Revisiting the Bithiophene Imide-Based Polymer Donors: Molecular Aggregation and Orientation Control Enabling New Polymer Donors for High-Performance All-Polymer Solar Cells^†

Bithiophene imide (BTI)-based polymers have been promising photovoltaic materials due to their high mobility and tunable energy levels. However, BTI polymers have rarely been revisited since organic solar cells (OSCs) entered the era of non-fullerene electron acceptors (NFEA) likely owing to their incompatibility with NFEAs. Herein, fine-tuning the aggregation and orientation of BTI-based donor-π-acceptor (D-π-A) polymer donors was achieved by incorporating the linear n-octyl group into thiophene π-bridge. The resulting polymer donor G15 shows excellent compatibility with NFEA L15 (polymer acceptor). The G15-based all-polymer OSCs achieve high power conversion efficiency of 15.17%. This is significantly higher than that (< 5%) of its analogue with isomerized branched alkyl chains and also among the highest values for all-polymer OSCs. The results highlight that well-tailored BTI polymer donors are attractive photovoltaic materials for further exploration in non-fullerene organic solar cells.