"N-π-N" Type Oligomeric Acceptor Achieves an OPV Efficiency of 18.19% with Low Energy Loss and Excellent Stability.

A novel "N-π-N" type oligomeric acceptor of 2BTP-2F-T, constructed by two small non-fullerene acceptor (NFA) units linked with a thiophene π bridge is reported. The 2BTP-2F-T not only combines the advantages of small NFA and polymeric acceptors (PYF-T-o) with similar units but also exhibits superior characteristics of high absorption coefficient and high electron moblity(µ e) ) with less dependence on molecular packing. Using PM6 as the donor, a remarkable efficiency of 18.19% is obtained with an open circuit (V oc ) of 0.911 V, short current circuit (J sc ) of 25.50 mA cm -2 , and fill factor (FF) of 78.3%, which is much better than that of the corresponding monomer (16.54%) and PYF-T-o (15.8%) based devices. The much-improved efficiency results from two aspects: 1) an enhanced FF due to the largely improved µ e and well-controlled morphology ; 2) a higher value of (J sc × V oc ) due to its higher absorption coefficient and efficient charge generation at a similar low energy loss. Furthermore, the PM6/2BTP-2F-T device possesses the longest T 80 lifetime to light-soaking and comparable high thermal stability with PM6/PYF-T-o. The results indicate that the "N-π-N" type oligomeric acceptor has a great application prospect due to its superior high efficiency and improved stability in organic solar cells.