Acetonitrile Solution Effect on Ru N749 Dye Adsorption and Excitation at TiO<sub>2</sub> Anatase Interface

We investigated stable structures and photoexcitation character of Ru N749 dye (black dye (BD)) adsorption to TiO<sub>2</sub> anatase (101) interface immersed in bulk acetonitrile (AN) solution, a most representative electrode interface in dye-sensitized solar cells (DSCs). Density-functional-theory-based molecular dynamics (DFT-MD) with explicit solvent molecules was used to take into account the fluctuations of solvation shells and adsorbed molecules. We demonstrated that BD adsorption via deprotonated carboxylate two anchors (d2) is the most stable at the interface, while the one protonated carboxyl anchor (p1) has the average energy only slightly higher than the d2. This indicates that the p1 state can still coexist with the d2 under equilibrium. It is in contrast with the calculated large stability of the p1 in vacuo. Inhomogeneous charge distribution and anchor fluctuation enhanced by AN solution causes this d2 stabilization. The calculated projected densities of states and the photoabsorption spectra clearly show that the d2 state has larger driving force of the electron injection into the TiO<sub>2</sub>, whereas the photoabsorption in the wavelength region over 800 nm, a characteristic of BD sensitizer, is mainly attributed to the p1 state even in the AN solution. Consequently, the better performance of BD DSC can be understood in terms of the cosensitizer framework of the d2 and p1 states.