Dye-sensitized Solar Cells

1. Preparation of TiO₂ Paste

1. Mass out 6 g of colloidal TiO₂ powder, and place it in a mortar.
2. Carefully add 2-3 mL of vinegar to the TiO₂, and begin grinding the suspension with the pestle until a uniform paste is obtained. The grinding serves to break up aggregated clumps in the powder.
3. Continue adding vinegar, in ~ 1 mL increments while grinding, up to ~ 9 mL total volume. Prior to each addition, the consistency of the paste should be uniform and free of lumps. The final paste should be thick, but not so thick that it cannot be squeezed out of a dropper bottle.
4. Add 1 drop of dish soap to 1 mL of distilled water, gently mix.
5. Add the dish soap solution to the TiO₂ suspension and gently mix, being careful not to produce bubbles.
6. Allow the suspension to equilibrate for 15 min. The dish soap serves as a surfactant, to help make the suspension more readily spread out into a uniform film on the glass.

2. Deposition of TiO₂ on Glass

1. Clean two conductive glass slides. Soak a kimwipe with ethanol and use it to wipe clean two conductive glass slides. Place the clean slides on a fresh kimwipe.
2. Determine which side of the glass is conductive. Using a multimeter set to ohms, touch both leads to one side of the glass. If a reading between 10 and 30 Ω is observed, it is the conductive side. A reading of 0 Ω indicates the non-conductive side.
3. Mask the slide. Place one glass slide with its conductive side up and the other with its conductive side down. Carefully keeping the slides touching, tape the glass slides to the bench top. Place tape on three of the four sides of the slides, making sure that ~ 5-8 mm of the slide is covered by tape on each of the three sides (Figure 4). Press the tape firmly, to ensure that there are no air bubbles.
4. Apply the TiO₂ paste. Using a glass rod, apply a thin line of paste across the masked top edge of the slide. Use the glass rod to carefully roll the paste down the length of the slide, and back up. Repeat this motion 2-3x without lifting the rod, or until a uniform film is obtained.
   1. If the film is not uniform, simply wipe it off with a kimwipe, clean the glass with ethanol, and once dry, try again.
5. Allow the film to dry a little, then carefully remove the tape from the glass. The slide with the TiO₂ film should be on the conductive side. The other slide can be cleaned and used later.
6. Anneal the TiO₂ film. Carefully place the slide (TiO₂ side up) on a hot plate that is set to 450 °C. Watch as the TiO₂ darkens to a purple/brown color, and regains its white color. At this point, turn the hotplate off and allow the film to slowly cool. If the slide is cooled too quickly, it may crack or shatter.
7. With a ruler, measure the surface area that is covered with the film, and note this value.

Figure 4. Deposition of TiO₂ on glass.

3. Stain the TiO₂ Film with Dye

1. Place a few blackberries, raspberries, or cherries in a mortar and crush them with a pestle.
2. Filter the solution through a coffee filter and into a Petri dish. It may be necessary to add a few mL of water to the juice.
3. Place the cooled TiO₂ film, face side down in the Petri dish. Be careful to not scratch off any TiO₂. Allow the dye to be adsorbed onto the film. This may take several minutes.
4. Once the film is fully coated (it should be dark red or purple and there are no white spots), lift the slide up with tongs (be careful to only tough the glass and not the film), and rinse the slide with water, then ethanol. Blot the film dry with a kimwipe, and use immediately.
   1. If not used immediately, then store the film in a Petri dish that contains acetic acid at pH 3-5, and cover the dish with the lid and wrap in foil.

4. Prepare the Counter Electrode

1. Using another conductive glass slide, follow steps 2.1-2.2.
2. Apply the carbon catalyst on the conductive side. Using tweezers, hold the slide, conductive side down, over the tip of a Bunsen burner. Move the slide around so that the soot collects on the entire surface, but for no longer than 30 s. Let the slide cool, and wipe the soot along one side of the slide with a cotton swab.
   1. Alternatively, using an HB pencil, cover the entire conductive surface with graphite. This gives a more robust electrode, but one that performs less well.

5. Assemble the Solar Cell

1. Dry the stained film. Rinse it with ethanol and place it on a kimwipe. Gently blot the film with a second kimwipe. The film must be dry as to not impact the electrolyte solution.
2. With the electrode film side up, gently place the carbon coated electrode on top (carbon face down). Be sure to offset the slides so that the exposed sides of both electrodes can be clipped to wires. Place two binder clips on the sides adjacent to the offset glass.
3. Place a few drops of the electrolyte solution along one edge of the slides, and carefully open/close each side of the cell by alternately opening/closing the binder clips. Be sure that all of the stained area is in contact with the electrolyte solution, and repeat step 5.2 if necessary.
4. Wipe off the excess electrolyte from the exposed areas using kimwipes and ethanol.
5. Fasten alligator clips to the two exposed sides of the solar cell.

6. Measuring Cell Performance

Note: Ideally, these measurements are to be done outside. However, if the weather is not permitting, they can be done inside using a halogen lamp. All measurements should be done with no movement of the cell so that they are performed under identical conditions.

1. Be sure to orient the cell so that the TiO₂ film is facing up towards the sun, and place a polycarbonate cover over the cell. This protects the cell from UV damage.
2. Connect the negative electrode (TiO₂-coated glass) to the negative wire of the multimeter, and the positive electrode (C) to the positive wire of the multimeter (Figure 5).
3. Set the multimeter to volts, and measure the voltage. This is the open circuit potential (maximum voltage at zero current). Cover the cell (with a hand or a solid object) to ensure that the voltage decreases.
4. Set the multimeter to milliamperes (mA), and measure the maximum current. This is the short circuit current (maximum current at zero voltage). Cover the cell with (with a hand) to ensure that the current decreases.
5. Record a full current-voltage curve using a 500-Ω potentiometer as a variable load.
   1. Determine which lead on the potentiometer is the central tap. This lead allows for the resistance to be varied. To do this, connect the multimeter (set to ohms) to two of the leads on the potentiometer, and vary the resistance on the potentiometer. Note if the resistance changes. Repeat this with the other two combinations of leads. Changes in resistance should be observed in two of the three combinations. The lead that was used in both combinations that gave changes is the central tap, and the other two are functionally identical.
   2. Assemble the circuit as shown in Figure 5 (right).
   3. Set the potentiometer to full (or zero) resistance, and note the current and voltage.
   4. Change the resistance on the potentiometer in small increments and note the current and voltage so that there are several points that span the entire range of the potentiometer. Be sure to not move the cell during these measurements. Once the current starts to change, be sure to collect many data points; fewer data points may be obtained when it is constant.

Figure 5. Circuit diagram to measure the open-circuit potential and short circuit current (left, steps 6.3 and 6.4), and to record the I-V curve (right).