We induce ultra-high carrier charge density in polycrystalline zinc oxide thin films on glass with a thickness of few tens of nm, achieving carrier concentrations as high as 2.2×1014 cm−2, well beyond the Ioffe-Regel limit for an insulator-metal transition in two dimensions. The sheet resistance is consequently lowered by up to 5 orders of magnitude to about 2 k Ω/◻ without alteration of transparency thanks to our space charge doping technique. Electrostatic doping of such a large band-gap semiconductor is quite challenging, and a high surface potential is required in order to induce conductivity at the interface. Through magneto-transport measurements performed at low temperature on the doped films, we show that both weak localization and weak anti-localization of charge carriers can be observed and that these quantum interference phenomena can be modulated by the carrier concentration and temperature.