  A near-field thermophotovoltaic (TPV) system with a multilayer emitter of
alternate tungsten and alumina layer is proposed in this paper. The
fluctuational electrodynamics along with the dyadic Green function for a
multilayered structure is applied to calculate the spectral heat flux, and the
charge transport equations are solved to get the photocurrent generation and
electrical power output. The spectral heat flux is much enhanced when plain
tungsten emitter is replaced with multilayer emitter. The mechanism of surface
plasmon polariton coupling in the tungsten thin film, which is responsible for
the heat flux enhancement, is analyzed. In addition, the invalidity of
effective medium theory to predict the optical properties of multilayer
structure in near-field radiation is discussed. The tungsten and alumina layer
thicknesses are optimized to match the spectral heat flux with the bandgap of
TPV cell. Practically, with a gold reflector placed on the back of TPV cell,
which also acts as the back electrode, and a 5-nm-thick indium tin oxide (ITO)
layer as the front contact, when the emitter and receiver temperature are
respectively set as 2000 K and 300 K, the conversion efficiency and electrical
power output can be achieved to 23.7% and 0.31 MW/m2 at a vacuum gap distance
of 100 nm.
