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 & AP;2.0 x 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. Two new carbazole-bridged pyrrolic polysquaraines are developed for near-infrared organic photodetectors (NIR OPDs) via a simple synthetic route. The fabricated OPDs not only show a broad spectral response of 400-1000 nm, but also deliver an ultralow dark current density of & AP;2.0 x 10-11 A cm-2 in a photovoltaic mode, leading to a high shot-noise-limited specific detectivity of over 1013 Jones in the NIR region.image