The physical properties of Li-doped g-C3N4 monolayer sheet investigated by the first-principles

•Systematically research on Li-doped g-C3N4 monolayer sheets by first-principles calculation.•Optimal dopant concentration for optical absorption is 7.12%.•Thermodynamics stability of the doped substrate g-C3N4 decreased with Li dopant concentration increasing.•The values of work function Φ decreased monotonously with the increasing of Li dopant concentration.