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.

RESEARCH SCHOOL OF ENGINEERING

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.