  We propose a concentrated thermionic emission solar cell design, which
demonstrates a high solar-to-electricity energy conversion efficiency larger
than 10\% under 600 sun, by harnessing the exceptional electrical, thermal and
radiative properties of the graphene as a collector electrode. By constructing
an analytical model that explicitly takes into account the non-Richardson
behavior of the thermionic emission current from graphene, space charge effect
in vacuum gap, and the various irreversible energy losses within the
subcomponents, we perform a detailed characterization on the conversion
efficiency limit and electrical power output characteristics of the proposed
system. We systematically model and compare the energy conversion efficiency of
various configurations of graphene-graphene and graphene-diamond and
diamond-diamond thermionic emitter, and show that utilizing diamond films as an
emitter and graphene as a collector offers the highest maximum efficiency, thus
revealing the important role of graphene in achieving high-performance
thermionic emission solar cell. A maximum efficiency of 12.8\% under 800 sun
has been revealed, which is significantly higher than several existing
solid-state solar cell designs, such as the solar-driven thermoelectric and
thermophotovoltaic converters. Our work thus opens up new avenues to advance
the efficiency limit of thermionic solar energy conversion and the development
of next-generation novel-nanomaterial-based solar energy harvesting technology.
