Ligand Dependent Oxidation Dictates the Performance Evolution of High Efficiency PbS Quantum Dot Solar Cells

TL;DR

This study reveals that ligand choice critically influences oxidation and stability in PbS quantum dot solar cells, with specific ligands preventing degradation and affecting performance evolution.

Contribution

It demonstrates that ligand-dependent oxidation processes dictate the performance and stability of PbS QD solar cells, highlighting ligand selection as a key factor for device longevity.

Findings

PbX2-coated PbS QDs show no oxidation, unlike EDT-coated QDs.

Oxidation reduces effective QD size, impacting charge transport and performance.

Proper ligand choice can significantly enhance stability of PbS QD solar cells.

Abstract

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.