COMMIT – Efficient and Reliable Wire Connection of Solar Cells

Duration: October 2013 - October 2016
Contracting Authority/ Sponsors: German Federal Ministry of Economic Affairs and Energy (BMWi)
Project Partners: Gebr. Schmid GmbH
Project Focus:
Homogeneous appearance of wire-connected cells in a module.
© Fraunhofer ISE

Homogeneous appearance of wire-connected cells in a module

Finite-Element simulation of cell deflection and thermo-mechanical stress after a one-sided solder process.
© Fraunhofer ISE

Finite-Element simulation of cell deflection and thermo-mechanical stress after a one-sided solder process.

Spatially resolved presentation of the series resistance of a contacted solar cell, recorded via photoluminescence measurement. Right: 3-busbar solar cell, left: Solar cell with wiring.
© Fraunhofer ISE

Spatially resolved presentation of the series resistance of a contacted solar cell, recorded via photoluminescence measurement. Right: 3-busbar solar cell, left: Solar cell with wiring.

Flat wire connectors are currently soldered on the busbars of the cell for wiring of crystalline solar cells. An alternative is wiring with thin round wires. This enables a more homogeneous power distribution on the cell, a lower silver consumption for front side metallization, and a higher module power due to improved light reflection. Beyond that, the many thin wires on the cell provide a homogeneous, aesthetically appealing appearance. Our measurements show that solar modules with this wiring technology show very good reliability under accelerated aging. They have potentially higher degrees of efficiency than standard modules.

 

In collaboration with plant manufacturer SCHMID, we develop a new generation of stringers. The solder-enclosed wires are soft-soldered on the solar cell. The advantages of the concept are simple retrofit of existing module productions and use of established tin-containing standard solder and solder processes.

So far, silver savings of 50 % could be achieved by adjusting front side metallization and direct contacting of backside aluminum. Further reductions are currently evaluated. The detailed characterization of joints and metallization is key. The PL-RS method for solar cell wiring was further developed. The 23% lower serial resistance losses were spatially resolved and quantified. Furthermore, a special preparation method enables the micro-structural analysis of the joints and the adhesion test towards the cell also after accelerated aging tests. Using validated FEM simulations, we calculate the thermo-mechanical stresses after the solder process depending on the different metallization designs. We were able to continuously improve the joining process and screen printing geometry. The pull-off forces are significantly above 1 N/mm soldering line width. The critical reliability tests from IEC 61215 for long-term stability verification were passed as well. It was demonstrated that the soldering points have very good mechanical strength and withstand the stresses of the solar module test.