Influence of parasitic plasma wrap-around during anti-reflective coating deposition on the formation of an aluminium back surface field

In industrial screen-printed aluminium-back-surface-field (Al-BSF) silicon solar cells the standard process for passivating the front surface and optimising its optical properties is the plasma-enhanced chemical vapour deposition (PECVD) of a silicon nitride anti-reflective coating (SiN-ARC). During PECVD of the front side SiNARC in an inline, tray-based reactor, the process plasma can wrap-around to the rear side causing a parasitic SiN deposition. We found, that the thickness of this parasitic SiN layer is in the order of 10 to 30 % of the thickness of the front side SiN-ARC. A corresponding SiN layer is investigated concerning its impact on the saturation current density of the aluminium-doped p+-region formed by two commercially available aluminium pastes. The SiN layer caused a significant decrease in thickness of the p+-region while its saturation current density increased. This implies a loss in open circuit voltage, which is calculated on the basis of a solar cell model and demonstrated experimentally.