Development of n-Type Small-Molecule Acceptors for Low Dark Current Density and Fast Response Organic Photodetectors.

Suppressing the dark current density ( J _d ) while maintaining sufficient charge transport is important for improving the specific detectivity ( D *) and dynamic characteristics of organic photodetectors (OPDs). In this study, we synthesized three novel small-molecule acceptors (SMAs) densely surrounded by insulating alkyl side chains to minimize the J _d in OPDs. Introducing trialkylated N-annulated perylene diimide as a terminal moiety to the alkylated π-conjugated core structure was highly efficient in suppressing J _d in the devices, resulting in an extremely low J _d of 4.60 × 10 ^-11 A cm ^-2 and 10-100 times improved D * values in the devices. In addition, SMAs with a geometrically aligned backbone structure exhibited better intermolecular ordering in the blended films, resulting in 3-10 times as high responsivity ( R ) values in the OPDs. Outstanding OPD performances with a D * of 8.09 × 10 ¹² Jones, -3 dB cutoff frequency of 205.2 kHz, and rising response time of 16 μs were achieved under a 530 nm illumination in photoconductive mode. Geometrically aligned core-terminal SMAs densely surrounded by insulating alkyl side chains are promising for improving the static and dynamic properties of OPDs.