P3HT-Based Polymer Solar Cells with Unfused Bithiophene–Rhodanine-based Nonfullerene Acceptors

We synthesized unfused bithiophene-based small molecules containing rhodanine (RH) dye end groups for use as acceptors in solution-processable polymer solar cells (PSCs). T2-ERH and T2-ORH were designed to have different alkyl side-chain lengths (ethyl and octyl, respectively), which enabled the optical properties and packing behavior of the corresponding films to be varied. PSCs were fabricated using poly(3-hexylthiophene) (P3HT) and one of the small molecules as a polymer donor and an acceptor, respectively. A power conversion efficiency of 1.81% was attained with the device based on P3HT and T2-ORH. The enhanced solubility resulting from the long octyl side chain resulted in a highly homogeneous morphology of the P3HT:T2-ORH film. In this study, we demonstrate the easy synthesis of RH-based bithiophenes, potentially providing a useful platform for developing nonfullerene small-molecule acceptors.