  Lead sulfide (PbS) quantum dot (QD) photovoltaics have reached impressive
efficiencies of 12%, making them particularly promising for future
applications. Like many other types of emerging photovoltaic devices, their
environmental instability remains the Achilles heel of this technology. In this
work, we demonstrate that the degradation processes in PbS QDs which are
exposed to oxygenated environments are tightly related to the choice of
ligands, rather than their intrinsic properties. In particular, we demonstrate
that while 1,2-ethanedithiol (EDT) ligands result in significant oxidation of
PbS, lead iodide/lead bromide (PbX2) coated PbS QDs show no signs of oxidation
or degradation. Consequently, since the former is ubiquitously used as a hole
extraction layer in QD solar cells, it is predominantly responsible for the
device performance evolution. The oxidation of EDT-PbS QDs results in a
significantly reduced effective QD size, which triggers two competing
processes: improved energetic alignment that enhances electron blocking, but
reduced charge transport through the layer. At early times, the former process
dominates, resulting in the commonly reported, but so far not fully explained
initial increase in performance, while the latter governs the onset of
degradation and deterioration of the photovoltaic performance. Our work
highlights that the stability of PbS quantum dot solar cells can be
significantly enhanced by an appropriate choice of ligands for all device
components.
