Design a thieno[3,2-b]thiophene bridged nonfullerene acceptor to increase open-circuit voltage, short-circuit current-density and fill factor via the ternary strategy

Abstract

In this study, we report a new small molecule acceptor (named TT-4F) which uses 3,6-dimethoxylthieno[3,2-b]thiophene (TT) as the π-bridge. Addition of 0.05 weight ratio amount of TT-4F into the host binary blend of PTB7-Th:IEICO-4F, resulting in a ternary blend in a weight ratio of 1:1:0.05, enables increased open-circuit voltage (Voc), short-circuit current-density (Jsc), and fill-factor (FF) at the same time. Finally, 12.1% efficiency is obtained. Compared to the 3-(2-ethylhexyloxylthiophene) bridge on IEICO-4F, the additional methoxyl group on the TT-6 position is involved in the lowest unoccupied molecular orbital (LUMO) and the larger π-system on TT increases the electron-donating nature, both of which help to raise the LUMO level, one reason of the increased Voc. Upon addition of 0.05 TT-4F, the hole mobility is increased, the monomolecular recombination is reduced, and the charge dissociation and collection is enhanced. All of these contribute to the increased Jsc and FF.