A novel soluble conjugated polymers, P2, with coplanar thiophene-phenylene-thiophene unit is designed and synthesized as suitable active material used in tandem cells to compensate the poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C71 butyric acid methyl ester bulk-heterojunction cell in this paper. P2 polymer bears advantages in both low optical bandgap (1.7 eV) and high hole mobility properties ( from field-effect transistor measurement). Furthermore, the electrochemical studies of P2 indicate desirable highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) band structure that enables a high open circuit voltage when pairing with PCBM acceptor. The best power conversion efficiency of this polymer solar cell thus far based on system with a weight ratio of 1:3 reached 4.4% with a short circuit current density of , an open circuit voltage of 0.81 V, and a fill factor (FF) of 0.53 under air mass (AM) 1.5 G . The preliminary data of the tandem cell with indium tin oxide (ITO) configuration has reached of , of 1.33 V, FF of 0.56 and an overall efficiency of 4.6% under AM 1.5 G .