Simple Bithiophene–Rhodanine-Based Small Molecule Acceptor for Use in Additive-Free Nonfullerene OPVs with Low Energy Loss of 0.51 eV

The introduction of rigid and extended ladder-type fused-ring cores, such as indacenodithiophene, has enabled the synthesis of a variety of nonfullerene small molecules for use as electron acceptors in high-performance organic photovoltaic cells. Contrasting with recent trends, a very simple-structured nonfullerene acceptor (NFA), T2-ORH, consisting of a bithiophene core and octyl-substituted rhodanine ends, is synthesized in two steps from inexpensive commercially available raw materials. Its relatively short π-conjugation results in a wide bandgap and a blue-shifted UV–vis absorption profile complementary to those of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th). Despite a sufficient offset between T2-ORH and PTB7-Th, the lowest unoccupied molecular orbital (LUMO) energy level of T2-ORH is still higher than the LUMOs of other NFAs (e.g., ITIC). Therefore, the PTB7-Th:T2-ORH blend film exhibits an efficiency of 9.33% with a high open-circuit voltage of 1.07 V and a short-circuit current of 14.72 mA cm ⁻² in an additive-free single-junction cell. Importantly, the optimized device displays a remarkably low energy loss of 0.51 eV, in which bimolecular and monomolecular charge recombination is effectively suppressed by solvent vapor annealing treatment. The blend film has a very smooth and homogeneous morphology, providing both vertical and parallel charge transport in the devices.