Electronically active impurities in colloidal quantum dot solids.
Colloidal quantum dot films have seen rapid progress as active materials in photodetection, light emission, and photovoltaics. Their processing from the solution phase makes them an attractive option for these applications due to the expected cost reductions associated with liquid-phase material deposition. Colloidally stable nanoparticles capped using long, insulating aliphatic ligands are used to form semiconducting, insoluble films via a solid-state ligand exchange in which the original ligands are replaced with short bifunctional ligands. Here we show that this ligand exchange can have unintended and undesired side effects: a high molecular weight complex can form, containing both lead oleate and the shorter conductive ligand, and this poorly soluble complex can end up embedded within the colloidal quantum dot (CQD) active layer. We further show that, by adding an acidic treatment during film processing, we can break up and wash away these complexes, producing a higher quality CQD solid. The improved material leads to photovoltaic devices with reduced series resistance and enhanced fill factor relative to controls employing previously reported CQD solids.
The Edward S. Rogers Department of Electrical and Computer Engineering, University of Toronto , 10 King's College Road, Toronto, Ontario M5S 3G4, Canada.
Graham H
GH
Carey
Illan J
IJ
Kramer
Pongsakorn
P
Kanjanaboos
Gabriel
G
Moreno-Bautista
Oleksandr
O
Voznyy
Lisa
L
Rollny
Joel A
JA
Tang
Sjoerd
S
Hoogland
Edward H
EH
Sargent
colloidal quantum dot; complex; electroluminescence; impurities; ligands; photovoltaics