An extended multi-band Hubbard model for the 2D CuO2 planes of high Tc superconductors is examined by exact diagonalization of the Hamiltonian for small clusters with periodic boundary conditions. Holes are shown to bind in the limit of strong onsite (Cu) and nearest neighbor (Cu-O) Coulomb repulsion. A simple explanation of the source of the binding energy is given in this strong-coupling limit.
Stephan, W. H., von der Linden, W., & Horsch, P. (1988). Hole binding in an extended multi-band hubbard model. International journal of modern physics / B, 02(05), 1005-1009. https://doi.org/10.1142/S0217979288000834