On the performance of ruthenium dyes in dye sensitized solar cells: a free cluster approach based on theoretical indexes.

The performance of ruthenium dye sensitized solar cells (DSSC) with different types of ligand was studied by means of a theoretical model where the ruthenium complex is bound to two [Ti(OH)3](+) units, instead of the more usual cluster TiO2 model. Electron injection is proposed to proceed from a thermalized (3)MLCT state rather than from higher vibrational excited states. The efficiency of the dye linked to the two [Ti(OH)3](+) units was determined in terms of a global index (ξ), calculated as the product of three theoretical indexes (FI) built from the results of time-dependent density functional theory (TDDFT) calculations. The index considers the harvested and delivered energy (F1), the charge transferred to the semiconductor (F2), and dye regeneration (F3). The results show that this set of parameters is unique for each dye, and allows the comparative evaluation of the performance of a series of dyes, with a different ancillary ligand at each stage of the cell operation. The method provides insights that can help explain the improved performance of N3 and black dyes compared to other dyes.