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2 Nov 2009

Volume 95, Issue 18, Articles (18xxxx)

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Appl. Phys. Lett. 95, 183504 (2009); http://dx.doi.org/10.1063/1.3256223 (3 pages)

Hoon-Sik Kim, Sang Min Won, Young-Geun Ha, Jong-Hyun Ahn, Antonio Facchetti, Tobin J. Marks, and John A. Rogers
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Oxidation of silver electrodes induces transition from conventional to inverted photovoltaic characteristics in polymer solar cells

Jong Bok Kim, Chang Su Kim, Youn Sang Kim, and Yueh-Lin Loo

Appl. Phys. Lett. 95, 183301 (2009); http://dx.doi.org/10.1063/1.3257361 (3 pages) | Cited 23 times

Online Publication Date: 3 November 2009

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Oxidation of silver top electrodes in polymer solar cells induces the devices to exhibit a transition from conventional to inverted photovoltaic characteristics. As silver oxidizes, its work function increases from 4.3 to 5.0 eV. The silver top electrode thus reverts from collecting electrons to collecting holes during device operation. We have quantified the extent of this transformation and find it to be correlated with the rate of silver oxidation, which we can influence by altering the annealing sequence of the polymer photoactive layer, ultimately varying the grain size of silver in the top electrode.
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72.40.+w Photoconduction and photovoltaic effects
73.30.+y Surface double layers, Schottky barriers, and work functions
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
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Low voltage operation in picene thin film field-effect transistor and its physical characteristics

Yumiko Kaji, Naoko Kawasaki, Xuesong Lee, Hideki Okamoto, Yasuyuki Sugawara, Shohei Oikawa, Akio Ito, Hiroyuki Okazaki, Takayoshi Yokoya, Akihiko Fujiwara, and Yoshihiro Kubozono

Appl. Phys. Lett. 95, 183302 (2009); http://dx.doi.org/10.1063/1.3257373 (3 pages) | Cited 12 times

Online Publication Date: 3 November 2009

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Low voltage operation of picene thin film field-effect transistor (FET) has been realized with 40 nm thick SiO2 gate dielectrics coated by two polymers, Cytop™ and polystyrene. The picene FETs operated in low absolute gate voltage |VG| below 15 V for Cytop™ coated SiO2 and 30 V for polystyrene coated SiO2 gate dielectrics, and they showed a significant O2 gas sensing effect down to ∼ 10 ppm. Photoemission spectrum clarified that O2 molecules penetrate into the thin films at O2/picene mole ratio of 1: 1. X-ray diffraction pattern of picene thin films showed highly oriented growth on the polymer-coated SiO2.
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85.30.Tv Field effect devices
68.55.-a Thin film structure and morphology
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
79.60.Dp Adsorbed layers and thin films
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Low-density band-gap states in pentacene thin films probed with ultrahigh-sensitivity ultraviolet photoelectron spectroscopy

Tomoki Sueyoshi, Hirohiko Fukagawa, Masaki Ono, Satoshi Kera, and Nobuo Ueno

Appl. Phys. Lett. 95, 183303 (2009); http://dx.doi.org/10.1063/1.3258351 (3 pages) | Cited 18 times

Online Publication Date: 3 November 2009

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We detected a very low density of electronic states in the band gap of a nondoped pentacene thin film by using ultraviolet photoelectron spectroscopy with ultrahigh sensitivity and ultralow background. The gap states, which may originate from the highest occupied molecular orbital (HOMO) state in imperfect molecular packing regions, are distributed exponentially up to the Fermi level and control the Fermi level relative to the HOMO band.
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71.20.Rv Polymers and organic compounds
79.60.Bm Clean metal, semiconductor, and insulator surfaces
79.60.Fr Polymers; organic compounds
68.55.ag Semiconductors
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Optical confirmation of biaxial nematic (Nb) phase in a bent-core mesogen

Yun Jang, Vitaly P. Panov, Antoni Kocot, J. K. Vij, A. Lehmann, and C. Tschierske

Appl. Phys. Lett. 95, 183304 (2009); http://dx.doi.org/10.1063/1.3255013 (3 pages) | Cited 11 times

Online Publication Date: 3 November 2009

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A bent-core mesogen with different end groups has been studied for different surface conditions in both planar and homeotropic cells using techniques for measuring biaxiality and optical switching. Biaxial nematic phase observed in between the uniaxial nematic and smectic phases is evidenced by a sharp increase in the biaxiality in a homeotropic cell measured using a photoelastic modulator. The material in this phase is switchable through the minor director with an in-plane electric field. In a planar cell, a step in the difference in the refractive indices resulting from the uniaxial to biaxial transition is also observed.
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61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures
64.70.mj Experimental studies of liquid crystal transitions
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Singlet energy transfer and singlet-singlet annihilation in light-emitting blends of organic semiconductors

A. Ruseckas, J. C. Ribierre, P. E. Shaw, S. V. Staton, P. L. Burn, and I. D. W. Samuel

Appl. Phys. Lett. 95, 183305 (2009); http://dx.doi.org/10.1063/1.3253422 (3 pages) | Cited 2 times

