Electron-hole pair spatial separation

After ionization of the excitons, the spatial separation of electrons and holes is small, i.e. <10 nm. Therefore, the charges attract each other by Coulomb interaction, and cannot be thought of as two independent freely moving species (Figure 13.2b). In the literature on organic solar cells this situation is described as bound electron-hole pairs , or geminate electron-hole pairs . The separation of electron and hole can be so small that their... 

The structure and function of this bacterial photosystem reveals important principles for the design of artificial photosystems. First, the sensitizer needs to be posi tioned close to secondary acceptors and donors which themselves are spatially iso lated from each other such that photoexcitation leads to rapid spatial separation of the electron hole pair. Second, compartmentalization of the photosynthetic assembly is likely to be necessary so as to prevent wasteful back reactions. For water splitting, a system in which H2 and O2 are generated in separate compartments would have both safety and efficiency advantages. 

Fig. 3 Schematic repersentation of generation of photocurrent in BHJ devices, (i) Generation of exciton on the absorption of a photon. Generation of exciton occurs in the donor materials, (ii) Diffusion of exciton to a D-A interface, (iii) Exciton dissociation via electron transfer to the acceptor to form a charge transfer state, CTS (iv) Complete separation of the electron-hole pair, (v) Transpotation of charge carriers to the respective electrodes, (vi) Collection of charges. These processes are illustrated according to (left) the spatial location at which they occur and (right) on a...

The lessened value of dV/dx within the semiconductor in the high surface state case tends to increase the loss of active electrons by means of hole—electron pair recombination because the rate of spatial separation of the electron from its hole, which was formerly encouraged by the electric fields sending the oppositely charged entities in die opposite directions, is now diminished. 

The existence of an electric field in the spatial charge region allows the separation of the photogenerated pairs. In the case of n-type semiconductors, electrons migrate toward the bulk whereas holes move to the surface (Fig. 6.10a). In the case of p-semiconductors, holes move towards the interior of the semiconductor and electrons toward the surface (Fig. 6.10b). 

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