  Highly efficient, environmental friendly and renewable sources of energy are
of great need today to combat with increasing energy demands and environmental
pollution. In this work, we have investigated the novel 2D allotropes i.e.,
$\beta$-Te$_2$X (X = S, Se) using first-principles calculations and study their
potential applications in light harvesting devices. Both the monolayers possess
to have the high stability and semiconducting nature with an indirect band gap.
The high carrier mobilities and excellent optical absorption of these
monolayers make them potential candidates for solar conversion applications. We
have proposed the type-II heterojunction solar cells and calculated their power
conversion efficiencies (PCEs). The small conduction band offset and
appropriate band gap of donor material in case of
$\beta$-Te$_2$S(S-Side)/$\alpha$-Te$_2$S(Te-Side) heterojunction results in the
PCE of ~ 21%. In addition to that, the band alignments of these monolayers
properly engulf the redox potentials of the water. The overpotentials required
to trigger the hydrogen reduction (HER) and water oxidation (OER) half
reactions reveal that HER and OER preferred the acidic and neutral mediums,
respectively. The calculated solar-to-hydrogen (STH) efficiencies of
$\beta$-Te$_2$S ($\beta$-Te$_2$Se) monolayers come out to be ~ 13 % (~12 %),
respectively, which implies their practical applications in water splitting.
Thus, our work provides strong evidence regarding the potential applications of
these materials in the field of light harvesting devices.
