  Multijunction solar cells in a tandem configuration could further lower the
costs of electricity if crystalline Si (c-Si) is used as bottom cell. However,
for direct monolithic integration on c-Si, only a restricted number of top and
bottom cell architectures are compatible, due to either epitaxy or high
temperature constraints, where the interface between subcells is subject to a
trade-off between transmittance, electrical interconnection, and bottom cell
degradation. Using polySi/SiOx passivating contacts for Si, this degradation
can be largely circumvented by tuning the polySi/SiOx stacks to promote
gettering of contaminants admitted into the Si bottom cell during the top cell
synthesis. Applying this concept to the low-cost top cell chalcogenides
Cu2ZnSnS4 (CZTS), CuGaSe2 (CGSe) and AgInGaSe2 (AIGSe), fabricated under harsh
S or Se atmospheres above 550 {\deg}C, we show that increasing the
heavily-doped polySi layer thickness from 40 to up to 400 nm prevents a
reduction in Si carrier lifetime by one order of magnitude, with final
lifetimes above 500 us uniformly across areas up to 20 cm2. In all cases, the
increased resilience was correlated with a 99.9% reduction in contaminant
concentration in the c-Si bulk, provided by the thick polySi layer, which acts
as a buried gettering layer in the tandem structure without compromising the Si
passivation quality. The Si resilience decreased as AIGSe > CGSe > CZTS, in
accordance with the measured Cu contamination profiles and higher annealing
temperatures. An efficiency of up to 7% was achieved for a CZTS/Si tandem,
where the Si bottom cell is no longer the limiting factor.
