  We describe strategies to estimate the upper limits of the efficiency of
photon energy harvesting via hot electron extraction from gapless absorbers.
Gapless materials such as noble metals can be used for harvesting the whole
solar spectrum, including visible and near-infrared light. The energy of
photo-generated non-equilibrium or hot charge carriers can be harvested before
they thermalize with the crystal lattice via the process of their internal
photo-emission (IPE) through the rectifying Schottky junction with a
semiconductor. However, the low efficiency and the high cost of noble metals
necessitates the search for cheaper abundant alternative materials, and we show
here that carbon can serve as a promising IPE material candidate. We compare
the upper limits of performance of IPE photon energy-harvesting platforms,
which incorporate either gold or carbon as the photoactive material where hot
electrons are generated. Through a combination of density functional theory,
joint electron density of states calculations, and Schottky diode efficiency
modeling, we show that the material electron band structure imposes a strict
upper limit on the achievable efficiency of the IPE devices. Our calculations
reveal that graphite is a good material candidate for the IPE absorber for
harvesting visible and near-infrared photons. Graphite electron density of
states yields a sizeable population of hot electrons with energies high enough
to be collected across the potential barrier. We also discuss the mechanisms
that prevent the IPE device efficiency from reaching the upper limits imposed
by their material electron band structures. The proposed approach is general
and allows for efficient pre-screening of materials for their potential use in
IPE energy converters and photodetectors within application-specific spectral
windows.
