Near-Infrared Nonfullerene Acceptors Based on 4H-Cyclopenta[1,2-b:5,4-b']dithiophene for Organic Solar Cells and Organic Field-Effect Transistors.

The rapid developments of nonfullerene small molecular acceptors (NF-SMAs) have dominated the improvement of photoelectron conversion efficiencies (PCEs) for organic solar cells. Light absorption reaching the near-infrared (NIR) region is the primary feature for NF-SMAs compared with fullerene derivatives, enabling their enhanced efficiencies in solar cells and further application in semitransparent or tandem devices. In this article, a series of acceptor-donor-acceptor-structured NF-SMAs (named CPICs ) containing 4H-cyclopenta[1,2-b:5,4-b']dithiophene (CPDT) electron donor and F-substituted 2-(3-oxo-2,3-dihydro-1Hinden-1-ylidene)malononitrile (2FIC) as electron acceptor were designed and synthesized. With the increase of CPDT units, the elongated conjugations have broadened the absorption range of the acceptors while tuned their energy levels sequentially. Simultaneously, their corresponding charge-transporting polarities were changed from electron-only mode to bipolar mode in single-crystal organic field-effect transistors. Moreover, these changes also influenced the voltages, current densities, and eventual PCEs of their corresponding solar cells. When blending with the polymer donor material PBDB-T, an open-circuit voltage of 0.69 V, a short-circuit current density of up to 22.72 mA/cm 2 , a fill factor value of 63.66%, and a PCE of 10.01% was achieved in CPIC-2 -based cells due to its appropriate energy levels and efficient charge transport. This work demonstrated the importance of absorptions as well as suitable energy levels and charge transports in improving the PCEs of organic solar cells.