Assessing the stability of p+ and n+ polysilicon passivating contacts with various capping layers on p-type wafers

Polysilicon (poly-Si)-on-oxide passivating contact structures (POLO/TOPCon) enable high-efficiency solar cells as they simultaneously provide a very high level of surface passivation and a high conductance for either electrons or holes. The ease of incorporation with existing manufacturing lines and their tolerance for high-temperature processing has increased the wide acceptance of this structure in the PV industry. In this report, we explore the effects of short high-temperature annealing required for effective hydrogenation and formation of ohmic screen-printed contacts across a wide temperature range (636 °C–846 °C) on the stability of passivating contact structures. We study this on p-type c-Si substrates with phosphorus-doped (n-type) or boron-doped (p-type) polysilicon contacts capped with either an AlOx or SiNx coating. Our experimental results show that irrespective of the poly-Si doping type, AlOx-capped samples suffer a loss in surface passivation across the investigated temperature range, while SiNx-capped samples show an improvement at lower annealing temperatures. Above 744 °C, severely ruptured blisters occur for the samples coated with a SiNx layer, leading to lift-off of the poly layer in extreme cases, and in all cases, significant surface passivation losses, up to 99%. A study of the long-term stability of these fired samples under 1-sun illumination @ 140 °C shows that they suffer from both bulk and surface-like instabilities. Two degradation cycles were observed: the first, a boron-oxygen light-induced degradation (BO-LID) observed after 5 min, with capture cross-section ratios of 15.8–19.2, and a slower secondary degradation, similar to light and elevated temperature-induced degradation (LeTID), with maximum degradation reached after ∼ 14 days. The presence of a silicon nitride layer does not appear to influence the kinetics of post-degradation recovery. Our results suggest that the effect of firing may be influenced by the polarity of the bulk c-Si or perhaps the chemistry of the SiNx film and highlight that passivating contact structures based on p-type c-Si may offer better long-term stability than those based on n-type c-Si.