The guidelines to pursuing and developing p-type conjugated polymers for organic solar cell applications, were studied. The requirements of specific intrinsic properties necessary for an ideal donor material include sufficient solubility to guarantee solution-processability as well as miscibility with n-type materials, a low optical band gap for a strong and broad absorption spectrum to capture more solar energy, and high hole mobility for accelerating charge transport. This in turn allows a thicker active layer required for increased light harvesting as well as reduced charge recombination and series resistance. Structural analysis of the low band gap conjugated polymers reveals that an alternating D-A arrangement is essential and must be combined with the necessary newly designed donor segments composed of multicyclic aromatic rings with enforced planarity. n-type polymers hold great promise and opportunity to realize a high-performance all-polymer BHJ solar cell if it is comparable to and optical properties superior to those of C60 derivatives can be realized.