Exciton shallow traps

We mentioned the main models for generation, transfer, and recombination of the charge carriers in polymers. Very often, these models are interwoven. For example, the photogeneration can be considered in the frame of the exciton model and transport in the frame of the hopping one. The concrete nature of the impurity centers, deep and shallow traps, intermediate neutral and charged states are specific for different types of polymers. We will try to take into account these perculiarities for different classes of the macro-molecules materials in the next sections. 

Webber and Swenberg (82) worked out a master equation theory for excitonic annihilation processes in low dimensional, finite lattices. This theory seems to describe well the process for polymers with few or shallow traps, as P2VN, but not for those containing many relatively deep traps, as PVCA. Finally, a list of those features which at the present time prevent a full understanding of T-T annihilation in dilute, solid solutions of aromatic polymers... 

Below 4 K the traps are effective. Around 4 K, however, the shallower traps (like Zn (3 nn). Mg (3 nn)) begin to lose their trapped excitation energy by thermally activated back-transfer to the exciton level. From here the energy may be trapped by deeper traps. Finally all the emitting traps are emptied and only the deep, non-emitting traps are operative. As a consequence the luminescence has been quenched. These quenching traps may be Ni and/or Fe ions (see Fig. 11). 

When the ideal case sketched here is realised (we will give some ejamples of it later), then the mobility /r can be directly determined from the I—Vcharacteristic j(V) (Eq. (8.33)). This is an important aspect, because the temperature dependence and the dependence on electric field of the mobility are the keys to understanding the coupling of the charge carriers to the remaining degrees of freedom, e.g. to the phonons, excitons, or shallow traps of the semiconductor. 

Like excitons, electrons, holes or polarons also carry out a hopping motion between the domains in a conjugated pol3rmer. Each domain provides unoccupied states around the LUMO and HOMO level for electrons and holes, respectively. They are delocalized within the domains and vary, as for the excitons, in their energy with the domain size. The tails of the corresponding density of states distributions act as shallow traps that keep the charges temporarily fixed. 

In a semiconductor, substitutional FAs from the same column of the periodic table as the one of the crystal atom they replace are usually electrically inactive and they are called isoelectronic with respect to the semiconductor. It can occur, however, that for some isoelectronic impurities or electrically-inactive complexes, the combination of the atomic potential at the impurity centre with the potential produced by the local lattice distortion produces an overall electron- or hole-attractive potential in a given semiconductor. This potential can bind an electron or a hole to the centre with energies much larger than those for shallow electrically-active acceptors or donors. The interaction of these isoelectronic impurities traps the free excitons producing isoelectronic bound excitons which display pseudo-donor or pseudo-acceptor properties. This is discussed later in this chapter in connection with the bound excitons, and examples of these centres are given in Chaps. 6 and 7. 

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