Doping quantum dots (QDs) is a problem that has been haunting researchers in the QD research community for years, even though doping techniques have been utilized for decades in conventional semiconductors. For the "self-purification" in colloidal QDs, engineering the surface ligands has emerged as an effective way to alter free carrier concentrations and doping types in colloidal QD solids. Halide-atomic ligands are the most popular ligands in producing PbS QD solids since they provide minimal dot-to-dot distance while maintain low in-gap trap states. However, previously reported halide surface treatment could only produce n-type QD solids. Here, we report the fabrication of p-type PbS QD solids using proton-assisted surface ligand exchange. We unveiled the origin of p-type doing in PbS QD solids, and it came from an unusual surface ligand; the HOH+ group formed using NH4X (X = Cl, Br, I) in methanol. We further fabricated QD solar cells using PbS-NH4Cl, a p-type QD solid predicted and proved by our theory and experiments. The champion device shows a high power conversion efficiency of 7.49%.
Keywords: deep trap; doping; ligand exchange; proton-assisted reaction; quantum dot; solar cell.