A Novel Approach Toward the Simultaneous Diffusion of Boron and Phosphorus in Silicon

Abstract

A novel technique is described for simultaneously diffusing boron and phosphorus on opposite sides of a silicon wafer for the fabrication of simple, high‐efficiency solar cells. The solid doping sources used in this approach are fabricated using standard and liquid sources and can be tailored to produce a wide range of boron and phosphorus diffusion profiles for a fixed thermal diffusion cycle. Uniformity of sheet resistance across a 49 cm2 area is greater than 95% in many cases. Unique to this approach is that the resulting diffusion glass is extremely thin, which allows for the in situ growth of a thin thermal oxide for surface passivation, without appreciably increasing the reflectance of the solar cell after applying the antireflection coating. Measurements of the saturation current density (Jo) by the photoconductance decay technique gave a low Jo of 33.3 fA/cm2 for a phosphorus‐diffused sample (70 Ω/□) with in situ oxide surface passivation, and a Jo of 292 fA/cm2 for a boron‐diffused sample (18.9 Ω/□). Initial solar cell fabrication experiments with simple, untextured n+pp+ structures have produced efficiencies as high as 17%, displaying fill factors of 0.79 and shunt resistances greater than 65,000 Ω‐cm2, demonstrating that no detectable cross‐doping takes place during this new simultaneous boron and phosphorus diffusion technique.