Black silicon is an interesting surface texture for solar cells because of its extremely low reflectance on a wide wavelength range and acceptance angle. In this paper we present how black silicon (b-Si) texturization can be applied on the boron doped front surface of an n-type solar cell resulting in an efficiency of 18.7%. We show that the highly boron doped emitter can be formed on black silicon without losing its good optical properties and that atomic layer deposited aluminum oxide provides good surface passivation on these boron doped b-Si emitters.Peer reviewe

Benick, Jan

Hermle, Martin

Repo, Päivikki

Savin, Hele

Schubert, Martin C.

Schön, Jonas

Steinhauser, Bernd

von Gastrow, Guillaume

Vähänissi, Ville

Aaltodoc Publication Archive

This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Repo, Päivikki & Benick, Jan & Vähänissi, Ville & Schön, Jonas & vonGastrow, Guillaume & Steinhauser, Bernd & Schubert, Martin C. &Hermle, Martin & Savin, HeleTitle: N-type Black Silicon Solar CellsYear: 2013Version: Post printPlease cite the original version:Repo, Päivikki & Benick, Jan & Vähänissi, Ville & Schön, Jonas & von Gastrow,Guillaume & Steinhauser, Bernd & Schubert, Martin C. & Hermle, Martin & Savin, Hele.2013. N-type Black Silicon Solar Cells. 3rd International Conference on CrystallineSilicon Photovoltaics. Energy Procedia. Volume 38. 866-871. ISSN 1876-6102 (printed).DOI: 10.1016/j.egypro.2013.07.358.Rights: © 2013 Elsevier BV. This is the post print version of the following article: Repo, Päivikki & Benick, Jan &Vähänissi, Ville & Schön, Jonas & von Gastrow, Guillaume & Steinhauser, Bernd & Schubert, Martin C. &Hermle, Martin & Savin, Hele. 2013. N-type Black Silicon Solar Cells. 3rd International Conference onCrystalline Silicon Photovoltaics. Energy Procedia. Volume 38. 866-871. ISSN 1876-6102 (printed). DOI:10.1016/j.egypro.2013.07.358, which has been published in final form athttp://www.sciencedirect.com/science/article/pii/S1876610213014446.All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)  SiliconPV: March 25-27, 2013, Hamelin, Germany n-type black silicon solar cells Päivikki Repoa,*, Jan Benickb, Ville Vähänissia, Jonas Schönb, Guillaume von Gastrowa, Bernd Steinhauserb, Martin C. Schubertb, Martin Hermleb,  and Hele Savina aAalto University, Tietotie 3, 02150 Espoo, Finland bFraunhofer Institute for Solar Energy Systems, Heidenhofstraße 2, 79110 Freiburg, Germany Abstract Black silicon is an interesting surface texture for solar cells because of its extremely low reflectance on a wide wavelength range and acceptance angle. In this paper we present how black silicon (b-Si) texturization can be applied on the boron doped front surface of an n-type solar cell resulting in an efficiency of 18.7 %. We show that the highly boron doped emitter can be formed on black silicon without losing its good optical properties and that atomic layer deposited aluminum oxide provides good surface passivation on these boron doped b-Si emitters.   © 2013 The Authors. Published by Elsevier Ltd.  Selection and/or peer-review under responsibility of the scientific committee of the SiliconPV 2013 conference  Keywords: black silicon; surface passivation; aluminum oxide; solar cell 1. Introduction Black silicon (b-Si) refers to a nanostructured silicon surface with extremely low surface reflectance. The first application of b-Si was the optimization of etching parameters in a reactive ion etching process (RIE) [1] but later on it became an interesting solar cell surface texture because of its low reflectance on a wide wavelength range and acceptance angle. Besides RIE, other methods for b-Si fabrication, such as laser texturing [2], metal-catalyzed wet chemical etching [3] or plasma immersion ion implantation [4] have been suggested. One issue related to the application of black silicon in an actual solar cell has been the increased surface recombination velocity due to the large surface area of the nanostructure. This causes poor spectral response especially at short-wavelengths [5]. Black silicon has been mostly applied on p-type cells using both thermal oxidation [3] and SiNx [4, 6] for the front surface passivation. However, the high surface recombination velocity has hindered the gain obtainable from the low reflectance and kept the efficiencies of black silicon cells relatively low, the highest efficiency to date being 18.2 % [7]. In recent years Al2O3 has been considered one of the most promising dielectric passivation layers for p- and n-type as well as p+ surfaces [8]. The passivation ability of Al2O3 is related to the combination of low defect density and negative fixed charge at the Si/Al2O3 interface. Emergence of ALD into the photovoltaic community has also offered a solution for the surface recombination issue of black silicon  * Corresponding author. Tel.: +358504361156 E-mail address: paivikki.repo@aalto.fi   and recent results show that atomic layer deposited aluminum oxide (ALD Al2O3) provides good passivation for p-type black silicon surfaces [9-10]. 2. Experimental details 2.1. Solar cell process PERL (passivated emitter with rear locally diffused) solar cells with black silicon on the front surface were fabricated on n-type 5 Ωcm FZ (400 µm) silicon wafers. The solar cell structure and the main process steps are shown in Figure 1. Reference cells with inverted pyramids and an Al2O3/SiNx stack on the front surface were processed alongside with the black silicon cells featuring only Al2O3 at the front. The only difference in the reference cell process was the front surface texturization and the deposition of 65 nm of SiNx with plasma enhanced chemical vapor deposition (PECVD). The process was started with a masking oxidation to define the cell areas of 2×2 cm2. Black silicon was etched on the window areas with a cryogenic deep reactive ion etching process at -120 °C using SF6/O2 plasma. Etching of 7 minutes leads to a random nanostructure shown in Figure 2a with an average pillar height of 1 µm and width of 200 nm.  Boron emitters were formed in a BBr3 tube furnace diffusion at three different temperatures: 890 °C, 910 °C and 930 °C. For the front surface passivation 10 nm of Al2O3 was deposited with plasma assisted atomic layer deposition (PA-ALD) whereas for the rear surface passivation the PassDop [11] process was applied. This includes the deposition of phosphorus doped amorphous SiCx:H with PECVD and the opening of point contacts through the passivation layer with a laser. During the laser process the phosphorus creates a local back surface field under the contact points. The rear surface metallization was done with evaporated aluminum followed by an anneal at 425 °C for 15 minutes to activate the Al2O3 passivation. The front surface contact grid was defined by photolithography and the contacts were formed by evaporated Ti/Pd/Ag thickened with electroplated Ag.   Fig. 1. Schematic of the applied solar cell structure and the main steps of the cell process.      Fig. 2. (a) Scanning electron microscope (SEM) image of a black silicon surface directly after etching. (b) The reflectance of black silicon after emitter formation at 910 °C, boron glass etching and Al2O3 deposition. As a comparison the reflectance from a cell with inverted pyramids and Al2O3/SiNx stack on the front surface (the reflectance of the contacts is omitted). The escape reflection is removed from the curves by assuming the front surface reflectance to be linear for λ > 900 nm and by extrapolating the measured reflectance. The AM1.5G spectrum weighted average reflectances are calculated by taking into account only the surface reflectance.  