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2 Jan 2012

Volume 100, Issue 1, Articles (01xxxx)

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Appl. Phys. Lett. 100, 013101 (2012); http://dx.doi.org/10.1063/1.3673334 (3 pages)

Patrice Genevet, Nanfang Yu, Francesco Aieta, Jiao Lin, Mikhail A. Kats, Romain Blanchard, Marlan O. Scully, Zeno Gaburro, and Federico Capasso
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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) | Cited 1 time

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) | Cited 1 time

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) | Cited 5 times

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) | Cited 4 times

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) | Cited 2 times

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) | Cited 2 times

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) | Cited 8 times

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) | Cited 5 times

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) | Cited 1 time

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) | Cited 8 times

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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