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Procedia Engineering 139 (2016) 147 – 154
MRS Singapore – ICMAT Symposia Proceedings
8th International Conference on Materials for Advanced Technologies
Ultra-low reflective silicon surfaces for photovoltaic applications
J.W.M. Lim a , S.Y. Huang, a* , C.S. Chan a , S.Xu a , D.Y. Wei a , Y.N. Guo a , L. Xu a , K.
Ostrikov b,c
a
Plasma Sources and Applications Center (PSAC), NIE, Nanyang Technological University, 1 Nanyang Walk, 637616, Singapore
Plasma Nanoscience Laboratories, Industrial Innovation, CSIRO Manufacturing Flagship, P.O. Box 218, Lindfield NSW 2070, Australia
c
Institute for Future Environments and School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology,
Brisbane, QLD 4000, Australia
b
Abstract
Silicon based photovoltaic cells still remain a mainstay in the industries due to its relatively low cost for
manufacturing and implementation. A good knowledge base of the material has also been built up over the years and
there is no doubt that silicon based photovoltaic cells would continue to lay the basis for renewable energy for many
years to come. However, it is widely known that conventional silicon photovoltaic cells have relatively lower power
conversion efficiencies as compared to its next generation counterparts. This is partly due to the high optical losses
on surfaces, resulting in poor harvesting of energy from incident light. In this work, an ICP process was developed
to fabricate ultra-low reflective silicon surfaces for photovoltaic applications. An Ar + H2 feedstock was used to
texture nanocones on the surface of silicon wafers, reducing the reflective losses and forming a high quality pn
junction simultaneously. Reflectivity of the samples were characterised with a Zolix SCS10-X150-DSSC UV-Vis
spectrometer with an attached integrating sphere, while the photovoltaic properties were measured with a PV
characterization suite from Sinton instruments. The low reflectivity with promising electronic properties of the
processed materials shows propitious potential for applications in the field of photovoltaics.
©
by by
Elsevier
Ltd.Ltd.
This is an open access article under the CC BY-NC-ND license
© 2016
2015The
TheAuthors.
Authors.Published
Published
Elsevier
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT.
Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT
Keywords: Plasma ; Materials processing; Silicon; Nanocones; Low-reflectivity
* Corresponding author. E-mail address: marklim84th@gmail.com
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT
doi:10.1016/j.proeng.2015.09.218
148
J.W.M. Lim et al. / Procedia Engineering 139 (2016) 147 – 154
1. Introduction
In recent years, there has been much concern over the continual depletion of non-renewable energy reserves such
as coal and oil [1, 2]. With the rapid advancement in technology in industries bringing sophisticated digital products
which consume a large amount of energy to the consumer on a personal level, the global energy crisis is becoming
an increasingly relevant problem that has yet to be solved thoroughly. On the other hand, huge concern has also
arisen while trying to meet the energy demands of consumers in the form of environmental degradation [3-5].
Conventional forms of energy conversion involves burning of fossil fuels which give rise to many associated
problems in damage to the environment and as a result, the health of the population living in areas which are unable
to mitigate the negative toxic byproducts of the conventional combustion of fuels.
Photovoltaics (PV) has received a renewed interest in recent years as a viable source of renewable energy [6]. A
few reasons which can be attributed to the popular reception of PV would be its relatively well developed field of
research which has seen large increments in power conversion efficiencies over the years [7, 8], as well as the small
environmental footprint that it has during the process of energy conversion, harnessing just incident light from an
external source, and converting it into usable energy for the masses without the need for combustion, leaving behind
undesirable byproducts [9-11]. In recent years, there has been a huge paradigm shift in the designs of PV modules,
with interesting novel metamaterials and tandem cells being studied and produced for high-efficiency harvesting of
solar energy [7, 12, 13]. However, industries still prefer to fall back on the conventional silicon based PV modules
for large-scale applications and implementation. This is due to the relatively low cost of silicon which drives down
the costs of fabrication of modules, and the “cost per watt” that would be borne by the industries and eventually, the
consumer. Silicon is also a mature semiconductor material and has a rich database of research in its backlog[14-16].
As such, there is no doubt that silicon based PV modules will continue to remain a mainstay in both the market as
well as a key area of research in laboratories globally.
