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10 Mar 2008

Volume 92, Issue 10, Articles (10xxxx)

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Appl. Phys. Lett. 92, 102101 (2008); http://dx.doi.org/10.1063/1.2890735 (3 pages)

Qing Wan, Jin Huang, Zhong Xie, Taihong Wang, Eric N. Dattoli, and Wei Lu
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Polymeric tandem organic light-emitting diodes using a self-organized interfacial layer

Seung Yoon Ryu, Jong Tae Kim, Joo Hyon Noh, Byoung Har Hwang, Chang Su Kim, Sung Jin Jo, Hyeon Seok Hwang, Seok Ju Kang, Hong Koo Baik, Chang Ho Lee, Seung Yong Song, and Se Jong Lee

Appl. Phys. Lett. 92, 103301 (2008); http://dx.doi.org/10.1063/1.2894072 (3 pages) | Cited 6 times

Online Publication Date: 10 March 2008

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The authors have demonstrated efficient polymeric tandem organic light-emitting diodes (OLEDs) with a self-organized interfacial layer, which was formed by differences in chemical surface energy. Hydrophilic poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT:PSS) was spin coated onto the hydrophobic poly(9,9-dyoctilfluorene) (PFO) surface and a PEDOT:PSS bubble or dome was built as an interfacial layer. The barrier heights of PEDOT:PSS and PFO in the two-unit tandem OLED induced a charge accumulation at the interface in the heterojunction and thereby created exciton recombination at a much higher level than in the one-unit reference. This effect was confirmed in both the hole only and the electron only devices.
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85.60.Jb Light-emitting devices
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
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Drift mobility and the frequency response of diode connected organic transistors

Brian Cobb, Yeon Taek Jeong, and Ananth Dodabalapur

Appl. Phys. Lett. 92, 103302 (2008); http://dx.doi.org/10.1063/1.2891877 (3 pages) | Cited 5 times

Online Publication Date: 10 March 2008

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A method to characterize the frequency response of an organic field effect transistor (FET) is presented. Analysis then shows a method to calculate the average drift mobility from the frequency at which a pole appears in the response. This pole is believed to appear at the point where charge carriers can no longer fully traverse the channel in one period of the input signal. The dc output characteristics of the device are also described, and saturation mobility values are derived. This saturation mobility and the drift mobility calculated from the frequency response are comparable. This method can be used in determining the drift mobility in other materials such as single nanowires in the FET configuration.
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85.30.Tv Field effect devices
85.35.-p Nanoelectronic devices
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Evidence for erbium-erbium energy migration in erbium(III) bis(perfluoro-p-tolyl)phosphinate

R. H. C. Tan, J. M. Pearson, Y. Zheng, P. B. Wyatt, and W. P. Gillin

Appl. Phys. Lett. 92, 103303 (2008); http://dx.doi.org/10.1063/1.2896105 (3 pages) | Cited 3 times

Online Publication Date: 10 March 2008

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Photoluminescence lifetime measurements of the erbium decay in samples of erbium(III) bis(perfluoro-p-tolyl)phosphinate, Er[(p-CF3C6F4)2PO2]3, as a function of erbium concentration have been made. It has been found that for concentrations greater than 50%, the lifetime could be fitted with a stretched exponential function, which is indicative of erbium-erbium energy migration to quenching sites, most likely at the surface of the finely divided powder. At concentrations below 50%, the decay was single exponential with a maximum lifetime of ∼ 700 μs.
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78.55.Kz Solid organic materials
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Surface plasmon coupled electroluminescent emission

D. M. Koller, A. Hohenau, H. Ditlbacher, N. Galler, F. R. Aussenegg, A. Leitner, J. R. Krenn, S. Eder, S. Sax, and E. J. W. List

Appl. Phys. Lett. 92, 103304 (2008); http://dx.doi.org/10.1063/1.2884268 (3 pages) | Cited 10 times

Online Publication Date: 10 March 2008

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Besides directly emitting light, electroluminescence in organic light emitting diodes (OLEDs) can excite surface plasmons (SPs) on the metal electrodes of the device. By designing a microcavity OLED with thin film metal electrodes, we can directly probe a SP mode by leaky wave extraction with a high refractive index glass prism coupler. Additional angle and polarization resolved reflection measurements on the OLED multilayer structure together with transfer matrix calculations allow us to unequivocally characterize the electroluminescent-driven SP mode.
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85.60.Jb Light-emitting devices
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Mixed ligands 8-hydroxyquinoline aluminum complex with high electron mobility for organic light-emitting diodes

