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 TiO 2 (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 + I 3 - ] 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 TiO 2 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 TiO 2 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 TiO 2 suspension to edge of plate 5 microliters per square centimeter Distribute TiO 2 over plate surface with stirring rod Dry covered plate for 1 minute in covered petri dish
Procedure: Deposition of TiO 2 Film Anneal TiO 2 film on conductive glass Tube furnace at 450 o C 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 H 2 O and filter (can use paper towel and squeeze filter)
Procedure: Staining TiO 2 Film Soak TiO 2 plate for 10 minutes in anthocyanin dye Insure no white TiO 2 can be seen on either side of glass, if it is, soak in dye for five more min Wash film in H 2 O then ethanol or isopropanol Wipe away any residue with a kimwipe
Procedure: Carbon Coating the Counter Electrode Apply light carbon film to second SnO 2 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 TiO 2 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 TiO 2 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 TiO 2 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
Analysis: Power Maximum Power: 21 mW Active Area: 0.7 in 2 Max. power per unit area: 30 mW/in 2
Questions Approximate TiO 2 particle size: assume ~25 nm diameter Number of TiO 2 units per nanoparticle: Volume of one nanoparticle = 8.18 * 10^-18 cm 3 Density of TiO 2 ~ 4 g/cm 3 Mass of one nanoparticle = 3.27 * 10^-17 g Molar mass of TiO 2 = 79.87 g/mol moles of TiO 2 in one nanoparticle = 4.10 * 10^-19 moles 4.10 * 10^-19 moles * 6.022 * 10^23 molecules/mole = 2.48 * 10^5 TiO 2 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 m 2 Surface area per gram = 3.06 * 10^16 nanoparticles/gram * 1.96 * 10^-15 m2/nanoparticle = 60.0 m 2 /gram
Questions Fraction of atoms that reside on the surface: Surface area of one particle = 1.96 * 10^-11 cm 2 Approximate atoms per unit area = 10 15 atoms/cm 2 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