Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)
Publication date: 5th February 2015
Colloidal quantum dots (QDs) stand among the most attractive light-harvesting materials to be exploited for solution-processed optoelectronic applications. To this aim, quantitative replacement of the bulky electrically-insulating ligands at the QD surface coming from the synthetic procedure is mandatory. Here we present a conceptually novel approach to design light-harvesting nanomaterials demonstrating that QD surface modification with suitable short conjugated organic molecules permits to drastically enhance light absorption of QDs, while preserving good long-term colloidal stability. Indeed, rational design of the pending and anchoring moieties which constitute the replacing ligand framework leads to a broadband increase of the optical absorbance larger than 300 % for colloidal PbS QDs also at high energies (> 3.1 eV), which could not be predicted by using formalisms derived from effective medium theory. We attribute such a drastic absorbance increase to ground-state ligand/QD orbital mixing, as inferred by density functional theory calculations; in addition, our findings suggest that the optical band gap reduction commonly observed for PbS QD solids treated with thiol-terminating ligands can be prevalently ascribed to 3p orbitals localized on anchoring sulfur atoms which mix with the highest occupied states of the QDs. More broadly, we provide evidence that organic ligands and inorganic cores are inherently electronically coupled materials thus yielding peculiar chemical species (the colloidal QDs themselves), which display arising (opto)electronic properties that cannot be merely described as the sum of those of the ligand and core components. We thus open a novel path for the design of light-harvesting nanomaterials to be applied in solution-processed photovoltaic and photodetection applications.
Short conjugated ligands enhance broadband optical absorption of PbS NCs, preserving their colloidal stability. It’s very surprising to observe a dark (NC) solution turning darker upon addition of a transparent liquid (the ligand solution). This picture represents the first description of the inherent orbital mixing between organic ligands and inorganic cores and the failure of formalisms derived from effective medium theory in predicting NC optical absorption.
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