Effect of Parasitic Resistances on Solar Cells
The key impact of parasitic resistance is reducing the fill factor in the majority of cases and for usual values of series and shunt resistance. Both the impact and the magnitude of series and shunt resistance is dependent on the geometry and shape of the solar cell, at the point of operation of the solar cell. As the value of resistance will be proportional to the area of the solar cell, when series resistance of solar cells that might have different areas is compared, a typical unit for resistance is in Ωcm2. This resistance normalized to area results from replacement of the current with current density in Ohm’s law as given in the equation below:
R’ (Ωcm2 ) = V/J
Resistive effects of solar cells reduce the efficiency of the solar cell by dissipation of power in the resistances. Series resistance in a solar cell is due to the following:
- The movement of the current through the pn materials of the solar cell
- The contact resistance between the silicon and the metal contact
- The top metal contacts and rear metal contacts resistance.
Similar to the way an electrical source can be represented with the internal resistance losses, solar cells commonly suffer from two distinct parasitic current losses (ISR and IPR) caused respectively by the series and the parallel resistance. The series resistance (RS), is basically a result of the bulk resistances of the semiconductor and all its metallic contacts in addition to the contact resistance between these two materials. The parallel resistance ( RP), usually can be due to leakage at the edge of the cell or any extended defects in the depletion zone.
These extended defects which are associated with low cost solar cells can include dislocations, grain boundaries and large precipitates. These aforementioned sites become areas which are likely to suffer doping diffusion or fine metallic bridges caused by contact metallisation.
If a resistance is connected to a solar cell, like when applying a resistance RA, two different parasitic effects can be found:
Current declines the higher the series resistance RS, while voltage declines the lower the parallel resistance RP. The two of the damaging cases reduce the FF as both IM and VM respectively are shifted towards the origin on an I-V graph.