Light trapping in ultrathin 25  μm exfoliated Si solar cells

Abstract

The optical absorption in 25-μm-thick, single-crystal Si foils fabricated using a novel exfoliation technique for solar cells is studied and improved in this work. Various light-trapping and optical absorption enhancement schemes implemented show that it is possible to substantially narrow the gap in optical absorption loss between the 25 μm Si foils and industry-standard 180-μm-thick Si wafer solar cells. An improvement of absorption by 58% in the near-infrared (740-1200 nm) range is observed for the 25 μm monocrystalline Si substrates with the use of antireflective coating and texturing. The back reflectance of the metal foil that provides mechanical support to the ultrathin Si semiconductor-on-metal foils is extracted to be ∼51.5%, based on the reflectance matching with the simulated escape reflectance in the sub-bandgap region. The back reflectance is enhanced to ∼58% by incorporating an intermediate silicon nitride layer on the back between the Si and the metal. The incorporation of Al as an improved metal reflector on top of the silicon nitride at the backside of the solar cell results in a 5.8 times enhancement in optical path length as a consequence of the improved effective back reflectance of ∼95%. A thin Si foil solar cell with an unoptimized amorphous Si/crystalline Si heterojunction with intrinsic-thin-layer design with implementation of such light-trapping schemes shows an efficiency of 13.28% with a short-circuit current density (JSC) of 35.97 mA/cm2, which approaches the JSC of industrial wafer-based Si solar cells.

Light trapping in horizontally aligned silicon microwire solar cells

“…4(g). Based on the measured absorption, the calculated maximum J sc for the F = 0.97 structure without a back-reflector was ≈ 29 mA·cm −2 (external quantum efficiency (EQE) of ≈ 67 %), a value comparable to that obtained for ultra-thin planar silicon wafers [27]. The measured value of 17.4 mA·cm −2 , which includes parasitic absorption of light in the ITO and amorphous layers, then gives a lower bound of 0.6 for the IQE in this prototype device.…”

Very Thin (56 μm) Silicon Heterojunction Solar Cells with an Efficiency of 23.3% and an Open‐Circuit Voltage of 754 mV

“…Experimentally, thin ( w < 100 μm) and very thin ( w < 70 μm) c‐Si cells have been investigated based on various technologies such as sliced/thinned free‐standing wafers, [ 6–11 ] epitaxially grown Si thin layers, [ 12–20 ] exfoliated Si films, [ 19,21 ] Si on insulator (SOI) wafers, [ 22–24 ] and polycrystalline Si thin films. [ 25,26 ] The first very thin c‐Si cell having η > 20% was demonstrated by Wang et al in 1996, [ 6 ] whereby a 21.5% efficient passivated emitter rear locally diffused (PERL) cell was fabricated using float‐zone (FZ) Si wafers ( w = 47 μm) and thinned by chemical etching.…”

“…It is worth noting that the efficiency limit is dependent on light trapping scheme and numerical simulation has predicted that η > 30% is possible for very thin c-Si cells (15 μm) by applying advanced light trapping using photonic crystals. [4,5] Experimentally, thin (w < 100 μm) and very thin (w < 70 μm) c-Si cells have been investigated based on various technologies such as sliced/thinned free-standing wafers, [6][7][8][9][10][11] epitaxially grown Si thin layers, [12][13][14][15][16][17][18][19][20] exfoliated Si films, [19,21] Si on insulator (SOI) wafers, [22][23][24] and polycrystalline Si thin films. [25,26] The first very thin c-Si cell having η > 20% was demonstrated by Wang et al in 1996, [6] whereby a 21.5% efficient passivated emitter rear locally diffused (PERL) cell was fabricated using float-zone (FZ) Si wafers (w ¼ 47 μm) and thinned by chemical etching.…”

Performance Analysis and Simulation of c-Si/SiGe Based Solar Cell

“…Bozzola et al [6] studied the effect of c-Si solar cell thickness variation in the range 10-80 µm to produce efficiency 15-19%. M. Hilali et al fabricated a 25 µm c-Si solar cell using exfoliation technique to improve the optical absorption [7]. A 30 µm thick c-Si solar cell is theoretically optimized using a double back reflector layer to enhance the efficiency about 19.28% [8].…”