Electrons and hole trapping

Considering the wide bandgap semiconductor crystal (there are mainly oxides, sulphides, selenides, and halides in this group of materials) the visible- or UV-light 

The lifetime of separated charges increases after electron and hole trapping in certain states, eg in the case of titanium dioxide, electrons are trapped as Tim centres [30,31] with the holes as [ rHIVOFI ]+ [30], Trapping of holes proceeds in 10-100ns, whereas this process is faster for electrons and requires a few hundred picoseconds. Charge-carrier recombination from the trapped states also proceeds in 10-100 ns. 

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By optically creating carriers with a pulse of above band-gap illumination, then monitoring the subsequent current transient due to thermal detrapping, Hurtes et al (1978) and Fairman et al (1979) were able to apply the DLTS method to bulk, high-resistivity materials. This method, however, is unable to distinguish between electron and hole traps, and the calculation of trap densities is difficult. 

The electron- and hole-trapping dynamics in the case of WS2 are elucidated by electron-quenching studies, specifically by the comparison of polarized emission kinetics in the presence and absence of an adsorbed electron acceptor, 2,2 -bipyridine [68]. In the absence of an electron acceptor, WS exhibits emission decay kinetics similar to those observed in the M0S2 case. The polarized emission decays with 28-ps, 330-ps, and about 3-ns components. For carrier-quenching studies to resolve the dynamics of electron trapping, it is necessary that the electron acceptor quenches only conduction-band (not trapped) electrons. It is therefore first necessary to determine that electron transfer occurs only from the conduction band. The decay of the unpolarized emission (when both the electron and the hole are trapped) is unaffected by the presence of the 2,2 -bipyridine, indicating that electron transfer docs not take place from trap states in the WS2 case. Comparison of the polarized emission kinetics in the presence and absence of the electron acceptor indicates that electron transfer does occur from the conduction band. Specifically, this comparison reveals that the presence of 2,2 -bipyridine significantly shortens the slower decay component of the polarized... 

It is possible to assess the approximate depths of the electron and hole traps from spectral evolution data. This has been done in the case of WS2 nanoparticles from the analysis of the unpolarized component of the emission [68]. The emission following hole trapping (but prior to electron trapping) is only partially polarized and the emission following electron trapping is unpolarized. Thus, the unpolarized... 

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. 

In general, it is accepted that recombination of electrons and holes, trapping of electrons by oxygen deficiency sites and a low mobility of the holes, cause a low conductivity and accordingly a low photoresponse for hematite. Electron mobility in the range 0.01 [60] to 0.1 cm2/V-s [17] has been reported. In the latter case, it was found that the electron mobility was independent of donor concentration. More recently, an electron mobility of about 0.1 cm2/V-s has been measured with doped single crystals and the mobility was also here independent of donor concentration [5]. A diffusion length of holes has been determined to be only of 2-4 nm [6], which is about 100 times lower than many other (III-V) oxides. 

Figure 7.10 Schematic illustration of electron and hole trapping at dopant sites and subsequent donor-acceptor-mediated photon emission. (Adapted from Ozawa and Itoh [33])...

Liu X, Zhang G, Thomas JK. (1997) Spectroscopic studies of electron and hole trapping in zeolites Formation of hydrated electrons and hydroxyl radicals. JPhys Chem B 101 2182-2194. 

The charge conservation law requires that the concentration of electrons and holes trapped in colour centres during irradiation in vacuo under steady-state conditions (since the concentration of free charge carriers can be neglected compared with that of trapped carriers) or after irradiation must be equal. That is... 

In the electrophosphorescent PLEDs made from the blends of Ir(HFP)3 PFO-F(l%) PFO, injected holes and electrons recombine by two processes direct recombination on the main chain (PFO) to produce blue emission in parallel with electron and hole trapping on the fluorenone units and on the Ir(HFP)3 followed by radiative recombination, with green light from PFO-F (1 %) and red light from the triplet excited state of Ir(HFP)3. 

Figure 9.32. Schematic band models for thermoluminescence (41) (a) simple model (fc) model with impurity present (c) model involving electron and hole traps and impurity ion recombination centers.

Iitg. 8,4. Energy band model showing the electronic transitions in a storage phosphor (a) generation of electrons and holes (b) electron and hole trapping (c) electron release due to stimulation (4) recombination. Solid circles are electrons, open circles are holes. Center I presents an electron trap, center 2 a hole trap... 

Fig. 1. Zeeman levels of an electron -hole pair in the presence of magnetic field. G, N, and n, designate generation rate, total number of pairs of electron and hole traps, and population of rth level, respectively. [From Moiigaki (1983).]...

The number of electron and hole traps (O2, Ti +, and O ) and the rate of formation of the short-lived hydroxyl radicals OH under UV irradiation were evaluated by EPR. A correlation was suggested among the amount of the charge carrier centers, the rate of formation of OH radicals, and the catalyst photoaclivity... 

In order to sustain the ON state by double injection or by tunneling it is necessary, as Lucas (1971) points out, that the carrier lifetime is longer than the transit time. Her model of switching is based on the idea that beyond a critical injection current both electron and hole traps are neutralized and as a consequence recombination is sharply decreased and the diffusion length becomes of the order of the film thickness. This sharply increases the bulk conductance and the sustaining field is again concentrated at the electrodes as in Figure 6.21(c). Lucas obtains for the critical current density at which the so-called recombination instability occurs... 

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