Solution-processed optoelectronic materials offer a route to low cost photodetectors, large area solar cells, and integrated optical sources. While significant progress has been reported in organic and polymer spin-cast optoelectronics, colloidal quantum dots offer a distinct further advantage---the convenient tuning of absorption onset via the quantum size effect. Electronic transport has recently been enhanced in size effect tuned colloidal quantum dot films using ligand exchange, resulting in ultrasensitive photodetectors in both visible and infrared wavelengths. Solid-film ligand exchange, however, generally results in rough film morphologies that are incompatible with high uniformity image sensors. Here, we report a new route to visible-wavelength spin-cast lead sulfide (PbS) nanocrystal photoconductive photodetectors with a sub 1% roughness, compared to the ∼10% roughness obtained using previously reported approaches. The new procedure yields devices that exhibit 10 AW-1 responsivities and reveals an added significant advantage: when illumination conditions change, the photodetectors respond with a single time constant of 20 ms. This compares very favorably to the multi second and multi-time-constant response of previously reported PbS-nanocrystal photoconductive photodetectors.
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