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

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 m(0)), and good defect tolerance, as well as strong visible-light absorption coefficients (10(5)-10(6) 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 mu m, 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.

Automated summary

This study reports three new two-dimensional (2D) Ge-Sb-Te compounds with superior stability, suitable electronic properties, and excellent optical performance. These compounds are predicted to have high photovoltaic conversion efficiencies and could serve as a prominent platform for future experimental research and nano-optoelectronic devices.