New two-dimensional Ge–Sb–Te semiconductors with high photovoltaic performance for solar energy conversion

Abstract

Atomically thin semiconductors with superior electronic and optical properties play a critical role in the continuous advances of electronic and optoelectronic devices. Here, using first-principles calculations, we report three new two-dimensional (2D) Ge–Sb–Te (GST) compounds, namely one septuple-layer Ge₁Sb₂Te₄ (Ge₁Sb₂Te₄-1 SL), one nonuple-layer Ge₂Sb₂Te₅ (Ge₂Sb₂Te₅-1 NL), and one undecuple-layer Ge₃Sb₂Te₆ (Ge₃Sb₂Te₆-1 UL), as very promising optoelectronic semiconductors for photovoltaic solar cells. It is demonstrated that these hitherto-unknown GST monolayers are highly stable, as they exhibit excellent thermodynamic, dynamical and room temperature thermal stability. Also, owing to the small exfoliation energy, they are expected to be synthesized experimentally by mechanical exfoliation. More importantly, comprehensive investigations show that Ge₁Sb₂Te₄-1 SL, Ge₂Sb₂Te₅-1 NL, and Ge₃Sb₂Te₆-1 UL possess suitable electronic bandgaps (0.837–1.031 eV), extremely low direct–indirect bandgap differences (≤0.031 eV), small carrier effective masses (≤0.26 m₀), and good defect tolerance, as well as strong visible-light absorption coefficients (10⁵–10⁶ cm⁻¹). Thanks to these favorable electronic properties and extraordinary optical performance, they are predicted to exhibit high photovoltaic conversion efficiencies of 26–30% at 0.1 μm, appreciably larger than the currently dominant commercial silicon solar cell. In addition, we also reveal that the strain has little effect on the light absorbance of 2D GST crystals, while it can significantly tune their electronic structures and thus the photovoltaic efficiency. Our explorations would render semiconducting 2D GST monolayers a prominent platform for future experimental research and nano-optoelectronic devices.