  We study by first-principles calculations a densely packed island of organic
molecules (F4TCNQ) adsorbed on graphene. We find that with electron doping the
island naturally forms a p-n junction in the graphene sheet. For example, a
doping level of ~ 3 * 10^{13} electrons per cm^2 results in a p-n junction with
800 meV electrostatic potential barrier. Unlike in a conventional p-n junction
in graphene, in the case of the junction formed by an adsorbed organic
molecular island we expect that the Klein tunneling is inhibited, even without
an applied external magnetic field. Here Klein tunneling is inhibited by the
ferromagnetic order that spontaneously occurs in the molecular island upon
doping. We estimate that the magnetic barrier in the graphene sheet is around
10 mT.
