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18 Aug 2008

Volume 93, Issue 7, Articles (07xxxx)

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Appl. Phys. Lett. 93, 073101 (2008); http://dx.doi.org/10.1063/1.2970055 (3 pages)

A. J. Du, Y. Chen, G. Q. Lu, and Sean C. Smith
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Effect of postannealing on the performance of pentacene single-crystal ambipolar transistors

Taishi Takenobu, Kazuhiro Watanabe, Yohei Yomogida, Hidekazu Shimotani, and Yoshihiro Iwasa

Appl. Phys. Lett. 93, 073301 (2008); http://dx.doi.org/10.1063/1.2969772 (3 pages) | Cited 15 times

Online Publication Date: 18 August 2008

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Ambipolar organic field-effect transistors (OFETs) with comparable hole and electron mobilities were fabricated using pentacene single crystals. Ambipolar single-crystal transistors enable us to determine the intrinsic effect of postannealing on the performance of OFETs, which remains controversial. The results confirm that postannealing predominantly causes hole dedoping due to oxygen desorption.
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85.30.Pq Bipolar transistors
85.30.Tv Field effect devices
61.72.Cc Kinetics of defect formation and annealing
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High efficiency, color stability, and stable efficiency roll off in three color hybrid white organic light emitting diodes

Kyoung Soo Yook, Soon Ok Jeon, Chul Woong Joo, and Jun Yeob Lee

Appl. Phys. Lett. 93, 073302 (2008); http://dx.doi.org/10.1063/1.2969821 (3 pages) | Cited 13 times

Online Publication Date: 18 August 2008

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High efficiency, good color stability, and stable efficiency roll off were achieved in three color white organic light emitting diodes by developing a device architecture with red and green phosphorescent materials codoped in a mixed host emitting structure. A fluorescent blue material was used in a blue emitting layer and a mixed interlayer structure was applied. A high quantum efficiency of 9.7% was obtained and color coordinate of (0.40, 0.40) was kept constant up to luminance of 10 000 cd/m2. In addition, efficiency decrease at 10 000 cd/m2 was only 2% of the maximum quantum efficiency.
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85.60.Jb Light-emitting devices
42.70.Jk Polymers and organics
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Organic photovoltaic cells made from sandwich-type rare earth phthalocyaninato double and triple deckers

Qi Wang, Yong Li, Xingzhong Yan, Monika Rathi, Michael Ropp, David Galipeau, and Jianzhuang Jiang

Appl. Phys. Lett. 93, 073303 (2008); http://dx.doi.org/10.1063/1.2966151 (3 pages) | Cited 13 times

Online Publication Date: 19 August 2008

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This work presents organic-inorganic hybrid solar cells, which possess the capability for broad band photon harvesting from an ultraviolet-visible to a near infrared range. These solar cells are bulk heterojunction devices, which have been fabricated by free base phthalocyanine and rare earth phthalocyaninato double or triple deckers (electron donors) with a perylenediimide derivative (electron acceptor). Two-type cell structures with or without nanostructured TiO2 layers have been presented. A characterization of the structures, steady state spectroscopy, fluorescence dynamics, and photoelectrical property of these cells and the active materials has been carried out. A cell structure of In2O3:SnO2/TiO2-active material-TiO2/Al has shown a significant improvement in conversion efficiency.
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85.60.-q Optoelectronic devices
84.60.Jt Photoelectric conversion
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Balanced ambipolar charge carrier mobility in mixed layers for application in hybrid white organic light-emitting diodes

Gregor Schwartz, Tung-Huei Ke, Chung-Chih Wu, Karsten Walzer, and Karl Leo

Appl. Phys. Lett. 93, 073304 (2008); http://dx.doi.org/10.1063/1.2973151 (3 pages) | Cited 22 times

