Free carrier-acceptor recombination

Comparison of the two elementary processes (2) and (3) shows the latter to be the most endothermic one. Indeed, decomposition of N2O yielding NO and N entails the rupture of the N=N bond, which requires 85 kcal. mole (3.6 e.v. molecule ) whereas for N2O decomposition into Na and 0, the breaking of the NO bond requires only 38 kcal. mole (1.6 e.v. molecule 0-This explains that thermally, in the absence of radiation, reaction (2) is always considerably favored, compared to reaction (3). Under irradiation, however, a great number of excess free carriers are produced in the conduction and valency bands these carriers tend to recombine. In this respect the adsorbed N2O molecule may behave like a recombination center. This phenomenon can be accounted for by considering the adsorbed N2O molecule to be an acceptor level. Under this hypothesis, the N2O chemisorption results from the capture, by the weakly adsorbed molecule, of an electron from the conduction band. At the moment of recombination with a positive hole from the valency band, a variable amount of energy can be recovered, depending on the position of the level constituted by the adsorbed N2O molecule. For the silica and alumina we have utilized, the width of the forbidden region is about 10 e.v. process (3) which only requires 3.6 e.v. may thus become possible. 

The schematic representation of the physical process that Braun proposed for the CT dissodation is illustrated in Figure 4. The lowest-lying CT state can be created dther by direct optical excitation or when an exdted neutral donor (acceptor) encounters an acceptor (donor). Then CT can dther dissodate into free carriers (ionized molecules of donor and acceptor) by a rate kd E) or decay to the ground state with a rate fef= l/rcr 10 1 s", where Ter is CT lifetime. A basic feature of this model is that during the CT lifetime one can observe many attempts to escape. The pair of ionized charges may undergo recombination to the CT state with a probability proportional to the constant rate... 

It is assumed that deeply trapped holes, h+ff, are chemically equivalent to surface-bound hydroxyl radicals. Weakly trapped holes, on the other hand, that are readily detrapped apparently posses an electrochemical potential close to that of free holes and can therefore be considered to be chemically similar to the latter. Their shallow traps are probably created by surface imperfections of the semiconductor nanocrystals. From these traps the charge carriers recombine or they are transferred by interfacial charge transfer to suitable electron acceptors or donors adsorbed at the surface of the semiconductor. 

M-S is an electrochemical impedance spectroscopy (EIS) technique [10-12] that can be difficult to perform and interpret if the system is not ideal. When the measurement is successful, it is able to determine both the fb and the free charge carrier density (donors or acceptors, A/Dopant) of the photoelectrode. Efb, along with the band gap (Eg) and the A dopant. can be used to determine the band structure of a photoelectrode and if it possesses the proper alignment with respect to the water splitting potentials (see chapter Introduction ). The A dopant also plays a role in the bulk and surface semiconductor properties such as the width of the depletion layer and rate of recombination. The conductivity type is also revealed by M-S analysis. The M-S plot will possess a negative slope for p-type materials and a positive slope for n-type materials (positive slope). In the case that the M-S measurement is not successful, then other techniques such as Hall Effect can still yield conductivity and A dopant for materials which can be deposited onto non-conductive substrates such as quartz. 

During each of the above-mentioned processes, energy can be lost, resulting in various loss mechanisms. First of all, all photons are not absorbed by the active layer due to limitations of the band gap, as described in the introduction and limited thickness of the active layer (1). Secondly, excitons will decay when created too far from the donor/acceptor interface (2). After the transfer of electron, recombination of the bound electron-hole pair can take place (3) as well as bimolecular recombination (4) of free charge carriers during transport to the electrodes can also occur. 

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