Dynamics of Photoinduced Processes in Dye-Sensitized Nanocrystalline Semiconductor Films--Reactions in the Photoactive Part of the Grätzel Solar Cell

Abstract: The key materials used to convert solar energy into electricity in the Grätzel-type solar cells are dye molecules and titanium dioxide particles attached to each other. The former are similar to those in common products like purple pigments in blueberry juice, while the latter are like sunscreen, white paint, or toothpaste. Their size, measured in nanometers, is a hundred thousand times smaller than a strand of hair. This thesis presents a series of investigations that attempt to give a better understanding of how the dye molecules, energized by sunlight, inject electrons into the titanium dioxide particles, which is an essential reaction in the functioning of the solar cell. As this reaction is one of the fastest ever studied, we have to use the world’s fastest camera. This camera uses laser flashes as short or even shorter in duration as the speed of the reaction, which occurs already in femtoseconds. One femtosecond is 10-15 seconds, that is, 0.000000000000001 seconds, which is to a second as a second is to 32 million years. With such a camera we are able to monitor the track of the electron in slow motion. We have seen for several dye molecule-titanium dioxide particle couples how the electron leaves the molecule and arrives into the particle as early as femtoseconds after laser flash illumination. For certain systems, the electron goes back to the dye molecule immediately after being injected in the titanium dioxide particle, without performing work in an outer electrical circuit. Obviously, those dyes are not suitable for the solar cell. The ultimate goal is to fully understand the mechanism of the electron transfer reaction, and according to the findings modify the materials to show better performance in the solar cell.