GaAs//Si Multijunction Solar Cells Fabricated via Mechanical Stack Technology Using Pd Nanoparticles and Metal-Assisted Chemical Etching

Multijunction solar cells (MJSCs) have attracted attention as next-generation solar cells. In particular, GaAs//Si-based MJSCs are highly efficient with low cost and are expected to gain new applications, such as on-vehicle integrations. In this article, we examined a highly efficient In0.49Ga0.51P/Al0.06Ga0.94As//Si three-junction solar cell. The bottom Si cell has a tunnel oxide passivated contact structure. The key technology used to fabricate this solar cell is a stacking method that uses Pd nanoparticles (Pd-NPs) and metal-assisted chemical etching (MacEtch) for the bonding interface, which is improved from our previous "smart stack" technology. The MacEtch method has a feature of selective etching for Si around a metal body. Pd-NPs selectively invade the Si cell through the surface of the Si oxide layer, thereby improving the bonding resistivity between the GaAs-based cell and Si cell. Further, this technology aids the management of the bonding gap width by controlling the Pd-NP invasion depth. As a result, an efficiency of 27.6% for the aperture area was attained. The proposed technology is useful for the connection of Si-based cells, enhancing the development of GaAs//Si-based tandem solar cells.