Silicon solar cells are metallized with thin strips printed on front and rear of a solar cell. These front and rear contact strips are referred to as busbars.
The purpose of busbars in solar cells is to conduct the electric DC power generated by the cell when photons hit the cells.
These contacts – the busbars and the fingers – are printed onto the surface via a technology called screen printing. Usually, solar cell busbars are made of a silver paste or similar high conductivity materials. The silver plating is necessary to improve current conductivity (front side) as well as to reduce oxidization (rear side).
Perpendicular to the busbars are the metallic and super-thin grid fingers , which are connected by the busbar.
Basically the fingers collect the generated DC current and deliver it to the busbars.
During the module manufacturing process, String and tab wires are soldered to the busbars to connect solar cells in a combination of series and parallel. The cluster of strings are then connected to the solar junction box.
Shown below is an image showing the busbars and the grid finders of a typical solar cell :-
An important point in solar cell design and production is to strike an optimal balance between busbar and finger resistance losses and shading and reflection losses – the former caused by using too few metallic contacts, thus creating greater distances between them, and the latter being the consequence of too many contacts on the cell surface, basically preventing light from entering the solar cell. Important parameters are in this regard are the height and width of the busbars, the width of the fingers, spaces between the fingers and the busbars as well as the metal type and quality.
The older practice was to have solar cells with typically 2 busbars.
However as the technology evolved and the industry moved towards higher efficiencies, the busbars in most solar cells were increased to 3 busbars.
However, to achieve even higher efficiencies the latest trend is to have 4 busbars and sometimes even 5 busbars.
This increased number of busbars reduces the internal resistance losses, which is due to the lesser distance between the busbars. They have also helped in increasing the cell efficiency and hence ultimately improving the module efficiency.
Even though the higher quantity of busbars increases the shading of the solar cell, yet overall performance of multi busbar cells is much better than that of conventional 2BB or 3BB cells. This is because of aspects such as the reduction of the effective finger length between the busbars, which reduces finger resistance losses.
In conclusion as compared to conventional 2BB and 3BB cells, the long-term reliability and performance of a multi busbar (Eg. 4BB / 5BB) cell is even higher.