  Solution-processable near-infrared (NIR) photodetectors are urgently needed
for a wide range of next-generation electronics, including sensors, optical
communications and bioimaging. However, there is currently a compromise between
low toxicity and slow (<300 kHz cut-off frequency) organic materials versus
faster detectors (>300 kHz cut-off frequency) based on compounds containing
toxic lead or cadmium. Herein, we circumvent this trade-off by developing
solution-processed AgBiS2 photodetectors with high cut-off frequencies under
both white light (>1 MHz) and NIR (approaching 500 kHz) illumination. These
high cut-off frequencies are due to the short transit distances of
charge-carriers in the AgBiS2 photodetectors, which arise from the strong light
absorption of these materials, such that film thicknesses well below 120 nm are
adequate to absorb >65% of near-infrared to visible light. By finely
controlling the thickness of the photoactive layer, we can modulate the
charge-collection efficiency, achieve low dark current densities, and minimize
the effects of ion migration to realize fast photodetectors that are stable in
air. These outstanding characteristics enable real-time heartbeat sensors based
on NIR AgBiS2 photodetectors.
  # equal contribution, * corresponding authors