Online Publication Date: 20 August 2008

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We investigate the electron and hole mobility in mixed layers of N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine and bis(2-methyl-8-quinolinato)-4-phenylphenolate aluminum with different mix ratios, using both space-charge limited currents of single-carrier devices with electrically doped charge transport layers and time-of-flight measurements. Both experimental methods yield consistent results. The 1:1 blend shows balanced ambipolar charge carrier transport, which is advantageous for the application as exciton blocking interlayer in hybrid white organic light-emitting diodes: The electroluminescence spectrum is rather stable against changes in interlayer thickness and driving current. Moreover, the external quantum efficiency is enhanced by a factor of 2.5 as compared to a device without interlayer.
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85.60.Jb Light-emitting devices
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
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Polymer thin film transistor without surface pretreatment on silicon nitride gate dielectric

Flora M. Li, Parul Dhagat, Hanna M. Haverinen, Iain McCulloch, Martin Heeney, Ghassan E. Jabbour, and Arokia Nathan

Appl. Phys. Lett. 93, 073305 (2008); http://dx.doi.org/10.1063/1.2927485 (3 pages) | Cited 2 times

Online Publication Date: 22 August 2008

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It is well known that surface modification of the gate dielectric in organic thin film transistors (TFTs) plays an important role in device performance, often giving rise to severalfold improvements in field-effect mobility. This paper reports on solution-processed polymer TFTs with mobilities comparable to high performance counterparts despite the absence of dielectric surface pretreatment. An effective mobility of 0.1 cm2/V s was obtained with poly(2,5-bis(3-dodecylthiophene-2-yl)thieno[3,2-b]thiophene) transistors on silicon nitride gate dielectric. The results indicate that by judicious preparation of the device layers, one can mitigate the need for dielectric surface pretreatment, thereby reducing fabrication complexity without compromising TFT performance.
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85.30.Tv Field effect devices
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Excitation of dielectric-loaded surface plasmon polariton observed by using near-field optical microscopy

Zheyu Fang, Xuejin Zhang, Dan Liu, and Xing Zhu

Appl. Phys. Lett. 93, 073306 (2008); http://dx.doi.org/10.1063/1.2973355 (3 pages) | Cited 9 times

Online Publication Date: 22 August 2008

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The excitation of dielectric-loaded surface plasmon polariton (DLSPP) by a CdS nanostripe placed on a Ag layer is observed by using a scanning near-field optical microscopy (SNOM). The spectroscopic redshift of 30 meV of photoluminescence is observed at different positions of the stripe by SNOM and can be explained as the result of Franz–Keldysh effect. Finite difference time domain calculations confirm the observations of the near-field optical images and spectroscopic data. This DLSPP excitation structure is important in the implementation of photonic integrated circuits as well as the optical guiding modes in plasmonic devices.
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73.22.Lp Collective excitations
78.68.+m Optical properties of surfaces
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
68.37.Uv Near-field scanning microscopy and spectroscopy
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Plasmon enhanced performance of organic solar cells using electrodeposited Ag nanoparticles

Seok-Soon Kim, Seok-In Na, Jang Jo, Dong-Yu Kim, and Yoon-Chae Nah

Appl. Phys. Lett. 93, 073307 (2008); http://dx.doi.org/10.1063/1.2967471 (3 pages) | Cited 112 times

Online Publication Date: 22 August 2008

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To enhance solar harvesting in organic solar cells, uniform-sized metal nanoparticles of ∼ 13 nm were incorporated to the device via pulse-current electrodeposition, which is a kind of simple and quick solution process that can control the density and size of metal nanoparticles. By incorporating plasmonic Ag nanoparticles on surface modified transparent electrodes, overall power conversion efficiency was increased from 3.05% to 3.69%, mainly resulting from the improved photocurrent density as a result of enhanced absorption of the photoactive conjugate polymer due to the high electromagnetic field strength in the vicinity of the excited surface plasmons.
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84.60.Jt Photoelectric conversion
81.15.Pq Electrodeposition, electroplating
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Indium-free transparent organic light emitting diodes with Al doped ZnO electrodes grown by atomic layer and pulsed laser deposition

J. Meyer, P. Görrn, S. Hamwi, H.-H. Johannes, T. Riedl, and W. Kowalsky

Appl. Phys. Lett. 93, 073308 (2008); http://dx.doi.org/10.1063/1.2975176 (3 pages) | Cited 32 times

