Excitons nonradiative decay

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 ... 

The PL quantum yield r)pl. While r]pl of many dyes is close to 100% in solution, in almost all cases that yields drops precipitously as the concentration of the dye increases. This well-known concentration quenching effect is due to the creation of nonradiative decay paths in concentrated solutions and in solid-state. These include nonradiative torsional quenching of the SE,148 fission of SEs to TEs in the case of rubrene (see Sec. 1.2 above), or dissociation of SEs to charge transfer excitons (CTEs), i.e., intermolecular polaron pairs, in most of the luminescent polymers and many small molecular films,20 24 29 32 or other nonradiative quenching of SEs by polarons or trapped charges.25,29 31 32 In view of these numerous nonradiative decay paths, the synthesis of films in which r]PL exceeds 20%, such as in some PPVs,149 exceeds 30%, as in some films of m-LPPP,85 and may be as high as 60%, as in diphenyl substituted polyacetylenes,95 96 is impressive. 

The radiative decay of singlet excitons is clearly an important process in the operation of polymer LEDs. This rate is denoted by kr, where for PPV, (k, rl 1200 ns.19 Radiative decay competes with various nonradiative decay processes, such as quenching of excitons by defects, exciton dissociation, and intersystem crossing to form triplet states. Assuming that both radiative and nonradiative decays are monoexponential, the photoluminescence quantum efficiency, PLeff, defined as the number of photons emitted per photon absorbed, is given by... 

A simple example of this is the case of a molecule (modeled as an oscillating dipole) close to a perfect mirror. If the dipole is parallel to the mirror, destructive interference between directly emitted light and reflected light causes a reduction in the radiative rate. In the presence of competing nonradiative decay processes, this leads to a reduction in the efficiency of emission. The variation of radiative rate with position and orientation for a molecule within an arbitrary planar dielectric structure has been modeled by Crawford.81 This model has been applied to polymer LEDs by Burns et al.,82 and Becker et al.,83 who predict significant variations in the efficiency of radiative decay in polymer LEDs depending on the distribution of exciton generation within the device. 

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. 

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. 

However, the process of free excitons binding in a deep local state need not be taken into consideration if no account is taken of the processes of nonradiative decay of single excitons, whereby the energy A goes over into the phonon energy. In crystals, where the quantum yield of exciton luminescence is close to unity (for instance, in anthracene crystals), the nonradiative decay of excitons cannot be realized within the exciton lifetime (otherwise we cannot regard the number of excitons in the crystal in consideration of collective processes as specified). 

In a molecular crystal the fluorescence decay reflects the sum of the rate constant for radiative and nonradiative decay of the excited state of the crystal. Information on energy transfer requires a fluorescent or nonfluorescent dopant that depletes the exciton reservoir. In a disordered system, in which the excited state is inhomo-geneously broadened a fluorescence decay study does yield information without requiring an excitation scavenger provided that the emission is spectrally resolved [see Section 3.2.3.1]. When measuring the decay within a spectrally narrow detection window one monitors the relaxation across the spectrally assessed energy slice of the EDOS. Such experiments were first done on films of PPV [70]. 

The photophysical processes of semiconductor nanoclusters are discussed in this section. The absorption of a photon by a semiconductor cluster creates an electron-hole pair bounded by Coulomb interaction, generally referred to as an exciton. The peak of the exciton emission band should overlap with the peak of the absorption band, that is, the Franck-Condon shift should be small or absent. The exciton can decay either nonradiatively or radiative-ly. The excitation can also be trapped by various impurities states (Figure 10). If the impurity atom replaces one of the constituent atoms of the crystal and provides the crystal with additional electrons, then the impurity is a donor. If the impurity atom provides less electrons than the atom it replaces, it is an acceptor. When the impurity is lodged in an interstitial position, it acts as a donor. A missing atom in the crystal results in a vacancy which deprives the crystal of electrons and makes the vacancy an acceptor. In a nanocluster, there may be intrinsic surface states which can act as either donors or acceptors. Radiative transitions can occur from these impurity states, as shown in Figure 10. The spectral position of the defect-related emission band usually shows significant red-shift from the exciton absorption band. 

Time-resolved fluorescence measurements on unsubstituted thiophene oligomers in solution indicate a sharp increase of the fluorescence quantum yield when the number of thiophene units is increased from two to seven [56, 70] in such experiments, we expect the migration of the excitons towards trapping centers to be minimized due to the finite size of the systems and the absence of interchain effects. The evolution of ( f with chain size has been related to a decrease in nonradiative decay rate /cnr, since the radiative decay constant is observed to be almost unaffected... 

Reynolds et al. [175] measured the recombination lifetime of the allowed (Fs) and forbidden (Fg, allowed by induced strain) free excitons at 2 K in a strained singlecrystal ZnO grown by the hydrothermal method as 259 and 245 ps, respectively. The lifetime for the F5 exciton was slightly higher, 322 ps, for an unstrained sample. They noted that free-exciton lifetimes are determined not only by the radiative decay but also by the nonradiative decay and capture processes leading to bound excitons [66]. Evidently, the measured single exponential decays reflect the effects from all three. 

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