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7 Jan 2008

Volume 92, Issue 1, Articles (01xxxx)

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Appl. Phys. Lett. 92, 011101 (2008); http://dx.doi.org/10.1063/1.2828458 (3 pages)

F. Pedaci, S. Barland, E. Caboche, P. Genevet, M. Giudici, J. R. Tredicce, T. Ackemann, A. J. Scroggie, W. J. Firth, G.-L. Oppo, G. Tissoni, and R. Jäger
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Reducing the contact resistance of bottom-contact pentacene thin-film transistors by employing a MoOx carrier injection layer

Daisuke Kumaki, Tokiyoshi Umeda, and Shizuo Tokito

Appl. Phys. Lett. 92, 013301 (2008); http://dx.doi.org/10.1063/1.2828711 (3 pages) | Cited 17 times

Online Publication Date: 2 January 2008

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We report on the reduced contact resistance in bottom-contact (BC) pentacene thin-film transistors (TFTs) with a molybdenum oxide (MoOx) carrier injection layer. MoOx layers were placed between the gate insulator and the source-drain (S-D) electrodes instead of the conventional adhesive layer such as Cr or Ti. The performance of the BC pentacene-TFT with the MoOx injection layer was significantly improved at low operating voltages. The contact resistance of the MoOx/Au S-D electrodes, estimated using the gated-transmission line method, was nearly two orders of magnitude smaller than that of conventional Cr/Au electrodes at the gate voltage of −10 V. The highest performance was obtained with a MoOx injection layer a few nanometers thick, which was comparable to the effective channel thickness of the pentacene-TFT on the gate insulator. This result indicated the importance of the direct connection between the MoOx injection layer and the effective channel to reduce the contact resistance.
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85.30.Tv Field effect devices
73.40.Cg Contact resistance, contact potential
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Spatially resolved photocurrent mapping of operating organic photovoltaic devices using atomic force photovoltaic microscopy

B. J. Leever, M. F. Durstock, M. D. Irwin, A. W. Hains, T. J. Marks, L. S. C. Pingree, and M. C. Hersam

Appl. Phys. Lett. 92, 013302 (2008); http://dx.doi.org/10.1063/1.2830695 (3 pages) | Cited 12 times

Online Publication Date: 4 January 2008

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A conductive atomic force microscopy (cAFM) technique, atomic force photovoltaic microscopy (AFPM), has been developed to characterize spatially localized inhomogeneities in organic photovoltaic (OPV) devices. In AFPM, a biased cAFM probe is raster scanned over an array of illuminated solar cells, simultaneously generating topographic and photocurrent maps. As proof of principle, AFPM is used to characterize 7.5×7.5 μm2 poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester OPVs, revealing substantial device to device and temporal variations in the short-circuit current. The flexibility of AFPM suggests applicability to nanoscale characterization of a wide range of optoelectronically active materials and devices.
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84.60.Jt Photoelectric conversion
85.60.-q Optoelectronic devices
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Extraction enhancement in organic light emitting devices by using metallic nanowire arrays

Shen-Yu Hsu, Ming-Chang Lee, Kuang-Li Lee, and Pei-Kuen Wei

Appl. Phys. Lett. 92, 013303 (2008); http://dx.doi.org/10.1063/1.2828712 (3 pages) | Cited 9 times

Online Publication Date: 4 January 2008

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The extraction efficiency of organic light emitting devices is enhanced by depositing metallic nanowires on the glass surface and indium tin oxide (ITO) anode. For the aluminum tris-(8-hydroxyquinoline) (Alq3) based devices, a 100 nm-width and 450 nm-period gold nanowire array increases light extraction up to 46% from glass substrate and 80% from the organic layer. Such metallic nanowire arrays double the brightness with small absorption, only 10% lower than ITO glass. In addition, colors of the devices can be selected by the period of nanowire array. We demonstrated blue to red light emission by using single Alq3-based device.
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85.60.Jb Light-emitting devices
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Hysteresis mechanisms of pentacene thin-film transistors with polymer/oxide bilayer gate dielectrics

D. K. Hwang, Min Suk Oh, Jung Min Hwang, Jae Hoon Kim, and Seongil Im

Appl. Phys. Lett. 92, 013304 (2008); http://dx.doi.org/10.1063/1.2830329 (3 pages) | Cited 11 times

Online Publication Date: 7 January 2008

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We have studied the electrical stability of organic poly-4-vinyl phenol (PVP)/inorganic oxide bilayer gate dielectrics for low-voltage pentacene thin-film transistors (TFTs). Curing conditions of spin-cast PVP influence on the drain current-gate bias hysteresis behavior; long term curing reduces the magnitude of the hysteresis, which can also be reduced by decreasing the PVP thickness. The electron charge injection from gate electrode plays as another cause of the electrical hysteresis. These instabilities are categorized into the following three: channel/dielectric interface-induced, slow polarization-induced, and gate charge injection-induced hystereses. By examining the hysteresis behavior of pentacene TFTs with five different combinations of bilayer dielectric, we clarified the instability mechanisms responsible for the electrical hysteresis.
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85.30.Tv Field effect devices
77.55.-g Dielectric thin films
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Hydrostatic pressure effects on poly(3-hexylthiophene) thin film transistors

Dominic D. Schroepfer, P. Paul Ruden, Yu Xia, C. Daniel Frisbie, and Sean E. Shaheen

Appl. Phys. Lett. 92, 013305 (2008); http://dx.doi.org/10.1063/1.2830330 (3 pages)

