Organic semiconductors are promising materials for optoelectronic applications, such as solar cells and light-emitting diodes (LEDs). Increasing the efficiency of solar cells beyond the traditional limit of 30%, for example, could give a boost to the energy transition. This thesis describes research into crystals of the material TIPS-tetracene using optical experiments at low temperatures (0.3–60 K), in strong magnetic fields (up to 33 T) and with intense terahertz radiation. We use this to study singlet fission, a promising energy conversion process whose intermediate steps are not yet fully understood. In particular, we examine a short-lived intermediate state formed immediately after light absorption: a strongly coupled pair of triplet excitons. The results show that in TIPS-tetracene, several such triplet pairs exist, with distinct (magneto-)optical characteristics. Furthermore, some of these pairs appear to emit light themselves when they become localised in the crystal at low temperatures. Analysis of magnetic field effects demonstrates that different emission lines correspond to different triplet pairs and distinct trajectories through the material. Many of the observed properties can be understood using a relatively simple model based on the interaction between two molecules and their microscopic environment.
Vishal Bechai (1993) studied Physics and Astronomy at Radboud University. At the HFML-FELIX Institute, he conducted PhD research into singlet fission, a photophysical process in organic semiconductors that is of interest for innovation in solar cells.