Disorders triplet excitons

The relatively low values of yss and Ds in quasi-amorphous solids might be underlain by disorder (see Sec. 2.4.3) and/or a contribution of triplet excitons in quenching of fluorescent singlets (cf. Sec. 2.5.1.2). The diffusion coefficient of triplets is expected to be lower than of singlets since both energy donor and acceptor transitions are disallowed. A low value of yss has been found for the triplet-triplet annihilation rate constant from biexcitonic quenching... 

Section IV is devoted to excitons in a disordered lattice. In the first subsection, restricted to the 2D radiant exciton, we study how the coherent emission is hampered by such disorder as thermal fluctuation, static disorder, or surface annihilation by surface-molecule photodimerization. A sharp transition is shown to take place between coherent emission at low temperature (or weak extended disorder) and incoherent emission of small excitonic coherence domains at high temperature (strong extended disorder). Whereas a mean-field theory correctly deals with the long-range forces involved in emission, these approximations are reviewed and tested on a simple model case the nondipolar triplet naphthalene exciton. The very strong disorder then makes the inclusion of aggregates in the theory compulsory. From all this study, our conclusion is that an effective-medium theory needs an effective interaction as well as an effective potential, as shown by the comparison of our theoretical results with exact numerical calculations, with very satisfactory agreement at all concentrations. Lastly, the 3D case of a dipolar exciton with disorder is discussed qualitatively. 

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