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January 2013

Volume 6, Issue 1

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A high-K ferroelectric relaxor terpolymer as a gate dielectric for organic thin film transistors

Shan Wu, Ming Shao, Quinn Burlingame, Xiangzhong Chen, Minren Lin, Kai Xiao, and Q. M. Zhang

Appl. Phys. Lett. 102, 013301 (2013); http://dx.doi.org/10.1063/1.4773186 (4 pages)

Online Publication Date: 3 January 2013

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Poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) is a ferroelectric terpolymer relaxor with a static dielectric constant of 50, which was developed using defect modification to eliminate remnant polarization in the normal ferroelectric PVDF. In this work, this solution processable terpolymer was used as the gate insulator in bottom gated organic thin-film transistors with a pentacene semiconductor layer. Due to the high dielectric constant of P(VDF-TrFE-CFE), a large capacitive coupling between the gate and channel can be achieved which causes a high charge concentration at the interface of the semiconductor and dielectric layers. In this device, an on/off ratio of 104 and a low minimum operation gate voltage (5-10 V) were attained.
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77.55.dj For nonsilicon electronics (Ge, III-V, II-VI, organic electronics)
77.22.Ch Permittivity (dielectric function)
77.22.Ej Polarization and depolarization
77.80.Dj Domain structure; hysteresis
77.84.Jd Polymers; organic compounds
85.30.Tv Field effect devices
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Spectral response tuning using an optical spacer in broad-band organic solar cells

Yuliang Zhang, Ajay K. Pandey, Chen Tao, Yuan Fang, Hui Jin, Paul L. Burn, and Paul Meredith

Appl. Phys. Lett. 102, 013302 (2013); http://dx.doi.org/10.1063/1.4773556 (5 pages)

Online Publication Date: 7 January 2013

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The effect of a zinc oxide optical spacer layer in broad-band polymer-fullerene solar cells is presented. The complimentary absorption in the donor and acceptor components allows photocurrent generation through photoinduced electron and hole-transfer mechanisms. Simulations of the optical-field distribution reveal that an optical spacer can be used to tune the spectral response to favor one photocurrent generation pathway via enhanced absorption in either the acceptor or donor component. Experimental results confirm these simulations, and the spacer is shown to enhance overall photocurrent in devices with thin active layers (<60 nm), with much less effect in thicker junctions (>90 nm).
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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Effects of cathode modification using spin-coated lithium acetate on the performances of polymer bulk-heterojunction solar cells

Liang Jiang, Aiyuan Li, Xianyu Deng, Shizhao Zheng, and King-Young Wong

Appl. Phys. Lett. 102, 013303 (2013); http://dx.doi.org/10.1063/1.4773570 (4 pages)

Online Publication Date: 7 January 2013

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In this report, we show that the performances of polymer bulk-heterojunction solar cells were improved by inserting thin films of lithium acetate layers between the active layer and the cathode using a spin-coating process. Comparing with the device without the cathode modification, significant enhancements of Voc (open circuit voltage) from 0.42 V to 0.55 V and device efficiency from 1.4% to 4.1% were achieved. X-ray and ultraviolet photoelectron spectroscopic studies indicate that both the improved damage tolerance of the active layer under the thermally evaporated metal and an n-type doping at the metal/organic interface play the crucial roles in the enhanced performances.
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88.40.hj Efficiency and performance of solar cells
88.40.jr Organic photovoltaics
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Efficient tandem organic light-emitting device based on photovoltaic-type connector with positive cycle

Huihui Liu, Fei Yan, Hua Wang, Yanqin Miao, Xiaogang Du, Shu Jing, Zhixiang Gao, Liuqing Chen, Yuying Hao, and Bingshe Xu

Appl. Phys. Lett. 102, 013304 (2013); http://dx.doi.org/10.1063/1.4773591 (4 pages)

Online Publication Date: 7 January 2013

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We designed a tandem organic light-emitting device based on an organic photovoltaic-type charge generation connector (CGC) of fullerene carbon 60/copper(II) phthalocyanine. The CGC can absorb a portion of photons radiated from emission zone and form excitons which disassociated into free charges at PN junction interface without energy barrier, leading to low driving voltage and better charge balance. The efficiency increases remarkably with increasing current density, even beyond two folds compared with single unit device under higher current density, meaning slower roll-off. The whole process is a positive cycle, and actually enhances the utilization of internal radiation and the overall performance of tandem device.
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85.60.Jb Light-emitting devices
81.05.ub Fullerenes and related materials
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Small molecule solution-processed bulk heterojunction solar cells with inverted structure using porphyrin donor

