Mobility, charge carrier trap limited

With mixed-valence compounds, charge transfer does not require creation of a polar state, and a criterion for localized versus itinerant electrons depends not on the intraatomic energy defined by U , but on the ability of the structure to trap a mobile charge carrier with a local lattice deformation. The two limiting descriptions for mobile charge carriers in mixed-valence compounds are therefore small-polaron theory and itinerant-electron theory. We shall find below that we must also distinguish mobile charge carriers of intennediate character. 

The most useful of the known photorefractives are LiNbC>3 and BaTiC>3. Both are ferroelectric materials. Light absorption, presumably by impurities, creates electron/hole pairs within the material which migrate anisotropically in the internal field of the polar crystal, to be trapped eventually with the creation of new, internal space charge fields which alter the local index of refraction of the material via the Pockels effect. If this mechanism is correct (and it appears established for the materials known to date), then only polar, photoconductive materials will be effective photorefractives. However, if more effective materials are to be discovered, a new mechanism will probably have to be discovered in order to increase the speed, now limited by the mobility of carriers in the materials, and sensitivity of the process. 

The total concentration of holes nh is a sum of the concentration of trapped (nht) and free (nhf) carriers. However, often rihf/nht —> 0, nh nht due to a large concentration of traps. Then, the excitons are quenched by trapped carriers and the annihilation rate constant yTq is equivalent to the mobile exciton-immo-bile (trapped) charge carrier interaction rate constant yxq. Under space-charge-limited conditions, the concentration of charge is simply proportional to the applied voltage (U), nht = (3/2) o U/ed2, where d is the sample thickness, e is the electronic charge, s is the dielectric constant of the sample material, and s0 is the permittivity of free space. Thus, it may be seen that the fractional change in the triplet exciton decay rate... 

From picosecond transient photoconductivity measurements on PPP films,22 we know that mobile charged states decay within 110 ps. In conventional routes to PPPs, defects like branched chains and large torsion angles of neighboring rings are known to occur. These defects act as shallow or deep traps for positive and negative polarons,38,39 which limit the mobility of charge carriers.40 The synthetic route toward the PPP-type ladder-polymers prevents the described defects and leads to a trap concentration of less than 1 trap per 1000 monomer units,28 whereas substi-... 

Figures 12-12 and 12-13 document that trap-free SCL-conduction can, in fact, also be observed in the case of electron transport. Data in Figure 12-12 were obtained for a single layer of polystyrene with a Cl 3-subsliluted vinylquaterphenyl chain copolymer, sandwiched between an ITO anode and a calcium cathode and given that oxidation and reduction potentials of the material majority carriers can only be electrons. Data analysis in terms of Eq. (12.5) yields an electron mobility of SxlQ- cm s . The rather low value is due to the dilution of the charge carrying moiety. The obvious reason why in this case no trap-limited SCL conduction is observed is that the CFs-quaterphenyl substituent is not susceptible to chemical oxidation.

Due to the saturation of the traps, changes in Q(x) have a super-linear effect on the local charge carrier mobilities. Since the charges have to pass the entire channel, exp/ n sim the effective mobility of the OFET are limited by the locally reduced mobility near the drain electrode, which explains the strong influence of Vr,. Finally, in Figure 8.9(a), the dashed lines indicate the variation of fnexp/ Dsim functiou of giiit at constant values of Vq by variation of Kpi. It can be seen in Figure 8.9(a) that the slopes (d(/n,-xp//ndni)/dQ ... 

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