Photoexcited state charge transport

Charge transport in nanocrystalline electrodes is clearly strongly influenced by the inter-penetration of the solid and liquid phases. If electron hole pairs are generated by band to band excitation, it is usually observed that one type of carrier is transferred to the solution, while the other is transported to the substrate contact. In the case of the dye sensitized nanocrystalline systems, an electron is injected into the conduction band from the photoexcited dye and is then transported to the substrate. The dye is regenerated by reaction of its oxidised state with a supersen-sitiser such as 1 as shown in Fig. 8.25. 

In order to study the charged photoexcitations in conjugated materials in detail their contribution to charge transport can be measured. One possible experiment is to measure thermally stimulated currents (TSC). Next, we will compare the results of the TSC-experiments, which are sensitive to mobile thermally released charges trapped after photoexcitation, to the temperature dependence of the PIA signal (see Fig. 9-17) which is also due to charged states as discussed previously. 

Thus weak disorder may be the main source of localization and conjugation length limitation. The variations in electronic properties such as absorption will then not always follow laws such as those given in Section II.C N 1 variations are well verified for polyenes, but then would not give the correct extrapolation for N — °°. Weak disorder also localizes excitations for instance, photoexcited charge carriers. It will induce the presence of polarons even if they were not bound states of the perfect chain, or of bipolarons, modify the transport properties, and increase the lifetime of charge carriers against recombination [98]. 

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