Charge generation excitonic

Organic semiconductor photovoltaic cells share many characteristics with both DSSCs and conventional cells. Charge generation occurs almost exclusively by interfacial exciton dissociation, as in DSSCs, but, in contrast, OPV cells usually contain no mobile electrolyte and thus rely on Vcharge separation. OPV cells may have planar interfaces, like conventional PV cells, or highly structured interfaces, like DSSCs. They provide a conceptual and experimental bridge between DSSCs and conventional solar cells. 

It is possible to define a quantum efficiency describing the true efficiency of the material in the photoactive layers, i.e., the efficiency of exciton-to-charge generation (QEC) [19]... 

L.A.A. Petterson, L.S. Roman, O. Inganas The quantum efficiency of exciton-to-charge generation in organic photovoltaic devices, J. Appl. Phys. 89 (10) 5564-5569 (2001)... 

Poly(phenylmethylsilane) (PMPS) was investigated by Kepler et al. (1983, 1984, 1987). Figure 32 shows the wavelength dependence of the absorption and the normalized photocurrent. The spectral dependence of the photocurrent indicates that carrier generation occurs by an extrinsic process which results primarily from an interaction of charge-transfer excitons with the surface. The argument is similar to that proposed earlier by Kepler (1976) to describe hole... 

There are other processes for charge generation these include two-photon absorption and single photon ionisation of singlet and triplet excitons. These processes are proportional to the square and higher powers of the light intensity and usually somewhat weaker than the processes described in the previous paragraph. 

A triplet exciton annihilation mechanism has been proposed for charge generation in molecularly doped polymers initiated with very fast, high-intensity, excitation [25b,c]. In this mechanism it is proposed that both charge separation, associated with the Onsager model, and the fusion process are very strongly field-dependent. This model has not been invoked in recent years. 

Electroabsorption of Azo-TPA (Scheme 5) is interpreted in terms of a charge transfer exciton rather than a Frenkel exciton as formed by Azo-FO [341]. From this and other studies it is suggested that carrier generation from charge transfer excitons is more efficient than from Frenkel excitons [34m]. 

Charge transfer from the 11 Bu to an adjacent molecule or segment of a chain, i.e., dissociation of the 11 Bu. This process may also be extremely fast.24 Indeed, so fast that it has been suspected that this charge transfer state, aka a spatially indirect exciton, charge transfer exciton (CTE), or intermolec-ular or interchain polaron pair, may be generated directly from the ground state.24... 

Within the bulk heterojimction, the donor and acceptor domains are generally disordered in volume. For exciton dissociation and charge generation a fine nanoscale intermixing is required, whereas for the efficient transport of charge carriers percolation and a certain phase separation are needed to ensure imdisturbed transport. Hence the optimization of the nanomorphology of the photoactive blend is a key issue for improving the efficiency of the photovoltaic operation [62,66,67]. 

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