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4 Feb 2008

Volume 92, Issue 5, Articles (05xxxx)

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Appl. Phys. Lett. 92, 053301 (2008); http://dx.doi.org/10.1063/1.2838457 (3 pages)

Michael S. Arnold, Gregory J. McGraw, Stephen R. Forrest, and Richard R. Lunt
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Direct vapor jet printing of three color segment organic light emitting devices for white light illumination

Michael S. Arnold, Gregory J. McGraw, Stephen R. Forrest, and Richard R. Lunt

Appl. Phys. Lett. 92, 053301 (2008); http://dx.doi.org/10.1063/1.2838457 (3 pages) | Cited 12 times

Online Publication Date: 4 February 2008

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Organic-vapor jet printing (OVJP) is used to directly pattern small molecular weight electrophosphorescent white organic light-emitting devices (WOLEDs) consisting of parallel stripes of separate red, green, and blue OLEDs. The spatial patterning of the WOLEDs with peak forward viewing external quantum and luminous efficiencies of 7.1±0.3% and 7.0±0.4 lm/W, respectively, is accomplished by transporting the vapors of organic semiconductors through a nozzle to a translating sample substrate. Using multiple color segments to generate white light allows for the individual optimization of each color, is continuously color tunable, and can be used to compensate for differential aging of the constituent dopants. The theory and experimental results of WOLED printing via OVJP are discussed.
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85.60.Jb Light-emitting devices
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Low operation voltage of inkjet-printed plastic sheet-type micromechanical switches

Shintaro Nakano, Tsuyoshi Sekitani, Tomoyuki Yokota, and Takao Someya

Appl. Phys. Lett. 92, 053302 (2008); http://dx.doi.org/10.1063/1.2840378 (3 pages) | Cited 10 times

Online Publication Date: 4 February 2008

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We have fabricated plastic micromechanical switches using inkjet printing technologies. By reducing the device dimensions, particularly the spacing between the electrodes for electrostatic attraction, the operation voltage is reduced to 6.6 V. The frequency response exceeds 1 kHz when the operation voltage is above 20 V. When the number of periodic cycles of the rectangular voltage waveforms that are applied to the control electrodes in air exceeds 2×106, the on resistance changes slightly from 25 to 30 Ω, thereby demonstrating the reliability and stability of the present plastic switches.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
84.32.Dd Connectors, relays, and switches
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Effects of ultraviolet light irradiation on poly(vinlycarbazole)

Lei Qian, Debasis Bera, and Paul H. Holloway

Appl. Phys. Lett. 92, 053303 (2008); http://dx.doi.org/10.1063/1.2840157 (3 pages) | Cited 6 times

Online Publication Date: 4 February 2008

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In this paper, we present a study of the photophysical and photochemical effects that influence phosphor-doped polymer light-emitting diodes based on poly(vinlycarbazole). With ultraviolet (UV) light irradiation, the relative concentration of partially overlapping carbazole groups [poly(vinlycarbazole), (p-PVK)] to the fully overlapping conformation (f-PVK) is decreased via photoisomerization, which affects both the energies and the probability of forming singlet versus triplet excitons. As a result, phosphor-doped polymer light-emitting diodes fabricated using UV irradiated PVK exhibited higher efficiencies as compared with control devices (doped PVK without UV light irradiation). These data demonstrate a new route for the production of more efficient electrophosphorescent light-emitting diodes.
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85.60.Jb Light-emitting devices
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Highly efficient green organic light-emitting diodes from single exciplex emission

Dan Wang, Wenlian Li, Bei Chu, Zisheng Su, Defeng Bi, Dongyu Zhang, Jianzhuo Zhu, Fei Yan, Yiren Chen, and Taiju Tsuboi

Appl. Phys. Lett. 92, 053304 (2008); http://dx.doi.org/10.1063/1.2841060 (3 pages) | Cited 10 times

Online Publication Date: 5 February 2008

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Spectral single and stable green exciplex emission was demonstrated from organic light-emitting diodes (OLEDs) with 4,4′,4″-tris[3-methylphenyl(phenyl)amino] triphenylamine and 4,7-diphenyl-1,10-phenanthroline that function as electron donor (D) and acceptor (A), respectively. As 8-hydroxyquinoline aluminum (Alq3) was attached to the acceptor layer, electroluminescent (EL) properties of the two exciplex-type OLEDs with D/A-bilayer and D:A mixture layer configurations were markedly improved, i.e., a peak current efficiency of 7.6 cd/A at 2.38 mA/cm2 in three-layer device and a maximum luminance of 6620 cd/m2 at 8.7 V in blend layer device were obtained, respectively, without changing the peak position (535 nm) and the shape of EL spectrum. Discussion is given on the harvest of the pure green exciplex emission and enhancement of luminance which is obtained by inserting Alq3 layer.
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85.60.Jb Light-emitting devices
85.60.Bt Optoelectronic device characterization, design, and modeling
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
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Moisture induced electron traps and hysteresis in pentacene-based organic thin-film transistors

