Core-shell colloidal quantum dots for photovoltaics with improved open circuit voltage*

High carrier recombination rates in lead sulfide (PbS) colloidal quantum dot (CQD) photovoltaics (PV) can result in a reduction in open circuit voltage (Voc). Surface states and sub-gap states are thought to increase recombination which leads to Voc loss. Here we present a method of forming an oxide shell on the CQD surface capped with native oleic acid ligands prior to the deposition of the CQD film and ligand swap. The core-shell QDs exhibit a narrowing in size distribution and the resulting devices yield improved Voc. In addition, films of core-shell QDs are more resilient to damage incurred during sputter deposition of overlaying films. Because the thickness and uniformity of the oxide shell can be precisely controlled, a natural balance between trap passivation and charge transport can be achieved.