3. Results and discussion Figure 2b presents an example of b-Si reflectance after emitter diffusion at 910 °C and the following Al2O3 deposition. As a comparison the reflectance from inverted pyramids with an Al2O3/SiNx stack is shown. In the short wavelength range the b-Si reflectance is much lower than in the reference samples whereas in the mid-wavelength range the reference sample exhibits slightly smaller reflectance values. The AM1.5G spectrum weighted average reflectance calculated from the front surface reflectance is however smaller in the black silicon sample even after processing and only with Al2O3 on the front surface. The solar cell parameters measured at one-sun with an aperture area of 4 cm2 as well as the pseudo fill factors are shown in Table 1. All cells, including the reference cell, exhibit a relatively low open-circuit voltage of ~630 mV. The comparison with the IQE reveals that all cells suffer from an insufficient rear side passivation. This is mainly due to the fact that the processing sequence was not adopted to meet the temperature requirements of both, the front side Al2O3 as well as the rear side PassDop layer, i.e. the high temperature applied for the annealing of the Al2O3 degraded the rear side passivation. The front side passivation, nevertheless is quite effective for all cells, as can be seen by the high IQE in the short wavelength range. This is coherent with our previous study on the surface passivation of boron doped b-Si emitters where a J0e value of 51 fA/cm2 was reached indicating an efficient surface passivation also on p+ surfaces on an n-type substrate [12]. The J0e values were extracted from the quasi-steady-state photoconductance measurements (QSSPC, Sinton WCT-120) from symmetrical emitter samples by using the high injection method [13]. Nevertheless, the highest IQE is reached with the reference cells. For the b-Si cells the IQE reduces with increasing diffusion temperature. An adoption of the diffusion process might increase the IQE to the same level as the reference cells. The reference cell with the inverted pyramids shows the highest short-circuit current of 39.9 mA/cm2. The short-circuit current of the black silicon cells is slightly lower and shows a small dependence of the diffusion temperature: the higher the diffusion temperature i.e. the deeper and more highly doped the   emitter, the smaller the short-circuit current density, as also can be seen in the IQE. Omitting the effect of the reflectance from the contacts the black silicon cells exhibit an average reflectance (Rw) of ~2-3 % which is higher than could be expected although still comparable to the average reflectance of the reference cells (see Figure 3). This naturally plays a role in the relatively low Jsc values. However, the problem causing the higher reflectance values has been identified and is easily avoided in future processing. The fill factor of all cells is relatively low. The root cause of this needs to be further investigated. As can be seen in the SEM image in Figure 4 showing a cross section of the b-Si surface under the evaporated metal contact the metal does not reach the extremely small gaps on the b-Si surface which might lead to an increased series resistance. However, also the reference cells suffer from a low FF. This can be related to the used base material with relatively high resistivity but more experiments are needed to have a more thorough analysis. The pseudo fill factor values are also lower than expected indicating that the low FF values cannot be completely explained by a high series resistance. No shunting could be observed based on the IV characteristics. Adoption of the cell processing to the b-Si process will allow much higher efficiencies in the future. In total a cell efficiency of 18.7% could be reached for these very first n-type b-Si cells. This proves that b-Si is a promising approach for the front side texturing. Most importantly we have been able to show that the nanostructured front surface is not destroyed during the multiple processing steps including diffusion, boron glass etching as well as resist application and stripping needed in lithography. Table 1. The solar cell parameters from the three different black silicon cells and the reference cell with inverted pyramids measured at one sun. The pseudo fill factors (PFF) were measured with Suns-Voc. Cell area is 4 cm2.  Surface structure Diffusion T (°C) Jsc (mA/cm2) Voc (mV) PFF (%) FF (%) η (%) Black silicon 890 39.3 628 80.1 75.8 18.7 Black silicon 910 39.2 632 79.8 75.8 18.7 Black silicon 930 38.4 630 80.3 76.1 18.4 Inverted pyramids 910 39.9 631 77.2 75.3 18.9     In order to optimize the emitter diffusion process for a black silicon surface it is important to gain more information about the emitter profile. This has already been and will further be addressed with process simulations whereas measuring the profile might prove to be unfeasible. Optimizing the emitter profile relates to the optimization of the surface passivation and the metallization and especially the metallization should be further addressed. The annealing treatments required by the Al2O3 and the PassDop passivation should also be fitted to the process accordingly.    Fig. 3. The internal quantum efficiencies (IQE) and reflectances (R) of the different black silicon cells and the reference cell with inverted pyramids and an Al2O3/SiNx stack. Spectrum weighted average reflectances (Rw) were calculated by taking into account only the front surface reflection although the escape is still shown in the figure. The reflection from the contacts was also omitted in the Rw calculation.       Fig. 4. A SEM image of the black silicon and front metal contact interface showing how the metal is mostly deposited on the nanostructure and does not reach the valleys of the structure. 4. Conclusions We have shown that black silicon passivated with ALD Al2O3 can be applied on the highly boron doped front surface of an n-type PERL solar cell. The best black silicon cells show an efficiency of 18.7 %. The high IQE in the short wavelength region proves the effective passivation of the boron doped black silicon surfaces with the PA-ALD Al2O3. We were able to show that the nanostructured front surface is not destroyed during the multiple processing steps including diffusion, boron glass etching as well as resist application and stripping needed in lithography. With more process optimization considerably   higher efficiencies can be expected. These optimization steps include the careful consideration of the diffusion process as well as the front surface passivation and contact formation. However, as the first trial in introducing Al2O3 passivated black silicon to an n-type solar cell the results are promising. Acknowledgements The authors thank Sonja Seitz, Felix Schätzle, Karin Zimmermann, Jutta Zielonka, Ulrich Jäger, Nicolas König, Nicole Hennig and Elisabeth Schäffer from Fraunhofer ISE for the support with processing and characterization and acknowledge the provision of facilities and technical support by Aalto University at Micronova Nanofabrication Centre. P. Repo acknowledges the Graduate School of the Faculty of Electronics, Communications and Automation for the financial support.  References [1] Jansen H, de Boer M, Legtenberg R, Elwenspoek M. J. Micromech. Microeng 1995; 5: 115-120. [2] Halbwax M, Sarnet T, Delaporte PH, Sentis M, Etienne H, Torregrosa F, Vervisch V, Perichaud I, Martinuzzi S. Thin Solid Films 2008; 516: 6791-6795. [3] Koynov S, Brandt MS, Stutzmann M. Phys. Stat. Solidi RRL. 2007; 1: R53-R55. [4] Xia Y, Liu B, Liu J, Shen Z, Li C. Sol. Energy 2011; 85: 1574-1578.  [5] Yuan HC, Yost VE, Page MR, Stradins P, Meier DL, Branz HM. Appl. Phys. Lett. 2009; 95: 123501. [6] Dimitrov DZ, Du C-H. Surf. Sci. 2012 ; Accepted for publication. [7] Oh J, Yuan H-C, Branz HM. Nature Nanotechnology 2012; 7: 743-748. [8] Dingemans G, Kessels WMM. Journal of Vacuum Science and Technolog y A 2012; 30: 040802.  [9] Repo P, Haarahiltunen A, Sainiemi L, Yli-Koski M, Talvitie H, Schubert MC, Savin H. Journal of Photovoltaics 2012; 3: 90-94. [10] Otto M, Kroll M, Käsebier T, Salzer R, Tünnermann A, Wehrspohn RB. Appl. Phys. Lett. 2012; 100: 191603. [11] Suwito D, Jäger U, Benick J, Janz S, Hermle M, Glunz SW. IEEE Transactions on Electron Devices 2010; 57: 2032-2036. [12] Repo P, Benick J, Vähänissi V, Schön J, Schubert MC, Hermle M, Savin H. To be submitted to Appl. Phys. Lett. [13] Kane DE, Swanson RM. Proceedings of the 18th IEEE Photovoltaic Specialists Conference 1985, Las Vegas, (IEEE, New York, 1985): 578-583.  

