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25 Jun 2012

Volume 100, Issue 26, Articles (26xxxx)

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Appl. Phys. Lett. 100, 261104 (2012); http://dx.doi.org/10.1063/1.4711253 (4 pages)

Marcelo Davanço, Jun Rong Ong, Andrea Bahgat Shehata, Alberto Tosi, Imad Agha, Solomon Assefa, Fengnian Xia, William M. J. Green, Shayan Mookherjea, and Kartik Srinivasan
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Optically pumped amplified spontaneous emission in an ionic liquid-based polymer light-emitting electrochemical cell

Tomo Sakanoue, Kosuke Sawabe, Yohei Yomogida, Taishi Takenobu, Shiro Seki, and Shimpei Ono

Appl. Phys. Lett. 100, 263301 (2012); http://dx.doi.org/10.1063/1.4730948 (4 pages)

Online Publication Date: 25 June 2012

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Optically pumped amplified spontaneous emission (ASE) is demonstrated in the active layer of the polymer light-emitting electrochemical cells (LECs). The active layer blend of the light-emitting polymer and the ionic liquid retained a high degree of transparency, enabling its use as an optical slab waveguide, allowing a low ASE threshold energy of 4.1 μJ cm−2 to be achieved. We also demonstrate a planar LEC that showed fast turn on, bright linear shape emission in the middle of the two contacts, and a relatively high current density of 36 A cm−2 with a low driving voltage of 6 V.
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85.60.Jb Light-emitting devices
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Probing the effect of relative molecular orientation on the photovoltaic device performance of an organic bilayer heterojunction using soft x-ray spectroscopies

S. W. Cho, A. DeMasi, A. R. H. Preston, K. E. Smith, L. F. J. Piper, K. V. Chauhan, and T. S. Jones

Appl. Phys. Lett. 100, 263302 (2012); http://dx.doi.org/10.1063/1.4731197 (4 pages) | Cited 2 times

Online Publication Date: 25 June 2012

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The orientation of the constituent molecules in organic thin film devices can affect significantly their performance due to the highly anisotropic nature of π-conjugated molecules. We report here an angle dependent x-ray absorption study of the control of such molecular orientation using well-ordered interlayers for the case of a bilayer heterojunction of chloroaluminum phthalocyanine (ClAlPc) and C60. Furthermore, the orientation-dependent energy level alignment of the same bilayer heterojunction has been measured in detail using synchrotron radiation-excited photoelectron spectroscopy. Regardless of the orientation of the organic interlayer, we find that the subsequent ClAlPc tilt angle improves the π-π interaction at the interface, thus leading to an improved short-circuit current in photovoltaic devices based on ClAlPc/C60. The use of the interlayers does not change the effective band gap at the ClAlPc/C60 heterointerface, resulting in no change in open-circuit voltage.
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71.20.Tx Fullerenes and related materials; intercalation compounds
79.60.-i Photoemission and photoelectron spectra
78.70.Dm X-ray absorption spectra
73.21.Ac Multilayers
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CuI interlayers in lead phthalocyanine thin films enhance near-infrared light absorption

Hyo Jung Kim, Hyun-Sub Shim, Ji Whan Kim, Hyun Hwi Lee, and Jang-Joo Kim

Appl. Phys. Lett. 100, 263303 (2012); http://dx.doi.org/10.1063/1.4730604 (4 pages) | Cited 2 times

Online Publication Date: 25 June 2012

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The insertion of a CuI interlayer improved the preferential alignment of lead phthalocyanine (PbPc) molecules in the monoclinic phase, as demonstrated by x-ray diffraction analysis of ultra-thin (5 nm) and thin (30 nm) PbPc films. CuI triclinic phases were not observed. The improved preferential alignment and crystallinity in the monoclinic phase enhanced near-infrared photon absorption. The power conversion efficiency of a PbPc/C60 planar heterojunction organic photovoltaic device fabricated using the CuI templating layer structure could be improved from 1.3% to 2.5%.
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88.40.jr Organic photovoltaics
78.30.Jw Organic compounds, polymers
78.66.Qn Polymers; organic compounds
68.55.am Polymers and organics
88.40.hj Efficiency and performance of solar cells
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Investigation of electronic trap states in organic photovoltaic materials by current-based deep level transient spectroscopy

