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4 Jan 2010

Volume 96, Issue 1, Articles (01xxxx)

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Appl. Phys. Lett. 96, 013107 (2010); http://dx.doi.org/10.1063/1.3280900 (3 pages)

L. Fernández, M. Corso, F. Schiller, M. Ilyn, M. Holder, and J. E. Ortega
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Correlation of energy band alignment and turn-on voltage in organic light emitting diodes

I-Wen Wu, Yu-Hung Chen, Po-Sheng Wang, Chao-Gong Wang, Shu-Han Hsu, and Chih-I Wu

Appl. Phys. Lett. 96, 013301 (2010); http://dx.doi.org/10.1063/1.3282682 (3 pages) | Cited 7 times

Online Publication Date: 4 January 2010

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The correlation of energy alignment and turn-on voltage of organic light emitting diodes (OLEDs) was investigated. With identical hole transport layers (HTLs) and electron transport layers (ETLs), the turn-on voltages of OLEDs are always the same, regardless of the cathode structures, such as Ca, Al, LiF/Al, and Cs2CO3/Al. For devices with various combinations of HTLs and ETLs, the turn-on voltages are equal to the energy difference between the lowest unoccupied molecular orbital of ETLs and the highest occupied molecular orbital of HTLs, taking into consideration of vacuum level shift at organic interfaces measured from ultraviolet photoemission spectroscopy.
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85.60.Jb Light-emitting devices
82.45.Fk Electrodes
07.60.Rd Visible and ultraviolet spectrometers
79.60.-i Photoemission and photoelectron spectra
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The electronic structure of C60/ZnPc interface for organic photovoltaic device with blended layer architecture

S. H. Park, J. G. Jeong, Hyo-Jin Kim, Seung-Han Park, Mann-Ho Cho, Sang Wan Cho, Yeonjin Yi, Min Young Heo, and Hyunchul Sohn

Appl. Phys. Lett. 96, 013302 (2010); http://dx.doi.org/10.1063/1.3285174 (3 pages) | Cited 15 times

Online Publication Date: 4 January 2010

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The interfacial electronic structures of fullerene (C60)/zinc-phthalocyanine (ZnPc) and C60/ZnPc:C60 (50 wt %) containing a blended layer were investigated by in situ ultraviolet photoelectron spectroscopy (UPS), in an attempt to understand the role of the blended layer in improving the performance of organic photovoltaic devices that contain such layers. From the UPS spectra, the band bending found to be 0.30 eV in the ZnPc layer and 0.43 eV in the C60 layer at the C60/ZnPc interface. On the other hand, the band bending was 0.25 eV in both of the organic layers at the ZnPc:C60/ZnPc interface and no significant band bending in the C60 layer at the C60/ZnPc:C60 interface was found. The observed interface dipole was 0.06 eV at the C60/ZnPc interface and 0.26 eV at the ZnPc:C60/ZnPc interface. The offset between the highest unoccupied molecular orbital of ZnPc and the lowest occupied molecular orbital of C60 was 0.75 eV at C60/ZnPc and was 1.04 eV at the ZnPc:C60/ZnPc interface. The increased offset can be attributed to an increase in the interface dipole, caused by the blending donor and acceptor material. The blending facilitates charge transfer between the donor and acceptor, resulting in an increase in the interface dipole, resulting in a larger offset.
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73.40.-c Electronic transport in interface structures
85.60.-q Optoelectronic devices
79.60.Jv Interfaces; heterostructures; nanostructures
73.50.Pz Photoconduction and photovoltaic effects
73.61.Wp Fullerenes and related materials
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High-Q conical polymeric microcavities

Tobias Grossmann, Mario Hauser, Torsten Beck, Cristian Gohn-Kreuz, Matthias Karl, Heinz Kalt, Christoph Vannahme, and Timo Mappes

Appl. Phys. Lett. 96, 013303 (2010); http://dx.doi.org/10.1063/1.3280044 (3 pages) | Cited 15 times

Online Publication Date: 4 January 2010

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We report on the fabrication of high-Q microresonators made of low-loss, thermoplastic polymer poly(methyl methacrylate) (PMMA) directly processed on a silicon substrate. Using this polymer-on-silicon material in combination with a thermal reflow step enables cavities of conical geometry with an ultrasmooth surface. The cavity Q factor of these PMMA resonators is above 2×106 in the 1300 nm wavelength range. Finite element simulations show the existence of a variety of “whispering gallery” modes in these resonators explaining the complexity of the measured transmission spectra.
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07.10.Cm Micromechanical devices and systems
42.15.-i Geometrical optics
42.70.Jk Polymers and organics
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Improved efficiency of hybrid solar cells based on non-ligand-exchanged CdSe quantum dots and poly(3-hexylthiophene)

Yunfei Zhou, Frank S. Riehle, Ying Yuan, Hans-Frieder Schleiermacher, Michael Niggemann, Gerald A. Urban, and Michael Krüger

Appl. Phys. Lett. 96, 013304 (2010); http://dx.doi.org/10.1063/1.3280370 (3 pages) | Cited 45 times

Online Publication Date: 7 January 2010

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We report on bulk-heterojunction hybrid solar cells based on blends of non-ligand-exchanged CdSe quantum dots (QDs) and the conjugated polymer poly(3-hexylthiophene) with improved power conversion efficiencies of about 2% under AM1.5G illumination after spectral mismatch correction. This is the highest reported value for a spherical CdSe QD based photovoltaic device. After synthesis, the CdSe QDs are treated by a simple and fast acid-assisted washing procedure, which has been identified as a crucial factor in enhancing the device performance. A simple model of a reduced ligand sphere is proposed explaining the power conversion efficiency improvement.
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88.40.jm Thin film III-V and II-VI based solar cells
88.40.hj Efficiency and performance of solar cells
72.40.+w Photoconduction and photovoltaic effects
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Bias and temperature dependent charge transport in high mobility cobalt-phthalocyanine thin films

S. Samanta, D. K. Aswal, A. Singh, A. K. Debnath, M. Senthil Kumar, Y. Hayakawa, S. K. Gupta, and J. V. Yakhmi

Appl. Phys. Lett. 96, 013305 (2010); http://dx.doi.org/10.1063/1.3284652 (3 pages) | Cited 11 times

Online Publication Date: 7 January 2010

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The temperature dependent current-voltage (J-V) characteristics of highly-oriented cobalt phthalocyanine films (rocking-curve width = 0.11°) deposited on (001) LaAlO3 substrates are investigated. In the temperature range 300–100 K, charge transport is governed by bulk-limited processes with a bias dependent crossover from Ohmic (J ∼ V) to trap-free space-charge-limited conduction (J ∼ V2). The mobility (μ) at 300 K has a value of ∼ 7 cm2 V−1 s−1 and obeys Arrhenius-type (ln μ ∼ 1/T) behavior. However, at temperatures <100 K, the charge transport is electrode-limited, which undergoes a bias dependent transition from Schottky (ln J ∼ V1/2) to multistep-tunneling (conductivity varying exponentially on the inverse of the square-root of electric field).
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73.61.Ph Polymers; organic compounds
68.55.ag Semiconductors
77.22.Jp Dielectric breakdown and space-charge effects
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.20.Ht High-field and nonlinear effects
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