However, silicon based PV modules as compared to its next-generation counterparts, does not perform as well in
terms of power conversion efficiencies (PCE). A few sources which contribute to the decreased PCE in silicon based
PV modules would be optical losses [17]as well as the inability of the PV module to successfully convert the
absorbed light in the material into usable light-generated charged carriers for production of a photocurrent. Bare
silicon has optical losses due to reflection on the surface which amounts to about 35% of incident light [18]. To
reduce optical losses from surface reflection, a common technique involving texturing [19-21] of the surface of
substrates are usually employed to increase the probability for light collection in the absorber material. When photogenerated carriers are produced in the absorber layer, a high-quality pn junction is required to sweep the carriers to
the different ends of the PV cell for collection and production of a photo-current. Conventional methods for
incorporating these functions in PV cells involve fabrication through wet chemical processes which require copious
amounts of reagents for processing, as well as a large amount of accumulated waste from washing and post process
treatment of substrates [22].
In this work, a unique method of processing silicon substrates in a low frequency inductively coupled plasma
(ICP) was developed for improving the optoelectronic properties of silicon substrates for PV applications. A
compound feedstock of Ar and H2 was used in a plasma discharge in an ICP reactor. Polycrystalline p-type wafers
that were subjected to the discharge were found to have dense nanocone structures etched onto the surface. The
nanocones were found to decrease the reflective losses to yield ultra-low values similar to that of black silicon. On
top of that, exposure to the discharge also simultaneously caused a “p-to-n type conductivity conversion” (PNTCC)
which resulted in a formation of a high-quality p-n junction in the material [23, 24], essential for the separation of
photo-generated carriers in a PV cell. The advantages of plasma processing as compared to the conventional wet
chemical processing include unprecedented control of discharge parameters to tailor the optoelectronic properties of
the resulting material to suit their applications, as well as the massive reduction of unwanted toxic byproducts
through treatment and washing in chemical processes. This truly makes the entire approach to utilizing PV modules
J.W.M. Lim et al. / Procedia Engineering 139 (2016) 147 – 154
149
as not only a viable alternative for renewable energy, but at the same time incorporating it as an environmentally
friendly source of energy from fabrication to implementation.
2. Experimental methods
A low frequency ICP reactor as shown in Fig. 1. was used exclusively in this work. The reactor is fully evacuated
to a base pressure of < 10-4 Pa with the aid of a 2 stage rotary and turbo-molecular pump set up. The reactor was
constructed out of stainless steel and feature double walls which enable coolants to flow throughout the sides of the
chamber. 4 portholes line the reactor radially, and enable the insertion of diagnostic probes and visual diagnosis of
the discharge process. Samples are loaded onto a processing stage which is connected to an external high voltage DC
power supply which enables a bias to be applied to the stage. A heating element was also attached to the sample
stage, enabling external heating of the substrates during processing. Feedstock gases are introduced into the chamber
through inlets which line the top of the reactor, and their flow rates are controlled with MKS 1100 mass flow
controllers externally. A RF coil, which enables the provision of 4000 W of power through a 460 kHz RF power
generator and matching network system, sits on top of an alumina lid which seals the reactor with a Viton O-ring.
The RF coil also features coaxial tubes to allow coolants to circulate to prevent over-heating and damage of the coil
and the reactor.
Fig. 1. Schematic of the ICP reactor used in this work.
3.0 cm x 3.0 cm boron-doped p-type polycrystalline silicon wafers were used exclusively in this work. Samples
were first washed with the standard RCA 1 clean prior to being placed in the loading transfer chamber, leading to
transfer onto the processing stage. Feedstock gases of Ar and H2 were introduced through the gas inlets, and the base
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J.W.M. Lim et al. / Procedia Engineering 139 (2016) 147 – 154
pressure of the reactor was varied manually through altering the width of the pump out valve. The discharge
conditions were kept constant throughout this work with the RF power supplied at 2000 W, temperature of 500 ºC,
process time of 30 minutes, and the base pressure of the discharge at 1.7 Pa. Only the ratio of Ar : H2 in the
feedstock recipe was varied for studies on how they affect the resulting optoelectronic properties of the processed
samples.
After processing, the surface morphology of samples were characterized with a JEOL JSM-6700F field emission
scanning electron microscope. This revealed details with regards to the type of nanostructures obtained, the
corresponding aspect ratios, and the density of the nanostructures through planar, tilted and cross-sectional analysis.
