Investigation of structural aspect in terms of atypical

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Investigation of structural aspect in terms of atypical
Research Article
1(1) 32-37
Advanced Materials Proceedings
Investigation of structural aspect in terms of
atypical phases within material deposited
for a-Si:H solar cell fabrication
Mansi Sharma1,2, Deepika Chaudhary1,2, S. Sudhakar1, Preetam Singh1, K.M.K. Srivatsa1,2,
Sushil Kumar1,2 *
1
Network of Institutes for Solar Energy (CSIR-NISE), Physics of Energy Harvesting Division, CSIR – National
Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012 India
2
Academy of Scientific and Innovative Research (AcSIR), CSIR-NPL Campus, Dr. K.S. Krishnan Marg, New Delhi
110012 India
*
Corresponding author, E-mail: [email protected]; Tel: (+91) 1126981717
Received: 30 March 2016, Revised: 01 August 2016 and Accepted: 03 August 2016
DOI: 10.5185/amp.2016/107
www.vbripress.com/amp
Abstract
The structural investigation of the a-Si:H material, deposited at different pressures by PECVD process, has been
carried out to analyze the signatures of diffused intermediate sort of crystalline phases within the amorphous silicon
matrix. Raman characterization along with the Photoluminescence (PL) and spectroscopic ellipsometry studies were
carried out to understand the microstructuree of these films. From Raman analysis the material was found to have
indistinguishable crystalline phase, which can also be named as “intermediate amorphous phase” (a phase defined
between amorphous and ultra nano-crystalline silicon) with crystalline volume fractions as 56 % and 62 % for 0.23
Torr and 0.53 Torr respectively. Here the contribution of ultra nano-crystallites results in higher crystalline fraction,
which is not visibly revealed from the Raman spectra due to its sub nano-crystallite characteristics. For the film
deposited at 0.53 Torr stable photo-conductance in conjunction with high photo-response under 10 hour light
soaking has been observed, which is as expected due to high crystalline volume fraction. The presence of these
phases might be the possible reason for the distinct device characteristics though having nearly the similar electrical
properties (photo-response ~104). These studies will help to make improvement in the individual layer properties,
other than the interface effect, in the fabrication of efficient p-i-n solar cells. Copyright © 2016 VBRI Press
Keywords: PECVD, a-Si:H, ellipsometer, raman spectra, PL spectra.
Introduction
Research and technology based on hydrogenated
amorphous silicon (a-Si:H) material and its alloys has
been well known from the past few decades with the
visible applicability of this material in the
photovoltaic area, where it has overcome the
requirement of cost effective device fabrication [1].
In a-Si:H its amorphous matrix offers the advantage
of tunable band gap along with high absorption > 10 4
over conventional bulk material and thus the recent
research has also been adopted the material suitably
for the fabrication of heterojunction solar cell [2-3].
Since the a-Si:H material lacks long range order, the
matrix highly engross towards the impurities and
defects [4]. The participation of such defective states
would significantly alter the material properties in
terms of carrier transport and stability. Thus from the
past few decades the study of such defects, as
observed with an effect of continuous light exposure,
has been a subject of investigation for the material
stability in terms of altered electronic properties.
Copyright © 2016 VBRI Press
These alterations in highly amorphous matrix were
recognized as material degradation effect, known as
Staebler-Wronski (SW) effect, where the continuous
light soaking will result in breaking of Si-Si bonds,
thereby raising the probability for intermediate range
ordering [5-9]. In consideration of such limitations of
conventional a-Si, the research has been directed
towards the micro/nano-crystalline (µ/nc-Si) silicon
thin films. In µ/nc-Si the accumulation of small
crystallite within the amorphous lattice would
significantly modify the optoelectronic properties of
the material as a result of which the matrix is
considered as biphasic where the properties are
almost dependent on the distribution of crystallites
and their sizes. Due to high band gap compared to aSi:H the nano-crystalline silicon was also considered
as good alternative for the amorphous silicon carbide
alloys (a-SiC:H) for intrinsic layer fabrication with
the advantage of improved material quality for PV
device fabrication [10-11]. These materials offer the
advantage of long term material stability (via
inclusion of small crystallites) under continuous light
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Research Article
1(1) 32-37
exposure for several hours but at the cost of poor
electrical response as per device requirement. In
view of this, several reports have already been
provided on the ultra-nano crystalline phase (unc-Si)
[12], the existence of which was expected just before
the formation of visible crystalline phase during
transition. Thus there is need to explore
characteristics of such material for required
properties, where the crystalline contribution adds on
to maintain the electric response as well.
