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January 2012

Volume 5, Issue 1

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Changing inter-molecular spin-orbital coupling for generating magnetic field effects in phosphorescent organic semiconductors

Liang Yan, Ming Shao, Carlos F. O. Graeff, Ivo Hummelgen, Dongge Ma, and Bin Hu

Appl. Phys. Lett. 100, 013301 (2012); http://dx.doi.org/10.1063/1.3673561 (3 pages)

Online Publication Date: 3 January 2012

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Phosphorescent organic semiconductors normally show negligible magnetic field effects in electronic and optic responses. These phenomena have been generally attributed to strong spin-orbital coupling which can dominate internal spin-dephasing process as compared with applied magnetic field. This paper reports both positive and negative magnetocurrents from phosphorescent organic semiconductors through dissociation and charge-reaction channels when the intermolecular spin-orbital coupling is changed based on materials mixing. Our experimental results indicate that inter-molecular spin-orbital coupling is essentially responsible for the generation of magnetic field effects in phosphorescent organic semiconductors.
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71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
78.55.Kz Solid organic materials
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Nanostructured barbed wire architecturing of organic conducting material blends by electrospinning

Sumeet R. Mishra, K. Ranjith, S. K. Swathi, and Praveen C. Ramamurthy

Appl. Phys. Lett. 100, 013302 (2012); http://dx.doi.org/10.1063/1.3673620 (4 pages)

Online Publication Date: 3 January 2012

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In this study, fibers of barbed wire structure were obtained by electrospinning blend of organic conducting crystalline material and polyethylene oxide. Thermal and structural characterization of the blend fibers has been carried out to study the fiber characteristics. An increase in crystallinity in the electrospun fibers was observed and was attributed to both electrospinning process as well as addition of organic conducting crystalline material. A mechanism for the formation of this barbed wire structure has also been proposed.
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81.16.-c Methods of micro- and nanofabrication and processing
65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
81.07.Gf Nanowires
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Reduction of charge injection barrier by 1-nm contact oxide interlayer in organic field effect transistors

Peter Darmawan, Takeo Minari, Akichika Kumatani, Yun Li, Chuan Liu, and Kazuhito Tsukagoshi

Appl. Phys. Lett. 100, 013303 (2012); http://dx.doi.org/10.1063/1.3673842 (3 pages)

Online Publication Date: 3 January 2012

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The enhancement of the charge injection process by the insertion of an ultrathin (∼1 nm) contact oxide interlayer (COI) at the metal/organic material interface in organic field effect transistors (OFETs) is reported. Six different oxides were used as COI, and Al2O3 was found to exhibit the highest OFET mobility with a reduction in the average contact resistance (Rc) from 19.9 to 1.9 kΩ⋅cm. Photoelectron yield spectroscopy analysis revealed that the insertion of COI increases the work function of an Au contact and reduces the charge injection barrier at the interface, which lowers Rc and, therefore, results in enhanced device performance.
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85.30.Tv Field effect devices
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Self-assembly of C60 monolayer on epitaxially grown, nanostructured graphene on Ru(0001) surface

G. Li, H. T. Zhou, L. D. Pan, Y. Zhang, J. H. Mao, Q. Zou, H. M. Guo, Y. L. Wang, S. X. Du, and H.-J. Gao

Appl. Phys. Lett. 100, 013304 (2012); http://dx.doi.org/10.1063/1.3673830 (4 pages)

Online Publication Date: 4 January 2012

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C60 molecules adsorbed on graphene/Ru(0001) substrate were investigated by scanning tunneling microscopy (STM) at 5 K. On high quality substrates, C60 molecules adopt a commensurate growth mode, leading to formation of a supramolecular structure with perfect periodicity and few defects. On under-annealed substrates with imperfections and domains, the molecules form the same closely packed hexagonal structures in spite of underlying corrugations, disorders or steps, indicating a weak molecule-substrate interaction—a conclusion that is also supported by DFT calculations. This system may be beneficial to the fabrication of carbon based devices and of other types of organic functional overlayers.
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81.16.Dn Self-assembly
81.05.ub Fullerenes and related materials
68.43.Fg Adsorbate structure (binding sites, geometry)
81.07.Bc Nanocrystalline materials
68.43.Mn Adsorption kinetics
61.48.-c Structure of fullerenes and related hollow and planar molecular structures
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Optical microcavities with a thiol-functionalized gold nanoparticle polymer thin film coating

