Enhanced Infrared Photodiodes Based on PbS/PbCl_x Core/Shell Nanocrystals

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.