Reactive Ion Etched Black Silicon Texturing: A Comparative Study · 2014. 7. 17. · Reactive Ion...
Transcript of Reactive Ion Etched Black Silicon Texturing: A Comparative Study · 2014. 7. 17. · Reactive Ion...
Reactive Ion Etched Black Silicon Texturing: A Comparative Study Thomas Allen1, James Bullock1, Andrés Cuevas1, Simeon Baker-Finch1,2 and Fouad Karouta3
1Research School of Engineering, College of Engineering and Computer Science, Australian National University, Canberra, ACT, 0200,
Australia 2PV Lighthouse, Coledale, NSW, 2515, Australia
3ANFF, Research School of Physics and Engineering, Australian National University, Canberra, ACT, 0200, Australia
Introduction
Reactive Ion Etched (RIE) black silicon (b-Si) offers a means of reducing
the broadband reflectance losses from the silicon-air interface to
near zero by a graded refractive index effect owing to the sub-
wavelength scale of the textured features. This could potentially increase
the photon collection, and therefore efficiency, of c-Si solar cells.
RIE etching offers other advantages over standard texturing techniques:
• Free of toxic/corrosive wet chemicals
• Crystal orientation independent – can be applied to multicrystalline
(mc-Si) wafers, cast-mono wafers, and kerfless technologies that
don’t results in a <100> surface
• Processing is inherently single-sided
• No definition (e.g. photolithography/masks, etc.) is required to form
the features
Motivation: to characterize the performance of b-Si etched surfaces
in terms of electrical (surface recombination) and optical
(reflectance/absorption) properties, and compare with random
pyramids and isotexture.
Methods: mc-Si, CZ and FZ substrates have been studied. Black silicon
texturing has been formed by RIE in an inductively coupled plasma (ICP)
system with SF6/O2 plasma. Isotexture was formed in a cooled
HF:HNO3:CH3COOH solution; random pyramids in a TMAH/IPA solution.
All surfaces were passivated with 20 nm PA-ALD Al2O3.
Results Surface Passivation Results:
• SEM images of RIE b-Si demonstrate the conformality of the PA-
ALD Al2O3 passivation layer.
• RIE b-Si poses no limitation on the minority carrier lifetimes of
bulk-limited Cz and mc-Si samples in mid-to-low injection
levels.
• The b-Si surface recombination can be characterized by a J0s of
20 fA.cm-2, compared to 11 fA.cm-2 for random pyramids and 3
fA.cm-2 for planar.
SEM images (clockwise from top left): b-Si with Al2O3 from above; from
an angle of 52 degrees; random pyramids; isotexture. Note the
difference in scale.
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Optical Results:
• b-Si front surface reflectance is approximately 1% as-etched;
reduces to below 0.4% after Al2O3 deposition; compared to 2.8%
for random pyramids and 6.1% for isotexture, both with an
optimized anti-reflection coating.
• Despite the extremely low reflectance off the front surface, the b-
Si suffers from poor light trapping in the NIR.
Conclusion
• RIE black silicon has been shown to provide very low front surface
reflection (SWR<0.4%), though poor light trapping compared to
both isotexture and random pyramids.
• PA-ALD Al2O3 is shown to passivate the nano-structured black
silicon surface with a measured J0s of 20 fA.cm-2 despite the
enhanced surface area.
• Black silicon texturing has the capacity to improve multicrystalline
silicon solar cell efficiencies.
• NIR absorption needs to be improved to compete with random
pyramids.