Charge generation, band model

Many authors have investigated the photoconductivity of the polydiacetylenes [142-171], The main problem discussed concerns the nature of the initial act of the photoeffect. At first, most authors considered the exciton formation to occur at the beginning with consequent dissociation on the free carriers. Then it was shown the broad band existence for directions along the chains. The unification of the excitonic and band model of the free charge carrier generation was developed [146-150],... 

Fig. 5.1. Charge generation process in a single-layer conjugated polymer device under short-circuit conditions in the MIM model. VB valence band, CB conduction band, Eg bandgap, P+, P positive and negative polarons...

To facilitate a self-contained description, we will start with well-established aspects related to the semiconductor energy band model and the electrostatics at semiconductor electrolyte interfaces in the dark . We shall then examine the processes of light absorption, electron-hole generation and charge separation at these interfaces. Finally, the steady-state and dynamic (i.e., transient or periodic) aspects of charge transfer will be considered. Nanocrystalline semiconductor films and size quantization are briefly discussed, as are issues related to electron transfer across chemically modified semiconductor electrolyte interfaces. 

The phthalocyanines are still of some interest, and Ayers has interpreted the photoelectrochemistry of copper phthalocyanine in terms of the band-model for semiconductors. Menzel et alf have studied the photoconductivity and electrical field-induced fluorescence quenching in metal-free phthalocyanine films. Doping of the film increases charge-carrier photogeneration, and it seems likely that the mechanism is extrinsic, involving field-assisted exciplex dissociation. These are complicated materials, which are difficult to purify, and the mechanisms of carrier generation in polycrystalline and amorphous films are still obscure. The porphyrins and monolayer assemblies of chlorophyll on SnOj electrodes 2 72,273 ijggjj studied. 

In the C=0 frequency region, the strong difference bands at 1748 cm , at 1702 cm and at 1682 cm correspond to changes of position and intensity of the 10a ester and the 9-keto C=0 groups of the BChl dimer. In this spectral region, band features very close to those previously obtained by photochemical charge separation (PQ P Q") [6] appear. They can be closely approximated by electrochemically-generated BChl model difference spectra [8]. 

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