Voltage offset and built-in polarization were investigated in in-situ reactively sputter deposited PZT thin films on Pt, as a function of composition, doping and cooling conditions. The voltage offset increases with increasing Ti content. Hot-poling treatments show that the voltage offset originates from non-mobile defects. Cooling the samples at various 02 partial pressures leads to a change in lattice parameters and orientation, showing a 'bulk' effect rather than a simple 'surface' effect. While doping with Nb does not change the status of the films, doping with Fe increases coercive fields but removes the asymmetrical behavior of the voltage offset after hot-poling at positive and negative voltages. Annealing/cooling cycles are reversible in the suppression/creation of voltage offset and built-in polarization, respectively, excluding the idea of oxygen and lead vacancy dipolar defects. Experimental data suggest that oxygen is lost and the vacancies diffuse during cooling without being compensated to equilibrate with surface or electrode band bending, resulting in the creation of an internal bias field.