Cuprous oxide (Cu2O) is naturally p-type, which has prevented an efficient Cu2O solar cell. n-Type doping of Cu2O is demonstrated during electrochemical deposition by adding a chlorine (Cl) precursor to the aqueous solution. Current-voltage characterization reveals that the resistivity of undoped Cu2O by electrochemical deposition is similar to 40 M Omega cm, while that of electrochemically doped Cu2O with Cl is significantly reduced to as low as similar to 7 Omega cm. X-ray diffraction confirms that the films are pure Cu2O. Photocurrent measurements verify that Cl-doped Cu2O is n-type. The solution-based doping method is particularly suitable for low-cost, large-area, and high-throughput fabrication of Cu2O solar cells.