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25 Aug 2008

Volume 93, Issue 8, Articles (08xxxx)

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Appl. Phys. Lett. 93, 083901 (2008); http://dx.doi.org/10.1063/1.2973167 (3 pages)

R. J. Martín-Palma, C. G. Pantano, and A. Lakhtakia
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Memory effect and negative differential resistance in tris-(8-hydroxy quinoline) aluminum/bathocuproine bilayer devices

Z. S. Su, M. K. Fung, C. S. Lee, W. L. Li, and S. T. Lee

Appl. Phys. Lett. 93, 083301 (2008); http://dx.doi.org/10.1063/1.2973902 (3 pages) | Cited 10 times

Online Publication Date: 26 August 2008

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Memory effect and negative differential resistance (NDR) were observed in simple tris-(8-hydroxyquinoline) aluminum/bathocuproine (BCP) bilayer devices. The devices could be switched from a low conductance state to a high conductance state when a negative bias was applied and could be restored to an OFF state when a positive bias was applied beyond the NDR region. The memory effect is nonvolatile, and an ON/OFF ratio of over 103 was achieved. The memory effect was observed only in the presence of both Alq3 and BCP layers, and the NDR is attributed to the defects formed in the BCP layer upon evaporation of an Al cathode.
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73.61.Ph Polymers; organic compounds
73.50.-h Electronic transport phenomena in thin films
73.40.-c Electronic transport in interface structures
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Low-voltage high-performance C60 thin film transistors via low-surface-energy phosphonic acid monolayer/hafnium oxide hybrid dielectric

Orb Acton, Guy Ting, Hong Ma, and Alex K.-Y. Jen

Appl. Phys. Lett. 93, 083302 (2008); http://dx.doi.org/10.1063/1.2975175 (3 pages) | Cited 12 times

Online Publication Date: 26 August 2008

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C60-based organic thin film transistors (OTFTs) have been fabricated using a n-octadecylphosphonic acid self-assembled monolayer/sol-gel processed hafnium oxide hybrid dielectric. With the combination of high capacitance (580 nF/cm2) and low leakage current density (8×10−9 A/cm2), this hybrid dielectric yields C60 OTFTs operating under 1.5 V with an average n-channel saturation field-effect mobility of 0.28 cm2/V s, high on-off current ratio of 105, and low subthreshold slope of 100 mV/decade. The low surface energy of the n-octadecylphosphonic acid allows C60 to form a thin film with large grains that provide an efficient charge carrier pathway for the low-voltage OTFTs.
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85.30.Tv Field effect devices
85.65.+h Molecular electronic devices
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Lifetime of organic light emitting diodes on polymer anodes

Karsten Fehse, Rico Meerheim, Karsten Walzer, Karl Leo, Wilfried Lövenich, and Andreas Elschner

Appl. Phys. Lett. 93, 083303 (2008); http://dx.doi.org/10.1063/1.2975369 (3 pages) | Cited 7 times

Online Publication Date: 26 August 2008

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We report on the use of a thin layer of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) as anode for bottom emission organic light emitting diodes (OLEDs). The combination of polymer anodes with OLEDs having either electrically doped or undoped hole transport layers in direct contact with the polymer is shown. We discuss the impact of the annealing conditions of the polymer on the OLED lifetime in comparison to indium tin oxide anodes. Supported by a differential thermal analysis of PEDOT:PSS, a strong influence of residual water in the polymer on the device lifetime is found. Additional heating of the polymer anode in a dry ambient prior to OLED deposition is necessary to achieve high device lifetimes. At a luminance of 260 cd/m2, pin-OLEDs on a PEDOT:PSS anode show no measurable device degradation during 5200 h of operation.
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85.60.Jb Light-emitting devices
85.60.Bt Optoelectronic device characterization, design, and modeling
85.30.Kk Junction diodes
85.30.De Semiconductor-device characterization, design, and modeling
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Mechanism of charge generation in p-type doped layer in the connection unit of tandem-type organic light-emitting devices

X. D. Gao, J. Zhou, Z. T. Xie, B. F. Ding, Y. C. Qian, X. M. Ding, and X. Y. Hou

Appl. Phys. Lett. 93, 083304 (2008); http://dx.doi.org/10.1063/1.2969293 (3 pages) | Cited 14 times