In our recent studies [13], for the a-Si:H films
deposited at pressure 0.23 Torr and 0.53 Torr the
photo-response and the required material band gap
were observed to be same, whereas for 0.43 Torr
similar photo-response (~ 104) was observed except
with slightly higher band gap. Keeping in view of
this, 0.23 Torr and 0.53 Torr pressures were
considered as the optimum pressures for further
doping and solar cell fabrication. However, out of
given set of samples deposited at different pressures
0.43 Torr may perhaps be one of the suitable
pressure with desired electric response (as for 0.23
Torr and 0.53 Torr) for absorber layer fabrication in
solar cell. Despite having the required properties in
terms of photo conduction, the film deposited at 0.43
Torr, when utilized as absorber layer in solar cell,
resulted lower efficiency and poor cell response in
terms of other solar cell parameters. To resolve this
anomaly aroused for distinct features of a-Si:H
materials, deposited at different pressures with
similar photo-response and almost similar electrical
characteristics, the study has been extended making
further investigations for the possible structural
aspects which could explain the observed behavior.
Present work insights into the detailed structural
identifications in support of our recent report [13].
These results highlight the existing diffused phase
consisting of fine sub- nano-crystallites with
dominating amorphous fraction, which overshadow
the contribution of these crystallites and thus
recognized as “intermediate amorphous phase”.
Experimental
Materials
5% silane diluted in hydrogen (99.9999% purity,
Matheson USA Inc.) is used as a precursor gas for the
deposition of thin film. Plane glass substrate (7059
corning glass), Indium Tin oxide (ITO) coated glass
and double side polished p-type silicon substrates
were used. The cleaning of the substrates was done
using semiconductor grades Isopropyl alcohol (IPA),
acetone, HF and Trichloroethylene chemicals (Merck
Ltd).
Material synthesis
Thin films of hydrogenated amorphous/mixed phase
silicon were grown on glass substrates by PECVD
technique at RF frequency of 13.56 MHz, where 5%
silane diluted in H2 was used as the precursor gas for
the deposition of device quality intrinsic layer under
Copyright © 2016 VBRI Press
Advanced Materials Proceedings
variable deposition pressure environment in the
range 0.13 Torr - 0.53 Torr (monitored using MKS
Baratron Digital Gauge meter).
The other
parameters, such as silane flow of 47 sccm (through
MKS Mass flow controller), applied power density
(35.3 mW/cm2) and substrate temperature of 270 ̊C
were kept fixed for all the deposited films [13].
Characterizations
The detailed characterizations of the films, deposited
on glass as well on silicon substrate were carried out
to extract the explicit structural profile for the
distributed crystalline within the amorphous phase.
The exploration of the material properties without
making any direct contact with material surface in
terms of optical constants have been carried out
using spectroscopic ellipsometer. Spectroscopic
ellipsometer (SE) studies were carried out for the
incident angle of 55o, 65o and 75o in the wavelength
range 250-1200 nm (using J. A. Woollam, model:
VASE32) from which the estimation was made for
the optical constants of the films. Ellipsometer is a
nondestructive technique and is based on theoretical
model fitting reference data rather than the solely
extracted experimental data [14], the fitting for
present set of samples utilized two basic models
(Tauc-Lorentz (TL) and Gaussian-Lorentz mode) in
order to have best fitting results with that of the
experimental data. From the mentioned fitting the
thickness of the films was found vary within the
range of 250-400 nm.
