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Cu2O is naturally p-type, which has prevented an efficient Cu2O solar cell. N-type doping of Cu2O is demonstrated during electrodeposition of Cu2O by adding a Cl precursor to the aqueous solution. Current-voltage characterization reveals that the resistivity of undoped Cu2O by electrodeposition is ~40 MÂ¿-cm, while that of Cl-doped Cu2O is significantly reduced to as low as ~7Â¿-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 solar cells. In addition, since the doping method substitutes chalcogen with halogen by co-precipitation of halide with chalcogenide, it is in principle universal for n-type doping in other solution-prepared chalcogenides. Several 3rd-generation concepts are enabled by the doping method through solution-prepared chalcogenide nanowires, including radial p-n junctions for enhanced charge separation in organic/inorganic hybrid cells and 3-dimensional p-n junctions for decoupling of photon absorption and charge separation in solar cells.