The cooling and Auger recombination of electron–hole pairs in PbSe quantum dots (QDs) and a series of nanorods (NRs) with similar diameter and varying length was studied by ultrafast pump–probe laser spectroscopy. Hot exciton cooling rates are found to be independent of nanocrystal shape. The energy relaxation rate decreases during cooling of charges, due to reduction of the density of electronic states. Auger recombination occurs via cubic third-order kinetics of uncorrelated charges in the QDs and NRs with length up to 29 nm. On increasing the NR length to 52 nm, a crossover to bimolecular exciton decay is found. This suggests a spatial extent of the one-dimensional exciton of 30–50 nm, which is significantly smaller than the value of 92 nm for the three-dimensional exciton diameter in bulk PbSe. The Auger decay time increases with NR length, which is beneficial for applications in nanocrystal lasers as well as for generation of free charges in photovoltaics.

The mobility and spatial distribution of photoexcited electrons in CdSe/CdS core/shell nanorods was studied using optical-pump THz-probe spectroscopy. Measurements were conducted on two samples, differing in rod length. After photoexcitation the hole localizes in the CdSe core within a picosecond, while the electron delocalizes around the core. Analysis of the THz mobility with a model of one-dimensional electron diffusion on a finite rod yields an electron delocalization of ∼25% into the CdS shell and a mobility of 700 cm2/(V s). This is one and a half times the mobility value for bulk CdS, which can be due to quantum confinement effects on electron–phonon scattering and electronic structure.

We have determined the Auger recombination kinetics of electrons and holes in colloidal CdSe-only and CdSe/CdS/ZnS core/shell nanoplatelets by time-resolved photoluminescence measurements. Excitation densities as high as an average of 18 electron–hole pairs per nanoplatelet were reached. Auger recombination can be described by second-order kinetics. From this we infer that the majority of electrons and holes are bound in the form of neutral excitons, while the fraction of free charges is much smaller. The biexciton Auger recombination rate in nanoplatelets is more than 1 order of magnitude smaller than for quantum dots and nanorods of equal volume. The latter is of advantage for application in lasers, light-emitting diodes, and photovoltaics.Keywords: Auger recombination; CdSe nanoplatelet; charge carrier; exciton; photoluminescence;

Organic semiconductors are of great interest for application in cheap and flexible solar cells. They have a typical absorption onset in the visible. Infrared light can be harvested by use of lead-chalcogenide quantum dot sensitizers. However, bulk-heterojunction solar cells with quantum-dot sensitizers are inefficient. Here we use ultrafast transient absorption and time-domain terahertz spectroscopy to show that charge localization on the quantum dot leads to enhanced coulomb attraction of its counter charge in the organic semiconductor. This localization-enhanced coulomb attraction is the fundamental cause of the poor efficiency of these photovoltaic architectures. It is of prime importance for improving solar cell efficiency to directly photogenerate spatially separated charges. This can be achieved when both charges are delocalized. Our findings provide a rationalization in the development of photovoltaic architectures that exploit quantum dots to harvest the near-infrared part of the solar spectrum more efficiently.Keywords: bulk heterojunction; infrared photovoltaic; organic semiconductor; quantum dot; terahertz spectroscopy; ultrafast spectroscopy

We show that in films of strongly coupled PbSe quantum dots multiple electron–hole pairs can be efficiently produced by absorption of a single photon (carrier multiplication). Moreover, in these films carrier multiplication leads to the generation of free, highly mobile charge carriers rather than excitons. Using the time-resolved microwave conductivity technique, we observed the production of more than three electron–hole pairs upon absorption of a single highly energetic photon (5.7Eg). Free charge carriers produced via carrier multiplication are readily available for use in optoelectronic devices even without employing any complex donor/acceptor architecture or electric fields.

The second peak in the optical absorption spectrum of PbSe nanocrystals is arguably the most discussed optical transition in semiconductor nanocrystals. Ten years of scientific debate have produced many theoretical and experimental claims for the assignment of this feature as the 1Pe1Ph as well as the 1Sh,e1Pe,h transitions. We studied the nature of this absorption feature by pump−probe spectroscopy, exactly controlling the occupation of the states involved, and present conclusive evidence that the optical transition involves neither 1Se nor 1Sh states. This suggests that it is the 1Ph1Pe transition that gives rise to the second peak in the absorption spectrum of PbSe nanocrystals.

Efficient carrier multiplication has been reported for several semiconductor nanocrystals: PbSe, PbS, PbTe, CdSe, InAs, and Si. Some of these reports have been challenged by studies claiming that carrier multiplication does not occur in CdSe, CdTe, and InAs nanocrystals, thus raising legitimate doubts concerning the occurrence of carrier multiplication in the remaining materials. Here, conclusive evidence is given for its occurrence in PbSe nanocrystals using femtosecond transient photobleaching. In addition, it is shown that a correct determination of carrier-multiplication efficiency requires spectral integration over the photobleach feature. The carrier multiplication efficiency we obtain is significantly lower than what has been reported previously, and it remains an open question whether it is higher in nanocrystals than it is in bulk semiconductors.