The structural investigation of these films were
carried out using micro Raman (InVia Renishaw
Raman spectrometer with 514 nm Ar + laser) and
photoluminescence measurements (PL) on the films
deposited on glass substrate. Further light soaking
measurements were performed to have a detailed
stability profile of the deposited material in terms of
its photoconductivity under continuous illumination
of light intensity of 100 mW/cm2 using Keithley
6487 programmable electrometer. For this the Al
metal contacts were deposited on the films in a
coplanar configuration having a gap of 0.078 cm and
width of 1.0 cm, using the method of thermal
evaporation in vacuum condition better than 10 -5
Torr.
Results and discussion
The complete structural analysis in terms of specific
distribution of crystallites within the material matrix
was recorded via mode of phonon vibrations from
Raman spectroscopy measurements. Fig. 1 shows
the deconvoluted Raman spectra of silicon thin film
within the wavelength range 400 cm-1-550 cm-1,
deposited at different pressures. The recorded
spectrum has been deconvoluted into two Gaussian
peaks which correspond to the particular distribution
of amorphous and crystalline/sub-nano-crystalline
content within the given set of films. The
deconvoluted peaks from the recorded spectra
observed at ~ 470 cm-1 corresponds to amorphous
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Research Article
1(1) 32-37
phase and another at ~ 490 cm-1 corresponds to the
contribution from fine sub-nano-crystalline phase
which may be termed as “intermediate amorphous
phase”. The interpretation out of these results was
further elaborated on the basis of estimated
crystalline content of the films. The crystalline
volume fraction for the same has been calculated
using the following relation:
𝑋𝑢𝑛𝑐 =
𝐼490
Advanced Materials Proceedings
mixed within the dominating amorphous phase (as
observed from the Raman spectra), the estimation of
exact particle size within nm range is difficult from
the normal morphological technique.
(a)
(1)
𝐼490 +𝐼470
where, I490 and I470 are the integrated intensities of
corresponding intermediate amorphous and highly
amorphous phase respectively. The crystalline
volume fraction was found to vary within range of
36% - 62%.
(b)
Fig. 2. (a) Spectral Variation of imaginary pseudo dielectric
constant (at incident angle 75o) for the films deposited at various
deposition pressure. (b) Experimental and model fitted tan ψ plots
films at different angle of incidence
Fig. 1. Deconvoluted Raman spectra for films deposited at
different pressure.
Table 1 provides the typical centered peak position
along with the estimated crystalline content. The
films deposited at 0.43 Torr was found to contribute
lowest crystalline content with a value of 36%
crystalline volume fraction whereas at 0.53 Torr the
material was found to have the highest contribution
of 62% volume fraction.
Table 1: Various calculated parameters of the a/unc-Si:H films
deposited under varying deposition pressure.
Deposition
Pressure (Torr)
0.23
Raman
Peak 1
472
Raman
Peak 2
492
Xunc (%)
<ε 2max>
56
0.33
472
497
37
19.3
22.5
0.43
0.53
469
471
497
497
36
62
21.4
18.6
This indicates that at 0.33 Torr and 0.43 Torr
pressures the plasma conditions are not favorable for
the growth of material with considerable crystallite
content within the amorphous matrix. In general, the
two extreme pressures 0.23 Torr and 0.53 Torr were
highly suitable for the device quality material within
low pressure regime (< 1 Torr). It is to be noted that,
since these intermediate phases seem to be almost
Copyright © 2016 VBRI Press
Fig. 2(b) represents the SE data for the Tan ψ
parameter of thin film silicon grown at 0.53 Torr
pressure from where it is evident that the recorded
experimental spectra (symbol) is well matched with
the theoretical modeling (solid line). The related
optical parameters have been extracted for different
incident angles 55̊, 65̊ and 75̊ respectively, out of
these, for the clarity other related results were
presented only for one angle of incidence75̊.
