Charge dynamics at heterojunctions for PbS/ZnO colloidal quantum dot solar cells probed with time-resolved surface photovoltage spectroscopy

Time-resolved laser-pump X-ray-photoemission-probe spectroscopy of a ZnO (101 ⎯ ⎯ 0 101¯0 ) substrate with and without PbS quantum dots (QDs) chemically linked to the surface is performed, using laser photon energies resonant with and below the band gap energy of the substrate (λ = 372 and 640 nm, hν = 3.33 and 1.94 eV). Charge injection from the photoexcited QDs to ZnO is demonstrated through the change in the surface photovoltage of the ZnO substrate observed when the heterojunction is illuminated with 1.94 eV radiation. The measured carrier dynamics are limited by the persistent photoconductivity of ZnO, giving dark carrier lifetimes of the order of 200 μs in a depletion layer at the interface. The chemical specificity of soft X-rays is used to separately measure the charge dynamics in the quantum dots and the substrate, yielding evidence that the depletion region at the interface extends into the PbS QD layer

Surface morphology and crystallinity of biaxially stretched PET films on the nanoscale

The surface morphology and crystallinity of biaxially drawn (BD) and amorphous poly(ethylene terephthalate) (PET) films were investigated by means of scanning probe microscopy. The PET surface is best imaged using non-contact mode atomic force microscopy (NC-AFM). Contact mode (C-AFM) under water can also offer a good resolution but the PET surface is not stable for long periods under such conditions. The ED film texture appears to be made up of "hillocks" of about 20 nm in diameter, while amorphous PET films appear featureless. It seems plausible to suggest that the observed hillocks represent small crystallites formed during the production process of BD films

Thermoelectric behaviour of Bi-Te films on polymer substrates DC-sputtered at room-temperature in moving web deposition

High-throughput roll-to-roll processing could be used to scale up the manufacture of flexible thermoelectric generators. Very thin thermoelectric layers can be manufactured at high throughput speed and low cost and, most importantly, are predicted to possess better thermoelectric properties than thicker layers. Here we present a study on a series of bismuth telluride films of different thickness (few nm to 370 nm), deposited on polymer substrates at room temperature using DC magnetron sputtering. Unlike previous studies of deposition of bismuth telluride films onto heated substrates, an island-growth mode, indicated by AFM, was observed for Bi-Te films grown at room temperature. A period of growth in which the layer only partially coats the substrate, with only imperfect connections between islands, was observed. In this partially coated region, the coating exhibited an extremely high Seebeck coefficient. An energy barrier mechanism, similar to the interface effect in nanomaterials, is proposed to explain this phenomenon, along with a possible quantum confinement effect. We found that a thinner Bi-Te film could generate a greater power factor because of a quasi-decoupling of Seebeck coefficient and electrical resistivity. In addition, ensuring that the sample passed directly under the sputtering target, and using a substrate smoothed with an acrylate layer were found to improve film properties, thus enhancing thermoelectric behaviour

Heterodyne force microscopy of PMMA/rubber nanocomposites: nanomapping of viscoelastic response at ultrasonic frequencies

We present measurements of the nanoscale elastic and viscoelastic properties of samples of poly(methylmetacrylate) (PMMA)/rubber nanocomposites. For these studies we have used a new technique based on atomic force microscopy (AFM) with ultrasonic excitation, heterodyne force microscopy (HFM), which provides a means of testing the viscoelastic response of polymeric materials locally (in tip-probed regions) at MHz frequencies. Phase-HFM contrast distinguishes local differences in the dynamic response of PMMA/rubber composites. Comparison of HFM with other AFM-based techniques (ultrasonic force microscopy, friction force microscopy and force modulation microscopy), while imaging the same surface region, emphasizes the unique capabilities of HFM for these kinds of studies, and reveals key nanostructural characteristics of the composites. Some of the toughening particles appear to be broken down, with areas of PMMA detached from the surrounding matrix

Fabrication and simulation of organic transistors and functional circuits

We report the development of a vacuum-evaporation route for the roll-to-roll fabrication of functioning organic circuits. A number of key findings and observations are highlighted which influenced the eventual fabrication protocol adopted. Initially, the role of interface roughness in determining carrier mobility in thin film transistors (TFTs) is investigated. Then it is shown that device yield is higher in devices fabricated on a flash-evaporated-plasma-polymerised tri(propyleneglycol) diacrylate (TPGDA) gate dielectric than for TFTs based on a spin-coated polystyrene (PS) dielectric. However, a degradation in mobility is observed which is attributed to the highly polar TPGDA surface. It is shown that high mobility and excellent stability are restored when the surface of TPGDA was buffered with a thin, spin-coated PS film. The resulting baseline process allowed arrays of functional circuits such as ring oscillators, NOR/NAND logic gates and S-R latches to be fabricated with high yield and their performance to be simulate

Static and dynamic post-annealing strategies for roll-to-roll fabrication of DC magnetron sputtered bismuth telluride thin films onto polymer webs

High-throughput roll-to-roll processes are desirable to scale up the manufacture of flexible thermoelectric generators. While vacuum deposition onto a heated dynamic substrate presents a considerable engineering challenge, viable postdeposition in-line annealing processes are considered as an alternative to improve the functional performance of as-deposited films. The effect of infrared and electron-beam irradiations of 1 μm thick bismuth telluride thin films, produced by a vacuum roll-to-roll process for use as thermoelectric materials, was examined. A static vacuum oven and pulsed high-energy electron beam were also studied as control groups. All annealing strategies increased the crystallite size and decreased the Te content. Only the static vacuum oven treatment was shown to significantly improve the film’s crystallinity. After 1 h annealing, the power factor improved by 400% (from 2.8 to 14 × 10–4 W/mK2), which, to the knowledge of the authors, is the highest reported thermoelectric performance of postannealed or hot-deposited Bi–Te films. As for in-line annealing, infrared and electron-beam post treatments improved the power factor by 146% (from 2.8 to 6.9 × 10–4 W/mK2) and 64% (from 2.8 to 4.6 × 10–4 W/mK2), respectively

Improving efficiency of MEH-PPV/TiO2 solar cells by lithium salt modification

The modi. cation of MEH-PPV/TiO2 photovoltaic devices with the lithium salt, Li[CF3-SO2](2)N, is shown to result in a 40% increase in both the short-circuit current and fill factor while not affecting the open-circuit voltage. The power conversion efficiency of the modified devices is 1.05% under 80 mW/cm(2) simulated solar illumination, which is twice that of the control device without the salt and the highest reported to date for a polymer/TiO2 solar cell. The increased performance is attributed to a large improvement in the hole mobility of the MEH-PPV, as measured by space-charge-limited current measurements, in the presence of the lithium salt. Modi. cation of the devices with LiClO4 is not as effective at improving device performance. Secondary ion mass spectroscopy measurements reveal that the poorer performance is due to a lower degree of Li diffusion into the polymer layer for LiClO4 than for Li[CF3SO2](2)N. (C) 2010 Elsevier B.V. All rights reserved