Low bandgap conjugated polymers have become promising materials for high efficiency organic photovoltaic (OPV) applications in recent years with the power conversion efficiency (PCE) up to > 10%. The main difference of these polymers with previously studied homopolymers in OPV, e.g., P3HT, is the intrinsic charge transfer character in the local repeating units. Intense ongoing effort has been focused on investigating the role of charge transfer characters in exciton splitting dynamics, charge carrier generation as well as the device performance using ultrafast transient spectroscopy. The poster will present our results showing that the charge transfer character in each repeating unit of the low band gap polymers could challenge some well recognized design principles, such as the criteria in design the materials with correct LUMO-LUMO bandgap between the polymers and fullerene derivatives. In this study, we demonstrated that the local electron density and potential gradient in each repeating unit, if not tuned properly, can give an apparent opposite trend of efficiency in devices from that predicted based on the LUMO-LUMO offset between the polymer and PCBM.Photoexcitation dynamics of pristine PTR series copolymers in solution and their blends with PC71BM were measured using the pump-probe photomodulation spectroscopy with ~50 fs time resolution in a spectral range from 450-1660 nm. This approach allows us to simultaneously monitor the dynamics of photoinduced absorption bands such as cation, charge transfer state, and exciton. Surprisingly, the cation and charge transfer state form instantaneously within 100 fs of the excitation, in both solution and heterojunction films with PC71BM. As the donor block in each repeating unit increases in size, its HOMO-LUMO gap decreases.At a certain point, the local charge transfer character diminishes because the cascade of potential energy level from the local donor to acceptor block no longer exists, resulting in the decrease of the device PCE, even the apparent LUMO-LUMO gaps of the polymer and PCBM are still in favor of exciton splitting or charge separation between the polymer and PCBM and the band gap increases as the donor block in each repeating unit increases in size. This suggests that due to the decreasing charge transfer state, the electron density cannot be transferred to the acceptor block that ultimately transfers the electron to PCBM. In comparison, when the local potential energies is in favor of charge transfer, the hole and electron has higher probability to dissociate into longer separation and evolve into free charge carriers. The study gives a better understanding of the role of local charge transfer characters for the improvement of the device PCE. This study is sponsored by National Science Foundation, Collaborative Sustainable Energy Pathways (NSF- SEP- 1230217 and 1229089).