Browsing by Author "Griffith, MJ"

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    Determining the orientation and molecular packing of organic dyes on a TiO(2) surface using x-ray reflectometry
    (American Chemical Society, 2011-11-01) Griffith, MJ; James, M; Triani, G; Wagner, P; Wallace, GG; Officer, DL
    The determination of the orientation and molecular density for several porphyrin dyes adsorbed on planar TiO2 surfaces using X-ray reflectometry (XRR) is reported. Adsorption of nanoscale water layers occurred rapidly upon exposure of freshly prepared TiO2 surfaces to ambient conditions; however, this was successfully eliminated, resulting in clearly discernible adsorbed dye layers for sensitized surfaces. Adsorbed dye orientations, determined from computations constrained by the measured dye layer thickness, were calculated to have a binding tilt angle of 35°?40°. Combining the XXR data with the orientation models indicates that the porphyrins form densely packed surfaces with an intermolecular spacing of 3?4 Ã…, consistent with Ï€?Ï€ stacking interactions. Changes in the molecular size of probe dyes were reflected in corresponding changes in the measured dye layer thickness, confirming the ability of this technique to resolve small variations in dye layer thickness and consequently adsorption orientation. Application of these results to understanding the behavior of dye-sensitized devices is discussed. © 2011, American Chemical Society
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    Significant performance improvement of porphyrin-sensitized TiO2 solar cells under white light illumination
    (American Chemical Society, 2011-01-13) Wagner, K; Griffith, MJ; James, M; Mozer, AJ; Wagner, P; Triani, G; Officer, DL; Wallace, GG
    A significant improvement in the photovoltaic performance of porphyrin-sensitized TiO2 solar cells under white light illumination is reported. The most significant improvement occurs within the first hour of light exposure and is irreversible within at least the studied 3-month period. Heat treatment in the dark produced only moderate performance improvement, whereas light treatment using a UV long-pass filter (>475 nm) led to an improvement similar to that obtained with the full simulated AM 1.5 spectrum, suggesting that the effect is linked to the photoexcitation of the porphyrin dye molecules. Light exposure resulted in simultaneous improvements in the short-circuit current (Jsc), the open-circuit voltage (Voc), and the fill factor (FF). The Jsc improvement is attributed to better charge injection demonstrated by thin-film APCE measurements. Photovoltage decay measurements showed a factor of 2−3 increase in the electron lifetime after light exposure, accompanied by a comparable decrease in the electron diffusion coefficient. The improved electron lifetime combined with the increased Jsc resulted in increased electron densities under open-circuit conditions, leading to improved Voc. Electrochemical impedance measurements showed a reduced charge-transfer resistance for I3− reduction at the Pt counter electrode, which is thought to be responsible for the increased filling factor. Furthermore, ATR-FTIR and X-ray reflectometry measurements indicated no significant change in the dye layer morphology after light exposure. An alternative mechanism involving the photogeneration of electronic states within the band gap of TiO2 leading to improved injection, slower electron transport, and improved electron lifetime is discussed. © 2011, American Chemical Society
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    A versatile binder-free TiO2 paste for dye-sensitized solar cells
    (Royal Society of Chemistry, 2015-03-19) Yune, JH; Karatchevtseva, I; Evans, PJ; Wagner, K; Griffith, MJ; Officer, DL; Triani, G
    In this study, binder-free TiO2 colloidal pastes have been prepared using a variety of heterocyclic bases with diverse characteristics to produce robust photoanodes for dye-sensitized solar cells (DSSC) from a single cast film thickness of 5 micron. The influence of the base on the electrode structure and film morphology, including its electron donor characteristics are investigated after low temperature thermal treatment and high temperature sintering. The results show that quinoline in the TiO2 paste is retained within the electrode structure in comparison to piperidine and pyridine after a short thermal treatment of 150 °C for 15 minutes. The presence of organic additives with π-conjugation in the photoanode enhances both electron injection efficiency and charge carrier lifetime resulting in higher Jsc and Voc. This formulation in combination with low temperature processing yields an energy conversion efficiency of over 5% in DSSC devices. In devices where high temperature sintering is permitted, the performance of TiO2 electrodes converges towards an efficiency of over 6%, irrespective of the organic additive within the paste. This formulation offers a high degree of versatility in casting electrodes onto polymer, glass or metal foil substrates from a single source of TiO2 paste, for the many variants of low-cost solar cells. © 2015 The Royal Society of Chemistry