N-type Black Silicon Solar Cells

AbstractBlack silicon is an interesting surface texture for solar cells because of its extremely low reflectance on a wide wavelength range and acceptance angle. In this paper we present how black silicon (b-Si) texturization can be applied on the boron doped front surface of an n-type solar cell resulting in an efficiency of 18.7%. We show that the highly boron doped emitter can be formed on black silicon without losing its good optical properties and that atomic layer deposited aluminum oxide provides good surface passivation on these boron doped b-Si emitters

Elsevier - Publisher Connector 

 Energy Procedia  38 ( 2013 )  866 – 871 
1876-6102 © 2013 The Authors. Published by Elsevier Ltd.
Selection and/or peer-review under responsibility of the scientifi c committee of the SiliconPV 2013 conference
doi: 10.1016/j.egypro.2013.07.358 
SiliconPV: March 25-27, 2013, Hamelin, Germany 
N-type black silicon solar cells 
Päivikki Repoa,*, Jan Benickb, Ville Vähänissia, Jonas Schönb, Guillaume von 
Gastrowa, Bernd Steinhauserb, Martin C. Schubertb, Martin Hermleb,  
and Hele Savina 
aAalto University, Tietotie 3, 02150 Espoo, Finland 
bFraunhofer Institute for Solar Energy Systems, Heidenhofstraße 2, 79110 Freiburg, Germany 
Abstract 
Black silicon is an interesting surface texture for solar cells because of its extremely low reflectance on a wide 
wavelength range and acceptance angle. In this paper we present how black silicon (b-Si) texturization can be applied 
on the boron doped front surface of an n-type solar cell resulting in an efficiency of 18.7 %. We show that the highly 
boron doped emitter can be formed on black silicon without losing its good optical properties and that atomic layer 
deposited aluminum oxide provides good surface passivation on these boron doped b-Si emitters.  
 
