Silicon nitride - silicon oxide stacks for solar cell rear side passivation

Plasma-enhanced chemical vapour deposited (PECVD) amorphous silicon nitride films (SiNx) are well known in the photovoltaic community for their good optical and electrical properties when being deposited as antireflection film on the solar cell's front and their low-temperature deposition process. The development of highefficiency solar cells brings up the need for passivating films for the solar cells' rear side when being contacted only pointwise (e.g. the passivated emitter and rear cell concept (PERC)). Compared to a thermally oxidised solar cell's rear side, silicon nitride already showed good but lower results in passivation and optical (reflection) quality in the past but it offers benefits in deposition costs, process time and heat load for the solar cells. In this work, a new surface passivating stack system consisting of a PECVD-silicon nitride and a PECVD-silicon oxide layer is presented that offers optical and electrical properties very close to those of thermal silicon dioxide without the drawback of heat load and with the benefit of using a quick and cheap process at a low temperature (~350°C). By sintering lifetime evaluation samples with the new stack system, the dependence of the thermal stability of the passivation on the thickness of the silicon oxide is shown. Furthermore, solar cells featuring this new stack system on their rear side are fabricated that show the expected optical and electrical benefits with a maximum solar cell efficiency of 19.3 %.