Ce Shi, Hong Seok Choi, and Andrea M. Armani

Appl. Phys. Lett. 100, 013305 (2012); http://dx.doi.org/10.1063/1.3673872 (3 pages)

Online Publication Date: 4 January 2012

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Polymer coatings endow ultra-high-Q dielectric resonators with nonlinear properties, impacting numerous applications. However, minimal research combining microcavities with polymer-nanoparticle coatings to tune or tailor the optical properties of the system has been performed. One challenge is maintaining the high performance of the optical device while in the presence of nanoparticles. In the present work, a toroidal microcavity is coated with a polymethylmethacrylate thin film containing thiol-functionalized gold nanoparticles. The thiol-functionalization ensures that the nanoparticles are uniformly distributed throughout the film. The quality factors of these devices are above 5 × 106 and are in good agreement with the theoretical predictions.
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42.79.Wc Optical coatings
42.70.Jk Polymers and organics
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Impact of unbalanced charge transport on the efficiency of normal and inverted solar cells

J. D. Kotlarski and P. W. M. Blom

Appl. Phys. Lett. 100, 013306 (2012); http://dx.doi.org/10.1063/1.3663860 (3 pages)

Online Publication Date: 5 January 2012

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In a normal solar cell, most charge carriers are generated close to the anode, such that electrons have to travel a longer distance as compared to the holes. In an inverted solar cell, holes have to travel a longer distance. We use a combined optical and electronic model to simulate the effect of unbalanced transport on the efficiency of normal and inverted single and tandem solar cells. When the electrons are ten times more mobile than the holes, the efficiency for a single cell with a thickness of 250 nm drops from 7.5% to 4.5% when changing from a normal to an inverted structure. For opposite mobility ratio, the inverted structure clearly outperforms the normal structure.
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88.40.hj Efficiency and performance of solar cells
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Can morphology tailoring improve the open circuit voltage of organic solar cells?

Biswajit Ray, Mark S. Lundstrom, and Muhammad A. Alam

Appl. Phys. Lett. 100, 013307 (2012); http://dx.doi.org/10.1063/1.3672221 (3 pages)

Online Publication Date: 5 January 2012

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While the effect of interfacial morphology on the short circuit current (ISC) of organic photovoltaic devices (OPVs) is well known, its impact on open circuit voltage (VOC) and fill-factor (FF) are less clear. Since the output power of a solar cell Pout = ISCVOCFF, such understanding is critical for designing high-performance, morphology-engineered OPVs. In this letter, we provide an explicit analytical proof that any effort to radically improve VOC by tailoring bulk heterojunction morphology is futile, because any increase in ISC due to larger interface area is counterbalanced by corresponding increase in recombination current, so that the upper limit of VOCBHJ cannot exceed that of the corresponding planar heterojunction devices, i.e., VOCBHJ ≤ VOCPHJ. We discuss the implication of this VOC-constraint on the efficiency optimization of organic solar cells.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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Surface plasmon-enhanced electroluminescence in organic light-emitting diodes incorporating Au nanoparticles

Y. Xiao, J. P. Yang, P. P. Cheng, J. J. Zhu, Z. Q. Xu, Y. H. Deng, S. T. Lee, Y. Q. Li, and J. X. Tang

Appl. Phys. Lett. 100, 013308 (2012); http://dx.doi.org/10.1063/1.3675970 (4 pages)

Online Publication Date: 6 January 2012

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Surface plasmon-enhanced electroluminescence (EL) in an organic light-emitting diode is demonstrated by incorporating the synthesized Au nanoparticles (NPs) in the hole injection layer of poly(3,4-ethylene dioxythiophene):polystyrene sulfonic acid. An increase of ∼25% in the EL intensity and efficiency are achieved for devices with Au NPs, whereas the spectral and electrical properties remain almost identical to the control device. Time-resolved photoluminescence spectroscopy reveals that the EL enhancement is ascribed to the increase in spontaneous emission rate due to the plasmonic near-field effect induced by Au NPs.
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85.60.Jb Light-emitting devices
78.55.-m Photoluminescence, properties and materials
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Extended spectral response in organic photomultiple photodetectors using multiple near-infrared dopants

Shao-Tang Chuang, Shang-Chieh Chien, and Fang-Chung Chen

Appl. Phys. Lett. 100, 013309 (2012); http://dx.doi.org/10.1063/1.3675573 (3 pages)

