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14 Jul 2008

Volume 93, Issue 2, Articles (02xxxx)

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Appl. Phys. Lett. 93, 023303 (2008); http://dx.doi.org/10.1063/1.2953179 (3 pages)

Takafumi Kawanishi, Takaaki Fujiwara, Megumi Akai-Kasaya, Akira Saito, Masakazu Aono, Junichi Takeya, and Yuji Kuwahara
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A gate dielectric that enables high ambipolar mobilities in polymer light-emitting field-effect transistors

R. C. G. Naber, M. Bird, and H. Sirringhaus

Appl. Phys. Lett. 93, 023301 (2008); http://dx.doi.org/10.1063/1.2957472 (3 pages) | Cited 19 times

Online Publication Date: 14 July 2008

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Polymer light-emitting field-effect transistors (LEFETs) require high and balanced electron and hole mobilities to achieve high current densities. Here we demonstrate a novel gate dielectric for polymer LEFETs that enables mobilities of 0.01 cm2/V s for both electrons and holes. The low-k dielectric polycyclohexylethylene is placed in direct contact with the poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) semiconductor. A second dielectric layer comprising the high-k material poly(vinylidene fluoride-trifluoroethylene) is used to apply a high electric field onto the low-k dielectric layer. The attainable electron-hole recombination current in such optimized polymer LEFETs is measured and the implications for achieving electrically pumped lasing in a LEFET are discussed.
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85.30.Tv Field effect devices
85.60.Jb Light-emitting devices
42.55.Px Semiconductor lasers; laser diodes
77.55.-g Dielectric thin films
77.84.Jd Polymers; organic compounds
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A photoelectron spectroscopy study of tunable charge injection barrier between metal/organic interface

Zheng Xu, Sheng-han Li, Liping Ma, Gang Li, Guanwen Yang, and Yang Yang

Appl. Phys. Lett. 93, 023302 (2008); http://dx.doi.org/10.1063/1.2957979 (3 pages) | Cited 2 times

Online Publication Date: 14 July 2008

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Photoelectron spectroscopy has been used to investigate a tunable charge injection barrier at the metal/organic interface. Results in this study show that the morphology of the Al electrode in the indium tin oxide (ITO)/LiF/Al/pentacene structure plays a critical role. When the sample is biased across ITO and Al electrodes, shifts in the binding energies of certain core-level electrons are observed on the surface of the discontinuous thin Al electrode. In contrast, no such shifts are observed on the thick Al electrode. Further studies indicate that applying a voltage bias changes the energy alignment between the discontinuous thin Al electrode and the pentacene layer deposited on it.
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79.60.Jv Interfaces; heterostructures; nanostructures
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.20.At Surface states, band structure, electron density of states
71.15.Nc Total energy and cohesive energy calculations
68.35.Ct Interface structure and roughness
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High-mobility organic single crystal transistors with submicrometer channels

Takafumi Kawanishi, Takaaki Fujiwara, Megumi Akai-Kasaya, Akira Saito, Masakazu Aono, Junichi Takeya, and Yuji Kuwahara

Appl. Phys. Lett. 93, 023303 (2008); http://dx.doi.org/10.1063/1.2953179 (3 pages) | Cited 4 times

Online Publication Date: 15 July 2008

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We demonstrate high-performance electric-field effects in submicrometer-channel organic transistors with rubrene single crystals. Platinum source and drain electrodes are embedded in silicon dioxide gate insulators to reduce thickness of the dielectrics and to minimize the short-channel effect. The miniaturized devices exhibit typical output characteristics with Ohmic linear region, well-defined current saturation, and on-off ratio of 106. Mobility values are in the range of 0.1–0.3 cm2/V s, which is comparable to those of the best submicrometer organic transistors. Anisotropy in the mobility is detected, indicating that bandlike transport is responsible for the high transistor performance of the short-channel devices.
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85.30.Tv Field effect devices
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
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Highly efficient polarized polymer light-emitting diodes utilizing oriented films of β-phase poly(9,9-dioctylfluorene)

