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December 2012

Volume 5, Issue 12

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The effect of carbon nanotube/organic semiconductor interfacial area on the performance of organic transistors

Narae Kang, Biddut K. Sarker, and Saiful I. Khondaker

Appl. Phys. Lett. 101, 233302 (2012); http://dx.doi.org/10.1063/1.4769439 (4 pages)

Online Publication Date: 4 December 2012

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We show that the performance of pentacene transistors can be significantly improved by maximizing the interfacial area at single walled carbon nanotube (SWCNT)/pentacene. The interfacial areas are varied by anchoring short SWCNTs of different densities (0–30/μm) to the Pd electrodes. The average mobility is increased three, six, and nine times for low, medium, and high SWCNT densities, respectively, compared to the devices with zero SWCNT. The current on-off ratio and on-current are increased up to 40 times and 20 times with increasing the SWCNT density. We explain the improved device performance using reduced barrier height of SWCNT/pentacene interface.
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85.30.Tv Field effect devices
85.35.Kt Nanotube devices
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Correlation between interface energetics and open circuit voltage in organic photovoltaic cells

A. Wilke, J. Endres, U. Hörmann, J. Niederhausen, R. Schlesinger, J. Frisch, P. Amsalem, J. Wagner, M. Gruber, A. Opitz, A. Vollmer, W. Brütting, A. Kahn, and N. Koch

Appl. Phys. Lett. 101, 233301 (2012); http://dx.doi.org/10.1063/1.4769360 (4 pages)

Online Publication Date: 4 December 2012

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We have used ultraviolet and inverse photoemission spectroscopy to determine the transport gaps (Et) of C60 and diindenoperylene (DIP), and the photovoltaic gap (EPVG) of five prototypical donor/acceptor interfaces used in organic photovoltaic cells (OPVCs). The transport gap of C60 (2.5 ± 0.1) eV and DIP (2.55 ± 0.1) eV at the interface is the same as in pristine films. We find nearly the same energy loss of ca 0.5 eV for all material pairs when comparing the open circuit voltage measured for corresponding OPVCs and EPVG.
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85.60.-q Optoelectronic devices
84.60.Jt Photoelectric conversion
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Tandem photovoltaic cells formed in single fullerene films by impurity doping

Norihiro Ishiyama, Masayuki Kubo, Toshihiko Kaji, and Masahiro Hiramoto

Appl. Phys. Lett. 101, 233303 (2012); http://dx.doi.org/10.1063/1.4769455 (3 pages)

Online Publication Date: 5 December 2012

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Tandem photovoltaic cells were formed in single fullerene films by doping with molybdenum oxide and cesium carbonate. A heavily doped n+p+-homojunction acted as an ohmic interlayer between the two pn-homojunction cells. The observed photovoltaic properties of the tandem cell were shown to be consistent with the energy band diagram mapped using a Kelvin probe.
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88.40.jp Multijunction solar cells
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Self-heating effects on the electrical instability of fully printed p-type organic thin film transistors

M. Rapisarda, G. Fortunato, A. Valletta, S. Jacob, M. Benwadih, R. Coppard, I. Chartier, and L. Mariucci

Appl. Phys. Lett. 101, 233304 (2012); http://dx.doi.org/10.1063/1.4769819 (4 pages)

Online Publication Date: 5 December 2012

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Bias stress instability has been investigated in printed p-channel organic thin film transistors. The observed instability is related to two mechanisms: one, dominating at low T and causing “mobile ions” like threshold voltage variations is probably due to creation/annihilation of acceptor-like states; the second one, causing charge-trapping like instability, dominates at high T. High drain voltage bias stress experiments, inducing device self-heating, present threshold voltage variations, suggest a channel temperature rise ranging from 50 to 60 °C. The results point out the role of self-heating on the bias-stress instability, which is related to a combination of bias and temperature conditions.
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85.30.Tv Field effect devices
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Energy level alignment at the interfaces between typical electrodes and nucleobases: Al/adenine/indium-tin-oxide and Al/thymine/indium-tin-oxide

Younjoo Lee, Hyunbok Lee, Soohyung Park, and Yeonjin Yi

Appl. Phys. Lett. 101, 233305 (2012); http://dx.doi.org/10.1063/1.4769438 (5 pages)

