  The conversion efficiency of ultra-thin solar cells based on layered
materials has been limited by their open-circuit voltage, which is typically
pinned to a value under 0.6 V. Here we report an open-circuit voltage of 1.02 V
in a 120 nm-thick vertically stacked homojunction fabricated with
substitutionally doped MoS2. This high open-circuit voltage is consistent with
the band alignment in the MoS2 homojunction, which is more favourable than in
widely-used TMDC heterostructures. It is also attributed to the high
performance of the substitutionally doped MoS2, in particular the p-type
material doped with Nb, which is demonstrated by the observation of
electroluminescence from tunnelling graphene/BN/MoS2 structures in spite of the
indirect nature of bulk MoS2. We find that illuminating the TMDC/metal contacts
decreases the measured open-circuit voltage in MoS2 van der Waals homojunctions
because they are photoactive, which points to the need of developing
low-resistance, ohmic contacts to doped MoS2 in order to achieve high
efficiency in practical devices. The high open-circuit voltage demonstrated
here confirms the potential of layered transition-metal dichalcogenides for the
development of highly efficient, ultra-thin solar cells.
