Results of an experimental study of fabrication processes for making phosphorous‐diffused silicon solar cells are presented. Solar cells having sheet resistance of 10 ohm/sq, high long‐wavelength collection efficiency, and efficiencies above 10% were fabricated by diffusion at 975 °C. Diffusions carried out at 875°C for ½ to 1 hr periods resulted in cells having high short‐ and long‐wavelength collection efficiency. Gridded contacts applied to such cells to minimize effects of their comparatively high sheet resistance raised the efficiencies of the cells to values above 10%. Phosphorous‐diffused cells in general have superior radiation resistance as compared to boron‐diffused cells because of the slower rate of degradation of lifetime of minority carriers in p‐type silicon as compared to n‐type silicon under bombardment by atomic particles. The high short‐wavelength collection efficiency of the 875 °C diffused cells results in increased radiation resistance of these cells. Cells having efficiencies above 10% were made from 13‐ohm‐cm material and found to have higher radiation resistance than cells made from 1‐ohm‐cm material. Low junction reverse currents and contact resistances of approximately 0.2 ohm have also been achieved.