Importance of High-Electron Mobility in Polymer Acceptors for Efficient All-Polymer Solar Cells: Combined Engineering of Backbone Building Unit and Regioregularity

The charge transport ability of polymer acceptors (P(A)s) is crucial for achieving high power conversion efficiencies (PCEs) of all-polymer solar cells (all-PSCs). However, the electron mobilities (mu(e)s) of most P(A)s are inferior to those of their small molecule acceptor (SMA) counterparts. Herein, the authors design a new series of the polymerized SMA-based P(A)s (Y5-A-B), where the donating moiety (A = selenophene (Se)/biselenophene (BiSe)) and the backbone regioregularity (B = In/Mix/Out) are 2D controlled, for enhancing both the mu(e) and PCEs. Interestingly, the effects of regioisomers on the mu(e) and all-PSC performance are the opposite depending on the donating unit. For the Y5-Se-based P(A)s, the PCEs increase in order of Out (7.52%) < Mix (9.33%) < In (13.38%). In contrast, for the Y5-BiSe-based P(A)s, the PCEs decrease in order of Out (10.67%) > Mix (9.58%) > In (8.52%). These opposite trends in each series originate from the different planarity and intermolecular assembly of P(A)s depending on the regioregularity. Thus, the Y5-Se-In blend exhibits the highest mu(e) and achieves the highest PCE (13.38%) among the all-PSCs in this study. Therefore, the authors report the importance of simultaneous engineering of the backbone building unit and regioregularity to realize high-mobility P-A and highly efficient all-PSCs.