Lower work-function electrode

Wlien an electrical coimection is made between two metal surfaces, a contact potential difference arises from the transfer of electrons from the metal of lower work function to the second metal until their Femii levels line up. The difference in contact potential between the two metals is just equal to the difference in their respective work fiinctions. In the absence of an applied emf, there is electric field between two parallel metal plates arranged as a capacitor. If a potential is applied, the field can be eliminated and at this point tire potential equals the contact potential difference of tlie two metal plates. If one plate of known work fiinction is used as a reference electrode, the work function of the second plate can be detennined by measuring tliis applied potential between the plates [ ]. One can detemiine the zero-electric-field condition between the two parallel plates by measuring directly the tendency for charge to flow through the external circuit. This is called the static capacitor method [59]. 

The positive charge is on the electrode with the lower work function. 

The positive charge is on the electrode with the lower work function. Thus under conditions of negligible ion spillover equations (7.11) and (7.12), are not valid. This is the case in aqueous electrochemistry and can also be the case in solid state electrochemistry when the temperature is... 

The standard electrode potential [1] of an electrochemical reaction is commonly measured with respect to the standard hydrogen electrode (SHE) [2], and the corresponding values have been compiled in tables. The choice of this reference is completely arbitrary, and it is natural to look for an absolute standard such as the vacuum level, which is commonly used in other branches of physics and chemistry. To see how this can be done, let us first consider two metals, I and II, of different chemical composition and different work functions 4>i and 4>ii-When the two metals are brought into contact, their Fermi levels must become equal. Hence electrons flow from the metal with the lower work function to that with the higher one, so that a small dipole layer is established at the contact, which gives rise to a difference in the outer potentials of the two phases (see Fig. 2.2). No work is required to transfer an electron from metal I to metal II, since the two systems are in equilibrium. This enables us calculate the outer potential difference between the two metals in the following way. We first take an electron from the Fermi level Ep of metal I to a point in the vacuum just outside metal I. The work required for this is the work function i of metal I. 

Development of collector electrode materials with lower work function ( 1.0 eV) is the most effective methods to improve energy conversion efficiency of a thermionic converter. At present refractory metals such as Mo, W, Nb with cesium adsorption are used as a collector with the work function values of about 1.7eV. It is suggested that metal oxide collectors can adsorb cesium more strongly and show lower values of work function. In the study, refractory metal oxides and AgO x were experimentally examined concerning the work function values and high temperature durability. A research thermionic converter of a W emitter and an AgO x collector was fabricated and power generation tested to examine the effectiveness of the AgO x collector. A new type of a FGM collector which integrates a... 

Quite pronounced photovoltaic effects have been observed in Mx 1150 nm -chlorophyll a M2 sandwich cells, where Mx = A1 or Cr and M2 = Hg or Au.7e>77 These are ascribed to a Schottky barrier at the junction with metal Mx which has a lower work function than M2. If the Mi junction is the front (illuminated) electrode then the photovoltaic action spectrum is identical with the absorption spectrum of the chlorophyll. If it is at the rear, the action spectrum shows an inner filter effect, because only light absorbed in the region of the barrier is effective. Figure 11 shows the performance of a typical cell, which has a power conversion efficiency of ca. 10-3% at 745 nm. The best efficiency, 5 x 10-a%, was achieved by a Cr chlorophyll a Hg cell. Photovoltaic properties have also been reported in the cell A11 Mg phthalocyanine Ag, which has a Schottky barrier of height ca. 0.6 eV at the A1 junction.78 At 690 nm, the power conversion efficiency was ca. 10-2%. It has been shown that oxidized A1 contacts to Cu phthalocyanine are blocking.79... 

The electron affinity ( s)> which is defined as the energetic difference between the lower edge of the CB and the vacuum level, of the conjugated polymers used in PLEDs is usually below 3 eV, so that metals with a very low work function are favored. Unfortunately, these metals (e.g., alkaline metals and alkaline earth metals) are known to be rather reactive and unstable under ambient conditions. Even if they can be applied in PLEDs under an inert atmosphere, chemical reactions between the polymer and the electrodes cannot be avoided [66]. Therefore, more stable metals with a higher work function (between 3.5 and 4.5 eV) are usually used. The most common metals in PLEDs used as low work function electrodes are Al, Ag, Mg, In, and several alloys of these materials. 

Similarly, timgsten electrodes used in tungsten inert gas welding and filaments used in light bulbs and vacuum tubes are strengthened by dispersions of thoria (Th02) particles (thoriated timgsten). The presence of thoria in W also lowers work function and is used to improve the efficiency of electron emitters in x-ray tubes and other electron beam devices. 

For PPV-imine and PPV-ether the oxidation potential, measured by cyclic voltammetry using Ag/AgCl as a reference are ,M.=0.8 eV and 0.92 eV, respectively. By adopting the values 4.6 eV and 4.8 eV for the work functions of a Ag/AgCl and an 1TO electrode, respectively, one arrives at zero field injection barriers of 0.4 and 0.55 eV. These values represent lower bounds because cyclic voltammetry is carried out in polar solvents in which the stabilization cncigy of radical ions exceeds that in a polymer film, where only electronic polarization takes place. E x values for LPPP and PPPV are not available but in theory they should exceed those of PPV-imine and PPV-ether. 

Modification of the top electrode may also be achieved. This was done by adding a small amount of surfactant, such as an ether phosphate or an ether sulfate, to the spin-coal solution of the luminescent polymer [89[. The lipophobic ether chains segregate at the surface of the (predominantly) hydrocarbon polymer, becoming available for complexation with the aluminum cathode which is deposited on top. Thus, the dipole in the surfactant points away from the electrode and lowers its work function to improve the injection of electrons. 

Friend et at. studied the influence of electrodes with different work-functions on the performance of PPV photodiodes 143). For ITO/PPV/Mg devices the fully saturated open circuit voltage was 1.2 V and 1.7 V for an ITO/PPV/Ca device. These values for the V c are almost equal to the difference in the work-function of Mg and Ca with respect to 1TO. The open circuit voltage of the ITO/PPV/A1 device observed at 1.2 V, however, is considerably higher than the difference of the work-function between ITO and Al. The Cambridge group references its PPV with a very low dark carrier concentration and consequently the formation of Schottky barriers at the PPV/Al interface is not expected. The mobility of the holes was measured at KT4 cm2 V-1 s l [62] and that for the electrons is expected to be clearly lower. 

The variation in quasireference electrode in presence of reactive gas mixtures. This is due to its high catalytic activity for H2 oxidation. Nevertheless the agreement with Eq. (7.11) is noteworthy, as is also the fact that, due to the faster catalytic reaction of H2 on Pt than on Ag and thus due to the lower oxygen chemical potential on Pt than on Ag,35 the work function of the Pt catalyst electrode is lower than that of the Ag catalyst-electrode over the entire UWr range (Fig. 7.8b), although on bare surfaces O0 is much higher for Pt than for Ag (Fig. 7.8b). 

By adsorbing alkali metals on a metal substrate, the work function of the substrate can be significantly altered in a similar manner to potentiostatically controlling the electrode potential.53 For instance, ca. 0.03 ML K lowers the work function of Pt(l 11) by 1.0 eV, but because of the low coverage, does not chemically interact with most adsorbates. The surface potential on the hydrogen electrode scale can be calculated using the relation... 

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