Online Publication Date: 7 January 2008

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Poly(3-hexylthiophene) thin-film transistors are subjected to hydrostatic pressure up to 1 GPa. The charge carrier mobility and threshold voltage are extracted from Id-Vgs curves. These parameters change linearly with pressure and retrace upon gradual pressure release. The mobility increases from 0.001 to 0.004 cm2/V s, and the threshold voltage falls from 36 to 2 V over the full pressure range. As a result, the current rises with increasing pressure up to 600 MPa and then falls as pressure is increased further. The increase in the mobility is attributed to enhanced π-orbital overlap under compression. The change in threshold voltage is interpreted as a modulation of trapped charge density.
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85.30.Tv Field effect devices
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4.8% efficient poly(3-hexylthiophene)-fullerene derivative (1:0.8) bulk heterojunction photovoltaic devices with plasma treated AgOx/indium tin oxide anode modification

Woo-Jun Yoon and Paul R. Berger

Appl. Phys. Lett. 92, 013306 (2008); http://dx.doi.org/10.1063/1.2830619 (3 pages) | Cited 7 times

Online Publication Date: 7 January 2008

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We report here an improved efficiency, up to 4.8% with a high fill factor of ∼ 63% under AM 1.5G spectral illumination and 100 mW/cm2 intensity, for poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester bulk heterojunction photovoltaic (PV) devices with a 1:0.8 weight ratio using surface modifications to the indium tin oxide (ITO) anodes through plasma oxidized silver. Here, an enhanced short-circuit current density was achieved without significant loss in the open-circuit voltage (>0.6 V) nor the fill factor (>63%), leading to an efficiency jump from 4.4% in the control devices to 4.8% with the surface modified ITO anode. The enhanced short-circuit density is attributed to an interface energy step between the ITO and the polymer hole transporting layer. It has been theorized that the introduction of an interface energy step could alter the charge collection efficiency, resulting in an improved overall efficiency in PV devices. In our study, the current density–voltage characteristics under darkness clearly show an increased current density, especially under forward bias, for the anode treated cell, suggesting the presence of an interface energy step between the ITO and the hole transporting layer with surface modified ITO anodes.
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85.60.Bt Optoelectronic device characterization, design, and modeling
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Transport energy in organic semiconductors with partially filled localized states

Ling Li, Gregor Meller, and Hans Kosina

Appl. Phys. Lett. 92, 013307 (2008); http://dx.doi.org/10.1063/1.2829863 (3 pages) | Cited 5 times

Online Publication Date: 10 January 2008

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The charge transport in organic semiconductors has been investigated theoretically. An analytical model describing the effect of partially filled localized states on the concept of transport energy is presented, based on variable range hopping theory. The results illustrate that at low enough temperature the partially filled localized states in organic semiconductor systems play an important role on the transport energy. Moreover, the charge concentration will change the transport energy dramatically at higher carrier concentration. The paper also discusses the carrier-concentration-dependent mobility in organic semiconductors.
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72.20.Ee Mobility edges; hopping transport
72.10.-d Theory of electronic transport; scattering mechanisms
72.20.Fr Low-field transport and mobility; piezoresistance
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Impedance spectroscopy investigation of electron transport in solar cells based on blend film of polymer and nanocrystals

Wenbo Huang, Junbiao Peng, Li Wang, Jian Wang, and Yong Cao

Appl. Phys. Lett. 92, 013308 (2008); http://dx.doi.org/10.1063/1.2831661 (3 pages) | Cited 14 times

Online Publication Date: 11 January 2008

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Bulk heterojunction solar cells, consisting of the active film of poly[2-methoxy,5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV) doped with cadmium selenium (CdSe) nanoparticles, were fabricated and investigated using impedance spectroscopy (IS) technique. The spectroscopy displayed a phenomenon of negative imaginary impedance (NII) related to the inductive effect in low frequencies in the device with low CdSe concentrations, and the NII phenomenon may disappear with increasing the CdSe concentrations. We propose an equivalent circuit to explain the IS and deduce that the NII phenomenon is related to inadequate channels for electron transport in the blend film of CdSe nanocrystals and MEH-PPV. The method based on the IS is available to probe the formation of a continuous network for electron transport in the nanocrystal dispersed polymer system.
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81.07.Bc Nanocrystalline materials
84.60.Jt Photoelectric conversion
68.55.ag Semiconductors
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Self-assembled deoxyguanosine based molecular electronic device on GaN substrates

H. Liddar, J. Li, A. Neogi, P. B. Neogi, A. Sarkar, S. Cho, and H. Morkoç

Appl. Phys. Lett. 92, 013309 (2008); http://dx.doi.org/10.1063/1.2828405 (3 pages) | Cited 1 time

Online Publication Date: 11 January 2008

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Nanoscale hybrid molecular organic photodetectors based on self-assembled guanosine molecules conjugated to wide-bandgap GaN semiconductors has been realized in the ultraviolet wavelength regime. Metal-semiconductor-metal based photodetector is fabricated using ordering of modified guanosine based semiconductor nanowires which exhibit I-V characteristics with high current response and higher rectification ratio compared to Si based hybrid photodetectors. Photocurrent response of a two-terminal device shows the typical characteristics of a semiconductor photodiode with a cutoff wavelength at ∼ 325 nm. The I-V characteristics have been elucidated using the induced polarization properties of self-assembled guanosine semiconductor.
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81.07.Pr Organic-inorganic hybrid nanostructures
81.07.Nb Molecular nanostructures
81.16.Dn Self-assembly
85.65.+h Molecular electronic devices
85.60.Gz Photodetectors (including infrared and CCD detectors)
73.40.Sx Metal-semiconductor-metal structures
73.50.Pz Photoconduction and photovoltaic effects
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