Theoretical investigation of the photophysical properties of black dye sensitizer [(H₃-tctpy)M(NCS)₃]^- (M = Fe, Ru, Os) in dye sensitized solar cells

The electronic properties and optical absorption spectrum of the photovoltaic sensitizer [(H₃-tctpy)M(NCS)₃]^- (M = Fe, Ru and Os) in ethanol solvent are investigated using density functional theory/time-dependent density functional theory. The calculated absorption spectrum is in agreement with experiment for ruthenium complex, thus allowing an assignment of UV-visible spectral features of the dye. Red shift in the absorption spectrum is observed moving down the group from iron to osmium. The observed red shift in ruthenium and osmium complexes appears to be related to the destabilization of the occupied orbitals with t_2g-NCS π* character. In osmium complex, the increase in the orbital splitting energy d (t_2g-e_g) is found due to the destabilization of the e _g orbitals. The weak d-d transitions are observed only in iron and ruthenium complexes.