The effect of surface passivation on the structure of sulphur-rich PbS colloidal quantum dots for photovoltaic application.

The use of PbS colloidal quantum dots in photovoltaic devices is very promising because of their simple and low cost production processes and their unique properties, such as bandgap tunability and potential multiple exciton generation. Here we report the synthesis of PbS nanocrystals used for application in solar cells. The sulphur-rich nature of their surface appears to be caused by the exposure to ambient conditions. The use of methanol as medium during the ligand exchange process has a crucial role in the removal of native oleate ligands. Without proper ligand exchange, the unpassivated surface is subject to ambient hydroxylation leading to the depletion of Pb atoms and the formation of a polysulfide phase. Devices assembled with this material showed good performance with an efficiency of 3.2%.