Molecular engineering of new phenothiazine-based D-A-π-A dyes for dye-sensitized solar cells

A new series of phenothiazine-based D-A-pi-A dyes PZ-1-4 have been designed, synthesized and fully characterized by H-1, C-13 NMR, HRMS, UV-vis, fluorescence, and cyclic voltammetry spectroscopy. Benzothiadiazole was introduced into the D-pi-A dyes as an additional acceptor. Various donors and new pi-spacers have been connected to phenothiazine skeleton on opposite sides to generate PZ-2-4 dyes. The DSSC devices have been systematically investigated by photoelectrochemical experiments, the solar cell of PZ-4 with a simple hexyloxyphenyl group achieves the highest photoelectric conversion efficiency of 6.35%. Density functional theory (DFT) calculation results demonstrate that the large dihedral angle (36.3 degrees) between the pi-spacer and phenothiazine in PZ-4 induces better charge separation. Its largest semicircle at intermediate frequencies in electrochemical impedance spectroscopy (EIS) prevents the recombination of photoelectrons injected into the titanium dioxide (TiO2) conductive band with I-3(-) in electrolyte, which might be caused by the blocking interaction of the hexyloxy group dye.