Concepedia

Surface Passivation of Boron-Diffused Junctions by a Borosilicate Glass and In Situ Grown Silicon Dioxide Interface Layer

Valentin D. Mihailetchi, Haifeng Chu, Jan Lossen, Radovan Kopecek

IEEE Journal of Photovoltaics · 2018 · 25 citations · 22 references

Abstract

An in situ oxidation during the boron tribromide diffusion process to form p + -doped junctions on crystalline Si solar cells leads to the formation of a layer stack system consisting of a borosilicate glass (BSG) (a binary B 2 O 3 -SiO 2 system) and a SiO 2 layer at the Si interface. We present a method to passivate the p + -doped regions by using this in situ grown SiO 2 in combination with a plasma-enhanced-chemical-vapor-deposited SiN x layer. We show that the etching rate of the BSG layer, in a hydrofluoric acid solution, varies over the wafer. The etching rate depends on its local B 2 O 3 content in the BSG and is markedly higher than that of the SiO 2 layer. This difference in the etching rates can be used to controllably etch back the BSG layer in order to obtain a thin and uniform passivating oxide layer for solar cell application. Using this oxide/SiN x stack, we obtained implied V oc of 705 mV and J 0e as low as 14 fA/cm 2 on symmetrically diffused boron emitters on n-type Czochralski wafers. These passivation results are comparable, on similar structure and boron emitters, with state-of-the-art Al 2 O 3 -based passivation methods. Moreover, we have successfully implemented this passivation method in the fabrication of n-type passivated emitter and rear totally diffused and interdigitated back contact solar cells, avoiding the need of adopting additional process steps and costs for boron emitter passivation.