Online Publication Date: 6 January 2012

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We demonstrate highly sensitive polymer photodetectors (OPDs) with spectral response extending from the ultraviolet to the near-infrared (NIR) region (∼1200 nm). After doping two NIR dopants, high external quantum efficiencies (∼5500%) and high responsivities (23.0 A/W) are achieved under a low reverse bias (−3.7 V). The high gains could be attributed to unbalanced carrier transport in the photoactive layer arising from the electron traps at the NIR dopants. This approach allows the ready preparation of OPDs exhibiting broad spectral responses and high quantum efficiencies simultaneously.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
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Efficient polymer solar cell employing an oxidized Ni capped Al:ZnO anode without the need of additional hole-transporting-layer

T. L. Chen, R. Betancur, D. S. Ghosh, Jordi Martorell, and V. Pruneri

Appl. Phys. Lett. 100, 013310 (2012); http://dx.doi.org/10.1063/1.3673843 (3 pages)

Online Publication Date: 6 January 2012

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We show that an effective transparent electrode for bulk-heterostructure organic solar cells (OSCs) can be produced by uniformly depositing a few nm of Ni on a film of aluminum-doped zinc oxide (AZO). After deposition, the Ni capping layer is O2 plasma treated to form a bilayer of Ni/NiO, as it is evidenced by x-ray photoelectron spectroscopy analysis. The oxidized Ni capped AZO electrode can act as anode and hole-transporting-layer in OSCs, providing an enhancement in transparency, environmental stability, and injection/collection of charges. The S-shaped feature of the IV curve for the OSC using AZO electrodes in conjunction with NiO transporting layer is not present in the case of the proposed electrode structure, clearly indicating the significant role of the Ni metallic interlayer in reducing the energy barrier. The relevant role played by the Ni was further confirmed when a NiO layer was deposited on top of the AZO/Ni bilayer. In that case, the S-shape was not present while a 90% photo-conversion efficiency relative to the ITO/NiO cell was achieved.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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Barrierless hole injection through sub-bandgap occupied states in organic light emitting diodes using substoichiometric MoOx anode interfacial layer

Maria Vasilopoulou, Leonidas C. Palilis, Dimitra G. Georgiadou, Stella Kennou, Ioannis Kostis, Dimitris Davazoglou, and Panagiotis Argitis

Appl. Phys. Lett. 100, 013311 (2012); http://dx.doi.org/10.1063/1.3673283 (4 pages)

Online Publication Date: 6 January 2012

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In this letter, highly efficient hole injection was demonstrated in hole only devices based on organic semiconductors with different highest occupied molecular orbital level and transport properties. The barrierless hole injection was achieved by using a substoichiometric MoOx thin film (consisting of 65% Mo+6 and 35% Mo+5) as a higly effective anode interfacial layer. The current in these devices was found to be space charge limited, achieved due to the formation of highly efficient anode ohmic contact via the excellent band alignment through occupied gap states at the ITO/MoOx and MoOx/organic semiconductor modified interface. Quite remarkably, the efficiency of hole injection was found to be almost independent of the MoOx thickness, which is indicative of perfect band alignment at the anode interface.
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85.60.Jb Light-emitting devices
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Role of n-dopant based electron injection layer in n-doped organic light-emitting diodes and its simple alternative

Young Wook Park, Jin Hwan Choi, Tae Hyun Park, Eun Ho Song, Hakkoo Kim, Hyun Jun Lee, Se Joong Shin, Byeong-Kwon Ju, and Won Jun Song

Appl. Phys. Lett. 100, 013312 (2012); http://dx.doi.org/10.1063/1.3674960 (4 pages)

Online Publication Date: 6 January 2012

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We investigate the enhancement mechanism of the electroluminescence (EL) of alkali metal based n-doped organic light-emitting diodes (OLEDs). The dual role of the n-dopant (carrier transport and lowering of the injection barrier) induces a trade-off. When the electron transport layer (ETL) is optimally doped by the n-dopant for the highest conductivity, the amount of n-dopant at the ETL/cathode interface is insufficient to form enough chemical bonds with the cathode for efficient carrier injection. This insufficient amount of n-dopant limits the carrier injection properties. To solve this problem, we demonstrated that the addition of an electron injection layer (EIL) comprised of the n-dopant could increase its presence at the interface and, thereby, improve the carrier injection properties and, consequently, the EL efficiency. Moreover, simply using an alkali-metal alloy (rather than co-deposition) on the n-doped ETL as a cathode, instead of using the additional EIL, greatly improves the EL efficiency of the OLEDs. The alkali-metal alloy cathode increased the interfaced states at the ETL/cathode. The proposed model was confirmed by x-ray photoemission spectroscopy experiments on the alkali-metal n-dopant/electrode interface.
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85.60.Jb Light-emitting devices
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High-performance short channel organic transistors using densely aligned carbon nanotube array electrodes

