  Charge redistribution at high-Tc superconductor interfaces and grain
boundaries on the one hand is problematic for technological application. On the
other hand, it gives rise to a great perspective for tailoring the local
electronic states. For prototypical (metallic) interfaces, we derive
quantitative results for the intrinsic doping of the CuO2-planes, i.e. for the
deviation of the charge carrier density from the bulk value. Our data are based
on ab-initio supercell calculations within density functional theory. A
remarkable hole-underdoping is inherent to the clean interface, almost
independent of the interface geometry. On the contrary, cation substitution as
well as incorporation of electronegative impurities can compensate the
intrinsic charge transfer and provide access to an exact adjustment of the
superconductor's doping. The effects of oxygen deficiency are discussed.