Fig. 2(a) shows the imaginary pseudo dielectric <ε2>
spectra for the samples deposited at different
pressures. The spectra hold the information in the
form of broad hump correspond to <ε 2> centered at
3.5 eV, which is close to the reported energy position
(3.4 eV) for a-Si:H dielectric peak. Moreover, we
found a noticeable shift in the <ε2> central peak
intensity which directly provides the value of
<ε2max>. The observed shift was segregated in two
distinct regions for ardent analysis where the films
deposited at 0.33 and 0.43 Torr have high value of
<ε2max> as compared to pressure 0.23 and 0.53 Torr.
This high <ε2max> corresponds to the lack of
roughness with the significant reduction in the
density deficits in the deposited material at 0.33 Torr
and 0.43 Torr and the details of which are given in
Table 1. The films deposited at 0.53 Torr exhibit a
comparatively broader hump with the least <ε2max>
value which may be attributed to the presence of
intermediate amorphous phases, thereby alters the
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Research Article
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interaction of incident light with the material. As a
consequence, the range of photon energy
corresponds to visible broadening gets widen for
conventional a-Si:H. In addition to this, the plot for
refractive index and the corresponding extinction
coefficient values as a function of wavelength were
presented in Fig. 3, which shows a significant shift
in the spectral distribution of extinction coefficient
and the refractive index for the films deposited at
different pressures. The corresponding shift in values
of <k> and higher <n> corresponds to the enhanced
scattering probability due to inclusion of fine
crystallites within few nm thick single layers. These
results are in correlation with the estimated
crystalline contribution as observed from Raman
spectrum.
Advanced Materials Proceedings
enhances the probability of light scattering within the
amorphous layer when used for device fabrication.
(a)
Fig. 4: Photoluminescence spectra (PL) for thin films deposited at
different pressure.
(b)
Thus, PL results further confirm the existence of
sub-nano-crystalline
phases
as
intermediate
amorphous material. These intermediate phases are
essentially diffused crystalline phases which are
quite different from conventional nano-crystalline or
micro-crystalline structures. Structurally they look
very similar to amorphous silicon, however, possess
properties predominately of nano/micro-crystalline
silicon.
Further, the validation of present results in the
form of intermediate amorphous phase for device
fabrication was ended up via performing light
soaking measurements on these films. Fig. 5 depicts
the results of observed variation in material
photoconductivity
under
continuous
light
illumination for 10 hrs.
Fig. 3: (a) Refractive index and (b) extinction coefficient
measured from ellipsometer for the films deposited at various
deposition pressures.
Fig. 4 shows the PL emission spectrum as a
function of wavelength for the films deposited at
different pressures where the samples were exposed
to suitable laser excitation which gives a very strong
and sharp emission peak around ~ 622 nm along
with a broaden peak at 656 nm, this corresponds to
crystalline contribution out of the grown amorphous
silicon films. These peaks evident the modified band
gap values as in accordance to the observed PL
spectra where it is considered to have a direct
relationship with the observed band gap value [13,
15-16]. These phases were observed only for the
films deposited at 0.23 Torr and 0.53 Torr which
concludes the required favorable conditions for the
growth of such unrevealed phases out of amorphous
matrix. The observed results of PL are in well
correlation with the mentioned growth possibilities
from Raman and ellipsometry measurements where
the contribution from such intermediate phases
Copyright © 2016 VBRI Press
Fig. 5. Photoconductivity plots for the films under light soaking
condition for 10 hrs.
From these results it has been observed that the
films deposited at 0.23 Torr and 0.53 Torr have
lesser degradation in terms of electrical stability
under light exposure as compared to 0.43 Torr. The
estimated values for the % degradation in terms of its
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Research Article
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photo response were given in Table 2 with the
corresponding device efficiency when the material
was utilized as absorber layer in the solar cell, as
mentioned in our earlier report [13]. These
observations can again be related to the unrevealed
crystalline phase within a-Si thin films where the
material deposited at 0.53 Torr has shown the lowest
degradation with the highest observed content of
sub-nano-crystalline phase.
