Exciton mean lifetime

In crystals where the probabilities of nonradiative decay processes are smaller (the latter takes place in luminescent crystals with a large quantum luminescence yield), the lifetime for singlet-excitons in pure crystals can be of the order of 10-9 s. For triplet-excitons this time can be a few orders of magnitude larger (for example, the lifetime of a triplet exciton in anthracene is of order 10-4 s). The characteristic time of exciton scattering by phonons is of the order of picoseconds and thus usually is much less than its radiative lifetime. This means that generally one may assume that during the exciton s lifetime thermodynamic equilibrium of excitons and phonons is established. 

The incorporation of oxadiazole moieties, which are highly electron deficient, into PPV/MEH-PPV as side chains, increases the exciton dissociation rate and promotes the electron transport. The oxadiazole moieties, c.f. Figure 3.13, are attached to the backbone via Cm alkyloxy Unks. The exciton dissociation rate is assumed to follow an exponential decay law. The decay constant (obviously the mean lifetime) x for oxadiazole-containing PPV/MEH-PPV is 0.4 ns, whereas the decay constant for pure MEH-PPV is around 0.65 ns. 

There remain, however, some questions about the real meaning of these data and the correctness of the evaluation procedure. First of all, the data presented in Table II suggest efficient trapping of the triplet excitons by intrinsic, not exclmeric traps of unknown structure. This is corroborated by reports of all authors who performed quenching and phosphoresence decay measurements that the phosphorescence lifetime is not shortened by adding the quencher. This is a clear proof that it is primarily not the free triplet which is observed in phosphorescence but rather a trapped species, since otherwise Equation (5) should apply. 

Here, th is the lifetime of the host excitons without traps. We furthermore make the simplifying assumption that every exciton is captured when it reaches a trap and that the excitation density is constant with time and remains homogeneous. If, in addition, the mean free path of the exciton is smaller than the capture radius R of the traps for excitons, and D is the exciton diffusion coefficient, then we obtain... 

Taking the lifetime of the triplet excitons to be r > 10 ms, we find the mean squared displacement or diffusion length L for triplet excitons in anthracene crystals at room temperature to be... 

Both the exciton/radical pair equilibrium model and the bipartite model predict formally the same kinetics and thus both give rise also to a biexponential fluorescence decay. However, the two models are fundamentally different. This difference consists in the entirely different meaning of the rate constants involved and thus in the entirely different origin of the two observed lifetimes. In the bipartite model the biexponentiality is due to the equilibration of the excitons between antenna and reaction center. Thus one of the lifetime components reflects an eneigy transfer process. In contrast to the exciton/radical pair equilibrium model the bipartite model basically describes a diffusion-limited kinetics. Despite the fact that it formaUy can describe correctly the observed kinetics, the application of the bipartite model on experimental data leads to physically unreasonable results. First it results in a charge separation time in the reaction centers which is by one to two orders of magnitude too high. 

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