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December 2010

Volume 3, Issue 12

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Light out-coupling enhancement of organic light-emitting devices with microlens array

J. P. Yang, Q. Y. Bao, Z. Q. Xu, Y. Q. Li, J. X. Tang, and S. Shen

Appl. Phys. Lett. 97, 223303 (2010); http://dx.doi.org/10.1063/1.3521413 (3 pages)

Online Publication Date: 1 December 2010

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Light out-coupling efficiency of organic light-emitting devices from high-index glass substrate into air is enhanced by attaching ordered microlens arrays, which are fabricated by a roll-to-roll mold transfer process. The dependence of microlens geometries on light extraction is analyzed experimentally and theoretically. An increase of 60% in the light out-coupling with an optimized elliptical microlens array is achieved over a conventional device without affecting the electroluminescent spectrum.
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42.79.Bh Lenses, prisms and mirrors
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Spin-polarized charge carrier injection by tunneling from ferromagnetic contacts into organic semiconductors

M. Yunus, P. P. Ruden, and D. L. Smith

Appl. Phys. Lett. 97, 223304 (2010); http://dx.doi.org/10.1063/1.3522657 (3 pages)

Online Publication Date: 1 December 2010

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Tunnel-injection of spin-polarized charge carriers from ferromagnetic contacts into organic semiconductors is modeled. Tunneling matrix elements and transition rates for the two spin types are calculated using a transfer Hamiltonian. The tunneling process occurs between extended states of the contact and model “molecular” orbitals. We explore the effects of the tunnel barrier height and of the ferromagnetic contact’s Fermi wave vectors on the level of spin injection. The barrier height and the majority and minority spin Fermi wave vectors of the contact have strong effects on the sign and magnitude of spin injection.
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85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
75.76.+j Spin transport effects
72.25.Mk Spin transport through interfaces
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
75.10.Dg Crystal-field theory and spin Hamiltonians
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Thermal nonlinear effects in hybrid optical microresonators

Hong Seok Choi and Andrea M. Armani

Appl. Phys. Lett. 97, 223306 (2010); http://dx.doi.org/10.1063/1.3520467 (3 pages)

Online Publication Date: 2 December 2010

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The inherent material properties of silica reduce the susceptibility of silica optical microcavities to many nonlinear phenomena, enabling the development of devices with improved stability. However, large nonlinear coefficients can be leveraged to create microcavities with additional functionality. Therefore, devices with controllable nonlinear behavior are of interest. In the present research, silica microcavities are coated with polystyrene or polymethylmethacrylate to form hybrid microresonators. Using these devices, the thermally induced nonlinear optical behavior is investigated. By precisely controlling the optical field overlap with the polymer film, an environmentally stable device, in which the resonant frequency is independent of the input power, is demonstrated.
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42.79.-e Optical elements, devices, and systems
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
78.20.N- Thermo-optic effects
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Enhanced performance of inverted polymer solar cells with cathode interfacial tuning via water-soluble polyfluorenes

Seok-In Na, Tae-Soo Kim, Seung-Hwan Oh, Junkyung Kim, Seok-Soon Kim, and Dong-Yu Kim

Appl. Phys. Lett. 97, 223305 (2010); http://dx.doi.org/10.1063/1.3522893 (3 pages)

Online Publication Date: 2 December 2010

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Enhanced performance of inverted polymer solar cells (PSCs) is demonstrated by indium tin oxide (ITO) interfacial tuning via a water-soluble polyfluorene (WPF-6-oxy-F). Kelvin probe studies and dark current-voltage curves demonstrated that the WPF-6-oxy-F layer reduces the ITO work-function because of the favorable interfacial dipole formed by the WPF-6-oxy-F interlayer, thereby enhancing the built-in potential and reducing the interface resistance. As a result, introduction of the WPF-6-oxy-F by simple solution processing into the inverted PSCs dramatically enhanced cell-performances. This approach could be very beneficial and an important step for the future development of all-solution-processed or roll-to-roll processed PSCs.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
73.30.+y Surface double layers, Schottky barriers, and work functions
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Polymer space-charge-limited transistor as a solid-state vacuum tube triode

Yu-Chiang Chao, Ming-Che Ku, Wu-Wei Tsai, Hsiao-Wen Zan, Hsin-Fei Meng, Hung-Kuo Tsai, and Sheng-Fu Horng

