  The monolithic tandem integration of third-generation solar energy materials
on silicon holds great promise for photoelectrochemistry and photovoltaics.
However, this can be challenging when it involves high-temperature reactive
processes, which would risk damaging the Si bottom cell. One such case is the
high-temperature sulfurization/selenization in thin film chalcogenide solar
cells, of which the kesterite Cu2ZnSnS4 (CZTS) is an example. Here, by using
very thin (<10 nm) TiN-based diffusion barriers at the interface, with
different composition and properties, we demonstrate on a device level that the
protection of the Si bottom cell is largely dependent on the barrier layer
engineering. Several monolithic CZTS/Si tandem solar cells with open-circuit
voltages (Voc) up to 1.06 V and efficiencies up to 3.9% are achieved,
indicating a performance comparable to conventional interfacial layers based on
transparent conductive oxides, and pointing to a promising alternative design
in solar energy conversion devices.
