Polaron optic

The charged quasiparticles can be probed by electrical dc conductivity measurements (for polarons), magnetic susceptibility (for polarons and bipolarons), electron-spin resonance (ESR) (for polarons) and optical measurements (for polarons and bipolarons). As ESR is well suited for studying spin-carrying polarons, optical modification of the ESR (optically detected magnetic resonance ODMR) can be applied to link the emissive or absorbing properties of the polymer with its spin state. 

If the exciton binding energy relative to the separate charge pair is larger than that of the polarons relative to their free particle bands, the polaronic optical transitions will be hidden under the excitonic ones, and a one-electron approximation of the polaron certainly fails. This is yet another reason why optical signatures of the polaron may give dubious results (see Section IV.C.4). Such a situation can have practical consequences, to which we return in the discussion of electroluminescence in Chapter 12, Section V.C. 

Figure 7-23. Schematic diagram of energy levels and optical irunsiciits of neutral molecule, polaron (P+) and bipolar. HP2+).

Kigurc 7-35. Comparison of the energy levels and optical transitions of oppositely-charged polarons and polaron pairs. 

Whereas the intermediate existence of polarons has been unequivocally proved by ESR measurements and optical absorption data, up to now, the existent of bipolarons has been only indirectly deduced from the absence of the ESR signal and the disappearance of the visible polaron bands from the optical absorption spectrum On the other hand, spinfree — diionic-charge — states in aromatics, whose optical properties bear a remarkably resemblence to the predictions of the bipolaron model, have long been known Further evidence of bipolarons is the fact that doped... 

EL), conjugated polymers are also of interest as materials for optically or electrically pumped stimulated emission. For effects of this type, the ratio of stimulated emission to photoinduced absorption (PA) is of particular interest for conjugated polymers. In this context, the orign of the PA is controversial the PA can be a result of the formation of either charge-separated polaron pair -states or excimers. Initial experiments support the conjecture that LPPP 26 is significantly superior [49], as the stimulated emission of LPPP 26 is markedly more intense than that of PPV under comparable conditions. 

Importantly, deep oxidation of polyaniline leads to a material that becomes insulating and spinless. This phenomenon was demonstrated in case of poly(fV-methylaniline) by monitoring ESR signal and electric conductivity of the sample (Wei et al. 2007). Deep oxidation results in the formation of the so-called polaron pairs that are evidenced by optical spectra. Because the hopping probability of two polarons on a single chain is too small, polaron pairs do not contribute to electric conductivity and ESR signal. 

Fesser K, Bishop AR, Campbell DK (1983) Optical-absorption from polarons in a model of polyacetylene. Phys Rev B 27 4804... 

In NiO the position is less clear, the optical band gap hv0 is about 4.3 eV (Terakura et al 1984), and conductivity measurements on pure NiO are hard, because the resistivity is high and measurements are very sensitive to surface conduction. Wittenauer and Van Zandt (1982) claimed Eat ihv0, which would imply that polarons are not formed, though their finding that Es < Ea suggests the opposite. 

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