Towards conjugated polymers with low exciton binding energy

We all love solar cells. Some of them are already placed on rooftops as big, heavy panels, known as silicon PV cells. In the laboratory, we are developing a new type of solar cells, known as plastic solar cells, or PSCs. Typically, it will only have less than 5% of the weight of a silicon PV panel, of the same area. PSCs generate voltage and electricity under illumination with (sun)light. Initially, a photon is absorbed by a photoactive, i.e. colored, molecule. The energy of the photon brings this molecule to an electronically excited state, which -in the language of physics- means that a bound electron-hole ( and + charges) pair is created. Such a bound electron-hole pair is named an exciton. Subsequent splitting of excitons gives free holes and electrons, which find their way out of the solar cells and drive electrical currents. However, this exciton splitting needs energy. The minimum energy it needed, is what we call the exciton binding energy, since this electron-hole pair is bound by the force between opposite charges. Efficient exciton splitting requires a low exciton binding energy. If it is low enough, the exciton would automatically split into free charges at room temperature. To learn how to make molecular materials that do this automatic splitting at room temperature, is the main goal of the work described in this thesis. We test new design principles of conjugated polymers, to see if we can reduce the exciton binding energy of organic photovoltaic materials.