Appl. Phys. Lett. 97, 223307 (2010); http://dx.doi.org/10.1063/1.3513334 (3 pages)

Online Publication Date: 3 December 2010

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We report the construction of a polymer space-charge-limited transistor (SCLT), a solid-state version of vacuum tube triode. The SCLT achieves a high on/off ratio of 3×105 at a low operation voltage of 1.5 V by using high quality insulators both above and below the grid base electrode. Applying a greater bias to the base increases the barrier potential, and turns off the channel current, without introducing a large parasitic leakage current. Simulation result verifies the influence of base bias on channel potential distribution. The output current density is 1.7 mA/cm2 with current gain greater than 1000.
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84.47.+w Vacuum tubes
85.30.Fg Bulk semiconductor and conductivity oscillation devices (including Hall effect devices, space-charge-limited devices, and Gunn effect devices)
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Conductance-dependent negative differential resistance in organic memory devices

Y. T. You, M. L. Wang, H. N. Xuxie, B. Wu, Z. Y. Sun, and X. Y. Hou

Appl. Phys. Lett. 97, 233301 (2010); http://dx.doi.org/10.1063/1.3524263 (3 pages)

Online Publication Date: 6 December 2010

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Single-layer organic memories made of organic material with good conductance have been characterized. Asymmetrical bistable behaviors under biases of opposite polarities are observed for devices with asymmetric electrodes. It is experimentally confirmed that a close correlation exists between the conductivity of the organic layer and the asymmetric bistability of the device under opposite biases. Inserting a block layer or thickening the organic layer will result in negative differential resistance under positive biases, leading to reversible symmetrical bistability. The phenomena are ascribed to the presence of filamentary microconducting channels in the organic layer.
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85.30.De Semiconductor-device characterization, design, and modeling
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Nongeminate carrier recombination rates in organic solar cells

Christopher Lombardo and Ananth Dodabalapur

Appl. Phys. Lett. 97, 233302 (2010); http://dx.doi.org/10.1063/1.3524025 (3 pages)

Online Publication Date: 7 December 2010

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Ambipolar organic thin-film transistors and lateral resistor structures have been used to study the transport of charge carriers in bulk heterojunction (BHJ) organic photovoltaic devices. Active layers of a phase-separated blend of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester were chosen due to their wide use in BHJ solar cell devices. A new method for determining nongeminate recombination rates is reported. Field dependent measurements of the recombination rate show that recombination decreases with increasing electric field.
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88.40.jr Organic photovoltaics
85.30.Tv Field effect devices
84.32.Ff Conductors, resistors (including thermistors, varistors, and photoresistors)
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Electroluminescence imaging of organic photovoltaic modules

U. Hoyer, M. Wagner, Th. Swonke, J. Bachmann, R. Auer, A. Osvet, and C. J. Brabec

Appl. Phys. Lett. 97, 233303 (2010); http://dx.doi.org/10.1063/1.3521259 (3 pages)

Online Publication Date: 8 December 2010

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We report on electroluminescence (EL) imaging of organic photovoltaic cells and modules with poly(3-hexylthiophene)/[6,6]-phenyl C61 butyric acid methyl ester as semiconductor layer. The dominant EL emission is found in a spectral regime between 1200 and 1400 nm and is identified as the radiative decay of the charge transfer complex formed between the polymer and the fullerene. Electroluminescence emission from the pristine compounds is either much weaker or completely absent. Overall, electroluminescence imaging is shown to give valuable information on the defects but also on the performance of organic solar modules.
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88.40.jr Organic photovoltaics
88.40.H- Solar cells (photovoltaics)
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Experimental observation of polarized electroluminescence from edge-emission organic light emitting devices

G. Z. Ran, D. F. Jiang, Q. Kan, and H. D. Chen

Appl. Phys. Lett. 97, 233304 (2010); http://dx.doi.org/10.1063/1.3525161 (3 pages)

Online Publication Date: 9 December 2010

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We have observed a strongly polarized edge-emission from an organic light emitting device (OLED) with a silicon anode and a stacked Sm/Au (or Ag) cathode. For the OLED with a Sm/Au cathode, the transverse magnetic (TM) mode is stronger than the transverse electric (TE) mode by a factor of 2, while the polarization ratio of TM:TE is close to 300 for that with a Sm/Ag cathode. The polarization results from the scattering of surface plasmon polaritons at the device boundary. Such a silicon-based OLED is potentially an electrically excited SPP source in plasmonics.
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85.60.Jb Light-emitting devices
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Charge accumulation dynamics in organic thin film transistors

