DFT/TD-DFT molecular design of porphyrin analogues for use in dye-sensitized solar cells

Density functional theory (DFT) and time-dependent DFT calculations have been employed to model Zn meso-tetraphenylporphyrin (ZnTPP) complexes having different beta-substituents, in order to design an efficient sensitizer for dye-sensitized solar cells. To calculate the excited states of the porphyrin analogues, at least the TD-B3LYP/6-31G* level of theory is needed to replicate the experimental absorption spectra. Solvation results were found to be invariant with respect to the type of model used (PCM vs. C-PCM). Most of the electronic transitions based on Gouterman's four-orbital model of ZnTPP-A and ZnTPP-B are pi -> pi* transitions, so that cell efficiency can be enhanced by increasing the pi-conjugation and electron-withdrawing capability of the beta-substituent. This proposition was tested by inserting thiophene into the beta-substituent of ZnTPP-A to form a new analogue, ZnTPP-C. Compared with ZnTPP-A and ZnTPP-B, ZnTPP-C has a smaller band gap, which brings LUMO closer to the conduction band of TiO2, and a red-shifted absorption spectrum with higher extinction coefficients, especially in the Q-band position.