Abstract
Quantum confined semiconductor nanoparticles, such as colloidal quantum dots, nanorods and nanoplatelets have broad extended absorption spectra at energies above their bandgaps. This means that they can absorb light at high photon energies leading to the formation of hot excitons with finite excited state lifetimes. During their existence, the hot electron and hole that comprise the exciton may start to cool as they relax to the band edge by phonon mediated or Auger cooling processes or a combination of these. Alongside these cooling processes, there is the possibility that the hot exciton may split into two or more lower energy excitons in what is termed carrier multiplication (CM). The fission of the hot exciton to form lower energy multiexcitons is in direct competition with the cooling processes, with the timescales for multiplication and cooling often overlapping strongly in many materials. Once CM has been achieved, the next challenge is to preserve the multiexcitons long enough to make use of the bonus carriers in the face of another competing process, non-radiative Auger recombination. However, it has been found that Auger recombination and the several possible cooling processes can be manipulated and usefully suppressed or retarded by engineering the nanoparticle shape, size or composition and by the use of heterostructures, along with different choices of surface treatments. This review surveys some of the work that has led to an understanding of the rich carrier dynamics in semiconductor nanoparticles, and that has started to guide materials researchers to nanostructures that can tilt the balance in favour of efficient CM with sustained multiexciton lifetimes.
Highlights
The process of carrier multiplication (CM) in semiconductors follows excitation with a high energy photon where the excess of energy above the bandgap, Eg, is at least twice the latter and may need to be considerably higher for the multiplication process to occur
Their pseudopotential model found the AR and direct carrier multiplication (DCM) processes to be highly sensitive to the quantum dots (QDs) surface whilst the presence of gaps in the hole manifold of states encountered for excess energies well above threshold, allowed Auger cooling the chance to compete strongly with DCM across ranges corresponding to the gaps
Midgett et al [81] observed CM in alloy QDs of PbSx Se1−x though in the lead chalcogenides the lattice is hexagonal and so the degeneracy of the band edge states is higher than cubic Hg1−x Cdx Te
Summary
The process of carrier multiplication (CM) in semiconductors follows excitation with a high energy photon where the excess of energy above the bandgap, Eg , is at least twice the latter and may need to be considerably higher for the multiplication process to occur. The process is known to occur in both bulk [1,2] and quantum confined semiconductors, i.e., quantum dots (QDs), and the latter are understood to offer advantages over their bulk counterparts owing to potentially lower multiplication thresholds and higher slope efficiencies [3] (the rate at which the carrier or exciton yield increases with excitation photon energy once the threshold has been exceeded, see Figure 1). If CM with near energy conservation limit threshold and high slope efficiency can be obtained, it remains a potentially interesting mechanism for improved solar cells along with other applications including. For higher multi-exciton occupancies, the triexciton and n-excitons decay even more rapidly than the biexciton [23] and so fast carrier extraction is a must for solar cell applications in order to reap the benefits of CM
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