More than \( 90\% \) of photovoltaic cells are now mostly built from silicon. After a p-n junction is made, a forward (negative) electrode is applied to the n-type semiconductor, made as a network of thin paths horizontal to the edge of the cell (called fingers), which collect charges from the entire cell and transfer them to ribbon connection paths perpendicular to the fingers, called busbars. A positive electrode (on p-type silicon) covers the entire back surface of the cell.
Busbar technology has been used since the early introduction of photovoltaics to the market. Polycrystalline cells first used 2 junction paths, or 2 busbars, achieving efficiencies of \( 12\% \).

They had 110 to 660 contact points with horizontal paths, that is, with fingers ( Fig. 1 ) [1].

As this technology was developed, the number of busbars was increased so that in more recent developments it is 5 busbars.

When 3 busbars are used, the finger length is reduced from 39 mm to 26 mm ( Fig. 2 ), which translates into a reduced finger cross section and at the same time the cell performance is increased.

Increasing the number of busbars helps to avoid the large impact of microcracks on photovoltaic cell performance.

Microcracks eliminate part of the photovoltaic cell from being able to receive electricity from it. The reason for microcracks, for example, is that cells are joined by soldering at temperatures between 240 and 340 \( _{}^{o}\textrm{C} \).

This exposes the entire structure to stresses associated with uneven heating of the cell, which can lead to degeneration of the entire panel. The lifetime of panels is shortened by microcracking, which increases the junction resistance.

The number of busbars and the number of fingers affect the performance of the cell (the so-called fill factor (FF) and series resistance) and, consequently, the total efficiency of the photovoltaic panel.

Using a larger number of busbars shortens the path that the electron and hole must travel, which decreases the internal resistance and facilitates current flow.
Increasing the number of busbars reduces the current flowing through a single busbar which results in lower operating temperature of the cell.
The resistance of the cell depends on the distance that must be overcome by the electric charge in the cell. Increasing the number of busbars decreases this distance. Increasing number of busbars [2] has an influence on the increase of cell efficiency, but also improves its work in shaded conditions or in case of mechanical damages reducing the cell surface. Increasing the number of busbars also increases the mechanical resistance of the panel to rain, snow, hail or wind.

MBB "Multi-Busbar" technology

In cells manufactured with MBB multi-path technology, wire electrodes are introduced in place of wide strip busbar electrodes, and the number of wires is 15 or more per cell. That is, each wire conducts less than 0.5 A rather than 4.5 A as with two busbars.

There are at least three different solutions in operation today: SmartWire technology from Swiss company Meyer Burger, Merlin technology, and Schmidt's Multi Busbar Connector.
Instead of the busbars described above, SmartWire technology is based on the use of a grid of wires on the plane of the photovoltaic cell. The number of electrical connections in such a single cell reaches up to 2660 ( Fig. 3 ). This provides resistance to mechanical stress and better performance in low light conditions.

Merlin technology is characterized by placing a specially formed copper grid over and under the photovoltaic cell ( Fig. 4 ) in the silicon cell. In this case, the internal connections are stronger, resulting in lighter and more durable photovoltaic panels.

Here, flexible copper mesh replaces traditional busbars, which reduces material consumption and thus costs. Furthermore, Merlin's technology can be easily integrated into existing cell and panel production lines.
Multi Busbar Connector Technology ( Fig. 5 ) is based on the use of 360 microns, coated with a thin layer of SnPbAg alloy about 15 microns thick. 12 busbars are placed on each cell. This results in an increased fill factor, which consequently increases the power produced by the photovoltaic cell.

Passivated Emitter Rear Cell - PERC

PERC cell differs from standard photovoltaic cell in the construction of the rear electrode of the cell.
In the rear electrode, a modification is introduced, consisting of a deposition of an additional layer, which performs reflection and passivation functions.
Passivation is used to protect against oxidation of the silicon surface and reduce the recombination at the silicon metal electrode interface.
The insulator layer ( Fig. 6 ) causes, among other things, reflection of sunlight and directing it to the cell to increase absorption, and thus the cell's power [3]. The increased reflection of electromagnetic radiation from the passivation layer increases the absorption and, therefore, also increases the obtained power of the cell.

Currently many laboratories are working on increasing the efficiency of photovoltaic cells, but many of the solutions developed have more laboratory than commercial application. In addition to the problems discussed above, there are also those related to shading, microcracks, reflection and, of course, lowering production costs. All these elements are motivation to search for solutions optimizing cell work.
Increasing the number of busbars on a cell decreases resistance, which results in an increase of power obtained from the cell. Most losses connected with microcracks are avoided. Increasing the number of busbars gives electrons more opportunities to reach the electrode.

The way of joining photovoltaic cells in PV panels made with busbar technology are shown in Fig. 7. The soldering places are particularly vulnerable. The soldering paces are particularly exposed to stress. This is due to the level of temperature at the time of joining the cells to each other.