Page 77 - Applied Photovoltaics

l ȡ w
R ȡ (4.5)
b b
A A
taking into account the thickness of the material, where l is the length of the
conducting (resistive) path, ȡ b is the bulk resistivity (inverse of conductivity) of the
bulk cell material (typically 0.5–5.0 ȍ.cm for a silicon solar cell), A is the cell area,
and w is the width of the bulk region of cell (see Fig. 4.13).

Figure 4.11. Current flow from point of generation to external contact in a solar
cell.

Similarly, for the top n-type layer, sheet resistivity (ȡ ) is defined as
†
ȡ
ȡ (4.6)
†
t
where ȡ is the resistivity of this layer. The sheet resistivity is normally expressed as
ohms/square or ȍ/†.
For non-uniformly doped n-type layers; that is, if ȡ is non-uniform

1
ȡ (4.7)
† t dx
³ 0 ȡ( x)

Sheet resistivity is very easy to measure experimentally using a ‘four point probe’, as
shown in Fig. 4.12.

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