Polarons and Polaron Pairs

Now the thermodynamic consequences will be eonsidered assuming that one has only Ps and PPs. The PP formation, according to 

(16) reflects the use of 2m for the PP extension. To compare with the former case, the dimensionless form 

One possible way out to resolve the discrepancy could be the assumption that PPs dominate at low and moderate concentrations, but an additional subsequent second oxidation step is possible at a standard potential near to the one of the first oxidation step. 

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

Observation of the charged carrier is even more favourable in the IR regime (photoinduced IRAV studies). In semiconducting, conjugated polymers, the quasi-one-dimensional electronic structure has to be strongly coupled to the chemical (geometrical) structure. As a result, nonlinear excitations (solitons, polarons and polaron pairs) are dressed with local structural... 

Further support for this model is provided by EPR experiments. With AsFj it was found that the susceptibility of all the PPP samples strongly deviated from Curie-Weiss behaviour at low temperatures, and this was interpreted as resulting from an equilibrium between isolated polarons and polaron pairs associated in singlet or triplet states [220]. 

Our results also shed light on the long-lived PA3 band detected in transient PM measurements of P3BT (see Fig. 7-19) and can explain changes in the PA spectra observed in several ps transient measurements of films of PPV derivatives at energies around 1.8 eV [9, 25, 60J. In good PPV films the transient PA spectrum shows a PA band of excitons at 1.5 eV whose dynamics match those of the PL and stimulated emission (SE) [9J. However, in measurements of oxidized [25] or C60-doped films 60, there appears a new PA band at about 1.8 eV whose dynamics are not correlated with those of the PL and SE. Based on our A-PADMR results here, we attribute the new PA band at 1.8 eV to polaron pair excitations. These may be created via exciton dissociation at extrinsic defects such as carbo-... 

Using a variety of transient and CW spectroscopies spanning the time domains from ps to ms, we have identified the dominant intrachain photoexcitations in C )-doped PPV films. These are spin-correlated polaron pairs, which are formed within picoseconds following exciton diffusion and subsequent dissociation at photoinduced PPV+/Cw> defect centers. We found that the higher-energy PA band of polaron pairs is blue-shifted by about 0.4 eV compared to that of isolated polarons in PPV. 

W Wohlgenannt, XM Jiang, C Yang, OJ Korovyanko, and ZV Vardeny, Spin-dependent polaron pair recombination in TT-conjugated polymers enhanced singlet exciton densities, Synth. Met., 139 921-924, 2003. 

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. 

A similar absorption bands have been reported for a regioregular P3HT pristine film by continuous wave photoinduced absorption (PIA) measurements at 10 K. The PIA spectrum exhibits an absorption peak around 990 nm and was assigned to localized polarons in the disordered portions of the film. Similar rapid decays were observed for the other polythiophene pristine films. Thus, these rapid decays are ascribed to the charge recombination of polaron pairs formed in the pristine film. 

Polaron Pair State. There are a number of experimental observations which can be interpreted neither by invoking charged excitations injected or photo-generated in the polymer, nor by excitons. However, it may happen that the singlet exciton is broken, as described above, and a pair of charges, negative P and positive P+ polarons, are separated onto adjacent chains, but still bound by the Coulomb attraction. These pairs will be referred to as polaron pairs. Polaron pairs are intermediate states between electronic molecular excitations and free charge carriers. They are formed by excitation of the photo-conductivity in polymers and other molecular solids, as well as... 

Electron back transfer is the opposite process to exciton dissociation and can positively influence the EL intensity, provided energy requirements are fulfilled. A detailed analysis of the transformation within polaron pairs is carried out in [70]. 

The elementary excitations of a conjugated polymer chain can be described within the mono-electronic approach as electron and hole quasiparticles [74] in a one-dimensional band structure, possibly weakly bound into extended Wannier-type excitons [71,75]. Within this framework, electron-phonon interactions lead to a peculiar family of exotic excitations including solitons, polarons, polaron pairs and bipolarons. In many cases, however, disorder is so significant that the polymer films are better described as an ensemble of relatively short conjugated segments [76], essentially behaving... 

The size of the soliton pair as on Fig. 6a is determined by the so-called confinement parameter y = Ae/(2XAo) [27,29]. Its meaning is qualitatively quite clear If the energy difference AE between the two structures increases, the extension of the high-energy one decreases and the pair is more confined. For very small y (i.e., Ae 0), the soliton pair or the bipolaron will tend to dissociate into independent solitons or polarons. 

The dynamics of the polarons and TEs, and their interactions with the SEs, have been the subject of numerous studies.20-25,28-36 Although the source of the EL is the recombination of a polaron pair in the antisymmetric singlet configuration to aSE ... 

Indeed, spin statistics mandate that if the rates of reactions (1) and (2) are the same, then the nongeminate polaron pairs generated by carrier injection in OLEDs would yield 3 TEs for every SE. This SE/TE branching ratio is one of the most important factors suppressing the efficiency of OLEDs based on the fluorescent decay of SEs. However, recent studies suggest that in luminescent -conjugated polymers the rate of reaction (1) is higher than that of (2), so the yield of SEs is... 

As mentioned in Sec. 1.2 above, from spin-statistics the ratio rSx of SEs to TEs formed from the recombining polaron pairs should be rsx = 0.25, since parallel spin pairs would recombine to TEs and antiparallel pairs would recombine equally to SEs and TEs. However, several recent independent studies suggest that the cross-section for SE formation (Eq. (1)) as is greater than that for TE formation (Eq. (2)) aT, so rSx > 0.25.37 39 However, the... 

The PL quantum yield r)pl. While r]pl of many dyes is close to 100% in solution, in almost all cases that yields drops precipitously as the concentration of the dye increases. This well-known concentration quenching effect is due to the creation of nonradiative decay paths in concentrated solutions and in solid-state. These include nonradiative torsional quenching of the SE,148 fission of SEs to TEs in the case of rubrene (see Sec. 1.2 above), or dissociation of SEs to charge transfer excitons (CTEs), i.e., intermolecular polaron pairs, in most of the luminescent polymers and many small molecular films,20 24 29 32 or other nonradiative quenching of SEs by polarons or trapped charges.25,29 31 32 In view of these numerous nonradiative decay paths, the synthesis of films in which r]PL exceeds 20%, such as in some PPVs,149 exceeds 30%, as in some films of m-LPPP,85 and may be as high as 60%, as in diphenyl substituted polyacetylenes,95 96 is impressive. 

Other interchain excited states are possible in PPV, including bound polaron pairs, where the positive and negative polarons reside on neighboring chains. 

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