Fabrication of solution-processed hydrogenated amorphous silicon single-junction solar cells

Article abstract of DOI:10.1063/1.4730614

Hydrogenated amorphous silicon solar cells were fabricated using solution-based processes. All silicon layers of the p-i-n junction were stacked by a spin-cast method using doped and non-doped polydihydrosilane solutions. Further, a hydrogen-radical treatment under vacuum conditions was employed to reduce spin density in the silicon films. Following this treatment, the electric properties of the silicon films were improved, and the power conversion efficiency of the solar cells was also increased from 0.01% to 0.30%-0.51% under the AM-1.5G (100 mW/cm²) illumination conditions.

Full text of DOI:10.1063/1.4730614

Fabrication of solution-processed hydrogenated amorphous silicon single-
junction solar cells
Takashi Masuda, Naoya Sotani, Hiroki Hamada, Yasuo Matsuki, and Tatsuya Shimoda
Citation: Appl. Phys. Lett. 100, 253908 (2012); doi: 10.1063/1.4730614
View online: http://dx.doi.org/10.1063/1.4730614
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v100/i25
Published by the American Institute of Physics.
Related Articles
Millisecond annealing for advanced doping of dirty-silicon solar cells
J. Appl. Phys. 111, 123104 (2012)
Exploration of nano-element array architectures for substrate solar cells using an a-Si:H absorber
J. Appl. Phys. 111, 123103 (2012)
Combined plasmonic and dielectric rear reflectors for enhanced photocurrent in solar cells
Appl. Phys. Lett. 100, 243903 (2012)
On-line thermal dependence study of the main solar cell electrical photoconversion parameters using low thermal
emission lamps
Rev. Sci. Instrum. 83, 063105 (2012)
Amorphous/crystalline silicon heterojunction solar cells via remote inductively coupled plasma processing
Appl. Phys. Lett. 100, 233902 (2012)
Additional information on Appl. Phys. Lett.
Journal Homepage: http://apl.aip.org/
Journal Information: http://apl.aip.org/about/about_the_journal
Top downloads: http://apl.aip.org/features/most_downloaded
Information for Authors: http://apl.aip.org/authors
Downloaded 24 Jun 2012 to 152.3.102.242. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

APPLIED PHYSICS LETTERS 100, 253908 (2012)
Fabrication of solution-processed hydrogenated amorphous silicon
single-junction solar cells
Takashi Masuda,1,a) Naoya Sotani,2,b) Hiroki Hamada,2,c) Yasuo Matsuki,1,3
and Tatsuya Shimoda1,4
1Japan Science and Technology Agency, ERATO, Shimoda Nano-Liquid Process Project, 2-13 Asahidai, Nomi,
Ishikawa 923-1211, Japan
2Solar Energy Research Center, SANYO Electric Co., 180 Ohmori, Anpachi-cho, Anpachi-Gun,
Gifu 503-0195, Japan
3Yokkaichi Research Center, JSR Corporation, 100 Kawajiri-cho, Yokkaichi, Mie 510-8552, Japan
4School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi,
Ishikawa 923-1292, Japan
(Received 7 March 2012; accepted 7 June 2012; published online 22 June 2012)
Hydrogenated amorphous silicon solar cells were fabricated using solution-based processes. All
silicon layers of the p-i-n junction were stacked by a spin-cast method using doped and non-doped
polydihydrosilane solutions. Further, a hydrogen-radical treatment under vacuum conditions was
employed to reduce spin density in the silicon films. Following this treatment, the electric properties
of the silicon films were improved, and the power conversion efficiency of the solar cells was also
increased from 0.01% to 0.30%–0.51% under the AM-1.5G (100 mW/cm2) illumination conditions.
C
V
2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4730614]
Solution-based processes for the fabrication of electronic
devices have attracted attention because of their wide applica-
tion and cost-effectiveness in comparison with conventional
vacuum-based processes. In particular, solution-processed
semiconducting materials are very useful in the development
of next-generation electronic devices such as large-area flexi-
ble solar cells and thin-film transistors. Recent advances in so-
lar cells using solution processes have principally found
application in organic semiconductors,1 some of which have a
power conversion efficiency comparable with that of a hydro-
genated amorphous silicon (a-Si:H) solar cell. However,
issues of reliability have remained. The stable and highly effi-
cient operation of solution-processed solar cells based on cop-
per indium gallium selenide has also been reported.2 Although
this class of materials appears to be an alternative to silicon,
the supply of rare metals is problematic. Therefore, research
efforts have concentrated on developing solution-processed
silicon solar cells in an efficient way. In this study, we report
the fabrication of a-Si:H solar cells using solution-based proc-
esses except for metal electrodes and modification treatment
to silicon layers. All silicon layers of the p-i-n junction were
prepared by a spin-cast method, and the quality of the silicon
layers was improved by a hydrogen-radical treatment under
vacuum condition. This latter procedure was applied because
our solution-processed a-Si:H film required a help of a vac-
uum process to achieve a good-quality silicon film for apply-
ing to solar cell devices. We have also demonstrated the
effectiveness of the hydrogen-radical treatment.
