The multi-busbars (MBB) approach aims to reduce resistive losses by reducing the amount of current that flows in both the fingers and the busbars. As more busbars are printed on a wafer, the space between them shortens, shortening the length that current flows in the fingers which is a large source of series resistance losses. As the resistive power loss Ploss scales with I2R, this means that if the current is reduced by half, the resistive losses are reduced by a factor of four. The MBB approach thus allows both busbars and fingers to be smaller in size and hence, a smaller volume of silver, one of the most expensive consumables, is used in solar cell manufacturing. The MBB approach has the potential to reduce silver usage by up to 50%-80%. Furthermore, less silver on the front in turn reduces finger shading as well.
Where MBB and the more traditional “more-busbars” approach differ is cross-section and function. Busbars are normally printed flat and require soldered flat ribbons to carry the current away from the cell, introducing more shading and resistive losses. MBB are thin, rounded copper wires that do not require ribbons across the solar cell, rather they carry current from the fingers through to interconnecting ribbons outside the front surface of the cell. As shown in Figure 1, their rounded cross-section also increases optical performance, allowing for more light to reflect onto the solar cell.
Furthermore, as bifacial technology gains a larger global market share, MBB has the potential to increase the bifaciality of PERC cells. Bifaciality is the ratio of the front power to rear power. By using MBB, smaller rear aluminium fingers can be printed, decreasing the shading on the rear side of the cell, improving bifacial light harvesting function of the cell. In addition to bifacialiality, the cell is less susceptible to microcracks. This is because MBB increases the chances of cracked parts of the cell maintaining electrical contact with the rest of the cell.
MBB technology has been implemented by two different techniques. Smart Wire Connection Technology (SWCT), which is Meyer Burger’s approach, and a more traditional soldering method originally developed by Schmid.
Smart Wire Connection Technology
SWCT moves away from traditional screen-printing of busbars. The technique involves coating thin wires in low melting point alloys and embedding them into a polymer foil. The foil is then laminated onto the solar cell allowing the rounded, thin wires to make contact with all the fingers of the solar cell. This lamination follows the traditional approach and hence, aside from bypassing the need to print busbars, implementing SWCT does not alter the production line too much. Currently, the standard is an 18 wire configuration.
SWCT introduces a couple of advantages over traditional screen-printed busbars:
- Low temperature processing as contact is made in the lamination process.
- No need for silver busbars, saving up to 80% of silver usage. This makes SWCT suitable for advanced bifacial architectures which normally require silver printed on both sides.
Currently, SWCT is applicable for bifacial, glass-glass and half cells, making Meyer Burger’s approach an optimal technique for higher efficiency solar cells. Meyer Burger themselves produced a 413 W module with 72 Heterojunction solar cells.
Soldered pads to contact the cell
Germany’s Schmid proposed a more traditional interconnection approach by soldering thin, rounded wires across the solar cell instead of flat ribbons. Although these wires still route the electrons from the front to the back of the next cell, they require soldering pads to attach the wires to the cells. Schmid has ceased its MBB development but Chinese manufacturers are now able to provide robust MBB processing tools to the PV market.
References:
[2] – T. Söderströma, P. Papetb, Y. Yao, J. Ufheilc, “Smartwire Connection Technology“, Meyer Burger.