  Improved passivation strategies to address the more complex surface structure
of large-diameter nanocrystals are critical to the advancement of infrared
photodetectors based on colloidal PbS. In this contribution, the performance of
short-wave infrared (SWIR) photodiodes fabricated with PbS/PbCl$_x$
(core/shell) nanocrystals versus their PbS-only (core) counterparts are
directly compared. Despite their inherently similar bulk properties, devices
using PbS cores suffer from shunting and inefficient charge extraction, while
core/shell-based devices exhibit greater external quantum efficiencies and
lower dark current densities. To elucidate the implications of the shell
chemistry on device performance, the thickness-dependent energy level offsets
and interfacial chemistry of the nanocrystal films with the zinc oxide
electron-transport layer are evaluated. The disparate device performance
between the two synthetic methods is attributed to unfavorable interface dipole
formation and surface defect states, associated with inadequate removal of the
native organic ligands in the core-only films. The core/shell system offers a
promising route to manage the additional nonpolar (100) surface facets of
larger nanocrystals that conventional halide ligand treatments fail to
sufficiently passivate.
