  Shortwave infrared (SWIR) has various applications, including night vision,
remote sensing, and medical imaging. SWIR organic photodetectors (OPDs) offer
advantages such as flexibility, cost-effectiveness, and tunable properties,
however, lower sensitivity and limited spectral coverage compared to inorganic
counterparts are major drawbacks. Here, we propose a simple yet effective and
widely applicable strategy to extend the wavelength detection range of OPD to a
longer wavelength, using resonant optical microcavity. We demonstrate a
proof-of-concept in PTB7-Th:COTIC-4F blend system, achieving external quantum
efficiency (EQE) > 50 % over a broad spectrum 450 - 1100 nm with a peak
specific detectivity (D*) of 1.1E13 Jones at 1100 nm, while cut-off bandwidth,
speed, and linearity are preserved. By employing a novel small-molecule
acceptor IR6, a record high EQE = 35 % and D* = 4.1E12 Jones are obtained at
1150 nm. This research emphasizes the importance of optical design in
optoelectronic devices, presenting a considerably simpler method to expand the
photodetection range compared to a traditional approach that involves
developing absorbers with narrow optical gaps.
