  A spectral splitting, light trapping dielectric metasurface is designed,
fabricated and integrated into a four-terminal perovskite/silicon hybrid tandem
solar cell to increase the absorption of light close to the bandgap of the
perovskite top cell, and enhance transmission of the near-infrared spectral
band towards the bottom cell. The metagrating is composed of a hexagonal array
of unit cells of 150-nm-tall hydrogenated amorphous silicon trimer
nanostructures with dielectric Mie resonances in the 600-800 nm perovskite
near-gap region, made using substrate-conformal imprint lithography. By
tailoring the metasurface resonant scattering modes and their interference with
the direct reflection paths we minimize specular reflection and obtain high
diffraction efficiency that leads to improved light trapping in the perovskite
top cell. The measured short-circuit current increase in the perovskite top
cell is 0.5 mA/cm2 corresponding to an estimated efficiency gain of 0.26%
(absolute) for the metasurface-integrated 4T perovskite/silicon tandem cell.
Simulations for a further optimized metasurface spectrum splitter geometry
predict a short-circuit current gain in the perovskite top cell of 1.4 mA/cm2
and an efficiency gain for the 4T tandem cell of 0.4% (absolute). The
metagrating approach for simultaneous spectral splitting, light trapping and
reflectance reduction provides a flexible platform that can be applied to many
tandem cell geometries.
