Electric field induced quenching

Ohta, N., Umeuchi, S., Nishimura, Y. and Yamazaki, I. (1998) Electric-field-induced quenching of exdplex fluorescence and photocurrent generation in a mixture of ethylcarbazole and dimethyl terephthalate doped in a PMMA polymer film. /. Phys. Chem. B, 102, 3784-3790. 

Veldman et al. [77] investigated the electric field-induced quenching of the emission from charge transfer states in a fluorine copolymer PCBM blend (Figure 4.16). There are three especially remarkable aspects of the results of these measurements. 

Electric field induced fluorescence quenching on a series of poly(p-terpheny-... 

These data demonstrate that QEext(EL) / QEext(PL) = 0.5. This is a lower limit for the ratio. Note that the EL data were obtained at electric fields of 3-4 X 105 V/cm at these high electric fields, field-induced quenching measurements show a reduction in PL efficiency by 10% [39, 40]. Thus, QEext(EL) / QEext(PL) > 0.5. 

The phthalocyanines are still of some interest, and Ayers has interpreted the photoelectrochemistry of copper phthalocyanine in terms of the band-model for semiconductors. Menzel et alf have studied the photoconductivity and electrical field-induced fluorescence quenching in metal-free phthalocyanine films. Doping of the film increases charge-carrier photogeneration, and it seems likely that the mechanism is extrinsic, involving field-assisted exciplex dissociation. These are complicated materials, which are difficult to purify, and the mechanisms of carrier generation in polycrystalline and amorphous films are still obscure. The porphyrins and monolayer assemblies of chlorophyll on SnOj electrodes 2 72,273 ijggjj studied. 

Since the jp-polarized radiation at cpi 0° has tangential and normal electric field components (1.4.4°), these components will interact with those dipoles within the film that are oriented parallel and perpendicnlar to the snrface, respectively, according to the selection mle (1.27). Fmthermore, under action of the external electric field normal to the surface, the dipoles experience an additional electric field induced by the polarized chemical bonds within the layer. It follows that in the jp-polarized spectra, the band near the longitudinal optical mode vlo will be observed, in addition to the band near vro resulting from the dipoles parallel to the film. In light of the discussion in Section 1.8.2, it is evident that because the dipole oscillations responsible for the vro band are directed along the surface, these oscillations will be quenched by a metal surface and, hence, be absent in p-polarized spectra of films on mefal substrates, as demonstrated in Fig. 3.2. 

At the donor/acceptor interface (step III in Figure 5), exciton D is quenched via electron transfer to the lowest unoccupied molecular orbital (LUMO) level of the acceptor molecule (A°). On the contrary, exciton A is quenched via hole transfer to the highest occupied molecular orbital (HOMO) level of the donor molecule (D ). Both pathways result in the formation of the same charge separated state D+ A . Positive and negative charges in this ion pair are bond by Coulomb attraction forces and also denoted as geminate polaron pair. This pair can dissociate in the electric field induced by the potential jump at the heterojunction and/or by the difference in the electrode work functions. At the same time, the energy difference... 

Several mechanisms associated with the application of an electric field may contribute to the PL quenching (field-induced quenching) in the device ... 

Deussen, M., Scheidler, M., and Bassler, H., Electric field-induced photoluminescence quenching in thin-film light-emitting diodes based upon poly(phenyl-p-phenylenevinylene), Synth. Met., 73, 123, 1995. 

Consider first the quenching of spin-polarized 2s ions in the absence of hyperfine structure or magnetic field effect . Then the 2sj state can decay by allowed Ml transitions, or by electric-field induced El or M2 transitions. An emitted photon is characterized by a propagation vector k and a polarization vector e, and the initial atomic state by a spin-polarization vector P. We choose a coordinate system such that k lies along the z-axis, and write e for the general case of elliptical polarization in the form... 

In addition to photoelectrical measurements, two fundamentally sound, non-electrical approaches based on the photophysics of the excited state have been reported. According to Figure 10.27(b), if the photoconductor is fluorescing, one can use the technique of electric field induced fluorescence quenching to probe the photogeneration process. The quantum yield of e-h pair formation (0eh) is given by ... 

Another possibility to determine PGr, which is free of these drawbacks, is electric-field modulation (EFM) of photoluminescence (PL). Electric-field effect on the effective charge separation efficiency (rj0 l) shows up in the varying population of CT states, and consequently, in the varying concentration of the emitting states. It is expected that the field-induced increase in the charge separation efficiency would translate into PL quenching. The ratio (<5) of the PL efficiency in the presence (external electric field (F) would give directly Pgr... 

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