Structural and electronic study of iron-based dye sensitizers for solar cells using DFT/TDDFT

Ruthenium-based molecules have proven their efficiency as photosensitizers in dye-sensitized solar cells. However, due to the high cost and scarcity of this noble metal, in this work we investigated the effects of replacing the ruthenium in photosensitizers with iron because it is less expensive and more abundant. DFT and TD-DFT methods were used to explore the resulting systems. The B3LYP functional was employed to compute the ground-state geometries and the frontier molecular orbitals for four complexes of general formula M(Rbpy)₂S₂N₂C₂ (M = Ru or Fe; R = COOH or COOEt). We also used TD-M06 to investigate the electronic properties and to simulate the absorption spectra of these Ru-based and Fe-based complexes. Finally, CPCM was applied to explore the effect of DMF solvent. The HOMOs of these Ru-based and Fe-based molecules were found to have metal d orbital and π(S₂C₂N₂) orbital character, while their LUMOs had π*(R-bpy) orbital character. In addition, values of the light-harvesting efficiency (LHE), open circuit voltage (V _oc), and driving force (∆G ^inject) were calculated for the Ru-based and Fe-based molecules. According to our results, the maximum absorbance, the LHE, V _oc, and ∆G ^inject values for complexes 2 and 4 (Fe-based dyes) are very close to those of complexes 1 and 3 (Ru-based dyes). Thus, our studies indicate that the Ru in photosensitizers can be replaced with the much less expensive metal Fe, as the resulting Fe-based dyes appear to be promising candidates for use in solar cells.