Molecular movie of dithizonatophenylmercury dissolved in methanol. The graph shows how the transmission (ΔmOD) of light through the sample changes with respect to time (t) and wavelength (λ). Image: Stellenbosch University
Charge propagation in a dye sensitised solar cell. Dye sensitised solar cells consist of a light absorbing material (dye) which is photo-excited by sunlight and donates charge to a nanoporous semiconductor that serves as the charge acceptor. Charge separation occurs at the dye/semiconductor interface thereby producing oxidised dye molecules, which are reduced by an electrolyte. Image: University of Stellenbosch
Dye-sensitised solar cell made by adsorption of the dye to the porous semiconductor surface. Image: Stellenbosch University
energy. The energy, when deposited into the molecule, in this case causes a rotation to occur within the molecule. This newly rotated molecule therefore looks different to the original one and consequently also has a different colour. Making a molecular movie of this orange to blue reaction allows scientists to determine the time it takes for the colour change to occur. This reaction takes place in roughly one picosecond, which is one millionth of one millionth of a second. In other words, seriously fast! The propagation of charges in a solar cell Using the electromagnetic radiation from the Sun to generate electricity is a concept that is well understood. In short, light from the Sun, when viewed on the quantum scale, consists of elementary particles known as photons. These photons interact with the solar cell and generate free charges (electrons) which, when connected in a closed electrical circuit, can do work. Currently there are various types of solar cells on the market, either based on high-purity silicon semiconductors or on blends of organic and inorganic materials. While the silicon solar cells are still more efficient, the novel cells based on organic matter offer the potential to be much cheaper and provide the option to be incorporated into flexible materials such a plastic films or even fabrics. Nevertheless, the efficiency is still a very important parameter to consider as it ultimately affects the plausibility of large-scale reproduction. In this regard, tracking how the charges propagate in a solar cell through a molecular movie is crucial, as it allows for viewing the various paths an electron can take in the solar cell. Knowing the paths and their probabilities gives one insight into the ultimate performance of a cell, as not all paths are beneficial for generating electricity. The projects discussed above form a subset of the research currently being conducted in the laboratory of Prof. Schwoerer, where the aim ultimately is to broaden our understanding of light-matter interactions. q Dr Gurthwin W Bosman is a laser physicist and researcher at the Laser Research Institute, Department of Physics, Stellenbosch University (SU). He works with Prof. Heinrich Schwoerer, who holds the South African Research Chair (SARChi) in Photonics at SU.
Image of a femtosecond pulse train. Image: Stellenbosch University
Image of the solar cell within the molecular movie setup. Image: Stellenbosch University
The author at work in the laboratory. Image: Stellenbosch University
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