C8: Photovoltaic & Dye-Sensitized Solar Cells
Looking at the examples on page 711, those compounds with alternating double and single bonds have conjugation. A and B show conjugation but C and D do not. For conjugated alkenes, the higher the degree of conjugation, the longer the wavelength of light that can be absorbed.
Photovoltaic cells made of semiconductors can absorb photons of light, resulting in electrons being knocked free from atoms and creating a potential difference. Semiconductor materials for such cells are often Group 14 elements such as silicon and germanium. Conductivity can be increased by doping the semiconductor with impurities from Group 15 elements such as phosphorous to create n-type semiconductors, or Group 13 elements such as boron to create p-type semiconductors. Page 712-713 go into detail about this. The below images come from the Pearson textbook.
In a dye-sensitized solar cell (DSSC), photons are absorbed by a dye in a way similar to the absorption of photons by chlorophyll in photosynthesis. Electrons in the dye are then injected into a titanium (IV) oxide nanoparticle layer, which conducts the electrons to the anode. Once a dye molecule as emitted its excited electron, it needs to gain another electron. To do this, dye-coated titanium (IV) oxide nanoparticles are immersed in a solution of I- ions. The I- ions release electrons to the dye on the titanium (IV) oxide, becoming oxidized to tri-iodide ions. They can accept electrons at the cathode, being reduced back to iodide ions. See page 714, Figure 7.