Electrode work function

D. Tsiplakides, and C.G. Vayenas, Electrode work function and absolute potential scale in solid state electrochemistry, J. Electrochem. Soc. 148(5), E189-E202 (2001). 

The non situ experiment pioneered by Sass uses a preparation of an electrode in an ultrahigh vacuum through cryogenic coadsorption of known quantities of electrolyte species (i.e., solvent, ions, and neutral molecules) on a metal surface. " Such experiments serve as a simulation, or better, as a synthetic model of electrodes. The use of surface spectroscopic techniques makes it possible to determine the coverage and structure of a synthesized electrolyte. The interfacial potential (i.e., the electrode work function) is measured using the voltaic cell technique. Of course, there are reasonable objections to the UHV technique, such as too little water, too low a temperature, too small interfacial potentials, and lack of control of ionic activities. ... 

These results are remarkable Coupled with other results for silver and platinum (19J they show that the emersed electrode work function cam be independent of electrode material (even oxide coated) and electrolyte. The tracks < g one-to-one over a large potential region, even after placement in UHV. The apparatus used allowed for emersion and placement in UHV without exposure to air at any time. 

Figure 3.27. Energy level scheme of the device in Figure 3.26, consisting of the electrode work functions and the molecular HOMOs and LUMOs. The relative energy level of HOMOs and LUMOs can he determined hy cyclic voltammetry and optical spectroscopy. Note the hole blocking character of the electron-transport layer. This feature is important since holes that proceed via the HOMO levels have much higher mobilities than electrons proceeding via the LUMO levels.

For proper operation of a bulk heterojunction photovoltaic cell, a special alignment of the HOMO and LUMO levels of the bulk heterojunction components must be accomplished, compatible with the electrodes work functions, as depicted in Scheme 5.8. If an exciton is formed in the polymer phase, then the electron is transferred to the NC phase and reaches the aluminum electrode via its percolating pathway. The remaining hole is transported to the ITO electrode through the polymer phase. In the alternative case, that is, the formation of an exciton in the NCs phase, the hole is transferred to the polymer phase and then transported to the ITO electrode, whereas the electron reaches the aluminum electrode through the NCs phase. 

Experimental results on the variation of the acceptor strength and on the variation of the top electrode work function are summarized in Figs. 5.34a... 

Fig. 5.34. (a) Voc versus acceptor strength and (b) Voc versus negative electrode work function. The slopes Si and Sa of the linear fits to the data are indicated... 

Fig. 5.13. Schematic variation of Voc with acceptor strength (solid double headed arrow, Voci) or/and electrode work function (dotted arrow, V0c2)> m a donor/acceptor BHJ solar cell. The electron transfer, occurring at the donor/acceptor interface after light excitation, is indicated by the bent arrow [134].

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