Colloidal quantum dot solar cells (CQDSCs) are attracting growing attention owing to significant improvements in efficiency. However, even the best depleted-heterojunction CQDSCs currently display open-circuit voltages (V _OCs) at least 0.5 V below the voltage corresponding to the bandgap. We find that the tail of states in the conduction band of the metal oxide layer can limit the achievable device efficiency. By continuously tuning the zinc oxide conduction band position via magnesium doping, we probe this critical loss pathway in ZnO-PbSe CQDSCs and optimize the energetic position of the tail of states, thereby increasing both the V_OC (from 408 mV to 608 mV) and the device efficiency. A fundamental loss mechanism in depleted-heterojunction colloidal quantum dot solar cells (CQDSCs) is identified to arise from the metal oxide conduction band tail. This loss is studied in ZnO-PbSe CQDSCs and minimized by optimizing the ZnO-PbSe conduction band alignment via magnesium-doping of the ZnO. Significant improvements in open-circuit voltage and efficiency are achieved.