High work function

One of the most important parameters of ITO, apart from the transparency, is the relatively high work function. In a comparative photoelectron spectroscopic study of ITO from different sources, and prepared by different techniques, a variation in the chemical composition and work functions are reported [101, 105]. For three carefully chosen types of ITO, the chemical composition and work functions... 

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. 

Parker [55] studied the IN properties of MEH-PPV sandwiched between various low-and high work-function materials. He proposed a model for such photodiodes, where the charge carriers are transported in a rigid band model. Electrons and holes can tunnel into or leave the polymer when the applied field tilts the polymer bands so that the tunnel barriers can be overcome. It must be noted that a rigid band model is only appropriate for very low intrinsic carrier concentrations in MEH-PPV. Capacitance-voltage measurements for these devices indicated an upper limit for the dark carrier concentration of 1014 cm"3. Further measurements of the built in fields of MEH-PPV sandwiched between metal electrodes are in agreement with the results found by Parker. Electro absorption measurements [56, 57] showed that various metals did not introduce interface states in the single-particle gap of the polymer that pins the Schottky contact. Of course this does not imply that the metal and the polymer do not interact [58, 59] but these interactions do not pin the Schottky barrier. 

A thin layer deposited between the electrode and the charge transport material can be used to modify the injection process. Some of these arc (relatively poor) conductors and should be viewed as electrode materials in their own right, for example the polymers polyaniline (PAni) [81-83] and polyethylenedioxythiophene (PEDT or PEDOT) [83, 841 heavily doped with anions to be intrinsically conducting. They have work functions of approximately 5.0 cV [75] and therefore are used as anode materials, typically on top of 1TO, which is present to provide lateral conductivity. Thin layers of transition metal oxide on ITO have also been shown [74J to have better injection properties than ITO itself. Again these materials (oxides of ruthenium, molybdenum or vanadium) have high work functions, but because of their low conductivity cannot be used alone as the electrode. 

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). 

Hyperthermal surface ionisation (HSI) is an ultrasensitive tuneable selective ion source [222,223] which is based on the very effective ionisation of various hyperthermal molecules upon their scattering from a surface with a high work function, such as rhenium oxide. Molecule-surface electron transfer constitutes the major and most important HSI mechanism for GC-MS. 

Surface Ionization Sources. In this system, a low ionization potential atom (e.g. caesium) is adsorbed on a high work function metal (e.g. iridium). The temperature is raised so that the rate of desorption exceeds the rate of arrival of the atoms at the surface, and the Cs is then desorped as ions with very small energy spread (< 1 eY). The spot size - current characteristics of these sources lie between liquid metal and plasma discharge sources. 

Underpotential deposition of heavy metals on H2 evolving electrodes is a well known problem [133], The existence of a direct correlation between H2 evolution activity and metal work function, makes UPD very likely on high work function electrodes like Pt or Ni. Cathode poisoning for H2 evolution is aggravated by UPD for two reasons. First, deposition potentials of UPD metals are shifted to more anodic values (by definition), and second, UPD favors a monolayer by monolayer growth causing a complete coverage of the cathode [100]. Thus H2 evolution may be poisoned by one monolayer of cadmium for example, the reversible bulk deposition potential of which is cathodic to the H2 evolution potential. 

The work functions of metals correlate with their surface energies Fig. 4.11 shows this for sp metals. Hence the surface energy of a metal with a high work function is lowered when it is covered with a monolayer of a metal with a lower work function and lower surface energy. 

When the secondary ion is close to the surface, there is a chance that an electron from the surface of the metal neutralizes it. This process becomes more likely if the work function of the metal is lower. In other words, a high work function prevents neutralization and is favorable for positive ion emission. 

High work function (WF) (< >4.1 eV) to promote efficient hole injection... 

On the substrate side, the same process occurs for the holes, but on a different energy level. The holes are injected by a high work function metal or semiconductor like the transparent ITO, which consists of a nonstoichiometric composite... 

In the photoelectric method, the measured average work function is always less than the true since patches of high work function tend to be excluded from the emission process. Thus, the nonuniform distribution of adsorbate on a patch surface may cause a slight discrepancy in the evaluation of A. Experimentally, the photoelectric method has various limitations. Photocurrents of the order of 10 A. must be measured accurately in the region of vo, and for films of work function greater than 5 v., the threshold frequency lies in the far ultraviolet—a practical disadvantage. Furthermore, the method is inapplicable at pressures in excess of 10 mm. Hg because of ionization of the gas by collision. 

Adsorption of hydrogen atoms by a metal with a high work function,... 

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