  As single junction thin-film technologies, both Si heterojunction (HIT) and
Perovskite based solar cells promise high efficiencies at low cost. One expects
that a tandem cell design with these cells connected in series will improve the
efficiency further. Using a self-consistent numerical modeling of optical and
transport characteristics, however, we find that a traditional series connected
tandem design suffers from low Jsc due to band-gap mismatch and current
matching constraints. It requires careful thickness optimization of Perovskite
to achieve any noticeable efficiency gain. Specifically, a traditional tandem
cell with state-of-the-art HIT (24%) and Perovskite (20%) sub-cells provides
only a modest tandem efficiency of ~25%. Instead, we demonstrate that a
bifacial HIT/Perovskite tandem design decouples the optoelectronic constraints
and provides an innovative path for extraordinary efficiencies. In the bifacial
configuration, the same state-of the-art sub-cells achieve a normalized output
of 33%, exceeding the bifacial HIT performance at practical albedo reflections.
Unlike the traditional design, this bifacial design is relatively insensitive
to Perovskite thickness variations, which may translate to simpler manufacture
and higher yield.
