Make a Dye-Sensitized Solar Cell

Background Information for Teachers

There is a growing need to investigate alternative energy sources due to the impacts of fossil fuels on global warming and clean air. Solar energy, or energy from the sun, is a free, readily available, plentiful resource that can be collected by solar cells to generate electricity.

Although solar cells have been around for a long time, their use for energy generation is not widespread. This is because traditional solar cells are expensive and inefficient (typically 11-18% of the sunlight they absorb is converted to electricity). To be considered a green chemistry technology, the technology must demonstrate three standards: performance, safety, and cost benefits. In this experiment, your students will make a dye-sensitized solar cell (DSSC) that is efficient, uses safe materials, and is inexpensive.

Unlike traditional solar cells that generate electricity through p/n junctions, the chemistry of the nanocrystalline TiO₂ is based on red-ox (reduction-oxidation) chemistry. This means that the excitement of electrons to generate electron movement through the system is what drives electricity, which can be measured in terms of voltage (V). The mechanism of a photovoltaic cell has three steps (Figure 1):

1. A dye, adsorbed on a layer of semiconductor (TiO₂), interacts with the visible light provided by the sun (just like the green pigment does in a leaf), promoting an electron from a lower-level orbital to an excited one.
2. The excited electron is injected by the dye into the semiconductor and, traveling through the bulk of it, reaches the electric contact with the outside circuit.
3. The electrons return to the cell to complete the circuit and bring the dye back to its "normal" state via an electrolyte solution that helps carry electrons through the cell.

The cells are a "sandwich" in which two conducting glass slides are overlapped. The photoanode is coated with the layer of TiO₂ sensitized with the dye, and the other is coated with graphite in order to enhance the interaction with the electrolytic solution that is contained between the glass slides themselves.

Image Credit: Beyond Benign
Figure 1. Mechanism of a dye-sensitized solar cell.

Prep Work

1. Order/acquire two conductive glass plates coated with ITO per group.
2. Purchase frozen blackberries from your local grocery store (you can use fresh, but they tend to be a lot more expensive).
3. Prepare iodide electrolyte solution (0.5 M potassium iodide mixed with 0.05 M iodine in propylene glycol). Propylene glycol is the more environmentally friendly antifreeze, as opposed to ethylene glycol, which is not so friendly.
4. Use a multimeter with its setting placed on resistance (Ω) to determine which side of the glass slide has the indium tin oxide coating.
5. "Mask," or cover, the glass slide along all four edges with 2 mm of tape with conductive side facing up. Secure the slide to the table by adhering the free side of the tape to the tabletop at a 45° angle. Your slide should look like the diagram below.

Image Credit: Beyond Benign

6. Prepare TiO₂ paste and coat 80% of the inside of the conductive slide, leaving a 2 mm border all the way around (1 per student group). Paste is produced by mixing the following:
   1. Grind 15% TiO₂ and 0.7% trimesic acid (1,3,5-tricarboxylbenzoic acid) with a mortar & pestle.
   2. Add 84.3% water by mass slowly and continue to grind as you add more water to make the paste.

Image Credit: Beyond Benign Image Credit: Beyond Benign

7. Add 3 drops of the TiO₂ solution uniformly to the conductive glass and spread it across the glass using the body of a stirring rod until the glass is covered completely.
8. Allow the TiO₂ to dry for 10 minutes, then remove tape slowly to avoid damaging the conductive glass.

Engage

1. Ask the class what they know about solar cells and if they know anyone who has them on their house.
2. Hand out the Student Background Worksheet. Have students read article #1 (How Green Are Those Solar Panels, Really?) independently or read it as a class. You may also want to assign this article as homework reading prior to class.
3. Discuss the article and ask the students what they now think about solar panels.
4. Explain that, as good scientists, we should scientifically evaluate the process of making solar energy panels.
5. Give students time to read articles #2 (How Are Solar Panels Currently Made?) and #3 (What is green chemistry?) in the Student Background Worksheet and answer the questions.
6. Discuss the results.
7. Explain to students that green chemists obviously have cause for concern when it comes to solar panels, and that there is an opportunity for a chemist to develop a better way of making solar panels.
8. Explain that they are now going to make a dye-sensitized solar cell, which is one of the ways that green chemists are working to improve solar energy production.

