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31 Mar 2008

Volume 92, Issue 13, Articles (13xxxx)

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Appl. Phys. Lett. 92, 133501 (2008); http://dx.doi.org/10.1063/1.2901684 (3 pages)

Eric Akmansoy, Emmanuel Centeno, Kevin Vynck, David Cassagne, and Jean-Michel Lourtioz
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Choice of electrode geometry for accurate measurement of organic photovoltaic cell performance

Myung-Su Kim, Myung-Gyu Kang, L. Jay. Guo, and Jinsang Kim

Appl. Phys. Lett. 92, 133301 (2008); http://dx.doi.org/10.1063/1.2895058 (3 pages) | Cited 39 times

Online Publication Date: 31 March 2008

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The relationship between the performance and the electrode geometry of organic photovoltaic devices was investigated to establish the proper electrode geometry for reproducible and accurate performance measurement. Photovoltaic cells (ITO/PEDOT:PSS/P3HT+PCBM/LiF/Al) having crossbar-type and island-type electrode geometries were fabricated. The crossbar-type cells varied greatly in performance depending on the illuminated light beam size relative to the overlap area of the crossbar-type electrodes due to excess photocurrent generated from the cell region outside the overlapped electrode area, where PEDOT:PSS serves as anode. We systematically investigated the relationship between the conductivity of the PEDOT:PSS, the illumination area, and the amount of excess photocurrent generated.
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85.60.-q Optoelectronic devices
72.40.+w Photoconduction and photovoltaic effects
79.60.Fr Polymers; organic compounds
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Enhanced hole mobility in ambipolar rubrene thin film transistors on polystyrene

Byoungnam Park, Insik In, Padma Gopalan, Paul G. Evans, Seth King, and Paul F. Lyman

Appl. Phys. Lett. 92, 133302 (2008); http://dx.doi.org/10.1063/1.2904964 (3 pages) | Cited 6 times

Online Publication Date: 31 March 2008

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We report amorphous rubrene thin film transistors with a polystyrene intermediate layer on the SiO2 gate dielectric that have hole mobilities up to 0.01 cm2/V s. This improvement by two orders of magnitude over devices formed on SiO2 alone occurs without the crystallization of rubrene. The enhanced charge transport is a result of the more planar growth and subsequent better geometrical connection of the first molecular layers of rubrene. Ambipolar conduction in the rubrene suggests that polystyrene minimizes the concentration of interfacial electron trap states.
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling
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Quantitative analysis of electronic transport through weakly coupled metal/organic interfaces

A. S. Molinari, I. Gutiérrez Lezama, P. Parisse, T. Takenobu, Y. Iwasa, and A. F. Morpurgo

Appl. Phys. Lett. 92, 133303 (2008); http://dx.doi.org/10.1063/1.2904629 (3 pages) | Cited 7 times

Online Publication Date: 31 March 2008

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Using single-crystal transistors, we have performed a systematic experimental study of electronic transport through oxidized copper/rubrene interfaces as a function of temperature and bias. We find that the measurements can be quantitatively reproduced in terms of the thermionic emission theory for Schottky diodes, if the effect of the bias-induced barrier lowering is included. Our analysis emphasizes the role of the coupling between metal and molecules, which in our devices is weak due to the presence of an oxide layer at the surface of the copper electrodes.
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73.40.Ns Metal-nonmetal contacts
85.30.Hi Surface barrier, boundary, and point contact devices
79.40.+z Thermionic emission
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Influence of doping profile on the efficiency of blue phosphorescent organic light-emitting diodes

Jonghee Lee, Jeong-Ik Lee, Ki-Im Song, Su Jin Lee, and Hye Yong Chu

Appl. Phys. Lett. 92, 133304 (2008); http://dx.doi.org/10.1063/1.2904632 (3 pages) | Cited 7 times

Online Publication Date: 31 March 2008

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We demonstrate that electroluminescent efficiency of blue phosphorescent organic light-emitting diodes could be improved by changing doping profile in the emissive layer (EML) of the device containing iridium(III)bis(4,6-difluorophenyl)-pyridinato-N,C2′) picolinate (FIrpic) and N,N′-dicarbazolyl-3,5-benzene (mCP). By examining the changes of recombination zone and current density with a partially doped EML, we found that most of the recombination occurred at the EML close to the electron transporting layer (ETL). Electron injection and transport were enhanced by increasing doping ratio at the EML close to the ETL and such a stepwise doping profile resulted in almost 40 % improved power efficiency compared to that of a conventional device.
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85.60.Jb Light-emitting devices
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The effect of molecule-molecule and molecule-substrate interaction in the formation of Pt-octaethyl porphyrin self-assembled monolayers

Nuri Oncel and Steven L. Bernasek

Appl. Phys. Lett. 92, 133305 (2008); http://dx.doi.org/10.1063/1.2904622 (3 pages) | Cited 3 times

