By introducing the narrow band gap unit(dithiophene-benzothiadiazole) to the organic dye system, two novel organic dyes(DD2, DH2) with two-dimensional structures were designed and synthesized. The influence of dithiophene-benzothiadiazole unit acting as side chain or bridge group on the photophysical, electrochemical, and photovoltaic properties of the two dye-sensitizers was studied systematically. Two as-synthsized dyes with dithiophene-benzothiadiazole unit exhibited broad absorption spectra. Under one s...