Today, triple-junction solar cells are typically delivered on 4 inch wafers. For the assembly process, the wafers are diced on a foil. The cells are picked automatically by a die bonding machine and are mounted onto a substrate. In a FLATCON module, the substrate consists basically of copper, which acts as heat sink and simultaneously as the electrical rear-side contact for the cell. This is achieved by using an electrically conducting adhesive to mount the cell on the copper plate. The front-side contact of the triple-junction solar cell is gold wire bonded to a layer that is electrically isolated to the copper substrate. Figure 14.9 shows the mounted solar cell on the cooper heat spreader. This unit is called solar cell assembly. In a pick-and-place process, the solar cell assembly is mounted with high precision onto the bottom glass plate of a FLATCON module . The top plate of a module carries the quadratic Fresnel lenses, which are arranged in a matrix. The bottom
Figure 14.9. The most important assembly steps necessary for a FLATCON concentrator assembly are shown. Upper left: the photo of a 4-in. wafer with about 1000 triple-junction concentrator solar cells is shown. Upper right: the cells are mounted onto a copper heat sink. The front side of the cell is contacted with gold wire bond technology. Lower right: many of the solar cell assemblies are mounted in the focus of their corresponding Fresnel lens— the position of one solar cell assembly in the module is marked. Lower left: many FLATCON modules are mounted on a two-axis tracking system—the position of one FLATCON module is marked.
glass plate with the solar cell assembly and the Fresnel lens plate are adjusted to each other. In the next step, the frames between the two plates are sealed together. A photo of the complete module is shown in Figure 14.9 . In order to complete a fUll FLATCON system, many modules are mounted on a two-axis tracking system. Figure 14.9 shows photos of how a FLATCON system is assembled.