Photovoltaic Cells

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Photovoltaic Cells
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Nanocrystalline Dye Sensitized Solar Cell
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Outline

• Cell Schematic
• Useful Physics
• Construction Procedure
• Preparation and deposition of TiO2 (10-50 nm
  diameter)
• Preparation of dye and staining semi-conducter
• Carbon Coating counter-electrode
• Assemblage
• Electric Output
• Data Analysis
• Conclusion

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Schematic of the Graetzel Cell
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Theory and Physics

• The adsorbed dye molecule absorbs a photon
  forming an excited state. dye
• The excited state of the dye can be thought of as
  an electron-hole pair (exciton).

• The excited dye transfers an electron to the
  semiconducting TiO2 (electron injection). This
  separates the electron-hole pair leaving the hole
  on the dye. dye
• The hole is filled by an electron from an iodide
  ion. 2dye 3I-? 2dye I3-

• Redox mediator is iodide/triiodide (I-/I3-)
• The dashed line shows that some electrons are
  transferred from the TiO2 to the triiodide and
  generate iodide. This reaction is an internal
  short circuit that decreases the efficiency of
  the cell.

• Electrons are collected from the TiO2 at the
  cathode.
• Anode is covered with carbon catalyst and injects
  electrons into the cell regenerating the iodide.

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Key Step Charge Separation

• Charge must be rapidly separated to prevent back
  reaction.
• Dye sensitized solar cell, the excited dye
  transfers an electron to the TiO2 and a hole to
  the electrolyte.
• In the PN junction in Si solar cell has a
  built-in electric field that tears apart the
  electron-hole pair formed when a photon is
  absorbed in the junction.

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Chemical Note

• Triiodide (I3-) is the brown ionic species that
  forms when elemental iodine (I2) is dissolved in
  water containing iodide (I-).

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Construction Procedure

• TiO2 Suspension Preparation
• TiO2 Film Deposition
• Anthrocyanin Dye Preparation and TiO2 Staining
• Counter Electrode Carbon Coating
• Solar Cell Assembly

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Preparing the TiO2 Suspension

• Begin with 6g colloidal Degussa P25 TiO2
• Incrementaly add 1mL nitric or acetic acid
  solution (pH 3-4) nine times, while grinding in
  mortar and pestle
• Add the 1mL addition of dilute acid solution only
  after previous mixing creates a uniform,
  lump-free paste
• Process takes about 30min and should be done in
  ventilated hood
• Let equilibrate at room temperature for 15 minutes

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Deposition of the TiO2 Film

• Align two conductive glass plates, placing one
  upside down while the one to be coated is right
  side up
• Tape 1 mm wide strip along edges of both plates
• Tape 4-5 mm strip along top of plate to be coated
• Uniformly apply TiO2 suspension to edge of plate
• 5 microliters per square centimeter
• Distribute TiO2 over plate surface with stirring
  rod
• Dry covered plate for 1 minute in covered petri
  dish

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Deposition of the TiO2 Film (cont.)

• Anneal TiO2 film on conductive glass
• Tube furnace at 450 oC
• 30 minutes
• Allow conductive glass to cool to room
  temperature will take overnight
• Store plate for later use

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Preparation photos
Mixing the TiO2
Safety first!
Applying the TiO2
Working under the hood
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Examples TiO2 Plate
Good Coating Mostly even distribution
Bad Coating Patchy and irregular
The thicker the coating, the better the plate
will perform
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Preparing the Anthrocyanin Dye

• Natural dye obtained from green chlorophyll
• Red anthocyanin dye
• Crush 5-6 blackberries, raspberries, etc. in 2 mL
  deionized H2O and filter (can use paper towel and
  squeeze filter)

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Dye Preparation
Dye comes from black berries
Crushing the berries
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Staining the TiO2 Film

• Soak TiO2 plate for 10 minutes in anthocyanin dye
• Insure no white TiO2 can be seen on either side
  of glass, if it is, soak in dye for five more min
• Wash film in H2O then ethanol or isopropanol
• Wipe away any residue with a kimwipe
• Dry and store in acidified (pH 3-4) deionized H2O
  in closed dark-colored bottle if not used
  immediately

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Filter and Staining the TiO2
Petri dish
TiO2 glass
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Carbon Coating the Counter Electrode

• Apply light carbon film to second SnO2 coated
  glass plate on conductive side
• Soft pencil lead, graphite rod, or exposure to
  candle flame
• Can be performed while TiO2 electrode is being
  stained

SnO2 pre-coated glass
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Assembling the Solar Cell

• Remove, rinse, and dry TiO2 plate from storage or
  staining plate
• Place TiO2 electrode face up on flat surface
• Position carbon-coated counter electrode on top
  of TiO2 electrode
• Conductive side of counter electrode should face
  TiO2 film
• Offset plates so all TiO2 is covered by
  carbon-coated counter electrode
• Uncoated 4-5 mm strip of each plate left exposed

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Assembling the Solar Cell

• Place two binder clips on longer edges to hold
  plates together (DO NOT clip too tight)
• Place 2-3 drops of iodide electrolyte solution at
  one edge of plates
• Alternately open and close each side of solar
  cell to draw electrolyte solution in and wet TiO2
  film
• Ensure all of stained area is contacted by
  electrolyte
• Remove excess electrolyte from exposed areas
• Fasten alligator clips to exposed sides of solar
  cell

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Measuring the Electrical Output

• To measure solar cell under sunlight, the cell
  should be protected from UV exposure with a
  polycarbonate cover
• Attach the black (-) wire to the TiO2 coated
  glass
• Attach the red () wire to the counter electrode
• Measure open circuit voltage and short circuit
  current with the multimeter.
• For indoor measurements, can use halogen lamp
• Make sure light enters from the TiO2 side

light
Multimeter
solar cell
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Testing Circuit
Ammeter
Photo Cell
Voltmeter
Potentiometer
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Measuring the Electrical Output

• Measure current-voltage using a 500 ohm
  potentiometer
• The center tap and one lead of the potentiometer
  are both connected to the positive side of the
  current
• Connect one multi-meter across the solar cell,
  and one lead of another meter to the negative
  side and the other lead to the load

Voltage Current
0.242 0
0.22 0.003
0.21 0.004
0.17 0.006
0.13 0.008
0.1 0.01
0.08 0.012
0.041 0.016
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Data Analysis

• Plot point-by-point current/voltage data pairs at
  incremental resistance values, decrease
  increments once line begins to curve
• Plot open circuit voltage and short circuit
  current values
• Divide each output current by the measured
  dimensions of stained area to obtain mA/cm2
• Determine power output and conversion efficiency
  values

Open circuit voltage ? 0.242mV
Excel generated plot of data
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Data Analysis Continued

• Max Power
• 1.025µW _at_ 0.14mV
• Max Power per unit area
• Photocell area 34.2 cm2
• 0.003µW/cm2

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Nanocrystalline nanoparticle calculations

• Assumed size of 20nm r 10nm, density TiO2
  3.84g/cm3
• Volume of spherical particle 4.19 10-18
  cm3/particle
• Amount of TiO2(4.1910-18)cm3 3.84g/cm31.61
  10-17g/particle
• SA 1.2610-11cm2/particle
• SA/g 1.2610-11/1.6110-17 78m2/g
• atoms on surface/atoms in volume
• 1.2610-11cm2 1015cm2 / 4.19 10-18 1022.5
  0.095

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Procedure Improvements

• Filter dye
• Dont get light source too close to photocell
  while performing data acquisition
• Be sure TiO2 layer is uniform and not too thin