Designing Alternative Non-Fullerene Molecular Electron Acceptors for Solution-Processable Organic Photovoltaics.

Until recently, solution-processable organic photovoltaics (OPVs) mainly relied on fullerene derivatives as the n-type material, paired with a p-type conjugated polymer. However, fullerene derivatives have disadvantages that limit OPV performance, thus fueling research of non-fullerene acceptors (NFAs). Initially, NFAs showed poor performance due to difficulties in obtaining favorable blend morphologies. One example is our work with 2,6-dialkylamino core-substituted naphthalene diimides. Researchers then learned to control blend morphology by NFA molecular design. To limit miscibility with polymer while preventing excessive self-aggregation, non-planar, twisted or 3D structures were reported. An example of a 3D structure is our work with homoleptic zinc(II) complexes of azadipyrromethene. The most recent design is a planar A-D-A conjugated system where the D unit is rigid and has orthogonal side chains to control aggregation. These have propelled power conversion efficiencies (PCEs) to ∼14 %, surpassing fullerene-based OPVs. These exciting new developments prompt further investigations of NFAs and provide a bright future for OPVs.