  Two-dimensional (2D) materials van der Waals heterostructures (vdWHs)
provides a revolutionary route towards high-performance solar energy conversion
devices beyond the conventional silicon-based pn junction solar cells. Despite
tremendous research progress accomplished in recent years, the searches of
vdWHs with exceptional excitonic solar cell conversion efficiency and optical
properties remain an open theoretical and experimental quest. Here we show that
the vdWH family composed of MoSi$_2$N$_4$ and WSi$_2$N$_4$ monolayers provides
a compelling material platform for developing high-performance ultrathin
excitonic solar cells and photonics devices. Using first-principle
calculations, we construct and classify 51 types of MoSi$_2$N$_4$ and
WSi$_2$N$_4$-based [(Mo,W)Si$_2$N$_4$] vdWHs composed of various metallic,
semimetallic, semiconducting, insulating and topological 2D materials.
Intriguingly, MoSi$_2$N$_4$/(InSe, WSe$_2$) are identified as Type-II vdWHs
with exceptional excitonic solar cell power conversion efficiency reaching well
over 20%, which are competitive to state-of-art silicon solar cells. The
(Mo,W)Si$_2$N$_4$ vdWH family exhibits strong optical absorption in both the
visible and ultraviolet regimes. Exceedingly large peak ultraviolet absorptions
over 40%, approaching the maximum absorption limit of a free-standing 2D
material, can be achieved in (Mo,W)Si$_2$N$_4$/$\alpha_2$-(Mo,W)Ge$_2$P$_4$
vdWHs. Our findings unravel the enormous potential of (Mo,W)Si$_2$N$_4$ vdWHs
in designing ultimately compact excitonic solar cell device technology.
