Slow Electron Cooling in Colloidal Quantum Dots

Abstract

Hot electrons in semiconductors lose their energy very quickly (within picoseconds) to lattice vibrations. Slowing this energy loss could prove useful for more efficient photovoltaic or infrared devices. With their well-separated electronic states, quantum dots should display slow relaxation, but other mechanisms have made it difficult to observe. We report slow intraband relaxation (>1 nanosecond) in colloidal quantum dots. The small cadmium selenide (CdSe) dots, with an intraband energy separation of approximately 0.25 electron volts, are capped by an epitaxial zinc selenide (ZnSe) shell. The shell is terminated by a CdSe passivating layer to remove electron traps and is covered by ligands of low infrared absorbance (alkane thiols) at the intraband energy. We found that relaxation is markedly slowed with increasing ZnSe shell thickness.

Multiple exciton generation in quantum dot-based solar cells

“…energetically removed from the discretised near-band edge states), these arguments do not hold and therefore cannot explain the apparent difference between the quantum efficiencies of PbSe and PbTe QD-based solar cells [26,28]. And yet, recent spectroscopic measurement on lead chalcogenide QDs are consistent with a transiently stable carrier population far away from the band edge in the energetically condensed energy manifold (see Figures 2 and 6C) [28,66]. As briefly discussed above, one explanation for this phenomenon could be the existence of a high-energy point in the material's band structure, which may act as a phonon scattering bottleneck.…”

“…In this theoretical framework the scattering wave vector of the emitted phonon has to change its direction in order to cool the hot carrier through the "critical" point in the energy manifold. This would increase the number of phonon emission events per energy interval, which consequently reduces the carrier cooling rate [66]. Note that in the context of MEG contributing to the device photocurrent, this bottleneck is only beneficial if it is located at or above MEG th .…”

“…At energies relevant for MEG (i.e. >2E g ) however, the density of states (DOS) is more condensed and should therefore enable rapid carrier cooling via single phonon emission (see Figure 2) [66].…”

Electron overheating during field emission from carbon island films due to phonon bottleneck effect

“…If spacing between allowed energy levels exceeds the maximum phonon energy (∼ 35 meV; given by Debye temperature Θ D ), direct emission of phonons is prohibited, and hot electron relaxation goes via slower processes. Relaxation times as large as τ = 0.1-1 ns have been observed for semiconductor QDs [8,9]. This phenomenon attracts substantial interest because of its possible applications in solar batteries, infrared sensors, etc.…”

“…Consequences of this assumption for the emission process are discussed below. For quantitative estimates, relaxation time value τ = 1 ns (determined in [8] for semiconductor QDs) will be used -because of the absence of experimental information more relevant for the considered object.…”