Natural photosynthetic complexes accomplish the rapid conversion of photoexcitations into spatially separated electrons and holes through precise hierarchical ordering of chromophores and redox centers. In contrast, organic photovoltaic (OPV) cells are poorly ordered, utilize only two different chemical potentials, and the same materials that absorb light must also transport charge; yet, some OPV blends achieve near-perfect quantum efficiency. Here we perform electronic structure calculations on large clusters of functionalized fullerenes of different size and ordering, predicting several features of the charge generation process, outside the framework of conventional theories but clearly observed in ultrafast electro-optical experiments described herein. We show that it is the resonant coupling of photogenerated singlet excitons to a high-energy manifold of fullerene electronic states that enables efficient charge generation, bypassing localized charge-transfer states. In contrast to conventional views, our findings suggest that fullerene cluster size, concentration, and dimensionality control charge generation efficiency, independent of exciton delocalization.

This review concerns the concept of multi-excitation entropy (MEE) and its consequences. When a fluctuation involving a large number of excitations occurs, for example, when a large activation barrier is overcome, there must be a large entropy associated with this fluctuation. First, the concepts of free energy and entropy, of activated processes and the Arrhenius and Eyring equations are reviewed. The tendency to neglect entropy, whose value is difficult to determine, in modelling kinetic processes, is briefly discussed. We then present a review of the experimental observations of the phenomenon which is variously known as the Compensation Law, the Isokinetic Rule and the Meyer-Neldel Rule (MNR). These observations include examples from chemistry, condensed matter physics, biology and geology. Arguments are then presented for the importance of entropy and particularly of MEE in both kinetics and thermodynamics, when activation energies are large. After a discussion of non-entropic models of compensation, we present results which support the MEE model as an explanation of MNR. The behaviour of systems with low activation energies, or at high temperatures, to which the MEE model does not apply, is then discussed.Several consequences of MEE, including applications to the interpretation of experimental data, particularly the unification of models of dc and ac electrical properties of materials are considered. The high temperature behaviour of systems which obey the MNR at low temperature is then explained, and the idea of a total entropy, of which the MEE is a part, is introduced, as is the correlation between the two empirical parameters encountered in MNR. Finally, these ideas lead to verified predictions of reasonable values of attempt frequencies and cross sections in kinetic processes, which initially appear unreasonable.

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We have recently demonstrated that the Meyer-Neldel {MN) rule {compensation law) may be understood as arising naturally when the activation energy for a process is significantly larger than both the typical excitations available and kT. This conclusion was supported by the results of two microscopic models, related to special cases. In the present paper we present arguments, based on general results from statistical physics, which lead to the same conclusion. We show that this simple explanation also leads to the solution of a number of puzzles which have been associated with Meyer-Neldel behavior. W'e show that phenomena in groups of similar materials yield similar MN slopes, Finally, we show that the values of the slope for semiconductors with gaps in the 1 -2-eV range are consistent with the suggestion that optical phonons are the source of the excitation energy in such processes.

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