Tuning the photovoltaic parameters of β-substituted porphyrin analogues: An experimental and theoretical approach

Spectroscopic and theoretical techniques were used to examine the effects of adsorption solvents and electrolyte additives on the photovoltaic performance of dye sensitized solar cells of two β-substituted zinc porphyrin analogues (A1-H and A1-M), where A1-M has an added methyl group at the phenyl ring attached to the meso-position of the porphyrin macrocycle. These results showed that the different solvents used in the sensitization process affects how the dyes are aggregated and how much of the dyes are adsorbed on the semiconductor surface. The highest efficiency of 4.2% (30 min soaking time) and 4.0% (1 h soaking time) was achieved for A1-H and A1-M analogues, respectively. The hypsochromic shift observed in the UV/Vis spectra of the analogues when THF was used as a solvent was due to the partial deprotonation of the hydrogen at the carboxylic moiety of the analogues induced by the interaction of the THF molecule. Theoretical calculations at the B3LYP/6-31G(d) level of theory indicated that when THF was used as an adsorption solvent, it tends to form six- or five-coordinate complexes causing the dyes to be largely separated when attached to the semiconductor thus lowering the amount of dye. The dye molecular plane of A1-M was 13° closer to the TiO2 surface than A1-H due to steric hindrance introduced by the methyl group. This increases the surface area occupied by the A1-M dye, as indicated by the smaller amount of dye adsorbed on the TiO2 surface. The addition of 4-tert-butylpyridine or the replacement of lithium ions with guanidinium thiocyanate decreased the solar cell efficiency by 12 and 37%, respectively, which was attributed to a decrease in the electron injection processes.