Effects of Selenophene Substitution in NDI-based n-Type Semiconducting Polymers for All-Polymer Solar Cells

We have investigated how substitution of selenophene for thiophene in the alternating naphthalene diimide (NDI)-thiophene copolymer (PNDIT-hd) resulting in NDI-selenophene copolymer (PNDIS-hd) influences the individual polymer properties and the performance of their binary blends with donor polymer PBDB-T in all-polymer solar cells (all-PSCs). The two polymers have identical electronic structures and similar bulk electron mobility; however, PNDIS-hd has a narrower optical bandgap of 1.70eV than that of PNDIT-hd (1.77eV). PBDB-T:PNDIS-hd all-PSCs were found to combine a high power conversion efficiency (PCE) of 8.4% with high external quantum efficiency (EQE=86%) and a high fill factor of 0.71, which are significantly enhanced compared to the corresponding PBDB-T:PNDIT-hd devices with 6.7%PCE and 73%EQE. The improved photovoltaic properties of the selenophene-containing acceptor copolymer are due to enhanced light harvesting, favorable molecular packing in blends and reduced charge recombination losses. These findings demonstrate that selenophene substitution for thiophene in donor-acceptor copolymers is an effective strategy that enhances the intrinsic polymer properties as well as the performance of their blend solar cells.