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21 Sep 2009

Volume 95, Issue 12, Articles (12xxxx)

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Appl. Phys. Lett. 95, 121104 (2009); http://dx.doi.org/10.1063/1.3231448 (3 pages)

E. H. Khoo, I. Ahmed, and E. P. Li
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Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films

M. Kröger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, and A. Kahn

Appl. Phys. Lett. 95, 123301 (2009); http://dx.doi.org/10.1063/1.3231928 (3 pages) | Cited 130 times

Online Publication Date: 22 September 2009

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The electronic structures of vacuum-deposited molybdenum trioxide (MoO3) and of a typical MoO3/hole transport material (HTM) interface are determined via ultraviolet and inverse photoelectron spectroscopy. Electron affinity and ionization energy of MoO3 are found to be 6.7 and 9.68 eV, more than 4 eV larger than generally assumed, leading to a revised interpretation of the role of MoO3 in hole injection in organic devices. The MoO3 films are strongly n-type. The electronic structure of the oxide/HTM interface shows that hole injection proceeds via electron extraction from the HTM highest occupied molecular orbital through the low-lying conduction band of MoO3.
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71.20.Ps Other inorganic compounds
73.20.At Surface states, band structure, electron density of states
79.60.Jv Interfaces; heterostructures; nanostructures
79.60.Bm Clean metal, semiconductor, and insulator surfaces
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On the long time holographic grating recording process in azo-polymer

Anna Sobolewska and Stanislaw Bartkiewicz

Appl. Phys. Lett. 95, 123302 (2009); http://dx.doi.org/10.1063/1.3232223 (3 pages) | Cited 3 times

Online Publication Date: 22 September 2009

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The authors report on a complex diffraction efficiency dynamics of the holographic grating recording process in azobenzene-functionalized polymer for a long recording time. The nonexponential dynamics was explain by the mechanism which assumes (i) simultaneous formation of three coupling phase gratings with constant, equal 0 or π, phase shifts between them and (ii) occurrence of gratings erasure processes when the recording time is long. Based on these assumptions and applying the Bessel function, the diffraction efficiency dynamics was calculated.
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42.40.Eq Holographic optical elements; holographic gratings
42.70.Jk Polymers and organics
02.30.Gp Special functions
42.70.Ln Holographic recording materials; optical storage media
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Varying polymer crystallinity in nanofiber poly(3-alkylthiophene): PCBM solar cells: Influence on charge-transfer state energy and open-circuit voltage

Koen Vandewal, Wibren D. Oosterbaan, Sabine Bertho, Veerle Vrindts, Abay Gadisa, Laurence Lutsen, Dirk Vanderzande, and Jean V. Manca

Appl. Phys. Lett. 95, 123303 (2009); http://dx.doi.org/10.1063/1.3232242 (3 pages) | Cited 21 times

Online Publication Date: 22 September 2009

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The effect of poly(3-alkylthiophene) (P3AT) crystallinity in (nanofiber P3AT):PCBM photovoltaic devices on the energy of the charge-transfer state (ECT) and on the open-circuit voltage (Voc) is investigated for poly(3-butythiophene), poly(3-pentylthiophene) and poly(3-hexylhiophene). P3AT crystallinity, expressed as the crystalline nanofiber mass fraction f to the total P3AT mass in the spin-coating dispersion, is varied between ∼ 0.1 and ∼ 0.9 by temperature control. ECT, as obtained by Fourier-transform photocurrent spectroscopy decreased with f as ECT = ECT0−0.2f eV. Alkyl side-chain length only influences ECT0. Voc relates to ECT as Voc = ECT/q−0.6 V.
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84.60.Jt Photoelectric conversion
61.41.+e Polymers, elastomers, and plastics
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
79.60.Bm Clean metal, semiconductor, and insulator surfaces
72.40.+w Photoconduction and photovoltaic effects
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Organic-based sensor for chemical detection in aqueous solution

A. Caboni, E. Orgiu, E. Scavetta, M. Barbaro, and A. Bonfiglio

Appl. Phys. Lett. 95, 123304 (2009); http://dx.doi.org/10.1063/1.3232252 (3 pages) | Cited 5 times

Online Publication Date: 22 September 2009

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We present a flexible, pentacene-based field-effect device, for the detection of chemical species in aqueous solution. The sensor consists in a double-gate transistor, where the detection is achieved by exploiting the charge sensing capabilities of the floating-gate terminal. To provide the pH-sensitivity, the floating gate is functionalized with thioamine groups as such groups protonize proportionally to the concentration of H3O+ ions in solution. With respect to the existing organic-based devices for pH monitoring, our sensor does not require a counterelectrode and the organic semiconductor is not affected by the contact with the monitored solution.
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82.80.Fk Electrochemical methods
85.30.Tv Field effect devices
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
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Organic field-effect transistors fabricated with N,N′-substituted dialkyl-1,3,8,10-tetramethylquinacridone compounds

Zong-Xiang Xu, Hai-Feng Xiang, V. A. L. Roy, Stephen Sin-Yin Chui, Yue Wang, P. T. Lai, and Chi-Ming Che

Appl. Phys. Lett. 95, 123305 (2009); http://dx.doi.org/10.1063/1.3233961 (3 pages) | Cited 7 times

