3-D multi‐ribbon interconnection tech for IBC cells

Researchers at the Belgian research institute Imec have tested a new 3-D multi‐ribbon interconnection technology in a few sample modules with interdigitated back‐contact (IBC) solar cells and have found that the modules show no significant degradation after 600 thermal cycles and very limited degradation after 800 cycles.

“This is the first time the technology is implemented in multi-cell modules with IBC cells,” Imec researcher Rik Van Dyck told pv magazine. “The technology was first tested using old, inefficient Metal Wrap Through (MWT) in 2019 and now we achieved nice reliable results and used proper IBC back-contact cells to show its potential.”

The proposed interconnection tech is based on a three-dimensional fabric of encapsulant with incorporated horizontal and vertical solder‐coated metal ribbons, which the scientists labeled as “cell‐to‐cell ribbons” and “busbar ribbons,” respectively. “These two layers of metal ribbons are separated by the encapsulant to provide electrical insulation between the different electrical phases,” they further explained. “At certain locations in the fabric, the busbar ribbons are stitched through the encapsulant, overlapping with the cell‐to‐cell interconnection ribbons.”

According to the Belgian team, this overlapping creates a floating connection point, which is a kind of connection ensuring that a connector adjusts its connection point when the shape moves. Each cell‐to‐cell ribbon has just one connection point with a busbar ribbon of each neighboring cell, which ensures lower thermal‐induced stress while increasing the reliability of the cells. “By interrupting the cell‐to‐cell ribbons on specific locations, the fabric gets a tapered design and one ribbon can be used for two opposite polarities, which can save on copper consumption,” highlighted the research group.

The combined lamination and soldering process used allows for soldering the busbar ribbons directly onto the cell fingers without the need for busbar printing on the cell. “This potentially cuts both processing steps and material consumption during cell manufacturing, further reducing the cost of the module,” the researchers stated. The technique must be implemented with relatively low lamination temperatures and a low-melting-temperature solder. “There is some adaptation required on [a] cell metallization level, but the used lamination temperatures and steps are compatible with current processing tools for module manufacturing,” Van Dyck added.

The interconnection technology was tested in one and four‐cell modules with front side and back side glass sheets, each with a thickness of 3mm. Heating and cooling cycles according to the IEC 61215 standard for thermal cycling reliability, were performed. The test showed that the fill factor and power output of both one-cell and four-cell devices was relatively stable, with imperceptible drops after 600 cycles. “These results demonstrate the module reliability in thermal cycling and prove the feasibility of the concept,” the scientists added. “They also prove that the encapsulant can act as a stable insulator between ribbons and cell metallization of opposite polarity.” The encapsulant is a glass-fiber-reinforced thermoplastic olefin.

The researchers are currently adapting standard encapsulation materials to enable their lamination process. The proposed interconnection technology is described in the paper Three‐dimensional multi‐ribbon interconnection for back‐contact solar cells, published in Progress in Photovoltaics.