th
15 International Photovoltaic Science & Engineering Conference (PVSEC-15) Shanghai China
2005
19-5
Surface Passivation of Silicon Solar Cells by
Amorphous Silicon/Silicon Nitride Dual Layers
Andreas Bentzen 1,3, Alexander Ulyashin 2, Aina Suphellen 3, Erik Sauar 1, Dieter Grambole 4, Daniel N. Wright 3, Erik S.
Marstein 3, Bengt G. Svensson 2, and Arve Holt 3
1
Renewable Energy Corp. AS, Veritasveien 14, P.O. Box 280, NO-1323 Høvik, Norway, E-Mail: andreas.bentzen@recgroup.com
2
Center for Material Science and Nanotechnology, University of Oslo, P.O. Box 1048 Blindern, NO-0316 Oslo, Norway
3
Section for Renewable Energy, Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller, Norway
4
Institut für Ionenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf, PF 510119, D-01314, Dresden, Germany
Abstract: We have investigated surface passivation of silicon by a dual structure consisting of a hydrogenated amorphous silicon thin
film capped by a silicon nitride anti-reflection layer, both layers deposited by plasma enhanced chemical vapor deposition. The results
show that a synergetic effect on the surface passivation is obtained from such a dual structure. Moreover, we find that the surface passivation can be significantly enhanced after a short anneal for temperatures up to about 500 °C, whereas anneals at higher temperatures
result in degradation of the passivation properties. From nuclear reaction analyses of the as-deposited and annealed structures, the enhanced surface passivation experienced after annealing is indicated to be due to hydrogen release from the structure towards the silicon
substrate, possibly followed by a redistribution of hydrogen and subsequent passivation of silicon dangling bonds in the sub-interface
region.
Key Words: Surface Passivation, Amorphous Silicon, Silicon Solar Cells.
measurements of the effective recombination lifetime by the
quasi-steady-state photoconductance (QssPC) technique, using a
generalized analysis to account for both quasi-steady-state and
quasi-transient contributions [2,3]. Finally, the H distributions
within the samples were investigated by nuclear reaction analysis (NRA) [4].
1 Introduction
Recently, surface passivation of Si using silicon nitride
(SiNx:H) or hydrogenated amorphous silicon (a-Si:H) has been
the subject of intense investigations for solar cell applications. It
has been established that a-Si:H films deposited at rather low
temperatures around 200 ºC can provide a better passivation of
Si surfaces compared to high-temperature thermal SiO2 or
plasma enhanced chemical vapor deposition (PECVD) SiNx:H
films deposited at temperatures around 400 ºC [1]. However, it
is generally found that the surface passivation properties of
a-Si:H exhibit non-reversible degradation after treatment at
temperatures above 300 – 350 ºC. Thus, for solar cell processing
employing a-Si:H layers, low temperature metallization steps
are commonly demanded in order to preserve the surface passivation.
In this work, we introduce a dual layer surface passivation
structure consisting of an a-Si:H thin film capped by a SiNx:H
anti-reflection layer, showing very promising results for application in solar cell processing. We study the temperature stability and the evolution of the surface passivation properties of the
structure, and investigate the evolution of the H distribution
through the structure upon annealing.
3 Results and Discussion
In Fig.1, the effective recombination lifetimes as measured
by QssPC at an injection level of ~1x1016 cm-3 on Cz-Si samples
both prior to and after annealing is summarized. Firstly, the figure shows the effective lifetime measured after deposition at 380
°C of an 80 nm SiNx:H film only, typical of anti-reflection
coatings used in industrial Si solar cells. Moreover, the figure
also shows the effective lifetime as measured after deposition at
230 °C of a 100 nm a-Si:H film only. The effective lifetime as
measured on these samples exhibit similar values, thus the surface passivation abilities of the two individual as-deposited
films are comparable. Also shown in the figure is the effective
lifetime after deposition of a dual 100 nm a-Si:H / 80 nm
SiNx:H structure, both layers deposited at 250 °C. As the figure
clearly reveals, there is a synergetic effect from the dual layer
structure.
It is further revealed in Fig.1 that the measured effective
lifetime of the samples employing an a-Si:H / SiNx:H dual
structure increases compared to the as-deposited value after a
subsequent 4 minutes anneal at temperatures up to 500 °C. At
higher temperatures, the measured effective lifetime commences
to decline, although at 550 °C the effective lifetime is still comparable to the as-deposited value. Thus, the temperature stability
of the dual layer structure is significantly higher than common
a-Si:H films alone, where significant degradation of the surface
passivation properties is commonly found to occur above
300-350ºC. After annealing at even higher temperatures, the
surface passivation property of the structure is observed to degrade considerably.
2 Experimental Details
Polished 2 Ωcm p-type (B doped) Czochralski Si (Cz-Si)
wafers were used for the investigations reported in this work.
