Flexible PV Technology Development Program at IIT Bombay
Siddhartha P. Duttagupta
Department of Electrical Engineering,
Indian Institute of Technology Bombay
Mumbai, India, Pin Code 400 076
Abstract-This
paper describes the ongoing flexible PV
technology program at the Indian Institute of Technology
Bombay. Flexible solar cells are fabricated by depositing
amorphous silicon (a-Si) on stainless steel substrate. We have
relied on hot-wire CVD technology to deposit fdms at 110 C, since
conventional PECVD processes have proven to he inadequate at
that temperature. The primary requirement was a high doping
concentration of the p-type and n-type a-Si layers. The efficiency
of the single-junction PV cells under simulated AM1.5 global
radiation initially was Z.S%, which improved to 4.8% following
optimization. Further improvements in efficiency will require
development of a technique for low temperature texturing of the
transparent conducting oxide film.
I. INTRODUCTION
Currently, the efficiency of commercial amorphous silicon
(a-Si) based PV technology is in the range of 6-9% [l]: Thus
a-Si technology can pose a challenge to the dominant
crystalline silicon (c-Si) technology only by offering radically
lower costs or application advantages. Such a possibility is
through the development of flexible PV technology. The a-Si
films are deposited on stainless steel [2] or polymer substrates
[3]. Flexible PV is lightweight and can be easily integrated
with various substrates. This can be a distinct advantage as an
energy source for space and military applications as well as
for Building Integrated PV (BIPV) systems. Currently,
PECVD based a-Si on polymer technology has been
commercialized with efficiencies of about 4% [4].
We are investigating the application of low temperature
hotwire CVD (HWCVD) based processes for fabricating PV
cells. HWCVD of a-Si:H films was firstly reported by
Wiesmann et al. [5] in 1979. At low deposition rates
HWCVD yields stable and dense films. At very high
deposition rates, as well at low temperature (< 250 "C) the
quality of the deposited films is superior to PECVD (no
plasma instability or powder formation). This makes it easier
to optimize device fabrication by slow growth of sensitive
components such as the thin window layer of solar cells on
one hand, while increasing the deposition rate to 1 n d s or
more in less critical parts, such as the intrinsic layer. Such
high deposition rates lead to lower module fabrication costs.
Also PV cells fabricated using HWCVD at low temperature
are expected to have higher efficiencies compared to those
fabricated using PECVD.
The first PV cell fabricated using HWCVD was reported by
Papadopulos et al. in 1993 [6]. They used a so-called
superstrate (pin) structure where only the intrinsic layer was
deposited by HWCVD. The device performance was q =
4.3%. In the following years the conversion efficiency of pin
(superstrate) as well as in nip (substrate) solar cells with
intrinsic H W C W absorber films has improved considerably.
The conversion efficiency was enhanced to q = 6.8 % in 1996
[7, 81 and q = 10.2 % [9] or q = 9.8 % [lo] in 1998 for the pin
or nip structure, respectively. In the nip cell yielding q = 9.8
% the i-layer was deposited with a very high rate of 1.6 n d s
[IO].
The a-Si:H films deposited by HWCVD at high
substrate temperatures (> 3 0 0 T ) also show enhanced stability
against light-induced degradation [9] as per the StaehlerWronski effect, which causes single-junction a-Si solar cells
to lose efficiency by as much as 30%. Therefore the HWCVD
method tums out to be a likely alternative to the industry
standard plasma-enhanced chemical vapor deposition
(PECVD).
11. EXPERIMENTAL
A . Facilifies
The Microelectronics Center at IIT Bombay has excellent
simulation, fabrication, and characterization facilities. The
PV cells were fabricated using a single-chamber HWCVD
tool which permits deposition of a-Si:H, pc-Si:H or a-Si-alloy
films (intrinsic, p-type, and n-type). It is also possible to
deposit very low-stress silicon nitride with the same tool.
B. Process details
The substrates used were Indiun Tin Oxide (ITO) coated
glass and ZnO coated Stainless Steel (SS). The polysilicon
films show excellent crystallinity and large grain size at high
filament temperatures (Figure 1). The a-Si films are grown at
110 OC. The doped films showed excellent conductivity
(Figure 2 ) . The final device structure is shown in Figure 3.
362
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Figure 4. PV I-V characteristics of flexible PV cell
C.Device Characteristics
10
20
30
40
28
50
60
The PV I-V characteristics of the devices were measured
under simulated AM1.5 (global) radiation (Figure 4). The
efficiency of the PV cells deposited on glass was 9.5%. The
efficiency of the PV cells deposited on Stainless Steel was
initially 2.8%. Further optimization increased the efficiency
to 4.8%. It was observed t h t further improvement in
performance would come from developing a low temperature
texturing technique on flexible substrates which would
improve the current collection efficiency (higher Jsc) and bring
it on par with the glass substrate based cells
70
Figure 1. Polysilicon and a-Si films deposited by HWCVD
111. CONCLUSION
Flexible PV cells with an efficiency of 4.8% have been
fabricated. We have also demonstrated high quality a-Si films
deposited at low temperatures (100 "C). Future work will
involve texturing of the TCO film in order to realize a 10%
efficient cell.
LR
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