Large-Area Nanosphere Gratings for Light Trapping and Reduced Surface Losses in Thin Solar Cells

Light trapping in thin silicon solar cells demands radically different fabrication approaches to standard commercial cells. Weaker optical absorption and increased sensitivity to surface recombination requires light trapping to be achieved over a broader spectral range and, ideally, without texturing the absorber itself. Nano-scale light trapping structures allow the strongest scattering to be tuned to wavelengths, where oblique scattering into the absorber is needed most. Furthermore, applying these structures “externally,” i.e., on a well-passivated planar silicon surface, reduces the surface area and permits optimal electronic conditions to be maintained. Despite these advantages, the challenges of balancing efficiency gain, cost, and lithographic fidelity have prevented the commercial use of nano-scale light trapping schemes. Here, we demonstrate the use of nanosphere lithography for producing high-quality and cost-effective nano-scale light trapping structures suitable for incorporation in thin solar cells. We have successfully fabricated large-area and uniform metal nanospheregrating structures, with embedded dielectric nanospheres, on 30 μm thick c-Si pseudo cells and measured their effectiveness for light trapping. Comparison between simulations and the fabricated pseudo cells’ characteristics highlighted key challenges in fabricating uniform structures, including the impact of air gaps within non-conformal coatings and minor changes in the geometry. Optical characterization via absorption spectroscopy and both spectral and spatially resolved photoluminescence showed a clear enhancement in the short-circuit current density of up to 4.33 mA/cm2 in comparison with a planar 30 μm thick device and a 3.7 times absorptance enhancement close to the bandgap of Si.