Moreover, in terms of device efficiency, the solar
cell fabricated with absorber layer grown at this
pressure found to have comparatively high efficiency
than at other pressures. This concludes that the
presence of these crystallites overcome the material
degradation effectively as compared to highly
micro/nano-crystalline silicon, thereby, maintaining
the conventional electrical properties of a-Si as per
the device requirement.
Advanced Materials Proceedings
Fig. 6: Schematic representation for typical role of intermediate
amorphous phases within absorber layer for solar cell.
(a)
(b)
Table 2: Results of photo-degradation as observed from light
soaking and the efficiency for the same when utilized as intrinsic
layer in p-i-n solar cell.
Intrinsic layer
deposition
pressure (Torr)
0.23
0.43
0.53
Material
Intermediate amorphous
Highly amorphous
Intermediate amorphous
Degradation (%) η (%)
in terms of photo
response
29
3.5 [13]
63
0.7
6
5.6 [13]
As the overall performance of the cell is highly
reliant on the properties of its absorber layer, the
material composition and structure for intrinsic layer
are of great importance. The dielectric and the
absorption profile of the material are considered as
its intrinsic property which is directly related to the
structure other than the geometry (roughness and
heterogeneity) which can vary for the same material.
Fig. 6(a-b) shows the schematic representation of pi-n solar cell with the intermediate amorphous and
highly amorphous intrinsic layer respectively, with
the help of which the important role of fine
crystallites has been highlighted via a typical
comparison for solar cell performance and effective
light trapping probabilities. From Fig. 6(a) it is
evident that the incident light encounters more
reflection within absorber layer due to distributed
fine crystalline content which thus leads to diffused
scattering and thereby enhances the probability for eh pair generation when compared with Fig. 6(b)
where highly amorphous silicon matrix was utilized
for absorber layer. In support of this we have given
the corresponding solar cell efficiency values in
Table. 2. From these results the cell with intrinsic
layer deposited at 0.53 Torr (highest contribution
from intermediate amorphous phase) had the
preeminent efficiency value as compared to others.
This will evident the effective role of intermediate
amorphous phases for improvement in the cell
response along with a stable intrinsic layer which
could be a better replacement for the conventional
a-Si:H with desired electrical response.
Copyright © 2016 VBRI Press
Conclusion
The detailed progressive study of the a-Si:H material
deposited as absorber layer in solar cell fabrication
using RF 13.56 MHz PECVD has been carried out.
The structural analysis using Raman and PL spectra
reveals the presence of dissolved fine nanocrystalline phases within the amorphous matrix,
which has been responsible for the material stability
under continuous light illumination. From these
results the contribution from such phases (~ 490 cm-1
in terms of crystalline volume fraction) was found to
vary from 36% to 62%. Further, from spectroscopic
ellipsometry study the results of observed optical
constants were in correlation with the above
mentioned results from where such phases were
identified as “intermediate amorphous phase”. The
observed shifting in the optical spectra for extinction
coefficient and refractive index suggest the enhanced
scattering via inclusion of sub-nano-crystalline
phases. These finding in terms of understanding the
micro structurally distributed phases of a-Si:H thin
film opens up with the scope of making further
improvement in the individual layer properties other
than the interface effects during the fabrication of
efficient p-i-n based solar cells.
Acknowledgements
The authors are grateful to Director, CSIR-National Physical
Laboratory, New Delhi (India) for his kind support. We are also
thankful to Dr. Bipin Gupta and Mr. Pawan for providing the PL
facility. The authors are also thankful to Ms Kalpana Lodhi and
Yamini Pant from NPL for providing help during experimentation.
We also acknowledge CSIR-India for TAPSUN program and
MNRE, Govt. of India for the research grant (sanction
31/29/2010-11/PVSE).
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Advanced Materials Proceedings
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