Soliton Models of Charge Generation and Transport

Photoconduction and absorption spectra of trans-polyacetylene are presented in Fig. 16 [104], 

The threshold of the photocurrent is at the energy of 1 eV. This value is less than the absorption edge enagy, what proves the existence of deep levels inside the forbidden gap. The absence of the structure at the beginning of the interband transitions excludes the possibility that the photocurrent is due to the surface exciton dissociation. Attempts to observe good photoconduction in ris-(CH) were unsuccessful. The photocunent in cis-(CH)n was three orders of magnitude lower than in trans-(CH) . 

These and other results on absorption and luminescence were the reason why the soliton model was proposed for the photoconduction process in trans-polyacetylene [103-110], 

The main scheme is shown in Fig. 17. The photogenerated electron hole pairs transfer to the soliton-antisoliton pairs in 10 13s. Two kinks appeared in the polymer structure, which separates the degenerated regions. Due to the degeneration, two charged solitons may move without energy dissipation in the electric field and cause the photoconductivity. The size of the soliton was defined as 15 monomer links with the mass equal to the mass of the free electron. In the scheme in Fig. 17, the localized electron levels in the forbidden gap correspond to the free ( + ) and twice occupied ( — ) solitons. The theory shows the suppression of the interband transitions in the presence of the soliton. For cis-(CH)n the degeneration is absent, the soliton cannot be formed and photoconductivity practically does not exist. 

A lot of research was carried out for proving existence of the soliton in trans-(CH)n by various methods - induced absorption and luminescence, electron spin resonance, transit photogeneration and so on. The references may be found in the monograph [14]. 

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