Abstract
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.
Highlights
Harvesting solar energy by photon absorption in metal nanostructures and the subsequent collection of photo-generated hot electrons via the processes of internal photo-emission has been actively explored as an alternative approach to traditional photovoltaics (PV), as well as for catalysis and photo-detection [1,2,3,4,5,6]
Once these hot electrons are injected into the semiconductor, the band gap prevents their recombination with holes and preserves their extra energy in excess of the Fermi level
It can be seen that thickness is comparable to or smaller than the hot carrier mean free path in the absorber material, the maximum efficiency of the graphite internal photo-emission (IPE) device at 130 K is almost double of that at 300 K, making it a potentially promising platform for airborne and space applications
Summary
Harvesting solar energy by photon absorption in metal nanostructures and the subsequent collection of photo-generated hot electrons via the processes of internal photo-emission has been actively explored as an alternative approach to traditional photovoltaics (PV), as well as for catalysis and photo-detection [1,2,3,4,5,6]. The efficiency and bandwidth of noble-metal absorptance be increased, it often requires revealed that the number of energy states available for electron transitions in the absorber material precise nano-patterning and/or external optical trapping schemes It has been recently imposes strict limitations on the upper limits of efficiency achievable in internal photo-emission (IPE). In contrast to metals, which have a partially-filled conduction instead of metals for harvesting both visible and near-infrared photons They can form rectifying band, semimetals are characterized by an overlap between the bottom of the conduction band and Schottky junctions with conventional semiconductor materials [24,25,26,27,28], making them promising the top of the valence band. Graphite and graphene as sample material candidates to evaluate and compare using this approach, and demonstrate higher efficiency limits for the solar spectrum harvesting of sunlight potentially achievable with the use of graphite
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