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February 2012

Volume 5, Issue 2

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Understanding metal doping for organic electron transport layers

Alexander Mityashin, David Cheyns, Barry P. Rand, and Paul Heremans

Appl. Phys. Lett. 100, 053305 (2012); doi:10.1063/1.3681383 (3 pages)

Online Publication Date: 1 February 2012

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This work concerns the physical mechanisms of metal n-doping in charge transport layers for optoelectronic devices, for which the doping level is constrained by transparency requirements so as to avoid parasitic absorption. Comparing various metal dopants, we claim that enhanced conductivity at low doping is initiated by the electrical doping effect, namely, metal-semiconductor charge donation. Electrical measurements show that doping effects at low concentration strongly depend on the work function of the introduced metal, and not every metal works as an efficient dopant. Practical applicability is demonstrated by introducing doped transport layers in prototypical bilayer solar cells in conventional and inverted architectures.
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61.72.up Other materials
65.40.gh Work functions
73.61.Ph Polymers; organic compounds
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Electrical phase transition of poly(4,4′-aminotriphenylene hexafluoroisopropylidenediphthalimide) by photogenerated charged carrier injection

Kyoung-Jae Lee, Dong Min Kim, Kyuwook Ihm, Moonhor Ree, Tai-Hee Kang, and Sukmin Chung

Appl. Phys. Lett. 100, 053306 (2012); doi:10.1063/1.3681776 (4 pages)

Online Publication Date: 1 February 2012

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We show a set-up of poly(4,4′-aminotriphenylene hexafluoroisopropylidenediphthalimide) (6F-TPA PI)/Al sample in which holes are injected by photoelectron emission process instead of direct charge carrier injection via metal electrode. In this process, an irreversible electrical phase transition of 6F-TPA PI is found in contrast to the Al/6F-TPA PI/Al structure, leading to a write-once-read-many behavior. The photoelectron spectroscopy results measured before and after the switching process revealed that the irreversible electrical phase transition of 6F-TPA PI is attributed to the chemical modification of the carbonyl group in phthalimide moiety.
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72.60.+g Mixed conductivity and conductivity transitions
79.60.Fr Polymers; organic compounds
78.70.Dm X-ray absorption spectra
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Optimal parameters for morphology of bulk heterojunction solar cells from simulations

Junwei Xue, Tingjun Hou, and Youyong Li

Appl. Phys. Lett. 100, 053307 (2012); doi:10.1063/1.3681947 (3 pages)

Online Publication Date: 2 February 2012

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We combine atomistic simulation with mesoscale simulation to predict the morphology of bulk heterojunction (BHJ) solar cells. We show that good morphology is formed for weight fraction of PCBM in the region of 0.41–0.62 for poly(3-hexylthiophene)/[6,6]-phenyl-C61 butyric acid methyl ester BHJ solar cells, which agrees with experimental results. Our simulation results indicate that the enhanced phase separation is achieved by increasing the temperature, which is consistent with transmission electron microscopy results. We find that Flory-Huggins parameter (χ) > 1 is required for phase separation and good morphology. Our simulation method is useful for predicting the morphology of BHJ polymer solar cells.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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Cumulative effects of electrode and dielectric surface modifications on pentacene-based transistors

Mélanie Devynck, Pascal Tardy, Guillaume Wantz, Yohann Nicolas, Luc Vellutini, Christine Labrugère, and Lionel Hirsch

Appl. Phys. Lett. 100, 053308 (2012); doi:10.1063/1.3681791 (4 pages)

Online Publication Date: 3 February 2012

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Surface modifications of the dielectric and the metal of pentacene-based field effect transistors using self-assembled monolayer (SAM) were studied. First, a low interfacial trap density and pentacene 2D-growth were favored by the nonpolar and low surface energy of octadecyltrichlorosilane-based SAM. This treatment leaded to increased mobility up to 0.4 cm2 V−1 s−1 and no observable hysteresis on transfer curves. Second, reduced hole injection barrier and contact resistance were achieved by fluorinated thiols deposited on gold contacts resulting in an increased mobility up to 0.6 cm2 V−1 s−1. Finally, a high mobility of 2.6 cm2 V−1 s−1 was achieved by cumulative effects of both treatments.
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85.30.Tv Field effect devices
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Cross-linking of a poly(3,4-ethylene dioxythiophene):(polystyrene sulfonic acid) hole injection layer with a bis-azide salt and the effect of atmospheric processing conditions on device properties

