Sensitive Shortwave Infrared Organic Photodetectors Enabled by Nonfullerene Acceptor Featuring an Ultralow Optical Bandgap of Less than 1.0 eV.

Shortwave infrared (SWIR) photodetectors are in high demand in modern applications, including night surveillance, biological imaging, and optical communication. Emerging organic semiconductors, featuring a tailorable spectral response and solution processability, open new avenues for SWIR light detection. However, SWIR organic photodetectors (OPDs) suffer from a scarcity of ultralow-bandgap organic semiconductors and low responsivity above 1000 nm. Here, we report a new electron-rich building block, thieno[3',2':4,5]cyclopenta[1,2-b]thieno[2,3-d]pyrrole (SNCS), that exhibits strong electron-donating ability. By applying acceptor-donor-acceptor and acceptor-quinoidal-donor-quinoidal-acceptor strategy, we developed two new nonfullerene acceptors: SNCS-4F and SNCSTT-4F. The latter, with thieno[3,4-b]thiophene moiety, exhibits strong SWIR absorption up to 1400 nm in thin films. The best-performing PTB7-Th:SNCSTT-4F-based OPD exhibits a record external quantum efficiency of 50.2%, a responsivity of 0.49 A W⁻¹ and remarkable specific detectivity of 4.47 × 10¹² Jones at 1200 nm under zero bias. This is the highest performance among reported SWIR organic photodiodes and is comparable with commercial InGaAs photodetectors. Ultraviolet photoelectron spectra, Mott-Schottky analysis and trap density of states analysis were applied to evaluate the OPDs' performances. Finally, we demonstrate that the OPDs can detect SWIR light with high sensitivity in photoplethysmography measurements and infrared audio communication applications.