Charge carrier recombination kinetics

From the analysis of their experimental results of the investigation of the charge carrier recombination kinetics in titanium dioxide colloidal solutions and in dispersions Serpone et al. and Bowman and co-workers have also assumed the existence of two different traps [5,6]. 

Biisslcr et ai [110-113] treated charge recombination in organic LEDs in terms of chemical kinetics. The probability of recombination depends on the ratio of recombination rate ynp-np (where y represents a bimolecular rate constant) and the transition time (itr=dlpE) of the charge carriers through the device. 

Besides spectral sensitization the chemical sensitization of PAC was revealed [281-282]. The treatment of the polymer with nucleic acids lead to an increase in the photocurrent by 3 orders of magnitude without changing the photoconductivity spectrum. The same results were obtained with adenine. The data obtained were explained by the model with new recombination centers leading to an increase in the life time of the predominant charge carriers. This was confirmed by kinetic investigations. 

Transient Photoconductivity. A solution of neutral molecules in a polar solvent shows only ohmic conductivity, but if ions are formed by the action of the photolytic flash these charge carriers generate an additional current which is proportional to the ion concentration. The observation of such transient photocurrents is the most direct experimental evidence for the formation of free, solvated ions in electron transfer reactions. The quantum yield of ion formation can be obtained through proper calibration procedures and the kinetics of ion recombination can be determined. Figure 7.37 gives an example of such transient photocurrent rise and decay. 

Studies on luminescence of CdS colloids provide useful knowledge on the energy and nature of recombination sites of charge carriers in the colloidal particles. The regularities of the colloid photoluminescence quenching provide the information on the dynamics of electrons and holes in semiconductor particles as well as on the kinetics of interfacial electron transfer. Of a particular interest are studies on the luminescence of colloidal solutions of the so-called Q-semiconductors, their properties depending on the size of semiconductor particles due to the quantum size effects. 

The recombination kinetics of the charge carriers have been studied in detail by the groups of Gratzel, Serpone and Colombo [4e, 5, 6]. Since recombina-tion of electrons and holes is monitored by transient absorption techniques most of the observed decay is due to reaction (7.15). 

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. 

Thus, it may be seen that, by reducing the particle radius, it is possible to obtain systems where transit from the particle interior to the surface occurs more rapidly than recombination, implying that quantum efficiencies for photoredox reaction of near unity are feasible. However, the achieving of such high quantum efficiencies depends very much upon the rapid removal of one or both types of charge carrier upon their arrival at the semiconductor surface, underlining the importance of the interfacial charge-transfer kinetics. This is the subject of the next section. 

Electrical Conductivity. Hayashi et al. (14) have made electrical conductivity measurements in irradiated styrene, a-methylstyrene, and isobutyl vinyl ether. The observed specific conductivities in these monomers were of the same order of magnitude, at a given dose rate, as that measured in cyclohexane. Since the G values for ion production in most hydrocarbon liquids are approximately the same, they concluded that the conducting species in the monomer systems must have had approximately the same molecular size as cyclohexane. Also, since the observed specific conductances of the three monomers and that of cyclohexane showed a square-root dependence on the dose rate, they concluded that the behavior of kinetically independent oppositely charged carriers was being observed, with very little effect of the applied field on the recombination process. 

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. 

If the oppositely charged carriers are generated independently far away of each other (e.g. injected from electrodes) volume-controlled recombination (VR) takes place, the carriers are statistically independent of each other, the recombination process is kinetically bimolecular. It naturally proceeds through a Coulombically correlated electron-hole pair (e h) leading to various emitting states in the ultimate recombination step (mutual carrier capture) (Fig. 3 for more details, see Figs. 11 and 27 in Sec. 2.3). As a result, the overall recombination probability becomes a product of the probability of the pair formation, Pr(1) = (1 + Tm/Tt) , and the capture probability, PR(2) = (1 + tc/t(1)... 

Figure 172 Two-step kinetic scheme of the volume-controlled recombination (VR), taking into account the motion (rm) of oppositely charged carriers forming a correlated e—h pair (CP) and its decay by either the back dissociation (id), direct transition (tcp) to the molecular ground state or the ultimate capture (tc) of each other leading to an excited singlet state (Si) which produces electrofluorescence (hv-Ei). Note that the capture can create other excited states as indicated in Fig. 11. After Ref. 598. Copyright 2001 Jpn. JAP, with permission.

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