© 2013 The Authors. Published by Elsevier Ltd.  
Selection and/or peer-review under responsibility of the scientific committee of the SiliconPV 2013 
conference 
 
Keywords: black silicon; surface passivation; aluminum oxide; solar cell 
1. Introduction 
Black silicon (b-Si) refers to a nanostructured silicon surface with extremely low surface reflectance. 
The first application of b-Si was the optimization of etching parameters in a reactive ion etching process 
(RIE) [1] but later on it became an interesting solar cell surface texture because of its low reflectance on a 
wide wavelength range and acceptance angle. Besides RIE, other methods for b-Si fabrication, such as 
laser texturing [2], metal-catalyzed wet chemical etching [3] or plasma immersion ion implantation [4] 
have been suggested. One issue related to the application of black silicon in an actual solar cell has been 
the increased surface recombination velocity due to the large surface area of the nanostructure. This 
causes poor spectral response especially at short-wavelengths [5]. Black silicon has been mostly applied 
on p-type cells using both thermal oxidation [3] and SiNx [4, 6] for the front surface passivation. 
 
* Corresponding author. Tel.: +358504361156 
E-mail address: paivikki.repo@aalto.fi 
Available online at www.sciencedirect.com
© 2013 The Authors. Published by Elsevier Ltd.
Selection and/or peer-review under responsibility of the scientifi c committee of the SiliconPV 2013 conference
ScienceDirect
Open access under CC BY-NC-ND license. 
Open access under CC BY-NC-ND license. 
 Päivikki Repo et al. /  Energy Procedia  38 ( 2013 )  866 – 871 867
However, the high surface recombination velocity has hindered the gain obtainable from the low 
reflectance and kept the efficiencies of black silicon cells relatively low, the highest efficiency to date 
being 18.2 % [7]. 
In recent years Al2O3 has been considered one of the most promising dielectric passivation layers for 
p- and n-type as well as p+ surfaces [8]. The passivation ability of Al2O3 is related to the combination of 
low defect density and negative fixed charge at the Si/Al2O3 interface. Emergence of ALD into the 
photovoltaic community has also offered a solution for the surface recombination issue of black silicon 
and recent results show that atomic layer deposited aluminum oxide (ALD Al2O3) provides good 
passivation for p-type black silicon surfaces [9-10]. 
2. Experimental details 
2.1. Solar cell process 
PERL (passivated emitter with rear locally diffused) solar cells with black silicon on the front surface 
were fabricated on n-
process steps are shown in Figure 1. Reference cells with inverted pyramids and an Al2O3/SiNx stack on 
the front surface were processed alongside with the black silicon cells featuring only Al2O3 at the front. 
The only difference in the reference cell process was the front surface texturization and the deposition of 
65 nm of SiNx with plasma enhanced chemical vapor deposition (PECVD). The process was started with 
a masking oxidation to define the cell areas of 2×2 cm2. Black silicon was etched on the window areas 
with a cryogenic deep reactive ion etching process at -120 °C using SF6/O2 plasma. Etching of 7 minutes 
leads to a random nanostructure shown in Figure 2a with an average pillar height of 1 μm and width of 
200 nm.  
Boron emitters were formed in a BBr3 tube furnace diffusion at three different temperatures: 890 °C, 
910 °C and 930 °C. For the front surface passivation 10 nm of Al2O3 was deposited with plasma assisted 
atomic layer deposition (PA-ALD) whereas for the rear surface passivation the PassDop [11] process was 
applied. This includes the deposition of phosphorus doped amorphous SiCx:H with PECVD and the 
opening of point contacts through the passivation layer with a laser. During the laser process the 
phosphorus creates a local back surface field under the contact points. The rear surface metallization was 
done with evaporated aluminum followed by an anneal at 425 °C for 15 minutes to activate the Al2O3 
passivation. The front surface contact grid was defined by photolithography and the contacts were formed 
by evaporated Ti/Pd/Ag thickened with electroplated Ag. 
868   Päivikki Repo et al. /  Energy Procedia  38 ( 2013 )  866 – 871 
Fig. 1. Schematic of the applied solar cell structure and the main steps of the cell process.
Fig. 2. (a) Scanning electron microscope (SEM) image of a black silicon surface directly after etching. (b) The reflectance of black
silicon after emitter formation at 910 °C, boron glass etching and Al2O3 deposition. As a comparison the reflectance from a cell with 
inverted pyramids and Al2O3/SiNx stack on the front surface (the reflectance of the contacts is omitted). The escape reflection is
removed from the curves by assuming the front surface reflectance to be linear for > 900 nm and by extrapolating the measured 
reflectance. The AM1.5G spectrum weighted average reflectances are calculated by taking into account only the surface reflectance.
3. Results and discussion
Figure 2b presents an example of b-Si reflectance after emitter diffusion at 910 °C and the following
Al2O3 deposition. As a comparison the reflectance from inverted pyramids with an Al2O3/SiNx stack is 
shown. In the short wavelength range the b-Si reflectance is much lower than in the reference samples 
whereas in the mid-wavelength range the reference sample exhibits slightly smaller reflectance values.
The AM1.5G spectrum weighted average reflectance calculated from the front surface reflectance is 
however smaller in the black silicon sample even after processing and only with Al2O3 on the front 
surface.
 Päivikki Repo et al. /  Energy Procedia  38 ( 2013 )  866 – 871 869
The solar cell parameters measured at one-sun with an aperture area of 4 cm2 as well as the pseudo fill 
factors are shown in Table 1. All cells, including the reference cell, exhibit a relatively low open-circuit 
voltage of ~630 mV. The comparison with the IQE reveals that all cells suffer from an insufficient rear 
side passivation. This is mainly due to the fact that the processing sequence was not adopted to meet the 
temperature requirements of both, the front side Al2O3 as well as the rear side PassDop layer, i.e. the high 
temperature applied for the annealing of the Al2O3 degraded the rear side passivation. The front side 
passivation, nevertheless is quite effective for all cells, as can be seen by the high IQE in the short 
wavelength range. This is coherent with our previous study on the surface passivation of boron doped b-
Si emitters where a J0e value of 51 fA/cm2 was reached indicating an efficient surface passivation also on 
p+ surfaces on an n-type substrate [12]. The J0e values were extracted from the quasi-steady-state 
photoconductance measurements (QSSPC, Sinton WCT-120) from symmetrical emitter samples by using 
the high injection method [13]. Nevertheless, the highest IQE is reached with the reference cells. For the 
