Forced Motion Activated Self-Alignment of Micro-CPV Solar Cells

In micro-concentrating photovoltaics (micro-CPV), the size of solar cells is reduced (<1x1 mm(2)) compared to conventional CPV. However, the quantity and requirement for placement accuracy of solar cells is increased. To be economically competitive, a promising possibility for the die assembly is a high throughput and relatively unprecise pick and place process combined with surface tension-driven self-alignment of the liquid solder. In this article, this approach is experimentally investigated, with a focus on the influences of solder volume, receiving pad layouts, and initial displacements of the cells on the self-alignment accuracy. Here, we show that an induced motion due to the initial displacement of the cells or due to solder flow along tracks leads to a more robust and accurate process. We found that less solder and rather smaller pads than cells (here by 92 mu m or 10.4% of the cell length) are beneficial for self-alignment accuracy. However, for micro-CPV, conductor tracks connected to the pad are required for electrical interconnection and heat dissipation. Here, all cells are self-aligned and reach an accuracy between -15 and +15 mu m, which is mainly due to the cell-to-pad size difference. Optical simulations show that this displacement would lead to an optical loss of 0.1%(abs) instead of 12.1%(abs) when displacing the cell by 150 mu m. Thus, the self-alignment using the surface tension of the liquid solder leads to sufficient accuracy.