Saddle-shaped porphyrins for dye-sensitized solar cells: new insight into the relationship between nonplanarity and photovoltaic properties

Abstract

We report on the theoretical and experimental studies of the new dye-sensitized solar cells functionalized with 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin zinc(II) complexes bearing 2- and 8-bromo substituents at the β positions. In agreement with the results of TD-DFT calculations, the absorption maxima of di- and octa-brominated Zn(II) complexes, ZnTCPPBr₂ and ZnTCPPBr₈, exhibited large red-shift compared to that of the non-brominated free base porphyrin (H₂TCPP). Furthermore, DFT calculations showed that the higher stabilization of the LUMO levels relative to the HOMO ones makes the HOMO–LUMO gap of the brominated Zn-porphyrins models smaller compared to that of the nonbrominated counterparts, which explains the red shifts of the Soret and Q bands of the brominated compounds. Solar cells containing the new saddle-shaped Zn(II) porphyrins were subjected to analysis in a photovoltaic calibration laboratory to determine their solar to electric energy conversion. In this regard, we found that the overall conversion efficiency of ZnTCPPBr₈ adsorbed on TiO₂ nanocrystalline films was 5 times as large as that of ZnTCPPBr₂ adsorbed on the same films. The effect of the increasing number of Br groups on the photovoltaic performance of the complexes was compared to the results of computational methods using ab initio DFT molecular dynamics simulations and quantum dynamics calculations of electronic relaxation to investigate the interfacial electron transfer (IET) in TCPPBr_x/TiO₂-anatase nanostructures. Better IET in ZnTCPPBr₈ compared to ZnTCPPBr₂, and in H₂TCPP was evaluated from interfacial electron transfer (IET) simulations. The IET results indicate that electron injection in ZnTCPPBr₈–TiO₂ (τ = 25 fs) can be up to 5 orders of magnitude faster than ZnTCPPBr₂–TiO₂ (τ = 125 fs). Both experimental and theoretical results demonstrate that the increase of the number of bromo-substituents at the β-pyrrole positions of the porphyrin macrocycle created a new class of porphyrin-based DSSC, which exhibits a remarkable increase in the photovoltaic performance compared to non-brominated porphyrins.