Semiconductors junction formation between

It has become clear that the potentials needed to form atomic layers shift negatively as the semiconductor films grow, especially over the first 25 cycles. The most probable reason is formation of a junction potential between the Au substrate and the depositing compound semiconductor. 

Fig. 7.21. When a junction between a p-type and an n-type of semiconductor is established (a), a diffusion of holes and electrons in the opposite direction takes place (b). This results in a separation of charge (c) and the formation of an electrical potential difference across the interface (d).

The photovoltaic effect in organic solar cells arises, not from formation of free charge carriers in one or both phases, as in a classical siliconp-n junction cell, but from exciton dissociation at the junction between the two phases. Photon absorption in organic semiconductors, whether small molecules or polymers, does not generate free holes and... 

Let us now consider the formation of the semiconductor/solution interface. The Fermi level in the solution phase, can be identified as Ji by (18.2.4) and is calculated in terms of values by the procedures described in Section 2.2. For most electrochemical purposes, it is convenient to refer values to the NHE (or other reference electrodes), but in this case it is more instructive to estimate them with respect to the vacuum level. This can be accomplished, as discussed in Section 2.2.5, by theoretical and experimental means with relaxation of thermodynamic rigor, so that one obtains an energy level value for the NHE at about —4.5 0.1 eV on the absolute scale (45) (Figure 18.2.5a). Consider the formation of the junction between an n-type semiconductor and a solution containing a redox couple 0/R, as shown in Figure 18.2.5. When the semiconductor and the solution are brought into contact, if electrostatic equilibrium is attained, in both phases must become equal (or equivalently the Fermi levels must become equal), and this can occur by... 

Figure 18.2.5 The formation of a junction between an n-type semiconductor and a solution containing a redox couple O/R. (a) Before contact in the dark. Typical values for energy levels shown referenced to NHE and to vacuum (E). (b) After contact in the dark and electrostatic equilibration.

Figure 18.2.8 Formation of the junction between a p-type semiconductor and a solution containing a redox couple O/R.

Figure 10 Schematic diagram showing the formation of ohmic and blocking junctions between metals and semiconductors. , , (f>v = work functions of n-type semiconductor, metal and p-type semiconductor, respectively. Ey 0 = vacuum level, Ec = conduction band edge, Ey = valence band edge, E = Fermi level...

First, the p-type material needs to have electronic levels into which holes can be injected from the oxidized or excited state of the dye. The redox levels of the dye and the p-type material therefore have to be adapted carefully. An intimate contact between the sensitized metal oxide and the p-type material is vital to assure fast injection and regeneration processes (Fig. 1). This implies either the growth or deposition of one semiconductor inside a preformed, sensitized porous film of its counterpart or the in situ formation of the sensitized composite. Direct formation of the sensitized junction would be appreciable however, charge collection within the two independent semiconductor networks, in which at least one semiconductor is formed from nanometer-sized inorganic semiconductor particles, demands intimate contact between the particles. Reduced... 

Such junctions mainly are of inorganic character, however, the dye present at the junction s interface still plays the central role in the initial formation of the charge separated state. The sensitizer takes on the role of both the light absorber and acts as the link between the two inorganic materials. The choice of inorganic wide band gap semiconductors applicable in such junctions is small, and only few... 

Formation of a junction between an n-type semiconductor and a solution (a) before the contact (b) at equilibrium. 

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