have fabricated a solution-processed polysilicon thin-film
transistor with the help of laser annealing.3 We have also
investigated the surface tension of CPS, the characteristics of
polydihydrosilane solution, and the stability of polydihydro-
silane film.4–6 Solution-processed a-Si:H films can be pre-
pared by the pyrolysis of polydihydrosilane at an appropriate
pyrolysis temperature (Tp). For the fabrication of the p-i-n
junction structure, we prepared doped and non-doped a-Si:H
films on the basis of similar techniques used for solution-
processed silicon films.3 To fabricate the p-type a-Si:H
(p-Si) film, the p-type polydihydrosilane was synthesised by
photo-induced ring-opening polymerisation of CPS, in which
decaborane was dissolved at 80 ꢀC. The dopant concentration
in the film was adjusted by changing the quantity of decabor-
ane in the CPS. The wavelength, intensity, and irradiation
time for the ultraviolet (UV) light-induced reaction were
365 nm, 15 mW/cm2, and 10–60 min, respectively. The re-
sultant polymer was dissolved in distilled CPS solvent. This
p-type polydihydrosilane solution was spin-coated on the
quartz substrate at 2000 rpm for 30 s and pyrolysed at
Tp ¼ 390 ꢀC for 30 min in a sealed chamber to prevent de-
sorption of the decaborane. For the n-type a-Si:H (n-Si) film,
the n-type polydihydrosilane was prepared in a similar way
to that employed for the p-type film.7 Instead of decaborane,
we used white phosphorus obtained by cracking red phos-
phorus. The wavelength, intensity, and irradiation time were
405 nm, 300 mW/cm2, and 5–120 min, respectively. Distilled
cyclooctane was used as a solvent for the n-type polydihy-
drosilane solution. The solution was spin-coated on the sub-
strate and pyrolysed at Tp ¼ 390 ꢀC for 30 min on a hot plate.
For the intrinsic a-Si:H (i-Si) film, the non-doped polydihy-
drosilane was also prepared without dopant. The wavelength,
intensity, and irradiation time were 365 nm, 15 mW/cm2, and
10 min, respectively. Distilled cyclooctane was used as a sol-
vent for non-doped polydihydrosilane solution. The i-Si film
was fabricated with similar way to that of n-Si film.
We have previously synthesised a silicon precursor
solution consisting of polydihydrosilane (-(SiH2)n-),
cyclopentasilane (CPS: Si5H10), and organic solvent and
a)Author to whom correspondence should be addressed. Electronic mail:
mtakashi@jaist.ac.jp.
b)Solar Business Unit, Energy Company, SANYO Electric Co., Ltd., Energy
Company of Panasonic Group.
c)Next-Generation Energy Device Development Center, Panasonic Corp.
C
V
0003-6951/2012/100(25)/253908/4/$30.00
100, 253908-1
2012 American Institute of Physics
Downloaded 24 Jun 2012 to 152.3.102.242. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

253908-2
Masuda et al.
Appl. Phys. Lett. 100, 253908 (2012)
rate of 15 sccm for 15 min. The chamber pressure and the
substrate temperature were maintained at 0.2 Pa and 200 ꢀC,
respectively.
The properties of the i-, p-, and n-Si films with and with-
out the hydrogen-radical treatment are listed in Table I. Typ-
ical properties of vacuum-processed i-Si films are also listed
as a reference. The hydrogen content, optical gap, and spin
density were measured by SIMS, UV–visible transmittance
and reflectance spectroscopy, and electron spin resonance,
respectively. The conductivity was measured on a coplanar
configuration of 250 lm gap width, using aluminum contacts
for samples. An increase in hydrogen content and drastic
reduction of spin densities by the hydrogen-radical treatment
were observed in all of the silicon films. The depth profile by
SIMS measurement (data not shown) showed that the hydro-
gen was increased not only surface but entire film. It indi-
cates that the hydrogen radical diffused in the film and
terminated the defect. As a result, with this treatment, a large
increase in photoconductivity (rp) and a decrease in rd in the
i-Si film were achieved, while the rd values of the p- and n-
Si films were increased. These properties of resultant a-Si:H
films showed comparable value to device-grade vacuum-
processed ones.10 These improvements may be attributed to
the reduction of spin densities.
