Role of Explicitly Included Solvents on Ultrafast Electron Injection and Recombination Dynamics at TiO₂/Dye...

Solvent effects are important for photovoltaic systems like dye-sensitized solar cells (DSCs) but remain largely unexplored, partly due to the complexity of explicitly including solvent molecules in atomistic simulations. To address these issues, we have systematically investigated the solvent effects in practical solar cells using ab initio excited state dynamics simulations. In this computational protocol and the extended model system, we considered giving a novel perspective on the excited state changes in response to solvation and finite temperature at the heterointerface of DSCs. By directly comparing the geometric stability of interface bonding, photoabsorption, interfacial electronic structure, and dynamics of vacuum and solvent systems, we obtain useful insights into how solvents influence the key factors that determine the efficiency of DSCs. Solvents significantly enhance the intensity of visible light absorption of chromophores (∼2 times) through two effects: (a) by inducing changes to the dye molecule structure due to intermolecular dye-solvent interactions, and (b) by the dielectric screening of the solvent. Furthermore, by adsorbing onto the TiO₂ surface, solvent molecules adjust the interfacial band alignment to a favorable level and screen out the attraction force between injected electrons in the semiconductor substrate and holes left on the chromophore to a large extent, dramatically slowing down the recombination process (>8 times). Our findings provide a comprehensive picture of the explicit solvent effects on individual energy conversion steps in DSCs at the microscopic scale and lead to more accurate prediction of the performance of nanodevices in practical environments, contributing to the optimization of realistic renewable energy devices.