Biddut K. Sarker and Saiful I. Khondaker

Appl. Phys. Lett. 100, 023301 (2012); http://dx.doi.org/10.1063/1.3675639 (4 pages)

Online Publication Date: 9 January 2012

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We report high-performance short channel pentacene field effect transistor (FET) using carbon nanotube aligned array electrodes. The devices show field effect mobility of up to 0.65 cm2/Vs and current on-off ratio of up to 1.7 × 106, which is the best for sub-micron pentacene FETs. The calculated cutoff frequency (fc) of the devices is up to 211 MHz which is among the best reported fc for organic transistors. The high performance of our short channel FET is attributed to improved charge injections from the aligned array carbon nanotube electrodes into the pentacene.
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85.30.Tv Field effect devices
72.20.Fr Low-field transport and mobility; piezoresistance
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Pentacene-based metal-insulator-semiconductor memory structures utilizing single walled carbon nanotubes as a nanofloating gate

A. Sleiman, M. C. Rosamond, M. Alba Martin, A. Ayesh, A. Al Ghaferi, A. J. Gallant, M. F. Mabrook, and D. A. Zeze

Appl. Phys. Lett. 100, 023302 (2012); http://dx.doi.org/10.1063/1.3675856 (4 pages)

Online Publication Date: 9 January 2012

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A pentacene-based organic metal-insulator-semiconductor memory device, utilizing single walled carbon nanotubes (SWCNTs) for charge storage is reported. SWCNTs were embedded, between SU8 and polymethylmethacrylate to achieve an efficient encapsulation. The devices exhibit capacitance-voltage clockwise hysteresis with a 6 V memory window at ± 30 V sweep voltage, attributed to charging and discharging of SWCNTs. As the applied gate voltage exceeds the SU8 breakdown voltage, charge leakage is induced in SU8 to allow more charges to be stored in the SWCNT nodes. The devices exhibited high storage density (∼9.15 × 1011 cm−2) and demonstrated 94% charge retention due to the superior encapsulation.
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85.30.Tv Field effect devices
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Effect of different electrodes on Fano resonance in molecular devices

Changfeng Fang, Dongmei Li, Bin Cui, Yuqing Xu, Guomin Ji, and Desheng Liu

Appl. Phys. Lett. 100, 023303 (2012); http://dx.doi.org/10.1063/1.3676190 (4 pages)

Online Publication Date: 10 January 2012

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By using nonequilibrium Green’s function in combination with density functional theory, we study the electronic transport properties of two typical π-conjugated molecules (dithiol-benzene and C4S2), sandwiched between two metallic electrodes made of different metals. The presence of two different electrodes leads to Fano resonances at certain energy. As a consequence, electronic transport in future molecular electric circuits can be substantially affected when the molecular devices placed between electrodes with different chemical potentials. The Fano line shapes reveal that there is nonresonant channel when two asymmetric electrodes are employed.
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85.65.+h Molecular electronic devices
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Excellent carrier mobility of 0.24 cm2/Vs in regioregular poly(3-hexylthiophene) based field-effect transistor by employing octadecyltrimethoxysilane treated gate insulator

Yi-Da Jiang, Tzu-Hao Jen, and Show-An Chen

Appl. Phys. Lett. 100, 023304 (2012); http://dx.doi.org/10.1063/1.3676444 (4 pages)

Online Publication Date: 11 January 2012

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The polymer field-effect transistor based on regioregular poly(3-hexylthiophene) (rr-P3HT) with excellent field-effect mobility up to 0.24 cm2/V s is demonstrated by spin-coating rr-P3HT onto octadecyltrimethoxysilane treated gate insulator followed by thermal annealing of rr-P3HT at 180 °C. This mobility is practically applicable in driving circuits of liquid crystal displays (greater than 0.1 cm2/V s) and close to the highest reported value (0.28 cm2/V s) for rr-P3HT but there a different coating method or dual dielectric layers was used. This impressive mobility can be attributed to the increase in structural ordering of rr-P3HT molecules as supported by x-ray diffraction measurement.
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85.30.Tv Field effect devices
72.20.Fr Low-field transport and mobility; piezoresistance
81.40.Tv Optical and dielectric properties related to treatment conditions
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Remanent polarization in a cryptand-polyanion bilayer implemented in an organic field effect transistor

