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applications and materials science
Fabrication of dye sensitized solar cell
using Cr doped Cu–Zn–Se type
chalcopyrite thin film
Phys. Status Solidi A 208, No. 9, 2215–2219 (2011) / DOI 10.1002/pssa.201026368
D. Paul Joseph , S. Ganesan , M. Kovendhan , S. Austin Suthanthiraraj , P. Maruthamuthu* ,
and C. Venkateswaran**
Materials Science Centre, Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai-600 025, India
Department of Energy, University of Madras, Guindy Campus, Chennai 600 025, India
Department of Physics, Presidency College, Chennai 600 005, India
Present address: Center for Condensed Matter Sciences, National Taiwan University, Taipei, 10617 Taiwan
Received 1 July 2010, revised 25 November 2010, accepted 18 April 2011
Published online 19 May 2011
Keywords chalcopyrite, DMS, ferromagnetism, dye sensitized solar cells
* Corresponding
** e-mail
authors: e-mail [email protected],
[email protected], Phone: þ91-44-22202803, Fax: þ91-44-22352870
Chalcopyrites are a versatile class of semiconductors known
for their potential in photovoltaic applications. Considering
the well established CuInSe2 as a prototype system, a new
compound of the chalcopyrite type, Cu1–xZn1–ySe2–d, by
replacing In with Zn, has been prepared (both undoped and
2% Cr doped) by the metallurgical method. Thin films have
been deposited by the thermal evaporation technique using the
stabilized polycrystalline compounds as charge. Structural,
compositional, morphological, and optical properties of the
films are analyzed and reported. Use of these films as electrodes
in dye sensitized solar cell (DSSC) is demonstrated.
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Chalcopyrites are a versatile class of
semiconductors known for their potential in photovoltaics
and non-linear applications [1–3]. They tolerate a large range
of anion to cation off stoichiometry termed as ‘‘Ordered
Defect Compound’’ (ODC) with large variations in cation to
anion ratio [4]. Recently, Tani et al. [5] have interestingly
shown in their materials design a positive mixing energy for
CuInSe2 and CuGaSe2 leading to spinodal nano-decomposition in CuIn1–xGaxSe2 for high efficiency solar energy
conversion. Also, there is an intense search for semiconducting materials possessing inherent ferromagnetic properties at
or above room temperature with appreciable magnetization
for spintronics applications. Several oxides such as ZnO,
TiO2, SnO2, etc. are explored as host materials for preparing
diluted magnetic semiconductors (DMS) [6–12]. Apart from
oxide system, chalcopyrites are also being explored for its
suitability as a DMS.
Recently, researchers disclosed the achievement of room
temperature ferromagnetism in ‘‘Mn’’ doped II–IV–V2
chalcopyrites such as CdGeP2, ZnGeP2, etc. [13–15]. First
principles study of 64 different types of magnetically doped
chalcopyrites by Erwin and Zutic [16], lead to the
identification of some compositions in which room temperature ferromagnetism can be achieved. On the experimental
side, the behavior of transition-metal-doped chalcopyrites
is yet to be clearly understood, especially in terms of
identifying and optimizing the properties. Few reports on
chalcopyrite compounds exist in literature with 3d elements
as cations A and B in the ABC2 structure, like CuFeS2
[17, 18]. As in the case of Cu doped ZnSe [19], Cu and Zn in
the ratio 1:1 in ZnSe may lead to the new combination,
CuZnSe2–d [20], with a chalcopyrite structure. Considering
the well established CuInSe2 as a prototype system, a new
compound of the chalcopyrite type has been recently
reported by replacing In with Zn [21]. The new I–II–VI2
compound CuZnSe2 is also magnetically doped and explored
for ferromagnetic ordering [21]. First, polycrystalline
undoped and Cr doped CuZnSe2 compounds were prepared
by the metallurgical method and using them as charge, thin
films have been deposited by thermal evaporation. The
motivation of this work is twofold: first, to prepare thin films
from previously stabilized [21] polycrystalline undoped and
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
D. Paul Joseph et al.: Fabrication of dye sensitized solar cell
Cr doped Cu1–xZn1–ySe2–d compound, second, to fabricate
dye sensitized solar cell (DSSC) using these films as
2 Experimental High pure elements of Cu, Zn, Se,
and Cr weighed for the stoichiometry CuZnSe2 and
CuZnSe2:Cr (2%), respectively, were taken in a quartz tube
which was evacuated (above 1 105 Torr) and carefully
sealed. The contents were induction melted initially
and mounted vertically in a box furnace and soaked at
1000 8C/12 h and furnace cooled. This process was repeated
three times by placing the tube upside down and vice versa to
attain homogeneity. The structural and magnetic characterization of the bulk samples are reported elsewhere [21].
