Cathodic hole injection

Next, we consider the anodic reaction current of redox electron transfer via the conduction band, of which the exchange reaction current has been shown in Fig. 8-16. Application of a slight anodic polarization to the electrode lowers the Fermi level of electrode fix>m the equilibrium level (Ep(sc)( n = 0) = eiiOTSDca)) to a polarized level (ep(8C)( n) = ep(REDox)- n)withoutchanging at the electrode interface the electron level relative to the redox electron level (the band edge level pinning) as shown in Fig. 8-20. As a result of anodic polarization, the concentration of interfacial electrons, n, in the conduction band decreases, and the concentration of interfadal holes, Pm, in the valence band increases. Thus, the cathodic transfer current of redox electrons, in, via the conduction band decreases (with the anodic electron im ection current, ii, being constant), and the anodic transfer current of redox holes, (p, via the valence band increases (with the cathodic hole injection... 

For cathodic hole injection, the overvoltage tip.sc(i) includes both diffusion and recombination of holes in the electrode this overvoltage occurs in the same cathodic direction as the cathodic hole injection so that tip. sc is the usual overvoltage (a negative quantity in the cathodic reaction) rather than the inverse overvoltage. Then, we obtain Eqn. 10-50 ... 

Fig. 10-24. Electron levels and polarization curves for a redox reaction of cathodic holes both at an n-type and at a p-type electrode of the same semiconductor in the dark curve (1) = polarization curve of cathodic hole injection in n -type electrode curve (2)= polarization curve of cathodic hole injection in p-type electrode (equivalent to a curve representing cathodic hole injection current as a i mction of quasi-Fermi level of interfodal holes in n-type electrode) = cathodic hole injection current N = polarization of cathodic hole ixu ection at potential nECi) of n-type electrode, P = polarization of cathodic hole iqjection at potential pE(.i) of p-type electrode.

In the above section, various dissolution mechanisms for III-V compound electrodes were proposed on the basis of stabilization studies. There is evidently a need for confirmation of these mechanisms from independent data. Such confirmation can be obtained by quantitatively studying the enhancement of the anodic dark current at n-type electrodes, caused by hole injection. Although, as mentioned in Sec. 2.1, this effect has been known for many years, only recently was it realized [41] that the relationship between the additional anodic current density 4 and the cathodic hole injection current density constitutes a powerful probe for in-... 

For the cathodic reaction of the corrosion, there are two different charge-transfer processes. One involves holes in the valence band as a cathodic hole-injecting reaction and the other involves electrons in the conduction band as a cathodic electron-emitting reaction ... 

On the substrate side, the same process occurs for the holes, but on a different energy level. The holes are injected with a high work fimction metal or semiconductor like the transparent indium-tin-oxide ITO, which consists of a nonstoichiometric composite of 10-20% Sn02 and 80-90% ln203. The work function of ITO depends strongly on the surface treatment and lies in the range of 4.4-S.2 eV [43,44]. As in the case of the cathode, hole injection can be improved by an additional layer of, e.g., copper phthalocyanine [45] or polyethylenedioxythiophene (FEDOT), doped with polystyrenesulfonic acid (PSS) [46]. The holes are injected into the hole transport layer and proceed... 

PPV and its alkoxy derivatives are /j-type conductors and, as a consequence, hole injection is more facile than electron injection in these materials. Efficient injection of both types of charge is a prerequisite for efficient LED operation. One approach to lowering the barrier for electron injection is the use of a low work function metal such as calcium. Encapsulation is necessary in this instance, however, as calcium is degraded by oxygen and moisture. An alternative approach is to match the LUMO of the polymer to the work function of the cathode. The use of copolymers may serve to redress this issue. 

One can, nevertheless, conclude that (i) there is only a very small barrier for hole injection from ITO to PTV, if any barrier at all, (ii) a finite energy should exist for hole transport across the PTVIDASMB interface, and (iii) PBD should act as an efficient internal blockade for hole transport towards the cathode. 

