Study on boron emitter formation by BBr₃ diffusion for n-type Si solar cell applications

Abstract

Solar cells based on ntype cSi wafers have raised growing interest since they feature clear advantages compared to the standard ptype Si substrates. A promising technology to establish the n-type solar cell’s pn junction is thermal diffusion of boron atoms into the Si surface from a boron tribromide (BBr3) source. Boron emitters are characterized in terms of surface doping density, depth and shape of the doping profile, resulting sheet resistance and its uniformity on a large-area wafer as well as emitter saturation current density. In addition, the emitter is to be utilizable in a solar cell production process aiming at an undiminished carrier lifetime in the Si substrate, a reduction of process duration and good removability of the borosilicate glass layer. By optimizing these emitter characteristics, the efficiency of ntype solar cells can be significantly enhanced. In order to achieve a desired boron emitter, diffusion process parameters such as temperatures, gas flows and periods of individual process steps are systematically varied. Predictions which kinds of emitters will result from various process parameters can be derived, helping to understand the B emitter formation from a BBr3 source.