Subgap absorptions

Fig. 9.11. Thickness dependence of the ESR and subgap absorption measurements of defects in undoped a-Si H. The inset is an expansion of the small thickness data. The non-zero intercept is due to surface or interface states (Jackson et at. 1983).

The absorption spectrum of m-LPPP (see Figs. 8.5 and 8.14) is characterized by a steep onset of the absorption above the energy gap (ow 1.1 x 105 cm-1 at 453 nm) and a very low subgap absorption ( ,], < 12 cm-1).4,5,77 The minimum value of a is more determined by scattering effects than by true absorption, so that this value can be drastically decreased if the film quality of the polymer film is further improved, which is essential for the application of this polymer in waveguide structures for optical23,78 79 and electrically pumped lasers. 

The species with the lower oxidation state characterized by the two sharply resolved subgap absorptions are observed for the oligothiophenes of trimer and compounds with a higher degree of polymerization. 

Table XV. Maximum of Subgap Absorptions Due to Charged Excitations in the Film State...

When a negative polaron is formed, one electron is added to the +wg level (Fig. 18c). In this case, three transitions are also expected. Within the Fesser-Bishop-Campbell model, the subgap absorptions due to the Pi and P2 transitions are much stronger than that due to the P3 transition, because the P3 transition is symmetry forbidden. Quantitatively, the intensities of these subgap absorptions are given as a function of the ratio aig/Ag. 

The transition from the —q level is missing, because the level is unoccupied. When a negative bipolaron is formed, two electrons are added to the -b o level and thus two transitions are again expected (Fig. 18e). Within the Fesser-Bishop-Campbell model, the subgap absorption due to the BPj transition is much stronger than that due to the BP3 transition,... 

The doping- and photoinduced absorption spectra of polythiophene, substituted polythiophenes, and their composites have been reported [145-166]. Let us discuss the subgap absorptions due to charged excitations in unsubstituted polythiophene. The optical absorption spectrum of BF -doped polythiophene [42,46] and the photoinduced absorption spectrum of polythiophene [145] are shown in Figure 20. In the spectrum of BFJ -doped polythiophene (Fig. 20a), doping-induced bands are observed at 0.73 and 1.68 eV, which are located below the gap edge, 1.94 eV. The 0.73-... 

Willekens J, Adriaenssens GJ, Aemouts T, Geens W, Vanlaeke P, Poortmans J (2004) Probing the subgap absorption of photovoltaic polymer blends by the constant photocurrent method. Thin Solid Films 451 26-28... 

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