Online Publication Date: 26 August 2008

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A p-type doped organic layer combined with a hole-blocking layer has been experimentally demonstrated to serve as the charge generation unit in tandem-type organic light-emitting devices. The p-type layer functions as the source of both holes and electrons. Charge separation is explained by the tunneling model that the hole-blocking layer reduces the energy barrier for the electrons generated in the p-type layer to tunnel through into one light-emitting unit, while the holes generated in the p-type layer can transport to the other light-emitting unit easily under operation voltage.
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85.60.Jb Light-emitting devices
61.72.U- Doping and impurity implantation
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Efficient tandem organic solar cells with an Al/MoO3 intermediate layer

D. W. Zhao, X. W. Sun, C. Y. Jiang, A. K. K. Kyaw, G. Q. Lo, and D. L. Kwong

Appl. Phys. Lett. 93, 083305 (2008); http://dx.doi.org/10.1063/1.2976126 (3 pages) | Cited 45 times

Online Publication Date: 26 August 2008

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We report efficient tandem organic solar cells with an Al and MoO3 intermediate layer. Such an intermediate layer with optimized thickness (1 nm Al and 15 nm MoO3) has high transparency ( ∼ 98% in the range from 350 to 900 nm) and efficient charge collections to realize electric connection in series. For polymer-small molecule tandem cell, due to the summation (1.01 V) of the open-circuit voltages of individual cells and a short-circuit current density of 6.05 mA/cm2, a power conversion efficiency (PCE) of 2.82% was obtained under 100 mW/cm2 illumination, which is larger than either of the individual cells. The PCE reached 3.88% when the tandem cell was illuminated under 300 mW/cm2. Additionally, we applied Al/MoO3 intermediate layer to realize a solution-processed polymer tandem cell with a high PCE (2.23%). The thick MoO3 (15 nm) provides a complete protection of the prior-deposited polymer layer from dissolving during the top cell polymer coating.
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84.60.Jt Photoelectric conversion
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Indium tin oxide modified transparent nanotube thin films as effective anodes for flexible organic light-emitting diodes

Jianfeng Li, Liangbing Hu, Jun Liu, Lian Wang, Tobin J. Marks, and George Grüner

Appl. Phys. Lett. 93, 083306 (2008); http://dx.doi.org/10.1063/1.2970049 (3 pages) | Cited 18 times

Online Publication Date: 26 August 2008

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Sn-doped In2O3 (ITO) modified single-walled carbon nanotube (SW-CNT) transparent electrodes are fabricated on flexible polyethyleneterephthalate (PET) substrates by stamp printing SW-CNT films, followed by room temperature ion-assisted deposition of ITO. Polymer light-emitting diodes (PLEDs) using such film as anodes exhibit superior performance versus CNT-only controls. Flexible PLEDs with the following structure: PET/CNT(30 nm)-ITO(45 nm)/poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate)/[poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)diphenylamine)]+{4,4′-bis[(p-trichlorosilyl propylphenyl)-phenylamino]biphenyl}/[poly(9,9-dioctylfluorene-co-benzothiadiazole)]/CsF/Al, achieve a maximum light output of 8900 cd/m2 with a current efficiency of 4.5 cd/A. Bending test comparisons with ITO/PET show the ITO modified CNT/PET electrodes to be far more mechanically flexible.
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85.60.Jb Light-emitting devices
85.35.Kt Nanotube devices
81.15.Jj Ion and electron beam-assisted deposition; ion plating
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Using thin film transistors to quantify carrier transport properties of amorphous organic semiconductors

C. H. Cheung, K. K. Tsung, K. C. Kwok, and S. K. So

Appl. Phys. Lett. 93, 083307 (2008); http://dx.doi.org/10.1063/1.2972125 (3 pages) | Cited 11 times

Online Publication Date: 27 August 2008

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The hole transport properties of two phenylamine-based compounds were evaluated by thin film transistor (TFT) measurement and time-of-flight (TOF) technique. The compounds were N,N′-diphenyl-N,N′-bis(1-naphthyl) (1,1′biphenyl)-4,4′diamine (NPB) and 4,4′,4″-tris[n-(2-naphthyl)-n-phenyl-amino] triphenylamine (2TNATA). With tungsten oxide/gold as the charge injecting electrode, the field effect mobility of NPB was found to be 2.4×10−5 cm2/V s at room temperature, which was about one order of magnitude smaller than that obtained from independent TOF experiments (3×10−4 cm2/V s). Similar observations were found for 2TNATA. Temperature dependent measurements were carried out to study the energetic disorder of the materials. It was found that the energetic disorder was increased in the neighborhood of a gate dielectric layer.
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72.80.Le Polymers; organic compounds (including organic semiconductors)
72.80.Ng Disordered solids
72.20.Fr Low-field transport and mobility; piezoresistance
85.30.Tv Field effect devices
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The density of states in thin film copper phthalocyanine measured by Kelvin probe force microscopy