Masahiro Misaki, Masayuki Chikamatsu, Yuji Yoshida, Reiko Azumi, Nobutaka Tanigaki, Kiyoshi Yase, Shuichi Nagamatsu, and Yasukiyo Ueda

Appl. Phys. Lett. 93, 023304 (2008); http://dx.doi.org/10.1063/1.2959073 (3 pages) | Cited 24 times

Online Publication Date: 18 July 2008

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Uniaxially oriented films of β-phase poly(9,9-dioctylfluorene) (PFO) were realized by a friction-transfer technique followed by thermal annealing and vapor treatments. Absorption and photoluminescence (PL) spectra show the characteristics of β-phase: an additional absorption peak at 433 nm and redshifted PL peaks compared with those of the usual nematic (N) phase. We fabricated polarized polymer light-emitting diodes utilizing oriented films of β-phase PFO. Highly polarized β-phase emission with an integrated polarization ratio of 51 was observed from the devices. The efficiency of the devices based on β-phase reached 2.0 cd/A, which is two times higher than that based on N-phase.
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85.60.Jb Light-emitting devices
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Plastic complementary microelectromechanical switches

Tomoyuki Yokota, Shintaro Nakano, Tsuyoshi Sekitani, and Takao Someya

Appl. Phys. Lett. 93, 023305 (2008); http://dx.doi.org/10.1063/1.2959644 (3 pages) | Cited 1 time

Online Publication Date: 18 July 2008

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We have fabricated plastic complementary microelectromechanical switches by using ink-jet printing technologies. Two vertically stacked regular plastic microelectromechanical switches that are complementary to each other realize the function of an inverter. While rectangular voltage waveforms were periodically applied to the control electrodes in the air, the delay times and durability were examined systematically. The frequency response was 50 Hz for an operation voltage of 60 V. When the number of periodic cycles exceeded 106, the changes in the on resistance of the top and bottom switches were 9% and 43%, respectively.
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84.32.Dd Connectors, relays, and switches
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
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Solid state organic laser emission at 970 nm from dye-doped fluorinated-polyimide planar waveguides

Shun Yuyama, Takahiro Nakajima, Kenichi Yamashita, and Kunishige Oe

Appl. Phys. Lett. 93, 023306 (2008); http://dx.doi.org/10.1063/1.2959729 (3 pages) | Cited 15 times

Online Publication Date: 18 July 2008

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We have demonstrated near-infrared amplified spontaneous emission and laser emission from fluorinated-polyimide waveguides doped with an organic dye, 5,6-dichloro-2[8-(p-dimethylaminophenyl)-2,4-neopentylene-1,3,5,7-octatetraenyl]-3-ethylbenzothiazolium perchlorate (LDS950). The planar waveguides doped with 1 wt % of the dye, 10 mm in length and 9.3 μm in thickness, have exhibited amplified spontaneous emission and optical gain around 960 nm under optical pulse pumping. A good optical gain coefficient of 1.4 mm−1 was obtained. Furthermore, by using the cleaved waveguide edges as reflective facets, sharp emission peaks showing laser oscillation at 970 nm were observed under the optical pulse pumping. The lasing threshold was 0.22 mJ/cm2. The lasing wavelength of 970 nm is the longest for solid state dye-doped polymeric lasers.
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42.79.Gn Optical waveguides and couplers
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
42.55.Rz Doped-insulator lasers and other solid state lasers
42.55.Mv Dye lasers
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Integration of amyloid nanowires in organic solar cells

S. Barrau, F. Zhang, A. Herland, W. Mammo, M. R. Andersson, and O. Inganäs

Appl. Phys. Lett. 93, 023307 (2008); http://dx.doi.org/10.1063/1.2949073 (3 pages) | Cited 9 times

