Ultrafast charge-transfer exciton dynamics in C$_{60}$ thin films

Abstract

The high flexibility of organic molecules offers great potential for designing the optical properties of light-active materials for the next generation of optoelectronic and photonic applications. However, despite successful implementations of molecular materials in todays' display and photovoltaic technology, many fundamental aspects of the light-to-charge conversion have still to be uncovered. Here, we focus on the ultrafast dynamics of optically excited excitons in C$_{60}$ thin films depending on the molecular coverage and the light-polarization of the optical excitons. Using time- and momentum-resolved photoemission with fs-XUV radiation, we follow the depopulation dynamics in the excited states while simultaneously monitoring the signatures of the excitonic charge character in the molecular valence states. Optical excitation with visible light results in the instantaneous formation of charge-transfer (CT) excitons, which transform stepwise into energetically lower Frenkel-like excitons. While the number and energetic position of energy levels within this cascade process are independent of the molecular coverage and the light polarization of the optical excitation, we find quantitative differences in the depopulation times and the optical excitation efficiency. Our comprehensive study reveals the crucial role of CT excitons for the excited state dynamics of homo-molecular fullerene materials and thin films.