Controlling Molecular Weight to Achieve High-Efficient Polymer Solar Cells With Unprecedented Fill Factor of 79% Based on Non-Fullerene Small Molecule Acceptor

In the fullerene based system, molecular weight (MW) of polymeric donor plays a critical role in determining the performance of polymer solar cells (PSCs) and higher MW often leads to better photovoltaic performance due to the enhanced carrier mobility and better morphology. However, for non-fullerene small molecule acceptors (SMAs) based PSCs, polymeric MW effect research is often ignored. Thus, in our research, MW impact of a series of wide band-gap polymers of PBT1-C with different number average molecular weights (M_n) on the optical, electrical, morphological, and photovoltaic properties of PSCs has been systematically studied based on non-fullerene SMAs. It is found that the variety of MW of PBT1-C results in dramatic effects on the molecular aggregations of the polymers and their blend morphology of SMA based PSCs. In particular, PBT1-C-MW with a medium MW gives the best power conversion efficiency of up to 12.8% and unprecedented fill factor of 79%, when blended with ITCPTC as acceptor due to the favorable morphology, the pronounced increase in the orientational coherence lengths and the highest domain purity. To the best of our knowledge, the fill factor of 79% is the highest value among non-fullerene SMAs based PSCs. This study reveals that even for a given SMA and polymer donor, rationally tailoring the MW of the polymer donor is a useful strategy for further improving their photovoltaic performance.