Colloidal Arenethiolate-Capped PbS Quantum Dots: Optoelectronic Properties, Self-Assembly and Application in Solution-Cast Photovoltaics

Suitable postsynthesis surface modification of lead-chalcogenide quantum dots (QDs) is crucial to enable their integration in photovoltaic devices. Here we exploit arenethiolate anions to completely replace pristine oleate ligands on PbS QDs in the solution phase, thus preserving the colloidal stability of QDs and allowing their solution-based processability into photoconductive thin films. Complete QD surface modification relies on the stronger acidic character of arenethiols compared to that of alkanethiols and is demonstrated by FTIR and UVvisNIR absorption spectroscopy analyses, which provide quantitative evaluation of stoichiometry and thermodynamic stability of the resulting system. Arenethiolate ligands induce a noticeable reduction of the optical band gap of PbS QDs, which is described and explained by charge transfer interactions occurring at the organic/inorganic interface that relax exciton confinement, and a large increase of QD molar absorption coefficient, achieved through the conjugated moiety of the replacing ligands. In addition, surface modification in the solution phase promotes switching of the symmetry of PbS QD self-assembled superlattices from hexagonal to cubic close packing, which is accompanied by further reduction of the optical band gap, ascribed to inter-QD exciton delocalization and dielectric effects, together with a drastic improvement of the charge transport properties in PbS QD solids. As a result, smooth dense-packed thin films of arenethiolate-capped PbS QDs can be integrated in heterojunction solar cells via a single solution-processing step. Such single PbS QD layers exhibit abated cracking upon thermal or chemical postdeposition treatment, and the corresponding devices generate remarkable photocurrent densities and overall efficiencies, thus representing an effective strategy toward low-cost processing for QD-based photovoltaics.