Excitons decay

Figure 1 Absorption spectra for system of excitons linear and locally coupled with strength S to nondispersive phonons with energy u>o calculated in NDCPA. B is excitonic bandwidth, and 7 is excitonic decay rate.

The interpretation of our CPG data is complicated by the presence of comparatively fast radiative and nonradiative decay channels for the singlet exciton, which compete with the field-induced dissociation. In order to provide a clear picture of the observed mechanism and disentangle it from the singlet exciton decay dynamics, we define the following phenomenological time-dependent parameter ... 

Exciton decay When an exciton decays radiatively a photon is emitted. When the excitons form in fluorescent materials radiative decay is limited to singlet excitons and emission occurs close to the recombination region [7] of the OLED due to the relatively short lifetime of the excited state (of the order of 10 ns). For phosphorescent materials, emission can occur from triplet excitons. Due to the longer excited state lifetime (of the order of hundreds of nanoseconds), triplet excitons can diffuse further before decaying. 

We conjecture that the dynamical robustness of the ultrafast exciton decay indeed results from the presence of multiple decay pathways. While the trans-... 

Figure 3.8. The exciton decay in photon and polariton states The time evolution (in units of w0r) of a 2D exciton K created at r = 0 (Kid). This decay, illustrated for various wave vectors (in units of iu0/c), is purely exponential for K < to0/c, but exhibits very complex transient oscillatory behavior in the region K - oj0/c. For K > o>Jc the 2D exciton is radiatively stable.

This type of laser produces output pulses which are typically between 1 and 10 ns duration and are well suited to provide initial excitation in the study of the decay of excited states and other transient effects in small molecules. Many fundamental processes, however, occur on a time scale much shorter than the 1—10 ns resolution available with dye lasers of the type discussed above. These processes, such as the relaxation of large biological molecules and dyes in solution, exciton decay and migration in solids, charge-transfer and other non-radiative transfer processes between molecules, and many more, take place on a picosecond time scale. 

The total concentration of holes nh is a sum of the concentration of trapped (nht) and free (nhf) carriers. However, often rihf/nht —> 0, nh nht due to a large concentration of traps. Then, the excitons are quenched by trapped carriers and the annihilation rate constant yTq is equivalent to the mobile exciton-immo-bile (trapped) charge carrier interaction rate constant yxq. Under space-charge-limited conditions, the concentration of charge is simply proportional to the applied voltage (U), nht = (3/2) o U/ed2, where d is the sample thickness, e is the electronic charge, s is the dielectric constant of the sample material, and s0 is the permittivity of free space. Thus, it may be seen that the fractional change in the triplet exciton decay rate... 

Figure 184 External quantum EPH efficiency data taken from Fig. 181 and represented by a < eph pl°t n order to fit with the trip 1 e t—charge-ca i rier interaction limit for triplet exciton decay according to Eq. (337) (solid lines). After Ref. 304. Copyright 2002 American Physical Society, with permission.

Excitons are neutral quasiparticles, so their creation by light absorption does not generate a current directly. Hence a test for the presence of excitons If photoconductivity is absent, absorption is excitonic the onset of band-to-band transitions would then be signaled by the photoconductivity threshold. It is, in fact, not so clear cut, since there are many ways by which exciton decay can generate charge carriers this is well documented in molecular crystals [128]. 

The presence of interfaces within a polymer LED can also introduce additional nonradiative decay channels. This is particularly important in proximity to a metal electrode. Excitons which are able to diffuse to the metal surface are liable to be quenched directly by interaction with the metal wave function. This mechanism is therefore active only within a few nanometers of the interface. At larger distances (up to about 100 nm), excited molecules can couple to the surface plasmon excitations in the metal, thus providing a further nonradiative decay channel. The combined effects of changes in the radiative and nonradiative rates in two-layer LED structures have been modelled by Becker et al.,83 who have been able to model the variation in EL efficiency with layer thickness due to changes in the efficiency of exciton decay. 

Band-to-band recombination of free charge carriers and exciton decay (step 1 ), and recombination and exciton decay through defects (step 8 ) are probably accompanied by luminescence. 

Polymers in Rigid Solution. The emission spectrum of PCVA in 2-methy1tetrahydrofuran (MTHF) at 77 K consists of prominent delayed fluorescence and phosphorescence bands(19). For this reason it was decided to investigate the rate of triplet exciton decay in these rigid solutions and to treat the data in terms of concurrent first and second order processes. For systems in which an equilibrium distribution of potential reactants may be assumed, eq 1 may be employed for data analysis. It is not clear, however, that such a distribution is valid for polymer solutions especially in light of evidence suggesting that T-T annihilations occur principally by intra-coil processes(14-15). 

Let us assume that in an initial state the exciton (//, k) propagates along the chain. If the operator (4.88) is considered as the cause of transitions, than in the framework of perturbation theory the probability per unit time that an exciton decays into a photon, is given by the relation... 

The hypothesis of a TFB F8BT exciplex is also supported by the annealing experiments shown in Fig. 2.15. The exciplex disappears when annealing causes the polymers to phase separate. We also see that the exciton decays slow down, as would be expected for a decreased transfer to the exciplex. 

In Fig. 2.16, the PL emission spectra of the pure polymers F8 and PFB are compared with a F8 PFB blend. In the blend, efficient PFB emission obscures the weak F8 emission but no additional emission peaks are visible in these CW spectra. Figure 2.17 compares the time-resolved PL decays of the pure polymers with that of the blend. The pure polymers show almost monoexponential decays over three orders of magnitude with 590ps (F8) and 1.7ns (PFB) lifetimes. In the blend, the fast excitonic decay is observed as well, but in addition a long-lived emission with a lifetime of roughly 26 ns (when fitted in the 30 to 90 ns window) is detected. 

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