Contacts band bending

Besides the classical Schottky contact, various surface mechanisms are known to influence polymer metal contacts. Band bending in metal/PPV interfaces is also discussed in terms of surface states or chemical reactions between the semiconductor and the metal [70-74]. An excellent review on conjugated polymer surfaces and interfaces is given by [129]. 

Although the observations for PPV photodiodes of different groups are quite similar, there are still discussions on the nature of the polymer-metal contacts and especially on the formation of space charge layers on the Al interface. According to Nguyen et al. [70, 711 band bending in melal/PPV interfaces is either caused by surface states or by chemical reactions between the polymer and the metal and... 

For Eredox more negative than the flat band potential (b) there is little band bending (no barrier) and the electrode is reversible with respect to the redox couple and is in ohmic contact (a). For Ere between the flat band potential and some positive potential represented in (c) the drop in potential occurs across the semiconductor and the behavior is ideal because the band bending varies following Equation 1. For more positive Ereadditional potential drop across the semiconductor does not occur because the semiconductor is inverted at the surface, and the band edges effectively shift more positive as the potential drop occurs across the Helmholtz... 

To understand the role of the noble metal in modifying the photocatalysts we have to consider that the interaction between two different materials with different work functions can occur because of their different chemical potentials (see [200] and references therein). The electrons can transfer from a material with a high Fermi level to another with a lower Fermi level when they contact each other. The Fermi level of an n-type semiconductor is higher than that of the metal. Hence, the electrons can transfer from the semiconductor to the metal until thermodynamic equilibrium is established between the two when they contact each other, that is, the Fermi level of the semiconductor and metal at the interface is the same, which results in the formation of an electron-depletion region and surface upward-bent band in the semiconductor. On the contrary, the Fermi level of a p-type semiconductor is lower than that of the metal. Thus, the electrons can transfer from the metal to the semiconductor until thermodynamic equilibrium is established between the two when they contact each other, which results in the formation of a hole depletion region and surface downward-bent band in the semiconductor. Figure 12.6 shows the formation of semiconductor surface band bending when a semiconductor contacts a metal. 

Figure 12.6 Plot showing the formation of semiconductor surface band bending when a semiconductor contacts a metal (Ec, the bottom of conduction band Ev, the top of valence band EF, the fermi energy level SC, semiconductor M, metal Vs, the surface barrier). (From Liqiang, J. et al., Solar Energy Mater. Solar Cells, 79, 133, 2003.)...

Fig. 2.3. Schematic view of a porous nanocrystaUine sensing layer with a one-dimensional representation of the energetic conduction band. A inter-grain band bending, eVs, occms as a consequence of smTace phenomena, and a band bending, eVc, occurs at the grain-electrode contact. Eb denotes the minimmn conduction band energy in the bulk tin oxide, and Ep is the Fermi-energy in the electrode metal...

One of the most distinguishing features of semiconductor nanoparticles for use in photoelectrocatalysis is the absence of band bending at the semiconductor-electrolyte interface, see Fig. 4.2. In contrast to bulk behavior, for a colloidal semiconductor or a semiconductor comprised of a nanociystalline network in contact with an electrolyte the difference in potentials between the center (r = 0) of the particle and a distance r from the center can be expressed [83] ... 

At equilibrium, the Fermi energy is the same on both sides of the interface. But since this energy is not, in general, the same in the bulk of the materials before contact, relative to vacuum level, the positions of the valence- and conduction-band levels in the semiconductor must adjust band bending occurs. The two most probable cases are sketched in Fig. 23, for a p-type semiconductor, since CPs are generally so. 

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