Surface passivation of boron diffused junctions by borosilicate glass and in situ grown silicon dioxide interface layer

An in situ oxidation during the boron tribromide (BBr
₃
) diffusion process to form p
⁺
doped junctions on crystalline Si solar cells leads to the formation of stack layer system consisting of borosilicate glass (BSG; a binary B
₂
O
₃
-SiO
₂
system) and a SiO
₂
layer at Si interface. We present a method to passivate the p
⁺
doped regions by using this in situ grown SiO
₂
in combination with a PECVD deposited SiN
_x
layer. We show that the etching rate of the BSG layer, in a HF acid solution, varies over the wafer, depending on its local B
₂
O
₃
content in 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 cells 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 Cz wafers. These passivation results are comparable, on similar structure and boron emitters, with today's state-of-the-art Al
₂
O
₃
based passivation methods. Moreover, we have successfully implemented this passivation method into mass-production of nPERT and pilot-production of IBC solar cells, avoiding the need of adopting additional process steps and costs, which are otherwise needed for p+ boron emitter passivation.