Synergistic ligand exchange and UV curing of PbS quantum dots for effective surface passivation
Lead sulfide (PbS) quantum dots (QDs) are promising materials in solution-processed photovoltaic (PV) devices due to their tunable bandgap, and low-cost processing. Replacing the long oleic acid ligands of the as-synthesized QDs with shorter ligands is a key step towards making functional QDs PVs with correctly tuned band energies and reduced non-radiative recombination centers. In this work, we study the effect of ultraviolet (UV) treatment of PbS QDs layers on the QDs surface states during ligand exchange. We demonstrate that this straightforward approach effectively reduces the surface trap states and passivates the surface of QDs. We find that UV treatment reduces the density of hydroxyl groups attached to QDs surface and improves bonding of short ligands to the QDs surface. Multiple analyses show the reduction of nonradiative recombination centers for UV-treated sample. The power conversion efficiency (PCE) of our optimized PbS QDs device reached 10.7% (vs. 9% for control device) and was maintained at above 10% after 230 h of constant illumination.