  Following the recent success of monolithically integrated Perovskite/Si
tandem solar cells, great interest has been raised in searching for alternative
wide bandgap top-cell materials with prospects of a fully earth-abundant,
stable and efficient tandem solar cell. Thin film chalcogenides (TFCs) such as
the Cu2ZnSnS4 (CZTS) could be suitable top-cell materials. However, TFCs have
the disadvantage that generally at least one high temperature step (>500 C) is
needed during the synthesis, which could contaminate the Si bottom cell. Here,
we systematically investigate the monolithic integration of CZTS on a Si bottom
solar cell. A thermally resilient double-sided Tunnel Oxide Passivated Contact
(TOPCon) structure is used as bottom cell. A thin (<25 nm) TiN layer between
the top and bottom cells, doubles as diffusion barrier and recombination layer.
We show that TiN successfully mitigates in-diffusion of CZTS elements into the
c-Si bulk during the high temperature sulfurization process, and find no
evidence of electrically active deep Si bulk defects in samples protected by
just 10 nm TiN. Post-process minority carrier lifetime in Si exceeded 1.5 ,s.
i.e., a promising implied open-circuit voltage (i-Voc) of 715 mV after the high
temperature sulfurization. Based on these results, we demonstrate a first
proof-of-concept two-terminal CZTS/Si tandem device with an efficiency of 1.1%
and a Voc of 900 mV. A general implication of this study is that the growth of
complex semiconductors on Si using high temperature steps is technically
feasible, and can potentially lead to efficient monolithically integrated
two-terminal tandem solar cells.
