Flexible Organic Photovoltaics with Star-Shaped Nonfullerene Acceptors End Capped with Indene Malononitrile and Barbiturate Derivatives

The design and synthesis of three star-shaped nonfullerene (NFA) acceptors, TPA-2T-INCN, TPA-2T-BAB, and TPA-T-INCN, based on a triphenylamine (TPA) core and linked through pi-conjugated thiophene (T) spacers to different terminal units (3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile, INCN, and 1,3-dimethylbarbituric acid, BAB), are reported. These materials are blended with the widely used poly(3-hexylthiophene-2,5-diyl) (P3HT) donor polymer and tested in flexible organic photovoltaics (OPVs). The NFAs capped with the strong electron-withdrawing INCN unit perform best in OPVs. Both P3HT:TPA-T-INCN and P3HT:TPA-2T-INCN blends also show the highest photoluminescence quenching efficiency (95.8% and 92.6%, respectively). Surprisingly, when reducing the number of T spacers from 2 to 1, the solubility of the NFAs in o-dichlorobenzene increases, leading to easier processing during the OPV fabrication and better surface morphology. This explains the best performance of TPA-T-INCN-based blends in OPVs, with a champion power conversion efficiency of 1.13%.