Materials Screening Approach to Thermochemically Stable Thin Film Optical Emitters for Thermophotovoltaics

Abstract

Thermophotovoltaics (TPVs) have the potential to exhibit higher power conversion efficiencies than traditional photovoltaics (PVs), with a broad range of applicability from waste recovery systems to aerospace solutions. They operate by preferentially radiating above bandgap photons via a high temperature optical emitter, whose spectrum is tuned through choice of materials and geometry. For TPV to be practically implemented, the emitters must be designed with a simple optical structure while remaining thermally stable. Here, we demonstrate coating/substrate bilayer thin films as a solution to these design criteria. With the optical data of 53 high melting point materials, we simulate the bilayer emissivity as a function of coating thickness for each thermochemically stable emitter operating at 1,800 °C. Emitter-cell systems are characterized by the cell power density and TPV conversion efficiency, constituting a TPV performance metric space. For a given bilayer and bandgap these coating thickness parameterized figures of merit form a performance metric curve, with the best points defining a tradeoff zone. We screen the resulting performance metric curves, identifying trends based on 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 systems with varying bandgap, we find unique emitter choices per PV cell for achieving the highest potential efficiency. Our materials screening approach uses physical data to identify improvements to emitters for experimental TPV designs.