Bingshe Xu, Liuqing Chen, Xuguang Liu, Hefeng Zhou, Huixia Xu, Xiaohong Fang, and Yanli Wang

Appl. Phys. Lett. 92, 103305 (2008); http://dx.doi.org/10.1063/1.2885708 (3 pages) | Cited 1 time

Online Publication Date: 10 March 2008

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A mixed ligands 8-hydroxyquinoline complex (Alq2A) with improved electron mobility was designed for organic light-emitting diodes. The electron mobility in Alq2A was determined via transient electroluminescence (EL) from bilayer devices with structure of indium tin oxide/4,4′-bis[N-(1-naphthyl)-N-Phenyl amino]biphenyl/Alq2A/LiF/Al. It was found that the electron mobility in Alq2A is between (2.7 and 4.4)×10−6 cm2/V s at electric fields ranging between 1.42×106 and 2.40×106 V/cm, which is higher than that in Alq3. The Alq2A also shows a higher EL efficiency in steady-state EL studies, which is considered to be derived from (1) improved electron mobility, (2) high fluorescene efficiency, and (3) good film-forming.
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72.20.Fr Low-field transport and mobility; piezoresistance
85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
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Control of the nanoscale crystallinity and phase separation in polymer solar cells

Chih-Wei Chu, Hoichang Yang, Wei-Jen Hou, Jinsong Huang, Gang Li, and Yang Yang

Appl. Phys. Lett. 92, 103306 (2008); http://dx.doi.org/10.1063/1.2891884 (3 pages) | Cited 14 times

Online Publication Date: 11 March 2008

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Grazing-incidence x-ray diffraction and atomic force microscopy were performed on bulk heterojunction regioregular poly(3-hexylthiophene) (RR-P3HT) [6,6]-phenyl-C71-butyric acid methyl esters spin-cast films with different film processing conditions to correlate the crystalline nanostructure of P3HT with the corresponding solar cell performance. The increase in long wavelength absorption for solvent annealed films is related to highly conjugated crystal structure of RR-P3HT phase-separated in the active layer. Upon thermal annealing, the solvent annealed 50-nm-thick device shows high solar cell performance with fill factor up to 73% and power conversion efficiency of 3.80%.
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84.60.Jt Photoelectric conversion
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Ferroelectric nanoparticles in low refractive index liquid crystals for strong electro-optic response

M. Kaczmarek, O. Buchnev, and I. Nandhakumar

Appl. Phys. Lett. 92, 103307 (2008); http://dx.doi.org/10.1063/1.2884186 (3 pages) | Cited 9 times

Online Publication Date: 11 March 2008

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Functional materials based on ferroelectric, inorganic nanoparticles, and low refractive index nematic liquid crystals show strong induced birefringence and dielectric anisotropy. Birefringence can increase by a factor of 2 and dielectric anisotropy by a factor of 3 as compared with nominally pure liquid crystals. The enhancement of the electro-optic performance is higher in liquid crystals with Sn2P2S6 (SPS) nanoparticles than with BaTiO3 nanoparticles. The shape and size distribution of both types of ferroelectric particles were characterized using atomic force microscopy. The average size of SPS nanoparticles was 45 nm and of BaTiO3 nanoparticles was 20 nm.
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77.80.-e Ferroelectricity and antiferroelectricity
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures
78.20.Fm Birefringence
78.20.Jq Electro-optical effects
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Polyfluorene-based light-emitting diodes with an azide photocross-linked poly(3,4-ethylene dioxythiophene):(polystyrene sulfonic acid) hole-injecting layer

Gustaf Winroth, Gianluca Latini, Dan Credgington, Loke-Yuen Wong, Lay-Lay Chua, Peter K.-H. Ho, and Franco Cacialli

Appl. Phys. Lett. 92, 103308 (2008); http://dx.doi.org/10.1063/1.2892685 (3 pages) | Cited 17 times

