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Exfoliated ∼25μm Si Foil for Solar Cells with Improved Light-Trapping

  • S. Saha (a1), E. U. Onyegam (a1), D. Sarkar (a2), M. M. Hilali (a1), R. A. Rao (a3), L. Mathew (a3), D. Jawarani (a3), D. Xu (a3), R. S. Smith (a3), U. K. Das (a4), J. G. Fossum (a2) and S. K. Banerjee (a1)...

Abstract

Investigation of optical absorption in ∼25μm thick, monocrystalline silicon (Si) substrates obtained from a novel exfoliation technique is done by fabricating solar cells with single heterojunction architecture (without using intrinsic amorphous silicon layer) with diffused back junction and local back contact. The ease of process flow and the rugged and flexible nature of the substrates due to thick metal backing enables use of various light-trapping and optical absorption enhancement schemes traditionally practiced in the industry for thicker (>120μm) substrates. Optical measurement of solar cells using antireflective coating, texturing on both surfaces, and back surface dielectric/metal stack as mirror to reflect the long wavelength light from the back surface show a very low front surface reflectance of 4.6% in the broadband spectrum (300nm-1200nm). The illuminated current voltage (IV) and external quantum efficiency (EQE) measurement of such solar cell shows a high integrated current density of 34.4mA/cm2, which implies significant internal photon reflection. Our best cell with intrinsic amorphous silicon (i-a-Si) layer with only rear surface textured shows an efficiency of 14.9%. EQE data shows improved blue response and current density due to better front surface passivation. Simulations suggest that with optimized light trapping and surface passivation, such thin c-Si cells can reach efficiencies >20%.

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