b-Si cells the IQE reduces with increasing diffusion temperature. An adoption of the diffusion process 
might increase the IQE to the same level as the reference cells. 
The reference cell with the inverted pyramids shows the highest short-circuit current of 39.9 mA/cm2. 
The short-circuit current of the black silicon cells is slightly lower and shows a small dependence of the 
diffusion temperature: the higher the diffusion temperature i.e. the deeper and more highly doped the 
emitter, the smaller the short-circuit current density, as also can be seen in the IQE. Omitting the effect of 
the reflectance from the contacts the black silicon cells exhibit an average reflectance (Rw) of ~2-3 % 
which is higher than could be expected although still comparable to the average reflectance of the 
reference cells (see Figure 3). This naturally plays a role in the relatively low Jsc values. However, the 
problem causing the higher reflectance values has been identified and is easily avoided in future 
processing. 
The fill factor of all cells is relatively low. The root cause of this needs to be further investigated. As 
can be seen in the SEM image in Figure 4 showing a cross section of the b-Si surface under the 
evaporated metal contact the metal does not reach the extremely small gaps on the b-Si surface which 
might lead to an increased series resistance. However, also the reference cells suffer from a low FF. This 
can be related to the used base material with relatively high resistivity but more experiments are needed to 
have a more thorough analysis. The pseudo fill factor values are also lower than expected indicating that 
the low FF values cannot be completely explained by a high series resistance. No shunting could be 
observed based on the IV characteristics. Adoption of the cell processing to the b-Si process will allow 
much higher efficiencies in the future. 
In total a cell efficiency of 18.7% could be reached for these very first n-type b-Si cells. This proves 
that b-Si is a promising approach for the front side texturing. Most importantly we have been able to show 
that the nanostructured front surface is not destroyed during the multiple processing steps including 
diffusion, boron glass etching as well as resist application and stripping needed in lithography. 
Table 1. The solar cell parameters from the three different black silicon cells and the reference cell with inverted pyramids measured 
at one sun. The pseudo fill factors (PFF) were measured with Suns-Voc. Cell area is 4 cm2.  
Surface structure Diffusion T (°C) Jsc (mA/cm2) Voc (mV) PFF (%) FF (%)  
Black silicon 890 39.3 628 80.1 75.8 18.7 
Black silicon 910 39.2 632 79.8 75.8 18.7 
Black silicon 930 38.4 630 80.3 76.1 18.4 
Inverted pyramids 910 39.9 631 77.2 75.3 18.9 
870   Päivikki Repo et al. /  Energy Procedia  38 ( 2013 )  866 – 871 
In order to optimize the emitter diffusion process for a black silicon surface it is important to gain
more information about the emitter profile. This has already been and will further be addressed with
process simulations whereas measuring the profile might prove to be unfeasible. Optimizing the emitter 
profile relates to the optimization of the surface passivation and the metallization and especially the
metallization should be further addressed. The annealing treatments required by the Al2O3 and the
PassDop passivation should also be fitted to the process accordingly.
Fig. 3. The internal quantum efficiencies (IQE) and reflectances (R) of the different black silicon cells and the reference cell with 
inverted pyramids and an Al2O3/SiNx stack. Spectrum weighted average reflectances (Rw) were calculated by taking into account 
only the front surface reflection although the escape is still shown in the figure. The reflection from the contacts was also omitted
in the Rw calculation.
Fig. 4. A SEM image of the black silicon and front metal contact interface showing how the metal is mostly deposited on the 
nanostructure and does not reach the valleys of the structure.
4. Conclusions
We have shown that black silicon passivated with ALD Al2O3 can be applied on the highly boron
doped front surface of an n-type PERL solar cell. The best black silicon cells show an efficiency of 18.7
 Päivikki Repo et al. /  Energy Procedia  38 ( 2013 )  866 – 871 871
%. The high IQE in the short wavelength region proves the effective passivation of the boron doped black 
silicon surfaces with the PA-ALD Al2O3. We were able to show that the nanostructured front surface is 
not destroyed during the multiple processing steps including diffusion, boron glass etching as well as 
resist application and stripping needed in lithography. With more process optimization considerably 
higher efficiencies can be expected. These optimization steps include the careful consideration of the 
diffusion process as well as the front surface passivation and contact formation. However, as the first trial 
in introducing Al2O3 passivated black silicon to an n-type solar cell the results are promising. 
Acknowledgements 
The authors thank Sonja Seitz, Felix Schätzle, Karin Zimmermann, Jutta Zielonka, Ulrich Jäger, 
Nicolas König, Nicole Hennig and Elisabeth Schäffer from Fraunhofer ISE for the support with 
processing and characterization and acknowledge the provision of facilities and technical support by 
Aalto University at Micronova Nanofabrication Centre. P. Repo acknowledges the Graduate School of the 
Faculty of Electronics, Communications and Automation for the financial support.  
References 
[1] Jansen H, de Boer M, Legtenberg R, Elwenspoek M. J. Micromech. Microeng 1995; 5: 115-120. 
[2] Halbwax M, Sarnet T, Delaporte PH, Sentis M, Etienne H, Torregrosa F, Vervisch V, Perichaud I, Martinuzzi S. Thin Solid 
Films 2008; 516: 6791-6795. 
[3] Koynov S, Brandt MS, Stutzmann M. Phys. Stat. Solidi RRL. 2007; 1: R53-R55. 
[4] Xia Y, Liu B, Liu J, Shen Z, Li C. Sol. Energy 2011; 85: 1574-1578.  
[5] Yuan HC, Yost VE, Page MR, Stradins P, Meier DL, Branz HM. Appl. Phys. Lett. 2009; 95: 123501. 
[6] Dimitrov DZ, Du C-H. Surf. Sci. 2012 ; Accepted for publication. 
[7] Oh J, Yuan H-C, Branz HM. Nature Nanotechnology 2012; 7: 743-748. 
[8] Dingemans G, Kessels WMM. Journal of Vacuum Science and Technolog y A 2012; 30: 040802.  
[9] Repo P, Haarahiltunen A, Sainiemi L, Yli-Koski M, Talvitie H, Schubert MC, Savin H. Journal of Photovoltaics 2012; 3: 
90-94. 
[10] Otto M, Kroll M, Käsebier T, Salzer R, Tünnermann A, Wehrspohn RB. Appl. Phys. Lett. 2012; 100: 191603. 
[11] Suwito D, Jäger U, Benick J, Janz S, Hermle M, Glunz SW. IEEE Transactions on Electron Devices 2010; 57: 2032-2036. 
[12] Repo P, Benick J, Vähänissi V, Schön J, Schubert MC, Hermle M, Savin H. To be submitted to Appl. Phys. Lett. 
[13] Kane DE, Swanson RM. Proceedings of the 18th IEEE Photovoltaic Specialists Conference 1985, Las Vegas, (IEEE, New 
York, 1985): 578-583. 
 


