Regioregular narrow bandgap copolymer with strong aggregation ability for high-performance semitransparent photovoltaics

Semitransparent organic solar cell (ST-OSC) has developed into one of the most promising direction for the actual application of OSCs. Balancing high power conversion efficiency (PCE) and high average visible transmittance (AVT) is the greatest challenge in this field currently, which mainly due to the shortage of appropriate materials, especially the high-performance narrow bandgap (NBG) donor materials. In this work, we demonstrated a synergetic strategy to design a new type of NBG polymer by combining main chain extension and side chain engineering, which aimed to down-shift the highest occupied molecular orbital energy level, red-shift the absorption edge and enhance the regioregularity and crystallinity of the polymer donor. Compared with the stateof-the-art NBG donor of PTB7-Th, the newly designed polymer-based traditional device presents the significant improvement of photovoltaic performances. More importantly, a new breakthrough for ST-OSC with more balanced PCE of 11.0% and AVT of 35% has been successfully obtained, indicating the suitability of the reported NBG polymer for the exploration of high-performance ST-OSCs.