Reducing silver use in heterojunction solar cells via low-cost copper wires
A research team led by German research center Forschungszentrum Jülich GmbH has proposed to use direct wire bonding (DWB) as a low-temperature method for interconnecting finger-free heterojunction (HJT) solar cells on the front side using low-cost, highly conductive copper (Cu) wires.
“Our study demonstrated the connection of low-cost Cu wires on HJT solar cells as electrodes with conductive paste applied in discrete pads to replace the traditional metallization and interconnection process,” the scientists stated. “The paste used in this structure is cured at low temperatures during the lamination process, making it suitable for heterojunction and perovskite-silicon tandem solar cells.”
In the paper “Silver reduction through direct wire bonding for Silicon Heterojunction solar cells,” which was recently published in Solar Energy Materials and Solar Cells, the research team explained that all their experiments were conducted on M2+ commercial metallization-free heterojunction solar cells with length of 156.75 mm and diameter of 210 mm. The devices were screen-printed with a 0-busbar metallization layout featuring 250 fingers on the rear side and and 96 fingers on the front side. The cells were cut into 5 cm × 5 cm squares from the edge, leaving only three non-passivated edges.
For the cell front side, they used copper wires with a diameter of 30 μm, with silver-coated copper paste being used in “tiny” dots along the wires. “After connecting six 250 μm diameter Cu tabbing wires coated with low-temperature solder wires with a pitch of 8.28 mm for the back contact, the cell is laminated in a module in a size of 20 cm × 20cm at 150 C for 1,060 s, during which the paste is annealed,” they further explained.
The research team then attached three 1 mm wide silver-coated Cu ribbons to the cells' printed busbars via electrically conductive adhesive dispensed in dots with a pitch of 1.65 cm.
The cells built with this architecture were tested in a series of simulations and the line resistance of the Cu wire was found to be six times lower than that of a silver finger and 13 times lower than that of fingers from silver-coated Cu paste, which the academics said ensures longer-distance current transport. “Combining the benefits of Ag-coated Cu paste at the contact points with the lower line resistance of Cu wires, DWB is promising in achieving performance comparable to traditional metallization and interconnection methods,” they emphasized.
Their analysis also showed that a solar module built with the proposed solar cells achieved a power conversion efficiency of 20.19%, which compares to 20.43% for a benchmark panel constructed with six conventional tabbing wires on the front side.
Despite the slightly lower efficiency, the proposed cell configuration offers considerable advantages in terms of paste consumption, with the scientists affirming it is comparable to that of modules with the lowest usage 0BB solution. “However, the DWB offers superior cost savings by eliminating the need for interconnect material,” they concluded.
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