X. Y. Chen, H. Zhu, and S. D. Wang

Appl. Phys. Lett. 97, 243301 (2010); http://dx.doi.org/10.1063/1.3526374 (3 pages)

Online Publication Date: 14 December 2010

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We have developed a transient current model to quantitatively describe the charge accumulation dynamics in organic thin film transistors. This model indicates that the charge accumulation process is faster at higher gate bias and/or higher field-effect mobility, and the experimental results are consistent with the theoretical expectations. A strong gate bias dependence of the field-effect mobility is observed, which suggests that the charge traps in the charge accumulation layer may limit the device performance not only at steady state but also at transient state.
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85.30.Tv Field effect devices
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Solution processed polymer light-emitting diodes utilizing a ZnO/organic ionic interlayer with Al cathode

Hongseok Youn and Minyang Yang

Appl. Phys. Lett. 97, 243302 (2010); http://dx.doi.org/10.1063/1.3526308 (3 pages)

Online Publication Date: 14 December 2010

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This letter reports polymer light-emitting diodes that employ a soluble zinc oxide (ZnO) nanoparticle (NP) and organic ionic interlayer as an electron-injection layer exhibits remarkable enhancement of device performance despite aluminum cathode. The ionic solution infiltrated into ZnO NP layer, which contains poly(ethylene oxide) and tetra-n-butylammonium tetrafluoborate, significantly lowers the large electron-injection barrier by forming a permanent interfacial dipole. The polymer, phenyl substituted poly(para-phenylene vinylene) known as “Super Yellow,” yellow light-emitting diodes employing the ZnO NP and ionic interlayer show a maximum efficiency of 6.3 cd/A at a 1209 cd/m2 and 5.4 V. The maximum brightness of the device reached 24 000 cd/m2 at 9 V.
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85.60.Jb Light-emitting devices
82.45.Fk Electrodes
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High-performance organic field-effect transistors based on copper/copper sulphide bilayer source-drain electrodes

Wen Gu, Weipeng Jin, Bin Wei, Jianhua Zhang, and Jun Wang

Appl. Phys. Lett. 97, 243303 (2010); http://dx.doi.org/10.1063/1.3526737 (3 pages)

Online Publication Date: 14 December 2010

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We demonstrate an organic field-effect transistor (OFET) using copper (Cu) and copper sulfide (CuxS) as the source-drain electrodes. The OFET using Cu/CuxS results in a fivefold higher field-effect mobility and a 12 V reduction in threshold voltage as compared with the conventional devices with gold electrodes. The improvements of device performances are attributed to the enhancement of charge-injection. X-ray photoelectron spectroscopy revealed that sulfate ions were formed during deposition, which is considered to be responsible for the improvements.
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85.30.Tv Field effect devices
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Causes of efficiency roll-off in phosphorescent organic light emitting devices: Triplet-triplet annihilation versus triplet-polaron quenching

Dandan Song, Suling Zhao, Yichun Luo, and Hany Aziz

Appl. Phys. Lett. 97, 243304 (2010); http://dx.doi.org/10.1063/1.3527085 (3 pages)

Online Publication Date: 16 December 2010

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Delayed electroluminescence measurements are used to probe and differentiate between triplet-triplet-annihilation (TTA) and triplet-polaron-quenching (TPQ) processes and their correlation with efficiency roll-off in fac-tris(2-phenylpyridine) iridium-based phosphorescent organic light emitting devices. Investigations on devices employing 4,4′-bis(9-carbazolyl)-1,1′-biphenyl (CBP) and 4,4′,4″-tris(N-carbazolyl) triphenylamine, two widely used host materials, show that the efficiency roll-off is primarily due to TPQ processes. Guest-guest TTA, on the other hand, is found to play no major role, contrary to speculations, especially at low guest concentrations. Evidence of host-host TTA in certain cases, and its possible contribution to exciton quenching in the case of devices with CBP host, is also reported.
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85.60.Jb Light-emitting devices
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Molecular n-type doping for air-stable electron transport in vacuum-processed n-channel organic transistors

