Photovoltaic characteristics of postdeposition iodine-doped amorphous carbon films by microwave surface wave plasma chemical vapor deposition

Article abstract of DOI:10.1063/1.2089168

The amorphous carbon thin films have been deposited on silicon and quartz substrates by microwave surface wave plasma chemical vapor deposition at low temperature (<100°C) in Ar CH4 phase gas. Doping of iodine has been done in the postdeposited films by exposing them in iodine vapor. The photovoltaic measurements of the films were carried out before and after iodine doping. The results show dramatic decrease of optical gap from 3.4 to 0.9 eV corresponding to nondoping to iodine doping conditions, respectively. The preliminary photovoltaic characteristics of the film deposited on n -type silicon substrate under light illumination (AM1.5, 100 mW cm2) reveal a short-circuit current density of 1.15 μA cm2, open-circuit voltage of 177 mV and fill factor of 21.7%.

Full text of DOI:10.1063/1.2089168

APPLIED PHYSICS LETTERS 87, 161912 ͑2005͒
Photovoltaic characteristics of postdeposition iodine-doped
amorphous carbon films by microwave surface wave plasma
chemical vapor deposition
a͒
Ashraf M. M. Omer and Sudip Adhikari
Department of Electrical and Electronic Engineering, Chubu University, 1200 Matsumoto-cho,
Kasugai 487-8501, Japan
Sunil Adhikary, Hideo Uchida, and Masayoshi Umeno
Department of Electronics and Information Engineering, Chubu University, 1200 Matsumoto-cho,
Kasugai 487-8501, Japan
͑
Received 25 February 2005; accepted 16 August 2005; published online 14 October 2005͒
The amorphous carbon thin films have been deposited on silicon and quartz substrates by
microwave surface wave plasma chemical vapor deposition at low temperature ͑Ͻ100°C͒ in
Ar/CH phase gas. Doping of iodine has been done in the postdeposited films by exposing them in
4
iodine vapor. The photovoltaic measurements of the films were carried out before and after iodine
doping. The results show dramatic decrease of optical gap from 3.4 to 0.9 eV corresponding to
nondoping to iodine doping conditions, respectively. The preliminary photovoltaic characteristics of
2
the film deposited on n-type silicon substrate under light illumination ͑AM1.5, 100 mW/cm ͒ reveal
2
a short-circuit current density of 1.15 ␮A/cm , open-circuit voltage of 177 mV and fill factor of
21.7%. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2089168͔
Carbon ͑C͒ is an attractive material for low cost and
reasonably high efficiency solar cells. This remarkable ele-
ment exists in different forms ranging from insulator dia-
mond to metallic graphite to conducting/semiconducting
source about 2.45 GHz microwave having up to 6 kW. Also,
this method is useful to avoid plasma induced damages of
the substrate surface. The CVD chamber was evacuated to a
base pressure at approximately 3.5ϫ10− Pa using a turbo-
molecular pump. For film deposition, we used Ar as carrier
4
1
1–7
nanotubes. In recent research, amorphous carbon ͑a-C͒
has been shown to behave as a semiconducting material,
which is able to accept dopants, shows photoconductivity
gas and CH as source gas, and the gas flow rates were
4
maintained as 280 and 10 sccm, respectively. The microwave
power and gas composition pressure in the CVD chamber
were maintained at 500 W and 60 Pa, respectively. The du-
ration of film deposition for this experiment was 60 min. The
substrates were cleaned beforehand by acetone and methanol
in ultrasonic bath and then rinsed by ultrapure water. The Si
substrates were etched with diluted hydrofluoric acid ͑10%͒
in order to remove the resistive native oxide layer over the
surface. For I doping, we employed the pyrolysis of the MW
SWP CVD deposited a-C films to I vapor for about 10 min-
2
and is suitable for optoelectronic devices. However, there
are some problems such as low conversion efficiency due to
2
3
the sp /sp bonding structure and the difficulties in control-
ling the conduction type, carrier concentration and optical
3
band gap. When we attempt to utilize a-C as an alternative
material in photovoltaic applications, control of the conduc-
4
tion type of a-C film is indispensable. Effective doping can
modify optical and electronic properties, in particular optical
band gap and photoconductivity, in semiconductor
5
materials. Doping of a-C with n- and p-type dopants such as
3
–7
P, N and B has been attempted by several researchers.
However, the doping prospect of a-C remains unclear. A
new method of doping by iodine ͑I͒ called oxidation has
8
been introduced by Shirakawa et al. for hydrocarbon
9
–11
polyacetylene ͑CH͒x.
In this letter, we report the effects of postdeposition solid
phase I doping on the photovoltaic properties of a-C thin
films deposited on n-type silicon ͑n-Si͒ and quartz substrates
by microwave ͑MW͒ surface wave plasma ͑SWP͒ chemical
vapor deposition ͑CVD͒ at low temperature ͑Ͻ100°C͒. The
films were deposited on quartz and n-Si substrates by MW
1
2,13
SWP CVD, a newly developed deposition method.
Fig-
ure 1 shows the schematic drawing of the CVD system. The
stage diameter of MW SWP CVD is about 20 cm, so that it
allows deposition of relatively large area thin films, which is
generally required for a high power electromagnetic wave
a͒Author to whom correspondence should be addressed; electronic mail:
e03801@isc.chubu.ac.jp
FIG. 1. Schematic diagram of MW SWP CVD system.
0003-6951/2005/87͑16͒/161912/3/$22.50
87, 161912-1
© 2005 American Institute of Physics

