Browsing by Subject "Dye-sensitized solar cells"

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    Fabrication Procedure Optimizations of Solar Cells
    (2014) Chen, Zhengtao; Xu, Zhihua
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    Synthesis and characterization of ensembles containing zinc oxide nanocrystals and organic or transition metal dyes to probe the early events in a dye-sensitized solar cell.
    (2011-08) Saunders, Julia Erin
    The synthesis of 3',4'-dibutyl-2-phenyl-2,2':5',2"-terthiophene-5"-carboxylic acid, and its behavior with monodispersed ZnO nanocrystals (NCs) having diameters from 2.7 to 3.2 nm are reported. The excited state of the dye (E0* = -1.61 V vs NHE) was quenched upon binding to ZnO Ncs. Adsorption isotherms were measured for the terthiophene dye in ethanol and fit with a Langmuir model, which gave a size-independent Kads of 2.3 ± 1.0 x 105 M-1. The maximum number of attached dyes per nanocrystal depended on the diameter and was consistent with each dye occupying 0.5 ± 0.1 nm2 at maximum coverage. Deviation from the Langmuir model observed at low dye concentrations was attributed to a small amount of free zinc ion present in solution that bind the carboxylate ions more strongly than do ZnO NCs. Incorporation of the equilibrium expression between zinc ion and free carboxylate into the model provided a satisfactory fit for both the adsorption isotherm experiments and the complex shape of the Stern-Volmer graphs. Treatment of the terthiophene dye-nanocrystal dyads with increasing concentrations of sodium acetate in ethanol resulted in gradual displacement of the dye. Time-resolved fluorescence and time- and frequency-resolved pump-probe spectroscopy confirm and characterize electron injection from the dye to the semiconductor nanocrystals in room temperature ethanol dispersions at a series of dye:ZnO NC concentration ratios. The spectrum of the oxidized dye was determined by spectroelectrochemistry. The singlet excited state of the dye (190 ps lifetime in ethanol) is quenched almost exclusively by electron transfer to the ZnO NC, and the electron transfer dynamics exhibit a single time scale of 3.5 ( 0.5 ps at all concentration ratios. In the measured transient responses at different dye:ZnO NC ratios, gain in the amplitude of the electron injection component is anticorrelated with loss of amplitude from unperturbed excited state dye molecules. The dependence of this amplitude on dye:ZnO NC ratio deviates significantly from the prediction of a standard Stern-Volmer model. This observation is in agreement with the static quenching studies. By identifying electron transfer as the quenching mechanism at all ratios, the work presented here helps to exclude concentration quenching as the basis for the complicated quenching results, and supports the model that incorporates competitive binding between ZnO NC s and free Zn2+ cations in solution.
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    Titanium dioxide nanostructures for photovoltaics and photocatalysis.
    (2011-08) Liu, Bin
    The dye-sensitized solar cell (DSSC) is a promising low cost photovoltaic device for the generation of carbon-free energy. DSSC consists of two conducting glass electrodes in a sandwich configuration, with a redox electrolyte filling the free space in between. During illumination of the cell, the dye molecules inject electrons into the semiconductor film and the injected electrons diffuse through the semiconductor nanoparticle network through hopping from particle to particle until being collected at the electron collecting photoanode. Meanwhile, the charged dye is regenerated by an electrochemical reaction with a redox couple in the electrolyte. The oxidized ionic species diffuse towards the counter photocathode and are reduced by electrons that have traveled from the photocathode through the load to complete the circuit. To date, DSSCs with light-to-electric conversion efficiencies of ~7 to 11% have been demonstrated with ~10 mm thick electrodes made of 10-30 nm diameter TiO2 nanoparticles sensitized with ruthenium-based dyes, but further device improvement is limited due to the competition between electron transport and recombination. Wide bandgap semiconductor nanowire electrodes have the potential to increase the DSSC performance by increasing the electron transport rate while keeping the electron recombination rate unaltered. Towards this end, the synthesis of single-crystalline TiO2 nanowires on substrates was studied. Mesoporous anatase TiO2 microspheres composed of abutted TiO2 nanoparticles were synthesized through a two-step hydrothermal method. Photoanodes assembled from alternating layers of these mesoporous TiO2 microspheres and TiO2 nanoparticles increase the overall power conversion efficiencies of DSSCs by as much as 26%. This increase is due to enhanced light scattering by porous TiO2 microspheres and is achieved without sacrificing the specific surface area. Single-crystalline TiO2 nanowire arrays were grown on flexible titanium foil using a three-step solution synthesis. The synthesis method relies on the ability to grow single crystal sodium titanate (Na2Ti2O5·H2O) nanowires on titanium foil through a novel alkali hydrothermal growth process. Following growth, the Na2Ti2O5·H2O nanowires are converted to protonated bititanate (H2Ti2O5·H2O) nanowires through an ion-exchange reaction without changing their morphology or crystal structure. Finally, the protonated bititanate nanowires are converted to single crystalline anatase TiO2 nanowires through a topotactic transformation by calcination. These three sequential steps yield a carpet of 2– 50 μm long single crystalline nanowires oriented in the [100] direction and primarily normal to the titanium foil. DSSC assembled from 12 μm thick TiO2 nanowire film gives a light-to-electric conversion efficiency of ~ 1.4%. Further improvements in the cell efficiency should be possible with longer nanowires. Single-crystalline rutile TiO2 nanorods were grown on transparent conductive fluorine-doped tin oxide (FTO) substrates using a facile, hydrothermal method. The diameter, length, and density of the nanorods could be varied by changing the growth parameters, such as growth time, growth temperature, initial reactant concentration, acidity, and additives. The epitaxial relation between the FTO substrate and rutile TiO2 with a small lattice mismatch plays a key role in driving the nucleation and growth of the rutile TiO2 nanorods on FTO. With TiCl4-treatment, a light-to-electricity conversion efficiency of 3% could be achieved by using 4 μm-long TiO2 nanorod films as the photoanode in a DSSC. Single crystal anatase TiO2 nanorods/nanoflakes were grown on FTO substrates though a TiCl4 evaporation-condensation-hydrolyzation process, following by a subsequent thermal treatment. DSSCs assembled from 1 μm long TiO2 nanorod and nanoflake films give a light to electricity conversion efficiency of ~ 2.1%.