Half-Planar-Half-Twisted Small Molecule Acceptors for Efficient Polymer Solar Cells

A series of asymmetric acceptor−π-donor−π′-acceptor-type (A-π-D-π′-A) non-fullerene acceptors (NFAs) were designed and synthesized, namely, h-IEICO, h-IEICO-2F, h-IEICO-4F, and h-IEICO-4Cl, which were prepared by simultaneously linking halogenated ending groups both at the 4-position of 3-ethylhexoxythiophene (π-segment) of one side and at the 5-position of 3-ethylhexoxythiophene (π′-segment) of another side. The asymmetric molecular structure, as well as the radically different steric hindrance caused by the two distinct π-bridged segments, successfully produced a wondrous half-planar-half-twisted backbone configuration. As a result of the special molecular structures of these materials, as well as the excellent electron-attracting properties of the fluorine and chlorine atoms attached to them, h-IEICO-4F and h-IEICO-4Cl are characterized by an absorption red-shift and a down-shifted molecular energy level among all four molecules. The J52:h-IEICO-4F device demonstrated preferable exciton dissociation and collection efficiency, suppressed charge recombination, a high and balanced charge transport ability, superior morphological properties, and better crystallinity, resulting in an efficiency of 11.65%. However, the efficiencies of the J52:h-IEICO-, J52:h-IEICO-2F-, and J52:h-IEICO-4Cl-based solar cells are 5.24, 9.96, and 10.43%, respectively. This work demonstrates the significant influence of the adjusting planarity of the conjugated backbone as well as the fine-tuning of the end-group halogenation substitution on the characteristic regulation and the photovoltaic performances of NFAs for efficient PSCs.