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

Abstract

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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> )) 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 NF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> treatment and damage-free neutral beam (NB) etching. The 2D Si-ND array shows high photon absorption coefficient (>10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) and high PL intensity at room temperature owing to high-density 2D Si-ND (>7×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ) and narrow spacing. The E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> and PL emission peaks can be easily controlled by changing the ND thickness. The E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> 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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> dimension. The NDs were separated by 1-2nm-thick SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 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.