Dye Sensitized Nanocrystalline Photovoltaic Cell

Group 1 – Luke, Matt, and Jeff

Theory

Schematic of Graetzel Cell

• 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

-]

Theory

Theory: 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.

Objective

Learn about the photovoltaic effect.

Understand the Scherrer formula.

Procedure: 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

Procedure: Deposition of 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

Procedure: Deposition of TiO2 Film

• 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

Procedure: Preparing 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)

Procedure: Staining 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

Procedure: 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

Procedure: 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

Procedure: Measuring the Electrical Output• 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

• Measure current-voltage using a 1 kohm potentiometer

• The center tap and one lead of the potentiometer are both connected to the positive side of the current

• Connect one multimeter across the solar cell, and one lead of another meter to the negative side and the other lead to the load

Results

Open circuit voltage: 0.388 V

Current vs. Voltage

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100

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200

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300

0 50 100 150 200 250 300 350

voltage (mV)

curr

ent

(mA

)

Analysis: Power

Maximum Power: 21 mW Active Area: 0.7 in2 Max. power per unit area: 30 mW/in2

Power vs. Voltage

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5

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25

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voltage (mV)

po

wer

(m

W)

Questions Approximate TiO2 particle size: assume ~25 nm diameter Number of TiO2 units per nanoparticle:

Volume of one nanoparticle = 8.18 * 10^-18 cm3

Density of TiO2 ~ 4 g/cm3 Mass of one nanoparticle = 3.27 * 10^-17 g

Molar mass of TiO2 = 79.87 g/mol moles of TiO2 in one nanoparticle = 4.10 * 10^-19 moles

4.10 * 10^-19 moles * 6.022 * 10^23 molecules/mole = 2.48 * 10^5 TiO2 units per nanoparticle

Nanoparticle surface area per gram: Number of nanoparticles per gram = 1/(3.27 * 10^-17) = 3.06 *

10^16 nanoparticles Surface area of one nanoparticle = 1.96 * 10^-15 m2

Surface area per gram = 3.06 * 10^16 nanoparticles/gram * 1.96 * 10^-15 m2/nanoparticle = 60.0 m2/gram

Questions Fraction of atoms that reside on the surface:

Surface area of one particle = 1.96 * 10^-11 cm2

Approximate atoms per unit area = 1015 atoms/cm2

Atoms on surface = 1.96 * 10^-11 cm2 * 10^15 atoms/cm2 = 1.96 * 10^4 atoms

Fraction of atoms on surface = (1.96 * 10^4)/(2.48 * 10^5) = 0.079

Way to improve experiment: Filter raspberry juice using a better filter system