  Colloidal nanocrystals (NCs), especially lead sulfide NCs, are promising
candidates for solution-processed next-generation photodetectors with
high-speed operation frequencies. However, the intrinsic response time of
PbS-NC photodetectors, which is the material-specific physical limit, is still
elusive, as the reported response times are typically limited by the device
geometry. Here, we use the two-pulse coincidence photoresponse technique to
identify the intrinsic response time of 1,2-ethanedithiol-functionalized PbS-NC
photodetectors after fs-pulsed 1560 nm excitation. We obtain an intrinsic
response time of ~1 ns, indicating an intrinsic bandwidth of ~0.55 GHz as the
material-specific limit. Examination of the dependence on laser power, gating,
bias, temperature, channel length and environmental conditions suggest that
Auger recombination, assisted by NC-surface defects, is the dominant mechanism.
Accordingly, the intrinsic response time might further be tuned by specifically
controlling the ligand coverage and trap states. Thus, PbS-NC photodetectors
are feasible for gigahertz optical communication in the third telecommunication
window.