Online Publication Date: 22 August 2008

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We present highly efficient transparent organic light emitting diodes (OLEDs) with Al doped ZnO (AZO) electrodes prepared by atomic layer deposition and pulsed laser deposition (PLD). The power and current efficiencies exceed 27 lm/W and 44 cd/A at a brightness level of 100 cd/m2, respectively. At the same time, the transmissivity of the devices is above 73% in the visible part of the spectrum. Owing to an efficient WO3 buffer layer and an optimized PLD process for the deposition of the top AZO electrode, the OLEDs show leakage current densities as low as 3×10−5 mA/cm2 at a reverse bias of 6 V. Therefore, our study paves the way for indium-free, see-through OLED displays.
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85.60.Jb Light-emitting devices
85.60.Pg Display systems
42.79.Kr Display devices, liquid-crystal devices
81.15.Fg Pulsed laser ablation deposition
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
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Efficient all-polymer solar cells based on blend of tris(thienylenevinylene)-substituted polythiophene and poly[perylene diimide-alt-bis(dithienothiophene)]

Zhan’ao Tan, Erjun Zhou, Xiaowei Zhan, Xiang Wang, Yongfang Li, Stephen Barlow, and Seth R. Marder

Appl. Phys. Lett. 93, 073309 (2008); http://dx.doi.org/10.1063/1.2975160 (3 pages) | Cited 50 times

Online Publication Date: 22 August 2008

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A narrow band-gap alternating copolymer of perylene diimide and bis(dithienothiophene) (2) and a polythiophene derivative substituted by a tris(thienylenevinylene) conjugated side chain (4) are used as acceptor and donor, respectively, in all-polymer solar cells (SCs). The optimized device based on the blend of 4 and 2 in the ratio 3:1 (w/w) gives a short circuit current (Jsc) of 5.02 mA cm−2 and a power conversion efficiency of 1.48%, under simulated AM 1.5 illumination at 100 mW cm−2. These values are among the highest values reported for all-polymer SCs.
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84.60.Jt Photoelectric conversion
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A conductor/insulator/conductor complex layer at anode for current Enhancement in a Polymer Solar Cell

Hsing-Wang Tsai, Zingway Pei, and Yi-Jen Chan

Appl. Phys. Lett. 93, 073310 (2008); http://dx.doi.org/10.1063/1.2974087 (3 pages) | Cited 6 times

Online Publication Date: 22 August 2008

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In this letter, a structure in the form of conductor/insulator/conductor was inserted into the anode and photoactive material of a polymer solar cell to enhance the photocurrent by magnifying the internal electrical field. Through this structure, a 15.8% enhancement in photocurrent and a 11.2% enhancement in power conversion efficiency over standard cell were obtained. The mechanism for the internal electrical field enhancement was proposed in terms of modifying the workfunction of the anode material and was proved by the ultraviolet photon spectroscopy.
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84.60.Jt Photoelectric conversion
73.40.Ty Semiconductor-insulator-semiconductor structures
73.30.+y Surface double layers, Schottky barriers, and work functions
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Thickness dependence of space charge limited current and injection limited current in organic molecular semiconductors

Ruchi Agrawal, Pramod Kumar, Subhasis Ghosh, and Ajit Kumar Mahapatro

Appl. Phys. Lett. 93, 073311 (2008); http://dx.doi.org/10.1063/1.2974084 (3 pages) | Cited 15 times

Online Publication Date: 22 August 2008

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We report the experimental investigations on space charge limited current (SCLC) and injection limited current (ILC) in copper phthalocyanine (CuPc), sandwiched between two metal electrodes. Thickness dependence of current-voltage characteristics of SCLC and ILC is accurately reproduced by the electric field and temperature dependent charge carrier mobility, without invoking charge density dependent mobility. These results are interpreted using a consistent description of SCLC and ILC, based on a unified model of hopping transport within Gaussian density of states in CuPc.
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72.20.Ht High-field and nonlinear effects
72.80.Le Polymers; organic compounds (including organic semiconductors)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.20.Ee Mobility edges; hopping transport
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