Metastability of a-SiOx:H thin films for c-Si surface passivation

The adoption of a-SiOx:H films obtained by PECVD in heterojunction solar cells is a key to further increase efficiency, because of its transparency in the UV with respect to the commonly used a-Si:H. On the other hand the application of amorphous materials like a-Si:H and SiNx on the cell frontside expose them to the mostly energetic part of the sun spectrum, leading to a metastability of their passivation properties. In this work we explore the reliability of a-SiOx:H thin film layers surface passivation on c-Si substrates under UV exposition, in combination with thermal annealing steps. Both p-and n-type doped c-Si substrates have been considered. To understand the effect of UV light soaking we monitored the minority carriers lifetime and Si-H and Si-O bonding, by FTIR spectra, after different exposure times to light coming from a deuterium lamp, filtered to UV-A region, and focused on the sample to obtain a power density of 500 uW/cm2. We found a certain lifetime decrease after UV light soaking in both p- and n-type c-Si passivated wafers. The role of a thermal annealing, which usually enhances the as-deposited SiOx passivation properties, is furthermore considered. In particular we monitored the UV light soaking effect on c-Si wafers after SiOx coating by PECVD and after a thermal annealing treatment at 250°C for 15'. We found that after UV exposure thermal annealing steps can be used as a tool for the c-Si passivation recovery.