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20 Oct 2008

Volume 93, Issue 16, Articles (16xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 93, 161101 (2008); http://dx.doi.org/10.1063/1.3000630 (3 pages)

E. Mujagić, L. K. Hoffmann, S. Schartner, M. Nobile, W. Schrenk, M. P. Semtsiv, M. Wienold, W. T. Masselink, and G. Strasser
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Area dependent efficiency of organic solar cells

Dhritiman Gupta, Monojit Bag, and K. S. Narayan

Appl. Phys. Lett. 93, 163301 (2008); http://dx.doi.org/10.1063/1.2998540 (3 pages) | Cited 23 times

Online Publication Date: 20 October 2008

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Efficiency estimations of organic solar cells are observed to be dependent on the dimensions of electrode defining the active area. We address this issue and explore the manner in which efficiency scales in polymer solar cells by studying these devices as a function of electrode area and incident beam size. The increase in efficiency for smaller active areas can be explained by the reduced electrical resistive loss, the enhanced optical effects, and the finite additional fraction of photogenerated carriers in the vicinity of the perimeter defined by the metal electrode
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84.60.Jt Photoelectric conversion
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High-purity white light from a simple single dopant host-guest white organic light-emitting diode architecture

Pavel Anzenbacher, Jr., Victor A. Montes, and Shin-ya Takizawa

Appl. Phys. Lett. 93, 163302 (2008); http://dx.doi.org/10.1063/1.3005424 (3 pages) | Cited 11 times

Online Publication Date: 20 October 2008

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White light with good color properties (color rendering index = 82–87) is generated in a simple organic light-emitting diode comprising an emissive layer, composed of an undoped tris(4-methyl-8-quinolinato)aluminum (Almq3) sublayer and region doped with an orange-red phosphorescent dopant, bis(2-phenyl-1-quinoline)iridium acetylacetonate (Ir(pq)2acac). Electron-hole recombination in a thin spacing Almq3 layer results in blue-green fluorescence, while the formed triplet excitons diffuse to the doped region and are harvested by the dopant to emit orange-red phosphorescence. The combination of blue-green and orange lights results in warm white light. This approach takes advantage of efficient migration of triplet excitons while being less demanding in terms of fabrication and color matching.
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85.60.Jb Light-emitting devices
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
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Polaron recombination in pristine and annealed bulk heterojunction solar cells

C. Deibel, A. Baumann, and V. Dyakonov

Appl. Phys. Lett. 93, 163303 (2008); http://dx.doi.org/10.1063/1.3005593 (3 pages) | Cited 42 times

Online Publication Date: 20 October 2008

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We determined the dominant polaron recombination loss mechanism in pristine and annealed polythiophene:fullerene blend solar cells by applying the photoinduced charge extraction by linearly increasing voltage method in dependence on temperature. In pristine samples, we find a strongly temperature-dependent bimolecular polaron recombination rate, which is reduced as compared to the Langevin theory. For the annealed sample, we observe a polaron decay rate which follows a third order of carrier concentration almost temperature independently.
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84.60.Jt Photoelectric conversion
72.80.Le Polymers; organic compounds (including organic semiconductors)
81.40.Gh Other heat and thermomechanical treatments
61.41.+e Polymers, elastomers, and plastics
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
71.38.-k Polarons and electron-phonon interactions
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Thin-film passivation by atomic layer deposition for organic field-effect transistors

Hayoung Jeon, Kwonwoo Shin, Chanwoo Yang, Chan Eon Park, and Sang-Hee Ko Park

Appl. Phys. Lett. 93, 163304 (2008); http://dx.doi.org/10.1063/1.3000017 (3 pages) | Cited 10 times

Online Publication Date: 21 October 2008

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The thin-film passivation of organic field-effect transistors (OFETs) using AlOx films grown by atomic layer deposition was investigated. A high-quality AlOx passivation layer was deposited on OFETs at 90 °C using trimethylaluminum and water. Despite the low deposition temperature, the 50-nm-thick AlOx passivation layers exhibited a low water-vapor-transmission-rate value of 0.0434 g/m2/day. In addition, the mobility of the AlOx-passivated OFETs was only slightly below that of the unpassivated devices (i.e., within 9%). Unlike unpassivated devices, the electric performance of the passivated OFETs remained almost unchanged after 2 months as a result of the excellent barrier properties of the passivation layer.
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85.30.Tv Field effect devices
81.65.Rv Passivation
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Near infrared organic light-emitting devices based on donor-acceptor-donor oligomers

Yixing Yang, Richard T. Farley, Timothy T. Steckler, Sang-Hyun Eom, John R. Reynolds, Kirk S. Schanze, and Jiangeng Xue

Appl. Phys. Lett. 93, 163305 (2008); http://dx.doi.org/10.1063/1.3006059 (3 pages) | Cited 20 times

