Surface chemistry of arenethiolate-capped PbS quantum dots and application as colloidally stable photovoltaic ink

Abstract

Suitable post-synthesis surface modification of lead-chalcogenide quantum dots (QDs) is crucial to enable their integration in photovoltaic devices. We have developed a solution-phase ligand exchange strategy that exploits arenethiolate anions to replace the pristine oleate ligands on PbS QDs, while preserving the long-term colloidal stability of QDs and allowing their solution-based processability into photoconductive thin-films. Complete QD surface modification is demonstrated by IR spectroscopy analysis, whereas UV–Vis–NIR Absorption Spectroscopy provides quantitative evaluation of stoichiometry and thermodynamic stability of the resulting system. Arenethiolate ligands permit to reduce the inter-particle distance in PbS QD solids, leading to a drastic improvement of the photoinduced charge transport properties. Therefore, smooth dense-packed thin-films of arenethiolate-capped PbS QDs obtained via a single solution-processing step are integrated in heterojunction solar cells: such devices generate remarkable photocurrent densities (14mAcm−2) and overall efficiencies (1.85%), which are outstanding for a single PbS QD layer. Solution-phase surface modification of QDs thus represents an effective intermediate step towards low-cost processing for all-inorganic and hybrid organic/inorganic QD-based photovoltaics.