Shingling is another advancement used to obtain cell-to-module (CTM) gains, the technique eliminates the need for interconnecting ribbons and hence reduces resistive losses. The main difference with other techniques is the aesthetic nature of shingled modules. The modules also look like panels of coloured glass, an excellent approach for aesthetic building design and hence, the rooftop solar market. Although companies such as Solaria and SunPower have made a considerable push for shingled modules, the International Technology Roadmap for Photovoltaics (ITRPV) predicts a small increase in market share of about 10% by 2029[2].
Shingling PV cells follow the same process for shingling roof tiles on a rooftop, however, standard cell formats cannot be used. It involves slicing complete cells along the busbars and forming interconnections by placing the rear busbar of one slice over the busbar of the next slice. Therefore the busbars are hidden in the overlap of the adjacent sliced cells. This approach removes the need for interconnection ribbons and eliminates the spacing between adjacent cells. The image below from SunPower indicates how the slices are placed and where the interconnection is formed[3].
In traditional solar cells, front and rear busbars are aligned such that ribbon interconnections can easily contact adjacent cells. From figure 2 above, cells used in shingling must have their busbars printed such that each resulting slice will have a front busbar on the ‘left’ and a rear busbar on the ‘right’ side. Therefore, this technique involves changes in the cell manufacturing stage as well. The small area of the individual slices also reduces the current that flows in the module and as the resistive power loss Ploss scales with I2R, this will result in a reduction in resistive losses. A smaller current also means the number of size of the fingers can reduced offering opportunities to reduce the cell manufacturing costs by a reduction in the usage of silver.
Furthermore, like many other PV module advancements, shingling can be combined with glass-glass and bifacial techniques. Since more of the module can be covered by solar cells, shingling is a very suitable method for bifacial modules. More light can be absorbed and ‘back-escape’ losses can be reduced, which normally occur when light passes through the gaps in traditional bifacial modules.
Challenges
Shingling also faces some challenges. Since interconnection is formed by physical contact of the sliced cells, the varying thermal expansion coefficients of silicon, silver and glass will create different levels of thermo-mechanical stress on the interconnection. Therefore, the interconnection must be flexible and adhesives can be used. The tools required are also very expensive and have a slower throughput.
Furthermore, the smaller area of the slices and the number of slices in a module increases the edge to area ratio. This results in larger recombination losses occurring at the edge of the cells. SunPower attempted to bypass this issue by using lower quality substrates such as multi-crystalline, with lower carrier lifetime, fewer edge losses are observed since the charge carriers do not ‘see’ the edge as much. However, Solaria has been able to deal with the edge recombination losses in a way that enables 20.3% efficient mono PERC PV modules at 20.3% efficient.
Another reason for the low expected market share by the ITRPV may be due to the number of patents surrounding the shingling technique. Solaria and SunPower hold important intellectual property rights to the technology required to produce shingled modules. However, with growing interest from Chinese manufacturers, they have begun to find technological methods around the patents.