161912-2
Omer et al.
Appl. Phys. Lett. 87, 161912 ͑2005͒
FIG. 2. Schematic diagram of cross-sectional view of the postdeposition
I-doped a-C based device.
utes. Pyrolysis was carried out using a double furnace setup,
one for heating the source material ͑I͒ placed in a quartz boat
and the other for controlling the sample temperature, using
1
4,15
Ar as a carrier gas.
A JASCO V-570 ultraviolet visible near-infrared ͑UV/
VIS/NIR͒ spectrophotometer is used to investigate the opti-
cal properties of the film. While Nanopics 2100/NPX200 is
used to measure the film thickness. A current-voltage ͑I-V͒
characteristic of a-C/n-Si structure was measured by a
JASCO SS-200W solar simulator.
The films deposited in this experiment are diamond-like
a-C, which promises as a good quality semiconducting thin
film for optoelectronic devices. Figure 2 shows the schematic
diagram of cross-sectional view of the postdeposition
I-doped a-C ͑a-C:I͒ based photovoltaic device.
FIG. 4. Plots of ͑␣h␯͒1/2 vs photon energy for the determination of the a-C
film’s optical gap.
a-C films results in more conductivity because the I content
in the films induced graphitization by decreasing the
optical gap.
Figure 5 shows the I-V characteristics of the a-C:I based
6,14
device. It shows a rectifying curve in the dark,
indicating
the formation of a heterojunction between the a-C:I film and
n-Si. When illuminated by the simulated solar light ͑AM1.5,
2
1
00 mW/cm ͒, this structure shows the photovoltaic behav-
To study the optical characteristics of a-C films, we car-
ried out the reflectance and transmittance measurements by
UV/VIS/NIR spectroscopy in the range of 200–2000 nm.
The absorption coefficient ͑␣͒ was calculated by the spectral
reflectance and transmittance, and the film thickness data.
ior with a maximum open-circuit voltage ͑V ͒ of 177 mV
and short-circuit current density ͑J ͒ of 1.15 ␮A/cm . Ac-
oc
2
sc
cording to calculation, the fill factor was found to be 21.7%.
Although undoped a-C is reported theoretically to have
weakly p-type conductivity in nature, the high density of
intrinsic defects restricts its ability to dope efficiently, which
is the main barrier for its electronic devices application when
we attempt to utilize a-C as alternative material in optoelec-
tronic devices. Hence, control of the conduction type of a-C
film is indispensable. Unlike Si the microstructure of C is
more complicated due to the presence of both ␴ and ␲ states
while Si and other group IV elements have only ␴ states.
Since ␲ states are more weakly bonded, they lie closer to the
1
6,17
The optical band gaps were obtained by Tauc plot.
The
Tauc optical band gap ͑E ͒ was obtained from the extrapola-
g
tion of the linear part of the curve at ␣=0 by using the Tauc
equation
␣h␯͒1/2 = B͑E − h␯͒,
͑
͑1͒
g
4
where B is the density of the localized state constant. Figure
shows a comparison of the dependence of ␣ on spectral
3
wavelength for the a-C film ͑thickness: 205 nm͒ before and
Fermi level ͑E ͒ than the ␴ states. Therefore, filled ␲ states
F
after I doping. The ␣ of the doped film in the visible wave-
5
−1
length region was found to be approximately 10 cm . Fig-
ure 4 shows that the optical gap dropped from 3.4 to 0.9 eV
corresponding to nondoping to I-doping conditions respec-
tively. Our results of optical gap suggest that the I doping in
FIG. 5. The current-voltage ͑I-V͒ characteristics of a-C/n-Si structure un-
2
FIG. 3. Absorption coefficients of the a-C film vs spectral wavelength
before and after I doping.
der dark and light illumination ͑AM1.5, 100 mW/cm ͒. Illuminated area is
2
about 0.5 cm .