Online Publication Date: 11 March 2008

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We used a water-soluble bis(fluorinated phenyl azide) to cross-link a poly(ethylene dioxythiophene):poly(styrene sulphonic acid) (PEDOT:PSS), hole-injection layer, with a view to its future use with water-soluble emitters. To enable direct comparison with conventional PEDOT:PSS, we studied the cross-linked films in diodes incorporating the organic-solvent soluble polymer poly(9,9′-dioctylfluorene-alt-benzothiadiazole). Kelvin probe characterization of the PEDOT:PSS and electroabsorption measurements of the devices consistently show a 0.2 eV increase of the PEDOT:PSS work function upon cross-linking. We also observe a 70-fold reduction in resistivity, an increase of the current above threshold and a decrease of the “leakage” current below threshold.
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85.60.Jb Light-emitting devices
78.20.Jq Electro-optical effects
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A single-layer permeation barrier for organic light-emitting displays

Prashant Mandlik, Jonathan Gartside, Lin Han, I-Chun Cheng, Sigurd Wagner, Jeff A. Silvernail, Rui-Qing Ma, Michael Hack, and Julie J. Brown

Appl. Phys. Lett. 92, 103309 (2008); http://dx.doi.org/10.1063/1.2890432 (3 pages) | Cited 12 times

Online Publication Date: 11 March 2008

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Films of a hybrid material with part-SiO2 part-silicone character are deposited as environmental barriers on bottom-emitting and on transparent organic light-emitting diodes. Devices coated with this barrier have lifetimes of up to ∼ 7500 h when stored at 65 °C and 85% relative humidity, by far exceeding the industrial requirement of 1000 h. The intensity of the Si–O–Si absorption at the wavenumber of 1075 cm−1, the wetting angle by water, and the indentation hardness support the interpretation of a homogeneous material with the properties of a SiO2-silicone hybrid. The films remain intact over 58 600 cycles of bending to ∼ 0.2% tensile strain.
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85.60.Jb Light-emitting devices
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Effects of substrates on photocurrents from photosensitive polymer coated carbon nanotube networks

Yumeng Shi, Hosea Tantang, Chun Wei Lee, Cheng-Hui Weng, Xiaochen Dong, Lain-Jong Li, and Peng Chen

Appl. Phys. Lett. 92, 103310 (2008); http://dx.doi.org/10.1063/1.2894503 (3 pages) | Cited 3 times

Online Publication Date: 11 March 2008

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We investigated the substarte (SiO2 and quartz) effects on the photoresponsitivity of photosensitive polymer coated single-walled carbon nanotube (SWNT) networks. As charge trapping on quartz surface is believed to be much less significant than that observed on conventional SiO2 dielectrics, using quartz as the supporting substrate allows us to probe the naïve photointeraction between the photosensitive polymers and SWNTs. The two-terminal SWNT resistor devices on quartz provide an effective yet simple methodology to discriminate electron- or hole-donating photosensitive polymers.
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78.67.Ch Nanotubes
73.61.Wp Fullerenes and related materials
72.40.+w Photoconduction and photovoltaic effects
42.70.Jk Polymers and organics
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Effect of electric-field-assisted thermal annealing of poly(4-vinylphenol) film on its dielectric constant

Ji Hoon Park and Eugene Kim

Appl. Phys. Lett. 92, 103311 (2008); http://dx.doi.org/10.1063/1.2896603 (3 pages)

Online Publication Date: 12 March 2008

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We investigated the effect of annealing a polymeric dielectric film in the presence of an oscillating electric field on its dielectric constant. Films were prepared with the vertical structure of electrode/poly(4-vinylphenol)/electrode and annealed at a temperature above the glass transition temperature while applying an ac electric field ranging from 0 to 6 V at various frequencies in the hope of altering the arrangement of polymer chains through conformational transitions influenced by the orientational polarization of the polar phenol groups. It was found that the dielectric constant increased significantly by about 30% when the annealing frequency corresponded to that at which dielectric relaxation takes place effectively.
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77.55.-g Dielectric thin films
77.22.Ch Permittivity (dielectric function)
77.22.Gm Dielectric loss and relaxation
81.40.Gh Other heat and thermomechanical treatments
64.70.pj Polymers
77.22.Ej Polarization and depolarization
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Determining the interfacial density of states in metal-insulator-semiconductor devices based on poly(3-hexylthiophene)

N. Alves and D. M. Taylor

Appl. Phys. Lett. 92, 103312 (2008); http://dx.doi.org/10.1063/1.2897238 (3 pages) | Cited 3 times