Online Publication Date: 18 July 2008

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Amyloid nanowires were incorporated in organic photovoltaic devices in order to enhance the transport properties. Amyloid fibrils act as a template for donor-acceptor materials. The current-voltage characteristics under illumination and in the dark display a maximum for the fill factor and the space charge limit current, respectively, at an amyloid nanowire-donor-acceptor mass ratio of 0.014:1:1, associated to a better charge transport in the donor-acceptor domains. The absorption experiments display a redshift associated to a more planar polymer backbone with increasing concentration of amyloid fibrils. Amyloid nanowires present a significant effect on the donor-acceptor materials organization.
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84.60.Jt Photoelectric conversion
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The role of the β-phase content on the stimulated emission of poly(9,9-dioctylfluorene) thin films

M. Anni

Appl. Phys. Lett. 93, 023308 (2008); http://dx.doi.org/10.1063/1.2957669 (3 pages) | Cited 6 times

Online Publication Date: 18 July 2008

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We investigated the optical gain properties of poly(9,9-dioctylfluorene) thin films as a function of the β-phase content. We demonstrate that the product between the gain cross section and the excited state lifetime of the β-phase is about 3.2 times larger than the glassy-phase one, indicating that the β-phase molecules, for a given pump density, have higher gain than the glassy-phase ones. The dependence of the amplified spontaneous emission threshold on the molecular properties, on the waveguide losses, and on the β-phase content is also quantitatively discussed.
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78.45.+h Stimulated emission
78.66.Qn Polymers; organic compounds
71.10.Li Excited states and pairing interactions in model systems
61.41.+e Polymers, elastomers, and plastics
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Reduced efficiency roll-off in phosphorescent organic light emitting diodes at ultrahigh current densities by suppression of triplet-polaron quenching

F. X. Zang, T. C. Sum, A. C. H. Huan, T. L. Li, W. L. Li, and Furong Zhu

Appl. Phys. Lett. 93, 023309 (2008); http://dx.doi.org/10.1063/1.2955527 (3 pages) | Cited 18 times

Online Publication Date: 18 July 2008

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High-performance phosphorescent organic light emitting devices with reduced efficiency roll-off at ultrahigh current densities were realized. The devices were Ir(ppy)3-based phosphorescent organic light emitting diodes that employed 1,3-bis[2-(2,2′-bipyridine-6-yl)-1,3,4-oxadiazo-5-yl]benzene as a high mobility electron transfer layer. The device’s brightness was enhanced while the efficiency roll-off was reduced. The device exhibits high current efficiency (21 cd/A) at high brightness (80 000 cd/m2), with a maximum luminescence of 136 000 cd/m2 at over 1 A/m2 (with an efficiency of 13 cd/A). This reduction in efficiency roll-off is attributed to the suppression of the triplet-polaron quenching rate through balancing the charge carrier ratio in the device.
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85.60.Jb Light-emitting devices
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Organic phototransistor based on poly(3-hexylthiophene)/TiO2 nanoparticle composite

Sheung Man Mok, Feng Yan, and Helen L. W. Chan

Appl. Phys. Lett. 93, 023310 (2008); http://dx.doi.org/10.1063/1.2957981 (3 pages) | Cited 32 times

Online Publication Date: 18 July 2008

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Light sensitive phototransistor based on the composite of poly(3-hexylthiophene) and TiO2 nanoparticles has been developed. The device shows a quick change in channel current under light exposure, which can be attributed to a positive shift of the threshold voltage, while no change in the field effect mobility and off current can be observed. The shift of the threshold voltage is induced by accumulated electrons trapped by the TiO2 nanoparticles in the channel. The photosensitivity of the device has been found to be dependent on the concentration of TiO2 nanoparticles, the incident wavelength and the voltage between the source and drain.
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85.60.Dw Photodiodes; phototransistors; photoresistors
85.30.Tv Field effect devices
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