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31 Aug 2009

Volume 95, Issue 9, Articles (09xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 95, 091901 (2009); http://dx.doi.org/10.1063/1.3212896 (3 pages)

Noy Bassik, George M. Stern, and David H. Gracias
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Micro-pixel array of organic light-emitting diodes applying imprinting technique with a polymer replica

Tae Hyun Park, Young Min Kim, Young Wook Park, Jin Hwan Choi, Jin-Wook Jeong, Ki Young Dong, Kyung Cheol Choi, and Byeong-Kwon Ju

Appl. Phys. Lett. 95, 093301 (2009); http://dx.doi.org/10.1063/1.3216051 (3 pages)

Online Publication Date: 1 September 2009

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Efficient micro-pixel array of small molecule organic light-emitting diodes (OLEDs) has been fabricated by an imprinting technique which uses a polymer replica. To confirm the effect of the oxygen plasma for removing the residual layer, the performance of two kinds of OLEDs with varying thicknesses of resin as the micro-pixel array, have been compared. The measured results of the OLEDs have shown comparable device performances that are significantly characterized depending on the residues on the substrate. The performance of enhanced device has achieved efficiencies of 3.6 cd/A and 1.9 lm/W at 20 mA/cm2.
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85.60.Jb Light-emitting devices
85.60.Bt Optoelectronic device characterization, design, and modeling
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High-resolution molecular images of rubrene single crystals obtained by frequency modulation atomic force microscopy

Taketoshi Minato, Hiroto Aoki, Hirokazu Fukidome, Thorsten Wagner, and Kingo Itaya

Appl. Phys. Lett. 95, 093302 (2009); http://dx.doi.org/10.1063/1.3184770 (3 pages) | Cited 9 times

Online Publication Date: 2 September 2009

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Frequency modulation atomic force microscopy (FM-AFM) was employed to study molecular structures of rubrene single crystals in ultrahigh vacuum. Molecularly flat and extraordinarily wide terraces were extended over the width of more than a few micrometers with monomolecular steps. Molecular packing arrangements and internal structures were revealed by FM-AFM. The unit cell determined by FM-AFM was consistent with the lattice parameters of bulk crystal within the experimental error, suggesting that the surface structure of rubrene is not reconstructed.
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61.66.Hq Organic compounds
68.35.bg Semiconductors
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Vertical coupled double organic microcavities

S. Stelitano, G. De Luca, S. Savasta, L. Monsù Scolaro, and S. Patané

Appl. Phys. Lett. 95, 093303 (2009); http://dx.doi.org/10.1063/1.3216838 (3 pages) | Cited 3 times

Online Publication Date: 2 September 2009

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A light emitting structure consisting of two coupled microcavities has been realized and studied. One of the two cavities contains a luminescent organic thin film of tetrakis(4-methoxyphenyl)porphyrin, whereas the other microcavity is a dielectric structure coupled to the organic one by means of a LiF/ZnS Bragg mirror. Reflectivity spectra show the presence of two well defined cavity dips. We observe an energy splitting of the two cavity modes. Despite the fact that only one cavity contains the active layer, the photoluminescence spectra display two peaks with comparable intensities at the same energy of the reflectivity dips. These observations indicate the strong coupling of the two cavities. The comparison of the diagonalized effective Hamiltonian with the observed resonances further confirms the strong coupling.
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85.60.Jb Light-emitting devices
78.55.Kz Solid organic materials
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The effect of molybdenum oxide interlayer on organic photovoltaic cells

Do Young Kim, Jegadesan Subbiah, Galileo Sarasqueta, Franky So, Huanjun Ding, Irfan, and Yongli Gao

Appl. Phys. Lett. 95, 093304 (2009); http://dx.doi.org/10.1063/1.3220064 (3 pages) | Cited 64 times

Online Publication Date: 3 September 2009

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Both small molecule and polymer photovoltaic cells were fabricated with molybdenum oxide interlayer at the indium tin oxide electrode. Enhancement in power efficiencies was observed in both small molecule and polymer cells. Specifically, the power conversion efficiencies of small molecule cells with the molybdenum oxide interlayer were enhanced by a maximum of 38% due to a significant enhancement in the fill factor. The improved fill factor is attributed to the reduction in series resistance. Our ultraviolet photoemission spectroscopy data indicate that the formation of band bending and the built-in field at the interface due to the interlayer leads to enhancement in hole extraction from the photoactive layer toward the anode.
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85.60.-q Optoelectronic devices
82.45.Fk Electrodes
79.60.Fr Polymers; organic compounds
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Interdiffusion of molecular acceptors through organic layers to metal substrates mimics doping-related energy level shifts

Steffen Duhm, Ingo Salzmann, Benjamin Bröker, Hendrik Glowatzki, Robert L. Johnson, and Norbert Koch

Appl. Phys. Lett. 95, 093305 (2009); http://dx.doi.org/10.1063/1.3213547 (3 pages) | Cited 8 times

