Charge-carrier traps

Along with the thermal and optical emptying of occupied traps, excitons can also empty them. In particular the long-lived triplet excitons Ti can be excited in a substantial stationary concentration. The excitation is usually carried out optically, but it can also be performed electrically through simultaneous injection of electrons and holes from both electrodes. In any case, the triplet excitons have a long lifetime and can couple, for example magnetically, to the charge carriers. 

---

Rapid e / h recombination, the reverse of equation 3, necessitates that D andM be pre-adsorbed prior to light excitation of the Ti02 photocatalyst. In the case of a hydrated and hydroxylated Ti02 anatase surface, hole trapping by interfacial electron transfer occurs via equation 6 to give surface-bound OH radicals (43,44). The necessity for pre-adsorbed D andM for efficient charge carrier trapping calls attention to the importance of adsorption—desorption equihbria in... 

The photoinduced absorption and the electrical characteristics of the conjugated LPPP show that the optoelectrical properties are strongly dependent on charge carrier traps in the bandgap. From aromatic molecular crystals it is known that impurities and structural imperfections form localized states [34]. LPPP forms homogeneous and dense films with a mean interchain distance of about 20 A and ncgligi-... 

An important aspect of semiconductor photochemistry is the retardation of the electron-hole recombination process through charge carrier trapping. Such phenomena are common in colloidal semiconductor particles and can greatly influence surface corrosion processes occurring particularly in small band gap materials, such... 

The results taken as a whole reveal the existence of at least three different trap species in the band gap of Sb(As)j Sei noncrystaUine semiconductors. These species are located at energies 0.22, 0.34, and 0.45 eV, respectively, below the conduction band edge and control the electron transport properties of the material. It seemed that Sb and As introduce a new set of detectable charge-carrier traps. 

In discussing low temperature-dependent mobility, we should mention charge transport by polarons, an intermolecular phonon-assisted hopping process 24>25>. Polarons (charge carriers trapped in their polarization field) arise from a strong electron-phonon interaction where there is a weak overlap of wave functions of... 

Rothenberger G, Moser J, Graetzel M, Serpone N, Sharma DK. Charge carrier trapping and recombination dynamics in small semiconductor particles. J Am Chem Soc 1985 107 8054. 

Charge carriers trapped in surface states may also react with reducible/ox-idisable species in solution ... 

The UV-visible absorption spectrum of the CdS nanotubes given in Fig. 2a shows a blue-shift in the excitonic absorption band to 460 nm. The blue-shift from the bulk value of 515 nm [12] is due to quantum confinement effects in the CdS nanotubes, the inner diameter of the nanotubes being less than the Bohr-exciton diameter of CdS (6 nm). Xiong et al. [13] have reported an absorption band at 459 nm for CdS nanotubes with an inner diameter 5 nm prepared by an in situ micelle-template-interface reaction. An absorption maximum around 450 nm has been reported in nanoparticles and hollow spheres of CdS [12,14], In Fig. 2b, we show the photoluminescence (PL) spectrum of the CdS nanotubes prepared by us, revealing a band centered at 610 nm. This band is due to charge carriers trapped at surface defects of the nanotubes [15,16],... 

The first picosecond laser spectroscopic study to examine charge carrier trapping and recombination dynamics was reported by Gratzel, Serpone and co-workers [15]. The mean lifetime of a single electron/hole pair was determined to be 30 15 ns at low occupancy of electron/hole pairs in the titanium dioxide particles. At high occupancies, where recombination followed second-order kinetics, the bulk rate coefficient for recombination was (3.2 1.4) x 10-11 cm3 s 1. 

Figure 15 The kinetic scheme illustrating the interplay between exciton (S) and charge carrier (q) trapping by crystal defects (S0t)-The PL spectrum of the crystal contains the excitonic emission (kr, hvm) and the trap center emission (kj., hi ). the latter being controlled by the number of the defect sites available for excitation. The exciton capture process (yst) competes directly with charge carrier trapping (yqt). The defects filled with charge reduce the emission resulting from radiative relaxation of the excited states produced at defect sites. For further explanations, see text.

P2VK which riiowed the largest upfield shift of aromatic protons exhibited a hole mobility of 1.4 x 10 cm /V sec. at an electric field of 4 x 10 V/cm. PVK with riightly smaller shift had mobility of 1.4 x 10 cm /V sec. and P3VK with the smallest shift had hole mobility otdy 2.4 x 10 cm /V at the same electric field. P2VK however exhibited the most severe charge carrier trapping. 

Charge Pair Separation and Charge Carrier Trapping... 

There are a number of models for polarization of heterogeneous systems, many of which are reviewed by van Beek (23). Brown has derived an exact, though unwieldly, series solution using point probability functions (24). For comparison to spectra for the thermoplastic elastomers of interest here, the most useful model seems to be the one derived by Sillars (25) and, in a slightly different form, by Fricke (26). The model assumes a distribution of geometrically similar ellipsoids with major radii, r-p and rj which are randomly oriented and randomly distributed in a dissimilar matrix phase. Only non-specific interactions between neighboring ellipsoids are included in the model. This model includes no contribution from the polarization of mobile charge carriers trapped on the interfacial surfaces. 

In this case, the thermostimulated luminescence is caused by free charge-carrier trapping that accompanies emission of photons (reaction 5.82). 

The electrical current of a coplanar interdigital gold/LPPP/gold device is space charge limited due to 79-type charge carrier traps localized in the bandgap [28], This can be inferred from the field dependence of the dark current at room temperature. The thermally stimulated current spectrum exhibits two peaks, corresponding to two distinct trap levels , and, which can be calculated from the rise in current, I, below the peak temperature ... 

---