Nature of charge carriers

Initial experimental studies [32] [using UV-Vis-NIR and electron spin resonance (ESR) techniques] of highly doped polythiophene showed the bipolaron structure to be dominant. Later, this finding was supported by theoretical studies [33]. However, the majority of the early theoretical studies used the Su-Schrieffer-Heeger (SSH) Pariser-Parr-Pople (PPP) model Hamiltonians and or low-level ab initio calculations such as HF/STO-3G. Semiempirical calculations predicted that two polarons on the oligothio-phene chain would be more stable than a bipolaron for oligomers longer than the dodecamer [34], and many 

Among the three dication species (singlet bipolaron, singlet polaron pair and triplet polaron pair), the most stable in all CAS/AMl calculations is the triplet polaron pair [41]. For 8T , next comes the singlet bipolaron, which lies 0.23-0.27 eV higher (depending on the active space size) than the triplet polaron pair. 

We further investigated the different electronic states of oligothiophene polycations such as tri-, tetra-, hexa- and octacations (nT , nT +, nT + and nT ) at the B3LYP/6-31G(d) level. 10-, 20-, 30- and 50-mers of oligothiophene polycations were studied [43]. The complete set of spin states was considered for tri- and tetracations (doublet and quartet for trications singlet, triplet and quintet for tetracations) 

S- UB3LYP/6-31Q(d) Singlet B- RB3LYP/6-31Q(d) Singlet triplet at B3LYP/6-31G(d) 

The doublet and quartet states of trications 20T, 30T and 50T + have very similar energies, whereas the doublet of lOT is significantly more stable (by 8.4 kcalmoP ) than the corresponding quartet [43]. 

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Cyclic voltammetric studies involving polymers, 558 and the nature of charge carriers, 561 and the nucleation loop, 557 of poly (3-methylthiophene), 564 and parallel-band electrodes, 570 Cyclic voltammograms as a function of scan rate, 559 involving polymerization, 559 with polyanaline, 566 of polypyrrole film, 581... 

Parris PE, Kenkre VM, Dunlap DH (2001) Nature of charge carriers in disordered molecular solids are polarons compatible with observations Phys Rev Lett 87 126601... 

D.c. electrical conductivity, thermal conductivity, Seebeck effect and Hall effect are some of the common electron-transport properties of solids that characterize the nature of charge carriers. On the basis of electrical properties, solid materials may be classified into metals, semiconductors, and insulators where the charge carriers move in band states (Fig. 6.1) there are other semiconductors and insulators where charge carriers are localized and their motion involves a diffusive process (Honig, 1981). We shall briefly present the important relations involved in interpreting the transport phenomena in solids. 

Rosenberg, B. Preliminary experiments to determine the nature of charge carriers in the electrical conductivity of proteins. Biopolymers Symposia 1, 453 (1964). 

Even though these materials are being studied from over two decades, there are many questions about, the nature of charge carriers and the dimensionality of electrical conduction. 

Nature of charge carriers. Once injected, an electron and a hole must be brought at small enough distance to allow their recombination. This will depend on the nature and transport properties of the charge carriers. All CPs used in EL are nondegenerate ground-state polymers. The relevant notions of polaron and bipolaron and their relative stability were introduced in Chapter 11, Section IV.C. Some characteristic times pertinent to polaron or bipolaron formation will be discussed first, then the influence of traps. 

Summarizing, whatever the nature of charge carriers, mobilities indicate hopping transport. At low-enough injection levels, most of the charges may reside in traps, but these can be filled completely at high current (as discussed in Section V.B.3 for FETs) their role in recombination and photon emission will depend on voltage. 

The effective cross section for trapping at such centres can exceed 10 cm, which means that once a carrier spatially samples a defect site the probability for trapping is very high. The highly extended nature of the electronic wavefunctions and mobile nature of charge carriers makes the prerequisite level of purity extremely high. Impurity/defect levels less than 1/10 are required just to keep the impurity density outside the mean free path of the carriers and carrier drift and diffusion raises the impurity requirement to much less than 1 ppm. Typical intrinsic defect densities of... 

Attempts to obtain transport number information by various methods such as pulsed field gradient NMR [62], radio tracer diffusion [77], and potentiostat-ic polarization technique [46] have suggested that both cation and anion mobilities are important for the total ionic conductivity seen. In general, however, the nature of charge carriers in polymer electrolytes is quite complex and ion aggregates such as triple ions have been implicated in conductivity [78-79]. 

In contrast to trans-polyacetylene, all other conjugated polymers, including ds-polyacetylene, possess nondegenerate ground states and this affects the nature of charge carriers they can support. In such... 

Wet PBI-PS films possess no electron conduction despite the fact that the main polymer chains are conjugated. To elucidate the nature of charge carriers in PBI-PS, the conductivity of PBI-PS films containing H2O and D2O was measured [7]. The results of measurements are presented in Fig. 22. 

Determining the nature of charge carriers is stiii the goai of research in various study areas such as aqueous solutions, organic solutions (where ion pairs are frequently observed), molten salts, polymer and solid state media. 

The first relates to the crucial and controversial question of the nature of charge carriers in high-tem-perature superconductors. Early work [25] had established the usefulness of the Mott-Littleton approach in modelling localised electron states in ionic crystals. Recent work of Catlow, Islam and Zhang [26] (which built on earlier studies of the same authors) examined the stability of hole pairs or bipolarons in the La2Cu04 superconductor. It is generally accepted that electron holes in these materials are 0 2p) in character, i.e. to a very simple approximation they correspond to localised 0 ions. Hole pairs would therefore correspond to i.e. peroxyanions, located at a pair of lattice sites. Mott-Littleton calculations indeed suggest that such species... 

We have reviewed the calculations related to the most important properties of thiophene-containing materials, such as the HOMO-LUMO gap (or bandgap for polymers), twisting, the effect of substitution, the nature of charge carriers and IR, Raman and UV-Vis spectra. In addition, promising new classes of thiophene type compounds, such as quinoid oligothiophenes and cyclic oligothiophenes, have been reviewed. This review covers the literature up to mid-2007. 

Tang, J.W., et al.. Mechanism of 0-2 Production from Water Splitting Nature of Charge Carriers in Nitrogen Doped Nanocrystalline Ti02 Films and Factors Limiting 0-2 Production. Journal of Physical Chemistry C, 2011. 115(7) p. 3143-3150. 

We now consider various types of charge carriers that can be found in electronically conducting polymers. As previously noted both experimental and theoretical evidence suggest that the precise nature of charge carriers present in conjugated polymer systems depends to a very large extent on the type of polymer. We discuss two representative polymer materials, polyacetylene and polypyrrole, which have been the subject of considerable study. 

In order to have a clearer understanding of the nature of charge carriers and the mechanisms of their conduction, the conducting polymers may be divided into two categories (Thomas et al. 2015 Bakhshi 2007) ... 

Apart from considerations of amplifier noise, good shielding and protection from other external interferences, there is one basic limitation of resolution to any electrical measurement, which is the discrete nature of charge carriers. The best known example of its consequences is the so-called resistor noise or thermal (Johnson) noise which is given by ... 

Shahin, M. M., 1969. Nature of Charge Carriers in Negative Coronas. Appl. Opt. Suppl., 3 106-110. 

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