Polymer solar cells (PSCs) have made a great achievement in the past decades. As a result, a record high power conversion efficiency (PCE) of 10.8% based on single junction PSC and 10.6% based on tandem junctions PSC are reported. Tandem cells by stacking two or more single cells together can absorb more photons from multiple active layers with complementary absorption ranges. In this way, the photon utilization efficiency can be significantly improved and a PCE of 15% can be expected. The challenge is to seek suitable donor materials for those sub cells in terms of their specific requirement, especially for the back cell material due to the trade-off between narrow band-gap and open circuit voltage. Herein, we have proposed a ternary regioregular copolymer strategy to tune the band-gap and energy levels for PSC applications, which consists of a weak donor component, a strong donor component and a strong acceptor component in each repeat unit. For instance, it is well known that indacenodithiophene (IDT) is a weak donor, cyclopentadithiophene (CPDT) is a strong donor and pyridyl-[2,1,3]thiadiazole (PT) is a strong acceptor. Thus, it is reasonable that the binary copolymer (PIPT-RG) of IDT and PT has a relative wide band-gap and the binary copolymer (PCPDTPT) of CPDT and PT has a very narrow band-gap. Normally, synthesizing random copolymers by mixing these three monomers with various ratios could tune the band-gap from the narrow band-gap (Eg of PCPDTPT) to wide band-gap (Eg of PIPT-RG). According to our previous study, regioregularity significantly affect the material performance in FET and OPV applications. Based on this knowledge, we have synthesized two regioregular binary copolymers PIPT-RG, PCPDTPT and a structurally precise regioregular ternary copolymer PIPCP. The regiorandom counterpart PIPC-RA with the same monomer ratio was also synthesized to compare their chemical properties and device performance. We have found that the band-gap of PIPCP was well controlled (1.47 eV) and the absorption was desirable for back layer in tandem cell applications. Hence the short current is high. Furthermore, the mobility is decent for OPV applications due to the regioregularity. Most importantly, we have obtained surprisingly high Voc (0.85~0.90 V) based on the PIPCP devices in considering of the low band-gap. Finally we obtained the power conversion efficiency around 6%~7%. These results make the polymer PIPCP is a promising candidate for the future high performance PSCs.