Photonic Crystal Back Reflectors for Enhanced Absorption in Amorphous Silicon Solar Cells
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Photonic Crystal Back Reflectors for Enhanced Absorption in Amorphous Silicon Solar Cells Benjamin Curtin1, Rana Biswas1,2 and Vikram Dalal1 1 Microelectronics Research Center; Dept. of Electrical and Computer Engineering, Iowa State University, Ames, Iowa 50011, U.S.A. 2 Physics & Astronomy; Ames Lab, Iowa State University, Ames, Iowa 50011, U.S.A. ABSTRACT Photonic crystal back reflectors offer enhanced optical absorption in thin-film solar cells, without undesirable losses. Rigorous simulations of photonic crystal back reflectors predicted maximized light absorption in amorphous silicon solar cells for a pitch of 700-800 nm. Simulations also predict that for typical 250 nm i-layer cells, the periodic photonic crystal back reflector can improve absorption over the ideal randomly roughened back reflector (or the ‘4n2 classical limit’) at wavelengths near the band edge. The PC back reflector provides even higher enhancement than roughened back reflectors for cells with even thinner i-layers. Using these simulated designs, we fabricated metallic photonic crystal back reflectors with different etch depths and i-layer thicknesses. The photonic crystals had a pitch of 760 nm and triangular lattice symmetry. The average light absorption increased with the PC back reflectors, but the greatest improvement (7-8%) in short circuit current was found for thinner i-layers. We have studied the dependence of cell performance on the etch depth of the photonic crystal. The photonic crystal back reflector strongly diffracts light and increases optical path lengths of solar photons. INTRODUCTION Light trapping and advanced photon management are key techniques for efficiently harvesting solar photons and improving solar cell efficiency. Commonly used amorphous silicon and crystalline silicon solar cells suffer from the large absorption length of long-wavelength photons with energies just above the band edge. For hydrogenated amorphous silicon (a-Si:H) the hole diffusion length (ld) is ~300-400 nm, limiting the solar cell absorber layer thickness to less than ld. For a-Si:H midgap cells, blue and green solar photons have absorption lengths (la) less than 250 nm[1,2], and are effectively absorbed in the thin absorber layer. la grows rapidly for λ>600 nm and exceeds 7 µm for photons near the band edge (750-800 nm). Thus red/near-IR photons are very difficult to absorb in thin aSi:H layers. Light-trapping is necessary to harvest these long-wavelength photons. Similarly in cSi solar cells, la>10 µm (for λ>900 nm) and harvesting of near-IR photons (λ=900-1100 nm) above the band edge is very difficult [3,4]. Conventional light trapping schemes utilize textured silver/aluminum-doped zinc oxide (Ag/ZnO:Al) back reflectors that scatter light within the a-Si:H absorber layer and increase the optical path length [5]. Although textured metallic back reflectors are widely used, they suffer intrinsic losses from surface plasmon modes generated at the granular metal-dielectric interface [6,7]. The plasmonic excitations in textured Ag/ZnO
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