Relaxation of Hot Carriers in Semiconductor Quantum Dots

As we commented before, the search for suitable PV absorbers where carrier cooling is slowed down or inhibited can have important implications in the development of hot carrier solar cells.16 Semiconductor QDs have been pro­posed as suitable candidates for hot carrier absorbers owing to their theoreti­cally expected slow carrier cooling (resulting from the ‘‘phonon bottleneck” effect). The phonon bottleneck in QDs was postulated as a consequence of the discretization of energy levels in QDs. As the characteristic spacing between QD energy levels (hundreds of meVs) is large compared to the LO phonon frequency (в 25 meV), electron thermalization via electron-phonon coupling in QDs will require multiphonon relaxation, which is a process with low prob­ability. While many works have reported bottleneck effects in colloidal and solid state QDs, there are also many reports that report the absence of a phonon bottleneck.69 Hence, the subject is still controversial and dependent on material selection and preparation (e. g. different surface chemistry defects). The controversy seems to have been influenced at least in part by the proper def­inition of what the bottleneck effect implies. In the limit of an ideally efficient bottleneck effect in QDs an infinite relaxation time for hot electrons within QDs can be expected (and monitored, for example, as total quenching of their luminescence). However, one can consider a less restricted definition implying that the bottleneck is manifested in QDs if slower carrier cooling is obtained compared with its bulk counterpart. For solar cell applications (hot carrier solar cells) a sufficient requirement for the ‘‘phonon bottleneck’’ to be useful is that the hot carrier relaxation time is long compared to the timescale necessary for carrier extraction.

A number of ultrafast carrier dynamics studies have been performed in colloidal dots.70 72 In these works ultrafast electron relaxation times of hun­dreds of femtoseconds were reported and attributed to an Auger process for electron relaxation bypassing the bottleneck effect. TRTS has been successfully applied in combination time-resolved photoluminescence to study the phonon bottleneck effect in CdSe QDs.73 In this work, thanks to the employment of THz frequencies, the authors were able to probe the rate of hole cooling fol­lowing photoexcitation of the QDs. It was shown that the hole relaxation rate critically depends on the electron excess energy. These results, constituting a quantitative measurement of electron-to-hole energy transfer, proved that in colloidal CdSe QDs the phonon bottleneck effect can be bypassed by an Auger like recombination (as suggested before by other authors). In this process the excess electron kinetic energy after photoexcitation is given to a hole which can undergo phonon relaxation owing to the lower discretization of hole energy levels in CdSe QDs.

The previous results imply, in any case, that slow carrier cooling can be achieved in QD materials with strong discretization of energy levels for both electron and holes (such as, for example, lead salts74), in QDs surrounded by ligands acting as hole scavengers or potentially in heteronanocrystals

(e. g. core-shell structures) where electron and hole are spatially separated and hence electron-hole energy transfer is inhibited.75

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