A comparative theoretical investigation of ruthenium dyes in dye-sensitized solar cells

Density functional theory (DFT) and time-dependent DFT method have been employed to analyze the tetradeprotonated complexes of polypyridyl ruthenium dyes cis-Ru(4,4′-COOH-2,2′-bpy)2(L)2 (L = NCS, CN and dcbpy; dcbpy = 2,2′-bipyridine-4,4′-carboxylate). The absorption spectra of these complexes in ethanol solution can be well reproduced with theoretical method. The nature of their absorption bands are assigned unambiguously. According to the computational results, energy levels of the singlet and triplet excited states of cis-Ru(4,4′-COO−-2,2′-bpy)2(NCS)2 are advantage for the charge injection. Its intense and broad absorption bands as well as favorable excited-state energy levels are key causes for its outstanding efficiency.