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8 Sep 2008

Volume 93, Issue 10, Articles (10xxxx)

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Appl. Phys. Lett. 93, 101905 (2008); http://dx.doi.org/10.1063/1.2977760 (3 pages)

Mei Wang, Yinwei Li, Tian Cui, Yanming Ma, and Guangtian Zou
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Low cost patterning of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) films to increase organic photovoltaic device efficiency

J. B. Emah, R. J. Curry, and S. R. P. Silva

Appl. Phys. Lett. 93, 103301 (2008); http://dx.doi.org/10.1063/1.2973342 (3 pages) | Cited 15 times

Online Publication Date: 8 September 2008

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A low-cost lithographic technique to pattern poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) films with 10 nm deep features of 700 nm periodicity is demonstrated. The use of these patterned films in poly(3-hexylthiophene):[6,6]-phenylC61-butyric acid methyl ester organic photovoltaic devices leads to an increase in short circuit current (Jsc), fill factor, and power conversion efficiency (PCE) with only a slight reduction in open circuit voltage. Patterning the PEDOT:PSS at 150 °C increases Jsc from 2.44 to 3.03 mA/cm2 improving the PCE from 0.63% to 0.81% with similar increases due to patterning also being obtained at other temperatures.
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73.61.Ph Polymers; organic compounds
81.65.-b Surface treatments
85.60.-q Optoelectronic devices
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Solvent-dependent electrical characteristics and stability of organic thin-film transistors with drop cast bis(triisopropylsilylethynyl) pentacene

Chang Su Kim, Stephanie Lee, Enrique D. Gomez, John E. Anthony, and Yueh-Lin Loo

Appl. Phys. Lett. 93, 103302 (2008); http://dx.doi.org/10.1063/1.2979691 (3 pages) | Cited 26 times

Online Publication Date: 8 September 2008

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The solvent from which the active layer is drop cast dramatically influences the electrical characteristics and electrical stability of thin-film transistors comprising bis(triisopropylsilylethynyl) pentacene. Casting from high boiling solvents allows slower crystallization; devices cast from toluene and chlorobenzene thus exhibit mobilities >0.1 cm2/V s and on/off ratios of ∼ 106. More importantly, the solvent choice influences the device stability. Devices from toluene exhibit stable characteristics, whereas devices from chlorobenzene show hystereses on cycling, with dramatic threshold voltage shifts toward positive voltages. The instability in chlorobenzene devices is attributed to the migration of water and solvent impurities to the charge transport interface on repetitive testing.
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73.61.Le Other inorganic semiconductors
71.20.Rv Polymers and organic compounds
81.05.Hd Other semiconductors
72.80.Le Polymers; organic compounds (including organic semiconductors)
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Modal analysis of a multimode polarization-maintaining plastic optical fiber fabricated using poly(methyl methacrylate/benzyl methacrylate) copolymer

Rei A. Furukawa, Akihiro Tagaya, and Yasuhiro Koike

Appl. Phys. Lett. 93, 103303 (2008); http://dx.doi.org/10.1063/1.2979705 (3 pages) | Cited 1 time

Online Publication Date: 8 September 2008

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We report an experimental demonstration of a multimode polarization-maintaining fiber that has extinction ratio (ER) of 14.0 dB at 1 m propagation, and this value was compared to those measured from different kinds of commercially available multimode fibers. The sample fiber was fabricated using poly(methyl methacrylate/benzyl methacrylate), which we named as polarization-maintaining graded-index plastic optical fiber. A measurement setup was developed in order to measure the ER in the cases when different points of the fiber core was launched. Birefringence and the output near field pattern for each launching points have exhibited a clear correlation to the measured ER.l
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42.70.Jk Polymers and organics
42.81.Gs Birefringence, polarization
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Highly efficient tandem p-i-n organic light-emitting diodes adopting a low temperature evaporated rhenium oxide interconnecting layer

Dong-Seok Leem, Jae-Hyun Lee, Jang-Joo Kim, and Jae-Wook Kang

Appl. Phys. Lett. 93, 103304 (2008); http://dx.doi.org/10.1063/1.2979706 (3 pages) | Cited 23 times