Takaki Yamamoto, Junichi Hatano, Takafumi Nakagawa, Shigeru Yamaguchi, and Yutaka Matsuo

Appl. Phys. Lett. 102, 013305 (2013); http://dx.doi.org/10.1063/1.4773910 (4 pages)

Online Publication Date: 8 January 2013

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Utilizing tetraethynyl porphyrin derivative (TE-Por) as a small molecule donor material, we fabricated a small molecule solution-processed bulk heterojunction (BHJ) solar cell with inverted structure, which exhibited 1.6% power conversion efficiency (JSC (short-circuit current) = 4.6 mA/cm2, VOC (open-circuit voltage) = 0.90 V, and FF (fill factor) = 0.39) in the device configuration indium tin oxide/TiOx (titanium sub-oxide)/[6,6]-phenyl-C61-butyric acid methyl ester:TE-Por (5:1)/MoOx (molybdenum sub-oxide)/Au under AM1.5 G illumination at 100 mW/cm2. Without encapsulation, the small molecule solution-processed inverted BHJ solar cell also showed remarkable durability to air, where it kept over 73% of its initial power conversion efficiency after storage for 28 days under ambient atmosphere in the dark.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
88.40.jp Multijunction solar cells
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Photo-assisted molecular engineering in solution-processed organic thin-film transistors with a blended semiconductor for high mobility anisotropy

Jaehoon Park, Chang-Min Keum, Jae-Hyun Kim, Sin-Doo Lee, Maricia Payne, Michael Petty, John. E. Anthony, and Jin-Hyuk Bae

Appl. Phys. Lett. 102, 013306 (2013); http://dx.doi.org/10.1063/1.4774001 (5 pages)

Online Publication Date: 8 January 2013

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This paper reports a viable method for enhancing mobility anisotropy in solution-processed organic thin-film transistors (TFTs) by irradiating linearly polarized ultraviolet (LPUV) light onto a 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene)/poly (vinyl cinnamate) (PVCi) blended semiconductor film. The needle-like crystalline domains in the blended semiconductor film were aligned along the polarization direction of the LPUV light, suggesting that TIPS-pentacene molecules are oriented anisotropically due to an intermolecular interaction and aggregation with the photo-aligned PVCi molecules. The mobility anisotropy reached up to about 50, which is the highest value achieved in solution-processed organic TFTs with a polymeric insulator.
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85.30.Tv Field effect devices
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Emission characteristics in solution-processed asymmetric white alternating current field-induced polymer electroluminescent devices

Yonghua Chen, Yingdong Xia, Gregory M. Smith, Yu Gu, Chuluo Yang, and David L. Carroll

Appl. Phys. Lett. 102, 013307 (2013); http://dx.doi.org/10.1063/1.4774317 (4 pages)

Online Publication Date: 9 January 2013

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In this work, the emission characteristics of a blue fluorophor poly(9, 9-dioctylfluorene) (PFO) combined with a red emitting dye: Bis(2-methyl-dibenzo[f,h]quinoxaline)(acetylacetonate)iridium (III) [Ir(MDQ)2(acac)], are examined in two different asymmetric white alternating current field-induced polymer electroluminescent (FIPEL) device structures. The first is a top-contact device in which the triplet transfer is observed resulting in the concentration-dependence of the emission similar to the standard organic light-emitting diode (OLED) structure. The second is a bottom-contact device which, however, exhibits concentration-independence of emission. Specifically, both dye emission and polymer emission are found for the concentrations as high as 10% by weight of the dye in the emitter. We attribute this to the significant different carrier injection characteristics of the two FIPEL devices. Our results suggest a simple and easy way to realize high-quality white emission.
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85.60.Jb Light-emitting devices
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Optical properties of fully and partially fluorinated rubrene in films and solution

F. Anger, R. Scholz, E. Adamski, K. Broch, A. Gerlach, Y. Sakamoto, T. Suzuki, and F. Schreiber

Appl. Phys. Lett. 102, 013308 (2013); http://dx.doi.org/10.1063/1.4773520 (5 pages)

