Percolation exciton

Kopelman, R., 1976, Exciton Percolation in Molecular Alloys and Aggregates, in Fong, F.K. ed.. Radiationless Processes in Molecules and Crystals, Vol. 15 (Springer-Verlag, Heidelberg, Germany). 

For proper operation of a bulk heterojunction photovoltaic cell, a special alignment of the HOMO and LUMO levels of the bulk heterojunction components must be accomplished, compatible with the electrodes work functions, as depicted in Scheme 5.8. If an exciton is formed in the polymer phase, then the electron is transferred to the NC phase and reaches the aluminum electrode via its percolating pathway. The remaining hole is transported to the ITO electrode through the polymer phase. In the alternative case, that is, the formation of an exciton in the NCs phase, the hole is transferred to the polymer phase and then transported to the ITO electrode, whereas the electron reaches the aluminum electrode through the NCs phase. 

Excitation spectroscopy Monitoring of the surface emission allows one to discriminate the upper excited surface states and their relaxation dynamics. Problems such as surface reconstruction, or quantum percolation of surface excitons upon thermal and static disorder, are connected with high accuracy to changes of the exciton spectra.61118,119,121... 

Fig. 5.5. Luminescence quenching (bullets, right hand, axis) and short-circuit current Ac (black squares, left hand axis) vs. molar fullerene concentration in a bulk hetero junction composite. The different onsets for percolation for the two phenomena (exciton diffusion versus ambipolar carrier transport) can be clearly seen...

The leitmotifs of these devices include bespoke dye sensitisers, space-quantised nanoscale structures that enable hot carrier or multiple exciton generation, molecular and solid-state junction architectures that lead to efficient exciton dissociation and charge separation, and charge collection by percolation through porous or mesoscale phases. Another common theme underlying the devices discussed in this book is the... 

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]. 

An extensive discussion of experiments on exciton transport in isotopically disordered crystals and numerical simulations of this phenomenon in the framework of a percolation model may be found in the review paper by Kopel-mann (20). A more recent review of this field, including the discussion of the Anderson model, may be found in the book by Pope and Swenberg (21). 

This new theory of the non-equilibrium thermodynamics of multiphase polymer systems offers a better explanation of the conductivity breakthrough in polymer blends than the percolation theory, and the mesoscopic metal concept explains conductivity on the molecular level better than the exciton model based on semiconductors. It can also be used to explain other complex phenomena, such as the improvement in the impact strength of polymers due to dispersion of rubber particles, the increase in the viscosity of filled systems, or the formation of gels in colloids or microemulsions. It is thus possible to draw valuable conclusions and make forecasts for the industrial application of such systems. 

Once the free charges (electrons and holes) are generated by exciton dissociation, they have to be transported to the respective electrodes. It is expected that combination of CPs (high hole mobility) with QCNs (relatively high electron mobility) will improve the charge transport and The use of elongated and branched QCNs (e.g., nanorods, TPs, or hyper-branched shaped) provides percolation at very low concentration of QCNs in hybrids. 

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