Τhe STEM Activity took place the days 26 and 27/4/2022, on the Science lab of 2nd General Lyseum of Oreokastro – Greece.
Teachers: Georgios Chatzigeorgiou (Biologist), Eftychia Karagianni (Physicist), Polyxeni Siorou (Physicist)
Mentors: Alexandros Terzopoulos, Christos Chatzigeorgiou
Students: E.K., D.K., E.A.,S.V., A.K., R.I.
We would like to express our utmost gratitude to Mr Paschalis Karakaris, Principal of the 2nd General Lyceum of Oraiokastro, for her continuous help and support in implementing the STEM activity in our school.
In this activity we study an alternative technology for the production of electricity from the conversion of solar energy, namely dye-sensitized solar cells (DSSC or Grätzel cells). Current commercially available photovoltaics consist of crystalline plates or films of semiconductors, typically silicon or GaAs. Dye-sensitized cells, on the other hand, rely on the photoelectrochemical system resulting from the adsorption of a dye (photosensitizer) to a mesoporous dielectric material (photodiode, usually titania TiO2), together with a cathode and an electrolyte. The electronic properties of the system are analogous to the conventional semiconductor; these involve transfer of electrons to the conduction band of the photodiode’s TiO2 to the photochemically excited photosensitizer.
In our proposed activity students constructed themselves some typical Grätzel cells (using a photodiode from FTO glass and pulverized TiO2, a cathode from identical FTO glass with surface-deposited soot, and iodine-triiodide electrolyte). The differentiation of our study concerns the photosensitizer, where instead of the expensive and toxic ruthenium dyes usually listed in literature, we used natural dyes extracted from different plants. The photovoltaic response of the cells was investigated by the students with measurements to record the current-voltage curve, from which characteristic quantities to compare the cells’ performance were obtained.
This activity explores topics such as new technologies for more accessible solar energy use, semiconductor properties, circuit measurements, and the use of natural materials in devices with positive environmental prospects. The goal is the successful construction of dye-sensitized cells in the school laboratory and the comparison of plant dyes as photosensitizers, wishing to demonstrate the most suitable for photovoltaic applications. The Municipality of Oraiokastro has provided financing for this work which has been an easy endeavor, as low cost is another of the advantages of the solar cell type that we have studied.
Experimental Methods—Materials
Steps
1. Add 6 g of TiO2 powder in a mortar, followed by 2–3 mL of vinegar. Grind until all the clumps are broken. Continue adding vinegar in 1 mL integrals whilst continuously grinding, until a total of 9 mL is added. The mixture (paste) must be uniform at the end.
2. Gently mix a drop of dishwashing liquid with 1 mL of distilled water. Add this solution slowly over the paste, taking care not to produce bubbles. Allow the suspension to equilibrate.
3. Clean two conductive SnO2-coated glass slides using ethanol. The students need to previously identify the conductive side with a multimeter (the conductive side must have a resistance of 10–30 Ω).
4. Place the two slides on the bench, one with the conductive side up and the other conductive side down, and cover the edges with adhesive tape (covering 5-8 mm) confirming that there are no air bubbles between the tape and glass.
5. Using a glass rod, spread a thin uniform layer of paste on the upper edge of the conductive side. Allow the film to dry slightly and remove the tape.
6. Dry the slide by placing it on a heating plate (450 °C for approx. 30 minutes), with the conductive side facing up. The paste layer will initially turn purple-brown and then eventually white. When this happens, stop the heating, keeping the slide on the hotplate.
7. After it cools down to room temperature, record the surface of the membrane (the piece of slide covered by the paste).
8. To prepare the counterelectrode, clean a second conductive glass slide and then apply the catalytic carbon coating (i.e. rub lightly with a soft pencil) on the conductive side.
9. Hold the conductive side with tweezers over a low flame. Let soot collect on the surface of the plate for no more than 30 seconds. Change the position on the plate with the tweezers and cover the rest of the corner with soot in the same way, making sure that the whole slide is covered.
Now that the electrodes have been prepared, we manufacture the dye-sensitized solar cell.
Cell construction and evaluation
A. Use a spatula to mix some blueberries or cherries in a beaker. Then filter the solution into a Petri dish using a coffee paper filter, adding a few drops of distilled water if necessary.
B. Using tweezers, place the photoelectrode in the Petri dish, with the conductive side facing down, taking care not to scratch the film.
C. When the staining is completed, carefully remove the plate and make sure that there are no white spots to be seen. Rinse the slide with ethanol and dry it.
D. Place the counterelectrode down on the slide, keeping a distance between the slides.
E. Attach the connecting clips to the edges of the slides. Place a few drops of electrolyte along the edge and let it pass through the strip, opening the clips slightly. The photovoltaic is now ready for operation.
We then measured the performance of the cell under a halogen lamp, by orienting the cell so that the photoelectrode sees the lamp in a consistent manner. We used a multimeter to measure the open circuit potential and short circuit current.
We then connected the cell to a 500 Ω potentiometer to create the circuit shown in the diagram, and sequentially increased the resistance through the potentiometer while using the multimeter to measure the voltage and current.