N-type Black Silicon Solar Cells 

Aaltodoc

This is an electronic reprint of the original article.
This reprint may differ from the original in pagination and typographic detail.
Author(s): Repo, Päivikki & Benick, Jan & Vähänissi, Ville & Schön, Jonas & von
Gastrow, Guillaume & Steinhauser, Bernd & Schubert, Martin C. &
Hermle, Martin & Savin, Hele
Title: N-type Black Silicon Solar Cells
Year: 2013
Version: Post print
Please cite the original version:
Repo, Päivikki & Benick, Jan & Vähänissi, Ville & Schön, Jonas & von Gastrow,
Guillaume & Steinhauser, Bernd & Schubert, Martin C. & Hermle, Martin & Savin, Hele.
2013. N-type Black Silicon Solar Cells. 3rd International Conference on Crystalline
Silicon Photovoltaics. Energy Procedia. Volume 38. 866-871. ISSN 1876-6102 (printed).
DOI: 10.1016/j.egypro.2013.07.358.
Rights: © 2013 Elsevier BV. This is the post print version of the following article: Repo, Päivikki & Benick, Jan &
Vähänissi, Ville & Schön, Jonas & von Gastrow, Guillaume & Steinhauser, Bernd & Schubert, Martin C. &
Hermle, Martin & Savin, Hele. 2013. N-type Black Silicon Solar Cells. 3rd International Conference on
Crystalline Silicon Photovoltaics. Energy Procedia. Volume 38. 866-871. ISSN 1876-6102 (printed). DOI:
10.1016/j.egypro.2013.07.358, which has been published in final form at
http://www.sciencedirect.com/science/article/pii/S1876610213014446.
All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, and
duplication or sale of all or part of any of the repository collections is not permitted, except that material may
be duplicated by you for your research use or educational purposes in electronic or print form. You must
obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or
otherwise to anyone who is not an authorised user.
Powered by TCPDF (www.tcpdf.org)
  
SiliconPV: March 25-27, 2013, Hamelin, Germany 
n-type black silicon solar cells 
Päivikki Repoa,*, Jan Benickb, Ville Vähänissia, Jonas Schönb, Guillaume von 
Gastrowa, Bernd Steinhauserb, Martin C. Schubertb, Martin Hermleb,  
and Hele Savina 
aAalto University, Tietotie 3, 02150 Espoo, Finland 
bFraunhofer Institute for Solar Energy Systems, Heidenhofstraße 2, 79110 Freiburg, Germany 
Abstract 
Black silicon is an interesting surface texture for solar cells because of its extremely low reflectance on a wide 
wavelength range and acceptance angle. In this paper we present how black silicon (b-Si) texturization can be applied 
on the boron doped front surface of an n-type solar cell resulting in an efficiency of 18.7 %. We show that the highly 
boron doped emitter can be formed on black silicon without losing its good optical properties and that atomic layer 
deposited aluminum oxide provides good surface passivation on these boron doped b-Si emitters.  
 
© 2013 The Authors. Published by Elsevier Ltd.  
Selection and/or peer-review under responsibility of the scientific committee of the SiliconPV 2013 
conference 
 
Keywords: black silicon; surface passivation; aluminum oxide; solar cell 
1. Introduction 
Black silicon (b-Si) refers to a nanostructured silicon surface with extremely low surface reflectance. 
The first application of b-Si was the optimization of etching parameters in a reactive ion etching process 
(RIE) [1] but later on it became an interesting solar cell surface texture because of its low reflectance on a 
wide wavelength range and acceptance angle. Besides RIE, other methods for b-Si fabrication, such as 
laser texturing [2], metal-catalyzed wet chemical etching [3] or plasma immersion ion implantation [4] 
have been suggested. One issue related to the application of black silicon in an actual solar cell has been 
the increased surface recombination velocity due to the large surface area of the nanostructure. This 
causes poor spectral response especially at short-wavelengths [5]. Black silicon has been mostly applied 
on p-type cells using both thermal oxidation [3] and SiNx [4, 6] for the front surface passivation. 
However, the high surface recombination velocity has hindered the gain obtainable from the low 
reflectance and kept the efficiencies of black silicon cells relatively low, the highest efficiency to date 
being 18.2 % [7]. 
In recent years Al2O3 has been considered one of the most promising dielectric passivation layers for 
p- and n-type as well as p+ surfaces [8]. The passivation ability of Al2O3 is related to the combination of 
low defect density and negative fixed charge at the Si/Al2O3 interface. Emergence of ALD into the 
photovoltaic community has also offered a solution for the surface recombination issue of black silicon 
 