The reflectivity of the substrates were derived with a Zolix QE-C2 integrating sphere attached to a UV-VIS
spectrometer which was selected to analyze the wavelengths of light corresponding to that from the solar spectrum
and within the range which is above the band gap of silicon (400 nm – 1100 nm). The PV properties of the samples
were measured with a Sinton instruments WCT-120 and a Suns-Voc add-on stage. This enabled the measurement of
the open-circuit voltage (Voc) of the samples without requirement of deposition of external electrodes. Finally, the
fill factor (FF) of the samples were obtained with a Keithley 4200-SCS semiconductor characterization system with
the Keithley interactive test environment for I-V characteristics. A Peccell PEC-L01 portable solar simulator was
used to simulate the conditions of an AM 1.5 spectrum.
3. Experimental results and discussion
Fig. 2. shows the SEM micrographs of the resulting silicon surfaces after being exposed to the Ar + H2 ICP
discharge at different Ar : H2 ratios. The figure shows the arrangement of nanostructures in increasing order their Ar
: H2 ratios. Table 1.1 summarizes the results from through SEM analysis. It is found that as the Ar : H2 ratio was
kept low, the size of the nanocones produced were coarse and less sharp. The densities of the nanostructure
distribution were relatively low initially, but increased gradually with Ar : H2 ratio. Similarly, the aspect ratios
(height: width) increased with Ar : H2 ratio.
Fig. 2. SEM micrographs of nanostructures produced at the following Ar : H2 ratios (a) 0.10; (b) 0.20; (c) 0.30; (d) 0.40.
J.W.M. Lim et al. / Procedia Engineering 139 (2016) 147 – 154
Table 1. Surface morphology of samples processed at different Ar : H2 ratio.
Physical appearance
Sample number
Ar : H2 ratio
Aspect ratio
(height : width)
Nanostructure density ( x 1010 cm-2)
1
0.10
Sparsely distributed nanostructures.
2
2.5
2
0.20
Dense nanostructures. Not well defined.
3
4.0
3
0.30
Dense nanocones with moderate heights
4
3.0
4
0.40
Well-defined sharp tipped nanocones
6
2.5
The resulting surface morphology as a result of Ar + H2 plasma processing is well understood by the different
roles and mechanisms that the radicals in the discharge play during fabrication [21, 23-26]. There are 3 main species
in the reactor which dynamically influence the resulting morphology of the samples. In the discharge, silicon may be
found after being removed from the surfaces of the samples through etching by the reactive species and high impact
sputtering. The silicon may go into the plasma phase as part of the discharge and may be re-deposited back on the
substrate again, forming new silicon structures. However, it is also important to note that the re-attachment of silicon
atoms is only possible on unpassivated surfaces (where dangling bonds are present). During plasma treatment, Ar
plays a major role of sputtering on the surface, thereby impacting the substrate at high energies resulting in removal
of material, whereas hydrogen plays the role in terminating the dangling bonds, and passivating the surfaces. This is
in agreement with experimental results. While the removal of material from the surface is due to the presence of Ar
species in the discharge, it is observed that with an increase in Ar in the feedstock ratio, the nanocones are moulded
to have high aspect ratios with sharp tips due to the rapid sputtering rate. As the concentration of Ar in the feedstock
recipe increases, the H2 concentration decreases consequently. This results in the decreased hydrogen species present
in the bulk which would be able to terminate the dangling bonds on the material. This in agreement with experiments
through the observations made when increasing the Ar : H2 ratio further (0.50), where the nanocones are no longer
seen on the surface. This can be attributed to the increased unpassivated sites on the surface of the substrate which
enable the re-deposition of sputtered silicon back onto the substrate, hence causing a less defined nanostructure
array. The densities of the nanostructures were also observed to decrease as a result of having more well defined
nanocones with high aspect ratios, since more material would be removed, giving rise to sharper nanocones with
larger bases as compared to the less defined nanostructures which are densely distributed throughout the surface.
Fig. 3. Reflectance spectra of the samples processed under different conditions in comparison with an untreated Si wafer.
The purpose of the nanostructures on the surface of the material was to reduce surface reflection. The reflectivity
of the samples and an untreated silicon wafer are shown in Fig. 3. It is shown that as the concentration of Ar in the
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feedstock increases, the aspect ratio increases accordingly, and it is the increase in aspect ratio which has a large
influence over the reflectivity of the samples as compared to the density of the nanostructures on the surface. The
results show that ultra-low reflective losses were conceivable with careful manipulation of the Ar : H2 ratio to be at
0.40 in order to obtain reflective losses of less than 2 % (black silicon) [19, 22, 27].
