Polaron excitonic

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.

Finally, it should be stressed that photoluminescence has usually a larger quantum yield in solution than in the solid state27. In the latter case, inter-chain interactions appear to quench the luminescence properties through a mechanism that is not fully understood yet. In this context, it is, however, useful to note that the possibility that the polaron-excitons evolve into excimers28 is ruled out by the absence of Stokes shifts in good-quality PPV oligomer and polymer samples14. 

One can say that the obtained by us experimental results upon 2D exciton localization (taking place due to the growth of the crystal dielectric permeability anisotropy parameter) with o are very close to [27] where the behaviour of polaron excitons in parabolic quantum dots were considered and shown that the dot size decrease results in increasing the exciton binding energy. 

D. Solitons, polarons and polaron excitons the elementary excitations of conducting polymers... 

According to a large number of experimental studies, the most stable photogenerated species in the lowest excited states of conjugated chains are electron-hole pairs bound by Coulomb attraction and associated to a local deformation of the backbone, i.e., polaron-excitons [18]. A good insight into the properties of these species can be provided by quantum-chemical calculations our recent theo-... 

Based on these direct experimental measurements, we conclude that the binding energy of the singlet polaron-excitons in conjugated polymers might be relatively small... 

We emphasize that the small value of the polaron-exciton binding energy results from a cancellation (indirect) of the electron-electron and electron-lattice contributions. This cancellation demonstrates the need for correlation effects to be taken into account when describing the excited state wavefunctions. 

Note that the binding energy is defined with respect to the single particle energy gap which evolves linearly with the inverse number of rings (1/n). Thus, the 0-0 transition should also evolve (approximately) as 1/n even thou the polarons and polaron-excitons wavefunctions extend over 3-4 rings (24,30). 

Figure 22.4c. Following the same arguments given previously for polaron transitions, we expect three strong transitions, PP1-PP3. For a loosely bound PP, these transitions are not far from transitions P1-P3 of polarons. However, for a tightly bound PP excitations, we expect a single transition, PP2 to dominate the spectrum, as PPi is considered to be intraband with traditional low intensity and PP3 is close to the fundamental transition and therefore difficult to observe. In this case, there are mainly two states in the gap, and the excitation is also known as a neutral BP (BP ) or a polaronic exciton. However, the PP2 transition is close in spirit to transition X2 discussed above for excitons, as a second electron is also promoted to the excited level in the case of PP. Then from the experimental point of view, it is not easy to identify and separate the transitions of a trapped exciton (2ft) from those of a tightly bound PP of (BP ) in the PM spectra. However, they may differ in their PADMR spectra [63].

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