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3 Mar 2008

Volume 92, Issue 9, Articles (09xxxx)

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Appl. Phys. Lett. 92, 093101 (2008); http://dx.doi.org/10.1063/1.2888164 (3 pages)

C. K. Lee, S. J. Kim, S. J. Shin, J. B. Choi, and Y. Takahashi
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Correlating reduced fill factor in polymer solar cells to contact effects

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

Appl. Phys. Lett. 92, 093301 (2008); http://dx.doi.org/10.1063/1.2841062 (3 pages) | Cited 10 times

Online Publication Date: 3 March 2008

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A probable limiting factor for efficiency and fill factors of organic solar cells originates from the cathode-polymer interface. We utilize various forms of cathode layer such as Al, Ca, oxidized Ca, and low melting point alloys in model systems to emphasize this aspect in our studies. The current-voltage (JV) response in the fourth quadrant indicates a general trend of convex shaped JV characteristics (d2J/dV2>0) for illuminated devices with good cathode-polymer interfaces and linear or concave JV responses (d2J/dV2<0) for inefficient cathode-polymer interfaces.
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84.60.Jt Photoelectric conversion
82.45.Wx Polymers and organic materials in electrochemistry
82.45.Fk Electrodes
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n-type organic field effect transistors from perylene bisimide block copolymers and homopolymers

Sven Hüttner, Michael Sommer, and Mukundan Thelakkat

Appl. Phys. Lett. 92, 093302 (2008); http://dx.doi.org/10.1063/1.2885712 (3 pages) | Cited 42 times

Online Publication Date: 3 March 2008

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We present organic field effect transistors (OFETs) based on solution-processable n-type polymers containing perylene bisimide as pendant groups. The OFET characteristics of a diblock copolymer consisting of polystyrene and poly(perylene acrylate) (PPerAcr) blocks and a PPerAcr homopolymer are compared. Thermal annealing improves the OFET performance by two to three orders of magnitude, which can be attributed to the improved order and interface properties in the transport layer, arising from the better alignment of the perylene bisimide moieties. Both polymers show excellent n-type performances with electron carrier mobility of 1.2×10−3 cm2/V s and low threshold voltages of 4–7 V.
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85.30.Tv Field effect devices
61.72.Cc Kinetics of defect formation and annealing
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Interface modification for highly efficient organic photovoltaics

Roland Steim, Stelios A. Choulis, Pavel Schilinsky, and Christoph J. Brabec

Appl. Phys. Lett. 92, 093303 (2008); http://dx.doi.org/10.1063/1.2885724 (3 pages) | Cited 71 times

Online Publication Date: 4 March 2008

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We present highly efficient inverted polymer:fullerene bulk-heterojunction solar cells by incorporation of a nanoscale organic interfacial layer between the indium tin oxide (ITO) and the metal oxide electron-conducting layer. We demonstrate that stacking of solution-processed organic and metal oxide interfacial layers gives highly charged selective low ohmic cathodes. The incorporation of a polyoxyethylene tridecyl ether interfacial layer between ITO and solution-processed titanium oxide (TiOx) raised the power conversion efficiency of inverted organic photovoltaics to 3.6%, an improvement of around 15% in their performance over comparable devices without the organic interfacial layer.
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84.60.Jt Photoelectric conversion
68.35.Ct Interface structure and roughness
82.45.Fk Electrodes
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Energy level alignments at tris(8-hydroquinoline) aluminum/8-hydroquinolatolithium/aluminum interfaces

Kwanghee Cho, Sang Wan Cho, Pyung Eun Jeon, Hyunbok Lee, Chung-Nam Whang, Kwangho Jeong, Seong Jun Kang, and Yeonjin Yi

Appl. Phys. Lett. 92, 093304 (2008); http://dx.doi.org/10.1063/1.2890163 (3 pages) | Cited 1 time

