Finger Resistance

For the optimum Design of efficient Silicone solar cells, we discussed many factors that should be considered. Now we look at another factor which is Finger resistance.First let us understand the Solar fingers.

Solar busbar and fingers

Silicon solar cells are metallized with thin strips that are printed on the front and rear surface of a solar cell. These front and rear contact strips are the busbars which conduct the electric DC power generated by the cell. Perpendicular to the busbars are the super-thin and metallic grid fingers , which are connected by the busbar. These fingers collect the generated DC current and deliver them to the busbars.  

Finger Resistance and Solar Cell Design

In solar cell design, the balance between finger resistance losses, shading and reflection losses  is optimized.  Even though the higher busbars quantity increases the shading of the solar cell, however the overall performance of multi busbar cells is way better than that of conventional 2 BB or 3 BB cells. This is based on other factors like the reduction of the effective finger length, which reduces finger resistance losses. To be able to provide higher conductivity for the top surface of a solar cell, a series of regularly spaced finger are used. While losses from tapered fingers are theoretically lower, technology limitations make fingers typically uniform in width. The resistive loss in a finger calculations and how it affects power loss are detailed below.

Calculation of Power Loss in the Fingers

For calculation of the power loss in a single finger, several assumptions are made. A constant width is assumed and a uniform generated current is also assumed, and also that the current flows perpendicular to the finger (no current flows into the busbar directly.) Consider an element dx which is at a distance x from the end of the finger. The current flowing through the element dx is: x J_MP S_f where J_mp: the current at maximum power point S_f: the finger spacing. Then the  resistance of the element dx can be calculated as: dx ρ_f  / w_f d_f where w_f: the finger width, d_f: the finger depth (or height) ρ_f: the effective resistivity of the metal. The power loss in the element dx can be caclculated using I²R as below: I²R= dx ρ_f  / w_f d_{f *}  (x J_MP S_f)²   By Integrating x from 0 to L we can calculate the finger power loss as per the equation below: Thus by minimizing finger resistance losses we can ensure we have an efficient design for our solar cell.