Dye Sensitized Nanocrystalline Photovoltaic Cell Group 1 – Luke, Matt, and Jeff Theory Schematic of Graetzel Cell 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-] 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 Current vs. Voltage 300 250 current (mA) 200 150 100 50 0 0 50 100 150 200 voltage (m V) Open circuit voltage: 0.388 V 250 300 350 Analysis: Power Pow er vs. Voltage 25 power (mW) 20 15 10 5 0 0 50 100 150 200 250 300 350 voltage (m V) Maximum Power: 21 mW Active Area: 0.7 in2 Max. power per unit area: 30 mW/in2 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