The materials used as the electron and hole injecting electrodes play a crucial role in the overall performance of the device and therefore cannot be neglected even in a brief review of the materials used in OLEDs. The primary requirements for the function of the electrodes is that the work function of the cathode be sufficiently low and that of the anode sufficiently high, to enable good injection of electrons and holes, respectively. In addition, at least one electrode must be sufficiently transparent to permit the exit of light from the organic layer. 

A typical multilayer thin film OLED is made up of several active layers sandwiched between a cathode (often Mg/Ag) and an indium-doped tin oxide (ITO) glass anode. The cathode is covered by the electron transport layer which may be A1Q3. An emitting layer, doped with a fluorescent dye (which can be A1Q3 itself or some other coordination compound), is added, followed by the hole transport layer which is typically a-napthylphenylbiphenyl amine. An additional layer, copper phthalocyanine is often inserted between the hole transport layer and the ITO electrode to facilitate hole injection. 

Hole Injection Materials and Cathode Interfacial Materials. 303... 

The simplest manifestation of an OLED is a sandwich structure consisting of an emission layer (EML) between an anode and a cathode. More typical is an increased complexity OLED structure consisting of an anode, an anode buffer or hole injection layer (HIL), a hole transport layer (HTL), a light-emitting layer, an electron transport layer (ETL), a cathode... 

Electron and hole injection from the cathode and the anode... 

When an electric field is applied between the anode and the cathode, electrons and holes are injected from the cathode and the anode, respectively, into the organic layers. With a matched energy barrier between the electron and the hole injection layers (EILs and HILs) and the cathode and the anode, electrons and holes are efficiently injected into the ETL and HTL. 

HOLE INJECTION MATERIALS AND CATHODE INTERFACIAL MATERIALS 3.3.1 Hole Injection Materials... 

Figure 3.26. Structure of an OLED. S = substrate (glass), ANO = anode (e.g., ITO — indium tin oxide), HIL = hole injection layer (e.g., Cu phthalocyanine), HTL = hole transport layer, EML = emission layer, ETL = electron transport layer, EIL = electron injection layer (e.g., LiF), KAT = cathode (e.g., Ag Mg, Al). The light that is generated by the recombination of holes and electrons is coupled out via the transparent anode.

The OCP etch rate of p-type and highly doped n-type Si electrodes in HF-HNO3 mixtures increases by an order of magnitude under sufficiently anodic bias [Le20]. In the cathodic regime significant dark-currents are observed for p-type electrodes, as shown in Fig. 4.12. This is ascribed to hole injection from the electrolyte [Kol4]. Note that hole injection is not observed in aqueous HF free of oxidants. 

Hole Injection under Cathodic Bias without Illumination... 

For homogeneously doped silicon samples free of metals the identification of cathodic and anodic sites is difficult. In the frame of the quantum size formation model for micro PS, as discussed in Section 7.1, it can be speculated that hole injection by an oxidizing species, according to Eq. (2.2), predominantly occurs into the bulk silicon, because a quantum-confined feature shows an increased VB energy. As a result, hole injection is expected to occur predominantly at the bulk-porous interface and into the bulk Si. The divalent dissolution reaction according to Eq. (4.4) then consumes these holes under formation of micro PS. In this model the limited thickness of stain films can be explained by a reduced rate of hole injection caused by a diffusional limitation for the oxidizing species with increasing film thickness. 

The holes injected by a cathodic redox reaction (Eqn. 10-47) diffuse toward the electrode interior and recombine with electrons of the m ority charge carriers in the same way as photogenerated holes, thereby producing a cathodic current inc, which is equivalent to the rate of recombination of holes. The cathodic current i actually observed is the sum of the current of recombination inc and the limiting diffusion current of holes ip.um as shown in Eqn. 10-48 ... 

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