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June 2013

Volume 6, Issue 6

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Role of intrinsic molecular dipole in energy level alignment at organic interfaces

Linda Lindell, Deniz Çakır, Geert Brocks, Mats Fahlman, and Slawomir Braun

Appl. Phys. Lett. 102, 223301 (2013); http://dx.doi.org/10.1063/1.4809567 (4 pages)

Online Publication Date: 5 June 2013

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The energy level alignment in metal-organic and organic-organic junctions of the widely used materials tris-(8-hydroxyquinoline)aluminum (Alq3) and 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) is investigated. The measured alignment schemes for single and bilayer films of Alq3 and NTCDA are interpreted with the integer charge transfer (ICT) model. Single layer films of Alq3 feature a constant vacuum level shift of ∼0.2–0.4 eV in the absence of charge transfer across the interface. This finding is attributed to the intrinsic dipole of the Alq3 molecule and (partial) ordering of the molecules at the interfaces. The vacuum level shift changes the onset of Fermi level pinning, as it changes the energy needed for equilibrium charge transfer across the interface.
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73.40.Ns Metal-nonmetal contacts
73.20.At Surface states, band structure, electron density of states
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Reversible luminance decay in polymer light-emitting electrochemical cells

Xiaoyu Li, Jun Gao, and Guojun Liu

Appl. Phys. Lett. 102, 223303 (2013); http://dx.doi.org/10.1063/1.4809603 (4 pages)

Online Publication Date: 6 June 2013

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The luminance decay of generic sandwich polymer light-emitting electrochemical cells has been investigated. Under constant current operation, the apparent luminance decay is caused by both the formation of non-emitting black spots, which decreases the active emitting area, and the in situ electrochemical doping, which quenches the luminescence of the light-emitting electrochemical cell film. The latter's effect on luminance, however, can be mostly reversed by letting the electrochemical doping relax. A dramatic recovery of luminance is observed when the device is stored without voltage bias and/or moderately heated between consecutive operations. The decay/recovery cycle can be repeated multiple times with little loss of luminance despite the high current density (167 mA/cm2) applied. At lower current density, a freshly made device loses less than 10% of its peak luminance after over 200 h of continuous operation. Polymer light-emitting electrochemical cells therefore possess vastly longer operating lifetime if allowed to recover from the effect of reversible doping.
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85.60.Jb Light-emitting devices
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Channel II photocurrent quantification in narrow optical gap polymer-fullerene solar cells with complimentary acceptor absorption

Yuliang Zhang, Ajay K. Pandey, Kristen Tandy, Gitish K. Dutta, Paul L. Burn, Paul Meredith, Ebinazar B. Namdas, and Satish Patil

Appl. Phys. Lett. 102, 223302 (2013); http://dx.doi.org/10.1063/1.4808386 (5 pages)

Online Publication Date: 6 June 2013

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Most charge generation studies on organic solar cells focus on the conventional mode of photocurrent generation derived from light absorption in the electron donor component (so called channel I). In contrast, relatively little attention has been paid to the alternate generation pathway: light absorption in the electron acceptor followed by photo-induced hole transfer (channel II). By using the narrow optical gap polymer poly(3,6-dithieno[3,2-b]thiophen-2-yl)-2,5-bis(2-octyldodecyl)-pyrrolo-[3,4-c]pyrrole-1,4-dione-5′,5″-diyl-alt-4,8-bis(dodecyloxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl with two complimentary fullerene absorbers; phenyl-C61-butyric acid methyl ester, and phenyl-C71-butyric acid methyl ester (70-PCBM), we have been able to quantify the photocurrent generated each of the mechanisms and find a significant fraction (>30%), which is derived in particular from 70-PCBM light absorption.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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Cesium-doped zinc oxide as electron selective contact in inverted organic photovoltaics

Achilleas Savva and Stelios A. Choulis

Appl. Phys. Lett. 102, 233301 (2013); http://dx.doi.org/10.1063/1.4811088 (5 pages)

