Valence electroluminescence

Polysilanes are cr-conjugated polymers composed of Si-Si skeletons and organic pendant groups. They are insulators with filled intramolecular valence bands and empty intramolecular conduction bands. However, because of strong cr conjugation, they have rather narrow band gaps of less than 4 eV [24,25] and are converted to conductors by photoexcitation or by doping electron donors or acceptors. Recently they have attracted much attention because of their potential utility as one-dimensional conductors, nonlinear optical materials, and electroluminescent materials [26-28]. 

Electroluminescence. In Section 6.3.2.5, we saw that some materials—in particular, semiconductors—can reemit radiation after the absorption of light in a process called photoluminescence. A related type of emission process, which is common in polymer-based semiconductors, called electroluminescence, results when the electronic excitation necessary for emission is brought about by the application of an electric field rather than by incident photons. The electric field injects electrons into the conduction band, and holes into the valence band, which upon recombination emit light. 

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 is an electronic analogue of photoluminescence and consists of the radiative recombination of the electrons and holes injected into the conduction and the valence band of the semiconductor, respectively (Fig. 6). Electroluminescence is intensively exploited in optoelectronic devices based on inorganic semiconductors such as light emitting diodes (LEDs) and lasers. 

As mentioned above, the operation of the device requires that electrons and holes be injected from opposite electrodes. Electrons are injected into the conduction band states of the polymer, and holes into the valence band states, and for a diode formed with a polymer such as PPV, a schematic energy level diagram as shown in Fig. 29.9 is considered appropriate. Note that there are barriers at the electrodes for injection of both electrons and holes from the aluminum and indium-tin oxide electrodes, respectively. It is difficult to make accurate predictions about the barriers to electron and hole injection (J e and respectively, in Fig. 29.9). Making reasonable assumptions about the polymer ionization potential and electrode work functions, it is clear that the barrier to injection of electrons from aluminum must be significantly larger than the barrier to injection of holes from ITO [90]. The majority of the current is therefore expected to be due to holes. Electroluminescence, however, requires the simultaneous injection of electrons, and the quantum efficiency will therefore depend strongly on the barrier to electron injection. 

A variant of photoiuminescence that in some cases requires the presence of a liquid junction is electroluminescence (EL) (28). In this case, the luminescence is not induced by optical excitation but by charge-transfer processes. The most widely used electrolyte for this purpose is alkaline peroxydisulfate (33) in which the SjOg ions are reduced by the conduction band electrons of an n-type semiconductor to yield the highly oxidizing sulfate radical anions that can inject a hole into the valence band. Recombination of an electron with the injected hole yields the luminescence. This process is much more localized to the surface as compared with photoiuminescence. Comparison between the two can... 

To measure the spin coherence time and to estimate the spin-polarized carrier injection efficiency from the electroluminescence data, the selection rules and the valence band structure in ZnO must be understood. The valence band in wurtzite materials is split into three bands (A, B, and C) due to crystal field and spin-orbit coupling as discussed before in Chapter 3. The spin degeneracy of these three bands and the conduction band is lifted in magnetic field resulting in small symmetric Zeeman splittings as shown in Figure 5.6 near the F point [48]. The allowed transitions following the selection rules AI = 1 (for 0 polarization) are indicated... 

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