Online Publication Date: 6 December 2012

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We investigated the interfacial electronic structures of Al/adenine/indium-tin-oxide (ITO) and Al/thymine/ITO using in situ ultraviolet and x-ray photoemission spectroscopy and density functional theory calculations. Adenine shows both an interface dipole and level bending, whereas thymine shows only an interface dipole in contact with ITO. In addition, thymine possesses a larger ionization energy than adenine. These are understood with delocalized π states confirmed with theoretical calculations. For the interface between nucleobases and Al, both nucleobases show a prominent reduction of the electron injection barrier from Al to each base in accordance with a downward level shift.
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73.20.At Surface states, band structure, electron density of states
79.60.Jv Interfaces; heterostructures; nanostructures
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
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Immerse precipitation as an efficient protocol to optimize morphology and performance of organic solar cells

Lijian Zuo, Zhen Cao, Xiaolian Hu, Zhuowei Gu, Hongbin Pan, and Hongzheng Chen

Appl. Phys. Lett. 101, 233306 (2012); http://dx.doi.org/10.1063/1.4769454 (4 pages)

Online Publication Date: 6 December 2012

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We developed a film-forming processing method for morphology control and organic solar cells (OSCs) optimization. In this protocol, the processing solvent inside a wet active layer is removed by dripping a soaking solvent that is selectively soluble for the processing solvents onto the wet active layer film. By this method, OSCs based on diketopyrrolopyrrole containing copolymers: [6,6]-phenyl-C61-butyric acid methyl ester blend were fabricated. Devices processed by this treatment show a significantly increased efficiency by a factor of 3 compared to devices fabricated by the traditional spin-coating method (from 1.03% to 3.2%), which is mainly attributed to morphology improvements.
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88.40.J- Types of solar cells
88.40.H- Solar cells (photovoltaics)
61.41.+e Polymers, elastomers, and plastics
61.50.-f Structure of bulk crystals
81.30.Mh Solid-phase precipitation
68.55.am Polymers and organics
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MoO3/Ag/MoO3 anode in organic photovoltaic cells: Influence of the presence of a CuI buffer layer between the anode and the electron donor

M. Makha, L. Cattin, Y. Lare, L. Barkat, M. Morsli, M. Addou, A. Khelil, and J. C. Bernède

Appl. Phys. Lett. 101, 233307 (2012); http://dx.doi.org/10.1063/1.4769808 (3 pages)

Online Publication Date: 7 December 2012

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MoO3/Ag/MoO3 (MAM) multilayer structures (layers thickness 20 nm/10 nm/35 nm) are used as anode in CuPc/C60/Alq3/Al organic photovoltaic cells. The averaged transmittance (400 nm-800 nm) of these MoO3/Ag/MoO3 multilayer structures is 70% ± 2% and their sheet resistance is 3.5 ± 1.0 Ω/sq. When these multilayer structures are used as anode, the power conversion efficiency of the MoO3/Ag/MoO3/CuPc/C60/Alq3/Al cells is around 1%, this efficiency is increased of 50% when a thin CuI film (3 nm) is introduced at the interface between the anode and the organic film. This improvement is attributed to the templating effect of CuI on the CuPc molecules.
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88.40.hj Efficiency and performance of solar cells
88.40.jp Multijunction solar cells
88.40.jr Organic photovoltaics
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Reduced graphene oxide based flexible organic charge trap memory devices

Adila Rani, Ji-Min Song, Mi Jung Lee, and Jang-Sik Lee

Appl. Phys. Lett. 101, 233308 (2012); http://dx.doi.org/10.1063/1.4769990 (5 pages)

Online Publication Date: 7 December 2012

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A nonvolatile organic transistor memory device was developed using layer-by-layer assembly of 3-aminopropyltriethoxysilane (APTES) and solution-processed, reduced graphene oxide (rGO) as the charge trapping layer on flexible substrates. Reduction of graphene oxide and successful adsorption of the rGO on APTES-covered substrates were confirmed. The organic memory devices based on rGO exhibited reliable programmable memory operations, confirmed by program/erase operations, data retention, and endurance properties. These methods can potentially play a significant role in the fabrication of next-generation flexible nonvolatile memory devices based on graphene materials.
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84.30.Sk Pulse and digital circuits
85.30.Tv Field effect devices
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The electrical and photoconductivity characteristics of donor-acceptor alternating copolymer using solution process

Shin Woo Jeong, Gi Back Lee, Hyeon-Jun Ha, Soon-Ki Kwon, Yun-Hi Kim, and Byeong Kwon Ju

Appl. Phys. Lett. 101, 243301 (2012); http://dx.doi.org/10.1063/1.4770074 (5 pages)

