Side-Chain Engineering of Nonfullerene Acceptors for Near-Infrared Organic Photodetectors and Photovoltaics

Narrow bandgap n-type molecular semiconductors are relevant as key materials components for the fabrication near-infrared organic solar cells (OSCs) and organic photodetectors (OPDs). We thus designed nearly isostructural nonfullerene electron acceptors, except for the choice of solubilizing units, which absorb from 600 to 1100 nm. Specific molecules include CTIC-4F, CO1-4F, and COTIC-4F, whose optical bandgaps are 1.3, 1.2, and 1.1 eV, respectively. Modulation of intramolecular charge transfer characteristics was achieved by replacing alkoxy groups with alkyl groups on thiophene spacers that connect an electron-rich cyclopentadithiophene core to peripheral electron-poor fragments. OSCs incorporating CTIC-4F and CO1-4F with PTB7-Th achieve power conversion efficiencies of over 10% with short-circuit current densities as high as a25 mA·cm^-2. The same blends achieve OPD responsivities of 0.52 A·W^-1 at a920 nm. These findings highlight outstanding opportunities to tune further molecular design so that OPDs may ultimately compete with their silicon counterparts.