Online Publication Date: 1 April 2008

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The adsorption of Pt-octaethyl porphyrin (Pt-OEP) molecules on highly ordered pyrolytic graphite (HOPG) and on 5-(octadecyloxy) isophthalic acid covered HOPG was studied with scanning tunneling microscopy at the solid-liquid interface. Pt-octaethyl porphyrin molecules lie flat on both surfaces and form a hexagonal lattice with an internal angle of about 60° and a lattice spacing of approximately 1.2 nm. The similar overlayer structure observed on the dramatically different substrates suggests that molecular packing in the Pt-OEP layer controls the structure.
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68.43.Mn Adsorption kinetics
68.43.Fg Adsorbate structure (binding sites, geometry)
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
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Exciton diffusion length in the organic semiconductor diindenoperylene

D. Kurrle and J. Pflaum

Appl. Phys. Lett. 92, 133306 (2008); http://dx.doi.org/10.1063/1.2896654 (3 pages) | Cited 3 times

Online Publication Date: 2 April 2008

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The photovoltaic behavior of Schottky barrier devices consisting of a single diindenoperylene (DIP) layer sandwiched between an indium tin oxide and Ag electrode has been investigated. Correlating the spectral dependence of the photocurrent and the absorption coefficient, we estimated the exciton diffusion length in DIP to ∼ 100 nm along the c′ direction. X-ray structural analysis yielded this length to be in agreement with the average crystallite size, thereby, revealing domain boundaries to be the limiting effect on the exciton transport. The corresponding exciton diffusion constant of 5×10−3 cm2/s resembles that of highly ordered single crystals of polyaromatic hydrocarbons.
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71.35.-y Excitons and related phenomena
72.40.+w Photoconduction and photovoltaic effects
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.30.+y Surface double layers, Schottky barriers, and work functions
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
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The surface engineering of top electrode in inverted polymer bulk-heterojunction solar cells

Jihwan Kim, Dahl-Young Khang, Ju-Hyung Kim, and Hong H. Lee

Appl. Phys. Lett. 92, 133307 (2008); http://dx.doi.org/10.1063/1.2904967 (3 pages) | Cited 2 times

Online Publication Date: 3 April 2008

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We present surface engineering of the top electrode in organic solar cells through soft contact lamination. A polymer conductor and a self-assembled monolayer material are utilized for the surface engineering of gold electrode in a polymer bulk-heterojunction solar cell. Approximately a threefold increase in the overall power conversion efficiency is shown to be realized with the surface engineering, which is made possible by the lamination.
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84.60.Jt Photoelectric conversion
68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces
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Low roll-off power efficiency organic light-emitting diodes consisted of nondoped ultrathin phosphorescent layer

Jun Wang, Junsheng Yu, Lu Li, Tao Wang, Kai Yuan, and Yadong Jiang

Appl. Phys. Lett. 92, 133308 (2008); http://dx.doi.org/10.1063/1.2907692 (3 pages) | Cited 3 times

Online Publication Date: 3 April 2008

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Low roll-off power efficiency (ηp) organic light-emitting diodes at high current density were fabricated using a nondoped ultrathin bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2′] iridium (acetylacetonate) phosphorescent layer and thin N,N′-di(naphthalene-2-yl)-N,N′-diphenyl-benzidine separate layer. A maximum ηp of 8.48 lm/W and luminance efficiency of 20.26 cd/A at 2014 cd/m2 were obtained from the device. The ηp roll-off percentage of ultrathin phosphor layer device was reduced to 20% at a current density of 20 mA/cm2, which is about 40% for conventional phosphorescent device. The low roll off of efficiency was attributed to the direct carrier trapping and confining emission zone function of the ultrathin phosphor layer.
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85.60.Jb Light-emitting devices
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Preparation and characterization of thermoconductive polymer nanocomposite with branched alumina nanofiber

Yuzuru Shimazaki, Fusao Hojo, and Yoshitaka Takezawa

Appl. Phys. Lett. 92, 133309 (2008); http://dx.doi.org/10.1063/1.2907315 (3 pages) | Cited 11 times

Online Publication Date: 4 April 2008

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A highly thermoconductive resin with branched filler is reported. The filler was fabricated by calcination of aluminium isopropoxide adsorbed onto paper filter (cellulose fibers). Scanning electron microscopy and x-ray diffraction measurement demonstrated that the filler consists of α-alumina nanofiber with a branched structure. The thermal conductivity of the filler-resin nanocomposite was two to five times larger than that predicted by the well-known Bruggeman equation, which postulates the composite with spherical filler in the matrix. The branched shape of the α-alumina nanofiber increased the probability of formation of phonon paths with lower thermal resistance, leading to the high thermal conductivity of the nanocomposite.
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81.16.-c Methods of micro- and nanofabrication and processing
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
61.46.-w Structure of nanoscale materials
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
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