Tunable phase transitions and high photovoltaic performance of two-dimensional In₂Ge₂Te₆ semiconductors

Ultrathin semiconductors with great electrical and photovoltaic performance hold tremendous promise for fundamental research and applications in next-generation electronic devices. Here, we report new 2D direct-bandgap semiconductors, namely mono- and few-layer In₂Ge₂Te₆, with a range of desired properties from ab initio simulations. We suggest that 2D In₂Ge₂Te₆ samples should be highly stable and can be experimentally fabricated by mechanical exfoliation. They are predicted to exhibit extraordinary optical absorption and high photovoltaic conversion efficiency (≥31.8%), comparable to the most efficient single-junction GaAs solar cell. We reveal that, thanks to the presence of van Hove singularities in the band structure, unusual quantum-phase transitions could be induced in monolayers via electrostatic doping. Furthermore, taking bilayer In₂Ge₂Te₆ as a prototypical system, we demonstrate the application of van der Waals pressure as a promising strategy to tune the electronic and stacking property of 2D crystals. Our work creates exciting opportunities to explore various quantum phases and atomic stacking, as well as potential applications of 2D In₂Ge₂Te₆ in future nanoelectronics.