Stefan Neugebauer, Julia Rauh, Carsten Deibel, and Vladimir Dyakonov

Appl. Phys. Lett. 100, 263304 (2012); http://dx.doi.org/10.1063/1.4731637 (4 pages) | Cited 2 times

Online Publication Date: 27 June 2012

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Current-based deep level transient spectroscopy was used to study trap states in poly(3-hexylthiophene-2,5-diyl) (P3HT), [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), and P3HT:PCBM blend. The obtained spectra showed traps of 87 meV activation energy in pure P3HT and 21 meV for PCBM. The blend shows a complex emission rate spectrum consisting of several different emission rate bands in the range of 0.1–30 s−1, yielding activation energies between about 30 meV and 160 meV.
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72.80.Le Polymers; organic compounds (including organic semiconductors)
79.10.Ca Deep-level photothermal spectroscopy
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Influence of film structure and light on charge trapping and dissipation dynamics in spun-cast organic thin-film transistors measured by scanning Kelvin probe microscopy

L. C. Teague, M. A. Loth, and J. E. Anthony

Appl. Phys. Lett. 100, 263305 (2012); http://dx.doi.org/10.1063/1.4720063 (5 pages)

Online Publication Date: 28 June 2012

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Herein, time-dependent scanning Kelvin probe microscopy of solution processed organic thin film transistors (OTFTs) reveals a correlation between film microstructure and OTFT device performance with the location of trapped charge within the device channel. The accumulation of the observed trapped charge is concurrent with the decrease in ISD during operation (VG = −40 V, VSD = −10 V). We discuss the charge trapping and dissipation dynamics as they relate to the film structure and show that application of light quickly dissipates the observed trapped charge.
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85.30.Tv Field effect devices
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Solution-processed ambipolar vertical organic field effect transistor

Ariel J. Ben-Sasson, Zhihua Chen, Antonio Facchetti, and Nir Tessler

Appl. Phys. Lett. 100, 263306 (2012); http://dx.doi.org/10.1063/1.4731774 (4 pages) | Cited 2 times

Online Publication Date: 28 June 2012

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We report on a solution-processed ambipolar patterned-electrode vertical organic field effect transistor (PE-VOFET) based on the P(NDI2OD-T2) polymer. The Schottky barrier-based VOFET operation uniquely facilitates an ambipolar transport using a single anode-cathode-electrode and a single semiconductor material. Pin-hole free sub-100 nanometer channel length devices are obtained with no high resolution patterning owing to both the polymer’s smooth morphology and the underlining patterned-electrode’s flatness. The VOFET exhibits n-type on/off ratio >103, current density >50 [mAcm−2] under VDS = 5 V, as well as p-type operation. Prone to design and optimization, the ambipolar PE-VOFET is a promising platform for organic complementary circuit technology.
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85.30.Tv Field effect devices
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Modeling the underlying mechanisms for organic memory devices: Tunneling, electron emission, and oxygen adsorbing

Yao Yao, Yintao You, Wei Si, and Chang-Qin Wu

Appl. Phys. Lett. 100, 263307 (2012); http://dx.doi.org/10.1063/1.4731879 (4 pages)

Online Publication Date: 29 June 2012

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We present a combined experimental and theoretical study to uncover the underlying mechanisms of organic memory devices. The theoretical model we propose is a metallic island array embedding within two electrodes, and the scattering operator method is used to evaluate the tunneling current among them. This theory rebuilds the bistable I-V curves and matches the experiments on several device structures very well. Basically, both memory and negative differential resistance effect in the device are understood comprehensively. To verify the theory further, the experimental correspondence, say, changing the pressure of oxygen, is addressed as well.
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84.30.Sk Pulse and digital circuits
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