Molecular design of distorted push–pull porphyrins for dye-sensitized solar cells

Abstract

A series of distorted push–pull meso-substituted porphyrin analogues with different acceptor groups and additional electron-donating substituents are investigated as organic sensitizers for application in dye-sensitized solar cells (DSSCs) using density functional theory (DFT) and time-dependent DFT approach. The donor was modified by interchanging methyl group with methoxy and extending the π-conjugation. The acceptor group was assessed based on cyanoacrylic (A analogues) or methylenemalonic (B analogues) acid groups. Benchmark calculations using YD1 as reference indicated that the best method to depict the excitation energies was with TD-ωB97X-D exchange–correlation (xc) functional while the computational protocol for computing redox potentials was found to be with the M06-2X xc functional based on vertical ΔSCF method. The absorption spectra of all the porphyrin analogues were red-shifted and produced higher oscillator strengths, especially at the Q-bands as compared to the reference molecule. Among the analogues, A2-OMe and B2-OMe are good candidates for sensitizers in DSSCs due to its larger hyperpolarizabilities, better light-harvesting efficiencies, proper matching of the ground-state oxidation potentials with respect to the \( I^{ - } /I_{3}^{ - } \) redox couple, and higher dipole moment of the adsorbed analogues. This study further enhances the role of theoretical calculations in the molecular design of sensitizers for DSSCs in an effort to produce a highly efficient dye.