Two 1D-2D asymmetric benzodithiophenes (BDTs) as donor building blocks are designed and synthesized, combining the advantages of both 1D and 2D symmetric BDTs. The photovoltaic properties of the asymmetric BDT-based polymers are improved greatly in comparison with corresponding symmetric BDT-based polymers. This work provides a new approach to design prospective organic optoelectronic materials employing the symmetry-breaking strategy.