Ultrafast Microscopy of Exciton Dynamics in Squaraine Films

Squaraine- (SQ) based organic semiconductor materials are promising for light-based applications, such as photovoltaics, photosensors, or biomedicine. Their popularity builds on the favorable optical properties of individual SQ molecules, as well as the ability to tune their optical functionality by changing the structural arrangement of individual molecules. Despite this potential, their optical properties have so far only been explored with conventional (steady-state) spectroscopy and only for the lowest-energy excitations. Their dynamical behavior upon photoexcitation, which is often more relevant than the static excitation energy, remains unexplored so far. To close this gap, we will investigate the dynamics following photoexcitation of SQ molecular crystals with ultrafast transient absorption microscopy (TAM), and compare the dynamics of different crystal structures. The TAM technique allows to observe light-matter interaction processes on their natural length- and time scale by providing (sub-) micrometer spatial and femtosecond temporal resolution. Interpretation of the TAM results will be supported by simulations within the essential states model and density functional theory. The aim of the project is a complete description of the excited-state optical properties of SQ organic semiconductors, including energetically higher states, ultrafast charge and energy transfer, and exciton-phonon coupling. The project is an international collaboration of TU Graz, University of Graz, University of Linz and Temple University (Philadelphia, USA).