Volume-Controlled Recombination

Since the probability of the initial recombination can be expressed by the separation step rates, the natural way of its determination is to measure the bulk-generated photocurrent. However, one should keep in mind that the measured photocurrent contains carrier mobilities in addition to the effective separation probability (f/oG). The carrier mobilities 

Another possibility to determine PGr, which is free of these drawbacks, is electric-field modulation (EFM) of photoluminescence (PL). Electric-field effect on the effective charge separation efficiency (rj0 l) shows up in the varying population of CT states, and consequently, in the varying concentration of the emitting states. It is expected that the field-induced increase in the charge separation efficiency would translate into PL quenching. The ratio ( 5) of the PL efficiency in the presence ( ppl(F)) and in the absence ( Ppl(0)) of an external electric field (F) would give directly Pgr 

A more detailed discussion of the GR and experimental examples of its manifestation are presented in Sec. 2.6. 

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) 

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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.

Since the design of the measuring cell for field modulation studies is not very critical it was relatively easy to study also the pressure-dependence of ion-pair dissociation and ionic recombination (1 4). Here also the values (Table II) for the activation volumes show the essentially diffusion controlled aspects of the association-dissociation phenomena, since the calculated values, essentially the pressure dependence of the viscosity, and the experimentally determined values agree rather well. 

The recombination of volume carriers in the field-free regions is diffusion-controlled. 

Yeasts Easy to grow in large-scale volumes Able to glycosylate. Improved glycosylating yeast Pichiapastorisand Hansenula polymorpha Easy to achieve secretion of recombinant proteins into growth medium Gene expression can be difficult to control Some post-translational modifications may not be correct... 

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