Online Publication Date: 8 September 2008

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High quality interconnection units (ICUs) with a high transparency and superior charge generating capability for tandem organic light-emitting diodes (OLEDs) are developed. The ICUs of rubidium carbonate-doped 4,7-diphenyl-1,10-phenanthroline/rhenium oxide (ReO3)-doped N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4′-diamine layers with or without an additional ReO3 interlayer produce high transmittance (88%–92% at 420–700 nm) and spontaneous internal charge generation properties. A very high efficiency of ∼ 129 cd/A has been demonstrated from only two stacked green p-i-n OLEDs by employing the developed ICUs. The relationship between the device efficiency and internal charge generation within the ICUs is further described by means of the capacitance measurements.
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85.60.Jb Light-emitting devices
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Enhancement of electron injection into a light-emitting polymer from an aluminum oxide cathode modified by a self-assembled monolayer

Yana Vaynzof, T. Joseph Dennes, Jeffrey Schwartz, and Antoine Kahn

Appl. Phys. Lett. 93, 103305 (2008); http://dx.doi.org/10.1063/1.2980425 (3 pages) | Cited 12 times

Online Publication Date: 8 September 2008

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A self-assembled monolayer (SAM) of octylphosphonate was deposited on an AlOx electrode using the tethering by aggregation and growth (T-BAG) procedure. Ultraviolet photoemission spectroscopy (UPS) measurements showed a decrease in the substrate work function from 3.8 to 3.3 eV. Poly[9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-diphenylamine] (TFB) films spin coated on the bare and the SAM-modified oxide surfaces were investigated by UPS. A shift in molecular levels, corresponding to a reduction in the electron injection barrier, was observed for the SAM-modified electrode. This barrier lowering was confirmed by current-voltage measurements showing a corresponding increase in electron current through the TFB/SAM/AlOx device.
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73.61.Ph Polymers; organic compounds
85.60.Jb Light-emitting devices
73.30.+y Surface double layers, Schottky barriers, and work functions
79.60.Jv Interfaces; heterostructures; nanostructures
73.50.Dn Low-field transport and mobility; piezoresistance
68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces
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The impact of water vapor transmission rate on the lifetime of flexible polymer solar cells

Jens A. Hauch, Pavel Schilinsky, Stelios A. Choulis, Sambatra Rajoelson, and Christoph J. Brabec

Appl. Phys. Lett. 93, 103306 (2008); http://dx.doi.org/10.1063/1.2975185 (3 pages) | Cited 32 times

Online Publication Date: 9 September 2008

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In this paper we perform accelerated lifetime testing on high efficiency flexible poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) solar cells encapsulated with food package quality barrier films with a water vapor transmission rate of 0.2 g/(m2 day) at 65 °C/85% relative humidity. We show that lifetimes exceeding 1250 h, even at high temperature/high humidity conditions, may be reached, proving that organic solar cells are significantly less sensitive against the environmental effects of water and oxygen than previously expected.
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84.60.Jt Photoelectric conversion
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Isotype heterojunction between organic crystalline semiconductors

Haibo Wang, Xiujin Wang, Haichao Huang, and Donghang Yan

Appl. Phys. Lett. 93, 103307 (2008); http://dx.doi.org/10.1063/1.2979707 (3 pages) | Cited 9 times

Online Publication Date: 9 September 2008

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We report the effect of n-n isotype organic heterojunction consisting of copper hexadecafluorophthalocyanine (F16CuPc) and phthalocyanatotin(IV) dichloride (SnCl2Pc). Their interfacial electronic structure was observed by Kelvin probe force microscopy (KPFM), and there is band bending in two materials, resulting in an electron accumulation region in F16CuPc layer and an electron depletion region in SnCl2Pc layer. The forming of organic heterojunction was explained by carriers flowing through the interface due to thermal emission of electrons. Furthermore, the carrier transport behavior parallel and vertical to heterojunction interface was also revealed by their heterojunction field-effect transistor with normally on operation mode and heterojunction diodes with rectifying property.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.40.Ei Rectification
73.20.At Surface states, band structure, electron density of states
85.30.Kk Junction diodes
85.30.Tv Field effect devices
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Charge transport in accumulation layers of organic heterojunctions

