Role of electronic structure of ruthenium polypyridyl dyes in the photoconversion efficiency of dye-sensitized solar cells: semiempirical investigation

Abstract

ZINDO/S calculations on cis-Ru(4,4'-dicarboxy-2,2'-bipyridine)₂(X)₂ and cis-Ru(5,5'-dicarboxy-2,2'-bipyridine)₂(X)₂ complexes where X = Cl^-, CN^-, and NCS^- reveal that the highest occupied molecular orbital (HOMO) of these complexes has a large amplitude on both the nonchromophoric ligand X and the central ruthenium atom. The lowest-energy metal to ligand charge transfer (MLCT) transition in these complexes involves electron transfer from ruthenium as well as the halide/pseudohalide ligand to the polypyridyl ligand. The contribution of the halide/pseudohalide ligand(X) to the HOMO affects the total amount of charge transferred to the polypyridyl ligand and hence the photoconversion efficiency. The virtual orbitals involved in the second MLCT transition in 4,4'-dicarboxy-2,2'-bipyridine complexes have higher electron density on the COOH group compared to the lowest unoccupied molecular orbital and hence a stronger electronic coupling with the TiO₂ surface and higher injection efficiency at shorter wavelengths. In comparison, the virtual orbitals involved in the second MLCT transition in 5,5'-dicarboxy-2,2'-bipyridine complexes have lesser electron density on the COOH group, leading to a weaker electronic coupling with the TiO₂ surface and therefore lower efficiency for electron injection at shorter wavelengths for these complexes.