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 Al_xC (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.