A-DA′D-A Type Acceptor with a Benzoselenadiazole A′-Unit Enables Efficient Organic Solar Cells

A new A-DA′D-A structured narrow bandgap small molecular acceptor (SMA) Se46 was designed and synthesized by using benzoselenadiazole as the A′ unit and branched 2-butyloctyl as upper side chains in its DA′D central fused ring. It was found that Se46 shows red-shifted absorption compared to that of L8-BO with a benzothiadiazole A′ unit and an up-shifted lowest unoccupied molecular orbital energy level compared to that of Y6 with a benzothiadiazole A′ unit and linear n-C11H23 upper side chains. The organic solar cells (OSCs) based on PM6:Se46 demonstrate a high power conversion efficiency over 18% with an increased short circuit current density compared to that of the L8-BO-based devices and a higher open circuit voltage than that of the Y6-based devices. The results indicate that the synergetic modification of selenium substitution in the A′ unit and branched alkyl upper side chain is an effective strategy to further promote photovoltaic performance for the narrow bandgap A-DA′D-A type acceptors.