Selective TDDFT with automatic removal of ghost transitions: application to a perylene-dye-sensitized solar cell model

We present an application of a selective time-dependent density-functional theory (TDDFT) scheme to a model for a dye-sensitized solar cell (DSSC) with a perylene sensitizer dye on a TiO2 nanoparticle model. In an earlier study on this system [De Angelis, Chem. Phys. Lett., 2010, 493, 323], it was reported that a large number of conduction-band excitations severely complicate the identification of the bright pi -> pi* excitations of the perylene dye. Here, we show that this problem can be overcome by applying a selective TDDFT solver based on a guess for the relevant orbital transition in combination with a suitable root-homing scheme. In order to enhance the efficiency of this algorithm we implement an automatic removal scheme for artificially low-lying long-range charge-transfer transitions from the TDDFT eigenvalue problem. A large number of such transitions appear in explicitly solvated systems in the form of inter-solvent or solvent-solute transitions. We study the characteristics of this removal scheme for a small water cluster and then apply it in a TDDFT calculation to a perylene-TiO2 nanoparticle model system and to perylene explicitly solvated in methanol. It is demonstrated that this scheme leads to a large reduction in the computational cost with essentially no loss in accuracy. Large differences in the effect of adsorption on the excited states of perylene dyes with two different anchor groups found in earlier work are confirmed.