Colloidal quantum dots (CQDs) are promising semiconducting materials to engineer photovoltaic and optoelectronic devices due to tunable size-dependent absorption and emission properties. Great efforts have been made to find suitable surface ligand to reduce spacing between the adjacent quantum dots to ease charge transport but there has been negligible attention given to purification strategies and systematic efforts to find the ideal ligand populations to optimize device performance. Charge transport could be hindered in the presence of excess free ligands forming interparticle barriers; on the contrary, an under-coated QD can have high density of trap states and could also undermine the nanocrystal’s stability toward coalescence. Precipitation and redissolution (PR) methods are widely used purification techniques that must typically be repeated to get clean CQDs with appropriate populations of surface-bound molecules (ligands). PR often results in loss of native ligands bound to the surface which induces more surface traps and/or compromise the chemical stability of these nanocrystals.<br/>Gel Permeation Chromatography (GPC) has been proven to be an effective technique to remove the byproducts and impurities from CQDs without damaging the QDs surface. In this study, we demonstrate the effectiveness of gel permeation chromatography (GPC) in removing byproducts and unbound ligands from pre and post ligand exchanged PbS CQDs.<br/>NMR and UV-Vis-NIR spectroscopy of oleate-capped PbS quantum dots shows that despite removal of unbound surfactants, PbS QDs remain stable in solution following GPC. These purified nanocrystals were used to made CQDs photodetectors and photovoltaic devices.<br/>We show that films fabricated using GPC purified PbS QD solutions exhibit higher charge transport, and low contact resistance, investigated through in-plane conductivity and photoresponse. The charge carrier mobility was extracted from the mobility lifetime product and recombination lifetime determined by the spatial profiles of the photocurrent obtained through scanning photocurrent microscopy (SPCM), and transient photocurrent measurements, respectively. Furthermore, our findings show that post synthetic purification and optimum ligand population can significantly improve physical characteristics including reduced surface roughness, suggesting that impurities and optimum ligand coverage play an important role in film morphology and device performance.

Role of post synthetic purification treatment and ligand population on optoelectronic properties of PbS quantum dot films and devices

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.