  A huge amount of thermal energy is available close to material surfaces in
radiative and non-radiative states, which can be useful for matter
characterization or for energy devices. One way to harness this near-field
energy is to scatter it to the far field. Another way is to bring absorbers
close to thermal emitters, and the advent of a full class of novel photonic
devices exploiting thermal photons in the near field has been predicted in the
last two decades. However, efficient heat-to-electricity conversion of
near-field thermal photons, i.e. the seminal building block, could not be
achieved experimentally until now. Here, by approaching a micron-sized infrared
photovoltaic cell at nanometric distances from a hot surface, we demonstrate
conversion efficiency up to 14% leading to unprecedented electrical power
density output (7500 W.m-2), orders of magnitude larger than all previous
attempts. This proof of principle is achieved by using hot graphite microsphere
emitters (~800 K) and indium antimonide cells, whose low bandgap energy matches
the emitter infrared spectrum and which are specially designed for the near
field. These results pave the way for efficient photoelectric detectors
converting thermal photons directly in the near field. They also highlight that
near-field thermophotovoltaic converters, which harvest radiative thermal
energy in a contactless manner, are now competing with other energy-harvesting
devices, such as thermoelectrics, over a large range of heat source
temperatures.
