Effects of alkoxylation position on fused-ring electron acceptors

Four fused-ring electron acceptors composed of the same naphtho[1,2-b:5,6-b']dithiophene-based core and 3-(1,1-dicyanomethylene)-5,6-difluoro-1-indanone end groups without or with hexyloxyl groups on core and/or phenyl side chains are compared to systematically study the effects of alkoxylation position on molecular packing, optical, electronic, and photovoltaic properties of the nonfullerene acceptors. Alkoxylation on the core red-shifts absorption and reduces bandgap, while that on side chains has little effect on absorption and bandgap. Alkoxylation on the core up-shifts HOMO and down-shifts LUMO, while that on side chains shows very little effect on energy levels. Alkoxylation on the core slightly improves electron mobility relative to that on side chains. Both methods of alkoxylation decrease open-circuit voltage, but increase short-circuit current density and fill factor, leading to improved efficiencies of organic solar cells. Finally, when blended with the polymer donor PM6, IOIC3/IOIC4 with alkoxylation on core or side chains yield efficiencies of 11.1-12.8%, which is higher than IOIC2 without alkoxylation (10.5%). IOIC5 with alkoxylation on both core and side chains yields the highest efficiency of 13.8%.