Online Publication Date: 10 December 2012

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The authors report on the electrical and photoconductivity characteristics of donor-acceptor alternating copolymer, poly(dioctyloxinapthalenediketopyrrolopyrrole) (PONDPP) with Al/PONDPP/p-Si/Al hybrid organic/inorganic Schottky diode for optoelectronic applications. The fabricated device shows ideality factor value of 2.6 and barrier height of 0.68 eV obtained from current-voltage characteristics. The high rectification ratio of 1.86 × 104 and photo-responsivity of 55 mA/W at 650 nm is achieved. From results, we found that the fine photo-response and electrical characteristics are attributed to the modified band-gap structure to have Schottky barrier at highest occupied molecular orbital to valence band of silicon and high hole mobility of PONDPP.
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72.40.+w Photoconduction and photovoltaic effects
73.40.Ei Rectification
85.30.Kk Junction diodes
71.20.Rv Polymers and organic compounds
72.20.Ee Mobility edges; hopping transport
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The Maxwell-Wagner model for charge transport in ambipolar organic field-effect transistors: The role of zero-potential position

Yasuhiro Mashiko, Dai Taguchi, Martin Weis, Takaaki Manaka, and Mitsumasa Iwamoto

Appl. Phys. Lett. 101, 243302 (2012); http://dx.doi.org/10.1063/1.4771989 (4 pages)

Online Publication Date: 12 December 2012

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Charge transport in an ambipolar organic field-effect transistors (OFETs) is discussed in accordance to the potential profiles reconstructed from the electric-field induced second-harmonic generation experiment. The Maxwell-Wagner model based on drift-diffusion equation in OFET is used for the potential profile analysis. A good agreement between dielectric model and the experiment suggests importance of the space-charge field effects in the design of the ambipolar light-emitting OFETs. Further, the highest enhancement of the electric field is on zero-potential position in the channel, which represents the meeting point of electrons and holes and is an origin of the electroluminescence.
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85.30.Tv Field effect devices
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Influence of phosphorescent dopants in organic light-emitting diodes with an organic homojunction

Changhun Yun, Jonghee Lee, Jaehyun Lee, Björn Lüssem, Fabian Ventsch, Karl Leo, and Malte C. Gather

Appl. Phys. Lett. 101, 243303 (2012); http://dx.doi.org/10.1063/1.4772014 (3 pages)

Online Publication Date: 12 December 2012

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We report on phosphorescent homojunction organic light-emitting diodes (HJOLEDs) using p-i-n structures based on a single ambipolar organic semiconductor, 4,4′-Bis(carbazol-9-yl)-biphenyl, as matrix organic materials. In HJOLEDs, the phosphorescent dopant molecules play an important role in controlling the charge balance inside the emissive layer. We observe a four-fold enhancement in the luminous efficacy at 1000 cd/m2 from 3.7% to 12.9% by varying the emitter molecule. The influence of the energy level of the emitter molecule on charge balance is investigated by analyzing current density vs. voltage curves with the trap-limited current theory and by analyzing the electroluminescence spectra.
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85.60.Jb Light-emitting devices
85.30.Kk Junction diodes
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Polyaniline micro-rods based heterojunction solar cell: Structural and photovoltaic properties

Savaş Sönmezoğlu, Recep Taş, Seçkin Akın, and Muzaffer Can

Appl. Phys. Lett. 101, 253301 (2012); http://dx.doi.org/10.1063/1.4772019 (4 pages)

Online Publication Date: 17 December 2012

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The present paper reports the fabrication and photovoltaic characterization of pure and dodecyl benzene sulfonic acid (DBSA)-doped polyaniline (PAni) micro-rods polymer/n-Si heterojunction solar cells, and also the morphological and structural properties of pure and micro-rods PAni doping with DBSA. The device shows a strong photovoltaic behavior with a maximum open-circuit voltage Voc of 0.83 V, a short-circuit current Jsc of 14.72 mA cm−2, fill factor FF of 0.54 resulting in an estimated device efficiency η of 6.13% under simulated solar light with the intensity of 100 mW/cm2. The results indicate that the Au/DBSA-doped PAni micro-rods/n-Si heterojunction structure might be promising for the solar cell applications.
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88.40.H- Solar cells (photovoltaics)
88.40.jj Silicon solar cells
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The penetration limit of poly(4-vinyl phenol) thin films for etching via holes by inkjet printing

Y. Zhang (张岩), C. Liu, and D. C. Whalley

Appl. Phys. Lett. 101, 253302 (2012); http://dx.doi.org/10.1063/1.4772796 (4 pages)

Online Publication Date: 19 December 2012

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This paper reports the penetration limit of via holes through dissolving dielectric polymer thin films by inkjet printing. It was found that both the outer diameter of via holes and the polymer thickness affect the penetration depth from the experimental results. Based on this finding, a more accurate relationship between the inner diameter of via holes and the diameter of in-flight droplets for different polymer thicknesses is obtained.
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81.65.Cf Surface cleaning, etching, patterning
77.55.-g Dielectric thin films
73.61.Ph Polymers; organic compounds
77.84.Jd Polymers; organic compounds
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
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Passivation of trap states in unpurified and purified C60 and the influence on organic field-effect transistor performance

