Industrial Rear Sin-Passivated Multicrystalline Silicon Solar Cells

The utilisation of progressively thinner wafers for solar cells leads to an increasing importance of rear surface passivation. In this work we investigate different solar cell processes for industrial rear side passivated solar cells using silicon nitride (SiN) on both sides and screen printed contacts. All solar cells were made by phosphorus emitter diffusion on p-type wafers. For a sufficient rear surface passivation, a parasitic rear emitter must be avoided. Therefore, we carefully compared different processes comprising rear SiN as barrier during phosphorus diffusion as well as parasitic emitter removal by etching after phosphorus diffusion. Furthermore two processes for rear side contacting through a silicon nitride layer were tested. The first process is firing of a screen printed aluminium grid through the backside SiN. The second process uses laser ablation to open a thick rear SiN followed by a full area screen printed rear contact. Additionally, a possible degradation of the SiN due to firing of aluminium contact fingers through the SiN layer was investigated using minority carrier lifetime topography before and after firing of the wafers. Efficiencies of up to 14.7% were obtained on 200 μm thick wafers using SiN layers on both sides. However, these processes are still excelled by the conventional solar cell process using a screen printed full area aluminium rear contact, which lead to an efficiency of 15.3% on a 200 μm thick neighbouring wafer.