Thermal Fluctuations on Förster Resonance Energy Transfer in Dyadic Solar Cell Sensitizers: A Combined Ab Initio Molecular Dynamics and TDDFT Investigation

Förster resonance energy transfer (FRET) is a key process in dyadic dye-sensitized solar cells (DSSCs) where an "antenna" donor has the role of collecting photons and redirecting the captured energy to an adsorbed-dye acceptor unit. Despite its popularity in, e.g., biology, where FRET rates are used to derive structural information on fairly complex systems, relatively few studies have appeared in the DSSCs field. These were based, to the best of our knowledge, either on a static modeling of FRET or on the so called isotropic regime assuming an isotropic motion of the donor/acceptor units and uncorrelated donor/acceptor relative distance and orientation. In this paper we carry out a combined Car-Parrinello molecular dynamics and TDDFT investigation to unravel the impact of thermal fluctuations on FRET in two dyadic carbazole-phenothiazine dye sensitizers. Both isolated and full-packed adsorption conditions are considered, mimicking the dye adsorption topology on TiO<inf>2</inf>. Results are discussed in relation to the above mentioned models and rationalized in terms of the structural differences of the considered dyes. We find a considerable difference between the FRET rates calculated at zero temperature and the results obtained by including thermal fluctuations, highlighting an important role of the latter in determining FRET rates in dyadic donor-acceptor dye-sensitized solar cells.