In situ modulated photoluminescence study of the hydrogenation processes of tunnel oxide passivating contacts during plasma processes

Compared to current c-Si solar cell technologies, fired passivating contacts (FPCs) can reduce the thermal budget of the solar cell fabrication process. In this study, a simplified process flow for the fabrication of these contacts is investigated along with the use of two hole transport materials: p-type nanocrystalline silicon ((p) nc-Si:H) and nanocrystalline silicon oxide ((p) nc-SiO_x:H). More specifically, the passivation properties provided by the FPC stack are assessed at different stages of the manufacturing. The hydrogenation process is studied using in situ modulated photoluminescence (MPL) which allows to measure in real time the effective minority carrier lifetime in the sample during a process step. The increase of passivation observed during the deposition of the capping silicon-nitride (a-SiN_x:H) layer by plasma-enhanced chemical vapor deposition is shown to be due to both annealing of the sample and the deposition in presence of silane in the plasma, while the exposure to an NH₃ and H₂ plasma is mostly detrimental for the passivation properties. The in situ MPL measurements additionally allow us to show that the improvement of the passivation properties happens during the first few minutes of a-SiN_x:H deposition.