We report here on low energy electron diffraction (LEED), Auger spectroscopy, scanning tunneling microscopy (STM) and angle-resolved photoemission (ARPES) studies of ultrathin films of K/Si(111)-√3×√3R30:B. A potassium-induced surface state is evidenced being maximum at the saturation coverage. ARPES data clearly evidence the folding of the K-induced surface band near the expected k_F attesting of a large Mott gap Δ=500 meV and a very narrow measured bandwidth W=140 meV. This clearly signs the strongly correlated nature of this material. Nevertheless, by decreasing the temperature, a novel insulator to insulator surface phase transition is observed characterized by a doubling of the unit cell together with a stabilization of the surface band. In addition, the linear temperature dependence of the ARPES linewidth suggests these materials are characterized by a strong electron-phonon coupling. Therefore, such a phase transition can be explained if the electron-phonon coupling is large enough to compensate the strong on-site repulsion defining a bi-polaronic insulating ground state instead of a Mott-Hubbard one. [DOI: 10.1380/ejssnt.2009.259]