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

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 B2O3-SiO2system) and a SiO2layer at the Si interface. We present a method to passivate the p+-doped regions by using this in situ grown SiO2in combination with a plasma-enhanced-chemical-vapor-deposited SiNxlayer. 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 B2O3content in the BSG and is markedly higher than that of the SiO2layer. 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/SiNxstack, we obtained implied Vocof 705 mV and J0eas low as 14 fA/cm2on 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 Al2O3-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.