Zn(II)-Porphyrin–Squaraine Dyads as Potential Components for Dye-Sensitized Solar Cells: A Quantum Chemical Study of Optical and Charge Transport Properties

In the present work, we theoretically investigate the push–pull effect in new dyads of chromophores formed by substituted Zn(II) porphyrin (P) and squaraine (SQ) that could be potential components of dye-sensitized solar cells (DSSCs). The effect of electron-donating moieties (amine, methoxy, and methyl) bound to porphyrin in meso-position C20 on the optical and charge transport properties of nine dyads (D1–D9) was studied, as well as the formation reaction of these. After a calibration procedure with 13 density functionals, the electronic spectra of the dyads were computed (TPSSh/def2-TZVP) with time-dependent density functional theory (TD-DFT). Dyads with amines (D5–D9) show a push effect denoted by the red-shifting of the Q-bands (up to 727 nm), while dyads with methyl or methoxy substituents (D1–D4) show no significative changes. Charge transfer bands computed with both TD-DFT and perturbative ΔSCF approaches preferably show a P → SQ transition, which indicates that porphyrin is a better electron-donating moiety (push effect), while squaraine is a better electron-withdrawing moiety (pull effect). Charge transport properties of the dyads calculated with the Keldysh nonequilibrium Green’s function (NEGF) formalism show a similar trend, with the current shifting P → SQ more favorably for dyads with amine groups. The latter would be more convenient for DSSCs.