Exciton bipolaron

Fig. 4. Energy level diagrams showing possible electronic configurations for positively-charged polaron (a) and bipolaron (b) defects and (c) a schematic bipolaron band model. The negatively-charged polaron would carry three electrons and the bipolaron four. Also shown is the neutral polaron-exciton (d) which would decay to restore the chain structure.

From photoinduced absorption, luminescence and electron spin resonance observations, the dominant photocarriers generated in the polymer were shown to be polarons and bipolarons [189-191]. It was found that the magnitude of photoinduced absorption is rather independent of the condition of sample preparation whereas the photoluminescence intensity is strongly influenced. The results suggest that the luminescent exciton does not play a primary role in the photogeneration of polaronic species. 

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... 

Mobile defects are Frenkel78 excitons, Mott-Wannier excitons, polarons, bipolarons, polaritons, and solitons. 

To summarize this section, we can say that the picture of CP excitation in terms of solitons, polarons, and bipolarons is certainly incomplete, particularly when considering optical and, to a lesser extent, magnetic properties. However, there is presently no clear-cut and generally accepted picture of how and when excitons are important, although they certainly are. 

Because of their special polymerization process [136], PDA crystals should not contain any dopant or associated residual polaron or bipolaron concentration. These negative results give some constraints on other low-lying states, especially exciton states. Triplet-state absorption from the ground state would probably be too strongly spin-forbidden to have absorption coefficients as high as 1 cm-1 [129]. As for g states, the absorption coefficient depends sensitively on how far from the intense absorption they are (for the polyene case, see Ref. 127). That they are not found in these experiments means that they are either above the main transition at 2 eV or not far below it and buried in its tail. Indeed, evidence of a weak absorption = 0.1 eV below the main transition has recently been found at low temperature [118] it would be buried in the absorption tail at higher temperatures. 

In PPV a PI A peak at 1.4 eV has been assigned to a triplet-triplet transition from the triplet exciton [148,150], as in PDA (it is not known if the near equality in the energies is accidental). But, in addition, two other induced absorptions are observed near 0.6 and 1.6 eV, and since they are associated with the characteristic IR bands (such as the 0.45-eV band in PA), they should be due to a charged state. The absence of ODMR signal suggests that they have no spin and would then be bipolarons. In improved PPV (see Fig. 15), the PIA spectrum contains only the triplet peak [151], suggesting that the presence of the other features is a consequence of strong localization in a defective polymer. Similar results are found in other CPs, but up to now evidence for PIA due to polarons is elusive. 

To summarize, the microscopic recombination processes in CPs are still debated. If recombination proceeds via the formation of a singlet exciton, a good CP for LED applications should have at least three properties It should not form bipolarons, or only slowly the u exciton should be the lowest singlet state and that state should, of course, have minimal non-radiative decay processes, in particular to the triplet state [129]. But, in addition, the diode geometry and the electrodes should be chosen so as to maximize carrier recombination and minimize carrier extraction through the electrodes. 

Poly(phenylene vinylene), PPV, and its soluble derivatives have emerged as the prototypical luminescent semiconducting polymers. Since PPV has a nondegenerate ground state, structural relaxation in the excited state leads to the formation of polarons, bipolarons, and neutral excitons. However, prior to treating the structural relaxation in the excited state, one needs to develop a satisfactory description of the electronic excited states. 

Figure 7-28. (a) H-PADMR and (b) A-PADMR spectra of u-bT film. The peaks of polarons, bipolarons, and triplet excitons are labeled. 

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