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Single-junction Si solar cell efficiencies are intrinsically limited to 29.4%. One common strategy to overcome this fundamental issue is to combine multiple semiconductor materials with different bandgaps in a multi-junction configuration so that light is effectively absorbed over a broad range in the solar spectrum. In particular, the combination of a III-V top cell (GaInP/GaAs) that is wafer-bonded to a planar Si bottom cell led recently to an overall record efficiency of 34.1%. The efficiency of this tandem design could be further improved if absorption in the Si cell near the bandgap (1000-1200 nm) is enhanced.
Here, we present a nanostructured metallodielectric back reflector placed at the rear of the Si cell that selectively steers incoming light to angles outside the escape cone of the tandem cell. The design is composed of a hexagonal array of Ag nanodisks embedded in a PMMA layer at the rear of the Si cell. Using finite-difference time-domain simulations we optimize pitch, radius, and height of the individual Ag scatterer such that we evenly distribute scattered power over the different diffraction orders. We analyze the scattering behavior in terms of plasmon scattering by the Ag disks and Mie scattering in the dielectric PMMA inclusions. To fully optimize light trapping inside the cell, we choose the geometry such that both 0th-order reflection and plasmonic losses in the Ag nanodisks are minimized.
We experimentally demonstrate photonic light trapping by fabricating large scale (2.5×2.5 cm) nanopatterns on untextured Si solar cells. Large-area patterning is performed via Substrate Conformal Imprint Lithography (SCIL) using silica sol-gel as a mask to etch patterns in PMMA, followed by thermal evaporation of Ag. Cross-section SEM shows excellent conformal deposition of Ag inside the patterned nanoholes. Light scattering spectroscopy shows a clearly reduced reflection of the Si cell in the desired wavelength range (1000-1200 nm) due to light trapping, in agreement with simulations. Experimental data of the full nanopatterned III-V/Si tandem geometry will be shown.
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