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The electrodes are made using a paste of extremely small particles of TiO2 that are spread out in a thin layer on transparent conductive glass electrodes. The particles provide a huge surface area for the dye molecules to stick to, and they provide an electron pathway for the generated electrical current to be collected. A significant portion of light is absorbed by the dye, even though only a single layer of dye molecules is attached to the surface. The dyed electrode goes from white to a dark purple.
The final steps include drying the electrode and then assembling the device with an additional electrode to form a "sandwich" solar cell. The device has two electrodes, the dyed TiO2 photoelectrode and a counter electrode. After the dye is excited with light, it gives an electron to the TiO2 layer and then gets an electron back from an electrolyte solution. The electrolyte solution is composed of potassium iodide and iodine and provides electrons to the dye attached to the TiO2 while generated current moves through an external load.
The solar conversion efficiency of these types of berry-sensitized TiO2 solar cells can reach 0.7 percent with demonstration cells reaching 1-3 mA/sq cm of photocurrent and 0.5 V when using an overhead projector as a simulated sun illumination source. The students will use the information about how much dye is used in each test cell, and then determine how many solar cells, and therefore how much power can be generated with the juice extracted from just one blackberry.
The dye-sensitized TiO2 solar cell was invented by Brian O'Regan and Michael Gratzel at the Ecole Polytechnique Federale De Lausanne in Switzerland. This approach has many advantages over other solar energy conversion technologies because of its simple device construction and inexpensive TiO2 particles and dyes which can be fine-tuned to increase their light absorbing properties. Although there is still much room for improvement, the state-of-the-art device converts solar energy into electricity with efficiencies over 10 percent, rivaling some Silicon-based technologies. These devices use specially prepared dyes that absorb a great deal more sunlight than the anthocyanin dyes extracted from blackberry juice.
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Diagram of the electron flow in a dye-sensitized TiO2 solar cell
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