Molecular-Level Understanding of Excited States of N-Annulated Rylene Dye for Dye-Sensitized Solar Cells

In organic push-pull dyes for photovoltaics, it is important to understand the character of intramolecular charge transfer states. Accordingly, the dynamics of charge carriers in photosensitizers based on donor-acceptor structures have been widely studied. Recently, photosensitizers based on N-annulated rylene derivatives have been extensively utilized in organic solar cells due to their outstanding optical properties and considerable power conversion efficiencies, but the excited-state dynamics in those materials have not been investigated yet. Here, we explore the ultrafast dynamics of intramolecular charge transfer (ICT) occurring in the excited states of a diphenylamine N-annulated naphthalene dicarboximide derivative (DND) and present the photovoltaic performance of DND. By using steady-state absorption/emission spectroscopy, femtosecond broadband transient absorption spectroscopy, and DFT calculations, we found that the ICT dynamics of DND vary sensitively depending on the solvent polarity, and the ultrafast transition from the Franck-Condon state to the intramolecular charge transfer state is correlated to the solvation dynamics. This correlation underlines that the ultrafast ICT is strongly coupled with the solvation, accounting for the dependence of the ICT dynamics on the solvent polarity.