how to solve pde in graphene lattice?

I want to solve poisson equation for 2D hexagonal structure (figure below). I have charge density on each angle and I want to know the potential value on that angles.the boundary condition is dirichlet with u=0;

 I know I should use assempde but I don't know how to mesh the structure and how to include boundary condition. actually how the code must be. does anyone have idea?