FIG. 1. Dark conductivity (rd) vs. dopant concentration for p- and n-Si films
at 25 ꢀC. The rd of the i-Si film is also plotted. The rd of p-, i-, and n-type
films are plotted by closed circles, triangle, and squares, respectively.
To confirm the formation of a-Si:H films, we employed
Raman scattering techniques. It was found that the Raman
spectrum of phonon bands from the p-, i-, and n-Si films was
almost identical to that of vacuum-processed a-Si:H films,
and thus these doped films were concluded to be in the amor-
phous state. The dark conductivity (rd) at 25 ꢀC for the p-
and n-Si films with the thickness of 50 nm is plotted as a
function of boron or phosphorus concentration in Figure 1,
where rd for the i-Si film is also given. The dopant concen-
tration was measured by secondary ion mass spectrometry
(SIMS). The rd of both the p- and n-Si films show a signifi-
cant increase with dopant concentration. These results sug-
gest that boron and phosphorus in the doped a-Si:H films act
as acceptors and donors, respectively. The conductivity of
our solution-processed doped a-Si:H films without the
hydrogen-radical treatment is smaller by one to three orders
of magnitude than that of device-grade vacuum-processed a-
Si:H films.8 The maximum concentration of boron and phos-
phorus is limited by the solubility of the decaborane and
white phosphorus in CPS, which were 7 ꢁ 1021 and
2 ꢁ 1021 cmꢂ3, respectively.
Figure 2 shows a schematic structure of the solution-
processed a-Si:H p-i-n single-junction solar cell. The solar
cells were fabricated according to the following four steps:
(1) a flat ZnO layer with a sheet resistivity of 100 X/sq and a
thickness of 200 nm was sputtered onto the glass substrate
with an area of 5 ꢁ 5 cm2, (2) the p-Si layer (thickness:
30 nm) was fabricated on the ZnO substrate followed by
stacking of the i-Si (thickness: 120 or 400 nm) and n-Si
layers (thickness: 30 nm) with Tp ¼ 390 ꢀC, (3) the hydrogen-
radical treatment was performed after fabricating the p-i-n
structure, and (4) a 200-nm thick Al electrode with an area
of 5 ꢁ 5 mm2 was deposited on the p-i-n structure by vacuum
evaporation. We removed the silicon layers with a diamond
pen to achieve contact with the ZnO electrode as shown in
Figure 2.
To apply the solution-processed doped and non-doped
a-Si:H films to solar cell devices, the film quality was
improved using the hydrogen-radical treatment via hot-wire
chemical vapour deposition. The apparatus used was nearly
identical to that described in the literature.9 A tungsten wire
with a diameter of 0.5 mm and a length of 100 cm was used
as a catalyser and heated at 1850 ꢀC. Hydrogen gas
(99.9999% purity) was introduced into the chamber at a flow
Three types of solar cell (cell 1, cell 2, and cell 3) were
fabricated by changing the thickness of the i-Si layer and
dopant concentration of the p-Si layer, as listed in Table II.
The dopant concentration was measured by SIMS.
For imaging the features of the p-i-n structure, the impu-
rity distribution in the cell was examined by SIMS. Figure 3
shows the depth profiles for boron, phosphorus, zinc, carbon,
TABLE I. Properties of i-, p-, and n-Si films with and without the hydrogen-radical treatment.
i-Si
p-Sia
n-Sib
i-Si (Ref)c
Without
Hydrogen radical treatment
With
Without
With
Without
With
Without
Hydrogen content (at. %)
Optical gap (eV)
9.9
9.8
10.5
1.67
10.0
1.66
12.0
1.67
9.7
1.67
10
1.64
1.64
1.72
Spin density (cmꢂ3
)
<2 ꢁ 1016
0.9 ꢁ 10ꢂ5
2.0 ꢁ 10ꢂ11
5 ꢁ 1017
1.5 ꢁ 10ꢂ7
4.0 ꢁ 10ꢂ11
1 ꢁ 1017
6 ꢁ 1017
6 ꢁ 1016
4 ꢁ 1017
<2 ꢁ 1016
1.6 ꢁ 10ꢂ5
2.0 ꢁ 10ꢂ11
Photoconductivity (S/cm)d
ꢃꢃꢃ
ꢃꢃꢃ
ꢃꢃꢃ
ꢃꢃꢃ
Dark conductivity (S/cm)
5.0 ꢁ 10ꢂ4
2.1 ꢁ 10ꢂ4
5.6 ꢁ 10ꢂ3
4.3 ꢁ 10ꢂ4
aBoron concentration is 7 ꢁ 1021 cmꢂ3
.
bPhosphorus concentration is 2 ꢁ 1021 cmꢂ3
.
cPrepared by plasma-enhanced chemical vapour deposition method.
dAM-1.5G solar simulator with an intensity of 100 mW/cm2 was employed.
