Semiconductors acceptor states

MLCT Excited States on Ti02 Experimental studies of MLCT states on Ti02 (and other semiconductors) are few mainly because of rapid interfacial charge separation that shortens their lifetimes considerably. Some aspects of MLCT excited states anchored to nanocrystalline Ti02 thin films are now becoming available through studies where the semiconductor acceptor states lie above (toward the vacuum level) the excited-state reduction potential of the sensitizer such that excited-state electron transfer from the thexi state is unfavorable. 

Figure 3.7 Schematic illustration of the relationship between structure and interfacial electron transfer properties in dye-sensitized nanostructured semiconductor materials. Control of the balance between electron injection, recombination and transport depends crucially on the physical separation and electronic coupling capabilities between the molecular donor and semiconductor acceptor states mediated by designated anchor and spacer groups.

Electron-hole recombination velocities at semiconductor interfaces vary from 102 cm/sec for Ge3 to 106 cm/sec for GaAs.4 Our first purpose is to explain this variation in chemical terms. In physical terms, the velocities are determined by the surface (or grain boundary) density of trapped electrons and holes and by the cross section of their recombination reaction. The surface density of the carriers depends on the density of surface donor and acceptor states and the (potential dependent) population of these. If the states are outside the band gap of the semiconductor, or are not populated because of their location or because they are inaccessible by either thermal or tunneling processes, they do not contribute to the recombination process. Thus, chemical processes that substantially reduce the number of states within the band gap, or shift these, so that they are less populated or make these inaccessible, reduce recombination velocities. Processes which increase the surface state density or their population or make these states accessible, increase the recombination velocity. 

Equation (6.96) is the relationship between the position of the Fermi level in an n-type semiconductor, and the partial pressure of the donor molecules in the gas phase. A similar relationship can be derived for the p-type semiconductor for which the ionization of a discrete acceptor state Na is given as Na + e = Na -... 

In addition, the presence of surface charges leads to band bending at the semiconductor-metal interface. For /(-type semiconductors, these states are acceptor-like and the semiconductor at equilibrium may exhibit upward (negative) band bending as the surface Fermi level moves towards the charged... 

Apart from fundamental transitions in direct-gap semiconductors, other processes may be responsible for radiative decay of the semiconductor excited states. The most common are processes associated with electron-hole annihilation involving donor and acceptor sites (Figure 7.10) [33],... 

The presence of this depletion layer has profound consequences if light of energy exceeding the bandgap is incident on the semiconductor, then the electron excited to the conduction band and the - hole left behind in the valence band can separate under the influence of the internal - electric field, with the electron drawn into the interior of the semiconductor and the hole driven to the surface, where, as can be seen from Fig. 3, it can be captured by an acceptor state in solution, driving an electrochemical reaction. The electron can pass round an external circuit to the counter electrode, and two types of electrochemical reaction are possible either a second different electrochemical couple... 

Figure 1 The schematic representation of various electronic excitation mechanisms due to ac or dc external electric fields (a) the tuneling electrons from the valence band (VB) to the conduction band (CB) and ionization of an acceptor state (-o-) (Zener effect) followed by electron-hole recombination, indicated by horizontal and vertical arrows, respectively (b) excitation or ionization by electron impact (c) recombination of electrons ( ) and (o) holes at a semiconductor p-n junction and (d) bulk recombination of electrons and holes injected from electrodes. Adapted from Ref. 2...

An alternate mechanism for spectral sensitization, not involving direct electron injection into the semiconductor, is energy transfer from the excited, adsorber dye to some unidentified acceptor state near the surface of the silver halide, followed by promotion of an electron into the conduction band. In an elegant series of... 

In semiconductors of high-dopant density and correspondingly thin depletion layers, tunneling may occur directly between the electrons in the conduction band and the surface of the electrode provided acceptor states or redox species in solution are available. The tunneling contribution to the total current has been considered by a number of workers [118-121] and the total anodic current can be written, with some generality, as... 

Several semiconductors have been studied using this technique the results from p-Zn3P2 are shown in Fig. 87 in which two localised and two VB -> trap transitions (which increase C for a p-type semiconductor) can clearly be seen. Results for n-CdSe in acetonitrile show something of the power of the technique acceptor states have been located at 1.40 and 1.57 eV above the VB edge these have been ascribed, respectively, to elemental Se and reconstructed polycrystalline CdSe domains. Two donor states, at 1.04 and 1.21 eV below the CB edge were also located in this study and appeared to be associated with an oxide film. It is in non-aqueous solvent, where faradaic reactions are often extremely slow, that this technique may have its best applications. 

The interfacial kinetics processes at semiconductor/liquid contacts for reactions with one-electron, outer-sphere, redox species can be understood in a conventional theoretical framework. The rate constant can be broken down into a term representing the attempt frequency, Vn, a term representing the electronic coupling between the electrons in the conduction band of the semiconductor and the redox acceptor state, k x, and a term representing the nuclear reorganization energy in the transition state from reactants to products, For outer-sphere electron transfer processes, the nuclear term is well-known to be ... 

Luminescence of rare earth ions can be understood, based on transitions between (almost) atomic eigenstates of the system [5.220, 5.221]. Forster and Dexter first described energy transfer between localized centers in luminescent material [5.222-5.224]. Besides orbital theory, semiconductor theory has also contributed to the understanding of radiative transitions Both band-to-band transitions and transitions involving localized donor and/or acceptor states fit within this framework. Nevertheless, there are also still open questions concerning the theoretical aspects. 

Defects or impurities in the semiconductor crystal structure create electronic states in the gap region. In the case of impurities, the valence character of the impurity determines whether the level acts as an electron donor or electron acceptor state. In doping semiconductors, impurities are deliberately used to generate either donor or... 

Moser J. E., Wolf M., Lenzmann F. and Gratzel M. (1999), Photoinduced charge injection from vibronically hot excited molecules of a dye sensitizer into acceptor states of wide-bandgap oxide semiconductors , Z. Phys. Chem B 212, 85-92. 

In this case Ep may be located between E and , but then not all of the more highly concentrated donors are ionized. Similar relations can be derived for acceptor states in a p-type semiconductor. 

Density of states at the lower edge of the conduction band Density of states at the upper edge of the valence band Density of donor states in the semiconductor Density of acceptor states Density of surface states... 

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