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29 Sep 2008

Volume 93, Issue 13, Articles (13xxxx)

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Appl. Phys. Lett. 93, 132101 (2008); http://dx.doi.org/10.1063/1.2988720 (3 pages)

Yueh-Nan Chen and Lukas Gilz
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Highly efficient organic light emitting devices with insulator MnO as an electron injecting and transporting material

Jiaxiu Luo, Lixin Xiao, Zhijian Chen, and Qihuang Gong

Appl. Phys. Lett. 93, 133301 (2008); http://dx.doi.org/10.1063/1.2960349 (3 pages) | Cited 18 times

Online Publication Date: 29 September 2008

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A facile way to fabricate highly efficient organic light emitting devices with insulator MnO as an electron injecting and transporting material was devised, which eliminates the problem of the oxidation of reactive dopants. The power efficiency of 1.1 lm/W by inserting 3-nm-thick MnO as the electron injecting layer was obtained, higher than the 0.8 lm/W efficiency for the reference device with 0.5-nm-thick LiF. A thermal coevaporation layer containing 10% weight of MnO and tris(8-hydroxyquinolato)aluminum (Alq3) as the electron transporting layer showed more efficient electron transport ability, with turn-on voltage of 3.8 V, lower than 7.4 V for the intrinsic Alq3.
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85.60.Jb Light-emitting devices
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Fluoropolymer indium-tin-oxide buffer layers for improved power conversion in organic photovoltaics

Bonan Kang, L. W. Tan, and S. R. P. Silva

Appl. Phys. Lett. 93, 133302 (2008); http://dx.doi.org/10.1063/1.2983742 (3 pages) | Cited 18 times

Online Publication Date: 29 September 2008

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We investigate the effects of polytetrafluoroethylene (PTFE) on poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) based organic photovoltaic (OPV) devices by inserting thermally evaporated PTFE films between indium-tin-oxide (ITO) and P3HT:PCBM layers. Significant improvement in terms of open-circuit voltage, short-circuit current, and thereby in its commensurate power conversion efficiency is achieved compared to devices with poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) layers. The OPVs performance enhancement is attributed to the formation of an artificial dipole layer resulting from the rich, negatively charged fluorine that facilitates the hole extraction process. This result shows the high potential of PTFE as a low cost and stable ITO buffer layer for OPV devices.
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85.60.Bt Optoelectronic device characterization, design, and modeling
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Electron trapping in pentacene based p- and n-type organic field-effect transistors

Christopher Siol, Christian Melzer, and Heinz von Seggern

Appl. Phys. Lett. 93, 133303 (2008); http://dx.doi.org/10.1063/1.2992031 (3 pages) | Cited 9 times

Online Publication Date: 30 September 2008

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We report on electron trapping in the channel of pentacene field-effect transistors due to electron injection over large energy barriers. Using scanning Kelvin-probe microscopy, one observes temporary and permanent electron trapping in the transistor channel for both n- and p-type organic field-effect transistors driven in ambipolar mode, proving the ability of electrons to be injected from Au contacts into pentacene despite a large energy barrier. The trapped negative charge carriers are determined to be the origin of hysteresis in n-type devices and appear to be the source of threshold voltage shifts for p-type transistors.
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85.30.Tv Field effect devices
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Realization of dual-channel organic field-effect transistors and their applications to chemical sensing

Yeon Taek Jeong, Brian H. Cobb, Shannon D. Lewis, Ananth Dodabalapur, Shaofeng Lu, Antonio Facchetti, and Tobin J. Marks

Appl. Phys. Lett. 93, 133304 (2008); http://dx.doi.org/10.1063/1.2952485 (3 pages) | Cited 3 times

Online Publication Date: 2 October 2008

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We report on the realization of dual-channel organic field-effect transistors (FETs). These devices have a four-terminal configuration with a polymeric semiconductor p-channel, a small molecule semiconductor n-channel, and a polymeric gate dielectric. The polymeric p-channel and the small molecule n-channel are coupled across the gate dielectric. Both the p-FET and the n-FET exhibit acceptable device characteristics at ∣VDS∣ ⩽ 50 V and ∣VG∣ ⩽ 50 V, in which the performances of the p-FET and the n-FET are comparable. The p-FET and n-FET respond to isopropyl alcohol and ethanol vapors with significant sensitivities.
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85.30.Tv Field effect devices
85.75.Hh Spin polarized field effect transistors
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Low-voltage C60 organic field-effect transistors with high mobility and low contact resistance

