  The cubic polytype of silicon (Si) is the most commercialized semiconductor
material and finds applications in numerous electronic and optoelectronic
devices such as solar cells. However, recent reports on the synthesis of the
hexagonal 4H Si polytype have attracted the attention of the scientific
community to understand its functional properties. Here we report the
electronic, vibrational, and optical properties of the 4H Si polytype obtained
with predictive first-principles calculations. We find that, compared to the
cubic polytype, 4H Si shows a slightly narrower indirect gap by $\sim$ 0.05 eV.
By calculating its direct and phonon-assisted optical spectra, we show that 4H
Si exhibits a stronger absorption coefficient than cubic Si across the visible
and IR spectral regions. We further evaluate the short-circuit current density
of textured thin-films, and we demonstrate that 4H Si can achieve the same
short-circuit current density for a five times thinner film compared to the
cubic polytype. Our work demonstrates the advantages of 4H Si for thin-film
silicon-based solar-cell applications.