Joon Hak Oh, Peng Wei, and Zhenan Bao

Appl. Phys. Lett. 97, 243305 (2010); http://dx.doi.org/10.1063/1.3527972 (3 pages)

Online Publication Date: 17 December 2010

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The effects of n-type doping on the air-stability of vacuum-processed n-channel organic transistors have been investigated using perylene diimides and pyronin B as the active layer and dopant, respectively. Systematic studies on the influence of doping location revealed the n-type doping of bulk active layer or channel region significantly improves air-stability by compensating for the trapped electrons with the donated mobile electrons. Although n-type doping at the electrode contact could readily turn on the devices, it could not confer air-stable electron transport. The described approach would open up opportunities to enable and improve the stability of n-channel organic transistors in air.
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85.30.Tv Field effect devices
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Effects of alignment layer thickness on the pretilt angle of liquid crystals

Jong-Ho Son and Wang-Cheol Zin

Appl. Phys. Lett. 97, 243306 (2010); http://dx.doi.org/10.1063/1.3528209 (3 pages)

Online Publication Date: 17 December 2010

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Mixture solutions of vertical- and planar-aligning polyimide precursors were coated on bare glass. The concentrations of the solutions were varied to control the thicknesses of the films. The resulting blend films were baked to induce imidization and then rubbed. The thicknesses (t) of the blend film and of the pure vertical-alignment film affected their surface energies; the pretilt angle can be fully controlled in the range 5.5° ≤ Θ0 ≤ 87° by adjusting t. The surface energy of pure planar-alignment layers was independent of t.
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61.30.Vx Polymer liquid crystals
61.25.H- Macromolecular and polymers solutions; polymer melts
61.41.+e Polymers, elastomers, and plastics
81.65.-b Surface treatments
65.40.gp Surface energy
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Reproducible and controllable organic resistive memory based on Al/poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate)/Al structure

Z. S. Wang, F. Zeng, J. Yang, C. Chen, Y. C. Yang, and F. Pan

Appl. Phys. Lett. 97, 253301 (2010); http://dx.doi.org/10.1063/1.3529455 (3 pages)

Online Publication Date: 20 December 2010

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We report reproducible and controllable bipolar resistive memory devices based on Al/poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate)/Al, which show an on/off current ratio as large as 104, reproducibility of more than 103 dc sweeping cycles, and retention time of 104 s. The switching mechanism is confirmed to be the filamentary switching. We show that the compliant current can effectively control the formation of filaments and the performance of the device, including the on-state resistance, the reset current, and the switching-off threshold voltage. In addition, the lowest reset power and the critical compliant current for resistive switching are determined.
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85.30.-z Semiconductor devices
84.30.Sk Pulse and digital circuits
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Subbandgap absorption in polymer-fullerene solar cells

M. Presselt, M. Bärenklau, R. Rösch, W. J. D. Beenken, E. Runge, S. Shokhovets, H. Hoppe, and G. Gobsch

Appl. Phys. Lett. 97, 253302 (2010); http://dx.doi.org/10.1063/1.3527077 (3 pages)

Online Publication Date: 20 December 2010

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We present external quantum efficiency (EQE) studies of poly(3-hexylthiophene-2,5-diyl):[6,6]-phenylC61-butyric acid methyl ester (P3HT:PCBM) based bulk heterojunction polymer solar cells with improved intensity resolution in the subbandgap (SBG) region, i.e., the energy range below the optical bandgaps of the pristine materials. Varying the P3HT:PCBM blending ratio, we find that in addition to a Gaussian profile an exponential tail is needed for a quantitative description of the SBG EQE spectra. While the exponential contribution can be reliably assigned to disorder effects, the SBG EQE Gaussian profile can be due to charge-transfer absorption between P3HT and PCBM or due to absorption of PCBM at the interface or in the polymer-rich phase.
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88.40.H- Solar cells (photovoltaics)
88.40.jr Organic photovoltaics
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Organic-inorganic magnetic tunnel heterojunctions based on dithiapyrannylidene ultrathin films grown on Fe3O4(111)

Stéphane Berny, Ludovic Tortech, Denis Fichou, Sylvia Matzen, and Jean-Baptiste Moussy

Appl. Phys. Lett. 97, 253303 (2010); http://dx.doi.org/10.1063/1.3529456 (3 pages)

