Bandgap Tailored Nonfullerene Acceptors for Low-Energy-Loss Near-Infrared Organic Photovoltaics

A series of A-pi-D-pi-A-type nonfullerene acceptors (NFAs) was designed and synthesized with the goal of optimizing light absorption and energy losses in near-infrared (NIR) organic solar cells (OSCs) principally through the use of side-chain engineering. Specific molecules include p-O1, o-IO1, p-IO2, and o-IO2 with optical bandgaps of 1.34, 1.28, 1.24, and 1.20 eV, respectively. Manipulating the optoelectronic properties and intermolecular organization by substituting bulky phenylhexyl (p-) for linear octyl chains (o-) and replacing bisalkoxy (-O2) with alkyl-alkoxy combination (-O1) allows one to target energy bandgaps and achieve a favorable bulk heterojunction morphology when in the presence of the donor polymer PTB7-Th. Solar cells based on o-IO1 and PTB7-Th exhibit an optimal power conversion efficiency of 13.1%. The excellent photovoltaic performance obtained with the o-IO1 acceptor can be attributed to a short-circuit current of 26.3 mA cm(-2) and energy losses on the order of 0.54 eV. These results further highlight how side-chain engineering is a straightforward strategy to tune the molecular design of n-type molecular semiconductors, particularly in the context of NIR high-efficiency organic photovoltaics.