Nanowire dye-sensitized solar cellsOutlineDye-Sensitized Cells(DSC)Nanowire DSCsFabrication of Nanowires and Solar CellResults and AnalysisResults and AnalysisSummary and ConclusionSlide 9NotesNanowire dye-sensitized solar cellsSung Hwan KimEE 235 Presentation 2UC Berkeley, Sung Hwan Kim 2OutlineDye-Sensitized Cells(DSC) / MotivationNanowire DSCFabrication of Nanowires and Solar CellResults and AnalysisSummary and ConclusionUC Berkeley, Sung Hwan Kim 3Dye-Sensitized Cells(DSC) A type of photochemical cell that consists of an electrolyte sandwiched between a cathod and transparent anodeAnode is a thick film of nanoparticles (~10μm TiO2) coated with a photosensitive dye(ruthenium-polypyridine) Electrolyte(iodide solution) consists of redox couplesGiving up electrons(accepting holes) oxidizesAccepting electrons changes from oxidized to reduced stateWhen sunlight enters through anode, photons strike the dye, injecting electrons into the conduction band of TiO2 filmElectrons are supplied to the dye from iodideOxidized iodide receives electron from cathodUC Berkeley, Sung Hwan Kim 4Nanowire DSCsLimitations of DSC:Electron transport in nanoparticle film(TiO2 layer) is a trap-limited diffusion process(diffusivity Dn≤10-4cm2/sec) => small diffusion lengthEfficiency is limited by Ln in the film, surface area of the electrodes, and low absorbance near 400-800nm where much of the solar spectrum is incidentNanowire DSCs For a single nanowire(ZnO), measured electron diffusivity(Dn) of 0.05-0.5cm2/sec is several hundred times larger than the highest reported diffusivity for TiO2 => provides faster carrier extractionProvides large surface area for dye loadingsOverall increase in carrier collection efficiencyUC Berkeley, Sung Hwan Kim 5Fabrication of Nanowires and Solar Cell3-4nm in diameter ZnO quantum dots deposited in FTO substrate and nanowires grown submerged in a complex solutionThermally platinized FTO counter electrodes were used to sandwich nanowires separated by 40μm thick spacers Internal space of the cell was filled with iodide electrolyte by capillary actionUC Berkeley, Sung Hwan Kim 6Results and AnalysisSolar cells were constructed for various surface areas(0.25-1.14cm2) and tested under 1 Sun(100mA/cm2)Jsc = 5.3–5.85 mA/cm2 Voc = 0.61–0.71V FF = 0.36–0.38 η = 1.2–1.5%FF for nanowire cells is relatively insensitive to device area => Nanowire cells are less affected by series resistanceUC Berkeley, Sung Hwan Kim 7Results and Analysis Fill factor falls off with increasing light intensity owing to the development of a large photo-shunt => efficiency is fairly constant above light intensity of 5mW/cm2For nanoparticle cells, there is a rapid saturation and decline of the current with increasing roughness factor => transport efficiency falls off above certain film thicknessUC Berkeley, Sung Hwan Kim 8Summary and ConclusionSome thoughts:Lifetime of DSC solar cellsSemiconductor-electrolyte operation => susceptibility of semiconductor to photoenhanced corrosion? Depends too heavily on dye loadingsNanowire electrodes increase the rate of electron transport Dye-sensitized solar cells are promising devices for inexpensive, large-scale solar energy conversionFurther work is required to accommodate the red region of the spectrum and to achieve higher dye loadingsUC Berkeley, Sung Hwan Kim 9UC Berkeley, Sung Hwan Kim 10NotesSlide 3: electrolyte – semiconductor or liquid // DSC low costSlide 3: , since drift transport is prevented by the ions in the electrolyteSlide 4: At the electrodes, since drift is not possible, carriers diffuse(percolate) to the contacts with transit times in miliseconds => small diffusion lengthRoughness factor = surface area x TiO2
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