Polaron-pairs

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. 

Figure 7-34. 2-PADMK specify of 10 mol% C(1 DOO-PPV, measured al the peak of Ihc spin-1/2 signal (polarons) or the broad resonant spectrum (polaron pairs). The H-PADMR spectrum is inset.

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

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. 

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. 

Deibel C, Strobel T, Dyakonov V (2009) Origin of the efficient polaron-pair dissociation in polymer-fulleiene blends. Phys Rev Lett 103 036402... 

Giebink NC, Lassiter BE, Wiederrecht GP, Wasielewski MR, Forrest SR (2010) Ideal diode equation for organic heterojunctions, n. The role of polaron pair recombination. Phys Rev B... 

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]. 

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... 

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... 

Charge transfer from the 11 Bu to an adjacent molecule or segment of a chain, i.e., dissociation of the 11 Bu. This process may also be extremely fast.24 Indeed, so fast that it has been suspected that this charge transfer state, aka a spatially indirect exciton, charge transfer exciton (CTE), or intermolec-ular or interchain polaron pair, may be generated directly from the ground state.24... 

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