Diffusion of B has been carried out inn‐type solar grade poly‐ and single‐crystalline silicon from a paint on source at 1075 °C for 10 min. From the measurements of the sheet conductivity versus depth, the dopant profiles were evaluated. Within the experimental errors the surface concentration is the same for both poly and single crystal, viz., 1.8×1020atoms cm−3. However, the junction depth is greater by about 850 A˚ in polycrystalline silicon than in single crystal. The shape of the profile observed in polycrystalline silicon is also quite different and this difference is attributed to fast diffusion through grain boundaries. The particular shape of the profile observed in polycrystalline silicon is more suited to solar cell fabrication than the shape of the profile in single crystal.