Downloaded 24 Jun 2012 to 152.3.102.242. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

253908-3
Masuda et al.
Appl. Phys. Lett. 100, 253908 (2012)
FIG. 2. Schematic of a solution-processed a-Si:H solar cell structure.
oxygen, and nitrogen for cell 1. The detection limits for bo-
ron, phosphorus, and zinc in Figure 3(a) are 2 ꢁ 1016,
2 ꢁ 1018, and 8 ꢁ 1018 cmꢂ3, respectively. It can be observed
that boron and phosphorus diffused in the i-Si layer, and the
concentration of these species in the i-Si layer fell below the
detection limit within 20–40 nm from each interface. We
also confirmed that the boron diffuses extensively into the i-
Si layer at Tp > 390 ꢀC (data not shown). Therefore, the
upper limit of the fabrication temperature for the p-i-n struc-
ture with a satisfactory interface is 390 ꢀC. Although a low
Tp is preferable to minimize the impurity diffusion, the lower
limit of the fabrication temperature for obtaining a-Si:H
layers was 360 ꢀC. The film oxidizes easily in the air at
Tp < 360 ꢀC because the pyrolytic transformation of the
polydihydrosilane into a-Si:H is not enough in low Tp.
Figure 3(b) shows that carbon, oxygen, and nitrogen are dis-
tributed in the p-i-n structure with a concentration of
1019–1021 cmꢂ3, which increases at each interface. This con-
centration may be affected by the tools, the solvent, and the
atmosphere in the glove box.
Figure 4 shows J–V characteristics at 25 ꢀC of the
solution-processed solar cells with and without the
hydrogen-radical treatment. The J–V curves denoted by c1,
c2, and c3 (c01, c02, and c03) correspond to cells 1, 2, and 3
with (without) the hydrogen-radical treatment, respectively.
Moreover, photovoltaic parameters such as the open-circuit
voltage (Voc), short-circuit current (Jsc), fill factor (FF), and
energy conversion efficiency (g) of cells 1, 2, and 3 as esti-
mates from c1, c2, and c3 curves, respectively, are shown in
the figure.
FIG. 3. Depth profiles of impurities in cell 1 measured by SIMS. (a) Boron,
phosphorus, and zinc. (b) Carbon, oxygen, and nitrogen. Tp ¼ 390 ꢀC.
Regarding the photovoltaic parameters of the cells with
the hydrogen-radical treatment, cell 3 shows the highest Jsc
among all the cells and is characterised by low Voc and the
lowest values of FF among the cells. The highest Jsc for cell
3 with a thicker i-Si layer is supported by the largest number
of photo-generated carriers in the i-Si layer compared with
those of cells 1 and 2. The lowest FF value might result
from the film quality of the i-Si layer, as shown in the micro-
graphs of the i-Si surface for cell 1 (Figure 5(a)) and cell 3
(Figure 5(b)).
Cell 3 alone displayed a few cracks (arrows in Figure
5(b)) and asperity generated by large shrinkage, which is
associated with pyrolysis of the thick (400 nm) i-Si layer.
These defects may become a leakage current path and result
in the decay of the insulation of the cell. Our experimental
results suggest that the defect- and crack-free i-Si layers may
improve the value of photovoltaic parameters. However, it
was difficult to fabricate a thick film (>300 nm) with high
A comparison of c1–c3 with c01–c03 indicates that the
hydrogen-radical treatment for the solution-processed solar
cells significantly improved Jsc from <0.2 mA/cm2 to
>1.7 mA/cm2. Accordingly, g improved from <0.01% to
>0.30% for all cells. This enhancement in Jsc by the
hydrogen-radical treatment may be because of the large
improvement in photoconductivity for the i-Si layer, as
shown in Table I. Therefore, we believe that the hydrogen-
radical treatment is essential for the fabrication of solar cells
using our solution-processed a-Si:H films.
TABLE II. Thickness and dopant concentrations of the p-, i-, and n-Si
layers in cells 1, 2, and 3.