Online Publication Date: 10 January 2013

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We present the optical properties of fully (C42F28, PF-RUB) and half-fluorinated (C42F14H14, F14−RUB) rubrene, both in thin films and as monomers in solution and compare them to hydrogenated rubrene (C42H28, RUB). All three compounds show similar optical absorption bands and photoluminescence line shapes. The results are interpreted with density functional calculations of the orbital energies and time-dependent density functional theory for the HOMO-LUMO transition. Red shifts induced by the surrounding solvent or organic thin films remain much smaller than for polyacenes, in keeping with previous observations for rubrene and existing models for the solvatochromic shifts.
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78.66.Qn Polymers; organic compounds
78.40.Me Organic compounds and polymers
78.55.Kz Solid organic materials
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The charge-trapping and triplet-triplet annihilation processes in organic light-emitting diodes: A duty cycle dependence study on magneto-electroluminescence

Qiming Peng, Ping Chen, and Feng Li

Appl. Phys. Lett. 102, 023301 (2013); http://dx.doi.org/10.1063/1.4788681 (5 pages)

Online Publication Date: 17 January 2013

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We studied the magneto-electroluminescence (MEL) in tri-(8-hydroxyquinoline)-aluminum (Alq3)-based organic light-emitting devices through both steady-state and transient methods. As the magnetic field increases, the MEL exhibits a rapid rise, followed by the saturation tendency at all voltages in the steady-state measurement, but in the transient measurement it first increases to a maximum and then decreases to negative values when the driving voltages are higher than 8V. Furthermore, we found that the MEL strongly depends on the duty cycle of the pulse voltage. Finally, by employing the triplet-triplet annihilation model combined with the charge trapping effects, we explained the duty cycle dependence of MELs.
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85.60.Jb Light-emitting devices
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Hole injection enhancement by sparsely dispersed Au nanoparticles on indium tin oxide electrode in organic light emitting devices

D. Wang, K. Yasui, M. Ozawa, K. Odoi, S. Shimamura, and K. Fujita

Appl. Phys. Lett. 102, 023302 (2013); http://dx.doi.org/10.1063/1.4776664 (3 pages)

Online Publication Date: 17 January 2013

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Gold nanoparticles stabilized by a hyper-branched polystyrene and adhered sparsely on indium tin oxide electrode can enhance hole injection to hole transport layer of N,N′-diphenyl-N,N′-bis(1-naphthyl)(1,1′-biphenyl)-4,4′-diamine. Although surface coverage by nanoparticles is less than 1%, the current density increases almost 1.5 orders of magnitude in hole-only device compared with the identical device without nanoparticles. This nanoparticle is also applied in typical organic light emitting diode. The driving voltage can be significantly lowered accompany with almost six times higher current density and luminance at 6 V. The local electric field induced by negatively charged nanoparticles should bring about the hole injection enhancement.
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85.60.Jb Light-emitting devices
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Organic field-effect transistor nonvolatile memories based on hybrid nano-floating-gate

Xu Gao, Xiao-Jian She, Chang-Hai Liu, Qi-Jun Sun, Jie Liu, and Sui-Dong Wang

Appl. Phys. Lett. 102, 023303 (2013); http://dx.doi.org/10.1063/1.4776677 (4 pages)

Online Publication Date: 17 January 2013

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High performance organic field-effect transistor nonvolatile memory is achieved by integrating gold nanoparticles and graphene oxide sheets as the hybrid nano-floating-gate. The device shows a large memory window of about 40 V, high ON/OFF ratio of reading current over 104, excellent programming/erasing endurance, and retention ability. The hybrid nano-floating-gate can increase the density of charge trapping sites, which are electrically separate from each other and thus suppress the stored charge leakage. The memory window is increased under illumination, and the results indicate that the photon-generated carriers facilitate the electron trapping but have almost no effect on the hole trapping.
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84.30.Sk Pulse and digital circuits
85.30.Tv Field effect devices
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Mapping electric field distributions in biased organic bulk heterojunctions under illumination by nonlinear optical microscopy

J. D. Morris, Timothy L. Atallah, Christopher J. Lombardo, Heungman Park, Ananth Dodabalapur, and X.-Y. Zhu

Appl. Phys. Lett. 102, 033301 (2013); http://dx.doi.org/10.1063/1.4788707 (4 pages)

