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

Nanocrystalline Dye Sensitized Solar Cell
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Outline
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Motivation
History
Cell Schematic
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Useful Physics
Construction Procedure
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Preparation and deposition of
TiO2 (10-50 nm diameter)
Preparation of dye and
staining semiconducter
Carbon Coating counterelectrode
Assemblage
Electric Output
Data Analysis
Conclusion
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Motivation
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Economically feasible harnessing
of solar energy
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Reduce fossil fuel usage and
subsequent pollution
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Provide usable energy to
inaccessible and economically
challenged communities
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Modeling of biological
photochemical systems
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Improvement of current
photographic methods
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History
• 1839: French physicist Antoine-Cesar
Becquerel observed that shining light on an
electrode submerged in electrolyte would create
an electric current.
• 1941: American Russell Ohl invented a PN
junction silicon solar cell
• The dye sensitized solar cell was developed in
1992 by Graetzel (EPFL, Laussane,
Switzerland) and utilizes nanocrystalline TiO2
as the photoabsorber
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Solar Panel Cost
• Initially solar panels were expensive (>$2000 per watt in
1950s). Thus their use was limited to very special
applications such as powering space satellites.
• Today solar panels are less than $4 per watt.
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What’s on the Horizon?
First Generation:
Single and polycrystalline wafer cells
Second Generation: Thin film cells
Third Generation:
Thin film cell efficiency is increased by using
multiple layers in tandem and matching the
band gaps of each layer to a different region of
the solar spectrum.
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Evolution of the Efficiency
of the Steam Engine
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Schematic of the
Graetzel Cell
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Useful Physics
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The adsorbed dye
molecule absorbs a
photon forming an
excited state. [dye*]
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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*+]
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The hole is filled by an electron from an iodide ion.
[2dye*+ + 3I- 2dye + I3-]
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Useful Physics
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Electrons are collected
from the TiO2 at the
cathode.
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Anode is covered with
carbon catalyst and
injects electrons into
the cell regenerating
the iodide.
Redox mediator is iodide/triiodide (I-/I3-)
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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.
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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-).
I2  I

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I

3
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
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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
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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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Examples: TiO2 Plate
Good Coating:
Bad Coating:
Mostly even distribution
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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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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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
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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
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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
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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
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Multimeter
light
solar cell
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 multimeter
across the solar cell, and one
lead of another meter to the
negative side and the other
lead to the load
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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
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Results
• Current:
– One solar cell: 0.11 - 0.19 mA
– Two cells in parallel: 0.164 0.278 mA
• Voltage:
– One solar cell: 0.30 – 0.40 V
• Resistance:
– Very large.
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Fig. 1: “How many nano physicists does it take to
screw in a lightbulb?”
Questions
• What have we learned about the relationship of
solar cell to photosynthesis and solar energy?
• How can you improve the procedure or design?
• How does this ultimately relate to other things
we've learned in NANOLAB?
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Further Reading
Konarka Technologies (Graetzel cells)
PV Power Resource Site
US DOE Photovoltaics
Key Center for Photovoltaic Engineering
National Center for Photovoltaics
NRELs Photovoltaic Information Index
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http://www.konarkatech.com/
http://www.pvpower.com/
http://www.eere.energy.gov/pv/
http://www.pv.unsw.edu.au/
http://www.nrel.gov/ncpv/
http://www.nrel.gov/ncpv/masterindex.html