Selina Olthof, Sanjeev Singh, Swagat K. Mohapatra, Stephen Barlow, Seth R. Marder, Bernard Kippelen, and Antoine Kahn

Appl. Phys. Lett. 101, 253303 (2012); http://dx.doi.org/10.1063/1.4772551 (4 pages)

Online Publication Date: 19 December 2012

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We investigate trap-state passivation by addition of ultra-low amounts of n-dopants in organic field-effect transistors (OFET) made of as-received and purified fullerene C60. We find a strong dependence of the OFET threshold voltage (VT) on the density of traps present in the layer. In the case of the unpurified material, VT is reduced from 17.9 V to 4.7 V upon trap passivation by a dopant:C60 ratio of ∼10−3, while the Ion/off current ratio remains high. This suggests that ultra-low doping can be used to effectively compensate impurity and defect-related traps.
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85.30.Tv Field effect devices
81.65.Rv Passivation
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Scanning photocurrent and photoluminescence imaging of a frozen polymer p-n junction

Alex Inayeh, Bryce Dorin, and Jun Gao

Appl. Phys. Lett. 101, 253305 (2012); http://dx.doi.org/10.1063/1.4773235 (4 pages)

Online Publication Date: 20 December 2012

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A polymer light-emitting electrochemical cell (LEC) is a solid-state polymer device operating on in situ electrochemical doping and the formation of a light-emitting polymer p-n junction. Electrochemical doping of the luminescence polymer quenches the photoluminescence. The chemical potential difference between the p- and n-doped regions creates a built-in potential/field in the junction region, which can be probed by measuring the optical beam induced current (OBIC). In this study, the OBIC and photoluminescence profiles of the LEC have been simultaneously measured by scanning a focused light beam across a large planar LEC that has been turned on and cooled to freeze the doping profile. The photoluminescence intensity undergoes a sharp transition between the p- and n-doped regions. The OBIC photocurrent is only observed in the transition region that is narrower than the width of the excitation beam, which is about 35 μm. The results depict a static planar polymer p-n junction with a built-in field pointing from n to p. The electrode interface and the neutral regions do not produce a measurable photocurrent.
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72.40.+w Photoconduction and photovoltaic effects
78.55.Kz Solid organic materials
78.66.Qn Polymers; organic compounds
85.60.Jb Light-emitting devices
61.72.up Other materials
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Comparing the emissive dipole orientation of two similar phosphorescent green emitter molecules in highly efficient organic light-emitting diodes

Philipp Liehm, Caroline Murawski, Mauro Furno, Björn Lüssem, Karl Leo, and Malte C. Gather

Appl. Phys. Lett. 101, 253304 (2012); http://dx.doi.org/10.1063/1.4773188 (4 pages)

Online Publication Date: 20 December 2012

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We investigate the average orientation of the transition dipole moments of two green phosphorescent emitters Ir(ppy)3 and Ir(ppy)2(acac) embedded in a CBP matrix, using in-situ angle resolved electroluminescence spectroscopy and optical simulations. The dipole orientation of Ir(ppy)3 is nearly isotropic while 77% of the dipoles are horizontally aligned for Ir(ppy)2(acac). Optimized organic light-emitting diodes based on these emitters achieve external quantum efficiencies of 18.3% (Ir(ppy)3) and 21.7% (Ir(ppy)2(acac)). This difference is partially explained by the different dipole orientations.
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85.60.Jb Light-emitting devices
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.50.Dq Fluorescence and phosphorescence spectra
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
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Efficiency enhancement in mesogenic-phthalocyanine-based solar cells with processing additives

Quang Duy Dao, Tetsuro Hori, Kaoru Fukumura, Tetsuya Masuda, Toshiya Kamikado, Akihiko Fujii, Yo Shimizu, and Masanori Ozaki

Appl. Phys. Lett. 101, 263301 (2012); http://dx.doi.org/10.1063/1.4773519 (3 pages)

Online Publication Date: 27 December 2012

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Bulk heterojunction (BHJ) solar cells, fabricated by spin casting processes, with phase-separated small molecules including a phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH2), and a fullerene derivative, 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM), have been demonstrated to have a power conversion efficiency exceeding 4.1%. The C6PcH2:PCBM BHJ surface morphology and the phase separation have been controlled by utilizing processing additives in various solvents. The effects of the processing additives on the crystallization of the discotic C6PcH2 columns in hexagonal structures have been discussed.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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