We report a systematic study of a family of statistical fluorene-carbazole-TITTBTT terpolymers with a variable fluorene-to-carbazole ratio. It has been shown that changing the molecular composition of the polymers affects significantly their characteristics such as absorption spectrum, extinction coefficients, position of the frontier energy levels and charge carrier mobility. Power conversion efficiencies of 6.4-7.0% were obtained for the best performing carbazole-rich and fluorene-rich terpolymers. Most importantly, we showed that optoelectronic properties (particularly, the extinction coefficients defining the light harvesting ability) and photovoltaic performances of conventional regioregular copolymers in the blends with [70]PCBM can be significantly improved simply by introducing a "rational disorder" in the polymer chains using statistical copolymerization of certain building blocks taken in appropriate ratios.