Charge generation and separation

The mechanisms for charge generation and separation upon optical excitation and light emission are different for the spherical nanoparticles and essentially linear chain polymers. 

Fig. 10-4. G neration of electron-hole pairs by photoexdtation and their recombination or separation in semiconductor (a) generation and recombination of photoexdted electron-hole pairs in a flat band state, (b) generation and separation of photoexdted electron-hole pairs in a space charge layer.

The earliest all-organic photoconductive compositions consisted of poly-vinyl(carbazole) (PVK, 1) and 2,4,7-trinitrofluorenone (TNF, 2) [25], In spite of its historical importance, the PVK/TNF system is an unusual one, because it is one of the few photoconductors which acts simultaneously as a photogenerator of primary charges and the medium in which the charge transport is accomplished. It is much more usual to employ a separate charge generator and a distinct transport medium. 

At short times the decay from the blend is faster than that from the pure polymers. This is due to the energy transfer from the excitons to the exciplex, as well as the charge separation that excitons undergo at the heterojunction. In Section 2.3.1 we investigate these phenomena and develop a model for charge generation and exciplex formation at the heterojunction. 

The charge generation and charge transport efficiency of a polymer can also be measured separately by other techniques. For example, laser flash photolysis can measure the charge generation efficiency directly, if the generated electron or hole (anion or cation) has a distinct absorp-... 

The problems with organic solid-state photovoltaic devices are well known the localized nature of the optical excitations in most cases and the high density of charge-carrier traps in polycrystalline dye films limit the efficiency of charge-carrier generation and separation. Nevertheless, Chamberlain et have... 

In this section we review theoretical and experimental results important for understanding the photophysics of polymer solar cells. We examine the states that are involved in the processes of photoinduced charge generation, and present a theoretical framework that describes the separation and recombination of charge-transfer states. Finally, we review the mechanisms for charge formation that operate in a fdm of a single conjugated polymer and the additional mechanisms, relevant to solar cell devices, that occur in binary donor acceptor blends. 

Figure 13.3 Charge-transfer state generation and separation. The energetics of charge-transfer state generation and subsequent separation are shown for (a) a highly excited singlet state in a pristine material, (b) thermal excitation or electric field assistance in a pristine material, and...

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