Online Publication Date: 4 March 2008

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The electronic structures of tris(8-hydroquinoline) aluminum (Alq3)/8-hydroquinolatolithium (Liq)/Al interfaces were studied using in situ ultraviolet and x-ray photoelectron spectroscopy. We constructed complete energy level diagrams and analyzed chemical interactions at the interface. When Liq was inserted between Al and Alq3, the electron injection barrier was reduced by 0.56 eV compared to the structure without Liq. Additionally, a gap state was observed in the gap of Liq, which is related to an interfacial reaction. The N 1s spectra revealed that there were destructive chemical reactions between Alq3 and Al, which could be prevented by inserting Liq between them.
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73.20.At Surface states, band structure, electron density of states
78.40.-q Absorption and reflection spectra: visible and ultraviolet
79.60.Jv Interfaces; heterostructures; nanostructures
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Very low turn-on voltage and high brightness tris-(8-hydroxyquinoline) aluminum-based organic light-emitting diodes with a MoOx p-doping layer

Guohua Xie, Yanlong Meng, Fengmin Wu, Chen Tao, Dandan Zhang, Mingjun Liu, Qin Xue, Wen Chen, and Yi Zhao

Appl. Phys. Lett. 92, 093305 (2008); http://dx.doi.org/10.1063/1.2890490 (3 pages) | Cited 2 times

Online Publication Date: 4 March 2008

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We have demonstrated an organic light-emitting diode based on molybdenum oxide (MoOx) doped 4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) as a p-type doping hole injection layer. The tris-(8-hydroxyquinoline) aluminum (Alq3)-based organic light-emitting diodes show high brightness at very low operating voltage, 100 cd/m2 at 3.2 V and 1000 cd/m2 at 4.4 V, corresponding to a low turn-on voltage of 2.4 V. Such improved properties are attributed to the formation of the charge transfer complex produced by doping MoOx into m-MTDATA, which provides much more free hole carriers, and the introduction of an efficient electron-injecting layer to improve the performance.
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85.60.Jb Light-emitting devices
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Multilayer organic solar cells with wet-processed polymeric bulk heterojunction film and dry-processed small molecule films

Youngkyoo Kim, Minjung Shin, Inhyuk Lee, Hwajeong Kim, and Sandrine Heutz

Appl. Phys. Lett. 92, 093306 (2008); http://dx.doi.org/10.1063/1.2890169 (3 pages) | Cited 7 times

Online Publication Date: 5 March 2008

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Here, we report multilayer organic solar cells fabricated using a mix of solution (wet) and thermal evaporation (dry) techniques, which consist of indium-tin oxide (ITO), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), poly(3-hexylthiophene) (P3HT):1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM), C60, bathocuproine (BCP), and aluminum layers. Results show that the short circuit current density (JSC) of a ITO/PEDOT:PSS/C60/BCP/Al device was greatly improved by inserting a pristine P3HT light-absorbing layer between the PEDOT:PSS and C60 layers. Addition of PCBM to the P3HT layer lowered the JSC in the devices compared to the pristine P3HT layer and, in general, the JSC continued to decline with increasing PCBM content.
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84.60.Jt Photoelectric conversion
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Solution-based formation of multilayers of small molecules for organic light emitting diodes

Kyung-Ho Kim, Sung-Yoon Huh, Soon-min Seo, and Hong H. Lee

Appl. Phys. Lett. 92, 093307 (2008); http://dx.doi.org/10.1063/1.2890845 (3 pages) | Cited 4 times

Online Publication Date: 6 March 2008

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We developed an approach for fabricating small molecule organic light emitting diodes by solution-based processing. The approach involves dissolving a small molecule organic in a solvent, spin coating it on a mold, and then transferring the layer onto the existing organic layer on a substrate. This ability to form multilayers of small molecule organics allows one to take advantage of both the efficiency offered by the multilayer structures and the low cost fabrication made possible by the solution processing.
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85.60.Jb Light-emitting devices
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Enhanced photovoltaic performance in nanoimprinted pentacene-PbS nanocrystal hybrid device

