Polaronic absorption

The final remark of this section concerns the polaronic transition of m-LPPP around 1.9 eV, where we can observe P2 with its vibronic replica P3 at 2.1 eV. In Figure 9-20 we show this polaronic absorption in m-LPPP as detected by photoin-duced absorption (a), chaige-induced absorption in conventional light-emitting devices (b), and chemical redox-reaction (c). Only under pholoexcilation, which creates both neutral and charged species, the triplet signal at 1.3 eV is also observed. 

Han and Elsenbaumer also note that the BP initially formed can react with neutral polymer to form two distinctly different polarons via interchain electron transfer. After twenty-four hours, our optical spectra are unchanged, and have measurable ESR activity. However, in contrast to alkoxy-PPV polymer, we do not observe a typical polaronic absorption spectrum, but rather one almost identical to the bipolaron obtained from SbCl5 doping of 10-5 M solutions in CH2CI2. A possible interpretation is one which allows for P and BP states coexisting in dynamic equilibrium, with the bipolaron dominating the optical absorption. The absorption characteristics of the protonically doped polyenes are shown in Table II compared to the same samples doped with SbCl5. 

For the mixture of OPV2 and MP-Ceo in ODCB, the PIA spectrum recorded with selective excitation of MP-Ceo at 528 nm (Fig. 1.30a) exhibits the transitions of the MP-Ceo(7i) state at 1.78 and 1.54 eV, which increase linearly with the excitation intensity (—AT oc Ip, p = 0.99-1.00) and correspond to a lifetime of 170-290 ps. Together with the concurrent absence of polaron absorption, we infer that intermolecular charge transfer between OPV2 and MP-Ceo does not occur in ODCB. 

These experiments yield an exciton binding energy of 0.5 eV relative to separate electron and hole. This is a large value. An important consequence, although one that is never mentioned, is that in such a case, the polaron binding energy u>p (see Fig. 7 of Chapter 11) must be larger than 0.25 eV for the absorption between polaron levels in the gap to occur below that of the exciton in practice, the polaron absorption, if it exists, will remain hidden under the excitonic absorption. 

Another absorbing species present both in photo excited and electrically excited para-phenylenes are polarons. In Fig. 8.9, we have summarized the absorption and emission spectra encountered in para-hexaphenyl (1) the triplet absorption, (2) the stimulated emission observed in time-resolved experiments with a 200-fs resolution, (3) the polaron absorption, and (4) the continuous-wave photoluminescence emission. A clearer picture for the polaronic state can be derived from experiments on the ladder-type PPPs. 

FIGURE 8.9. The solid line peaking at 1.78 eV (1) describes the triplet continuous-wave (cw) absorption spectrum of hexaphenyl squares (2) represents the femtosecond pump/probe spectra at zero delay between pump and probe (solid line to guide the eye), (3) the polaronic absorption peaks at at 2.3 eY, and (4) the cw emission spectrum of peaks at roughly 2.9 eV. 

Riischer CH, Schrader G (1996) Temperature dependence of small polaron absorption in biotites. Phys Chem Minerals 23 243-245... 

In Figure 14.13, spectacular optical changes are observed even at moderate potentials. A very strong excitonic absorption at 1.8 eV is observed, but which is more surprising is the very high polaronic absorption. Since, from the Raman results as well as from the XPS analysis [74], the number of carriers is lower in pH3 solution than in pH I solution, it means that the absorption... 

Figures 14.18 and 14.19 present the variations of the excitonic absorption as a function of the polaronic absorption, during the equlibration time and for different potentials. It is interesting to see that, at each pH, the points corresponding to the different polarization potentials are on the same slope therefore, the ratio between the number of polarons and excitions (i.e., between the rates of protonation and oxidation reactions) depends exclusively on the pH (and not on the potential). Two different steps can be separated in the first step, the polaron stength increases more rapidly than that of the exciton, then the polaron strength reaches its steady value and the exciton continues to increase. The two steps are found for the two pH.

Titanium dioxide exhibits optical properties very similar to those of tungsten oxide. Electrons in the conduction band become localized by the electron-phonon interaction and give rise to polaron absorption. Coatings of titanium oxide are less stable in an electrochromic device than films of tungsten oxide, and have therefore not been used so much. 

---