Xiaodong Wang, Ari Laiho, Magnus Berggren, and Xavier Crispin

Appl. Phys. Lett. 100, 023305 (2012); http://dx.doi.org/10.1063/1.3677663 (3 pages)

Online Publication Date: 11 January 2012

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We investigate the possibility to maintain an electric polarization in an organic bilayer via ion trapping, i.e., without any external bias. In the cryptand-polyanion bilayer, ions of specific size can be strongly coordinated with organic macrocyclic molecules. Cations move from the polyanion layer to the cryptand layer upon applying a bias and are trapped in this layer. As a result, the voltage dependence of the polarization displays a hysteresis. The bilayer is then advantageously used as an electronic insulating layer in an organic field effect transistor. The ions trapping and de-trapping can be followed by the amplitude of the threshold voltage (Vth) shift as well as its temporal evolution.
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85.30.Tv Field effect devices
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Local charge transfer doping in suspended graphene nanojunctions

Jeffrey H. Worne, Hemtej Gullapalli, Charudatta Galande, Pulickel M. Ajayan, and Douglas Natelson

Appl. Phys. Lett. 100, 023306 (2012); http://dx.doi.org/10.1063/1.3677686 (4 pages)

Online Publication Date: 11 January 2012

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We report electronic transport measurements in nanoscale graphene transistors with gold and platinum electrodes whose channel lengths are shorter than 100 nm and compare them with transistors with channel lengths from 1 μm to 50 μm. We find a large positive gate voltage shift in charge neutrality point (NP) for transistors made with platinum electrodes but negligible shift for devices made with gold electrodes. This is consistent with the transfer of electrons from graphene into the platinum electrodes. As the channel length increases, the disparity between the measured NP using gold and platinum electrodes disappears.
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85.65.+h Molecular electronic devices
85.30.-z Semiconductor devices
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Organic photovoltaic cells based on unconventional electron donor fullerene and electron acceptor copper hexadecafluorophthalocyanine

J. L. Yang, P. Sullivan, S. Schumann, I. Hancox, and T. S. Jones

Appl. Phys. Lett. 100, 023307 (2012); http://dx.doi.org/10.1063/1.3671181 (3 pages)

Online Publication Date: 12 January 2012

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We demonstrate organic discrete heterojunction photovoltaic cells based on fullerene (C60) and copper hexadecafluorophthalocyanine (F16CuPc), in which the C60 and F16CuPc act as the electron donor and the electron acceptor, respectively. The C60/F16CuPc cells fabricated with conventional and inverted architectures both exhibit comparable power conversion efficiencies. Furthermore, we show that the photocurrent in both cells is generated by a conventional exciton dissociation mechanism rather than the exciton recombination mechanism recently proposed for a similar C60/F16ZnPc system [Song et al., J. Am. Chem. Soc. 132, 4554 (2010)]. These results demonstrate that new unconventional material systems are a potential way to fabricate organic photovoltaic cells with inverted as well as conventional architectures.
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84.60.Jt Photoelectric conversion
72.40.+w Photoconduction and photovoltaic effects
71.35.-y Excitons and related phenomena
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Charge transport in dual-gate organic field-effect transistors

J. J. Brondijk, M. Spijkman, F. Torricelli, P. W. M. Blom, and D. M. de Leeuw

Appl. Phys. Lett. 100, 023308 (2012); http://dx.doi.org/10.1063/1.3677676 (4 pages)

Online Publication Date: 13 January 2012

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The charge carrier distribution in dual-gate field-effect transistors is investigated as a function of semiconductor thickness. A good agreement with 2-dimensional numerically calculated transfer curves is obtained. For semiconductor thicknesses larger than the accumulation width, two spatially separated channels are formed. The cross-over from accumulation into depletion of the two channels in combination with a carrier density dependent mobility causes a shoulder in the transfer characteristics. A semiconducting monolayer has only a single channel. The charge carrier density, and consequently the mobility, are virtually constant and change monotonically with applied gate biases, leading to transfer curves without a shoulder.
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85.30.Tv Field effect devices
72.20.Fr Low-field transport and mobility; piezoresistance
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The influence of gate dielectrics on a high-mobility n-type conjugated polymer in organic thin-film transistors

Jinhua Li, Jun Du, Jianbin Xu, Helen L. W. Chan, and Feng Yan

Appl. Phys. Lett. 100, 033301 (2012); http://dx.doi.org/10.1063/1.3678196 (4 pages)