* Corresponding author. Tel.: +358504361156 
E-mail address: paivikki.repo@aalto.fi 
  
and recent results show that atomic layer deposited aluminum oxide (ALD Al2O3) provides good 
passivation for p-type black silicon surfaces [9-10]. 
2. Experimental details 
2.1. Solar cell process 
PERL (passivated emitter with rear locally diffused) solar cells with black silicon on the front surface 
were fabricated on n-type 5 Ωcm FZ (400 µm) silicon wafers. The solar cell structure and the main 
process steps are shown in Figure 1. Reference cells with inverted pyramids and an Al2O3/SiNx stack on 
the front surface were processed alongside with the black silicon cells featuring only Al2O3 at the front. 
The only difference in the reference cell process was the front surface texturization and the deposition of 
65 nm of SiNx with plasma enhanced chemical vapor deposition (PECVD). The process was started with 
a masking oxidation to define the cell areas of 2×2 cm2. Black silicon was etched on the window areas 
with a cryogenic deep reactive ion etching process at -120 °C using SF6/O2 plasma. Etching of 7 minutes 
leads to a random nanostructure shown in Figure 2a with an average pillar height of 1 µm and width of 
200 nm.  
Boron emitters were formed in a BBr3 tube furnace diffusion at three different temperatures: 890 °C, 
910 °C and 930 °C. For the front surface passivation 10 nm of Al2O3 was deposited with plasma assisted 
atomic layer deposition (PA-ALD) whereas for the rear surface passivation the PassDop [11] process was 
applied. This includes the deposition of phosphorus doped amorphous SiCx:H with PECVD and the 
opening of point contacts through the passivation layer with a laser. During the laser process the 
phosphorus creates a local back surface field under the contact points. The rear surface metallization was 
done with evaporated aluminum followed by an anneal at 425 °C for 15 minutes to activate the Al2O3 
passivation. The front surface contact grid was defined by photolithography and the contacts were formed 
by evaporated Ti/Pd/Ag thickened with electroplated Ag. 
 
 
Fig. 1. Schematic of the applied solar cell structure and the main steps of the cell process. 
 
  
 
 
Fig. 2. (a) Scanning electron microscope (SEM) image of a black silicon surface directly after etching. (b) The reflectance of black 
silicon after emitter formation at 910 °C, boron glass etching and Al2O3 deposition. As a comparison the reflectance from a cell with 
inverted pyramids and Al2O3/SiNx stack on the front surface (the reflectance of the contacts is omitted). The escape reflection is 
removed from the curves by assuming the front surface reflectance to be linear for λ > 900 nm and by extrapolating the measured 
reflectance. The AM1.5G spectrum weighted average reflectances are calculated by taking into account only the surface reflectance. 
 
3. Results and discussion 
Figure 2b presents an example of b-Si reflectance after emitter diffusion at 910 °C and the following 
Al2O3 deposition. As a comparison the reflectance from inverted pyramids with an Al2O3/SiNx stack is 
shown. In the short wavelength range the b-Si reflectance is much lower than in the reference samples 
whereas in the mid-wavelength range the reference sample exhibits slightly smaller reflectance values. 
The AM1.5G spectrum weighted average reflectance calculated from the front surface reflectance is 
however smaller in the black silicon sample even after processing and only with Al2O3 on the front 
surface. 
The solar cell parameters measured at one-sun with an aperture area of 4 cm2 as well as the pseudo 
fill factors are shown in Table 1. All cells, including the reference cell, exhibit a relatively low open-
circuit voltage of ~630 mV. The comparison with the IQE reveals that all cells suffer from an insufficient 
rear side passivation. This is mainly due to the fact that the processing sequence was not adopted to meet 
the temperature requirements of both, the front side Al2O3 as well as the rear side PassDop layer, i.e. the 
high temperature applied for the annealing of the Al2O3 degraded the rear side passivation. The front side 
passivation, nevertheless is quite effective for all cells, as can be seen by the high IQE in the short 
wavelength range. This is coherent with our previous study on the surface passivation of boron doped b-
Si emitters where a J0e value of 51 fA/cm2 was reached indicating an efficient surface passivation also on 
p+ surfaces on an n-type substrate [12]. The J0e values were extracted from the quasi-steady-state 
photoconductance measurements (QSSPC, Sinton WCT-120) from symmetrical emitter samples by using 
the high injection method [13]. Nevertheless, the highest IQE is reached with the reference cells. For the 
b-Si cells the IQE reduces with increasing diffusion temperature. An adoption of the diffusion process 
might increase the IQE to the same level as the reference cells. 
The reference cell with the inverted pyramids shows the highest short-circuit current of 39.9 mA/cm2. 
The short-circuit current of the black silicon cells is slightly lower and shows a small dependence of the 
diffusion temperature: the higher the diffusion temperature i.e. the deeper and more highly doped the 
  
emitter, the smaller the short-circuit current density, as also can be seen in the IQE. Omitting the effect of 
the reflectance from the contacts the black silicon cells exhibit an average reflectance (Rw) of ~2-3 % 
which is higher than could be expected although still comparable to the average reflectance of the 
reference cells (see Figure 3). This naturally plays a role in the relatively low Jsc values. However, the 
problem causing the higher reflectance values has been identified and is easily avoided in future 
processing. 
The fill factor of all cells is relatively low. The root cause of this needs to be further investigated. As 
can be seen in the SEM image in Figure 4 showing a cross section of the b-Si surface under the 
evaporated metal contact the metal does not reach the extremely small gaps on the b-Si surface which 
might lead to an increased series resistance. However, also the reference cells suffer from a low FF. This 
can be related to the used base material with relatively high resistivity but more experiments are needed to 
have a more thorough analysis. The pseudo fill factor values are also lower than expected indicating that 
the low FF values cannot be completely explained by a high series resistance. No shunting could be 
observed based on the IV characteristics. Adoption of the cell processing to the b-Si process will allow 
much higher efficiencies in the future. 
In total a cell efficiency of 18.7% could be reached for these very first n-type b-Si cells. This proves 
that b-Si is a promising approach for the front side texturing. Most importantly we have been able to show 
that the nanostructured front surface is not destroyed during the multiple processing steps including 
diffusion, boron glass etching as well as resist application and stripping needed in lithography. 
Table 1. The solar cell parameters from the three different black silicon cells and the reference cell with inverted pyramids measured 
at one sun. The pseudo fill factors (PFF) were measured with Suns-Voc. Cell area is 4 cm
2.  
Surface structure Diffusion T (°C) Jsc (mA/cm
2) Voc (mV) PFF (%) FF (%) η (%) 
Black silicon 890 39.3 628 80.1 75.8 18.7 
Black silicon 910 39.2 632 79.8 75.8 18.7 
Black silicon 930 38.4 630 80.3 76.1 18.4 
Inverted pyramids 910 39.9 631 77.2 75.3 18.9 
  