Oliver Fenwick, Kate Oliver, and Franco Cacialli

Appl. Phys. Lett. 100, 053309 (2012); doi:10.1063/1.3680606 (4 pages)

Online Publication Date: 3 February 2012

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We investigate the role of atmospheric conditions during processing when cross-linking a poly(3,4-ethylene dioxythiophene):(polystyrene sulfonic acid) hole injection layer with a bis-azide salt. In particular, we investigate nitrogen atmosphere and air, since there is a competing reaction of the cross-linker with oxygen. We show enhanced work function when processing under inert conditions, with device properties otherwise unaffected. When processing is done in air, higher irradiation dosages are needed to cross-link and the resulting films show lower work functions. Surprisingly, the finished devices display an unexpected two-fold increase in efficiency which we attribute to increased electron trapping in these films.
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73.30.+y Surface double layers, Schottky barriers, and work functions
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
61.41.+e Polymers, elastomers, and plastics
68.55.-a Thin film structure and morphology
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
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Four-mode plasmonic structure based on a prism-grating anticrossing bandgap

R. Chuliá-Jordán and D. Santamaría-Pérez

Appl. Phys. Lett. 100, 063301 (2012); doi:10.1063/1.3682081 (4 pages)

Online Publication Date: 7 February 2012

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We have developed a plasmonic structure based on an anticrossing bandgap prism-grating coupling technique. We observe that up to four surface plasmons can be excited simultaneously in this plasmonic structure. Initial experiments confirm that such a plasmonic structure can improve sensor performance by tuning the amplitude of the grating. From numerical simulations, we conclude that plasmon modes with equal propagation constants at a different wavelength can only be excited at different grating amplitudes of our plasmonic structure, in such a way that the increase of wavelength implies the increase of the amplitude for a fixed propagation constant.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
42.79.Bh Lenses, prisms and mirrors
42.79.Dj Gratings
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Mixed self-assembled monolayer of molecules with dipolar and acceptor character—Influence on hysteresis and threshold voltage in organic thin-film transistors

Abdesselam Jedaa, Michael Salinas, Christof M. Jäger, Timothy Clark, Alexander Ebel, Andreas Hirsch, and Marcus Halik

Appl. Phys. Lett. 100, 063302 (2012); doi:10.1063/1.3682301 (4 pages)

Online Publication Date: 7 February 2012

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In this report, we investigate the impact of the molecular dipole moment and redox active head groups (C60) in pure and mixed self-assembled monolayers (SAMs), which serve as an ultra-thin hybrid dielectric layer in low-voltage operating organic thin-film transistors. We show that the dipole of the SAM-forming molecules affects the threshold voltage, while the concentration of redox-active C60 moieties determines the hysteresis in devices with α,α′-dihexyl-sexithiophene and pentacene as organic semiconductors.
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68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces
85.30.Tv Field effect devices
77.80.Dj Domain structure; hysteresis
77.55.-g Dielectric thin films
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Fullerene acceptor for improving open-circuit voltage in inverted organic photovoltaic devices without accompanying decrease in short-circuit current density

Yutaka Matsuo, Junichi Hatano, Takayuki Kuwabara, and Kohshin Takahashi

Appl. Phys. Lett. 100, 063303 (2012); doi:10.1063/1.3683469 (3 pages)