Explore

1. Allow students to work in small groups (working in pairs is ideal, but can be adjusted based on class needs/materials).
2. Hand out the Student Lab Worksheet which has directions for the students to follow and post-lab questions. The video shows how to do the experiment as do steps 4–21.
   

   https://www.youtube.com/watch?v=_PyuApQnrWg

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3. Review the safety instructions with students:
   1. Handle glass slides with care to prevent injury to yourself and breakage of the glass.
   2. Do not ingest any materials (students may be tempted with the blackberries).
   3. It is recommended that only teachers should handle the knife when cutting out the center of the parafilm.
4. Place the blackberry in the aluminum pan.
5. Using a spatula, crush the blackberry to extract the juices. Scoop out the solid pulp.
6. Remove the glass slide containing the white TiO₂ coating from its bag. Handle the glass slide by the edges only. Determine which side has the TiO₂ coating.
7. Place the glass slide with the TiO₂ face down into the aluminum pan. Allow to sit for 3-5 minutes.
8. Remove the ITO-coated glass slide from its bag. Determine which side the coating is on by using a multimeter with its setting placed on resistance (Ω). The indium tin oxide coating is on the side of the slide that gives a non-zero reading on the multimeter.
9. Using the tip of a graphite pencil, lay down the carbon catalyst by shading the indium tin oxide coated side of the slide. The graphite may not leave a visible mark.
10. Remove the TiO₂ slide from the blackberry juice. Use the paper towel to gently blot the excess juices off the slide. Dry the slide as much as possible, but do not remove any of the TiO₂ coating. Do not wipe the slide, as this may remove some of the TiO₂ coating.
11. Remove and discard the wax paper backing from the parafilm and place the parafilm on top of the dye-coated TiO₂ slide. Use the eraser end of the pencil to press the parafilm to the glass slide in the area that borders the TiO₂.
12. Read the entire step carefully before beginning: Using a razor blade, carefully cut out the area of the parafilm that sits on top of the TiO₂. Press lightly with the blade, so that the conductive coating does not scratch off. Reinforce the parafilm seal around the edges of the TiO₂ area with the eraser end of the pencil.

Image Credit: Beyond Benign Image Credit: Beyond Benign Image Credit: Beyond Benign

13. Place 1-2 drops of the iodide electrolyte solution on top of the TiO₂. The parafilm should act as a wall that prevents the electrolyte solution from leaking out.
14. Place the ITO-coated glass slide on top of the TiO₂ slide so that the conductive sides face each other. Stagger the slides to expose as much of the glass slide as possible and to cover the entire TiO₂.

Image Credit: Beyond Benign

15. Use the 2 small binder clips to hold the slides together along the longer sides.
16. Carefully push back a small amount of the parafilm wall to expose a tiny part of the conductive side of the slide.
17. Place the multimeter probes on opposite ends of the solar cell's conductive glass slides.
    
    
    Image Credit: Beyond Benign
    
    
18. Place the solar cell under either sunlight or a flashlight.
19. With the multimeter set to measure electric potential, measure the voltage of the solar cell.
20. Record on the data table how many millivolts (mV) are generated for the light source.
21. Repeat steps 15–17 for 1 or 2 more light sources.
22. Have student groups share their data with the rest of the class. Sum up how many volts the class would make as a whole if the cells were connected in a series.
23. Disposal information: The paper towel, parafilm, parafilm paper backing, and used blackberry can be thrown into the trash. Teachers can rinse or wipe down the glass slides, aluminum dish, binder clips, spatula, knife, and multimeter probes (if necessary).

Reflect

1. Review student responses to post-lab questions.
2. Discuss what kinds of applications the volts generated by the class can power. Could the solar cell power a calculator, for instance?
3. Discuss the different locations that a small, flexible solar cell can be placed.