Project 025 – Realising a concept

Design of a conductive foil for MWT solar cell panels

I designed this patterned conductive foil for a research institute for renewable energies within a project that aimed to half the cost price of PV solar panels. I also guided the production of the prototypes.

The foil is used to interconnect metallisation-wrap-through (MWT) type solar cells. In this solar panel type, the electrical series connections of the solar cells are all made on the rear side of the solar cells by an electrically conductive foil. This is very similar to the way surface mount components (SMDs) are mounted to printed circuit boards (PCBs) in the electronics industry.

These MWT type solar panels avoid the shadow losses and the mechanical stress from the electrically conductive copper ribbons that have been the standard in solar cell industry. These ribbons connect the solar cell in series from the top side of one cell to the back of the next cell. This needed to obtain a usable voltage from a solar panel.

The challenge was to meet all requirements for efficiency and production:

  • cost price
  • manufacturability
  • lifetime
  • degradation
  • high electrical conductivity / low electrical resistance
  • aesthetic
  • electrical insulation between individual cells and cell strings
  • electrical insulation between cells and the solar panel edge/frame
  • moisture proof (creep currents, corrosion, electrochemical damage)
  • protection from shorts due to rough solar cell backsides
  • allow cell dimension tolerances
  • allow pick-and-place alignment tolerances
  • allow for foil shrinkage during lamination (optional local stress relief cutouts)
  • good adhesion to EVA encapsulants
  • three electrical terminals (for two external bypass diodes)

The present-day MWT solar panel backsheets are made in a different way than was possible at the time this project was executed. Nowadays, the separation of the conductive areas in the foil is usually done by CNC milling. Back then, the foils were made using a wet etching process. An insulating varnish was added on top of the conductive layer to prevent short circuits with the back side of the solar cells.

Both the etch-resist and the varnish were applied by screen-printing. The pattern was designed to have no grooves perpendicular to the screen-print direction. This was to prevent air entrapment (voids) in the grooves between the conductive areas.

I verified the design using electrical current density simulations.

For more information see also Project 024 and Project 026.


Currently, these MWT-type solar panels are in full production around the world. A few links: