NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

17.1%-Efficient Multi-ScaleTextured Black Silicon Solar 

Cells without Dielectric 

Antireflection Coating 

Preprint

Fatima Toor, Matthew R. Page, 

Howard M. Branz, and Hao-Chih Yuan

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50707

July 2011
NOTICE

The submitted manuscript has been offered by an employee of the Alliance for Sustainable Energy, LLC 

(Alliance), a contractor of the US Government under Contract No. DE-AC36-08GO28308. Accordingly, the US 

Government and Alliance retain a nonexclusive royalty-free license to publish or reproduce the published form of 

this contribution, or allow others to do so, for US Government purposes.

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, 

express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of 

any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately 

owned rights. Reference herein to any specific commercial product, process, or service by trade name, 

trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, 

or favoring by the United States government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/help/ordermethods.aspx

Cover Photos: (left to right) PIX 16416, PIX 17423, PIX 16560, PIX 17613, PIX 17436, PIX 17721

Printed on paper containing at least 50% wastepaper, including 10% post consumer waste.
1 

17.1%-Efficient Multi-Scale-Textured Black Silicon Solar Cells without Dielectric 

Antireflection Coating

Fatima Toor, Matthew R. Page, Howard M. Branz and Hao-Chih Yuan

National Renewable Energy Laboratory (NREL), 1617 Cole Blvd., Golden, CO 80401, USA

ABSTRACT

In this work we present 17.1%-efficient p-type single 

crystal Si solar cells with a multi-scale-textured surface 

and no dielectric antireflection coating. Multi-scale 

texturing is achieved by a gold-nanoparticle-assisted 

nanoporous etch after conventional micron scale KOHbased pyramid texturing (pyramid black etching). By 

incorporating geometric enhancement of antireflection, this 

multi-scale texturing reduces the nanoporosity depth 

required to make silicon ‘black‘ compared to nanoporous 

planar surfaces. As a result, it improves short-wavelength 

spectral response (blue response), previously one of the 

major limiting factors in ‘black-Si’ solar cells. With multiscale texturing, the spectrum-weighted average 

reflectance from 350- to 1000-nm wavelength is below 2% 

with a 100-nm deep nanoporous layer. In comparison, 

roughly 250-nm deep nanopores are needed to achieve 

similar reflectance on planar surface. Here, we 

characterize surface morphology, reflectivity and solar cell 

performance for the multi-scale textured solar cells.

INTRODUCTION

While dielectric antireflection (AR) coatings remain the 

photovoltaic industry standard, many alternative 

approaches to achieve AR in Si for photovoltaic and other 

optoelectronic applications are being explored by research 

groups [1]. Some of these approaches include anodic 

electrochemical etch [2], metal-assisted etch [3], dry 

plasma etch [4] and laser-induced etching [5]. Our group 

at NREL has previously demonstrated a 16.8%-efficient Si 

solar cell with Au nanoparticle-assisted black Si AR layer 

in place of the conventional quarter-wavelength SiNx AR 

coating [6,7]. The black Si layer is made by a one-step 

liquid etch and would eliminate a vacuum-coating step 

from the Si solar cell production. The physics of 

nanostructured black Si AR is based on the existence of a 

graded index of refraction: if the nanostructure is 

comprised of structures smaller than the wavelength of 

incident light and the Si density is graded across a 

thickness more than about half the wavelength of light, 

reflection is strongly suppressed [8]. The black Si provides 

broader band and wider angle-of-view AR than quarterwavelength interference coatings [9]. However, blue 

response in the black Si cells is poor, due to high 

recombination in the nanoporous surface layer [7]. One of 

the major suspects for high recombination is the high 

surface area of the nanoporous structure. Unfortunately, 

as mentioned previously, a good AR puts stringent 

requirements on both feature size and the depth of the 

nanostructure and inevitably results in high surface area. 

By combining the nanostructured black Si layer with a 

conventional micron-scale KOH-etched pyramidal texture 

as a multi-scale texture, we can reduce the amount of 

surface area required to achieve good AR properties. The 

reason is that micron-scale pyramidal texture enhances 

AR in a purely geometric optics effect; some of the 

reflected light is incident on neighboring pyramids for a 

second chance of absorption. So even though 

nanostructured black Si must still have feature sizes 

smaller than the wavelength of light, when combined with 

pyramid texture its depth can be greatly reduced. Our 

study found pyramid texture with nanostructures only 100-

nm deep provides reflectivity below 2% across a 

wavelength range of 350-1000 nm. In comparison, 

nanostructures roughly 250-nm deep are needed to 

achieve similar reflectance on a planar surface where 

pyramid texture is absent. With the reduced nanostructure 

depth (and surface area) we have achieved a conversion 

efficiency of 17.1% on a multi-scale textured black Si solar 

cell that exhibits improved blue response and higher shortcurrent density (Jsc) compared to our best planar black Si 

cell.

MULTI-SCALE TEXTURED BLACK SI 

SURFACE FORMATION

We first perform conventional alkaline etch and then the 

one-step black Si etch to create multi-scale textured 

surface. A p-type c-Si (100) FZ wafer with resistivity of 2.7 

Ω-cm and thickness of 300 μm is immersed in 10% HF for 

1 min to remove any native oxide. Then, we perform 

pyramidal texturing of both faces in a 600-ml solution of 

2.5% KOH in deionized (DI) water with 200-ml isopropanol 

and bubbling N2 [10] for 25 min at 80o C. The textured 

wafer is then decontaminated from potassium impurities in 

HCl:H2O2:H2O 1:1:5 mixture at 80o C for 10 min [11]. Next, 

a one-step black etch is performed on the front side of the 

substrate (back side protected with paraffin) by immersing 

the pyramidal textured Si wafer in 0.4-mM HAuCl4 solution 

and adding an equal quantity of HF:H2O2:H2O 1:5:2 

mixture in a sonication bath. After the black etch, residual 

Au impurities are removed by immersing the wafer in I2/KI 

solution. Finally, the protective paraffin is removed using 

an organic solvent.
2 

Fig. 1. Cross-sectional SEM images of as-etched 

multi-scale textured Si with (a) 1.5-min and (b) 3-min 

Au-assisted black etch. Magnification on both images 

is identical and the white scale bar represents 500 nm.

Multi-scale textured samples were prepared for various 

black etch times from 1 min to 3 min. Figure 1 shows the 

cross-sectional SEMs of two representative multi-scale 

textured samples with 1.5- and 3-min black etch time. The 

vertical nanopores are along the [100] direction even 

though the exposed pyramid facets are {111}. Thus, the 

pore structure on pyramidal textured black Si is similar to 

black Si etched into the planar (100) surface shown in [7].

SOLAR CELL FABRICATION AND ITS EFFECTS ON 

NANOPOROUS LAYER MORPHOLOGY

We clean the multi-scale textured black Si substrates with 

RCA-1, 10% HF and RCA-2 to remove any organic and 

inorganic contaminants. The substrates are then 

processed with one-step POCl3 and dry thermal oxidation 

to obtain a diffused emitter and surface passivation layer 

in one tube furnace run. The emitter sheet resistance is 75 

Ω/☐ on a planar witness sample. The combined oxide and 

phosphosilicate glass (PSG) thickness measured by an 

ellipsometer on the planar witness wafer is 470 Å. After 

removing the backside oxide layer with dilute HF, Al paste 

is applied on the back followed by 800o C alloying for 5 

min in a tube furnace to form the back-surface field. A 

front grid is then formed by chemically removing the oxide 

and PSG layer through a photolithographically defined 

photoresist pattern, e-beam evaporating metal 

(50/60/1000/100 nm Ti/Pd/Ag/Pd), and metal lift-off. 

Finally, 1x1-cm2 cells are defined by photolithography 

followed by plasma etching to remove the exposed emitter 

and isolate the cells.

Previous studies on planar Si (100) surfaces [7,8] have 

shown that the pore depth of the as-etched nanostructure, 

dE, is directly proportional to black etch time. However 

during the solar cell processing steps such as POCl3

diffusion and oxidation the depth of the nanostructured 

porous layer is reduced such that the processed solar cell 

nanostructure depth, dSC, is about two-thirds of dE. This 

decrease in the nanoporous layer depth is mainly due to 

the consumption of Si during thermal oxidation. The 

remaining height of Si nanostructures (dSC) may vary from 

pore to pore since the starting nanostructure feature size 

varies and oxidation consumes Si from all sides of the 

nanostructure. The result is the variations of pore width 

and depth (dSC) as depicted in the schematic of Figure 2. 

Fig. 2. Schematic depicting the effect of solar cell 

processing on nanoporous Si layer for as-blacketched, dE, (square) and processed solar cell, dSC, 

(circle).
3 

PLANAR AND PYRAMID-TEXTURED BLACK SILICON 

OPTICAL CHARACTERISTICS

The goal of combining nanostructured black Si with 

micron-scale pyramid texture is to utilize geometric optics 

of of pyramids to obtain low reflectivity with a shallow 

nanostructure surface layer. We processed both planar 

and pyramid textured Si with various black etch times to 

compare the reflection and absorption. The reflection and 

transmission of the black etched Si are measured after the 

one-step emitter and passivation formation by a 

spectrophotometer (Varian, Cary 6000i) with an integrating 

sphere. Figure 3 shows the measured results at 400 nm 

and 1000 nm wavelengths. Absorption is calculated by 

subtracting reflection (%) and transmission (%) values 

from 100%.

Fig. 3. (a) Reflection and (b) absorption of processed 

pyramid (solid symbols) and planar (open symbols) Si

at 400 nm and 1000 nm after different black etch (BE) 

times. The 0-min BE planar sample refers to two-sided 

polished Si and the 0-min BE pyramid refers to twosided pyramidal textured Si without any BE.

We calculate spectrum-weighted average reflection (Rave) 

between 350- and 1000-nm wavelengths from the 

measured reflection spectra. The 1.5- and 2-min blacketched planar samples have Rave of 6.9% and 2.7%, 

respectively. In comparison, black-etched pyramid Si 

etched for the same times have very low Rave of 1.8% and 

1.4%, respectively. So it is clear that geometric optics 

effect from pyramid texture enhances AR and enables 

short black etch time. Moreover, the absorption data of 

Figure 3(b) shows that the pyramidal textured black Si has 

higher absorption at the 1000 nm wavelength range due to 

the light-trapping effect of the random pyramids. This 

increased absorption in the near-IR can provide additional

Jsc gain especially in thin film solar cells.

Hence a 1.5-min black etched multi-scale textured Si has 

Rave of 1.8% with an extremely shallow pore depth of 

about 100 nm. In comparison, the pore depth needs to be 

about 250 nm on planar Si to achieve similar AR. 

Therefore multi-scale texture provides low reflection for

high short-circuit current density (Jsc) with a nanoporous 

layer only around 100 nm deep. 

HIGHER JSC AND IMPROVED BLUE RESPONSE OF 

MULTI-SCALE TEXTURED BLACK SI SOLAR CELLS

Our previous 16.8%-efficient black Si solar cell [7] was a 

3-min black etched cell on planar Si and processed using 

separated POCl3 and oxidation steps where PSG was 

removed from the cell surface before thermal oxidation. 

Rave for that cell was ~3% with Jsc of 34.1 mA/cm2

, opencircuit voltage (Voc) of 612 mV and Fill Factor (FF) of 

80.6%. Our best cell with the multi-scale texturing requires 

only a 1.5-min black etch on pyramidal textured Si to 

achieve Rave of 1.8% and it has an independently 

confirmed conversion efficiency of 17.1%, Jsc of 35.6 

mA/cm2

, Voc of 615 mV, and FF of 78.2%. Figure 4 

compares the illuminated AM1.5 current density-voltage 

(J-V) curves of our best multi-textured black Si solar cell 

and of the planar black Si cell. We attribute the 1.5-

mA/cm2 increase of Jsc in the multi-scale textured cell to a 

combination of lower Rave and better blue response, as 

explained below. 
4 

Fig. 4. Illuminated current density-voltage curves for 

1.5-min black etched (BE) pyramidal textured Si (solid) 

and 3-min black etched (BE) planar Si (dashed) solar 

cells under 1-sun condition.

Figure 5 (a) shows improved blue response of the multiscale textured cell in the internal quantum efficiency (IQE). 

Furthermore, the ratio of external quantum efficiency 

(EQE) spectra for 1.5-min black etched pyramid Si to 3-

min black etched planar cell in Figure 5(b) shows that the 

blue EQE between 400-450 nm is enhanced by a factor of 

about 1.5. This improvement in blue response contributes 

to higher Jsc and is most likely related to the shallower 

nanopore depth. Further improvement in the blue 

response will be possible if surface passivation can be 

improved or surface area can be further reduced. Figure 5 

(b) also shows a small improvement of EQE, around 10% 

in the near-IR (1000-1200 nm) for the multi-scale textured 

cell, which can be attributed to the improved light trapping 

of pyramid texture.

In conclusion, we have fabricated a 17.1%-efficient multiscale textured solar cell without any vacuum-deposited 

dielectric AR coating. Instead, the AR is achieved by 

combining the advantages of geometric optics from 

micron-scale pyramid texture and of a density graded 

nanoporous black Si layer. We achieve Rave of 1.8% with a 

nanoporous layer merely 100-nm thick on this multi-scale 

textured cell. Blue response is improved by about 1.5 

times with the reduced nanopore depth and hence lower 

front-surface recombination. Our results suggest that most 

high efficiency nanostructured solar cells will be limited in 

efficiency by high surface recombination due to their high 

surface areas, and will therefore benefit from incorporation 

of light management strategies that minimize the 

nanostructured surface area without eliminating the 

benefits.

Fig. 5. (a) Internal quantum efficiency (IQE) and (b) 

external quantum efficiency (EQE) enhancement 

spectra as a function of wavelength for 1.5 min black 

etched (BE) pyramidal textured Si (solid) and 3 min 

black etched (BE) planar Si (dashed) solar cells.

ACKNOWLEDGEMENT

This work was supported by the U.S. Department of 

Energy (DOE) under Contract No. DE-AC36-08-

GO28308, through a DOE American Recovery and 

Reinvestment Act (ARRA) Photovoltaic Supply Chain and 

Crosscutting Technologies grant. We thank Falah 

Hasoon, Jihun Oh, Anna Duda and Scott Ward for helpful 

discussions, and Keith Emery and Paul Ciszek for currentvoltage measurements.
5 

REFERENCES

[1] G. Koroteenkov and B. K. Cho, “Silicon Porosification: 

State of the Art”, Crit. Rev. in Solid State Mat. Sci. 35, 

2010, pp. 153-260.

[2] D. R. Turner, “Electropolishing silicon in hydrofluoric 

acid solutions”, J. Electrochem. Soc. 105, 1958, pp. 402-

408.

[3] X. Li and P. W. Bohn, “Metal-assisted chemical etching 

in HF/H2O2 produces porous silicon”, Appl. Phys. Lett. 77, 

2000, pp. 2572.

[4] Y-F. Huang et al., “Improved broadband and quasiomnidirectional anti-reflection properties with biomimetic 

silicon nanostructures”, Nature Nanotech. 2, 2007, pp. 

770-774.

[5] M. Shen et al., “Femtosecond laser-induced formation 

of submicrometer spikes on silicon in water”, Appl. Phys. 

Lett. 85, 2004, pp. 5694-5696.

[6] H-C. Yuan et al., “Efficient black silicon solar cell with a 

density-graded nanoporous surface: Optical properties, 

performance limitations, and design rules”, Appl. Phys. 

Lett. 95, 2009, pp. 123501.

[7] H-C. Yuan et al., “Efficient black silicon solar cells with 

nanoporous anti-reflection made in a single-step liquid 

etch”, Thirty-fifth IEEE PVSC, 2009, pp. 141-145. 

[8] H. M. Branz et al., “Nanostructured black silicon and 

the optical reflectance of graded -density surfaces”, Appl. 

Phys. Lett., 94, 2009, pp. 231121. 

[9] H. Sai et al., “"Wide-angle antireflection effect of 

subwavelength structures for solar cells," Jpn. J. Appl. 

Phys. 46 (6A), 2007, pp. 3333-3336.

[10] J. D. Hylton et al., “Uniform Pyramid Formation on 

Alkaline-etched Polished Monocrystalline (100) Silicon 

Wafers”, Prog. Photo. Res. Appl. 4, 1996, pp. 435-438.

[11] 

http://snf.stanford.edu/Equipment/wbgeneral/KOHEtch.ht

ml, last access date 11/05/2011