Near‐Infrared Organic Photodetectors with Ultralow Dark Currents Based on Carbazole‐bridged Pyrrolic Polysquaraines.

High specific detectivity (D*) is one of the key performance metrics for near‐infrared (NIR) organic photodetectors (OPDs), which however is always limited by the high dark current density (Jd). Herein, high‐performance NIR‐OPDs with ultralow Jd are successfully achieved by developing new conjugated pyrrolic polysquaraines as photoactive materials. By changing the linkage sites of the carbazole bridge, two low‐bandgap pyrrolic polysquaraines, i.e., PSQ‐3,6‐Cz and PSQ‐2,7‐Cz, are prepared via a facile synthetic route, both showing strong absorption in the visible and NIR region (600–1000 nm), respectable hole mobility, and enhanced compatibility with the PC61BM acceptor. The fabricated polysquaraine‐based NIR‐OPDs deliver impressive Jd values as low as ≈2.0 × 10−11 A cm−2 in a photovoltaic mode, which to the best of the knowledge is among the lowest values for NIR‐OPDs so far, and thus enables a high shot‐noise‐limited specific detectivity (Dsh*) over 1013 Jones in 650–950 nm. With a more accurate noise spectral current measurement, the PSQ‐2,7‐Cz‐based OPDs show lower noise currents and better detection performance than the PSQ‐3,6‐Cz‐based OPDs, which is attributed to the lower energetic disorder degree and fewer traps of the former, both resulting in suppressed trap‐assisted recombination. [ABSTRACT FROM AUTHOR]