Direct and quasi-direct band gap silicon allotropes with low energy and strong absorption in the visible for photovoltaic applications

Abstract

Four new sp3-hybridized silicon allotropes (Si10, Si14, Si20-I, and Si20-II) in the space groups of P21/m and P2/m were developed in this study. Their values for properties related to stability, and mechanical, electronic, and optical properties were investigated using the first-principles method. Compared with most previously predicted and synthesized silicon phases, these allotropes exhibit lower energy, which are only 0.042, 0.037, 0.056, and 0.120 eV/atom higher than that of diamond-Si. These silicon allotropes are direct or quasi-direct semiconductors with band gaps in the range of 1.193–1.473 eV, and exhibit stronger photon absorption in the visible and ultraviolet regions than diamond-Si. In addition, they exhibit excellent mechanical properties, such as stronger resistance to linear compression than diamond-Si and higher hardness than most previously reported phases of silicon. Given their low energy, direct or quasi-direct band gap, and strong capacity for absorbing solar energy, these four silicon materials have potential for use in thin-film solar cells and photovoltaic devices.

Keywords

• Si allotrope
• Density functional theory
• Electronic properties
• Photovoltaic application
• Mechanical properties