Online Publication Date: 21 October 2008

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We report strong and efficient near infrared emission from organic light-emitting devices (OLEDs) based on two donor-acceptor-donor oligomers. These oligomers have fluorescent quantum yields of up to 20% and their energy gap can be tuned by changing the strengths of the donor and acceptor components. Electroluminescence with peak emission wavelengths of 692 and 815 nm were observed from the two oligomers studied here. External quantum efficiencies up to 1.6% and electrical-to-optical power efficiencies up to 7.0 mW/W were achieved in OLEDs based on these near-infrared emitters.
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85.60.Jb Light-emitting devices
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Small molecule sensitizers for near-infrared absorption in polymer bulk heterojunction solar cells

J. Peet, A. B. Tamayo, X.-D. Dang, J. H. Seo, and T.-Q. Nguyen

Appl. Phys. Lett. 93, 163306 (2008); http://dx.doi.org/10.1063/1.3001802 (3 pages) | Cited 32 times

Online Publication Date: 22 October 2008

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A low band gap small molecule chromophore has been incorporated into a polymer/fullerene bulk heterojunction (BHJ) solar cell yielding increased carrier generation in the near infrared and increased overall short circuit current. The use of a small concentration of a soluble oligothiophene with a diketopyrrolopyrrole core can extend the absorption and photocurrent of poly(3-hexyl thiophene):[6,6]-phenyl C71 butyric acid methyl ester solar cells to 800 nm. Photocurrent from the dye embedded within the polymer BHJ is demonstrated, and the use of soluble small molecule sensitizers as a path toward high efficiency solar cells is discussed.
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84.60.Jt Photoelectric conversion
61.48.-c Structure of fullerenes and related hollow and planar molecular structures
61.41.+e Polymers, elastomers, and plastics
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Gain coefficient method for amplified spontaneous emission in thin waveguided film of a conjugated polymer

I. Silvestre, P. W. B. Marques, M. Valadares, and L. A. Cury

Appl. Phys. Lett. 93, 163307 (2008); http://dx.doi.org/10.1063/1.3007983 (3 pages) | Cited 5 times

Online Publication Date: 23 October 2008

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A method based on the unidimensional gain equation has been developed in order to fit the experimental data due to amplification of spontaneous emission in a thin film of conjugated polymer waveguide. The results have confirmed not only a dependence of the gain coefficient on the laser intensity but also on the length of the excitation laser stripe. The results are presented as a function of the average intensity in W/cm2, which is a manner to express the threshold intensity for a direct comparison between different materials, independent of the setup used.
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42.79.Gn Optical waveguides and couplers
42.50.Md Optical transient phenomena: quantum beats, photon echo, free-induction decay, dephasings and revivals, optical nutation, and self-induced transparency
42.70.Jk Polymers and organics
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Stabilization of the work function of indium tin oxide using organic surface modifiers in organic light-emitting diodes

Asha Sharma, Bernard Kippelen, Peter J. Hotchkiss, and Seth R. Marder

Appl. Phys. Lett. 93, 163308 (2008); http://dx.doi.org/10.1063/1.2998599 (3 pages) | Cited 16 times

Online Publication Date: 24 October 2008

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We herein report on the performance and improved stability of organic light-emitting diodes (OLEDs) in which the transparent indium tin oxide (ITO) electrode is modified using organic surface modifiers based on phosphonic acid anchoring groups. In contrast to air plasma treatment, a commonly used technique to increase the work function of ITO, treatment of the ITO surface with a partially fluorinated phosphonic acid results in a comparable change in work function but with a higher stability over time. The resultant lifetime of OLEDs also increased when this phosphonic acid modified ITO was used.
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85.60.Jb Light-emitting devices
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Highly polarized white-light emission from a single copolymer based on fluorene

D. X. Zhu, H. Y. Zhen, H. Ye, and X. Liu

Appl. Phys. Lett. 93, 163309 (2008); http://dx.doi.org/10.1063/1.3009195 (3 pages) | Cited 6 times

Online Publication Date: 24 October 2008

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Polarized white-light emission was realized from a single copolymer with green and red-light-emitting chromophores incorporated into the blue-light-emitting poly(9,9-dioctylfluorene) backbone through intrachain energy transfer. Based on the good thermal stability and nematic liquid crystalline state of the copolymer, the emissive layer spin coated on a rubbed poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) layer was aligned under thermal treatment at a temperature of 180 °C for 5 min. The polarized white-light emission was stable with a Commission Internationale de L’Eclairage coordinates of (0.33, 0.35). A high polarization ratio of up to 24 with a luminance of 419 cd/m2 at voltage of 15 V was achieved.
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85.60.Jb Light-emitting devices
42.79.Kr Display devices, liquid-crystal devices
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