Gong Gu and Michael G. Kane

Appl. Phys. Lett. 92, 053305 (2008); http://dx.doi.org/10.1063/1.2841918 (3 pages) | Cited 14 times

Online Publication Date: 6 February 2008

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Previous work revealed that electron traps in pentacene formed in air cause hysteresis in thin-film transistor characteristics. Here, we experimentally showed that water vapor, rather than oxygen, is responsible for these hysteresis-causing trap states. Photogenerated and injected electrons are trapped at the pentacene-dielectric interface under positive gate bias, and induce extra holes, resulting in the observed extra drain current. The electron detrapping causes the decay of the extra hole population with time under negative gate bias and, therefore, that of the drain current.
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85.30.Tv Field effect devices
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Organic semiconductor-carbon nanotube bundle bilayer field effect transistors with enhanced mobilities and high on/off ratios

Shuhong Liu, Stefan C. B. Mannsfeld, Melbs C. LeMieux, Hang W. Lee, and Zhenan Bao

Appl. Phys. Lett. 92, 053306 (2008); http://dx.doi.org/10.1063/1.2841033 (3 pages) | Cited 5 times

Online Publication Date: 6 February 2008

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Organic field-effect transistors containing pentacene or α sexithiophene on random arrays of carbon nanotube bundles were fabricated. The impact of nanotube bundles on the performance of devices with different geometries and surface treatments was studied. Upon incorporation of an appropriate amount of nanotube bundles, we observed an increase in the “effective” field effect mobility as high as 20 times while maintaining the high on/off ratios. Furthermore, our preliminary results show that nanotube bundles might template the growth of organic crystals under certain conditions, resulting in the formation of organic nucleates with preferred orientations.
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85.30.Tv Field effect devices
85.35.Kt Nanotube devices
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Optimal negative electrodes for poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester bulk heterojunction photovoltaic devices

Matthew O. Reese, Matthew S. White, Garry Rumbles, David S. Ginley, and Sean E. Shaheen

Appl. Phys. Lett. 92, 053307 (2008); http://dx.doi.org/10.1063/1.2841067 (3 pages) | Cited 55 times

Online Publication Date: 6 February 2008

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The role of the work function and interfacial chemistry on organic device performance was investigated by using a series of contact materials. The active layer was a standard blend of poly(3-hexylthiophene) and [6-6]-phenyl C61-butyric acid methyl ester. Over 100 devices were fabricated and measured to obtain good statistics. Ba/Al and Ca/Al electrodes performed best, with similar open-circuit voltages and power conversion efficiencies. Device stability studies showed devices with these two electrodes remained similar after six weeks with degradation of 11%–16% in net conversion efficiency observed. The incorporation of silver into the electrodes led to considerably more degradation than other electrode types.
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85.60.-q Optoelectronic devices
84.30.Sk Pulse and digital circuits
84.70.+p High-current and high-voltage technology: power systems; power transmission lines and cables
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Influence of polymer ionization potential on the open-circuit voltage of hybrid polymer/TiO2 solar cells

T. Ishwara, D. D. C. Bradley, J. Nelson, P. Ravirajan, I. Vanseveren, T. Cleij, D. Vanderzande, L. Lutsen, S. Tierney, M. Heeney, and I. McCulloch

Appl. Phys. Lett. 92, 053308 (2008); http://dx.doi.org/10.1063/1.2840608 (3 pages) | Cited 5 times

Online Publication Date: 7 February 2008

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We report studies of the dependence of the open-circuit voltage (VOC) of polymer/titanium dioxide hybrid devices on the ionization potential of the polymer (IP). Once corrected for differences in photocarrier generation by the polymers, the measured VOC values vary linearly with the polymer IP, with a slope of 0.8±0.1. This behavior agrees with recent studies of polymer/fullerene photovoltaic devices and is consistent with the hypothesis that VOC of an organic donor-acceptor solar cell is limited by the energy difference between the highest occupied molecular orbital of the donor (in this case, the polymer) and the lowest unoccupied electronic level of the acceptor (in this case, the conduction band edge of the TiO2).
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84.60.Jt Photoelectric conversion
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Evolution of the unoccupied states in Cs-doped copper phthalocyanine