Online Publication Date: 22 September 2009

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A series of robust and inexpensive p-type organic semiconductors 1,3,8,10-tetramethyl quinacridone compounds was prepared. These quinacridone compounds bearing N,N′-disubstituted long N-alkyl chains self-organize into highly oriented crystalline films, leading to high performance organic thin film transistors with the best field-effect mobility, on/off ratio, and threshold voltage being 1.6×10−1 cm2 V−1 s−1, 1×104, and −17 V, respectively. The effects of methyl substituent and N-alkyl chain length of the quinacridone compounds together with the molecular packing on the field-effect mobility are discussed.
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85.30.Tv Field effect devices
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Inversion of magnetic field effects on electrical current and electroluminescence in tri-(8-hydroxyquinoline)-aluminum based light-emitting diodes

Linyuan Xin, Chuannan Li, Feng Li, Shiyong Liu, and Bin Hu

Appl. Phys. Lett. 95, 123306 (2009); http://dx.doi.org/10.1063/1.3233973 (3 pages) | Cited 17 times

Online Publication Date: 22 September 2009

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The magnetic field effects on electrical current and electroluminescence have been investigated for tri-(8-hydroxyquinoline)-aluminum based organic light emitting diodes through the transient electroluminescence method. By applying rectangular pulse to drive the devices, the influence of the degradation is dramatically decreased; and a full-range evolvement for the magnetoconductance (MC) and magnetoelectroluminescence (MEL) with the driving voltage has been obtained. The transition voltages for MC (1.6 V) from the negative to positive value and MEL (7 V) from the positive to negative value are different, which suggests that the magnetic field effect on the MC and MEL may have different mechanisms.
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85.60.Jb Light-emitting devices
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Influence of interlayer on the performance of stacked white organic light-emitting devices

Ping Chen, Qin Xue, Wenfa Xie, Guohua Xie, Yu Duan, Yi Zhao, Shiyong Liu, Liying Zhang, and Bin Li

Appl. Phys. Lett. 95, 123307 (2009); http://dx.doi.org/10.1063/1.3234379 (3 pages) | Cited 6 times

Online Publication Date: 23 September 2009

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Stacked white organic light-emitting devices (WOLEDs) comprising of blue fluorescent and orange phosphorescent emissive units employing tri(8-hydroxyquinoline) aluminum (Alq3):Mg/MoO3 as charge generation layer are fabricated. The working mechanism of Alq3:Mg/MoO3 is also discussed using a simple method. We demonstrate charge-carrier separation takes place only in MoO3 layer. Stacked WOLED with better performance was obtained by adjusting the thickness of MoO3. The stacked WOLED with efficiency of 39.2 cd/A has excellent color stability with the Commission Internationale de l’Eclairage coordinates only changing from (0.407, 0.405) to (0.398, 0.397) when luminance increases from 22 to 10 000 cd/m2.
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85.60.Jb Light-emitting devices
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Band dispersion of quasi-single crystal thin film phase pentacene monolayer studied by angle-resolved photoelectron spectroscopy

Manabu Ohtomo, Tadamasa Suzuki, Toshihiro Shimada, and Tetsuya Hasegawa

Appl. Phys. Lett. 95, 123308 (2009); http://dx.doi.org/10.1063/1.3232205 (3 pages) | Cited 10 times

Online Publication Date: 24 September 2009

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Intermolecular band dispersion of thin film phase pentacene grown epitaxially on step-bunched math×math Bi–Si(111) was investigated using angle resolved photoemission spectroscopy at 130 K. We evaluated the transfer integrals using two-dimensional tight-binding fit to the experimental dispersion and found that adjacent molecules in Γ-M direction have the strongest coupling as predicted. The estimated effective mass at the top of the valence band was isotropic and ranged between 0.8 and 1.0 of the electron mass.
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81.05.Hd Other semiconductors
71.20.Rv Polymers and organic compounds
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.55.ag Semiconductors
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
79.60.Fr Polymers; organic compounds
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Tilted bulk heterojunction organic photovoltaic cells grown by oblique angle deposition

Ning Li and Stephen R. Forrest

Appl. Phys. Lett. 95, 123309 (2009); http://dx.doi.org/10.1063/1.3236838 (3 pages) | Cited 28 times

Online Publication Date: 25 September 2009

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We demonstrate small molecule bulk heterojunction organic photovoltaic cells using oblique angle vacuum deposition. Obliquely deposited donor chloroaluminum phthalocyanine (ClAlPc) films on indium tin oxide have surface feature sizes of ∼ 30 nm, resulting in ClAlPc/C60 donor-acceptor heterojunctions (HJs) with approximately twice the interface area of HJs grown at normal incidence. This results in nearly twice the external quantum efficiency in the ClAlPc absorption band compared with analogous, planar HJs. The efficiency increase is attributed to the increased surface area presented by the donor-acceptor junction to the incident illumination by ClAlPc protrusions lying obliquely to the substrate plane formed during deposition. The power conversion efficiency improves from (2.0±0.1)% to (2.8±0.1)% under 1 sun, AM 1.5G simulated solar illumination. Similarly, the power efficiency of copper phthalocyanine/C60 organic photovoltaic cells is increased from (1.3±0.1)% to (1.7±0.1)%.
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85.60.-q Optoelectronic devices
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.55.ag Semiconductors
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
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