  Transition metal dichalcogenides (TMDs) are layered semiconductors with
indirect band gaps comparable to Si. These compounds can be grown in large
area, while their gap(s) can be tuned by changing their chemical composition or
by applying a gate voltage. The experimental evidence collected so far, points
towards a strong interaction with light, which contrasts with the small
photovoltaic efficiencies $\eta \geq 1$ % extracted from bulk crystals or
exfoliated monolayers. Here, we evaluate the potential of these compounds by
studying the photovoltaic response of electrostatically generated PN-junctions
composed of approximately ten atomic-layers of MoSe$_2$ stacked onto the
dielectric $h$-BN. In addition to ideal diode-like response, we find that these
junctions can yield, under AM-1.5 illumination, photovoltaic efficiencies
$\eta$ exceeding 14 %, with fill-factors of ~ 70 %. Given the available
strategies for increasing $\eta$ such as gap tuning, improving the quality of
the electrical contacts, or the fabrication of tandem cells, our study suggests
a remarkable potential for photovoltaic applications based on TMDs.
