High-density and well-ordered Si-nanodisk array with controllable band gap energy and high photon absorption coefficient for all-silicon tandem solar cell applications

All-Si tandem solar cells comprising quantum dot superlattice (QDSL) have attracted much attention. However, a fabrication to realize uniform and controllable QD size (determines band gap (E_g)) and spacing between QDs (generates miniband) in QDSL is still a big challenge. In this study, we created a two-dimensional sub-10nm-Si-nano-disk array (2D Si-ND array) with a high-density and well-ordered arrangement using bio-template (Iron-oxide core) as a φ-7-nm-etching-mask and an advanced etching process that included NF₃ treatment and damage-free neutral beam (NB) etching. The 2D Si-ND array shows high photon absorption coefficient (>10⁵ cm^-1) and high PL intensity at room temperature owing to high-density 2D Si-ND (>7×10¹¹ cm^-2) and narrow spacing. The E_g and PL emission peaks can be easily controlled by changing the ND thickness. The E_g varied from 2.2 to 1.4 eV. To realize tandem solar cells, we also investigated fabricating stacked NDs, which is to stack the NDs along the 3^rd dimension. The NDs were separated by 1-2nm-thick SiO₂ that was formed by our developed NB oxidation (NBO) at 300°C. The tunneling current can be controlled by changing the tunneling junction thickness using NBO. Those results support the feasibility of our proposed processes for the high-efficiency all-Si tandem solar cells comprising QDSL.