Impact of the cathode metal work function on the performance of vacuum-deposited organic light emitting-devices

Received: 7 September 1998 / Accepted: 27 November 1998 / Published online: 24 February 1999     

Pulsed laser deposition of conductive metallo-dielectric optical filters

We describe the fabrication by room-temperature pulsed laser deposition of a transparent conductor comprising alternating layers of silver and aluminum oxide, forming a metallo-dielectric filter. Transmittances of 0.7 over specific wavelength bands were achieved with resistivities as low as 6.0×10^-6 Ω cm, almost two orders of magnitude lower than that of the best single-substrate thin films, such as indium tin oxide. The resistivity can be predicted without adjustable parameters and designed using a simple parallel-circuit model; the optical properties are well described by standard matrix transmission calculations. This demonstrates that pulsed laser deposition may be used to fabricate prototypes of high-quality transparent conductors with predictable properties for conducting windows where low-temperature deposition is critical, as in organic light-emitting diodes and for non-linear optical films. Received: 10 June 2001 / Accepted: 9 August 2001 / Published online: 20 December 2001     

Influence of phase transitions on the spontaneous voltage in P(VDF/TrFE) copolymers

Spontaneous voltages were observed in a structure made of poly(vinylidene fluoride-trifluoroethylene) copolymers films sandwiched by metals evaporated onto both surfaces. Open voltages and short-circuit currents were measured as a function of the temperature using different combinations of evaporated metals as electrodes: aluminum, gold, nickel and copper. The ferroelectric-to-paraelectric transition was also observed in such structures for 60:40, 70:30 and 80:20 of VDF/TrFE molar ratios, including the temperature hysteresis characteristic of the first-order transition. Received: 9 February 2000 / Accepted: 28 March 2000 / Published online: 9 August 2000     

Properties of conductive polymer films deposited by infrared laser ablation

Thin films of the conducting polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) were deposited by resonant infrared laser vapor deposition (RIR-LVD). The PEDOT:PSS was frozen in various matrix solutions and deposited using a tunable, mid-infrared free-electron laser (FEL). The films so produced exhibited morphologies and conductivities that were highly dependent on the solvent matrix and laser irradiation wavelength used. When deposited from a native solution (1.3% by weight in water), as in matrix-assisted pulsed laser evaporation (MAPLE), films were rough and electrically insulating. When the matrix included other organic “co-matrices” that were doped into the solution prior to freezing, however, the resulting films were smooth and exhibited good electrical conductivity (0.2 S/cm), but only when irradiated at certain wavelengths. These results highlight the importance of the matrix/solute and matrix/laser interactions in the ablation process.     

Effect of modifying a methyl siloxane-based dielectric by a polymer thin film for pentacene thin-film transistors

We report on the fabrication of pentacene thin-film transistors (TFTs) utilizing a spun methyl siloxane-based spin-on-glass (SOG) dielectric and show that these devices can give a similar electrical performance as achieved by using pentacene TFTs with a silicon dioxide (SiO₂) dielectric. To improve the electrical performance of pentacene TFTs with the SOG dielectric, we employed a hybrid dielectric of an SOG/cross-linked poly-4-vinylphenol (PVP) polymer. The PVP film was deposited onto the spun SOG dielectric prior to pentacene evaporation, resulting in an improvement of the saturation field effect mobility (μ_sat) from 0.01 cm²/(V s) to 0.76 cm²/(V s). The good surface morphology and the matching surface energy of the SOG dielectric that was modified with the polymer thin film allow the optimized growth of crystalline pentacene domains whose nuclei are embedded in an amorphous phase.     

Electronic and structural properties of graphene-based transparent and conductive thin film electrodes

We demonstrate that graphene-based transparent and conductive thin films (GTCFs), fabricated by thermal reduction of graphite oxide, have very similar electronic and structural properties as highly oriented pyrolytic graphite (HOPG). Electron spectroscopy results suggest that the GTCFs are also semi-metallic and that the individual graphene sheets of the film are predominantly oriented parallel to the substrate plane. These films may therefore be considered as a technologically relevant analogue to HOPG electrodes, which cannot be easily processed into thin films.     

