Polaron and Exciton

Kuhn C (1989) SoUtons, polarons, and excitons in polyacety lene Step-potential model for electron-phonon couphng in r-electron systems. Phys Rev B 40 7776-7787 Kuki M, Koyama Y and Nagae H (1991) Triplet-sensitized and thermal isomerization of all-trans, 1-cis, 9-cis, 13-cis, and 15-cis isomers of carotene Configurational dependence ofthe quantum yield of isomerization via the Tj state. J Phys Chem 95 7171-7180... 

A useful simplification to this expression arises by noting that the geometric distortions of the polarons and exciton polarons (namely the or states) from the ground state structure are similar (as described in Chapter 7). Thus, the Huang-Rhys parameter (proportional to AQi, as defined in Fig. 9.14) for the 1 B and 1 E states relative to the positive polaron is negligible. Therefore, to a good approximation,... 

Recently the theoretical models developed for polyacetylene have been applied to PDAs (00, 115, 116). A degenerate ground state does not occur for PDA so that this discussion is limited to polarons. The potential surface for the ground state structures deduced by this method is very similar to that calculated by the ab initio method (00, 106) indicating that the alternative butatriene structure is metastable. A further conclusion is that the band gap is primarily due to the bond alternation produced when the TT-electrons are added to the bare PDA backbone. These calculations are currently being extended to make realistic exciton calculations for PDAs (117). The occurrence of acousticmode polarons and exciton-polarons in PDAs has also been discussed recently (95, 145). 

In Fig. 1 the absorption spectra for a number of values of excitonic bandwidth B are depicted. The phonon energy Uq is chosen as energy unit there. The presented pictures correspond to three cases of relation between values of phonon and excitonic bandwidths - B < ujq, B = u)o, B > ujq- The first picture [B = 0.3) corresponds to the antiadiabatic limit B -C ljq), which can be handled with the small polaron theories [3]. The last picture(B = 10) represents the adiabatic limit (B wo), that fitted for the use of variation approaches [2]. The intermediate cases B=0.8 and B=1 can t be treated with these techniques. The overall behavior of spectra seems to be reasonable and... 

The combination of a hole polaron and an electron polaron, with binding energies Ep+ and Ep-, respectively, results in the formation of an exciton. Their difference corresponds to Et and is also referred to as the single particle energy gap Egsp -... 

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. 

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. 

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

The foregoing section attempted to provide an introduction to the dynamics of singlet excitons, generated either by photoexcitation or by polaron recombination, and the effects of polarons and TEs on the SE dynamics. We now turn to the basic structure and dynamics of OLEDs, which obviously reflect the basic processes described above. 

Polaron kinetics is also unaffected by variations in the applied voltage, as shown in Figure 8-14b. The inset of Figure 8-14b shows CPG efficiency as a function of the applied electric field. Symmetry with respect to the LED bias voltage rules out space charge effects and exciton-carrier interactions. In addition, we note that has a quadratic dependence on the electric field, sim-... 

Chang, J.F., Clark, J., Zhao, N., Sirringhaus, H., Breiby, D.W., Andreasen, J.W., Nielsen, M.M., Giles, M., Heeney, M. and McCulloch, I., Molecular-weight dependence of interchain polaron delocalization and exciton bandwidth in high-mobility conjugated polymers, Phy. Rev. B, 74, 15318, 2006 (submitted). 

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