Carrier Transport in PbS and PbSe QD Films Measured by Photoluminescence Quenching

The temperature-dependent quantum yield of photoluminescence (PL) has been measured in films of various sizes of PbS and PbSe quantum dots (QDs) capped with alkanedithiol ligands with lengths varying from 4 to 20 A. We demonstrate that PL within QD films can provide information about transport in a regime that is relevant to solar photoconversion. The ligand-length dependent PL quenching reveals behavior similar to that of ligand-length dependent carrier mobility. determined from field-effect transistor (FET) measurements in the dark. The data are described by a model in which band tail luminescence is quenched upon thermal activation by charge separation and hopping followed by nonradiative recombination. We extract the tunneling parameter beta and find values of 1.1 +/- 0.2 angstrom(-1) except for a value of 0.7 for the smallest QD sample. Changes in the transport mechanism may be due to unique surface faceting or QD-ligand coupling that occurs in small QDs. Furthermore, we compare all-organic capped PbS QD films with those infilled by Al2O3, discovering a surprisingly small value of beta less than 0.3 for the latter, which may be related to a graded potential barrier because of amorphous Al2O3 at the QD surface or interfacial chemistry inherent in the atomic layer deposition process.