p-Type PbSe and PbS Quantum Dot Solids Prepared with Short-Chain Acids and Diacids

We show that ligand exchange with short-chain carboxylic acids (formic, acetic, and oxalic acid) can quantitatively remove oleic acid from the surface of PbSe and PbS quantum dot (QD) films to yield p-type, carboxylate-capped QD solids with field-effect hole mobilities in the range of 10(-4)-10(-1) cm(2) V(-1) s(-1). For a given chemical treatment, PbSe devices have 10-fold higher mobilities than PbS devices because of stronger electronic coupling among the PbSe QDs and possibly a lower density of surface traps. Long-term optical and electrical measurements (i) show that carboxylate-capped PbSe QD films oxidize much more gradually in air than do thiol-capped PbSe films and (ii) quantify the slower and less extensive oxidation of PbS relative to PbSe QDs. We find that whereas the hole mobility of thiol-capped samples decreases continuously with time in air, the mobility of carboxylate-capped films first increases by an order of magnitude over several days before slowly decreasing over weeks. This behavior is a consequence of the more robust binding of carboxylate ligands to the QD surface, such that adsorbed oxygen and water initially boost the hole mobility by passivating surface states and only slowly degrade the ligand passivation to establish an oxide shell around each QD in the film. The superior hole mobilities and oxidation resistance of formic- and acetic-treated QD solids may prove useful in constructing efficient, stable QD photovoltaic devices.