Electronic and optical aspects

1 Junction formation, band line-up and charge distribution 

It is in principle possible to determine the energy diagram for the whole heterojunction cell in this way. Indeed Mahrov et al. (2004) have recently carried out photo- 

When the doping in the two transport layers is due to shallow donors and acceptors, the charge distribution across the junction is as shown in the middle part of Fig. 6.16a. The electric-field profile and the potential profile can be obtained from Poisson s equation (Sze, 1981). For the planar case the depletion width w on each side 

When a thin absorber layer is embedded between the transport layers, the main ingredients of this qualitative description can still be apphed. Since the electric displacement vector across the interfaces is continuous and since the absorber layer is assumed thin and of low conductivity, it is clear that the absorber layer is subject to electric fields similar to the junction field. If the junction curvature is not too small. 

The second case is where both transport layers are transparent metallic condnctors with a work function difference A4 . If the metallic layers are separated by an extremely thin absorber of thickness da, the work function difference will be dropped nearly completely across the absorber layer. This will give rise to an electric field of strength = A lqd. Clearly, if the semiconductor is sufficiently thin this field can also be quite large, and this configuration would therefore also produce a useful driving force for charge separation. 

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