X.-H. Zhang and B. Kippelen

Appl. Phys. Lett. 93, 133305 (2008); http://dx.doi.org/10.1063/1.2993349 (3 pages) | Cited 13 times

Online Publication Date: 2 October 2008

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State-of-the-art C60 organic transistors are reported here by engineering the essential electrode/semiconductor and dielectric/semiconductor interfaces. By using calcium (Ca) as the source and drain electrodes, the width-normalized contact resistance (RCW) at the electrode/semiconductor interface could be reduced to a constant value of 2 kΩ cm at a gate-source voltage (VGS) of 2.6 V, for devices with channel lengths ranging from 25 to 200 μm. Channel transconductance is observed to follow channel length scaling, and charge mobility average value of 2.5 cm2/Vs at VGS<5 V is found independent of channel length within the studied range.
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85.65.+h Molecular electronic devices
85.30.Tv Field effect devices
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Improvement in carrier transport and recombination of white phosphorescent organic light-emitting devices using a composite blue emitter

Meng-Ting Lee, Jin-Sheng Lin, Miao-Tsai Chu, and Mei-Rurng Tseng

Appl. Phys. Lett. 93, 133306 (2008); http://dx.doi.org/10.1063/1.2994697 (3 pages) | Cited 14 times

Online Publication Date: 2 October 2008

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We demonstrate high-efficiency white phosphorescent organic light-emitting devices (PHOLEDs) based on a yellow and composite blue emitters. The composite blue emitter is constructed from a wide-band-gap host, organometallic iridium dopant, and a carrier-transporting material. Under the same driving current density and emissive color, the current efficiency of the white PHOLEDs can be enhanced by a factor of 1.4 comparing to that of using typical blue emitter composed of host and dopant only. Attaching an outcoupling enhancement film onto glass substrate, the white PHOLEDs with a current efficiency of 47 cd/A, a power efficiency of 32 lm/W, and a CIEx,y (CIE: Commission Internationale d'Eclairage) of (0.40,0.44) at a practical brightness of 1000 cd/m2 can be achieved.
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85.60.Jb Light-emitting devices
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Ideal laminate theory for water transport analysis of metal-coated polymer films

Changsoo Jang, Young-Rae Cho, and Bongtae Han

Appl. Phys. Lett. 93, 133307 (2008); http://dx.doi.org/10.1063/1.2996018 (3 pages) | Cited 3 times

Online Publication Date: 2 October 2008

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We propose and validate a diffusion model, based on the ideal laminate theory (ILT), to describe water transport through metal-coated polymer films for barrier applications. The physical properties required for numerical implementation are studied. The analysis reveals that the solubility of the metallic coating is not required for an accurate water transport analysis, which makes the ILT-based model practical. The model is implemented numerically and the results are compared with the experimentally measured transient water weight gain of PET films with Al and Cr coatings to corroborate the validity of the model.
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66.30.-h Diffusion in solids
64.75.Bc Solubility
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Carrier mobility, structural order, and solar cell efficiency of organic heterojunction devices

R. A. Street

Appl. Phys. Lett. 93, 133308 (2008); http://dx.doi.org/10.1063/1.2996029 (3 pages) | Cited 16 times

Online Publication Date: 2 October 2008

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The loss mechanisms limiting solar cell efficiency in organic heterojunction photodiodes are discussed. We show that the energy loss when the exciton is split, and absence of Langevin recombination at the interface, are related to the magnitude of the electron-phonon interaction and the wavefunction extent of the electron and hole. In contrast to several recent publications, it is argued that well-ordered, high mobility organic materials will provide higher efficiency cells.
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84.60.Jt Photoelectric conversion
85.60.Dw Photodiodes; phototransistors; photoresistors
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Low voltage and very high efficiency deep-blue phosphorescent organic light-emitting devices

