Download OPTICAL AND PASSIVATION PROPERTIES OF DOUBLE LAYER A

22nd European Photovoltaic Solar Energy Conference, 3-7 September 2007, Milan, Italy
OPTICAL AND PASSIVATION PROPERTIES OF DOUBLE LAYER A-SI:H/SINX ANTI-REFLECTIVE
COATINGS FOR SILICON SOLAR CELLS
A. Ulyashin1, D.N. Wright1, A. Bentzen2, A. Suphellen2, E. Marstein1, A. Holt1
1
Section for Renewable Energy, Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller, Norway
Renewable Energy Corp. AS, Veritasveien 14, P.O. Box 280, NO-1323 Høvik
Corresponding author: Alexander Ulyashin, e-mail: [email protected]
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ABSTRACT: The goal of this work is to study main trends regarding the transmittance and passivation properties of
SiNy/a-Si:H ARC structures by variation and adjusting of the a-Si:H layer thickness. Experimental results on the
transmittance for SiNx/a-Si:H/glass, a-Si:H/glass, SiNx/glass and on the effective lifetime measurements for SiNy/aSi:H/p-Si/a-Si:H/SiNx, SiNy/p-Si/SiNx, a-Si:H/p-Si/a-Si:H structures with different a-Si:H thicknesses are presented.
In some cases properties of similar structures in which SiNx layer is substituted by a transparent conductive oxide
(TCO) film (ITO, ZnO:Al) are studied for a comparison also. It is found that the passivation and optical properties of
an a-Si:H individual layer and an a-SiH/SiNx ARC depend essentially on the thickness of a-Si:H layer and can be
modified by heat treatments. It is shown also that passivation properties of an a-Si:H/SiNx stack are better than those
of a-Si:H/TCO double layer in case of magnetron sputtered ITO and ZnO:Al films. By iterative simulations optical
properties of a double stack antireflection coating (ARC) consisting of SiNx(ITO, ZnOx)/a-Si:H layers have been
investigated.
Keywords: silicon, a-Si, passivation, antireflection coating, PECVD, silicon-nitride
1
P-type Cz Si substrates were used in these
investigations. All as-received Si substrates were
subjected to the RCA-1 cleaning before any further
treatments. All individual layers, which the
investigated ARC consists of, were deposited using a
plasma enhanced chemical vapour deposition
(PECVD) set up at temperatures around 200 ºC. TCO
layers (ITO, ZnO:Al) were deposited by magnetron
sputtering at temperatures around 200 °C and below.
The surface passivation properties were investigated
by means of the effective recombination lifetime as
measured by the quasi-steady-state photoconductance
(QssPC) technique, using a generalized analysis to
account for both quasi-steady-state and quasitransient contributions. Optical properties were
investigated by the transmittance (Corning glass
substrates were used in this case) and ellipsometry
measurements.
INTRODUCTION
Processing of silicon based solar cells requires
application of an antireflection coating (ARC), which can
be formed from silicon nitride (SiNx) or transparent
conductive oxide (TCO) thin layers. It is well established
that not only optical but also passivation properties of
any ARC are an important issue for the high efficiency
solar cell processing. It is shown recently that the
passivation properties of an ARC in case of silicon based
solar cells can be improved essentially by including of an
intermediate a-Si:H layer deposited by the plasma
enhanced chemical vapor deposition (PECVD) method
[1-3]. Less attention was paid so far to the investigation
of optical properties of such ARC. Since quality of an
ARC layer is a trade between passivation and optical
properties and both properties are thickness dependent, it
is necessary to find the optimum relationship between
them. In spite of some remarkable recent achievements
for the silicon based solar cell processing, which is
utilizing the double stack a-Si:H/SiNx ARC, the detailed
investigations concerning the influence of the thickness
of an a-Si:H layer on optical and at the same time on
passivation properties of such ARC are not discussed in
literature.
The goal of this work is to find the main trends,
which allow to optimize the transmittance and at the
same time pasivation properties of SiNy/a-Si:H ARC
structures by variation and adjusting of the a-Si:H layer
thickness.
In some cases properties of similar ARC structures in
which SiNx layer is substituted by a transparent
conductive oxide (TCO) film (ITO, ZnO:Al) are studied
for a comparison also.
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2.2 Optical model and simulation method
The optical model used for the optical simulations in
this work is illustrated in Figure 1. The layers in question
are sandwiched between an air ambient and a flat (i.e. no
texture) Si substrate. The light enters the layers from the
ambient at normal incidence. The individual layers are
represented by ñLm and dLm, where m is an index
indicating the layer number, counted from the ambient
towards the substrate. In using this model, R(λ) refers to
light reflected back into the ambient, while the A(λ)
refers to the light absorbed by the layers and the T(λ) is
the light which reaches the substrate. This model is
appropriate for our purpose because we are interested in
the amount of light that reaches the substrate and can be
converted into current in a solar cell.
