Mobility of Frenkel Excitons

Frenkel excitons in the standing film and, in the lying film, by short-range inter molecular annihilation of singlet excitons highly mobile between adjacent PHP molecules (Rc 0.8 nm). A fit to the experimental data (from Eq. 5) yields (3 = 10.8 x 108 s-1 and yrio = 1-3 x 1010 s values close to those determined from Eq. 11 (see Fig. 12). 

The formation of photoactive films on metal electrodes is not restricted to inorganic materials. Copper, for example, can be anodised to form polymeric phenylacetylide [33] and acetylide [34] layers that appear to behave as p-type organic semiconductors. The photoconducting properties of the arylethynyl polymers have been known for some time, although the mechanism of photoconductivity is not well understood. It seems probable that charge carriers are created by the annihilation of mobile Frenkel excitons at electron traps such as adsorbed oxygen rather than by direct interband excitation. 

In Chap. 8, we treated organic crystals which are composed of a single type of molecules. As we expect of organic substances, these crystals are semiconductors or insulators. The LUMOs of the molecules form the conduction band, the HO-MOs form the valence band, and the energy gap is large compared to ksT. At room temperature, typical values of the conductivity are less than about lO" (S2 cm) , and values of the mobilities are less than around 1 cm /Vs. The lowest-lying optical excitation states of these substances are Frenkel excitons. 

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