Al_xC Monolayer Sheets: Two-Dimensional Networks with Planar Tetracoordinate Carbon and Potential Applications as Donor Materials in Solar Cell

We perform a global search of the most stable structures of 2D stoichiometric AlxC (x = 1/3, 1, 2, and 3) monolayer sheets. In the most stable 2D planar AlC network, every carbon atom is tetracoordinated. In addition to the structure of AlC, structures of the most stable Al2C and Al3C monolayer sheets are also predicted for the first time. AlC and Al2C monolayers are semiconducting, while Al3C monolayer is metallic. In particular, Al2C monolayer possesses a bandgap of 1.05 eV (based on HSE06 calculation), a value suitable for photovoltaic applications. Moreover, three Al2C/WSe2, Al2C/MoTe2, and AlC/ZnO van der Waals heterobilayers are investigated, and their power conversion efficiencies are estimated to be in the range of 12-18%. The near-perfect match in lattice constants between the Al2C monolayer and PdO (100) surface suggests strong likelihood of experimental realization of the Al2C monolayer on the PdO (100) substrate.