The Matrix Method [4] was used in calculating the
R( ), A( ) and T( ) spectra. From these spectra the
total values of R, A and T were calculated by weighting
EXPERIMENTAL
2.1 Processing of ARC layers
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22nd European Photovoltaic Solar Energy Conference, 3-7 September 2007, Milan, Italy
against the photon flux of the AM1.5 spectrum in the
wavelength range 300nm to 1150nm.
1,0
Transmittance
0,9
0,8
0,7
a-Si:H ~10 nm
a-Si:H ~8 nm
0,6
0,5
a-Si:H ~5 nm
0,4
0,3
400
600
800
1000
1200
Wavelength, nm
Figure 3: Transmittance of a-Si:H thin layers with
different thicknesses deposited on a Corning glass at 230
°C. Calibration is done using the pure glass substrate.
Figure 1: A sketch of the physical structure used in this
study to implement the Matrix Method for calculating
T( ) , R( ) and A( ). For a single layer, Layer 2 is
omitted.
From Fig.4 it can be seen that the absorption of the aSi:H layer is essentially increased after such treatment.
Thus, this effect has to be taken into account in case of
an implementation of the a-Si:H for the ARC.
The refractive index (n) and extinction coeeficient (k)
dispersion relationships used in the simulations in this
work are displayed in Figure 2.
1,0
Transmittance
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0
a-Si:H (~5 nm) + 450 C 2 min
a-Si:H (~5 nm) as deposited
400
600
800
1000
1200
Wavelength, nm
Figure 2: Optical dispersion relationships for SiNx:H,
ITO, ZnO and a-Si used in this work.
Figure 4: Transmittance of ~5 nm a-Si:H thin layer after
heat treatment at 450 °C for ~2 min.
The optical constants for ZnO were taken from
Gumos et al. [5] while the constants for SiNx:H were
derived from spectroscopic ellipsometry done on a
sample prepared in the PECVD chamber used. The TaucLorentz oscillator model, which has previously been used
with success to characterise the SiNx:H films [6, 7] was
applied. For ITO and a-Si:H layers , the optical constants
were taken from the database of the optical simulation
software VWASE32 from Woollam & co. The origin of
the ITO data was not given, but they was modelled by
two Lorentz oscillators located at 0eV and 5.75eV. The
data for a-Si:H were taken from Palik [8]
Fig. 5 shows the transmittance of SiNx layers (~70
nm thick), deposited at different temperatures on Corning
glass substrates.
1,0
o
SiNx 300 C
Transmittance
0,9
0,8
0,7
0,6
o
0,5
SiNx 230 C
0,4
0,3
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RESULTS AND DISCUSSIONS
0,2
400
600
800
1000
1200
Wavelength, nm
Fig. 3 shows the transmittance of a-Si:H thin layers
with different thicknesses deposited on a Corning glass at
230 °C. From Fig.3 it can be seen that the absorption of
the a-Si:H layer in the ultraviolet range is remarkable and
rapidly increasing with the increasing of the a-Si:H layer
thickness.
Fig. 4 shows the transmittance of a thin (~5 nm) aSi:H layer after a short (~2 min) heat treatment at 450 °C.
Figure 5: Transmittance of ~70 nm SiNx layers
deposited on a glass substrate at different temperatures.
From Fig.5 it can be seen that an increase in the
deposition temperature leads to the increase of the SiNx
layer transmittance. It is interesting to note also that the
transmittance is not changed after a short heat treatment
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22nd European Photovoltaic Solar Energy Conference, 3-7 September 2007, Milan, Italy
TCO(ITO, ZnO:Al)/a-Si:H/p-Si/a-Si:H/TCO structures
after heat treatments at 450 °C for ~2 min.
at 450 °C (Fig. 6) in contrast to the case of the a-Si:H
layer (Fig. 4). Thus, the optical properties of SiNx layers
used in this work depend on the deposition temperature
but stable at annealing temperatures around 450 °C.
Transmittance
0,9
o
Minority carrier lifetime, µs
1,0
2000
0
SiNx 230 C + 450 C 2 min
0,8
0,7
0,6
o
0,5
SiNx 230 C
~ a-Si:H thickness
a-SiH/p-Si/a-Si:H
white collums - as deposited
~50 nm
1600 black collums - 450 o C annealed
15
-3
injection level 10 cm
~10 nm
1200
~8 nm
~6 nm
800
400
0
0,4
0,3
0,2
400
600
800
1000
1200
Figure 8: Effective lifetime of as deposited and annealed
a-Si:H/p-Si/a-Si:H structures with different a-Si:H layers.
Wavelength, nm
Figure 6: Transmittance of ~70 nm SiNx layers
deposited on a glass substrate at 230 °C and then heated
at 450 °C for ~2 min.
2000
Minority carrier lifetime, µs
SiNx/a-SiH(~10nm)/p-Si/a-Si:H/SiNx)
a-Si:H/SiNx
800
a-Si:H
400
0
Figure 9: Effective lifetime of as deposited and annealed
SiNy/a-Si:H/p-Si/a-Si:H/SiNx, structures with ~70nm
thick SiNx and ~ 10 nm a-Si:H layer.