Online Publication Date: 11 June 2013

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Water based sol-gel processed Cesium-doped Zinc oxide (CZO) with low processing annealing temperature is introduced as an efficient electron selective contact in inverted Organic Photovoltaics (OPVs). The corresponding inverted OPVs not only demonstrate similar performance compared to the well-established sol-gel processed ZnO inverted devices but also maintain their functionality when thick layers of CZO, suitable for the up scaling scenario of OPVs have been used. The three orders of magnitude higher conductivity of CZO than ZnO in combination with the high transmittance above 80%, makes this doped oxide a suitable electron selective contact for the low-cost, roll-to-roll printing process of OPVs.
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88.40.jm Thin film III-V and II-VI based solar cells
84.32.Dd Connectors, relays, and switches
88.40.hj Efficiency and performance of solar cells
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A flexible high potential printed battery for powering printed electronics

Abhinav M. Gaikwad, Daniel A. Steingart, Tse Nga Ng, David E. Schwartz, and Gregory L. Whiting

Appl. Phys. Lett. 102, 233302 (2013); http://dx.doi.org/10.1063/1.4810974 (5 pages)

Online Publication Date: 12 June 2013

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Mechanically flexible arrays of alkaline electrochemical cells fabricated using stencil printing onto fibrous substrates are shown to provide the necessary performance characteristics for driving ink-jet printed circuits. Due to the dimensions and material set currently required for reliable low-temperature print processing of electronic devices, a battery potential greater than that sourced by single cells is typically needed. The developed battery is a series interconnected array of 10 low resistance Zn-MnO2 alkaline cells, giving an open circuit potential of 14 V. This flexible battery is used to power an ink-jet printed 5-stage complementary ring oscillator based on organic semiconductors.
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88.80.ff Batteries
73.61.Ph Polymers; organic compounds
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
85.30.-z Semiconductor devices
81.05.Fb Organic semiconductors
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Contact properties of high-mobility, air-stable, low-voltage organic n-channel thin-film transistors based on a naphthalene tetracarboxylic diimide

R. Rödel, F. Letzkus, T. Zaki, J. N. Burghartz, U. Kraft, U. Zschieschang, K. Kern, and H. Klauk

Appl. Phys. Lett. 102, 233303 (2013); http://dx.doi.org/10.1063/1.4811127 (5 pages)

Online Publication Date: 12 June 2013

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Air-stable bottom-gate, top-contact n-channel organic transistors based on a naphthalene diimide exhibiting electron mobilities up to 0.8 cm2/Vs at low voltages were fabricated. Transistors with channel lengths of 1 μm show a transconductance of 60 mS/m, but are significantly limited by the contact resistance. Transmission line measurements in combination with contact resistance models were applied to investigate this influence. Both contact resistance and contact resistivity are proportional to the inverse gate overdrive voltage. Organic complementary ring oscillators were fabricated on a flexible plastic substrate showing record signal delays down to 17 μs at a supply voltage of 2.6 V.
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85.30.Tv Field effect devices
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
07.10.Cm Micromechanical devices and systems
73.40.Cg Contact resistance, contact potential
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Improved electron injection and transport by use of baking soda as a low-cost, air-stable, n-dopant for solution-processed phosphorescent organic light-emitting diodes

Taeshik Earmme and Samson A. Jenekhe

Appl. Phys. Lett. 102, 233305 (2013); http://dx.doi.org/10.1063/1.4811087 (4 pages)