161912-3
Omer et al.
Appl. Phys. Lett. 87, 161912 ͑2005͒
from valence bands and empty ␲ states from conduction
This work was supported by the New Energy and Indus-
trial Technology Development Organization ͑NEDO͒ under
the Ministry of Economy, Trade and Industry ͑METI͒, Gov-
ernment of Japan. One of the authors, A.M.M.O., is grateful
to ASJA International for its financial support.
4
,5
bands determine the characteristics of gap states. The
bonds ␴ and ␲ are fixed and immobile. They form the cova-
lent bonds between C atoms. The electrons in a conjugated
double bond system are also relatively localized, though not
as strongly bound as the electrons. Before a current can flow
along the molecule, one or more electrons have to be re-
moved or inserted. If an electrical field is then applied, the
electrons constituting the bonds can move rapidly along the
molecule chain. As mentioned before, the doping by I is
1
K. M. Krishna, T. Soga, K. Mukhopadhyay, M. Sharon, and M. Umeno,
Sol. Energy Mater. Sol. Cells 48, 25 ͑1997͒.
S. M. Mominuzzaman, K. M. Krishna, T. Soga, T. Jimbo, and M. Umeno,
2
Jpn. J. Appl. Phys., Part 2 38, 658 ͑1999͒.
Y. Hayashi, S. Ishikawa, T. Soga, M. Umeno, and T. Jimbo, Diamond
3
called oxidation, which was introduced for ͑CH͒ , and it is
x
Relat. Mater. 12, 687 ͑2003͒.
M. Rusop, S. M. Mominuzzaman, T. Soga, T. Jimbo, and M. Umeno, Jpn.
4
shown in the following formula:
J. Appl. Phys., Part 1 42, 2339 ͑2003͒.
+
n
−
͑
CH͒ + ͑3n/2͒I → ͓͑CH͒ ͔ + nI ,
͑2͒
5
x
2
x
3
C. W. Chen and J. Roberson, Carbon 37, 839 ͑1999͒.
X. M. Tian, M. Rusop, Y. Hayashi, T. Soga, T. Jimbo, and M. Umeno, Sol.
6
where n is the number of molecules. In this doping, the io-
dine molecule ͑I ͒ attracts an electron from the ͑CH͒ chain
and becomes I . The ͑CH͒ molecule is now positively
Energy Mater. Sol. Cells 77, 105 ͑2003͒.
V. S. Veerasamy, J. Yuan, G. A. J. Amaratunga, W. I. Milne, K. W. R.
7
2
x
−
3
x
Gilkes, M. Weiler, and L. M. Brown, Phys. Rev. B 48, 17954 ͑1993͒.
H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang, and A. J.
8
charged. The lonely electron of the double bond, from which
an electron was removed, can move easily. Consequently, the
double bond successively moves along the molecule. The
positive charge, on the other hand, is fixed by electrostatic
attraction to the iodide ion, which does not move so readily.
Since we need to increase the conductivity of a-C we applied
this new doping method that was successful in conducting
polymer.
In conclusion, we investigated the photovoltaic charac-
teristics of the postdeposition solid-phase I-doped a-C films
deposited by MW SWP CVD. To achieve our target to de-
velop a carbon-based device for photovoltaic applications,
we need to improve the optoelectronic properties of the
films, such as absorption and photoconductivity. We suc-
ceeded to narrow the band gap of the films to 0.9 eV by I
doping. Further research is in progress to optimize the dop-
ants and to improve quality of the films by reducing the
bonding defects.
Heeger, J. Chem. Soc., Chem. Commun. ͑1977͒ p. 578.
C. K. Chiang, C. R. Fincher, Jr., Y. W. Park, A. J. Heeger, H. Shirakawa,
9
E. J. Louis, S. C. Gau, and A. G. MacDiarmid, Phys. Rev. Lett. 39, 1098
10͑
1977͒.
C. K. Chiang, Y. W. Park, A. J. Heeger, H. Shirakawa, E. J. Louis, and
A. G. MacDiarmid, J. Chem. Phys. 69, 5098 ͑1978͒.
C. K. Chiang, M. A. Druy, S. C. Gau, A. J. Heeger, E. J. Louis, A. G.
MacDiarmid, Y. W. Park, and H. Shirakawa, J. Am. Chem. Soc. 100, 1013
11
2͑
1978͒.
1
1
1
1
M. Nagatsu, T. Sano, N. Takada, N. Toyoda, M. Tanga, and H. Sugai,
Diamond Relat. Mater. 11, 976 ͑2002͒.
M. Nagatsu, T. Sano, N. Takada, W. X. Guang, T. Hirao, and H. Sugai,
Jpn. J. Appl. Phys., Part 2 39, L929 ͑2000͒.
K. M. Krishna, M. Umeno, Y. Nukaya, T. Soga, and T. Jimbo, Appl. Phys.
Lett. 77, 1472 ͑2000͒.
3
4
5
K. M. Krishna, Y. Nukaya, T. Soga, T. Jimbo, and M. Umeno, Sol. Energy
Mater. Sol. Cells 48, 163 ͑2001͒.
16
J. Tauc, Amorphous and Liquid Semiconductors ͑Plenum, London, 1974͒,
Chap. 4.
1
7
J. Tauc, R. Grigorovici, and A. Vancu, Phys. Status Solidi 15, 627 ͑1966͒.