Effects of Oxygen Position in the Alkoxy Substituents on the Photovoltaic Performance of A-DA′D-A Type Pentacyclic Small Molecule Acceptors

In this work, we explore the reason that the alkoxy substituent on the terminal thiophene in a fused DA′D unit results in an abnormal blue-shifted absorption of A-DA′D-A type small molecule acceptors (SMAs), and we further propose a simple molecular strategy of altering the oxygen position along the alkoxy side chains to fine-tune the optoelectronic properties. By replacing the n-hexyl side chains of our previously reported pentacyclic acceptor Y26 (BZ4F) with hexyoxyl groups and moving the oxygen atom to the second and third positions along the alkoxy chains, we synthesize BZ4F-O-1, BZ4F-O-2, and BZ4F-O-3. BZ4F-O-1 shows blue-shifted absorption, while BZ4F-O-2 and BZ4F-O-3 exhibit red-shifted absorption. Using PM6 as the polymer donor, the BZ4F-O-3 based device achieves a favorable efficiency of 14.69%. This is the first work to study the effects of the oxygen position along the alkoxy side chains on the photovoltaic performance of the A-DA′′D-A type SMAs, which can provide important guidance toward future molecular design.