Huanjun Ding and Yongli Gao

Appl. Phys. Lett. 92, 053309 (2008); http://dx.doi.org/10.1063/1.2842384 (3 pages) | Cited 3 times

Online Publication Date: 7 February 2008

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The evolution of both the occupied and unoccupied states for Cs-doped copper phthalocyanine (CuPc) has been investigated with photoemission and inverse photoemission spectroscopy. As the Cs doping ratio increases, the lowest unoccupied molecular orbital (LUMO) of CuPc shifts downwards, reaching the Fermi level. After the saturation, the LUMO intensity decreases monotonically, while a gap state grows in the valence spectra, which gives direct evidence for the origin of the doping-induced gap state in CuPc molecules. The intensity of the LUMO, as well as the gap state, suggest the formation of multiply charged CuPc spices in heavily doped film.
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71.20.Rv Polymers and organic compounds
61.72.up Other materials
79.60.Bm Clean metal, semiconductor, and insulator surfaces
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Simultaneous heterojunction organic solar cells with broad spectral sensitivity

Fan Yang, Richard R. Lunt, and Stephen R. Forrest

Appl. Phys. Lett. 92, 053310 (2008); http://dx.doi.org/10.1063/1.2839408 (3 pages) | Cited 10 times

Online Publication Date: 7 February 2008

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We demonstrate organic solar cells incorporating two simultaneously active donor-acceptor heterojunctions contained within a single film with sensitivity extending across the visible solar spectrum into the near infrared (NIR). The NIR absorbing material, tin(II)-phthalocyanine (SnPc), is grown into nanocrystalline islands using organic vapor phase deposition. Using an optimized copper phthalocyanine (CuPc)/SnPc/C60 nanocrystalline network, we obtain a power conversion efficiency of (2.9±0.2)% under 100 mW/cm2, AM1.5G illumination. We find that the open circuit voltage of the simultaneous heterojunction is determined by the lowest voltage of the separate donor-acceptor constituent junctions.
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84.60.Jt Photoelectric conversion
78.40.Me Organic compounds and polymers
78.30.Hv Other nonmetallic inorganics
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Reduced efficiency roll-off in high-efficiency hybrid white organic light-emitting diodes

Gregor Schwartz, Sebastian Reineke, Karsten Walzer, and Karl Leo

Appl. Phys. Lett. 92, 053311 (2008); http://dx.doi.org/10.1063/1.2836772 (3 pages) | Cited 17 times

Online Publication Date: 8 February 2008

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White organic light emitting diodes harvesting triplet excitons from the fluorescent blue emitter N,N′-di-1-naphthalenyl-N,N′-diphenyl-[1,1′:4′,1″:4″,1‴-quaterphenyl]-4,4‴-diamine (4P-NPD) are presented. Direct doping of the phosphorescent orange iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) into 4P-NPD results in a strongly reduced efficiency roll-off as compared to separate emission layers, and yields 49.3 lm W−1 total external power efficiency (24.1% quantum efficiency) at a luminance of 1000 cd m−2 [CIE 1931 chromaticity coordinates (0.49,0.41)], measured in an integrating sphere. Introduction of an exciton balancing interlayer improves the chromaticity (0.43,0.43) toward the CIE illuminant A warm white point and keeps a high efficiency of 40.7 lm W−1, 20.3%.
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85.60.Jb Light-emitting devices
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Improved performance of polymer/TiO2 nanorod bulk heterojunction photovoltaic devices by interface modification

Yun-Yue Lin, Tsung-Hung Chu, Chun-Wei Chen, and Wei-Fang Su

Appl. Phys. Lett. 92, 053312 (2008); http://dx.doi.org/10.1063/1.2839405 (3 pages) | Cited 33 times

Online Publication Date: 8 February 2008

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In this article, the polymer photovoltaic devices based on the poly(3-hexylthiophene)/TiO2 nanorods hybrid material is present. An enhancement in the device performance can be achieved by removing or replacing the insulating surfactant on the TiO2 nanorod surface with a more conductive ligand, which can play the role to assist charge separation efficiency or also to prevent from back recombination, giving a large improvement in the short circuit current and fill factor. The relatively high power conversion efficiency of 1.7% under simulated AM 1.5 illumination (100 mW/cm2) can be achieved, providing a route for fabricating low-cost, environmentally friendly polymer photovoltaic devices by all-solution processes.
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85.30.De Semiconductor-device characterization, design, and modeling
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