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.