Narrow bandgap conjugated polymers based on a high-mobility polymer template for visibly transparent photovoltaic devices

In this contribution, starting from a high mobility polymer P2 with a regioregular D-A-D-A structure, we set out to further optimize the optical bandgap and energy levels for photovoltaic applications. By introducing different donor segments to construct a modified D'-A-D-A backbone, we successfully synthesized three polymers named as PBT, PFT and PPT. Our results demonstrate that the rotation barrier between the D' and A or D and A significantly increases after using asymmetric A unit PT, compared to either an asymmetric FBT unit or the original BT unit. So, PPT has a more planar backbone conformation, stronger intermolecular contacts and higher degree of structural order than PBT and PFT, and also possesses a deeper HOMO level. When blended with PC61BM, a fullerene acceptor, the PPT-based device showed a power conversion efficiency (PCE) of 7.3%, which is higher than that of PFT-(6.7%) and PBT-based (6.2%) devices; when blended with ITIC, a non-fullerene acceptor, the PPT-based device showed a PCE of 5.4%, which is also higher than that of the PFT (4.4%) and PBT-based (4.7%) device. Most importantly, we discover that these blends consisting of either PC61BM or ITIC as the electron acceptor exhibit an average transmission of similar to 60% over the 400 to 600 nm range and a maximum transmission of similar to 70% at similar to 500 nm, which provides opportunities and possibilities of achieving high performance, stable and visibly transparent PVs in the future.