Relating the Physical Structure and Optoelectronic Function of Crystalline TIPS-Pentacene

Theory and experiment are combined to investigate the nature of low-energy excitons within ordered domains of 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-PEN) polycrystalline thin films. First-principles density functional theory and many-body perturbation theory calculations, along with polarization-dependent optical absorption spectro-microscopy on ordered domains, show multiple low-energy absorption peaks that are composed of excitonic states delocalized over several molecules. While the first absorption peak is composed of a single excitonic transition and retains the polarization-dependent behavior of the molecule, higher energy peaks are composed of multiple transitions with optical properties that can not be described by those of the molecule. The predicted structure-dependence of polarization-dependent absorption reveals the exact inter-grain orientation within the TIPS-PEN film. Additionally, the degree of exciton delocalization can be significantly tuned by modest changes in the solid-state structure and the spatial extent of the excitations along a given direction is correlated with the degree of electronic dispersion along the same direction. These findings pave the way for tailoring the singlet fission efficiency of organic crystals by solid-state structure.