In recent years a lot of research has been done on solar cells based on organic semiconductors. These materials are fundamentally different from the more traditional inorganic semiconductors such as silicon: organic semiconductors are based on carbon chemistry. Due to their special chemical structure they are able to conduct current. Potential advantages of solar cells made from organic semiconductors are their flexibility and low weight. They can also be produced cheaply, for example by printing the materials with an inkjet printer. However to make organic solar cells really successful the efficiency with which light in the solar cells is converted into electrical current must be further increased.
One of the most important causes of the loss of efficiency in organic solar cells is the recombination of positive and negative charges (holes and electrons). When holes and electrons recombine, these charge carriers disappear, as a result of which the solar cell provides less current and power. Physicists have proposed various conflicting recombination mechanisms. For example, recombination could occur in the bulk of the semiconductor, or instead at the boundary with the contacts. Both processes would require a different approach to increase the efficiency. So researchers must know which mechanism predominates.
Bulk or electrode?
FOM PhD student Niels van der Kaap (University of Groningen) and scientists from University of Potsdam (Germany) have studied the consequences of both mechanisms. The research team in Potsdam determined the strength of bulk recombination. Van der Kaap subsequently used their experimental data to study the different recombination processes with the help of a computer simulation. With this he demonstrated that bulk recombination is far more important than recombination at the boundaries with the electrodes. This even applies to solar cells with very weak bulk recombination. The FOM focus group 'Next-Generation Organic Photovoltaics' in Groningen is now trying to further suppress this recombination process by developing materials with a higher dielectric constant – as a result of this electrons and holes attract each other less strongly and so the recombination is reduced.