Formulation of Colloidal Suspensions of 3-mercaptopropionic acid capped PbS Quantum Dots as Solution Processable QD â€œInksâ€ for Optoelectronic Applications
The use of colloidal quantum dots (QDs) for photovoltaic energy conversion is a nascent field that has been dominated for well over a decade by the use of 3-mercaptopropionic acid (3-MPA) capped PbS QDs. These QDs are routinely deposited via an in situ solid state ligand exchange process that displaces the native oleate ligand on the PbS QD surface. This ligand exchange procedure is wasteful of material and has been demonstrated to leave numerous impurities that limit electronic performance of the as-deposited QD devices. Until the last few years there was very little understanding in chemical literature as to many important aspects of QD chemistry for this material pairing outside the framework of a QD solid. In this work, a colloidal suspension of 3-MPA capped PbS QDs in DMSO was formulated and investigated to probe ligand dynamics and optical properties of the suspended colloid. QD bound 3-MPA was found to be in dynamic exchange with â€œfreeâ€ ligand in solution by 1H-NMR spectroscopy. Optical properties and colloidal stability were found to be heavily dependent on the presence of a significant excess of free ligand. PbS QDs were also found to be highly photo-catalytic towards oxidative dimerization of 3-MPA to its dimer, dithiodipropionic acid (dTdPA).
After an initial colloidal suspension was achieved, attempts were made to directly deposit the colloid as a QD â€œinkâ€ to form optoelectronic devices. While photo-switchable devices were obtained, ultimately it was determined that DMSO was a largely incompatible solvent choice for solution processing methodologies. Subsequently, 3-MPA capped PbS QD colloids were obtained in volatile organic solvents amenable to solution processing by the addition of a stabilizing ammonium salt. These QD colloids maintained excellently resolved optical properties and were able to form conformal coatings from simple evaporative deposition. The ligand chemistry of this colloid was extensively investigated via NMR and optical spectroscopy. These QDs were also found to be highly photo-catalytic towards conversion of monomer 3-MPA to dTdPA.