Fabrication and magnetic properties of highly ordered Co16Ag84 alloy nanowire array

Highly ordered Co-Ag alloy nanowire arrays embedded in the nanochannels of anodic alumina membranes (AAMs) were successfully fabricated using electrodeposition. Scanning electron microscopy and transmission electron microscopy observations revealed that the ordered Co-Ag alloy nanowires were uniformly assembled into the hexagonally ordered nanochannels of the AAMs. Magnetic measurements showed that the perpendicular coercivity (H_c⊥) of the ordered nanowire arrays increased dramatically as the annealing temperature (T_a) rose from 300 °C, reached its maximum (183 Oe) at 400 °C and then decreased sharply as T_a further increased beyond 400 °C. However, there was little change in the parallel coercivity (H_c∥) of the nanowire arrays during the annealing process. The mechanism of this phenomenon was attributed to the special structure of the AAMs and nanowires. Received: 27 November 2000 / Accepted: 3 May 2001 / Published online: 25 July 2001     

Phase separation and partial devitrification in soda-lime silicate glasses

2 O-rich droplets dispersed throughout the SiO₂-rich matrix and nanoparticles of metallic silver. In exchanged specimens the joint effect of colloidal silver and electric field results in partial transformation of the amorphous droplets into crystalline Na₂O particles. Received: 11 February 1998 / Accepted: 20 November 1998 / Published online: 24 February 1999     

Pulsed laser deposition of (110) oriented semiconductive SrFeO3−x thin films

 The semiconductive perovskite-type oxide SrFeO_3-x (x<0.16) (SFO) thin films have been directly fabricated on (001)SrTiO₃ and (001)LaAlO₃ single crystal substrates by pulsed laser deposition(PLD) under high oxygen partial pressure of 100 Pa. The SFO thin films were (110) oriented. The x-ray photoelectron spectroscopy (XPS) analysis showed that the surface of SFO thin film has strong gas absorption capability. The resistance versus temperature has been measured in the temperature range from 77 K to 300 K. The SFO thin film showed typical semiconductive property. Dependence of resistance of SFO thin film on oxygen pressure was measured and result showed that the SFO thin film had better oxygen sensitive property. Received: 14 May 1996/Accepted: 15 August 1996     

Function and analytical formula for nanocrystalline dye-sensitization solar cells

Based on the experimental observations that the three-phase nano-TiO₂/F:SnO₂/I^-/I^- ₃ electrolyte front contact has to have pronounced rectifying properties (reverse reaction with electrolyte suppressed) for efficient operation of the dye-sensitization solar cell and plays an active part in the generation of photoelectrochemical energy, an analytical formula is derived which allows the understanding of the relevance and involvement of a variety of kinetic and cell parameters. Essentially, the TiO₂ layer is treated as a photocathode, donating electrons to a kinetically controlled front contact, with the counter-charges being transported by the electrolyte within the pores. The formula was expanded to include photochemical kinetics of the sensitizer, for which photodegradation properties were also calculated. The branching ratio, the ratio of regeneration-rate constant of the sensitizer and of product-formation rate, turned out to be critical for long-term stability. It may have to be improved by one order of magnitude for efficient cells to reach a lifetime of 20 years. The degree of rectifying character of the nano-TiO₂/F:SnO₂/I^-/I^- ₃ electrolyte interface (electric-field-dependent charge transfer to the front contact versus recombination-rate constant with I₃ ^- distinguishes between a low-efficiency (‘dynamic’) Galvani-type solar cell (efficiency determined by photoinduced chemical potential gradients, no rectifying contact) and a more highly efficient ‘junction-type’ solar cell (separation and collection of charges additionally assisted by junction potential). Several controversial subjects are addressed. The key challenges for the improvement of such cells are discussed, especially with respect to photodegradation and to solid-state devices. Received: 18 September 2000 / Accepted: 17 January 2001 / Published online: 20 June 2001     

Investigation of electrical properties of organic 4-tricyanovinyl-N,N-diethylaniline thin film

Thin films of 4-tricyanovinyl-N,N-diethylaniline (TCVA) with different thickness were prepared using thermal evaporation technique. A relative permittivity, ?_r, of 3.04 was estimated from the dependence of capacitance on film thickness. The current density-voltage (J-V) characteristics of TCVA thin films have been investigated at different temperatures. At low-voltage region, the current conduction in the Au/TCVA/Au sandwich structures obeys Ohm's law. At the higher-voltage regions, the charge transport phenomenon appears to be space-charge-limited current (SCLC) dominated by an exponential distribution of traps with total trap concentration of 1.21 × 10²² m⁻³. In addition, various electrical parameters were determined.     

