Theoretical insight into the spectral characteristics of Fe(II)-based complexes for dye-sensitized solar cells: Functionalized bipyridyl chromophores

Fine tuning of electronic properties by virtue of the proper choice of functionalized chromophores would improve performance of absorption spectrum for dye-sensitized solar cells (DSSCs). We report herein a theoretical investigation on a series of bipyridyl Fe(II)-based complexes with general formula [FeLL′(NCS)2] (L, L′ = 4,4′-substituted-2,2′-bipyridine) by density functional theory (DFT) and time-dependent DFT (TD-DFT). Molecular geometries, electronic structures, and optical absorption spectra are predicted in methyl cyanide (MeCN) solution. [FeLL′(NCS)2] derivatives display Fe → bipyridine metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT) in the range of 350–700 nm. Structural modifications by enhancing π-conjugation and introducing heteroaromatic groups on ancillary ligands lead to the upshift of most of molecular orbital energies, and remarkable increase in absorption intensity. Compared with Ru(II)-based sensitizers, bipyridyl Fe(II)-based complexes exhibit similar absorption spectral characteristics but better improving trend along with structural optimizations.