Online Publication Date: 23 January 2013

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How charge carriers are distributed in a bulk heterojunction (BHJ) under illumination is central to the understanding of organic photovoltics and photodetectors. Here, we apply nonlinear optical microscopy to quantitatively map the spatial distributions of electric fields in two lateral organic BHJs: poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (PCBM) and poly(4,4-dioctyldithieno(3,2-b:2′,3′-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl) and PCBM. For the former, we observe the development with time of a depletion region adjacent to the electron-collecting electrode. In the latter, the device is stable and characterized by a nearly linear potential drop. We discuss the origins of field distributions and space charge accumulation in organic BHJs.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
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Strong interfacial dipole formation with thermal evaporation of lithium cobalt oxide for efficient electron injections

Hyunbok Lee, Soohyung Park, Jeihyun Lee, Younjoo Lee, Dongguen Shin, Kwangho Jeong, and Yeonjin Yi

Appl. Phys. Lett. 102, 033302 (2013); http://dx.doi.org/10.1063/1.4789394 (4 pages)

Online Publication Date: 24 January 2013

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We investigated the electronic structures at the interface of Al/lithium cobalt oxide (LiCoO2)/tris(8-hydoxyquinoline) aluminum (Alq3) to elucidate the origin of the electron injection enhancement with the insertion of the LiCoO2 layer in organic light-emitting devices using in situ photoelectron spectroscopy experiments. We discovered that LiCoO2 was decomposed into lithium oxide (Li2O) by thermal evaporation, and only Li2O was deposited on the desired substrate. Li2O forms a strong interfacial dipole, which reduces the surface potential on Alq3 due to its extremely low work function. As a result, the electron injection barrier was dramatically decreased by the Li2O layer. Furthermore, there is no strong chemical interaction at the interface of Al/Li2O/Alq3; hence, this would contribute to extend the device lifetime.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
81.05.Fb Organic semiconductors
79.60.Jv Interfaces; heterostructures; nanostructures
85.60.Jb Light-emitting devices
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
73.30.+y Surface double layers, Schottky barriers, and work functions
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Improved cathode buffer layer to decrease exciton recombination in organic planar heterojunction solar cells

Bregt Verreet, Pawel E. Malinowski, Bjoern Niesen, David Cheyns, Paul Heremans, Andre Stesmans, and Barry P. Rand

Appl. Phys. Lett. 102, 043301 (2013); http://dx.doi.org/10.1063/1.4789852 (5 pages)

Online Publication Date: 29 January 2013

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We show that an advanced cathode buffer design, consisting of bathocuproine/3,4,9,10-perylenetetracarboxylic bis-benzimidazole/Ag, increases the short-circuit current of organic planar heterojunction cells and reduces the J-V slope at reverse voltages. We study the physical origin of these effects by measuring reflectivity, voltage dependent external quantum efficiency, and voltage dependent photoluminescence. Our findings suggest that the observed effects are mainly associated with a voltage dependent polaron-induced exciton quenching in the C60 layer. Finally, this improved cathode buffer design is applied to a diindeno[1,2,3-cd:1′,2′,3′-lm]perylene/C70 based cell, leading to a considerable planar heterojunction efficiency of 5.7%.
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88.40.jr Organic photovoltaics
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Efficient organic solar cells with solution-processed carbon nanosheets as transparent electrodes

Seok-In Na, Yong-Jin Noh, Su-Young Son, Tae-Wook Kim, Seok-Soon Kim, Sungho Lee, and Han-Ik Joh

Appl. Phys. Lett. 102, 043304 (2013); http://dx.doi.org/10.1063/1.4789980 (5 pages)

Online Publication Date: 30 January 2013

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We demonstrate that solution-processed carbon nanosheet (CNS) films can efficiently serve as transparent electrodes for organic solar cells (OSCs). The CNS was obtained by spin-coating of polyacrylonitrile (PAN) dissolved in dimethylformamide on quartz substrates, followed by stabilization and carbonization processes to convert polymer into CNS. The thickness of the newly developed CNS films was easily controlled by varying the PAN solution concentration. The polymer-converted CNS films were intensively examined for the feasibility of the use as transparent anodes in solar cells. This approach could be highly desirable for all-solution-processed or printed OSCs.
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88.40.jr Organic photovoltaics
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
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Kinetics of light induced defect creation in organic solar cells

