Quantum chemical study of the effect of π-bridge on the optical and electronic properties of sensitizers for DSSCs incorporating dioxythiophene and thiophene units

Twelve molecules were theoretically studied through density functional theory with the M06 density functional and the 6-31G(d) basis set. The molecular systems have potential application as sensitizers for dye-sensitized solar cells; these molecular structures are composed of triphenylamine as the donor moiety, different conjugation orders of thiophene and dioxythiophene as the pi-bridge, and cyanoacrylic acid as the acceptor moiety. This study focused on the effect of the pi-bridge on the properties of interest. Ground-state geometry optimization, the highest occupied molecular orbital, the lowest unoccupied molecular orbital, and their energy levels were calculated and analyzed. Absorption wavelengths, vertical energy, oscillator strength, and electron transitions were calculated through time-dependent density functional theory with the M06-2X and CAM-B3LYP functionals using the 6-31G(d) basis set. Driving force of injection (Delta G(inj)) was calculated and analyzed from the ground-state oxidation potential of the dye and the energy associated with the maximum absorption wavelength. As an important element presented in this study, chemical reactivity parameters are discussed, such as chemical hardness, electrodonating power, electroaccepting power, and electrophilicity index. In conclusion, a reliable methodology was presented and discussed to predict properties in triphenylamine derivative dyes considering the modification of the pi-bridge.