Schottky barrier band bending

Inorganic Schottky barrier rectifiers are commercially available. Of course, for a monolayer, one cannot speak of band bending as in a bulk semiconductor, but the ideas of a barrier, and of dipoles across it, are valid. 

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. 

Hetero junctions, forming a Schottky barrier like a metal-semiconductor junction, normally change the energy levels of conduction and valence bands. When the Fermi level of the semiconductor equilibrates with the energy level of the redox couple in the solution, the electric energy level at the surface is pinned and a depletion layer is formed. This is postulated since the rectified current can be observed at semiconductor plate electrodes. The bending of the band in the semiconductor at the surface can be described as a solution of the one-dimensional Poisson-Boltzmann equation... 

Flat-band potential — In the energy barrier formed for example at metal-semiconductor junctions (- Schottky barrier), metal-insulator-semiconductor junctions, and solution-semiconductor interfaces the flat-band potential corresponds to the potential at which the electric field equals zero at the semiconductor interface, i.e., there is no -+ band bending. In case of solution-semiconductor interfaces, the flat-band potential corresponds to the condition of absence of excess charge and consequently, depletion layer, in the semiconductor. See also -> Mott-Schottky plot, and -> semiconductor. 

The most inaccurate assumption, especially at low band-bending, is undoubtedly the first. However, the mathematical derivation leading to eqns. (396) and (397) cannot be easily modified to take account of depletion-layer recombination owing to the way in which the depletion layer is considered. In order to develop the theory to take into account recombination in the depletion layer, it is necessary to solve explicitly the transport equation in the depletion layer as well as the bulk. If we persevere, for the moment, with the Schottky barrier model and we continue, for the moment, with the assumption that recombination does not occur in the depletion layer (x < VF) then the transport equations for x < VF, x > VF are... 

Ionized donors inside the material are indicated by , and the electrons they have released to the conduction band, by —. If a metal were in electrical contact with the semiconductor, electrons would be redistributed until the Fermi level, Ei-, separating the occupied from the unoccupied levels in the metal, were near the conduction-band edge of the semiconductor. Then, if the electrons near the surface were eaten up by surface reconstruction, as shown in part (b), an electric dipole layer would arise, giving the potential hill, or Schottky barrier, shown in part (c). The hill or htirrier would bend the bands, as indicated in part (d). One can say that the Fermi level is pinned midgap at the surface, though it is near the conduction-band edge in the interior. 

A similarly high Voc for ITO/PPV/Al photovoltaic devices also was observed by other groups. Jenekhe et al. [63, 64] report the observation of a quantum efficiency IPCE of 5% in ITO/PPV/Al photodiodes and of a power conversion efficiency of approximately 0.1% under low light intensities of 1 mW/cm. The typical film thickness of their devices was varied between 100 to 600 nm. The open circuit voltage of these devices, as defined with respect to the ITO electrode, was measured as 1.2 V. The high open circuit voltage was explained by the formation of a Schottky barrier at the Al/PPV interface. The predicted band bending due the PPV/Al interface formation was verified by XPS measurements [65, 66]. 

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