Material Screening for Thermophotovoltaic Optical Emitters

Thermophotovoltaics (TPVs) have the potential to exhibit higher power conversion efficiencies than traditional photovoltaics (PVs) through the use of a selective optical emitter, with a broad range of applicability from waste recovery systems to aerospace solutions. For TPV to be practically implemented, the emitters must be designed with a simple optical structure while remaining thermally stable. Despite this, most efforts to date have focused on nanostructuring, which is challenging to scale up, while also employing a limited selection of refractory materials. Here, we present a material screening paradigm entailing coating/substrate bilayer thin films as a solution to these design criteria. With the optical data of 53 high melting point materials (including oxides, nitrides, carbides, refractory metals, etc.), we calculate the bilayer emissivity as a function of coating thickness for thermochemically stable emitters operating at 1,800 °C. Emitter-cell systems are characterized by the cell power density and TPV conversion efficiency, constituting a universal performance metric space. For a given bilayer and bandgap, these figures of merit are parametrized by coating thickness, forming a performance metric curve, with the best points defining a “trade-off zone.” We screen the resulting performance metric curves, identifying trends based on the optical properties of the system, finding a high degree of tunability through the material selection step. For GaSb cells, > 49% efficiency is achieved using AlN/W, a 5.6% increase over bulk W. By calculating the figures of merit for all bilayers with varying bandgap, we find unique emitter choices per PV cell for achieving the highest potential efficiency. Our material screening approach uses physical insight to identify improvements to emitters for experimental TPV designs and could be expanded to consider features such as light emission directionality and polarizability.