Towards Photoinduced Charge Separation

Although this possibility has still to be confirmed, the above discussion points out the importance of kinetic data obtained with model systems for a rational design of photochemical molecular devices. 

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Fig. 15 Rate data for photoinduced charge separation and subsequent charge recombination in the dyads 18(h)- Charge separation rates, cs, in THF at 20°C were determined both from fluorescence lifetimes92,102 and by pump-probe (time-resolved transient absorption) spectroscopic measurements.105 The mean lifetimes towards charge recombination, rcn were obtained from time-resolved conductivity measurements in 1,4-dioxane.99 101,103,104...

Important progress has been made toward creating bulk D/A heterojunction materials [61,93,94,136].As shown in Figure 8.49, the short circuit current, /sc = 0.5 mA/cm under 20 mW/cm illumination, corresponding to a collection efficiency of t)c = 7.4% of electrons per incident photon [61] is approximately two orders of magnitude higher than that of pure MEH-PPV tunnel diodes as well as of the MEH-PPV/C60 heterojunction device described in the previous section. The electroluminescence quantum efficiency of this blend device was 10 -10 times less than in pure MEH-PPV devices, consistent with the ultrafast photoinduced charge separation which quenches the emission of the donor [61]. 

After photoinduced electron injection, the strong interfacial electric field at the semiconductor solid-liquid junction draws the injected electron (or hole) into the semiconductor and towards the electrical contact. This process facilitates charge separation and reduces the chances of hole-electron recombination. The most important prerequisites for this process include, as described, good redox matching of the dye species excited state and the conduction band of the semiconductor, as well as strong orbital coupling between the immobilized dye and semiconductor. 

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