Lateral electromigration of Cu+in a Cu2−&agr;S/CdS solar cell induced by light generated current causes variation in composition of copper sulfide leading to increased sheet resistance and power losses of the device. These effects have been studied in short‐circuit mode by fine probe measurements of lateral potential distribution and are clearly evident inI‐Vcharacteristics. The theory of mixed ionic‐electronic transport in copper sulfide explains steady‐state logarithmic potential distribution between grid wires and correctly predicts its kinetics which depend on Cu+diffusion coefficient in Cu2−&agr;S and grid density. The effects of grid defects on cell degradation are also demonstrated.