  We report the fabrication and measurement of thermophotovoltaic (TPV) cells
with efficiencies of >40%, which is a record high TPV efficiency and the first
experimental demonstration of the efficiency of high-bandgap tandem TPV cells.
TPV efficiency was determined by simultaneous measurement of electric power
output and heat dissipation from the device via calorimetry. The TPV cells are
two-junction devices comprising high-quality III-V materials with band gaps
between 1.0 and 1.4 eV that are optimized for high emitter temperatures of
1900-2400{\deg}C. The cells exploit the concept of band-edge spectral filtering
to obtain high efficiency, using high-reflectivity back surface reflectors to
reject unusable sub-bandgap radiation back to the emitter. A 1.4/1.2 eV device
reached a maximum efficiency of (41.1 +/- 1)% operating at a power density of
2.39 W/cm2 under an irradiance of 30.4 W/cm2 and emitter temperature of
2400{\deg}C. A 1.2/1.0 device reached a maximum efficiency of (39.3 +/- 1)%
operating at a power density of 1.8 W/cm2 under an irradiance of 20.1 W/cm2 and
emitter temperature of 2127{\deg}C. These cells can be integrated into a TPV
system for thermal energy grid storage (TEGS) to enable dispatchable renewable
energy. These new TPV cells enable a pathway for TEGS to reach sufficiently
high efficiency and sufficiently low cost to enable full decarbonization of the
grid. Furthermore, the high demonstrated efficiency also gives TPV the
potential to compete with turbine-based heat engines for large-scale power
production with respect to both cost and performance, thereby enabling possible
usage in natural gas or hydrogen-fueled electricity production.