Online Publication Date: 20 December 2010

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We report on the morphology, transport, and magnetic properties of ultrathin films of 4,4′-bis(diphenyl-2,6-thiapyrannylidene) (DITPY-Ph4), a planar quinoïd organic compound with potential applications in electronics. Atomic force microscopy (AFM) shows that highly homogeneous and continuous DITPY-Ph4 ultrathin films with thicknesses as low as 2 nm can be grown on epitaxial Fe3O4(111). Current-sensing AFM performed on ultrathin layers evidences an insulating behavior with a nonresonant tunneling conduction regime. Vibrating sample magnetometry achieved on Fe3O4/DITPY-Ph4/Co organic-inorganic heterojunctions reveals the decoupling between the two magnetic electrodes and demonstrates that DITPY-Ph4 can be used to build magnetic tunnel junctions.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
72.25.Mk Spin transport through interfaces
73.40.-c Electronic transport in interface structures
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High-performance optoelectrical dual-mode memory based on spiropyran-containing polyimide

Qisheng Liu, Kejian Jiang, Yongqiang Wen, Jingxia Wang, Jia Luo, and Yanlin Song

Appl. Phys. Lett. 97, 253304 (2010); http://dx.doi.org/10.1063/1.3529453 (3 pages)

Online Publication Date: 21 December 2010

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A thermally stable polyimide (PI-SP) was designed as a functional material for the fabrication of memory device, and its optoelectrical dual-mode memory was studied. In an Al/PI-SP/Al device, the memory can be switched on with the negative or positive voltage with the on/off current ratios of about 104. Besides, the PI-SP can also undergo the reversibly photochromic reactions in solution or solid state with high fatigue resistance, which can serve as optical recording material. The PI-SP based device can exhibit the electrical and optical switching properties independently, while their cooperative switching properties cannot be realized simultaneously.
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42.79.Vb Optical storage systems, optical disks
42.65.Pc Optical bistability, multistability, and switching, including local field effects
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Quantification of energy loss mechanisms in organic light-emitting diodes

Rico Meerheim, Mauro Furno, Simone Hofmann, Björn Lüssem, and Karl Leo

Appl. Phys. Lett. 97, 253305 (2010); http://dx.doi.org/10.1063/1.3527936 (3 pages)

Online Publication Date: 22 December 2010

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The external quantum efficiency of organic light-emitting diodes (OLEDs) is limited by several loss mechanisms. By applying a numerical model for the efficiency analysis of OLED devices, we analyze the distribution of the different energy loss mechanisms in bottom and top emission organic light-emitting diodes. We validate the findings by the comparison with experimental data measured on red state-of-the-art p-i-n devices containing the red phosphorescent emitting dye iridium(III)bis[2-methyldibenzo-(f, h)quinoxaline](acetylacetonate) [Ir(MDQ)2(acac)]. The model is used to design extremely efficient bottom and top emission diodes with 21% and 27% external quantum efficiencies, respectively.
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85.60.Jb Light-emitting devices
85.30.Kk Junction diodes
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Improvement of power conversion efficiency of phthalocyanine/C60 heterojunction solar cells by inserting a lithium phthalocyanine layer at the indium-tin oxide/phthalocyanine interface

Senku Tanaka, Toshiyuki Hanada, Koji Ono, Kazuya Watanabe, Katsumi Yoshino, and Ichiro Hiromitsu

Appl. Phys. Lett. 97, 253306 (2010); http://dx.doi.org/10.1063/1.3529452 (3 pages)

Online Publication Date: 22 December 2010

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Improvement of power conversion efficiency of a zinc phthalocyanine (ZnPc)/C60 heterojunction solar cell was achieved by inserting a lithium phthalocyanine (LiPc) layer at the indium-tin oxide (ITO)/ZnPc interface. The results of photoelectron spectroscopy suggest that the barrier height for the hole transport at the ITO/ZnPc interface is reduced by the LiPc layer. A similar improvement of the power conversion efficiency by the insertion of a LiPc layer was also observed in M-phthalocyanine (M = H2, Cu, and TiO)/C60 cells.
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88.40.jr Organic photovoltaics
85.65.+h Molecular electronic devices
88.40.hj Efficiency and performance of solar cells
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Solution processed inverted tandem polymer solar cells with self-assembled monolayer modified interfacial layers