1,0
0
o
0,9 a-Si:H (~5 nm), 230 C + SiNx, 230 C
0,8
0,7
0,6
0,5
0
firing at 450 C for ~ 2 min.
0,4
0,3
0,2
0,1
0,0
400
600
800
1000 1200
Transmittance
Transmittance
Fig. 7 shows the transmittance of a-Si:H/SiNx stack
layers before (as deposited) and after an annealing at 450
°C. An essential decrease of the transmittance can be
seen from Fig.7, which can be attributed mainly to a
degradation of the optical properties of the a-Si:H layer
(see also Fig. 4).
1600 white collums - as deposited
o
black collums - 450 C annealed
15
-3
a-Si:H/SiNx
injection level 10 cm
1200
Wavelength, nm
Figure 7: Transmittance of a-Si:H(~5 nm)/SiNx (~70nm)
stack layer deposited on a glass substrate at 230 °C and
then heated at 450 °C for ~2 min.
1,0
o
ITO (160 C)
o
0,8
ITO (230 C)
0,6
0,4
ITO (RT)
0,2
200
400
600
800
1000 1200
Wavelength, nm
Figure 10: Transmittance of ITO layers deposited at
different temperatures.
Fig. 8 shows the effective lifetime evolution for the
a-Si:H/p-Si/a-Si:H structures with different thicknesses
of a-Si:H layers after heat treatments at 450 °C for ~2
min.
Fig. 9 shows the effective lifetime evolution for the
SiNy/a-Si:H/p-Si/a-Si:H/SiNx, structures with ~70nm
thick SiNx layer and ~10 nm thickness of the a-Si:H layer
after heat treatments at 450 °C for ~2 min. From Figs. 8,9
it can be seen that the effective lifetime in all cases
exhibit some general trends: (i) it depends on the a-Si:H
thickness and (ii) it can be increased by the annealing.
ZnO:Al/glass
Transmittance
1,0
Figs. 10,11 shows Fig. 3 shows the transmittance of
ITO and ZnO layers deposited on a Corning glass at
different conditions. From these figures it can be seen
that the transmission of TCO layers is better than that for
SiNx layers (see Fig. 5), which indicates some advantages
of TCO ARCs compare to silicon nitride based ones.
Fig. 12 shows the effective lifetime evolution for the
0,8
undoped ZnO/glass
0,6
0,4
0,2
200
400
600 800 1000 1200
Wavelength, nm
Figure 11: Transmittance of doped and un-doped ZnO
layers deposited on a Corning glass.
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22nd European Photovoltaic Solar Energy Conference, 3-7 September 2007, Milan, Italy
From Fig. 12 it can be concluded that the passivation
properties of an a-Si:H/SiNx stack better than those of aSi:H/TCO ARC. This means that the magnetron
sputtering process introduces some not desirable changes
of the thin a-Si:H layer properties and has to be
optimised.
4
Results of our investigations can be summarized as
follows:
• Optical properties of double stack a-Si:H/SiNx(ITO,
ZnO) ARC have to be optimised because of the high
absorption coefficient of the a-Si:H layer.
• Passivation and optical properties of an a-Si:H
individual layer and an a-SiH/SiNx ARC depend
essentially on the thickness of the a-Si:H layer and
can be modified by heat treatment at 450 °C. These
modifications have to be taken into account for
iterative optical simulations in each concrete case.
• TCO layers deposited by magnetron sputtering
exhibit better optical properties compare to those for
SiNx ARC.
• Passivation properties of an a-Si:H/SiNx stack are
better than those of a-Si:H/TCO in case magnetron
sputtered of TCO layers, which means that the
sputtering process has to be optimised or substituted
by another “softer” processes.
• By iterative simulations it is shown that optical
properties of the double stack a-Si:H/SiNx(ITO,
ZnO) passivation layer depend essentially on the
properties of individual layers and exhibit some
advantages for a-Si:H/SiNx and a-Si:H/ITO ARCs.
Minority carrier lifetime, µs
1000
800
TCO/a-SiH(~10nm)/p-Si/a-Si:H/TCO
TCO-magnetron sputtering
white collums - as deposited
o
black collums - 450 C annealed
600 injection level 1015 cm-3
400
a-Si:H/ZnO:Al
a-Si:H
SUMMARY
a-Si:H/ITO
200
0
Figure 12: Effective lifetime of as deposited and
annealed TCO/a-Si:H/p-Si/a-Si:H/TCO structures with
~70nm thick TCO(ITO,ZnO:Al) and ~ 10 nm a-Si:H
layer.
Fig. 13 shows results of simulations for the AM1.5
transmittance versus thickness of a-Si:H and ARC layers.
From Fig.13 it can be concluded that the AM1.5
transmittance depends mainly on the thickness of the aSi:H layers and has to be optimised for each chosen ARC
design.
ACKNOWLEDGEMENT
This work was supported by the Research Council of
Norway (NANOMAT).
REFERENCES
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Figure 13: AM1.5 transmittance versus thickness of a-Si
and ARC layers
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