Defects across the back‐surface‐field junction (BSF) can seriously degrade the performance of high‐efficiency solar cells. Poor alloying, diffusion pipes, random contact metal penetration, and impurity segregation and clustering can all cause partial or complete electrical short circuits across the BSF junction. Performance degradation from defects distributed in the bulk across the BSF junction is analyzed using the newdeveloped‐perimetermodel. A defective unit cell, containing one defect, is characterized by a three‐region developed‐perimeter device model. The width of the first region surrounding the defect is characterized by the range or the distance‐of‐influence of the defect and this range is shown to be about two diffusion lengths. The defect itself is characterized by three parameters: the defect area, the defect areal density, and the surface recombination velocity at the defective area. This defect model applies to both a few defects and a large number of defects. Family of curves and numerical examples are presented to show that there is very substantial degradation of open‐circuit voltage in thin and high‐efficiency cells, even if there are only a few defects. It is also shown that the degradation is not sensitive to the defect area but highly dependent on the defect density. Two defects of half area will cause almost twice as much open‐circuit‐voltage degradation than one defect of twice the area due to the two‐diffusion‐length defect range or distance‐of‐influence. Due to the strong density dependence, bulk defects, even at low density, could pose a serious limitation to the attainment of theoretical high open‐circuit voltage and efficiency in thin and high‐efficiency back‐surface‐field solar cells.