Steven K. Hau, Hin-Lap Yip, Kung-Shih Chen, Jingyu Zou, and Alex K.-Y. Jen

Appl. Phys. Lett. 97, 253307 (2010); http://dx.doi.org/10.1063/1.3530431 (3 pages)

Online Publication Date: 23 December 2010

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Inverted tandem bulk-heterojunction solar cells with comparable efficiency to single layer devices have been demonstrated by utilizing two thin layers ( ∼ 50 nm) of poly-(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester as the active material and a fullerene self-assembled monolayer (C60-SAM) to modify the interfaces between the ZnO buffer layer and the active layer. Single and tandem solar cells without the SAM modification have much lower efficiencies than the ones with modification. The successful demonstrations of inverted tandem devices with SAM modification give promise of further device improvements if active materials with complementary absorption can be used.
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88.40.J- Types of solar cells
88.40.H- Solar cells (photovoltaics)
81.16.Dn Self-assembly
68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces
68.55.-a Thin film structure and morphology
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
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Top-emitting organic light-emitting diodes: Influence of cavity design

Simone Hofmann, Michael Thomschke, Patricia Freitag, Mauro Furno, Björn Lüssem, and Karl Leo

Appl. Phys. Lett. 97, 253308 (2010); http://dx.doi.org/10.1063/1.3530447 (3 pages)

Online Publication Date: 23 December 2010

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We report on red top-emitting organic light-emitting diode structures with higher order cavities. The emission zone is placed in the first, second, and third antinodes of the electric field in the cavity by increasing the hole transport layer thickness. Furthermore, the thicknesses of the cathode and the capping layer are varied to achieve high efficiencies. Using doped charge transport layers and a phosphorescent emitter, we reach up to 29%, 17%, and 12% external quantum efficiencies for first, second, and third order devices, respectively. An optical model is further used to analyze the angular dependent emission.
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85.60.Jb Light-emitting devices
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The vertically stacked organic sensor-transistor on a flexible substrate

Shin Woo Jeong, Jin Wook Jeong, Seongpil Chang, Seung Youl Kang, Kyoung Ik Cho, and Byeong Kwon Ju

Appl. Phys. Lett. 97, 253309 (2010); http://dx.doi.org/10.1063/1.3530448 (3 pages)

Online Publication Date: 23 December 2010

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The authors report on the photo-response characteristics of flexible sensor-transistor circuits (ST-circuits) made with (poly(3-hexylethiophene)/phenyl-C61-butryic acid methyl ester) (P3HT/PCBM) bulk heterojunction polymer and pentacene-based organic field-effect transistors, which are stacked via poly(dimethylsiloxane) (PDMS) on the plastic substrate. The results indicate that the anode-source current is variable because of both the charge separation of the photogenerated excitons and the accumulated charges at the OFET channel layer. The light dependent photo response (ΔI/I0) is modulated from 0.47 to 1.9 by the gate-source voltage at the fixed anode-source voltage of the ST-circuits.
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85.60.-q Optoelectronic devices
85.30.Tv Field effect devices
84.30.-r Electronic circuits
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
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Interface formation between tris(8-hydroxyquinoline) aluminum and ZnO nanowires and film

Jung Han Lee, Jeong Ho Shin, Jae Yong Song, and Yeonjin Yi

Appl. Phys. Lett. 97, 263302 (2010); http://dx.doi.org/10.1063/1.3531812 (3 pages)

Online Publication Date: 28 December 2010

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The energy level alignments at the interface between tris(8-hydroxyquinoline) aluminum (Alq3) and ZnO nanowires and thin film were studied with in situ x-ray and ultraviolet photoemission spectroscopy. The changes of work functions, highest occupied molecular orbitals, and core levels were measured with step-by-step deposition of Alq3 on each ZnO substrate. Although both substrates show similar electronic structures, a larger interface dipole is induced at the interface between Alq3 and ZnO nanowires. This results in the reduction of the electron injection barrier at the interface of Alq3/ZnO nanowires. Thus, the ZnO nanowire substrate is expected to show better performance than that of ZnO film when used as a cathode. We discussed the different interface dipole formation at each interface.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
79.60.-i Photoemission and photoelectron spectra
73.30.+y Surface double layers, Schottky barriers, and work functions
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