Online Publication Date: 8 February 2012

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A poly(3-hexylthiophene) (P3HT)-based inverted organic photovoltaic (OPV) device with a fullerene electron acceptor, (dimethyl(o-anisyl)silylmethyl)(dimethylphenylsilylmethyl)[60]fullerene (SIMEF2), exhibited 2.9% power conversion efficiency (PCE; JSC = 7.9 mA/cm2, VOC = 0.66 V, and FF = 0.56) in the device configuration, indium tin oxide/ZnO/P3HT:SIMEF2/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/Au under AM1.5 G illumination at 100 mW/cm2. Through a comparison with a [6,6]-phenyl-C61-butyric acid methyl ester-based device (PCE = 2.4%, JSC = 7.8 mA/cm2, VOC = 0.56 V, and FF = 0.55), SIMEF2 was found to give higher VOC without a decrease in JSC. This result marks progress toward overcoming the trade-off relationship between VOC and JSC in the development of highly efficient OPV devices.
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85.60.Bt Optoelectronic device characterization, design, and modeling
85.65.+h Molecular electronic devices
84.70.+p High-current and high-voltage technology: power systems; power transmission lines and cables
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Efficient hybrid white polymer light-emitting devices with electroluminescence covered the entire visible range and reduced efficiency roll-off

Sujun Hu, Minrong Zhu, Qinghua Zou, Hongbin Wu, Chuluo Yang, Wai-Yeung Wong, Wei Yang, Junbiao Peng, and Yong Cao

Appl. Phys. Lett. 100, 063304 (2012); doi:10.1063/1.3682526 (4 pages)

Online Publication Date: 8 February 2012

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We report efficient hybrid white polymer light emitting devices (WPLEDs) fabricated via simple solution-proceeded process from a newly synthesized wide band-gap fluorene-co-dibenzothiophene-S,S-dioxide copolymer, which dually function as fluorescent blue emitter and host material for electrophosphorescent sky-blue, yellow, and saturated-red dyes. The Commission Internationale d’Énclairage coordinates of the best devices are (0.356, 0.334), with electroluminescence covered the entire visible light spectrum from 400 to 780 nm, resulting in a high color rendering index of 90. Incorporation of a bilayer electrode consisting of water/alcohol-soluble conjugated polymer and Al as electron-injection cathode boosts an enhancement of 50% in device efficiency, leading to external quantum efficiency of 12.6%, and peak power efficiency of 21.4 l m W−1 as measured in an integrating sphere. Both the efficiency and the color quality of the obtained device are ranking among one of the highest values for WPLEDs reported to date. Furthermore, as compared with those all-phosphorescent WPLEDs, the hybrid WPLEDs studied here exhibit a significantly reduced efficiency roll-off due to the very low doping concentration.
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85.60.Jb Light-emitting devices
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Bloch surface waves-controlled fluorescence emission: Coupling into nanometer-sized polymeric waveguides

Mirko Ballarini, Francesca Frascella, Emanuele Enrico, Pietro Mandracci, Natascia De Leo, Francesco Michelotti, Fabrizio Giorgis, and Emiliano Descrovi

Appl. Phys. Lett. 100, 063305 (2012); doi:10.1063/1.3684272 (4 pages)

Online Publication Date: 10 February 2012

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The lateral confinement of Bloch surface waves on a patterned multilayer is investigated by means of leakage radiation microscopy (LRM). Arrays of nanometric polymeric waveguides are fabricated on a proper silicon-nitride/silicon-oxide multilayer grown on a standard glass coverslip. By exploiting the functional properties of the polymer, fluorescent proteins are grafted onto the waveguides. A fluorescence LRM analysis of both the direct and the Fourier image plane reveals that a substantial amount of emitted radiation couples into a guided mode and then propagates into the nanometric waveguide. The observations of the mode are supported by numerical simulations.
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42.70.Jk Polymers and organics
42.79.Gn Optical waveguides and couplers
42.30.Kq Fourier optics
42.79.-e Optical elements, devices, and systems
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Investigation of the device instability feature caused by electron trapping in pentacene field effect transistors

Chang Bum Park

Appl. Phys. Lett. 100, 063306 (2012); doi:10.1063/1.3680248 (3 pages)

