Dye Sensitized Nanocrystalline Photovoltaic Cell

1. Dye Sensitized Nanocrystalline Photovoltaic Cell Group 1 – Luke, Matt, and Jeff

2. Theory • Schematic of Graetzel Cell

3. Theory • 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-]

4. 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.

5. Objective • Learn about the photovoltaic effect. • Understand the Scherrer formula.

6. 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

7. 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

8. 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

9. 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)

10. 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

11. 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

12. 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

13. 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

14. Results • Open circuit voltage: 0.388 V

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

16. 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

17. 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