Fabrication of highly-ordered TiO(2) nanotube arrays and their use in dye-sensitized solar cells.

Highly ordered TiO(2) nanotubes were successfully fabricated using a nanoporous alumina templating method. A modified sol-gel route was used to infiltrate the alumina pores with Ti(OC(3)H(7))(4) which was subsequently converted into TiO(2) nanotubes. The average external diameter, tube lengths, and wall thickness achieved were 295 nm, 6-15 microm, and 21-42 nm, respectively. Diffraction data reveals that the nanotubes consist solely of the anatase phase. Dye-sensitized solar cells using TiO(2) nanotube arrays as the working electrode yielded power conversion efficiencies as high as 3.5% with a maximum incident photon-to-current conversion efficiency of 20% at 520 nm.

Bistability in doped organic thin film transistors.

Organic thin film transitors (TFTs) with the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid), PEDOT:PSS, as the active layer and cross-linked, layer-by-layer assembled poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA) multilayers as the gate dielectric layer were investigated. A combination of spectroscopic data and device performance characteristics was used to study the behavior of these TFT devices under a variety of controlled environmental test conditions. It was shown that depletion and recovery of the device can be induced to occur by a means that is consistent with the electrochemical oxidation and reduction of water contained in the film. In addition to acting as a reactant, moisture also acts as a plasticizer to control the mobility of other species contained in the film and thereby permits bistable operation of these devices. Raman spectroscopy was used to show that the observed device switching behavior is due to a change in the PEDOT doping level.

Fabrication of organic thin-film transistors using layer-by-layer assembly.

Layer-by-layer assembly is presented as a deposition technique for the incorporation of ultrathin gate dielectric layers into thin-film transistors utilizing a highly doped organic active layer. This deposition technique enables the fabrication of device structures with a controllable gate dielectric thickness. In particular, devices with a dielectric layer comprised of poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA) bilayer films were fabricated to examine the properties of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the transistor active layer. The transistor Ion/off ratio and switching speed are shown to be controlled by the gate bias, which is dependent upon the voltage applied and the number of bilayers deposited for the gate dielectric. The devices operate in the depletion mode as a result of dedoping of the active layer with the application of a positive gate bias. The depletion and recovery rate are highly dependent on the level of hydration in the film and the environment under which the device is operated. These observations are consistent with an electrochemical dedoping of the conducting polymer during operation.