Online Publication Date: 10 February 2012

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The phenomenon of device instability governed by electron trapping was investigated in pentacene field-effect transistors. We found that the threshold voltage shift (ΔVT) increases significantly when the device is exposed to photo-irradiation with the depletion gate bias. This effect reveals that a number of electron trapping sites are located in the pentacene band gap, presenting the mechanism of the photo-induced ΔVT. In addition, we found the specific shift of bias-stressed ΔVT under a monochromatic of 690-500 nm, which reflects that the electron trap states would be distributed between the energy levels of 1.8 and 2.5 eV in the gap state.
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85.30.Tv Field effect devices
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Organic photovoltaic cells with nano-fabric heterojunction structure

June Hyoung Park, Austin R. Carter, Lynetta M. Mier, Chi-Yueh Kao, Sharlene A. M. Lewis, Raju P. Nandyala, Yong Min, and Arthur J. Epstein

Appl. Phys. Lett. 100, 073301 (2012); doi:10.1063/1.3679097 (4 pages)

Online Publication Date: 13 February 2012

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Organic photovoltaic cells containing electron-transporting organic nanofibers in the form of “nanofabrics” are investigated. Nano-fabric heterojunctions of poly(3-hexylthiophene) and electron-transporting nanofibers significantly improve short-circuit current density in organic photovoltaic cells. The nanofibers and nanofabric are synthesized from organic electron-transporting material bis(octyl)-perylenediimide (PDI-C8). The PDI-C8 based nano-fabric’s electron mobility is measured to be 0.08 cm2/V s. The nanofabric improves charge collection by expanding the interfacial acceptor-donor area while simultaneously providing dedicated electron transport pathways to the LiF/Al electrodes. An increase in fill factor is observed for photovoltaic cells incorporating the nanofabric heterojunctions and is attributed to efficient removal of space charge.
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88.40.jr Organic photovoltaics
88.40.H- Solar cells (photovoltaics)
88.40.jp Multijunction solar cells
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Sign change in the organic magnetoresistance of tris(8-hydroxyquinolinato)aluminum upon annealing

Hoju Kang, Ik-Jae Lee, and Choon Sup Yoon

Appl. Phys. Lett. 100, 073302 (2012); doi:10.1063/1.3684873 (4 pages)

Online Publication Date: 13 February 2012

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We report the sign change in the magnetoresistance of a tris(8-hydroxyquinolinato)aluminum film with the morphological change from amorphous to crystalline state upon annealing. The negative component of the magnetoresistance followed power law behavior, whereas the positive one showed non-Lorentzian function behavior. The decreasing absolute values of the negative component with increased annealing temperature may be understood by both intersystem-crossing-based mechanism and quenching of triplet excitons. The increasing values of the positive component with increased annealing temperature may be explained by the increase in the hopping probability of charge carriers with increased crystallinity of the film.
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73.61.Ph Polymers; organic compounds
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
81.40.Gh Other heat and thermomechanical treatments
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.50.Dn Low-field transport and mobility; piezoresistance
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Electron mobility determination of efficient phosphorescent iridium complexes with tetraphenylimidodiphosphinate ligand via transient electroluminescence method

Ming-Yu Teng, Song Zhang, Sheng-Wei Jiang, Xu Yang, Chen Lin, You-Xuan Zheng, Leyong Wang, Di Wu, Jing-Lin Zuo, and Xiao-Zeng You

Appl. Phys. Lett. 100, 073303 (2012); doi:10.1063/1.3684971 (4 pages)

Online Publication Date: 13 February 2012

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The electron mobility of Alq3 and iridium complexes was determined via transient electroluminescence (EL) method based on ITO (indium tin oxide)/di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane/complex/LiF/Al with short and rectangular driving voltage pulses. Apparent values of the electron mobility (μe) in complexes have been investigated from their onset of EL upon different driving voltages. The result reveals that the μe, 4.31 × 10−6 cm2/V⋅s, of the efficient phosphorescent material Ir(tfmppy)2(tpip) [1, tfmppy = 4-trifluoromethylphenylpyridine, tpip = tetraphenylimido-diphosphinate] under electric field of 1300 (V/cm)1/2 is as high as that of Alq3, which indicates that the good device performances of 1 are partly due to its high electron mobility.
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85.60.Jb Light-emitting devices
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