Ultrafast Exciton-to-Polaron Conversion in Densely Packed Small Organic Semiconducting Molecules

In the rapidly developing field of organic photovoltaics, the material design and device engineering are key factors that eventually determine the device efficiency. Design of the active layer material and intermolecular interactions largely determine the efficiency of organic solar cells. In this study, the authors discuss ultrafast photophysics of four star-shaped molecules (SSMs) as benchmark materials with time-resolved photoinduced absorption and photoluminescence spectroscopy as experimental tools. The authors show that efficient exciton-to-charge conversion occurs in SSM films even without an external acceptor. This results in the lowering of the Coulomb binding between intermolecular electron- hole polaron pairs which, in turn, can lead to an increased open-circuit voltage. The findings suggest that promoting intermolecular interactions in films of small organic molecules is one of the pathways to highly efficient organic solar cells.