Online Publication Date: 3 September 2009

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Photoemission measurements reveal energy level shifts toward the Fermi level when a strong electron acceptor (tetrafluoro-tetracyanoquinodimethane, F4-TCNQ) is deposited on pristine layers of 4,4′,4″-tris(N,N-diphenyl-amino)triphenylamine (TDATA) or 4,4′-bis(N-carbazolyl)biphenyl (CBP). The shifts of the TDATA and CBP energy levels toward the Fermi level of the Au substrate could, in principle, arise from p-type doping of the intrinsic organic layers. While this indeed takes place in TDATA, doping of CBP by F4-TCNQ, i.e., charge transfer complex formation, does not occur. The shifts observed in CBP arise from the diffusion of F4-TCNQ toward the Au substrate, which modifies the buried metal surface potential, leading to a realignment of the energy levels of the organic overlayer.
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66.30.Ny Chemical interdiffusion; diffusion barriers
68.35.Fx Diffusion; interface formation
79.60.Fr Polymers; organic compounds
61.72.up Other materials
71.20.Rv Polymers and organic compounds
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Multilayer barrier films comprising nitrogen spacers between free-standing barrier layers

Jimmy Granstrom, Michael Villet, Tirtha Chatterjee, Jeffrey A. Gerbec, Evan Jerkunica, and Anshuman Roy

Appl. Phys. Lett. 95, 093306 (2009); http://dx.doi.org/10.1063/1.3222971 (3 pages) | Cited 1 time

Online Publication Date: 4 September 2009

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An encapsulation architecture for organic electronic devices utilizing nitrogen gas-phase spacers between free-standing barrier films is demonstrated. The nitrogen spacers act as sinks for permeating H2O and O2, delaying establishment of steady-state chemical potential gradients across the barriers and thereby reducing permeation rates. Water vapor transmission through nitrogen-spaced barriers was measured via the calcium optical transmission test. Substantial reductions in permeation rate were observed for a variety of barrier materials and configurations, suggesting a general and cost-effective approach for improving encapsulation performance. A low-cost polyethylene terephthalate film increases the calcium lifetime of a Cytop™-Kureha structure from 7000 to 12000 min.
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68.35.Fx Diffusion; interface formation
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
78.66.Qn Polymers; organic compounds
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
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Influence of the donor/acceptor interface on the open-circuit voltage in organic solar cells

Z. T. Liu, M. F. Lo, H. B. Wang, T. W. Ng, V. A. L. Roy, C. S. Lee, and S. T. Lee

Appl. Phys. Lett. 95, 093307 (2009); http://dx.doi.org/10.1063/1.3222975 (3 pages) | Cited 21 times

Online Publication Date: 4 September 2009

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The donor/acceptor interface in a standard CuPc/C60 organic solar cell was modified by insertion of a thin layer of molybdenum trioxide (MoO3). An ultrathin layer of MoO3 between the donor and acceptor increased the open-circuit voltage (VOC) from 0.45 to 0.85 V. The enhancement in VOC is explained by the increase in the energy level offset between the lowest unoccupied molecular orbital of the acceptor and the highest occupied molecular orbital of the donor (EDHOMO-EALUMO). The explanation is supported by the energy level analysis of the donor/acceptor interface by ultraviolet photoemission spectroscopy and x-ray photoemission spectroscopy.
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84.60.Jt Photoelectric conversion
73.20.At Surface states, band structure, electron density of states
79.60.Jv Interfaces; heterostructures; nanostructures
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Photoimpedance characterization of polymer field-effect transistor

C. S. Suchand Sangeeth, Manu Jaiswal, and Reghu Menon

Appl. Phys. Lett. 95, 093308 (2009); http://dx.doi.org/10.1063/1.3222978 (3 pages) | Cited 4 times

Online Publication Date: 4 September 2009

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The small signal ac response is measured across the source-drain terminals of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) field-effect transistor under dc bias to obtain the equivalent circuit parameters in the dark, and under a monochromatic light (540 nm) of various intensities. The numerically simulated response based on these parameters shows deviation at low frequency which is related to the charge accumulation at the interface and the contact resistance at the electrodes. This method can be used to differentiate the photophysical phenomena occurring in the bulk from that at the metal-semiconductor interface for polymer field-effect transistors.
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85.30.Tv Field effect devices
73.40.Cg Contact resistance, contact potential
61.41.+e Polymers, elastomers, and plastics
73.40.Ns Metal-nonmetal contacts
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Floating-gate memory based on an organic metal-insulator-semiconductor capacitor

S. William, M. F. Mabrook, and D. M. Taylor

Appl. Phys. Lett. 95, 093309 (2009); http://dx.doi.org/10.1063/1.3223606 (3 pages) | Cited 8 times

Online Publication Date: 4 September 2009

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A floating gate memory element is described which incorporates an evaporated gold film embedded in the gate dielectric of a metal-insulator-semiconductor capacitor based on poly(3-hexylthiophene). On exceeding a critical amplitude in the voltage sweep, hysteresis is observed in the capacitance-voltage (C-V) and current-voltage (I-V) characteristics of the device. The anticlockwise hysteresis in C-V is consistent with strong electron trapping during the positive cycle but little hole trapping during the negative cycle. We argue that the clockwise hysteresis observed in the negative cycle of the I-V plot, arises from leakage of trapped holes through the underlying insulator to the control gate.
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84.32.Tt Capacitors
85.30.Tv Field effect devices
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
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