The electrical properties of the metallic interface in LaAlO3/SrTiO3 (LAO/STO) bilayers are investigated with focus on the role of cationic defects in thin film STO. Systematic growth-control of the STO thin film cation stoichiometry (defect-engineering) yields a relation between cationic defects in the STO layer and electronic properties of the bilayer-interface. Hall measurements reveal a stoichiometry-effect primarily on the electron mobility. The results indicate an enhancement of scattering processes in as-grown non-stoichiometric samples indicating an increased density of defects. Furthermore, we discuss the thermodynamic processes and defect-exchange reactions at the LAO/STO-bilayer interface determined in high temperature equilibrium. By quenching defined defect states from high temperature equilibrium, we finally connect equilibrium thermodynamics with room temperature transport. The results are consistent with the defect-chemistry model suggested for LAO/STO interfaces. Moreover, they reveal an additional healing process of extended defects in thin film STO.