Publication date: 27th June 2014
Colloidal quantum dots (QDs) are among the most attractive photo-active materials to be exploited for solution-processed optoelectronic applications. To this aim, replacement of the surface-bound bulky electrically-insulating ligands coming from the synthetic procedure is mandatory. Here we demonstrate that tailoring QD surface chemistry with suitable short molecules permits to (i) tune their optical absorption properties and (ii) mediate non-covalent interactions with the surroundings, while preserving good long-term colloidal stability. Indeed, rational design of anchoring and pending moieties which constitute the replacing ligand framework is exploited to (i) enhance broadband solar-light absorption of colloidal QDs and to (ii) dictate the morphology of hybrid composites with conjugated polymers during formation from blend solutions, thus markedly affecting the resulting optoelectronic properties. The relevance of our approach is demonstrated by (i) achieving an optical absorbance increase in the UV-Vis-NIR spectral range larger than 300 % for colloidal PbS QDs and by (ii) fabricating hybrid solar cells based on poly(3-hexylthiophene)/PbS QD nanocomposites which display a power conversion efficiency that reaches 3 %, far beyond any previous report for this blend. Interfacial photo-induced processes, investigated by (quasi)steady-state and time-resolved photo-induced absorption and luminescence spectroscopy, are discussed.
We thus provide straightforward evidence that ligands at the QD surface do not merely confer colloidal stability, while hindering charge separation and transport, but represent a simple yet powerful tool to gain control over the macroscopic optoelectronic properties of colloidal QD and solids thereof. More broadly, organic ligands and inorganic cores are inherently coupled materials which may be considered as a whole system rather than the sum of the components.
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