysis by ultraviolet spectroscopy suggests that the two devices have the same energy level structure and the performance improvement should not be due to change in the PEDOT/polymer interface. It was further found that incorporating a suitable amount of glycerol into the PEDOT:PSS layer can increase its conductivity by six times. This leads to a better balance in the hole and electron currents and thus improved device efficiency. © 2002 American Institute of Physics.
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85.60.Jb Light-emitting devices
78.66.Qn Polymers; organic compounds
78.40.Me Organic compounds and polymers

Photocurrent multiplication in organic single crystals

Masahiro Hiramoto, Ayako Miki, Manabu Yoshida, and Masaaki Yokoyama

Appl. Phys. Lett. 81, 1500 (2002); http://dx.doi.org/10.1063/1.1501764 (3 pages) | Cited 10 times

Online Publication Date: 9 August 2002

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A photocurrent multiplication of up to 200 times has been observed in single crystals of naphthalene tetracarboxylic anhydride sandwiched between metal electrodes. Photocurrent multiplication arises from photoinduced electron injection occurring at the crystal/metal interface. The high-speed response of the multiplied photocurrent reached 500 ms. © 2002 American Institute of Physics.
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72.40.+w Photoconduction and photovoltaic effects
72.80.Le Polymers; organic compounds (including organic semiconductors)
85.60.Gz Photodetectors (including infrared and CCD detectors)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Direct measurement of the absolute value of the interaction force between the fiber probe and the sample in a scanning near-field optical microscope

D. A. Lapshin, V. S. Letokhov, G. T. Shubeita, S. K. Sekatskii, and G. Dietler

Appl. Phys. Lett. 81, 1503 (2002); http://dx.doi.org/10.1063/1.1499736 (3 pages) | Cited 4 times

Online Publication Date: 9 August 2002

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The absolute values of the force exerted by the fiber probe of a scanning near-field optical microscope onto the surface were measured using an atomic force microscope in ambient conditions. We demonstrate that a usually neglected static attraction force is dominant at small dither amplitudes and is of the order of 200 nN. The tapping component of the force, often referred to as shear force, is of the order of 1 nN at these conditions for both the tuning fork-based and optical in resonance detection schemes. Other peculiarities of the shear force interaction are also discussed. © 2002 American Institute of Physics.
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07.79.Fc Near-field scanning optical microscopes

Gallium arsenide photodetectors for imaging in the far ultraviolet region

M. Caria, L. Barberini, S. Cadeddu, A. Giannattasio, A. Rusani, A. Sesselego, A. Lai, S. D’Auria, and F. Dubecky

Appl. Phys. Lett. 81, 1506 (2002); http://dx.doi.org/10.1063/1.1497996 (3 pages) | Cited 7 times

Online Publication Date: 9 August 2002

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The aim of the present work is to systematically investigate the response and stability of commercial GaAs devices in the 200–400 nm UV range with a view to establishing their potentiality in imaging devices. The irradiation results of GaAs detectors with various geometries are presented and discussed. The detectors were reverse biased in fully depleted condition and in partially depleted condition (5 V reverse bias) in order to investigate the possibilities of integration with the standard bias values of read-out-integrated circuits. The results show that fabrication technology for nondedicated devices is still immature. © 2002 American Institute of Physics.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
42.79.Pw Imaging detectors and sensors

Highly efficient polymer light-emitting devices using a phosphorescent sensitizer

Gufeng He, Shun-Chi Chang, Fang-Chung Chen, Yongfang Li, and Yang Yang

Appl. Phys. Lett. 81, 1509 (2002); http://dx.doi.org/10.1063/1.1502442 (3 pages) | Cited 28 times

Online Publication Date: 9 August 2002

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Show Abstract
Highly efficient single-layer polymer light-emitting diodes that employ Nile Red as a fluorescent dye, with a green phosphorescent-sensitizer, bis(2-phenyl pyridinato-N,C2′) iridium (acetylacetonate) doped in a PVK/PBD host are demonstrated. The function of the phosphorescent sensitizer is to convert the triplet exciton into a singlet exciton during the energy transfer process. Therefore, ideally, all the excitons can be utilized. For comparison, devices with the same structure, but using a fluorescent sensitizer instead of a phosphorescent sensitizer, were fabricated. The efficiency of the phosphor-sensitized device is 6.4 cd/A, almost triple that of lumophor-sensitized or nonsensitized devices. This result indicates that not only singlet excitons but also triplet excitons are efficiently transferred from the host to the fluorescent dye when a phosphorescent material is used as a sensitizer. © 2002 American Institute of Physics.
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85.60.Jb Light-emitting devices
42.70.Jk Polymers and organics

Electroluminescence from low-dimensionally confined crystals of thiophene/p-phenylene co-oligomers

Hisao Yanagi, Takayuki Morikawa, and Shu Hotta

Appl. Phys. Lett. 81, 1512 (2002); http://dx.doi.org/10.1063/1.1502023 (3 pages) | Cited 10 times

Online Publication Date: 9 August 2002

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Organic electroluminescence from epitaxially oriented needle crystals of a thiophene/p-phenylene co-oligomer was studied. The crystals grown by vapor deposition on a KCl (100) surface were confined in a p-sexiphenyl (p-6P) layer. After deposition of an Al cathode the film was wet transferred onto an indium tin oxide (ITO) coated glass. At a low bias voltage, this cell exhibited homogeneous blue light emission from the p-sexiphenyl layer due to carrier recombination in the ITO/p-6P/Al region. With an increase of the voltage, the electroluminescence band shifted to the longer wavelengths corresponding to the green spectrum of the co-oligomer. This emission was concentrated on the needle crystals and suggested that the injected holes were confined inside this low-dimensional structure and recombined with electrons injected from the p-6P layer. © 2002 American Institute of Physics.
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78.60.Fi Electroluminescence
78.66.Qn Polymers; organic compounds
85.60.Jb Light-emitting devices

Feasibility of a semiconductor superlattice oscillator based on quenched domains for the generation of submillimeter waves

R. Scheuerer, E. Schomburg, K. F. Renk, A. Wacker, and E. Schöll

Appl. Phys. Lett. 81, 1515 (2002); http://dx.doi.org/10.1063/1.1500770 (3 pages) | Cited 13 times

Online Publication Date: 9 August 2002

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We discuss the feasibility of a semiconductor superlattice oscillator which exploits the quenching of propagating dipole domains for the generation of submillimeter waves. We studied the dynamics of electrons in a semiconductor superlattice by performing a simulation based on a drift-diffusion model, taking into account feedback from a resonant circuit. The simulation delivers propagating dipole domains which are quenched before they reach the anode. The periodic formation and quenching of domains creates a self-sustained oscillation of the current through the superlattice. The frequency of the oscillation can be more than three times higher than without feedback. We suggest that with already existing superlattices an oscillator working in the quenched domain mode can be realized up to almost 500 GHz. © 2002 American Institute of Physics.
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84.30.Ng Oscillators, pulse generators, and function generators
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
07.57.Hm Infrared, submillimeter wave, microwave, and radiowave sources
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