Open-Circuit Voltage Degradation by Dye Mulliken Electronegativity in Multi-Anchor Organic Dye-Based Dye-Sensitized Solar Cells

Four dyes consisting of a phenoxazine–triphenylamine double-donor assembly, on which one to four cyanoacrylic acid acceptor groups were grafted, have been synthesized and characterized. The UV–vis measurements revealed a complex intramolecular charge transfer, a feature supported by the density functional theory calculations. We found that both highest occupied molecular orbitals and lowest unoccupied molecular orbitals negatively shift as more anchors are attached to the donor core, leading to an increase of the dye Mulliken absolute electronegativity. Dye-sensitized solar cell batches impregnated with two different electrolytes have been fabricated with the dyes. Their open-circuit voltage was found to decrease with the increase in the anchor number due to the electron interception by the electrolyte. The dye participation in the electron interception is deduced from the Bode diagrams and the dark current analysis. Moreover, the Nyquist spectra revealed that the dye structure affects the electron diffusion in TiO2. We show that the dye absolute electronegativity is responsible for the modulation of electron diffusion, which stands for the mechanism by which the dye structure catalyzes the electron interception by the electrolyte. Consequently, the increase of the Mulliken dye electronegativity as more anchors are attached to the donor core is identified as the cause of the open-circuit voltage degradation observed experimentally.