Page 76 - Applied Photovoltaics

Figure 4.9. Typical external quantum efficiency and responsivity in actual solar
cells, illustrating the impact of optical and recombination losses.

4.4 TOP CONTACT DESIGN
Metallic top contacts are necessary to collect the current generated by a solar cell.
Busbars are connected directly to the external leads, while fingers are finer areas of
metallisation that collect current for delivery to the busbars. A simple top contact
design is shown in Fig. 4.10. Top contact design aims to optimise current collection
against losses owing to internal resistances and cell shadowing.

busbar

finger

Figure 4.10. Top contact design of a solar cell.

4.4.1 Bulk and sheet resistivities

Generated current typically flows perpendicular to the cell surface from the bulk of
the cell and then laterally through the top doped layer until it is collected at a top
surface contact, as shown in Fig. 4.11.
The resistance to the current of the bulk component of the cell, or the bulk resistance
(R b ) is defined as

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