Growth of polycrystalline silicon solar cell on epitaxial thickening of AIC seed layer by
hot-wire CVD
Shui-Yang Lien a*, Jui-Hao Wang b, Dong-Sing Wuu c
a
b
Department of Materials Science and Engineering, MingDao University, ChungHua 52345, Taiwan,
Republic of China
Department of Materials Science and Engineering, National Chiao-Tung University, Hsinchu 300,
Taiwan, Republic of China
c
Department of Materials Engineering, National Chung Hsing University, Taichung 402, Taiwan, Republic
of China
E-mail address: syl@mdu.edu.tw ; Fax number: +886-48871020
Abstract
Large-grain polycrystalline silicon (poly-Si) films were prepared on foreign substrates by
epitaxial thickening of seed layers. The seed layers were formed by the aluminum-induced
crystallization (AIC) processes. Large-grain poly-Si layers were deposited on epitaxial seeds using
by hot-wire chemical vapor deposition (HWCVD). High crystalline fractions of 93%, a lateral grain
size of 5 µm and non-incubation growth of intrinsic layer have been made by this study. Using
these techniques, large-grain n-i-p poly-Si thin-film solar cell has been prepared. An initial
efficiency of 5.6 % was obtained for an ITO/n-i-p (HWCVD)/seed (AIC)/Ti/glass structured solar
cell.
Key words: Aluminum induced crystallization; Polycrystalline silicon; Hot-wire chemical vapor
deposition
The formation of high-quality polycrystalline silicon (poly-Si) on low cost substrates has
important applications in the development of electrical devices. Aluminum-induced crystallization
(AIC) is a promising technology for the low-temperature fabrication of large-area poly-Si as a
seeding layer for solar cell application. One of the characteristics that play a device role in the
suitability of any deposition process for device application is the deposition rate. Maintaining
device quality while increasing the deposition rate is a challenge to materials researchers. Hot-wire
chemical vapor deposition (HWCVD) is a popular technique for producing amorphous silicon (a-Si)
and poly-si material at a high deposition rate. Device quality a-Si films can be obtained by
HWCVD using pure silane (SiH4) without hydrogen dilution. Good quality poly-Si films are
obtained when the silane is diluted in hydrogen at a higher pressure. However, the deposition rate
decreases with increasing hydrogen dilution. Poly-Si films that deposited on glass directly are an
inhomogeneous growth process. The deposition generally begins with an amorphous phase. It
needs a minimum thickness called the incubation phase before a localized phase transformation
takes place, which is called nucleation. In order to make polycrystalline films at high deposition
rates without an incubation phase, a seed-layer deposition process is applied. The p+-type poly-Si
films (seed layer) is prepared on glass substrate. This seed layer is epitaxially thickened. We
focus on the properties of poly-Si deposited on seed layer using by high deposition rate, and present
the solar cell performance in this article.
In this study, the p+-type seed layers were prepared by AIC technology. The thin Titanium,
Aluminum and Si layers with thickness of 200 nm were deposited on glass substrate (Corning 1737)
using the Electron-beam evaporation and HWCVD, respectively. Thermal annealing was
performed in a conventional furnace at 450 
C for 5 hours. After the AIC process the Al(+Si) layer
on top of seed layer has to be removed. In this study, tow main types of polycrystalline silicon exist:
one with columnar oriented crystals (called Poly1) and the other with V shaped columns crystals
(called Poly2). Poly1 that as seed layers produced by AIC process. Poly2 is deposited using a low
hydrogen dilution of the silane source gas, at a much higher deposition rate of 5 nm/s.
Fig. 1 shows the Raman spectra of the Si films prepared under different growth techniques.
From Raman results, line (a) and line (b) shows the a-Si and poly-Si film that deposited by
HWCVD on the glass directly. This was deposited using low hydrogen dilution (30%) and high
hydrogen dilution (98%), respectively. It is indicate that the increase of hydrogen dilution leads to
increase of the crystallinity (Rc) of the material. Line (c) shows the properties of the poly-Si film
produced by AIC process. This material does not show any incubation phase (Raman shift between
480 and 510 cm-1). Line (d) shows that by using a seed layer which formed by AIC method, high
crystalline layers could be deposited by HWCVD using a lower hydrogen dilution. Without the use
of the seed layer, this lower hydrogen dilution yields amorphous layers.
The TEM cross-sectional image of Poly 2 (1200 nm)/Poly 1 (200 nm)/glass was shown in Fig.
2. poly-Si formed by AIC technique (Poly1) with columnar grains forming a compact structure. On
the other hand, films made by HWCVD (Poly 2) at a low hydrogen dilution on it were growth
along the AIC grains. The HWCVD poly-Si films show fully polycrystalline, with a V shaped
columns. The TEM image of poly-Si (HWCVD)/AIC seed layer/glass structure and its diffraction
pattern is shown in Fig. 3. The lateral grain size of the poly-Si films deposited on seed layer was
approximately 5 µm, which was large than that the poly-Si films deposited directly on glass by
HWCVD with high hydrogen dilution.
The current-voltage characteristic of poly-Si solar cell is shown in Fig. 4. The structure of
poly-Si thin film solar cells investigated here is schematically shown in Fig. 4 (inlet). This cell has
a fill factor of 0.63, an open-circuit voltage of 0.475 V, a short-circuit current density of 19
mA/cm2 and an initial efficiency of 5.6 %. The large-grain poly-Si films were produced by AIC
and HWCVD techniques. High efficiency of poly-Si thin film solar cell was obtained for an
Ag/ITO/n-i-p/AIC(p+)/Ti structured on glass substrate.
Intensity (arb. units)
Incubation
phase
Rc= 93%
(d)
(c)
Rc= 99%
(b)
Rc= 81%
Poly-Si
(HWCVD)
(a)
Rc= 5 %
420
460
500
540
Poly-Si (AIC)
glass
580
500 nm
-1
Raman shift (cm )
Fig. 1. Raman spectra of samples: (a) a-Si
films deposited by HWCVD, (b) poly-Si
films deposited by HWCVD, (c) AIC poly-Si
films and (d) Poly-Si (HWCVD)/Poly-Si
(AIC)/glass structure.
Fig. 2. Transmission electron microscopy
cross-section image of poly-Si film grown by
HWCVD on an AIC seed layer/glass.
Current density (mA/cm2)
20
16
Voc = 4.75 V
Jsc = 19 mA/cm2
FF = 62.3%
η= 5.6%
12
8
n
i
p
Ag
ITO
p-i-n (HWCVD)
+
4
p poly-Si (AIC)
Glass
Ti
0
1 µm
0
0.1
0.2
0.3
0.4
0.5
Voltage (V)
Fig. 3. Transmission electron microscopy
plane view of poly-Si (HWCVD)/AIC seed
layer/glass structure and its diffraction
pattern.
Fig. 4. Current-voltage characteristic of the
best poly-Si thin-film solar cells so far.