Gap states induce soft Fermi level pinning upon charge transfer at ZnO/molecular acceptor interfaces

The deposition of strong molecular electron acceptors onto ZnO induces a substantial work function (\ensuremath{\phi}) increase due to electron transfer from the inorganic semiconductor to the molecules. The \ensuremath{\phi} increase results from two mechanisms: (i) a change of the surface band bending within ZnO and (ii) an interface dipole between the inorganic surface and the negatively charged acceptors. The molecule adsorption induced upward band bending in ZnO is, however, found to be limited to a few 100 meV, while the \ensuremath{\phi} increase is significantly larger (up to 2.8 eV). We elucidate the origin of limited upward band bending by revealing a notable gap state density-of-states (GDOS) using high-sensitivity photoemission spectroscopy. Upon acceptor-induced upward band bending, the GDOS with a wide energy distribution becomes increasingly unoccupied. This, in turn, makes the interface dipole dominant and limits the ZnO surface band bending changes due to a ``soft'' pinning at the Fermi level.