Trapped charge carriers

In electroluminescence devices (LEDs) ionized traps form space charges, which govern the charge carrier injection from metal electrodes into the active material [21]. The same states that trap charge carriers may also act as a recombination center for the non-radiative decay of excitons. Therefore, the luminescence efficiency as well as charge earner transport in LEDs are influenced by traps. Both factors determine the quantum efficiency of LEDs. 

All TSRs involve the release of trapped charge carriers into either the conduction band or valence band and their subsequent capture by recombination centers and recapture by other traps (retrapping). Their experimental investigation is undertaken with the goal of determining the characteristic properties (parameters) of traps cap-tnre cross sections, thermal escape rates, activation energies, concentration of traps. 

This experimental method, as well as the formal kinetics of the process, is closely related to trap level spectroscopy by thermally stimulated release of trap charge carriers. 

Here, the responses are normalized to the maximum concentration r>o of excitations. The signal evolution in a bi-exponential decay is therefore n(t) = Ani(t) + Bn2(t), where A and B are proportional to the radiative (or non-radiative) rates of the two levels. For solids, a monoexponential PL decay can be explained by the thermally activated recombination of highly mobile electrons and holes trapped onto radiative defects. Such a mechanism requires that the spatial separation of the trapped charge carriers be small. 

These trapped charge carriers exhibit strong optical absorptions. The position of the absorption maximum is strongly affected by the presence of suitable electron acceptors and donors in the surrounding aqueous phase. Exploiting this effect it has been shown in early laser flash photolysis studies that the trapped electron exhibits a strong optical absorption around 650 nm (Fig. 7.3, top) while the trapped hole absorbs predominantly at shorter wave-lengths, i.e., around 430 nm or even shorter (Fig. 7.3, bottom) [4c, 4d]. 

Thus, it might concluded that the observed absorption spectra resulted from the superposition of the spectra of the trapped charge carriers, i.e., the extremely broad and featureless spectra indicate the simultaneous observation of trapped electrons and trapped holes [5,7]. 

Serpone et al. have examined colloidal titanium dioxide sols (prepared by hydrolysis of TiCl4) with mean particle diameters of 2.1, 13.3, and 26.7 nm by picosecond transient absorption and emission spectroscopy [5]. Absorption decay for the 2.1 nm sols was found to be a simple first-order process, and electron/hole recombination was 100% complete by 10 ns. For the 13.3 and 26.7 nm sols absorption decay follows distinct second-order biphasic kinetics the decay times of the fast components decrease with increase in particle size. 10 ns after the excitation pulse, about 90% or more of the photogenerated electron/hole pairs have recombined such that the quantum yield of photooxidations must be 10% or less. The faster components are due to the recombination of shallow-trapped charge carriers, whereas the slower components (x > 20 ns) reflect recombination of deep-trapped electrons and holes. 

In these systems the conductivity increases in the presence of oxidant gases that generate charge carriers (holes) and decreases by electron-donating gases which trap charge carriers [233], in a process in which the formation of five-or six-coordinate species seems to occur. 

A fast conversion of A and D + to secondary, more stable products (process 7 in Figure 7.14) diminishes the probability of these reactions. An increased lifetime of trapped charge carriers, IFET rate, and rates of the primary redox product conversion results in higher quantum yields of an overall photocatalyzed reaction. 

Trapped charge carriers (e-f, At) can further participate in radiative and non-radiative recombination processes ... 

The total concentration of holes nh is a sum of the concentration of trapped (nht) and free (nhf) carriers. However, often rihf/nht —> 0, nh nht due to a large concentration of traps. Then, the excitons are quenched by trapped carriers and the annihilation rate constant yTq is equivalent to the mobile exciton-immo-bile (trapped) charge carrier interaction rate constant yxq. Under space-charge-limited conditions, the concentration of charge is simply proportional to the applied voltage (U), nht = (3/2) o U/ed2, where d is the sample thickness, e is the electronic charge, s is the dielectric constant of the sample material, and s0 is the permittivity of free space. Thus, it may be seen that the fractional change in the triplet exciton decay rate... 

The observation of space charge limited currents (SCLC) in nanoscaled pure and chromium-doped titania was reported [310] and both the free-charge carrier density and the trapped-charge carrier density were given. 

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