Band bending formation

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... 

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.)...

Band bending at the semiconductor surface causes a depletion of the majority carriers (electrons for n-type CdSe) underneath the surface (depletion layer). Formation of a depletion layer gives rise to a system equivalent to a Schottky barrier between a metal and a semiconductor. 

Fig. 4.12 Diagram illustrating space charge layer formation in microcrystalline and nanocrystalline particles in equilibrium in a semiconductor-electrolyte interface. The nanoparticles are almost completely depleted of charge carriers with negligibly small band bending.

The magnitude of the injection barrier is open to conjecture. Meanwhile there is consensus that energy barriers can deviate significantly from the values estimated from vacuum values of the work-function of the electrode and from the center of the hole and electron transporting states, respectively. The reason is related to the possible formation of interfacial dipole layers that are specific for the kind of material. Photoelectron spectroscopy indicates that injection barriers can differ by more than 1 eV from values that assume vacuum level alignment [176, 177]. Photoemission studies can also delineate band bending close to the interface [178]. 

Figure 7.5 Schematic representation of an n-type semiconductor-solution electrolyte junction showing the formation of depletion layer, band bending and Helmholtz layer (a) before immersion and (b) after immersion in solution.

Fig. 4.38. Energy band diagrams at In2S3/ZnO interfaces as determined from photoelectron spectroscopy. The material used as substrate during interface formation is shown to the left. I112S3 films were deposited by evaporation onto substrates held either at room temperature or at 250°C. ZnO Al films were prepared by dc magnetron sputtering at room temperature in pure Ar or with 10% O2 in the sputter gas as indicated at the top. All values are given in electronvolt. Band bending at the interface is <0.2 eV in all experiments. Because of the uncertainty in the band gap, the conduction band positions of In2S3 are given as dotted lines. Reproduced with permission from [136]...

Fig. 1. (a and b) Specimen geometries used in the field-effect experiments. S, D, and G represent the source, drain, and gate electrodes Q is a thin quartz dielectric, (c) Band diagram showing the formation of an electron accumulation layer near the surface between x = 0 and x — X. The electric field e from the positive gate electrode induces a charge —qin the a-Si H. The tail-state distribution TS between Ec and EA is likely to limit the band bending at the surface. 

Fig. 9.3. Band bending and space charge layer formation at an n-type semiconductor-electrolyte interface (a) accumulation layer,...

Fig. 9.4. Comparison of the band bending, space charge layer formation and Fermi levels (E,r) for a large particle when r = r throughout the depletion layer and equation (9.18) applies, and for a small particle when r = tv and equation (9.19) applies. The semiconductor particles are considered to be in thermodynamic equilibrium with a redox pair of Nernst...

Fig. 61. Schematic description of the surface band bending of ZnO (top) and TiOa (bottom). 1, after degassing in vacuo (n-type semiamductors) 2, after adsorption of H O (decrease of band bending by formation of positively charged species on the surface) 3, after adsorption of O2 (increase of band bending by the formation of negatively charged species on the surface) [reproduced with permission from Anpo et a . (224)].

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