explain in details the semiconductor/electrolytes interface for solar cell application by drawing figures
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Answer:
Semiconductors are characterized by an absolute energy gap, where thermal excitation of electrons from the valence band to the conduction band results in a conductivity that depends on temperature and the energetic width of the bandgap. Doping with foreign atoms allows the conductivity to be tuned over a potentially broad range if the energy levels of the dopants, ED,A, are located close enough to the band edges. In this case, thermal excitations of electrons from ED (donors) to the conduction band of an n-type semiconductor or from the valence band to EA (acceptors) in a p-type semiconductor increases the majority carrier concentration and thus the conductivity. For an n-type semiconductor, the conductivity is given by σ=e n μ, and increases as the carrier concentration in the conduction band, n, is increased for a given mobility, μ. The semiconductor Fermi level, EF, is given by the electroneutrality condition and can be expressed by the relation of the donor doping concentration, ND, and the effective density of states at the conduction band edge, NCB, which defines the energetic distance of EF from the conduction band edge, ECB:
image file: BK9781782625551-00001-t1.tif (1.1)for n-type semiconductors; accordingly, the Fermi level for p-type semiconductors, with acceptor doping concentration NA, is located above the valence band maximum according to (Figure 1.2):
image file: BK9781782625551-00001-t2.tif (1.1a)
Fig. 1.2 Shift of the energetic position of the Fermi level in Si for n-type doping with the donor doping concentration ND. Also shown is the position of the intrinsic Fermi level (undoped Si), located slightly above the middle of the energy gap because the effective density of states at the conduction- and valence band edges differ (NVB=1.83×1019 cm−3, NCB=3.2×1019 cm−3).
The Fermi level is an electrochemical potential and is the sum of a concentration and an electrical term, i.e. of the chemical potential μ and the Galvani potential φ:
μ*=μ+eφ (1.2a)
μ=kT ln c0 (1.2b)
The Galvani term arises because equilibria between charged phases are considered and the depletion of, for example, electrons on one side of a contact results in its positive charging, whereas the electron-receiving phase will be negatively charged. The energetic position of the Fermi level (or electrochemical potential) in two phases that are contacted defines the flow of charges upon contact formation. The phase with larger electrochemical potential (1EF>2EF or 1μ*>2μ*) will provide the electrons during contact formation. This charge exchange continues until the redistribution of charges is compensated by the built-up electrical field. Accordingly, both phases are charged. For an asymmetric semiconductor p–n junction in which the n-type doping concentration is higher than the p-type doping concentration, an energy schematic is shown in Figure 1.3.
Fig. 1.3 Si pn junction such as used in the classical asymmetrically doped crystalline Si solar cell; the doping concentrations here are 1015 cm−3 for the p-type side and 1017 cm−3 for the n-type side of the junction. From the doping concentrations, the positions of the Fermi levels away from the junction have been determined using eqn (1.1), (1.1a) to be 0.15 eV below the CB edge (n-type) and 0.26 eV above the VB edge (p-type). The total contact potential difference of 0.71 eV drops, however, almost exclusively in the lower doped p-type part, where a pronounced space charge layer is formed.
Answer:
this is the diagram..
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