  Multi-junction solar cells provide a path to overcome the efficiency limits
of standard silicon solar cells by harvesting more efficiently a broader range
of the solar spectrum. However, Si-based multi-junction architectures are
hindered by incomplete harvesting in the near-infrared (near-IR) spectral
range, as Si sub-cells have weak absorption close to the band gap. Here, we
introduce an integrated near-field/far-field light trapping scheme to enhance
the efficiency of silicon-based multi-junction solar cells in the near-IR
range. To achieve this, we design a nanopatterned diffractive silver
back-reflector featuring a scattering matrix that optimizes trapping of
multiply-scattered light into a range of diffraction angles. We minimize
reflection to the 0th-order and parasitic plasmonic absorption in the silver by
engineering destructive interference in the patterned back contact. Numerical
and experimental assessment of the optimal design on the performance of
single-junction Si TOPCon solar cells highlights an improved external quantum
efficiency (EQE) over a planar back-reflector (+1.52 mA/cm2). Nanopatterned
metagrating back-reflectors are fabricated on GaInP/GaInAsP//Si two-terminal
triple-junction solar cells via Substrate Conformal Imprint Lithography (SCIL)
and characterized optically and electronically, demonstrating a power
conversion efficiency improvement of +0.9%abs over the planar reference.
Overall, our work demonstrates the potential of nanophotonic light trapping for
enhancing the efficiency of silicon-based multi-junction solar cells, paving
the way for more efficient and sustainable solar energy technologies.