Simulating percolating behavior of conductive particles in anisotropically conductive composite films

A numerical scheme is developed to simulate the percolating behavior of conductive particles within a non-conductive matrix film with a preferential alignment of particles achieved via externally imposed deterministic driving forces. The sharp transition from non-conducting to conducting of the composite film is successfully revealed with the model. The percolation behavior is studied in terms of four percolation parameters, including the percolation probability, the normalized shortest percolation path, normalized gyration radius and density of the percolation cluster, subject to variation in five important system parameters. These include particle concentration, relative importance of the externally applied force, film thickness, film width and particle size. The threshold particle concentration can be reduced by increasing the strength of the deterministic driving force, decreasing film thickness, increasing film width or using smaller size particles. Our study suggests that using stronger applied force for wider and thinner films containing smaller particles may be a good practice to obtain anisotropically conductive films with a light particle loading that possess good conduction capability in the thickness direction and good insulation in the planar direction. Received: 19 February 2001 / Accepted: 30 May 2001 / Published online: 30 August 2001     

Characteristics of the conductive polyimide film surfaces induced by ultraviolet laser beam

-1 cm^-1 stripped in a solvent from KrF-laser-irradiated polyimide thin film is taken as a sample to determine the microstructure of the conducting layer. Fourier-transform infrared and X-ray photoelectron spectroscopies show the formation of the carbon-rich clusters after irradiation. The element analysis gives the atomic ratio of C:H:N:O for the carbon-rich cluster as 60:20:3:1. Wide-angle X-ray diffraction indicates that the conducting layer is mainly amorphous carbon with a small amount of the short-range ordered carbon-rich clusters. This study suggests a structural model with three-layer carbon sheets linked together in a random fashion for the short-range ordered carbon-rich clusters. The interplanar spacing is 3.87 Å and the layer diameter 25 Å. The transport model of variable-range hopping in three dimensions is used to explain the conducting behavior of the conducting layer. In our case, the short-range ordered carbon-rich clusters are assumed to be conducting islands dispersed in the amorphous carbon-rich cluster matrix. Received: 26 May 1997/Accepted: 8 September 1997     

Escape time model for the current self-oscillation frequencies. in weakly coupled semiconductor superlattices

A semiclassical model was developed to predict the frequencies of current self-oscillations in weakly coupled semiconductor superlattices (SLs). The calculated frequency is derived from the classical round trip time in one well and the tunneling probability through the barrier, using the well and barrier width, effective masses and band offsets as well as the resulting energies of the sub- and minibands as input parameters. For all SLs, the measured frequency dependence on the sample parameters can be well described by our model. For weakly (strongly) coupled SLs, the calculated frequencies are somewhat above (below) the observed ones. The changeover from one behavior to the other occurs for SLs with miniband widths of a few meV. Received: 2 August 2000 / Accepted: 27 October 2000 / Published online: 28 February 2001     

Nonlinear optical spectroscopy of embedded semiconductor clusters

Received: 20 September 1998 / Revised version: 11 January 1999     

Electrically conducting ion tracks in diamond-like carbon films for field emission

Electrically conducting channels in an insulating carbon matrix were produced by 140-MeV Xe ion irradiation. The high local energy deposition of the individual ions along their pathes causes a rearrangement of the carbon atoms and leads to a transformation of the insulating, diamond-like (sp³-bonding) form of carbon into the conducting, graphitic (sp²-bonding) configuration. The conducting ion tracks are clearly seen in the current mapping performed with an atomic force microscope (AFM). These conducting tracks are of possible use in field emission applications. Received: 4 May 1999 / Accepted: 5 May 1999 / Published online: 1 July 1999     

Novel nanotubes and encapsulated nanowires

2 mixtures. Prolonged electron irradiation of these nanowires leads to axial growth and to dynamic transformations. These observations suggest ways in which materials may be modified by microencapsulation and irradiation. Received: 31 July 1997/Accepted: 6 October 1997     

Polymer gate dielectrics with self-assembled monolayers for high-mobility organic thin-film transistors based on copper phthalocyanine

Organic thin-film transistors based on polycrystalline copper phthalocyanine (CuPc) were fabricated by using poly(vinyl alcohol) as gate dielectric. After treatment of the gate dielectric using an octadecyltrichlorosilane self-assembled monolayer, a mobility of up to 0.11 cm²/V s was achieved, which is comparable to that of single-crystal CuPc devices (0.1–1 cm²/V s). The surface morphology was analyzed and the possible reasons for the enhanced mobility are discussed.     

Transparent,flexible and low‐resistive precision fabric electrode for organic solar cells

We report the use of conducting precision fabrics as transparent and flexible electrode for organic semiconductor‐based thin film devices. Precision fabrics have well‐defined mesh openings, excellent flexibility and are fabricated by high‐throughput roll‐to‐roll manufacturing. Optimized fabrics reached light transmittance over 95% throughout the visible and near infrared spectra. A significant part of the transmitted light is scattered, which is particularly advantageous for solar cell applications. Surface resistivity is as low as ～3 Ohms/square, which decreases Ohmic losses when scaling up to large area devices. We demonstrate that solar cells fabricated onto these electrodes show very similar characteristics to those prepared on ITO. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)