 
 
In order to optimize the emitter diffusion process for a black silicon surface it is important to gain 
more information about the emitter profile. This has already been and will further be addressed with 
process simulations whereas measuring the profile might prove to be unfeasible. Optimizing the emitter 
profile relates to the optimization of the surface passivation and the metallization and especially the 
metallization should be further addressed. The annealing treatments required by the Al2O3 and the 
PassDop passivation should also be fitted to the process accordingly.  
 
 
Fig. 3. The internal quantum efficiencies (IQE) and 
reflectances (R) of the different black silicon cells and the 
reference cell with inverted pyramids and an Al2O3/SiNx 
stack. Spectrum weighted average reflectances (Rw) were 
calculated by taking into account only the front surface 
reflection although the escape is still shown in the figure. 
The reflection from the contacts was also omitted in the Rw 
calculation. 
 
 
    
Fig. 4. A SEM image of the black silicon and front metal contact interface showing how the metal is mostly deposited on the 
nanostructure and does not reach the valleys of the structure. 
4. Conclusions 
We have shown that black silicon passivated with ALD Al2O3 can be applied on the highly boron 
doped front surface of an n-type PERL solar cell. The best black silicon cells show an efficiency of 18.7 
%. The high IQE in the short wavelength region proves the effective passivation of the boron doped black 
silicon surfaces with the PA-ALD Al2O3. We were able to show that the nanostructured front surface is 
not destroyed during the multiple processing steps including diffusion, boron glass etching as well as 
resist application and stripping needed in lithography. With more process optimization considerably 
  
higher efficiencies can be expected. These optimization steps include the careful consideration of the 
diffusion process as well as the front surface passivation and contact formation. However, as the first trial 
in introducing Al2O3 passivated black silicon to an n-type solar cell the results are promising. 
Acknowledgements 
The authors thank Sonja Seitz, Felix Schätzle, Karin Zimmermann, Jutta Zielonka, Ulrich Jäger, 
Nicolas König, Nicole Hennig and Elisabeth Schäffer from Fraunhofer ISE for the support with 
processing and characterization and acknowledge the provision of facilities and technical support by 
Aalto University at Micronova Nanofabrication Centre. P. Repo acknowledges the Graduate School of the 
Faculty of Electronics, Communications and Automation for the financial support.  
References 
[1] Jansen H, de Boer M, Legtenberg R, Elwenspoek M. J. Micromech. Microeng 1995; 5: 115-120. 
[2] Halbwax M, Sarnet T, Delaporte PH, Sentis M, Etienne H, Torregrosa F, Vervisch V, Perichaud I, Martinuzzi S. Thin Solid 
Films 2008; 516: 6791-6795. 
[3] Koynov S, Brandt MS, Stutzmann M. Phys. Stat. Solidi RRL. 2007; 1: R53-R55. 
[4] Xia Y, Liu B, Liu J, Shen Z, Li C. Sol. Energy 2011; 85: 1574-1578.  
[5] Yuan HC, Yost VE, Page MR, Stradins P, Meier DL, Branz HM. Appl. Phys. Lett. 2009; 95: 123501. 
[6] Dimitrov DZ, Du C-H. Surf. Sci. 2012 ; Accepted for publication. 
[7] Oh J, Yuan H-C, Branz HM. Nature Nanotechnology 2012; 7: 743-748. 
[8] Dingemans G, Kessels WMM. Journal of Vacuum Science and Technolog y A 2012; 30: 040802.  
[9] Repo P, Haarahiltunen A, Sainiemi L, Yli-Koski M, Talvitie H, Schubert MC, Savin H. Journal of Photovoltaics 2012; 3: 
90-94. 
[10] Otto M, Kroll M, Käsebier T, Salzer R, Tünnermann A, Wehrspohn RB. Appl. Phys. Lett. 2012; 100: 191603. 
[11] Suwito D, Jäger U, Benick J, Janz S, Hermle M, Glunz SW. IEEE Transactions on Electron Devices 2010; 57: 2032-2036. 
[12] Repo P, Benick J, Vähänissi V, Schön J, Schubert MC, Hermle M, Savin H. To be submitted to Appl. Phys. Lett. 
[13] Kane DE, Swanson RM. Proceedings of the 18th IEEE Photovoltaic Specialists Conference 1985, Las Vegas, (IEEE, New 
York, 1985): 578-583. 
 


English

file:///data/remote/core/dit/data/elsevier/pdf/dfd/aHR0cDovL2FwaS5lbHNldmllci5jb20vY29udGVudC9hcnRpY2xlL3BpaS9zMTg3NjYxMDIxMzAxNDQ0Ng==.pdf

N-type Black Silicon Solar Cells

Abstract

Similar works

Full text

Available Versions

Aaltodoc Publication Archive

Elsevier - Publisher Connector

Aaltodoc

Elsevier - Publisher Connector