Online Publication Date: 18 January 2012

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Organic thin-film transistors based on a high mobility n-type semiconductor poly{[n,n9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)} P(NDI2OD-T2) and different polymer gate dielectrics are fabricated. The average electron mobility decreases from 0.76 to 0.08 cm2/Vs with the increase of the gate dielectric constant from 2.6 to 7.8. The P(NDI2OD-T2) film shows unconventional face-on molecular packing, which results in short distances and pronounced interactions between electrons and gate dielectric. Therefore, the decrease of the electron mobility with the increasing dielectric constant is attributed to the Fröhlich polaron effect for the interaction between electrons in the channel and ionic polarization cloud in the gate dielectric.
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85.30.Tv Field effect devices
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Publisher’s Note: “Quantitative analysis of the guest-concentration dependence of the mobility in a disordered fluorene-arylamine host-guest system in the guest-to-guest regime” [Appl. Phys. Lett. 99, 203303 (2011)]

H. T. Nicolai, A. J. Hof, M. Lu, P. W. M. Blom, R. J. de Vries, and R. Coehoorn

Appl. Phys. Lett. 100, 039902 (2012); http://dx.doi.org/10.1063/1.3679550 (1 page)

Online Publication Date: 19 January 2012

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Abstract Unavailable
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99.10.Fg Publisher's note
82.80.-d Chemical analysis and related physical methods of analysis
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Morphological control of hybrid polymer-quantum dot solar cells with electron acceptor ligands

Mathieu Boivin, Sébastien Lamarre, Jonathan Tessier, Marie-Ève Lecavalier, Ahmed Najari, Sophie Dufour-Beauséjour, Evelyne Brown Dussault, Pierre Collin, and Claudine Nì. Allen

Appl. Phys. Lett. 100, 033302 (2012); http://dx.doi.org/10.1063/1.3678603 (3 pages)

Online Publication Date: 20 January 2012

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We integrate the electro-attractive conjugated molecule tetrafluoro-tetracyano-quinodimethane (F4TCNQ) in the active layer of polymer-CdSe colloidal quantum dot (cQD) solar cells. The addition of this molecule enhances cQD dispersion inside the polymer. In tuning its concentration, we can optimize the active layer morphology for charge separation and transport. A smoother morphology is likely the result of polymer chain adsorption on cQDs via F4TCNQ which increases the steric barrier between cQDs. Our most optimized device has a F4TCNQ:cQDs weight ratio of 0.5% improving the power conversion efficiency by a factor ∼2.3.
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88.40.H- Solar cells (photovoltaics)
88.40.J- Types of solar cells
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Micro-imprinted prism substrate for self-aligned short channel organic transistors on a flexible substrate

Jin Jeon, Benjamin C.-K. Tee, Boris Murmann, and Zhenan Bao

Appl. Phys. Lett. 100, 043301 (2012); http://dx.doi.org/10.1063/1.3679119 (4 pages)

Online Publication Date: 23 January 2012

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Using a simple micro-imprinting process on flexible substrates, we demonstrate fabrication of self-aligned short channel organic thin film transistors (OTFTs) with significantly reduced parasitic capacitance. The surface topology resulting from the imprinted prism-like structures enables accurate alignment of both the gate and source-drain electrodes. The parasitic overlap capacitance was reduced by 80%, which enables twice higher transition frequency (fT = 10.1 kHz) compared with conventional top-contact OTFT devices. The prism-OTFTs were applied to both p-type (pentacene) and n-type (C60) organic semiconductors to implement low voltage complementary inverters.
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85.30.Tv Field effect devices
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Enhanced carrier injection in pentacene thin-film transistors by inserting a MoO3-doped pentacene layer

Zhaokui Wang, Mir Waqas Alam, Yanhui Lou, Shigeki Naka, and Hiroyuki Okada

Appl. Phys. Lett. 100, 043302 (2012); http://dx.doi.org/10.1063/1.3680249 (4 pages)

Online Publication Date: 27 January 2012

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We report on the enhanced carrier injection in pentacene thin-film transistors with a thin MoO3-doped pentacene layer between pentacene semiconductor and the source-drain electrodes. Device performance including drain current, field effect mobility, and threshed voltage are improved by employing a MoO3-doped pentacene thin layer. The barrier height at the Au/pentacene interface is lowered from 0.12 to 0.05 eV after inserting a MoO3-doped pentacene thin layer between them. The reduced barrier height is attributed to the formation of a good contact between MoO3-doped pentacene and Au owing to smoothed surface morphology of pentancene and suitable band bending by MoO3 doping.
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85.30.Tv Field effect devices
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