Non-Fullerene Small Molecule Acceptors Containing Barbituric Acid End Groups for Use in High-performance OPVs

We synthesized two new bithiophene-based small molecules, TT-BBAR, and TT-OBAR, having butyl- and octyl-substituted barbituric acid (BAR) groups, respectively, via a well-known synthetic method, the Knoevenagel condensation, in high yield. These small molecules displayed solubilities and thermal stabilities sufficient for the fabricating organic photovoltaic cells (OPVs) and were designed to have relatively low molecular orbital energy levels and act as non-fullerene acceptors (NFAs) for use in OPVs upon introduction of electron-withdrawing BAR groups at both ends. For example, the LUMO and HOMO energy levels of TT-OBAR were −3.79 and of −5.84 eV, respectively, clearly lower than those of a polymer donor, PTB7-Th. Importantly, the small molecules featured an energy offset with PTB7-Th sufficient for achieving exciton dissociation. The optical and electrochemical properties of TT-BBAR and TT-OBAR did not depend on the alkyl chain length. Finally, OPV devices were fabricated in an inverted structure using a solvent process. The power conversion efficiency of TT-OBAR (1.34%) was found to be slightly higher than that of TT-BBAR (1.16%). The better performance and higher short-circuit current value of TT-OBAR could be explained based on a morphological AFM study, in which TT-OBAR displayed a more homogeneous morphology with a root-mean-square value of 1.18 nm compared to the morphology of TT-BBAR (11.7 nm) induced by increased alkyl chain length.