The wafers were cleaned in H2SO4:H2O2 (4:1) followed by an
oxide removal in diluted HF. Then, the samples were introduced
into a PECVD chamber, and a-Si:H films were deposited using
SiH4 as the sole precursor gas. Subsequently, a SiNx:H layer
was deposited atop the a-Si:H film, using SiH4 and NH3 as precursor gases. Both layers were deposited at the same temperature of 250°C, and deposition of the dual structure was performed on both surfaces of the samples. After deposition, selected samples were annealed in an infrared belt furnace for 4
minutes in the temperature range 300-650 ºC.
The surface passivation properties were investigated via
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measured after annealing up to 500 °C. H in the subsurface region of the substrate is likely to redistribute easily at these temperatures, possibly leading to passivation of Si dangling bonds
in the sub-interface region. At even higher temperature anneals,
breakage of Si-H bonds and out-effusion of H through the
SiNx:H layer may cause a collapse in the surface passivation at
temperatures around 600°C, as seen from the apparently complete lack of H below the SiNx:H layer in the NRA measurements of these samples.
In order for a-Si:H films to be used as a front surface passivation in solar cells, the optical properties must be considered.
Significant photon absorption occurs in a-Si:H, causing reduced
current generation in the underlying solar cell. Thus, the benefits
of the a-Si:H / SiNx:H dual structure must be considered in conjunction with the loss of optical generation on the bulk of the
solar cell. To investigate this, we performed simulations of the
total reflection loss, absorption loss, and transmitted photon flux
of the dual structure in the wavelength range 300-1150 nm,
weighted towards the AMG1.5 incoming photon flux spectrum.
The results of these simulations, where the thickness of the
SiNx:H was kept constant at 80 nm, show that the absorption
loss is as high as 5 % at an a-Si:H layer thickness of 10 nm, increasing to 9 % for a thickness of 20 nm. Thus, for successful
implementation of the proposed surface passivation structure on
the front surface of solar cells, achieving good surface passivation with a very thin a-Si:H layer is crucial.
We have recently achieved very promising results by the
use of the above presented dual structure with thinner a-Si:H on
6 Ωcm p-type (B doped) Cz-Si wafers. Through QssPC measurements, we extracted surface recombination velocities (SRV)
as low as 7 cm/s at an injection level of 1x1015 cm-3 for a layer
thickness of ~50 nm, increasing to 25 cm/s for a ~20 nm thickness. These values for the SRVs were calculated as in [1] by
assuming recombination only by the Coulomb-enhanced Auger
mechanism [5]. Representing an upper estimation of the SRV,
these results encourage further work on surface passivation by
the dual structure as presented herein.
Fig.1 Effective lifetimes as measured by QssPC at an injection
level of ~1×1016 cm-3 on Cz-Si samples with different surface
treatments
In order to investigate the possible reasons behind this enhanced effective lifetime after annealing up to 500 °C, we
measured the H concentration distribution through the
as-deposited and annealed structures by NRA. The results of
these measurements are shown in Fig.2.
Fig.2 Hydrogen distributions in the a-Si:H/SiNx:H structures on
a Cz-Si substrate as measured by nuclear reaction analyses both
prior to and after annealing for 4 minutes
4 Conclusions
We have found that a combined structure consisting of an
a-Si:H film capped by SiNx:H is capable of providing excellent
surface passivation of Si. Furthermore, we have observed that
this structure has a temperature stability that is superior to
a-Si:H alone, even revealing increased effective lifetimes upon
annealing up to 500 °C. The results are attributed to a release of
H towards the substrate, possibly leading to a passivation of Si
dangling bonds in the sub-interface region.
From the as-deposited H distribution, shown as the solid
line with open circles in the figure, the interfaces between the Si
substrate and the a-Si:H film, and between the a-Si:H film and
the SiNx:H layer is clearly revealed as the positions in the sample where the H concentration changes markedly. Upon inspection of the H distribution in the structures annealed at temperatures up to 550 °C, it is seen that the H penetration in the Si
substrate is increased as compared to the as-deposited sample.
Thus, it is indicated that H is released towards the substrate, either directly from the a-Si:H film or from the overlying SiNx:H.
It is seen from Fig.2 that the H concentration in the SiNx:H decreases upon annealing, however H may also effuse out towards
the atmosphere. Nevertheless, it is seen from the figure that the
H concentration within the a-Si:H film does not significantly
decrease for anneals up to 450 °C, indicating the possibility of a
partial release of H from the SiNx:H towards the substrate. Upon
annealing at temperatures exceeding 450°C, the H concentration
in the a-Si:H film is observed to decrease, however there is still
a significant amount of H in the sub-interface region of the substrate. It is believed that this release of H towards the substrate
is at least partly responsible for the increased effective lifetime
References
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