Online Publication Date: 13 June 2013

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Sodium bicarbonate (baking soda, NaHCO3) is found to be an efficient low-cost, air-stable, and environmentally friendly n-dopant for electron-transport layer (ETL) in solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). A 2.0-fold enhancement in power efficiency of blue PhOLEDs is observed by use of NaHCO3-doped 4,7-diphenyl-1,10-phenanthroline (BPhen) ETL. The bulk conductivity of NaHCO3-doped BPhen film is increased by 5 orders of magnitude. Enhanced performance of PhOLEDs is similarly observed by use of NaHCO3-doped 1,3,5-tris(m-pyrid-3-yl-phenyl)benzene ETL. These results demonstrate that sodium bicarbonate is an effective n-dopant in organic electronics.
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85.60.Jb Light-emitting devices
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Improved cation valence state in molybdenum oxides by ultraviolet-ozone treatments and its applications in organic light-emitting diodes

Xiao-Bo Shi, Mei-Feng Xu, Dong-Ying Zhou, Zhao-Kui Wang, and Liang-Sheng Liao

Appl. Phys. Lett. 102, 233304 (2013); http://dx.doi.org/10.1063/1.4811267 (4 pages)

Online Publication Date: 13 June 2013

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The authors demonstrate a thick MoO3 layer (60 nm) as a good short reduction layer in organic light emitting diodes (OLEDs), which is especially useful for large-area and flexible OLEDs to prevent short circuit issues. The crystallization of organic material and the increase of driving voltage induced by a thick MoO3 layer in OLEDs were resolved by a simple ultraviolet-ozone (UV-ozone) treatment. Ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, contact angle, and atomic force microscope analyses revealed that a longer UV-ozone treatment resulted in an optimized fraction of oxygen vacancies in MoO3, which is responsible for the improved device performance.
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85.60.Jb Light-emitting devices
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The nature and role of trap states in a dendrimer-based organic field-effect transistor explosive sensor

Guoqiang Tang, Simon S. Y. Chen, Kwan H. Lee, Almantas Pivrikas, Muhsen Aljada, Paul L. Burn, Paul Meredith, and Paul E. Shaw

Appl. Phys. Lett. 102, 243301 (2013); http://dx.doi.org/10.1063/1.4810914 (5 pages)

Online Publication Date: 17 June 2013

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We report the fabrication and charge transport characterization of carbazole dendrimer-based organic field-effect transistors (OFETs) for the sensing of explosive vapors. After exposure to para-nitrotoluene (pNT) vapor, the OFET channel carrier mobility decreases due to trapping induced by the absorbed pNT. The influence of trap states on transport in devices before and after exposure to pNT vapor has been determined using temperature-dependent measurements of the field-effect mobility. These data clearly show that the absorption of pNT vapor into the dendrimer active layer results in the formation of additional trap states. Such states inhibit charge transport by decreasing the density of conducting states.
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85.30.Tv Field effect devices
72.20.Fr Low-field transport and mobility; piezoresistance
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C60/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine:MoO3 as the interconnection layer for high efficient tandem blue fluorescent organic light-emitting diodes

Xiaoming Wu, Wentao Bi, Yulin Hua, Jing Sun, Zhihui Xiao, Li Wang, and Shougen Yin

Appl. Phys. Lett. 102, 243302 (2013); http://dx.doi.org/10.1063/1.4811551 (4 pages)

Online Publication Date: 18 June 2013

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The high efficient tandem blue fluorescent organic light emitting diodes (OLEDs) with the transparent interconnection layer (ICL) of fullerence (C60)/Molybdenum oxide (MoO3)-doped N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) were presented. A stack consisting of 0.5 nm of LiF and 1 nm of Ca, which is located from C60 to adjacent electron transporting layer is used as an electron injection layer. The experiment results indicate that the luminance of the tandem device is basically equal to that of the traditional single-unit device, but the current density of the tandem device is much less than that of the single-unit device under a same luminance. The current efficiency and the maximal power efficiency of tandem device with LiF/Ca/C60/NPB:MoO3/MoO3-based interconnection layer have been approximately enhanced by 250% and 126%, respectively. In addition, we also analyze that the mechanism of the efficiency enhancement is ascribed to the effective charge separation and transport of the ICL in tandem OLEDs.
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85.60.Jb Light-emitting devices
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