Hot carrier transfer

Short wavelength photons (ofenergymuch greater than Eg) create hot carriers. If, somehow, thermalization of these carriers can be avoided, photoelectrochemical reactions that would otherwise be impossible with the cooled counterparts, that is, at very negative potentials for n-type semiconductors, would be an intriguing possibility. The key issue here is whether the rate of electron transfer across the interface can exceed the rate of hot electron cooling. The observation of hot carrier effects at semiconductor-electrolyte interfaces is a controversial matter [3,7,11,171] and practical difficulties include problems with band edge movement at the interface and the like [4]. Under certain circumstances (e.g. quantum-well electrodes, oxide film-covered metallic electrodes), it has been claimed that hot carrier transfer can indeed be sustained across the semiconductor-electrolyte interface [7,172,173]. 

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Hot carrier transfer is most likely expected for semiconductors having high carrier mobility, low minority carrier effective mass and of high doping density [167]. The first experiments were reported by Nozik and co-workers for p-GaP and p-InP liquid junctions [168, 166]. Especially InP was a good candidate, because of its high electron mobility. The authors used p-nitrobenzene (U edox = — 0-86 V (SCE)) as an electron acceptor, because the standard potential occurs 0.44 eV above the conduction band at the interface, as shown in Fig. 36. A photocurrent was observed at potentials negative of Ue = -I- 0.15 V. 

With sufficiendy thin absorber layers, particnlarly with quantum-dot absorbers, it also appears possible to harvest hot carriers and reduce the thermalisation loss in solar cells. This idea was pursned at an early time by Cooper et al. (1983). Hot electron transfer has been demonstrated in an electrochemical arrangement, and the possibility of hot-carrier transfer across solid interfaces is starting to be realised. Furthermore, the associated phenomenon of mnltiple exciton generation from a single hot electron-hole pair created by a single photon within a semicondnctor qnantnm dot has now been realised, as Art Nozik discnsses in Chapter 3. 

Photo-induced electron transfer reactions from quantum well electrodes into a redox system in solution represent an intriguing research area of photoelectrochemistry. Several aspects of quantized semiconductor electrodes are of interest, including the question of hot carrier transfer from quantum well electrodes into solution. The most interesting question here is whether an electron transfer from higher quantized levels to the oxidized species of the redox system can occur, as illustrated in Fig. 9.31. In order to accomplish such a hot electron transfer, the rate of electron transfer must be competitive with the rate of electron relaxation. It has been shown that quantization can slow down the carrier cooling dynamics and make hot carrier transfer competitive with carrier cooling. 

Photo-induced electron transfer reactions from quantum well electrodes into a redox system in solution represent an intriguing research area of photoelectrochemistry. Several aspects of quantized semiconductor electrodes are of interest, including the question of hot carrier transfer from quantum well electrodes into... 

Here the transfer time of the hot carriers is balanced against their thermalization time constant. 

In the second group of experiment, the surface of BPH particle were covered with Ca(OH>2 in aqueous solution. TG analysis of the BPH both in pure state and covered with 2.46 wt.% Ca2+ were performed under nitrogen atmosphere at 10 °C/min heating rate. Fig. 2 shows the results of TG analysis. As can be seen from Fig. 2, the covered BPH particles with Ca(OH)2 dehydrate slowly with respect to pure state. This phenomena involves the simultaneous transfer of heat to evaporate the liquid and transfer of vapor within the solid and vapor from the surface into the hot carrier gas. In the case of pure BPH at temperature higher than 300 °C, all... 

The electrons excited into the different levels within in the single well, could be transferred to an acceptor molecule in the electrolyte either by thermionic emission across the outer barrier layer Otherm) or tunneling through it (Jiun) (F g- 9.33). The photocurrent spectrum does not give any information about whether a hot electron was transferred. The observed structure in these spectra could in principle be caused simply by quantized absorption followed by a complete hot carrier relaxation and electron transfer from the lowest quantum level. 

Is Ultrafast Electron Injection Useful for Solar Energy Conversion While ultrafast injection is necessary for sensitizers with inherently short excited-state lifetimes, such as those based on iron polypyridyl compounds, it remains unclear whether ultrafast injection is necessary, or even desirable, for solar energy conversion. A recent example of trapping hot carriers provides some clues as to how ultrafast electron transfer might be exploited for enhanced energy conversion efficiency.123,124... 

A class of futuristic solar cells, often called hot carrier solar cells, seeks to harvest the full energy of solar photons. Such cells would utilize the additional energy content of a blue photon relative to ared one.126 In present-day solar cells, equilibrated carriers are collected and hence all absorbed photons with energy greater than the bandgap contribute equally to the measured efficiency. The realization of such hot carrier solar cells therefore requires electron transfer processes that are competitive with nonradiative decay of molecules or phonon relaxation in solids.126 Literature data indicate that such relaxation occurs on a femtosecond timescale. The ultrafast... 

During Convection Drying, a hot carrier gas is introduced this either flows over or through the product. Heat is transferred from the gas to the product. The liquid volatilising from the solids is carried away by the gas that is cooling down during this process (Figure 12.3). 

Hot carrier charge transfer processes are important in solid state devices [51]. One fundamental question concerns whether these hot carriers can be transferred across the semiconductor-liquid junction before they completely relax to the band edge [91]. This problem is not only of interest from a fundamental point... 

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