R. A. Street and D. M. Davies

Appl. Phys. Lett. 102, 043305 (2013); http://dx.doi.org/10.1063/1.4789993 (3 pages)

Online Publication Date: 30 January 2013

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The kinetics of light-induced recombination centers in bulk heterojunction organic solar cells are measured as a function of exposure time, intensity, and the illumination photon energy. The density of induced centers increases with exposure but stabilizes partially due to self-annealing. UV exposure is roughly 50 times more effective for defect creation than white light or yellow-filtered white light. Light-induced breaking of C-H bonds to create H-related localized states is proposed as the underlying mechanism.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
88.40.jp Multijunction solar cells
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Photoelectron spectroscopic study of band alignment of polymer/ZnO photovoltaic device structure

T. Nagata, S. Oh, Y. Yamashita, H. Yoshikawa, N. Ikeno, K. Kobayashi, T. Chikyow, and Y. Wakayama

Appl. Phys. Lett. 102, 043302 (2013); http://dx.doi.org/10.1063/1.4790298 (4 pages)

Online Publication Date: 30 January 2013

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Using x-ray photoelectron spectroscopy, we investigated the band alignment of a Ag/poly(3-hexylthiophene-2,5-diyl) (P3HT)/ZnO photovoltaic structure. At the P3HT/ZnO interface, a band bending of P3HT and a short surface depletion layer of ZnO were observed. The offset between the highest occupied molecular orbital of P3HT and the conduction band minimum of ZnO at the interface contributed to the open circuit voltage (Voc) was estimated to be approximately 1.5 ± 0.1 eV, which was bigger than that of the electrically measured effective Voc of P3HT/ZnO photovoltaic devices, meaning that the P3HT/ZnO photovoltaic structure has the potential to provide improved photovoltaic properties.
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79.60.Jv Interfaces; heterostructures; nanostructures
81.40.Lm Deformation, plasticity, and creep
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
62.20.F- Deformation and plasticity
73.20.At Surface states, band structure, electron density of states
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A plasmonically enhanced charge generation layer for tandem organic light emitting device

Fei Yan and Xiao Wei Sun

Appl. Phys. Lett. 102, 043303 (2013); http://dx.doi.org/10.1063/1.4789979 (4 pages)

Online Publication Date: 30 January 2013

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We studied silver nanoparticles (Ag NPs) as an n-type dopant in the charge generation connector of tandem organic light-emitting diodes. Due to the localized surface plasmonic resonance and quantum size effects, Ag NPs are more active and easy to lose electron comparing to bulk or film forms of Ag, and can work as excellent n-type dopant. For the tandem device with Ag NPs in the charge generation layer, the current efficiency is more than doubled and the driving voltage is less than twice compared to one single emission unit device.
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85.60.Jb Light-emitting devices
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Enhancing crystallinity of C60 layer by thickness-control of underneath pentacene layer for high mobility C60/pentacene ambipolar transistors

Kwangseok Ahn, Jong Beom Kim, Hyunjun Park, Hyunjung Kim, Moo Hyung Lee, Beom Joon Kim, Jeong Ho Cho, Moon Sung Kang, and Dong Ryeol Lee

Appl. Phys. Lett. 102, 043306 (2013); http://dx.doi.org/10.1063/1.4789873 (5 pages)

Online Publication Date: 31 January 2013

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We present systematic control of the crystallinity and electrical transport properties of C60 films that are deposited onto pentacene layers, based on simple tuning of the underneath pentacene layer thickness. With increasing the pentacene layer thickness from 0 to 2 monolayers, we observed improvement in crystallinity and grain size of the C60 layer, which led to dramatic enhancement in electron conduction. Also, hole transport in this bilayer structure could be generated when the thickness of the pentacene layer was above one monolayer. The resulting ambipolar transport thin-film transistors yielded electron and hole mobilities as high as 2.8 and 0.3 cm2 V−1 s−1, respectively, and complementary inverters with gain value above 20.
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81.05.Fb Organic semiconductors
73.61.Cw Elemental semiconductors
73.61.Ph Polymers; organic compounds
73.61.Wp Fullerenes and related materials
85.30.Tv Field effect devices
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