By introducing various low concentrations of Iridium complexes to the famous donor polymer of PTB7-Th backbone, new heavy metal containing terpolymers have been demonstrated. When blended with PC71BM, an obvious increase of power conversion efficiency (PCE) is obtained in 1 mol% Ir containing polymer for different photovoltaic devices either using Ca or PDIN as cathode interface layers. The impact of molecular weight on the photovoltaic performance has been particularly considered by using three batches of control polymer PTB7-Th to ensure a fair and more convincing comparison. At similar molecular weight conditions (Mn: ~60 kg mol−1, Mw: 100–110 kg mol−1), the 1 mol% Ir containing PTB7-ThIr1/PC71BM blends exhibits enhanced PCE to 9.19% compared with 7.92% of the control PTB7-Th. Through a combination of physical measurement, such as optoelectrical characterization, GIWAXS and pico-second time-resolved photoluminescence, the enhancement are contributed from comprehensive factors of higher hole mobility, less bimolecular recombination and more efficient slow process of charge separation.