Fundamental understanding, impact, and removal of boron-rich layer on n-type silicon solar cells

Most boron diffusion technologies result in the formation of an undesirable boron-rich layer (BRL) on the emitter surface. This paper reports on a study of the impact of gradual etching of the BRL on n-type silicon solar cell performance. It is found that gradual removal of the BRL improves surface passivation and bulk lifetime in the finished cell, while over-etching of the BRL results in a sharp decrease in fill factor due to the increased n-factor and series resistance. It is shown that the optimum chemical etching of the BRL formed as a byproduct of the screen-printed boron emitter diffusion used in this study raised the cell efficiency by ~0.5%, resulting in 20.0% efficient large area (239 cm2) n-type solar cells. The change in BRL thickness and morphology as a function of chemical etching time was investigated by TEM and AES measurements to explain the quantitative impact of BRL removal on cell performance.