K. Celebi, P. J. Jadhav, K. M. Milaninia, M. Bora, and M. A. Baldo

Appl. Phys. Lett. 93, 083308 (2008); http://dx.doi.org/10.1063/1.2976634 (3 pages) | Cited 5 times

Online Publication Date: 28 August 2008

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The density of states (DOS) is an important factor in understanding charge transport in organic semiconductors. We use Kelvin probe force microscopy to find the DOS in thin films of copper phthalocyanine (CuPC). We find an exponential DOS with a characteristic energy of 0.11 eV over a 0.5 eV range of the highest occupied molecular orbital of CuPC. We also find that the technique is limited by charge trapping and hysteresis at low DOSs, and nonuniform potential profiles within the CuPC film at high DOSs.
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71.20.Rv Polymers and organic compounds
73.61.Ph Polymers; organic compounds
68.37.Ps Atomic force microscopy (AFM)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
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A comparison of CuO and Cu2O hole-injection layers for low voltage organic devices

G. B. Murdoch, M. Greiner, M. G. Helander, Z. B. Wang, and Z. H. Lu

Appl. Phys. Lett. 93, 083309 (2008); http://dx.doi.org/10.1063/1.2966140 (3 pages) | Cited 13 times

Online Publication Date: 29 August 2008

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Cu2O and CuO have been grown with an aim to reduce junction electrical resistance when interfaced with N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′ biphenyl 4,4′-diamine (NPB). Organic light-emitting diodes employing Cu/CuO anodes have equivalent driving voltages as devices made with indium tin oxide. Hole-injection barriers are calculated from current-voltage characteristics of CuO/NPB/Cu and Cu2O/NPB/Cu devices via theoretical simulation. Photoelectron spectroscopies are used to measure oxide valence band spectra, interfacial dipole formation, and band bending during in situ sequential deposition of NPB on each oxide. Calculated hole-injection barriers and those derived from photoemission results accord well, explaining the superior hole injection at the CuO-NPB interface.
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85.60.Jb Light-emitting devices
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A combined experimental and simulation study on thickness dependence of the emission characteristics in multicolor single layer organic light-emitting diodes

N. A. Stathopoulos, M. Vasilopoulou, L. C. Palilis, D. G. Georgiadou, and P. Argitis

Appl. Phys. Lett. 93, 083310 (2008); http://dx.doi.org/10.1063/1.2977479 (3 pages) | Cited 3 times

Online Publication Date: 29 August 2008

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The impact of the active layer thickness on the emission characteristics of multicolor single layer organic light-emitting diodes based on poly(9-vinylcarbazole) is examined by combining experimental results with model simulations. We compare experimental electroluminescence spectra with simulations using photon-emitting point dipoles and find a very good agreement. We also simulate the location of the recombination zone, considering that the emission probability distribution has a peak located 25 nm from the cathode, which decays exponentially above and below that point. Simulated radiation patterns show that microcavity effects dominate the thickness dependent emitting properties of these devices.
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85.60.Jb Light-emitting devices
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Contact formation at the C60/alkali-metal fluoride/Al interface

M. G. Helander, Z. B. Wang, and Z. H. Lu

Appl. Phys. Lett. 93, 083311 (2008); http://dx.doi.org/10.1063/1.2976307 (3 pages) | Cited 15 times

Online Publication Date: 29 August 2008

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Efficient contact formation is critical in organic electroluminescence and photovoltaic devices that utilize fullerene (C60). Unlike traditional electron transport molecules, such as tris-(8-hydroxy-quinolinato)aluminum (Alq3), C60 is found to be highly selective of injection layers. Charge injection properties of alkali-metal fluoride injection layers at the C60/Al cathode interface of organic light emitting diodes were studied. LiF is found to be unique amongst the alkali-metal fluorides in producing an Ohmic contact. The device performance is strongly linked to the size of the vapor phase fluoride molecules. The observed phenomena are explained by an intercalation region at the metal/organic interface.
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73.40.Ns Metal-nonmetal contacts
78.60.Fi Electroluminescence
85.60.Jb Light-emitting devices
72.20.-i Conductivity phenomena in semiconductors and insulators
72.80.-r Conductivity of specific materials
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