Nanowire dye-sensitized solar cells

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Nanowire dye-sensitized solar cells

• Sung Hwan Kim
• EE 235 Presentation 2

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Outline

• Dye-Sensitized Cells(DSC) / Motivation
• Nanowire DSC
• Fabrication of Nanowires and Solar Cell
• Results and Analysis
• Summary and Conclusion

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Dye-Sensitized Cells(DSC)

• A type of photochemical cell that consists of an
  electrolyte sandwiched between a cathod and
  transparent anode
• Anode is a thick film of nanoparticles (10µm
  TiO2) coated with a photosensitive
  dye(ruthenium-polypyridine)
• Electrolyte(iodide solution) consists of redox
  couples
• Giving up electrons(accepting holes) oxidizes
• Accepting electrons changes from oxidized to
  reduced state
• When sunlight enters through anode, photons
  strike the dye, injecting electrons into the
  conduction band of TiO2 film
• Electrons are supplied to the dye from iodide
• Oxidized iodide receives electron from cathod

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Nanowire DSCs

• Limitations of DSC
• Electron transport in nanoparticle film(TiO2
  layer) is a trap-limited diffusion
  process(diffusivity Dn10-4cm2/sec) gt small
  diffusion length
• Efficiency 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
  incident
• Nanowire 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
• gt provides faster carrier extraction
• Provides large surface area for dye loadings
• Overall increase in carrier collection efficiency

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Fabrication of Nanowires and Solar Cell

• 3-4nm in diameter ZnO quantum dots deposited in
  FTO substrate and nanowires grown submerged in a
  complex solution
• Thermally 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 action

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Results and Analysis

• Solar cells were constructed for various surface
  areas(0.25-1.14cm2) and tested under 1
  Sun(100mA/cm2)
• Jsc 5.35.85 mA/cm2
• Voc 0.610.71V
• FF 0.360.38
• ? 1.21.5
• FF for nanowire cells is relatively insensitive
  to device area

• gt Nanowire cells are less affected by series
  resistance

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Results and Analysis

• Fill factor falls off with increasing light
  intensity owing to the development of a large
  photo-shunt gt efficiency is fairly constant
  above light intensity of 5mW/cm2
• For nanoparticle cells, there is a rapid
  saturation and decline of the current with
  increasing roughness factor gt transport
  efficiency falls off above certain film thickness

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Summary and Conclusion

• Some thoughts
• Lifetime of DSC solar cells
• Semiconductor-electrolyte operation
• gt susceptibility of semiconductor to
  photoenhanced corrosion?
• Depends too heavily on dye loadings
• Nanowire electrodes increase the rate of electron
  transport
• Dye-sensitized solar cells are promising devices
  for inexpensive, large-scale solar energy
  conversion
• Further work is required to accommodate the red
  region of the spectrum and to achieve higher dye
  loadings

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Notes

• Slide 3 electrolyte semiconductor or liquid //
  DSC low cost
• Slide 3 , since drift transport is prevented by
  the ions in the electrolyte
• Slide 4 At the electrodes, since drift is not
  possible, carriers diffuse(percolate) to the
  contacts with transit times in miliseconds gt
  small diffusion length
• Roughness factor surface area x TiO2 weight