Mg2Si is the new black: Introducing a black silicide with >95% average absorption at 200–1800 nm wavelengths

Abstract

Textured silicon surface structures, in particular black silicon (b-Si), open up possibilities for Si-based solar cells and photodetectors to be extremely thin and highly sensitive owing to perfect light-trapping and antireflection properties. However, near-infrared (NIR) performance of bare b-Si is limited by Si band gap of 1.12 eV. This work reports a simple method to increase NIR absorption of b-Si by in vacuo silicidation with magnesium. Obtained Mg2Si/b-Si heterostructure has a complex geometry where b-Si nanocones are covered by Mg2Si shells and crowned with flake-like Mg2Si hexagons. After growth optimization, Mg2Si cover atop b-Si resulted in 5-fold lower total reflectivity (3.7%) and optical absorption to be no lower than 88% over 200–1800 nm spectral range. More importantly, Mg2Si/b-Si heterostructure is more adjusted to match AM-1.5 solar spectrum with theoretically higher photogenerated current density. The maximal advantage is demonstrated in the NIR region compared to bare b-Si in full accordance with one‘s expectations about NIR-sensitive narrow band gap (0.75 eV) semiconductor with high absorption coefficient, which is Mg2Si. Results of optical simulation confirmed the superiority of Mg2Si/b-Si NIR performance. Therefore, this new wide-band optical absorber called black silicide proved rather competitive alongside state-of-the-art approaches to extend b-Si spectral blackness.