Decreased Synthesis Costs and Waste Product Toxicity for Lead Sulfide Quantum Dot Ink Photovoltaics

Use of lead sulfide (PbS) colloidal quantum dot (QD) films as photoactive layers in photovoltaic (PV) devices typically requires replacement of native QD ligands with lead-based capping ligands (i.e., PbX2, X = Br, I) for the best-performing QD PVs. This ligand replacement process often requires additional solvents and toxic reagents. In the present study, an alternative PbS QD PV fabrication method with a lead-free tetrabutylammonium iodide (TBAI) ligand source and lower material requirements and toxicity is demonstrated, yielding 10% power conversion efficient PVs with more than 1000 h of storage stability under ambient conditions. Evaluation of the economic and toxicological benefits of this new ligand exchange protocol reveals a 72% reduction in synthesis costs, an 80% reduction in solvent volumes, and a nearly 250-fold reduction in lead waste generated compared to the previous PbX2-based protocol for highest-performing QD PVs. Most significantly, it is shown that unencapsulated PbS QD PVs made from this TBAI method leach less lead than U.S. Environmental Protection Agency limits and thus do not require hazardous waste disposal at the end of life. This finding implies a further decrease in expected device costs, extending progress toward large-scale production of commercial PbS QD PVs with low environmental risk.