Online Publication Date: 3 November 2009

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Excitation energy transfer from host to guest is studied in spin-cast blends of 4,4′-bis(N-carbazolyl)biphenyl (CBP) and a phosphorescent fac-tris(2-phenylpyridyl)iridium(III)-cored dendrimer using time resolved fluorescence. The kinetics of energy transfer are consistent with homogeneous dispersion of the dendrimers in the CBP host. Diffusion-controlled singlet-singlet exciton annihilation is observed in the CBP host at moderate excitation densities, similar to those encountered in high brightness light-emitting devices and organic lasers. The results are important for organic lighting and the understanding of exciton diffusion in guest-host systems for electrophosphorescence.
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71.35.-y Excitons and related phenomena
66.30.H- Self-diffusion and ionic conduction in nonmetals
78.55.Kz Solid organic materials
78.47.jd Time resolved luminescence
85.60.Jb Light-emitting devices
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Studies of charge transfer processes across donor-acceptor interface using a field effect transistor geometry

Manohar Rao and K. S. Narayan

Appl. Phys. Lett. 95, 183306 (2009); http://dx.doi.org/10.1063/1.3259629 (3 pages)

Online Publication Date: 5 November 2009

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The existence of donor-type polymer field effect transistors (FETs), which are FETs exhibiting p-type characteristics and acceptor-type molecular FETs with n-type characteristics, provide an interesting possibility of a combined active bilayer system, especially under photoexcitation. We present a device structure and methodology that is conducive to study donor-acceptor interfacial processes using a FET platform. The changes in FET characteristics initiated by the photogenerated carriers in the donor coated acceptor device are studied using steady state and pulsed photoexcitation.
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85.30.Tv Field effect devices
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Stable and reversible optoelectrical dual-mode data storage based on a ferrocenlylspiropyran molecule

Ying Ma, Chengshan Niu, Yongqiang Wen, Guo Li, Jingxia Wang, Heng Li, Shixuan Du, Lianming Yang, Hongjun Gao, and Yanlin Song

Appl. Phys. Lett. 95, 183307 (2009); http://dx.doi.org/10.1063/1.3259647 (3 pages) | Cited 3 times

Online Publication Date: 5 November 2009

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A spiropyran molecule bonded with ferrocene (SPFc) was synthesized as data storage medium. Optical recording pattern was obtained through the reversible modulation of UV and visible light on the SPFc film, which exhibited good reversibility compared with the spiropyran without ferrocene moiety. In addition, the SPFc thin film also possessed reversible electrical switching property with high ON/OFF ratio, low threshold voltage, and long retention time. Accordingly, reliable, stable, and reversible nanoscale data storage was achieved on the SPFc thin film by scanning tunneling microscopy. The results will be significant to develop multifunctional molecule and reversible multimode data storage.
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42.79.Vb Optical storage systems, optical disks
85.60.-q Optoelectronic devices
85.65.+h Molecular electronic devices
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Influence of molecular weight on the solar cell performance of double-crystalline donor-acceptor block copolymers

Michael Sommer, Sven Hüttner, Ullrich Steiner, and Mukundan Thelakkat

Appl. Phys. Lett. 95, 183308 (2009); http://dx.doi.org/10.1063/1.3257367 (3 pages) | Cited 21 times

Online Publication Date: 6 November 2009

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We investigate the influence of the molecular weight of double-crystalline donor-acceptor block copolymers comprised of poly(3-hexylthiophene) as donor and poly(perylene bisimide acrylate) as acceptor segments on the device performance of polymer solar cells. Two block copolymers 1 and 2 exhibiting different molecular weights but the same composition are compared. Block copolymer 2 with the higher molecular weight shows an improvement in the hole carrier mobility μOFET of more than two orders of magnitude, and an improvement in the external quantum efficiency of one order of magnitude reaching 31%, which is the highest reported value for a block copolymer system.
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82.35.Jk Copolymers, phase transitions, structure
82.45.Wx Polymers and organic materials in electrochemistry
72.20.Fr Low-field transport and mobility; piezoresistance
72.80.Le Polymers; organic compounds (including organic semiconductors)
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Chemical changes on the green emitter tris(8-hydroxy-quinolinato)aluminum during device aging of p-i-n-structured organic light emitting diodes

Sebastian Scholz, Björn Lüssem, and Karl Leo

Appl. Phys. Lett. 95, 183309 (2009); http://dx.doi.org/10.1063/1.3257380 (3 pages) | Cited 8 times

Online Publication Date: 6 November 2009

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Metal organic fluorescent and phosphorescent emitters are widely used in organic light emitting devices (OLEDs). Iridium-based triplet emitters are known to undergo chemical reactions with other materials during OLED aging. The material tris(8-hydroxy-quinolinato)aluminum (Alq3), which is widely used as electron transporting material and green fluorescent emitter, degrades mainly during hole transport. We investigate the chemical changes in Alq3 during device aging: using laser desorption ionization time-of-flight mass spectrometry, we study the reaction products found in degraded OLEDs. Similar to the reactions known from the phosphorescent iridium-based emitters, Alq3 undergoes a dissociation and further reaction with the hole blocking material 4,7-diphenyl-1,10-phenanthroline.
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85.60.Jb Light-emitting devices
85.30.Kk Junction diodes
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