Online Publication Date: 13 March 2008

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Low frequency admittance measurements are used to determine the density of interface states in metal-insulator-semiconductor diodes based on the unintentionally doped, p-type semiconductor poly(3-hexylthiophene). After vacuum annealing at 90 °C, interface hole trapping states are shown to be distributed in energy with their density decreasing approximately linearly from ∼ 20×1010 to 5×1010 cm−2 eV−1 over an energy range extending from 0.05 to 0.25 eV above the bulk Fermi level.
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85.30.Tv Field effect devices
85.30.Kk Junction diodes
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High-mobility, low-power, and fast-switching organic field-effect transistors with ionic liquids

S. Ono, S. Seki, R. Hirahara, Y. Tominari, and J. Takeya

Appl. Phys. Lett. 92, 103313 (2008); http://dx.doi.org/10.1063/1.2898203 (3 pages) | Cited 53 times

Online Publication Date: 13 March 2008

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We report high-mobility rubrene single-crystal field-effect transistors with ionic-liquid (IL) electrolytes used for gate dielectric layers. As the result of fast ionic diffusion to form electric double layers, their capacitances remain more than 1 μF/cm2 even at 0.1 MHz. With high carrier mobility of 1.2 cm2/V s in the rubrene crystal, pronounced current amplification is achieved at the gate voltage of only 0.2 V, which is two orders of magnitude smaller than that necessary for organic thin-film transistors with dielectric gate insulators. The results demonstrate that the IL/organic semiconductor interfaces are suited to realize low-power and fast-switching field-effect transistors without sacrificing carrier mobility in forming the solid/liquid interfaces.
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85.30.Tv Field effect devices
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Gold nanoparticle-pentacene memory transistors

Christophe Novembre, David Guérin, Kamal Lmimouni, Christian Gamrat, and Dominique Vuillaume

Appl. Phys. Lett. 92, 103314 (2008); http://dx.doi.org/10.1063/1.2896602 (3 pages) | Cited 14 times

Online Publication Date: 14 March 2008

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We demonstrate an organic memory-transistor device based on a pentacene-gold nanoparticles active layer. Gold (Au) nanoparticles are immobilized on the gate dielectric (silicon dioxide) of a pentacene transistor by an amino-terminated self-assembled monolayer. Under the application of writing and erasing pulses on the gate, large threshold voltage shift (22 V) and on/off drain current ratio of ∼ 3×104 are obtained. The hole field-effect mobility of the transistor is similar in the on and off states (less than a factor of 2). Charge retention times up to 4500 s are observed. The memory effect is mainly attributed to the Au nanoparticles.
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85.30.Tv Field effect devices
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Carrier trapping and scattering in amorphous organic hole transporter

K. K. Tsung and S. K. So

Appl. Phys. Lett. 92, 103315 (2008); http://dx.doi.org/10.1063/1.2894013 (3 pages) | Cited 5 times

Online Publication Date: 14 March 2008

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The effects of dopants on the hole transporting properties of N,N′-diphenyl-N,N′-bis(1-naphthyl)(1,1’-biphenyl)-4,4’diamine (NPB) have been studied by time of flight. Five dopants: copper phthalocyanine (CuPc), 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyrle)-4H-pyran (DCM1), 4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetra-hydro-1H,5H-benzo[ij] quinolizin-8-yl)vinyl]-4H-pyran (DCM2), 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (tBu-PBD), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) are used in this study. The dopant molecules behave like hole traps or scatterers. Their detailed behaviors are determined by their highest occupied molecular orbital relative to that of NPB. Generally, traps are found to induce significant reduction in hole mobility while there is a slight reduction for scattering. Two different underlying charge transport mechanisms are proposed.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.20.Fr Low-field transport and mobility; piezoresistance
72.80.Le Polymers; organic compounds (including organic semiconductors)
61.72.up Other materials
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Solvent mixtures for improving device efficiency of polymer photovoltaic devices

Fang-Chung Chen, Hsin-Chen Tseng, and Chu-Jung Ko

Appl. Phys. Lett. 92, 103316 (2008); http://dx.doi.org/10.1063/1.2898153 (3 pages) | Cited 35 times

Online Publication Date: 14 March 2008

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In this work, we used solvent mixtures, consisting of 1-chloronaphthalene (Cl-naph), one solvent with a high boiling point, and o-dichlorobenzene, to prepare the polymer films for polymer photovoltaic devices. Because of the lower vapor pressure of the solvent mixtures, the polymer films dried slower. With higher Cl-naph concentration in the organic solvent, the polymer chains had longer time to self-organize themselves. As a result, the higher degree of crystalline led to lower device series resistance, thereby increasing the performance of the photovoltaic devices.
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84.60.Jt Photoelectric conversion
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