Fluorene-Based Sensitizers with a Phenothiazine Donor: Effect of Mode of Donor Tethering on the Performance of Dye-Sensitized Solar Cells

Two types of fluorene-based organic dyes featuring T-shape/rod-shape molecular configuration with phenothiazine donor and cyanoacrylic acid acceptor have been synthesized and characterized as sensitizers for dye-sensitized solar cells. Phenothiazine is functionalized at either nitrogen (N10) or carbon (C3) to obtain T-shape and rod-like organic dyes, respectively. The effect of structural alternation on the optical, electrochemical, and the photovoltaic properties is investigated. The crystal structure determination of the dye containing phenyl linker revealed cofacial slip-stack columnar packing of the molecules. The trends in the optical properties of the dyes are interpreted using time-dependent density functional theory (TDDFT) computations. The rod-shaped dyes exhibited longer wavelength absorption and low oxidation potentials when compared to the corresponding T-shaped dyes attributable to the favorable electronic overlap between the phenothiazine unit and the rest of the molecule in the former dyes. However, the T-shaped dyes showed better photovoltaic properties due to the lowest unoccupied molecular orbital (LUMO) energy level favorable for electron injection into the conduction band of TiO₂ and appropriate orientation of the phenothiazine unit rendering effective surface blocking to suppress the recombination of electrons between the electrolyte I₃^– and TiO₂. The electrochemical impedance spectroscopy investigations provide further support for the variations in the electron injection and transfer kinetics due to the structural modifications.