Feng Zhu, Haibo Wang, De Song, Kun Lou, and Donghang Yan

Appl. Phys. Lett. 93, 103308 (2008); http://dx.doi.org/10.1063/1.2980023 (3 pages) | Cited 13 times

Online Publication Date: 9 September 2008

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We studied the charge transport in organic heterojunction films consisting of copper phthalocyanine (CuPc) and copper hexadecafluorophthalocyanine (F16CuPc). The heterojunction effect between CuPc and F16CuPc induced high-density carriers at both sides of heterojunction. The Hall effect was observed at room temperature, which demonstrated the existence of free carriers and their delocalized transport under heterojunction effect. The Hall mobility of 1.2 cm2/V s for holes and 2.4 cm2/V s for electrons indicated that the transport capability of the heterojunction films is comparable to single crystals. The transport process was further explained by the multiple trap-and-release model according to the temperature dependence of conduction.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
72.20.Ee Mobility edges; hopping transport
73.61.Ph Polymers; organic compounds
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High response organic ultraviolet photodetector based on blend of 4,4′,4″-tri-(2-methylphenyl phenylamino) triphenylaine and tris-(8-hydroxyquinoline) gallium

Zisheng Su, Wenlian Li, Bei Chu, Tianle Li, Jianzhuo Zhu, Guang Zhang, Fei Yan, Xiao Li, Yiren Chen, and Chun-Sing Lee

Appl. Phys. Lett. 93, 103309 (2008); http://dx.doi.org/10.1063/1.2980025 (3 pages) | Cited 17 times

Online Publication Date: 9 September 2008

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The authors demonstrate high response organic ultraviolet (UV) photodetector (PD) using 4,4′,4″-tri-(2-methylphenyl phenylamino) triphenylaine (m-MTDATA) and tris-(8-hydroxyquinoline) gallium (Gaq3) to act as the electron donor and acceptor, respectively. The m-MTDATA:Gaq3 blend device shows a photocurrent of 405 μA/cm−2 at −8 V, corresponding to a response of 338 mA/W under an illumination of 365 nm UV light with an intensity of 1.2 mW/cm2. The high response is attributed to the enhanced dissociation of geminate hole-electron pairs in the distributed heterojunction of the blend and suppression of radiative decay. Photophysics of the PD involved is also discussed in terms of the performance and device structures.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
42.79.Pw Imaging detectors and sensors
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Improved air stability of n-channel organic thin-film transistors with surface modification on gate dielectrics

Fang-Chung Chen and Cheng-Hsiang Liao

Appl. Phys. Lett. 93, 103310 (2008); http://dx.doi.org/10.1063/1.2980421 (3 pages) | Cited 29 times

Online Publication Date: 9 September 2008

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The air stability of n-channel organic thin film transistors based on N,N′-dioctyl-3,4,9,10-perylene tetracarboxylic diimide is improved when modifying the dielectric surfaces with polymer insulators. The hydrophobic nature of the polymer surface inhibits protonation of the siloxyl groups on the SiO2 surface, leading to fewer SiO− groups that can behave as electron traps. Among the polymer insulators tested, the devices modified with hydroxyl-free polymers exhibited the best air stabilities.
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85.30.Tv Field effect devices
68.47.Mn Polymer surfaces
68.47.Gh Oxide surfaces
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Improved light harvesting in tin-doped indum oxide (ITO)-free inverted bulk-heterojunction organic solar cells using capping layers

J. Meiss, N. Allinger, M. K. Riede, and K. Leo

Appl. Phys. Lett. 93, 103311 (2008); http://dx.doi.org/10.1063/1.2981525 (3 pages) | Cited 23 times

Online Publication Date: 11 September 2008

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We show that ultrathin metal layers (Ag or Al/Ag) are feasible as transparent top contacts for zinc phthalocyanine: C60 bulk-heterojunction inverted organic solar cells thermally evaporated on glass substrates. Furthermore, it is demonstrated that the introduction of an organic capping layer drastically increases light incoupling and photon harvesting, in accordance with optical simulations. Proof of principle tin-doped indium oxide (ITO)-free solar cells employing a transparent metal contact and a capping layer reach efficiencies of 1.06%, compared to 0.69% without addition of the capping layer.
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84.60.Jt Photoelectric conversion
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