Nano-photonic and nano-plasmonic enhancements in thin film silicon solar cells

Abstract

We design, simulate and fabricate enhanced thin film silicon solar cells using periodically textured photonic crystal substrates. We utilize a thin film absorber layer consisting of a superlattice of alternating hydrogenated amorphous silicon (a-Si:H) and nano-crystalline silicon (nc-Si). Rigorous vectorial simulations optimized the periodically patterned solar cells by solving Maxwell׳s equations in Fourier space. Simulations found optimized architectures for a triangular lattice of metallic nano-cones as a back-reflector, and a conformal solar cell geometry. The periodically patterned photonic crystal based substrates achieve (1) high diffraction, enhancing the path length of light in thin absorber layers and (2) plasmonic concentration of light intensity. Simulations predict an absorption enhancement of 43% for a 12-period superlattice of 800nm thickness. The optimized pitch of the photonic lattice is near 700nm. Experimentally the periodically patterned substrates were fabricated with nano-imprint lithography, and utilized as a substrate for the superlattice cells. We measured a large photo-current enhancement between the textured photonic crystal based superlattice cell and the flat cell of 21%, together with long-wavelength quantum efficiency enhancements beyond 600nm. This is an approach to achieving thin film solar cells with high currents through advanced light-trapping techniques on novel materials.