Table 2 shows the photovoltaic responses and reflectivities of the samples processed at different Ar : H2 ratios.
The results concur very well with previous work done on the origins of the conductivity conversion of p-type wafers
when exposed to a plasma discharge containing hydrogen species. It is understood that the conductivity conversion
occurs in a 2 step process. Firstly, impurity dopant atoms (boron in the case of this experiment utilizing p-type Si
wafers) diffuse to the surface of the samples when initially being exposed to the plasma discharge from heating
processes through ion, electron and radical related heating mechanisms as well as the sputtering with heavier Ar ions
impacting the surface. The boron impurities are then removed from the surface of the samples through interaction
with atomic hydrogen during the formation of volatile boron hydrides [28]. At the same time, weakly bonded
interstitial hydrogen atoms as well as oxygen related thermal donors play a more significant role in determining the
overall conductivity behaviour of the resulting materials, since the concentration of acceptor species have decreased
significantly. This results in the observed PNTCC seen in the samples.
Table 2. Photovoltaic response and reflectivity of samples processed at different Ar : H2 ratio
Reflectivity from 400 nm – 1100 nm (%)
FF (%)
Sample number
Ar : H2 ratio
Voc (mV)
Aspect ratio (height : width)
Bare wafer
-
35
-
-
-
1
0.10
2.5
72
525
2
2
0.20
2.0
74
528
3
3
0.30
1.8
75
530
4
4
0.40
< 1.8
71
524
6
As can be seen in the Table 2, the resulting PV response in terms of Voc and FF increase generally with increasing
aspect ratio. This can be attributed to the increased amount of available sites on the surface realized by the increased
surface area to volume ratio, for the removal of substitutional acceptor (boron) atoms as they diffuse to the surface
and get removed by impinging hydrogen atoms as boron hydrides. However, it is also noticed that as the aspect ratio
increases beyond 4, the corresponding photovoltaic properties started to decline. This is due to the decreased
concentration of hydrogen species present in the discharge recipe to obtain the increased aspect ratio. Hydrogen
plays a vital role in determining the resulting conductive character of the sample. A decrease in hydrogen species
results in reduced passivation of dangling bonds found on the surface, leading to trap states which reduces the
minority carrier lifetimes, thereby influencing the resulting PV properties. The decrease in hydrogen also decreases
the rate of removal of boron atoms on the surface, and the formation of hydrogen related shallow thermal donors
[29]. This is illustrated in sample 4 where there is a relatively lower concentration of hydrogen present in the
discharge resulting in a decrease in PV response which continued to degrade further as the Ar : H2 ratio was
increased further. It is therefore reasonable to conclude that an Ar : H2 ratio of 0.30 would be the most practical for
PV applications since it returns the best FF and Voc while retaining the ultra-low reflectivity properties which is only
marginally higher than that processed with a ratio of 0.40 which compromises the resulting FF and Voc.
4. Conclusion
This work demonstrates the ability of an ICP process to simultaneously produce an array of nanostructures on
silicon in order to reduce reflective losses as well as to form a pn junction which are vital requirements for high
performance PV cells. The effect of the ratio of the discharge feedstock was investigated with respect to how it
affected the resulting surface morphology which influences both the reflectivity as well as the PV response of the
samples. It was found that Ar and H2 both play important and competing roles in the dynamic process resulting in
J.W.M. Lim et al. / Procedia Engineering 139 (2016) 147 – 154
the optoelectronic properties obtained. An optimum processing condition which balances the reflective losses with
photovoltaic response was realised. Ultra-low reflective surfaces of 1.8 % were obtained with relatively high FF and
Voc. This paves way for exciting developments which could further reduce reflective losses through implementation
of anti-reflection coatings [30], as well as to improve the FF and Voc by introducing novel structures which could
increase the minority carrier lifetimes of the samples [31-34]. The processes are also highly controllable through
variation of process parameters and scalable for large scale implementation in industrial fabrication lines. Most
importantly, the entire approach towards fabrication is environmentally friendly which is in tandem with the ideals
of a green renewable source of energy.
Acknowledgements
The authors thank S.Q Xiao, H.P Zhou and C.Y Lim for fruitful collaborations, discussions, and critical
comments. This work was partially supported by Academic Research Fund Tier 1 (RP 6/13 XS), A*STAR and
National Research Foundation of Singapore.
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