  The energetic positions of molecular electronic states at molecule/electrode
interfaces are crucial factors for determining the transport and optoelectronic
properties of molecular junctions. Strong light--matter coupling offers a
potential for manipulating these factors, enabling to boost in the efficiency
and versatility of these junctions. Here, we investigate electroluminescence
from single-molecule junctions in which the molecule is strongly coupled with
the vacuum electromagnetic field in a plasmonic nanocavity. We demonstrate an
improvement in the electroluminescence efficiency by employing the strong
light--matter coupling in conjunction with the characteristic feature of
single-molecule junctions to selectively control the formation of the
lowest-energy excited state. The mechanism of efficiency improvement is
discussed based on the energetic position and composition of the formed
polaritonic states. Our findings indicate the possibility to manipulate
optoelectronic conversion in molecular junctions by strong light--matter
coupling and contribute to providing design principles for developing efficient
molecular optoelectronic devices.
