Grain boundary space charge

The effective terms ct" and ct1 may be broken down into core (co) and space charge (sc) contributions,285 q>L is the volume fraction of the grain boundaries (core and space charge zone), p = 1/3 and Pi = 2/3 in the ideal brick layer model. 

In poly crystalline semiconductor samples, the excited-state lifetime of electron-hole pairs is so short that photocurrent collection is efficient only for carriers created within the space charge (depletion) region. Thin-film processes offer an inexpensive way to prepare large solar arrays, but the semiconductors formed by such processes are almost inevitably polycrystalline. It is not wise to use semiconductor films thicker than the depletion width in such devices because the additional thickness contributes only extra grain barrier boundaries for the majority of carriers to surmount on their way to the back contact. The additional thickness does not provide any additional photocurrent. 

In the case of an enhanced charge carrier concentration in the space charge (cf. Fig. 4b), mainly the current parallel to the grain boundary plane is important. The corresponding parallel excess grain boundary conductance... 

In the case of electrical transport across and along space charge layers at grain boundaries, there is usually no fundamental difference between electronic and ionic charge carriers. At electrodes, however, electrons and ions behave completely differ-... 

Serious problems with respect to a quantitative analysis can occur if the assumption of identical grain boundaries is violated. In sintering processes, inhomogeneities are the rule rather than the exception and grain boundary cores in one and the same polycrystal can easily differ in terms of structure and chemistry [109, 120, 242-246], Hence, core as well as space charge conductivities might also vary from boundary to... 

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