Recombination zone

The zone of recombination can be very small as was shown by Aminaka et al. [225] by doping only a thin layer (5 nm) in the device by a red emission material. By observing the ratio of host and dopant emission, the authors were able to show that the recombination zone of the device was as thin as 10 nm. The emitted light is usually coupled out at the substrate side through the transparent anode. As a rule, the electroluminescence spectrum does not differ much from the photoluminescence spectrum. 

Building on the foundation of the hydrocarbon oxidation mechanisms developed earlier, it is possible to characterize the flame as consisting of three zones [1] a preheat zone, a reaction zone, and a recombination zone. The general structure of the reaction zone is made up of early pyrolysis reactions and a zone in which the intermediates, CO and H2, are consumed. For a very stable... 

The recombination zone falls into the burned gas or post-flame zone. Although recombination reactions are very exothermic, the radicals recombining have such low concentrations that the temperature profile does not reflect this phase of the overall flame system. Specific descriptions of hydrocarbon-air flames are shown later in this chapter. 

Hydrocarbon flames have rapid dissociation reactions within the primary flame zone, leading to a mole-number overshoot, which is followed by a relatively long recombination zone. Important rapid-dissociation reactions include... 

The second part of Eqn. (1.23) is obtained from Eqn. (1.22). From the requirement of eleetroneutrality and the definition of a (linearized) defect recombination zone of width R, Eqns. (1.22) and (1.23) yield... 

Thus, if the length of the one dimensional recombination zone rs> di the steady state condition in Eqn. (4.121) for the minority species in this zone simplifies to... 

If we denote the point defect injected by the applied field into the wrong sublattice of AX by i (e.g A ), and the conjugate defect that carries the flux in AX by j (e.g., V ), then the steady state condition for both fluxes (i, j) in the defect recombination zone r is... 

In a zeroth order approach, integration of Eqn. (10.36) gives y0 as a function of the electrical potential drop AC/ in the recombination zone... 

Results of computer simulations of defect accumulation in a two-dimensional crystal of length L. The area of the recombination zone is lx l, the number of accumulated defects is N [113]... 

Here z = WqLR is identical to the EM parameter provided Wo = k-el. Z(D) behavior for inner starts is qualitatively similar for contact starts from the exponential reaction zone. The noncontact starts from the exponential reaction zone are also similar to the starts from inside the rectangular recombination zone. Only for starts outside this zone, Z(D) is a monotonically increasing function of the diffusion coefficient [Fig. 3.26(h)]. For thin layers (qL << 1) this dependence coincides with that predicted by the CA Eq. (3.209). This is a consequence of the artificially sharp borders of the rectangular recombination layer. Ions that are born outside do not react in principle unless diffusion delivers them into the reaction zone. 

In the NI case most of the starts are from inside the recombination zone, while in the IN region ions always start from outside. In the latter case Zo is a bit smaller than z, but the difference is insignificant and the free-energy dependence of Zo results mainly from the z(AGr) dependence. According to the EM equations (3.194) and (3.207), z should reproduce the PEG law peculiar to Wr(ct). This is the symmetric curve represented by the dotted line in Figure 3.37, while the true kinetic efficiency represented by the thick solid line in the same figure is asymmetric and wider. This is due to the X(r) dependence, which broadens the PEG law for recombination as it did for ionization [compare the dashed and solid lines (a) in Fig. 3.16], This dependence is accounted for when z is calculated from the integral (3.337). 

The very existence of the maximum in the viscosity dependence of Z seen in Figure 3.40(a) (bottom) is conditioned by a clearcut separation of the near contact ionization and remote recombination zones in the NI situation... 

Fig. 3.40(a), top]. In situation II ( AG/ = AGj > Xc) the effect is completely lost since both reaction zones have exactly the same shape [Fig. 3.40(b), top]. Thus the initial ion distribution, even when it coincides with one of them, cannot be inside the other. As a consequence, only the descending (diffusion-controlled) branch of this dependence is seen in Figure 3.40(a) (bottom). Such a high sensitivity of the results to the shape and relative location of the ionization and recombination zones makes any model simplifications of these zones undesirable. 

Fig. 3.42(a)], but at AGr > AG the recombination zone is stretched while the ionization zone is squeezed as compared to the exponential one [Fig. 3.42(b)]. If the exponential rates are used in IET, the recombination can be accelerated only by speeding up the encounter diffusion. For the Marcus rates this is true only for AGrwell-defined maximum. These qualitative expectation were proved in Ref. 152, where the straightforward calculation of Z(D) with the exact Marcus rates (3.343) and their exponential simplification (3.53) were undertaken. Figure 7 in Ref. 152 demonstrates that all curves representing the exponential transfer rates monotonously increase with diffusion approaching... 

One should parenthetically note that the measurement of OH concentration and temperature in the flame recombination zone provides a method of determining the 0 atom concentration as well. The reactions G1 - G3 are usually fast consequently, if one assumes partial equilibrium,... 

Figure 58 The spatial distribution of the EL intensity in a 90 um-thick tetracene crystal at two different voltages 17= 750 V (°) and 17=950 V ( ). The semi transparent hole-injecting Au anode is located at x = 0, and a thick layer of a Na/K alloy forms the electron-injecting contact at the rear crystal side, x = d = 90 pm. The black field patterns simulate the light intensity distribution for these two voltages. The upper part illustrates the position and width of the recombination zone as predicted by Eq. (155) for a trap-free tetracene crystal and ohmic contacts. From Refs. 2, 51.

The width of the recombination zone is directly related to the EL efficiency of LEDs, through its definition as a distance traversed by a carrier during the recombination time, Trec [2]... 

We note that such defined recombination zone width must not be identified with the geometrical limits imposed on the charge recombination by the device structure. 

Accordingly, the recombination zone generally varies with electric field. For a strongly field-dependent injection-limited-currents (ILC) (Sec. 4.3.2), Equation (154)... 

Figure 59 Location (xT) and width (w) of the recombination zone for a small space-charge overlap in a plate shaped EL material of thickness d, provided with two injecting ohmic electrodes (anode, cathode), dh and de denote the penetration depths of injected holes and electrons, respectively. We note that xr = dh, d /de = /ih/and dh de = d for w —> 0.

Its solution gives a spatial distribution of the electron concentration within the recombination zone ... 

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