Remarkable Isomer Effect on the Performance of Fully Non-Fused Non-Fullerene Acceptors in Near-Infrared Organic Photodetectors

Two fully non-fused small-molecule acceptors BTIC-1 and BTIC-2 are reported for application in near-infrared organic photodetectors (NIR OPDs). Both acceptors contain the same conjugated backbone but differing sidechain regiochemistry, affording significant differences in their optical properties. The head-to-head arrangement of BTIC-2 results in a reduction of optical band gap of 0.17 eV compared to BTIC-1, which contains a head-to-tail arrangement, with absorption spanning the visible and near-IR regions up to 900 nm. These differences are rationalized on the basis of non-covalent intramolecular interactions facilitating a more co-planar conformation for BTIC-2. OPDs based on PM6:BTIC-2 deliver a low dark current density of 2.4 × 10⁻⁷ A cm⁻², leading to a superior specific detectivity of 1.7 × 1011 Jones at 828 nm at -2 V. The optimized device exhibits an ultrafast photo response of 2.6 µs and a high -3 dB cut-off frequency of 130 kHz. This work demonstrates that fully non-fused small-molecule acceptors offer competitive device performance for NIR OPDs compared to fused-ring electron acceptors, but with reduced synthetic complexity. Furthermore, the study presents an efficient strategy to enhance device performance by varying conformational locks.