Redox couples electroluminescence

The process shown in Fig. 90(b) is favoured by the known position of the band edges in alkali, the similarity with the electroluminescence induced by persulphate, and the energy of the [Fe(CN)6]3, 4 redox couple. Either mechanism would, however, be consistent with the observation that the onset of electroluminescence coincides, approximately, with the flat-band potential, as shown in Fig. 91. Interestingly, the photoluminescence reaches a maximum at more negative potentials, a result not predicted by either model, as shown in Fig. 91 [167],... 

Electroluminescence was first observed with n-GaP electrodes using hole donors such as [Fe(CN)(,] in alkaline or S2O8 in acid solutions [112]. In these two cases the corresponding standard potentials occur at or even below the valence band edge (see Table in Appendix). In the case of [Fe(CN)(s] no luminescence was found in acid solutions although the current-potential curve indicates that the redox species is reduced. The differences between alkaline and acid solutions can be explained by the pH-dependence of the position of the energy bands at the surface, as shown in Fig. 7.62. Since is far below Ep.redox at pH 1 no charge transfer between the redox couple and the valence band is possible anymore, and the cathodic current is only due to an electron transfer via the conduction band. 

Electroluminescence, (EL), is a versatile probe for studying such carrier injection processes. Thus, hole injection into the VB of a n-type semiconductor leads to cathodic EL, whereas electron injection into the GB of a p-type semiconductor leads to anodic EL [67]. Examples of studies of cathodic EL are commonplace [68-70] however, anodic EL is not very common because the energy requirement for the redox couple has a very negative redox potential. Nonetheless, anodic EL has been reported for the p-InP-[Cr(CN)6] interface [71]. Radical intermediates can also cause EL as discussed later for multielectron redox processes. EL is treated in more depth in another chapter. 

In-situ luminescence measurements have been used to study the semiconductor/ electrolyte interface for many years (e.g. Petermann et al., 1972). Luminescence may result from optical excitation of electron/hole pairs that subsequently combine with the emission of light (photoluminescence). Alternatively, minority carriers injected from redox species in the electrolyte can recombine with majority carriers and give rise to electroluminescence. The review by Kelly et al. (1999) summarises the main features of photoluminescence (PL) and electroluminescence (EL) at semiconductor electrodes. The experimental arrangements for luminescence measurements are relatively straightforward. Suitable detectors include a silicon photodiode placed close to the sample, a conventional photomultiplier or a cooled charge-coupled silicon detector (CCD). The CCD system is used with a grating spectrograph to obtain luminescence spectra. 

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