D. M. N. M. Dissanayake, A. A. D. T. Adikaari, and S. R. P. Silva

Appl. Phys. Lett. 92, 093308 (2008); http://dx.doi.org/10.1063/1.2890848 (3 pages) | Cited 3 times

Online Publication Date: 6 March 2008

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Pentacene and PbS nanocrystal bilayer photovoltaic devices are fabricated after the pentacene layer is subjected to nanoimprinting using a laser textured silicon stamp. Increased short circuit current densities are observed for the imprinted devices, which are attributed to increased charge mobility in the pentacene film caused by the decrease in the intermolecular distances during nanoimprinting. This work is consistent with previous reports where hydrostatic pressure induced mobility increases have been observed in polyacenes under gigapascal pressure regimes. It is believed that the pentacene film undergoes localized high pressures during nanoimprinting, giving rise to the increased hole mobilities.
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85.60.-q Optoelectronic devices
85.35.-p Nanoelectronic devices
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Influence of H2O and O2 on threshold voltage shift in organic thin-film transistors: Deprotonation of SiOH on SiO2 gate-insulator surface

Daisuke Kumaki, Tokiyoshi Umeda, and Shizuo Tokito

Appl. Phys. Lett. 92, 093309 (2008); http://dx.doi.org/10.1063/1.2890853 (3 pages) | Cited 9 times

Online Publication Date: 6 March 2008

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The influence of H2O and O2 on the transistor characteristics in p- and n-type organic thin-film transistors (OTFTs) fabricated on the SiO2 gate insulator was investigated. In both p- and n-type OTFTs, the threshold voltage (Vth) shifted drastically to positive direction after exposure to ambient air and dry air, although the field-effect mobilities in saturation regime were almost unchanged before and after the Vth shift. The Vth shifts to the positive direction indicate that negative charges are generated on the SiO2 gate-insulator surface by exposure to ambient air and dry air. The influence of SiO− on the gate-insulator surface and deprotonation processes of SiOH caused by H2O and O2 were discussed as the origin of the significantly positive Vth shift.
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85.30.Tv Field effect devices
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Polymer hot-carrier transistor with low bandgap emitter

Yu-Chiang Chao, Ming-Hong Xie, Ming-Zhi Dai, Hsin-Fei Meng, Sheng-Fu Horng, and Chain-Shu Hsu

Appl. Phys. Lett. 92, 093310 (2008); http://dx.doi.org/10.1063/1.2839395 (3 pages) | Cited 11 times

Online Publication Date: 7 March 2008

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Vertical polymer hot-carrier transistor using the low bandgap material poly(3-hexylthiophene) as both the emitter and the collector are studied. The common emitter current gain is shown to depend on the LiF thickness and the emitter thickness, with maximal value at 31. Current density as high as 31 mA/cm2 is achieved when collector voltage is −10 V. For the device using blend of poly(3-hexylthiophene) and high bandgap polymer poly(9-vinylcarbazole) as the emitter, the current density rises sharply to 428 mA∕cm2. The brightness of 3000 cd/m2 is obtained as a polymer light-emitting diode is driven by the transistor with the same area. The transistor can be operated at 100 kHz.
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85.30.Tv Field effect devices
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Experimental determination of the rate law for charge carrier decay in a polythiophene: Fullerene solar cell

C. G. Shuttle, B. O’Regan, A. M. Ballantyne, J. Nelson, D. D. C. Bradley, J. de Mello, and J. R. Durrant

Appl. Phys. Lett. 92, 093311 (2008); http://dx.doi.org/10.1063/1.2891871 (3 pages) | Cited 22 times

Online Publication Date: 7 March 2008

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We use transient photovoltage and differential charging experiments, complemented by transient absorption data, to determine charge carrier lifetimes and densities in a poly(3-hexylthiophene): methanofullerene solar cell at Voc as a function of white light-bias intensity. For a typical device, the charge carrier decay dynamics are observed to exhibit an approximately third order dependence on charge density (dn/dt∝n3).
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84.60.Jt Photoelectric conversion
85.65.+h Molecular electronic devices
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