Sang-Hyun Eom, Ying Zheng, Neetu Chopra, Jaewon Lee, Franky So, and Jiangeng Xue

Appl. Phys. Lett. 93, 133309 (2008); http://dx.doi.org/10.1063/1.2996274 (3 pages) | Cited 36 times

Online Publication Date: 2 October 2008

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We report on very high efficiency deep-blue phosphorescent organic light-emitting devices (PHOLEDs) based on iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6). Dual emissive layers consisting of an N,N′-dicarbazolyl-3,5-benzene layer doped with 4 wt % FIr6 and a p-bis(triphenylsilyly)benzene layer doped with 25 wt % FIr6 were employed to maximize exciton generation on FIr6 molecules. Combined with the p-i-n device structure, we achieved a low turn-on voltage of 3.2 V and very high power efficiencies of 25±2 lm/W at 100 cd/m2 and 20±2 lm/W at 1000 cd/m2 for such deep-blue PHOLEDs with peak emission at a wavelength of 458 nm.
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85.60.Jb Light-emitting devices
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Degradation studies on high-voltage-driven organic light-emitting device using in situ on-operation method with scanning photoelectron microscopy

J. Lee, S. Sohn, H. J. Yun, and H. J. Shin

Appl. Phys. Lett. 93, 133310 (2008); http://dx.doi.org/10.1063/1.2994668 (3 pages) | Cited 1 time

Online Publication Date: 3 October 2008

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We investigated the degradation behavior of a high-voltage-driven organic light-emitting device (OLED) by operating the device in an ultrahigh-vacuum environment. In situ on-operation method provided the initial degradation process when the OLED was biased inside an analysis chamber. The degraded area was probed by scanning photoelectron microscopy (SPEM) using synchrotron. SPEM showed that the degradation was accompanied by a local drift of indium tin oxide (anode) toward Al (cathode) and that the heat from the degraded area separated the cathode from the Alq3 layer, forming large bubbles. These results also indicate that microbubbles were formed under the Alq3, implying, before popping, the existence of local high-temperature degradation spots.
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85.60.Jb Light-emitting devices
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Effect of increased surface area of stainless steel substrates on the efficiency of dye-sensitized solar cells

Ho-Gyeong Yun, Yongseok Jun, Jongdae Kim, Byeong-Soo Bae, and Man Gu Kang

Appl. Phys. Lett. 93, 133311 (2008); http://dx.doi.org/10.1063/1.2996017 (3 pages) | Cited 28 times

Online Publication Date: 3 October 2008

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In order to increase the electrical contact area between TiO2 particles and stainless steel (StSt) substrates of the dye-sensitized solar cells (DSSCs), StSt foil was roughened electrochemically using sulfuric acid with some additives. Compared with the DSSC with nontreated StSt substrate, DSSC with this roughened StSt substrate showed a 33% increase in light-to-electricity conversion efficiency with negligible effect on open circuit voltage (Voc) and fill factor. Electrochemical impedance spectroscopy clearly confirmed that the increased performance was due to a decreased electrical resistance at the TiO2/StSt interface.
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84.60.Jt Photoelectric conversion
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Highly efficient, deep-blue phosphorescent organic light emitting diodes with a double-emitting layer structure

H. Fukagawa, K. Watanabe, T. Tsuzuki, and S. Tokito

Appl. Phys. Lett. 93, 133312 (2008); http://dx.doi.org/10.1063/1.2996572 (3 pages) | Cited 37 times

Online Publication Date: 3 October 2008

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We have demonstrated a highly efficient, deep-blue organic light-emitting diode (OLED) using a host material with a high triplet energy. The OLED device that we have prepared utilizes a phosphorescent guest material, iridium(III)bis(4′,6′,-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate, exhibits a peak quantum efficiency of about 15.7%. We employed a double-emitting layer (DEL) structure that distributes the carrier recombination region within the device. In this DEL structure, the emission mechanism is such that the energy transfers from the host material in one emitting layer, and the other emitting layer provides for direct charge trapping in the guest material. This DEL structure proved to be quite useful in achieving the reported device characteristics.
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85.60.Jb Light-emitting devices
78.55.Kz Solid organic materials
78.66.Qn Polymers; organic compounds
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