Infrared band gap semiconductor nanocrystals are promising materials for exploring generation III photovoltaic concepts that rely on carrier multiplication or multiple exciton generation, the process in which a single high-energy photon generates more than one electron-hole pair. In this work, we present measurements of carrier multiplication yields and biexciton lifetimes for a large selection of PbS nanocrystals and compare these results to the well-studied PbSe nanocrystals. The similar bulk properties of PbS and PbSe make this an important comparison for discerning the pertinent properties that determine efficient carrier multiplication. We observe that PbS and PbSe have very similar biexciton lifetimes as a function of confinement energy. Together with the similar bulk properties, this suggests that the rates of multiexciton generation, which is the inverse of Auger recombination, are also similar. The carrier multiplication yields in PbS nanocrystals, however, are strikingly lower than those observed for PbSe nanocrystals. We suggest that this implies the rate of competing processes, such as phonon emission, is higher in PbS nanocrystals than in PbSe nanocrystals. Indeed, our estimations for phonon emission mediated by the polar Fröhlich-type interaction indicate that the corresponding energy-loss rate is approximately twice as large in PbS than in PbSe.

Carrier multiplication (CM) is the process in which absorption of a single photon produces multiple electron-hole pairs. Here, we evaluate the effect of particle shape on CM efficiency by conducting a comparative study of spherical nanocrystal quantum dots (NQDs) and elongated nanorods (NRs) of PbSe using a time-resolved technique that is based on photon counting in the infrared using a superconducting nanowire single-photon photodetector (SNSPD). Due to its high sensitivity and low noise levels, this technique allows for accurate determination of CM yields, even with the small excitation intensities required for quantitative measurements, and the fairly low emission quantum yields of elongated NR samples. Our measurements indicate an up to ∼60% increase in multiexciton yields in NRs versus NQDs, which is attributed primarily to a decrease in the electron-hole pair creation energy. These findings suggest that shape control is a promising approach for enhancing the CM process. Further, our work demonstrates the effectiveness of the SNSPD technique for the rapid screening of CM performance in infrared nanomaterials.