Recent Progress on Solar Cell Performance Based on Structural Tailoring on DA'D Units of Nonfullerene Acceptors

In the recent years, designing and synthesizing high-performance nonfullerene acceptor (NFA) materials have become the mainstream on research of organic solar cells (OSCs). Fused-ring based NFAs show high absorption coefficients, adjustable energy levels and bandgaps, straightforward synthesis, and excellent electrical and photovoltaic properties. The power conversion efficiencies ( PCEs) have been increased from 3% similar to 4% to 18% just in 7 years. In 2019, Zou et al. reported a smart acceptor molecule, named Y6, which supplied a very impressive PCE of 15. 7% when pairing with PM6. Y6 and its analogs were featured with DA'D type fused- ring as the central electron-donating (D) units. The electron-accepting (A') unit was fused through nitrogen with two D units, which helped to downshift the Frontier molecular orbitals and enhance light absorption. Again, the steric effect between the two N-linked alkyl chains and the decorated chains on the fused thienothiophene-beta-positions helped to increase the solubility and tune crystallinity After the report of the smart Y6, intense investigations have been made on structure cutting of the molecule and tens of new structures have been reported. Among these new acceptors, structural tailoring on the DA' D moiety plays a vital role in improving device efficiency and solar cell performance. In this review , we focus on the recent advances on the structural modifications on A' and D units and side chains. We set up several sets of acceptors, by which we classify the recently reported molecules and correlate their optical, electrochemical, electrical and photovoltaic properties with the precise structural modulations so as to present a comprehensive review on the structure-property relationship.