Photovoltaic Cells

Nanocrystalline Dye Sensitized Solar Cell

Outline Cell Schematic Useful Physics Construction Procedure Preparation and deposition of TiO 2 (10-50 nm diameter) Preparation of dye and staining semi-conducter Carbon Coating counter- electrode Assemblage Electric Output Data Analysis Conclusion

Schematic of the Graetzel Cell

Theory and Physics 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-] Electrons are collected from the TiO2 at the cathode. Anode is covered with carbon catalyst and injects electrons into the cell regenerating the iodide. Redox mediator is iodide/triiodide (I-/I3-) 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.

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.

Chemical Note Triiodide (I 3 - ) is the brown ionic species that forms when elemental iodine (I 2 ) is dissolved in water containing iodide (I - ).

Construction Procedure TiO 2 Suspension Preparation TiO 2 Film Deposition Anthrocyanin Dye Preparation and TiO 2 Staining Counter Electrode Carbon Coating Solar Cell Assembly

Preparing the TiO 2 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

Deposition of the 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

Deposition of the TiO 2 Film (cont.) 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

Preparation photos Safety first! Mixing the TiO 2 Working under the hood Applying the TiO 2

Examples: TiO 2 Plate Good Coating: Mostly even distribution Bad Coating: Patchy and irregular The thicker the coating, the better the plate will perform

Preparing the 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)

Dye Preparation Dye comes from black berries Crushing the berries

Staining the 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 Dry and store in acidified (pH 3-4) deionized H 2 O in closed dark-colored bottle if not used immediately

Filter and Staining the TiO 2 Petri dish TiO 2 glass

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 Can be performed while TiO 2 electrode is being stained SnO 2 pre-coated glass

Assembling the Solar Cell Remove, rinse, and dry TiO 2 plate from storage or staining plate Place TiO 2 electrode face up on flat surface Position carbon-coated counter electrode on top of TiO 2 electrode Conductive side of counter electrode should face TiO 2 film Offset plates so all TiO 2 is covered by carbon-coated counter electrode Uncoated 4-5 mm strip of each plate left exposed

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

Measuring the Electrical Output To measure solar cell under sunlight, the cell should be protected from UV exposure with a polycarbonate cover 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 Multimeter light solar cell

Testing Circuit Photo Cell Voltmeter Ammeter Potentiometer

Measuring the Electrical Output Measure current-voltage using a 500 ohm potentiometer The center tap and one lead of the potentiometer are both connected to the positive side of the current Connect one multi-meter across the solar cell, and one lead of another meter to the negative side and the other lead to the load VoltageCurrent

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/cm 2 Determine power output and conversion efficiency values Excel generated plot of data Open circuit voltage  0.242mV

Data Analysis Continued Max Power 0.14mV Max Power per unit area –Photocell area = 34.2 cm 2 –0.003µW/cm 2

Nanocrystalline nanoparticle calculations Assumed size of 20nm: r = 10nm, density TiO 2 = 3.84g/cm 3  Volume of spherical particle = 4.19 * cm 3 /particle  Amount of TiO 2 =(4.19* )cm 3 *3.84g/cm 3 =1.61 * g/particle  SA= 1.26* cm 2 /particle  SA/g = 1.26* /1.61* = 78m 2 /g  atoms on surface/atoms in volume = 1.26* cm 2 * cm 2 / 4.19 * * = 0.095

Procedure Improvements Filter dye Don’t get light source too close to photocell while performing data acquisition Be sure TiO 2 layer is uniform and not too thin