Transcript Solar Cell - Homer L. Dodge Department of Physics and

Photovoltaic Cells
Nanocrystalline Dye Sensitized Solar Cell
Outline
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•
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Cell Schematic
Useful Physics
Construction Procedure
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•
•
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Preparation and
deposition of TiO2 (10-50
nm diameter)
Preparation of dye and
staining semi-conducter
Carbon Coating counterelectrode
Assemblage
•
Electric Output
•
•
Data Analysis
Conclusion
Schematic of the
Graetzel Cell
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-]
•Electrons are collected from the TiO2 at
the cathode.
•Redox mediator is iodide/triiodide (I-/I3-)
•The dashed line shows that some
electrons are transferred from the TiO2 to
•Anode is covered with carbon catalyst and the triiodide and generate iodide. This
reaction is an internal short circuit that
injects electrons into the cell regenerating
decreases the efficiency of the cell.
the iodide.
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.
Chemical Note
Triiodide (I3-) is the brown ionic species that
forms when elemental iodine (I2) is dissolved
in water containing iodide (I-).
I2  I

I

3
Construction Procedure
• TiO2 Suspension Preparation
• TiO2 Film Deposition
• Anthrocyanin Dye Preparation and TiO2 Staining
• Counter Electrode Carbon Coating
• Solar Cell Assembly
Preparing the TiO2 Suspension
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Begin with 6g colloidal Degussa P25 TiO2
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Incrementaly add 1mL nitric or acetic acid
solution (pH 3-4) nine times, while grinding
in mortar and pestle
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Add the 1mL addition of dilute acid solution
only after previous mixing creates a uniform,
lump-free paste
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Process takes about 30min and should be
done in ventilated hood
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Let equilibrate at room temperature for 15
minutes
Deposition of the TiO2 Film
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Align two conductive glass plates, placing one upside
down while the one to be coated is right side up
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Tape 1 mm wide strip along edges of both plates
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Tape 4-5 mm strip along top of plate to be coated
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Uniformly apply TiO2 suspension to edge of plate
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5 microliters per square centimeter
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Distribute TiO2 over plate surface with stirring rod
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Dry covered plate for 1 minute in covered petri dish
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
Preparation photos
Mixing the TiO2
Safety first!
Applying the TiO2
Working under the hood
Examples: TiO2 Plate
Good Coating:
Bad Coating:
Mostly even distribution
Patchy and irregular
The thicker the coating, the better the plate will perform
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)
Dye Preparation
Dye comes from black berries
Crushing the berries
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
Filter and Staining the TiO2
Petri dish
TiO2 glass
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
Assembling the Solar Cell
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Remove, rinse, and dry TiO2
plate from storage or staining
plate
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Place TiO2 electrode face up on
flat surface
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Position carbon-coated counter
electrode on top of TiO2
electrode
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Conductive side of counter
electrode should face TiO2
film
Offset plates so all TiO2 is
covered by carbon-coated
counter electrode
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Uncoated 4-5 mm strip of
each plate left exposed
Assembling the Solar Cell
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Place two binder clips on longer
edges to hold plates together
(DO NOT clip too tight)
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Place 2-3 drops of iodide
electrolyte solution at one edge
of plates
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Alternately open and close each
side of solar cell to draw
electrolyte solution in and wet
TiO2 film
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Ensure all of stained area is
contacted by electrolyte
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Remove excess electrolyte from
exposed areas
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Fasten alligator clips to exposed
sides of solar cell
Measuring the Electrical Output
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To measure solar cell under
sunlight, the cell should be
protected from UV exposure
with a polycarbonate cover
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Attach the black (-) wire to the
TiO2 coated glass
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Attach the red (+) wire to the
counter electrode
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Measure open circuit voltage
and short circuit current with
the multimeter.
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For indoor measurements, can
use halogen lamp
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Make sure light enters from
the TiO2 side
Multimeter
light
solar cell
Testing Circuit
Ammeter
Voltmeter
Photo
Cell
Potentiometer
Measuring the Electrical Output
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Measure current-voltage
using a 500 ohm
potentiometer
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The center tap and one lead
of the potentiometer are both
connected to the positive
side of the current
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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
0.242
0.22
0.21
0.17
0.13
0.1
0.08
0.041
Current
0
0.003
0.004
0.006
0.008
0.01
0.012
0.016
Data Analysis
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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
VI characteristic
Current
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0.018
0.016
0.014
0.012
0.01
0.008
0.006
0.004
0.002
0
Series1
0
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Divide each output current
by the measured
dimensions of stained area
to obtain mA/cm2
Determine power output and
conversion efficiency values
0.1
0.2
0.3
Voltage
Open circuit voltage  0.242mV
Excel generated plot of data
Data Analysis Continued
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Max Power
Power curve
– 1.025µW @ 0.14mV
0.0012
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Max Power per unit area
– Photocell area = 34.2 cm2
Power, mA
0.001
0.0008
0.0006
Series1
0.0004
0.0002
– 0.003µW/cm2
0
0
0.1
0.2
Voltage, mV
0.3
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=(4.19*10-18)cm3 *3.84g/cm3=1.61 * 1017g/particle
SA= 1.26*10-11cm2/particle
SA/g = 1.26*10-11/1.61*10-17 = 78m2/g
atoms on surface/atoms in volume =
1.26*10-11cm2 * 1015cm2 / 4.19 * 10-18 * 1022.5 =
0.095
Procedure Improvements
• Filter dye
• Don’t get light source too close to
photocell while performing data acquisition
• Be sure TiO2 layer is uniform and not too
thin