Low-work-function

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

Thermionic Emission - Because of. the nonzero temperature of the cathode, free electrons are continuously bouncing inside. Some of these have sufficient energy to overcome the work function of the material and can be found in the vicinity of the surface. The cathode may be heated to increase this emission. Also to enhance this effect, cathodes are usually made of, or coated with, a low work-function material such as thorium. 

The simplest and most widely used model to explain the response of organic photovoltaic devices under illumination is a metal-insulaior-metal (MIM) tunnel diode [55] with asymmetrical work-function metal electrodes (see Fig. 15-10). In forward bias, holes from the high work-function metal and electrons from the low work-function metal are injected into the organic semiconductor thin film. Because of the asymmetry of the work-functions for the two different metals, forward bias currents are orders of magnitude larger than reverse bias currents at low voltages. The expansion of the current transport model described above to a carrier generation term was not taken into account until now. 

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. 

Eq. (14.1) is known as the Mott-Schotlky equation. We note llial for a given n-lype semiconductor, the harrier height increases as the work function of the metal increases. It is therefore expected that high work function metals will give a rectifying junction, and low work function metals an ohmic contact (it is the reverse for a p-type semiconductor). 

Tungsten has the necessary high melting temperature (3660 K) to be employed as a thermionic source, and lanthanum hexaboride (LaB6) is also employed because of its low work function. 

The catalytic significance of Fig. 9.12 is that it represents the differences in the effective work functions that a molecule experiences upon adsorption at different positions on the surface. As explained in the Appendix, a low work function of the substrate enhances the capability of the substrate to donate electrons into empty chemisorption orbitals of the adsorbate. If such an orbital is antibonding with respect to an intramolecular bond of the adsorbed molecule, the latter is weakened due to a higher electron occupation. 

We consider two cases (see Fig. A.13). First, the metal has a work function that is between electron affinity (the energy of the o -level) and the ionization potential (the energy of the o-level) of the molecule. Upon adsorption, the levels broaden. However, the occupation of the adsorbate levels remains as in the free molecule. This situation represents a rather extreme case in which the intramolecular bond of the adsorbate molecule stays about as strong as in the gas phase. The other extreme occurs if both the a-level and the o -1evel fall below the Fermi level of the metal. Because the antibonding G -level is filled with electrons from the metal, the intramolecular bond breaks. This is the case for hydrogen adsorption on many metals. Thus, a low work function of the metal and a high electron affinity of the adsorbed molecule are favorable for dissociative adsorption. 

The picture sketched above is a gross oversimplification, although it comes close to the situation for H2 chemisorption on aluminum. It also explains correctly the role that the work function plays in the dissociation of a molecule. However, a low work function is not the only reason for dissociation. The interaction of the adsorbed molecule with the d-levels of the metal is at least as important. 

To optimize the performance of SMOLEDs and PLEDs, it is important to choose electrodes whose work functions are well matched to the bands of EL organic materials in order to minimize the barriers for charge injections. Although cathodes made of low-work-function... 

Although the exact mechanism of the fluorenone formation is not known, it is believed that the monoalkylated fluorene moieties, present as impurities in poly(dialkylfluorenes), are the sites most sensitive to oxidation. The deprotonation of rather acidic C(9)—H protons by residue on Ni(0) catalyst, routinely used in polymerization or by metal (e.g., calcium) cathode in LED devices form a very reactive anion, which can easily react with oxygen to form peroxides (Scheme 2.26) [293], The latter are unstable species and can decompose to give the fluorenone moiety. It should also be noted that the interaction of low work-function metals with films of conjugated polymers in PLED is a more complex phenomenon and the mechanisms of the quenching of PF luminescence by a calcium cathode was studied by Stoessel et al. [300],... 

Even in the absence of illumination (darkness) some electrons, excited by thermal energy, are emitted from the photocathode. Since photocathodes are materials with low working functions, the thermal energy can be high enough to induce the emission of electrons. These emitted electrons give rise to what is known as the dark current or, sometimes, the thermo-ionic current. The dark current varies randomly with time, so that it is considered as noise. It has been experimentally determined that the thermo-ionic current, U, due to photoelectrons emitted by a photocathode in the absence of illumination is given by... 

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