Nowadays, while the inorganic based solar cells have already commercialized and matured for several years, the polymer based counterpart are still on their way to hit the 10% or even higher power conversion efficiency. Generally speaking, the power conversion efficiency is the multiply of open voltage, current density and fill factor. As is known to all that the absolute energy gap between the LUMO Levels of donor and acceptor should be no less than 0.3 eV. However, this thumb rule seemed having been challenged from recent reports. It should be an encouraging result if we can minimize the gap without deteriorates the charge dissociation, given the fact while the necessary gap provide the required driving force but at the same time it is one of the source of energy loss for larger open circuit voltage. In order to fulfill this aim, side chain decorating has been widely studied, so did the introducing of heteroatoms, such as fluorine. However, it is less studied the introducing of chlorine into the side chain. In this study, chlorination as an effective method to enlarge the open voltage was realized by chlorine substitution of either the electron rich or electron deficient part of PTB7-Th precursor. By combining this newly designed polymer with the widely used molecular ITIC, an evident enhancement of power conversion efficiency (PCE) up to 8.21% (no additive) with an open circuit voltage up to 1.03 V was realized, which is much higher than that of PTB7-Th counterpart (6.80%) with 0.25% DIO additive.