Cell 1
Cell 2
Cell 3
n-Si: thickness
(concentration)a 30 nm (2 ꢁ 1021
FIG. 4. J–V characteristics of the solar cells with the top electrode size of
5 ꢁ 5 mm2. The curves denotes by c1, c2, and c3 (c01, c02, and c03) represent
the J–V curves for cells 1, 2, and 3 with (without) the hydrogen-radical treat-
ment, respectively. The inset shows the photovoltaic parameters and energy
conversion efficiency for cells 1, 2, and 3 with the hydrogen-radical treat-
ment. The curves were measured using the solar simulator under the illumi-
nation condition of AM-1.5G (100 mW/cm2).
)
)
30 nm (2 ꢁ 1021
)
)
30 nm (2 ꢁ 1021
)
)
i-Si: thickness
p-Si: thickness
120 nm
120 nm
400 nm
(concentration)a 30 nm (7 ꢁ 1021
30 nm (1 ꢁ 1021
30 nm (7 ꢁ 1020
aUnit is in cmꢂ3
.
Downloaded 24 Jun 2012 to 152.3.102.242. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

253908-4
Masuda et al.
Appl. Phys. Lett. 100, 253908 (2012)
In conclusion, we have fabricated a-Si:H p-i-n structure
by employing a solution-based process using doped and non-
doped polydihydrosilane solutions. Further, we have demon-
strated the effectiveness of hydrogen-radical treatment for
improving the conductivity of a-Si:H layers by a reduction in
spin density. The solution-processed a-Si:H solar cells are
shown to have g of 0.31%–0.51% under AM-1.5G (100mW/
cm2) illumination. Even though g is low, the formation of the
solution-processed silicon p-i-n structure indicates that the
solution-based process might be a promising technology for
future application to silicon devices. We also believe that the
characteristics of our solar cells will be improved by optimiza-
tion of processes and introducing the light-trapping structure.
FIG. 5. Optical micrographs in the i-Si surface of (a) cell 1 and (b) cell 3.
uniformity and no cracks using the spin-cast method under
the conditions employed at this time. Therefore, it is neces-
sary when fabricating high-performance solution-processed
solar cells to understand the liquid engineering principles for
coating a thick film on a substrate and also to employ suita-
ble coating techniques for a large substrate, such as an ink-
jet or slit coating method.
1J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T. Q. Nguyen, M. Dante, and
A. J. Heeger, Science 317, 222 (2007).
2W. Wang, Y.-W. Su, and C.-H. Chang, Sol. Energy Mater. Sol. Cells 95,
2616 (2011).
3T. Shimoda, Y. Matsuki, M. Furusawa, T. Aoki, I. Yudasaka, H. Tanaka,
H. Iwasawa, D. Wang, M. Miyasaka, and Y. Takeuchi, Nature (London)
440, 783 (2006).
4T. Masuda, Y. Matsuki, and T. Shimoda, J. Colloid Interface Sci. 340, 298
(2009).
In addition to improving the quality of thick i-Si layer,
there are three problems still unsolved, which are as follows:
(1) reduce the concentration of carbon, oxygen, and nitrogen
(of the order of 1019–1021 cmꢂ3) causing the donor-like
state,11 (2) decrease the Schottky-barrier height at the p-Si/
ZnO interface, which results in a large deviation from ideal
J–V curves, and (3) remove native silicon oxide layers
between each silicon layer, which acted as resistive compo-
nents in the cells. We believe that increasing g can be
achieved by overcoming these issues as well as preparing a
high-quality thick i-Si layer.
5T. Masuda, Y. Matsuki, and T. Shimoda, Polymer 53, 2973 (2012).
6T. Masuda, Y. Matsuki, and T. Shimoda, Thin Solid Films 520, 5091
(2012).
7H. Tanaka, H. Iwasawa, D. Wang, N. Toyoda, T. Aoki, I. Yudasaka, Y.
Matsuki, T. Shimoda, and M. Furusawa, Jpn. J. Apply. Phys. 46, L886
(2007).
8S. Kalbitzer, G. Muller, P. G. Le Comber, and W. E. Spear, Philos. Mag.
B 41, 439 (1980).
9A. Izumi and H. Matsumura, Jpn. J. Appl. Phys. 41, 4639 (2002).
10Thin-Film Silicon Solar Cells, edited by A. Shah (EEPL, 2010), pp. 17–96.
11T. Kinoshita, M. Isomura, Y. Hishikawa, and S. Tsuda, Jpn. J. Appl. Phys.
35, 3819 (1996).
Downloaded 24 Jun 2012 to 152.3.102.242. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions