A series of organic D-pi-A sensitizers composed of different triarylamine donors in conjugation with the thienothiophene unit and cyanoacrylic acid as an acceptor has been synthesized at a moderate yield. Through tuning the number of methoxy substituents on the triphenylamine donor, we have gradually red-shifted the absorption of sensitizers to enhance device efficiencies. Further molecular engineering by the substitution of two hexyloxy chains in place of the methoxy groups allows fabricating a solvent-free dye-sensitized solar cell with a power conversion efficiency of 7.05% measured under the air mass 1.5 global sunlight. Time- and frequency-domain photoelectrical techniques have been employed to scrutinize the aliphatic chain effects with a close inspection on effective electron lifetime, diffusion coefficient, and diffusion length.