Midgap absorption

Careful experiments have not revealed any significant midgap absorption in the undoped polymer. It has been suggested that in the undoped polymer the energy levels due to solitons are close to the band edges. The simple theory which predicts midgap levels in the undoped polymer does not take into account the electron-electron interactions and is very approximate. 

The 0.5 eV midgap state absorption band is quite prominent but is rather broad. The difference in the midgap energy is 0.7 eV and the absorption peak at 0.5 eV is due to the effect of the electron-electron interaction on the transitions [8], Since the initial state involves two electrons in the midgap level and the final state involves only one, the Coulomb interaction lowers the energy of transition. The 0.5 eV band is quite similar to the midgap absorption in the doped r-PA shown in Fig. 2.2. This demonstrates that this band is associated with the same center in both the photoirradiated and the doped polymers. 

Tl)e appearance of the new peak is practically independent of the oxidizing reagents used and must, therefore, be related to a change of the electronic structure of the polymer backbone. The new peak in the near IR is generally called a "midgap" absorption since its energy is approximately situated at half of the interband (or excitonic) transition of the initial polymer. Its intensity is proportional to the extent of the reaction ("the dopant level") over the whole range of conversion. 

Figure VE-1 Ultrafast relaxation of photoexcited carriers into polaronic states with their associated midgap electronic states. We show schematically the band diagrams and corresponding absorption spectra for a non-degenerate ground state polymer.

The optical properties of (CH) are of interest, since these directly give information about the band gap and/or the levels in the midgap, which is considered to be closely related with the electric transport property. Currently available experimental data on the photoabsorption of (CH), polymers are listed in Table IV. It seems to be reasonable to regard these absorption onsets as the it - tt interband transition energy from the HO to the LU band, that is, the band gap between the valence and the conduction bands in the sense of the one-electron approximation based on the one-dimensional Peierls transition mentioned in the Section II,A. 

Figure 4. When a t/ans-polyacetylene chain is n>doped with a divalent countercation the charge could either be a. distributed between two or more chains leading to the formation of two soliton states with their corresponding midgap soliton absorptions (Eg) orb. localized on a single chain leading to the formation of a bipolaron state with its two interband absorptions...

Optical absorption experiments on the thin semitransparent t/ans-polyacetylene revealed essentially no absorption until the band gap of 1.4 eV was reached. The strong absorption beyond 1.4 eV in the interband region, is consistent with previous studies on undoped polyacetylene(lO). After n-doping with either Ca or Eu" 2 countercations, increased absorption is found in the midgap region (0.7 - 0.9 eV). 

Both solitons and polarons have their characteristic absorption bands below the band gap energy. Then, for the identification of the nonlinear excited states, the optical absorption and the electron spin resonance spectra must be studied, to get information about the midgap states and the spin, respectively. Studies of the electrical properties are also needed to get information about the charge. In this report, the excited states in single crystals of (Pt(en)2][Pt(en) ] 2 4 are studied by the experimental methods mentioned above, and the photo-induced excited state in this material is shown to be polarons, which are also produced by halogen-doping. 

Semiconducting photoelectrodes are almost always doped to improve then-properties. In most cases, the aim is to enhance the n- or p-type conductivity, as described in Sect. 2.3. Certain dopants may enhance the optical absorption of wide bandgap semiconductors [19], increase the minority carrier diffusion length [20, 21], or enhance the catalytic activity at the surface of the semiconductor [22]. Other dopants adversely affect the properties, for example, by introducing midgap bulk or surface states that act as recombination centers [23, 24]. 

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