Geometrical Analysis of Solar Cells With Partial Rear Contacts

The analysis of solar cells where the posterior metal contact is formed only on part of the rear surface, which is mostly covered by a nonconductive, passivating layer, is both important and complex. A possible approach, based on a geometrical representation of the device structure, is examined here. As minority carriers flow toward the localized rear contact, they crowd inside a diminishing cross-sectional area, resulting in a high current density. The latter demands a strong gradient in their concentration, which leads to an increase of the open-circuit voltage V_oc. Similarly, the crowding of majority carriers requires a strong gradient of the electrostatic potential, which leads to an increased series resistance Rs. These effects of carrier crowding are described here with simple mathematical expressions that permit an approximate evaluation of V_oc and Rs for partial rear contact solar cells.