Finally, the retrieved sample, brittle and light green in color,
was then powdered using a clean pestle and mortar.
Thin film deposition was carried out in a thermal coating
unit (Hindhivac 1200 Vacuum Coating Unit, Model 12A4D)
using the stabilized polycrystalline un-doped and 2% Cr
doped Cu1–xZn1–ySe2–d compounds with a unique vaporization point allowing formation of a uniform thin film of the
multi-component alloy. Ultrasonically cleaned <100>
oriented Si, quartz, fluorine doped tin-oxide (FTO) glass,
and plain glass substrates were mounted over a stainless steel
substrate holder cum heater positioned above (’15 cm) the
tantalum boat. A K-type thermocouple was used to monitor
the substrate temperature. A vacuum of ’106 Torr was
achieved using a diffusion pump attached with a liquid
nitrogen trap. However, the vacuum during deposition was
’105 Torr due to the vapors from the samples. The
temperature of the tantalum boat containing the stabilized
polycrystalline pure and 2% Cr doped Cu1–xZn1–ySe2–d
powders was raised by resistive heating (above 100 A,
’ 1100 8C) using a transformer to sublime the charge. After
deposition, the substrate heater was switched off and the
vacuum was maintained till the substrates reached ambient
temperature so as to prevent accidental oxidation.
3 Results and discussion The XRD patterns of
Cu1–xZn1–ySe2–d and 2% Cr doped thin films deposited at
150 8C over Si wafers are shown in Fig. 1. The characteristic
peak of a chalcopyrite phase (112) confirm the formation of the
chalcopyrite type Cu1–xZn1–ySe2–d with a tetragonal structure
[21, 22]. The chalcopyrite phase with (112) orientation was
reported to be beneficial for efficient solar energy conversion
in CuInSe2-based solar cells [23]. The (112) and (323) peaks
belonging to the tetragonal phase of CuInSe2 chalcopyrite
are the characteristics of the polycrystalline nature of
the films (JCPDS 81-1936). The average crystallite size
estimated from the full-width at half the maximum of the
(112) plane using Scherer’s relation [24] were 22 and 25 nm
for the un-doped and Cr doped Cu1–xZn1–ySe2–d films,
Microstructural and chemical composition studies using
scanning electron microscope (SEM) (Hitachi-S-3400NSEM) were performed on the fabricated thin films for
examining the morphology and distribution of the
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1 XRD patterns of as deposited Cu1–xZn1–ySe2–d and
Cu1–xZn1–ySe2–d:Cr (2%) thin films over silicon substrates.
constituent elements. Elemental mapping was conducted
on a micron scale to check for Cr clustering if any. Figure 2
shows the SEM micrographs and elemental mapping of the
Cu1–xZn1–ySe2–d and Cu1–xZn1–ySe2–d:Cr (2%) thin film
samples. Green, pink, and blue represent the Cu, Zn, and Se,
respectively. The pink at the right end represents Cr. Cu
(green dots), Zn (pink dots), and Cr (pink dots) seen in the
films are quite uniform without major segregation. However,
a variation in stoichiometry of the constituent elements of the
synthesized chalcopyrite compounds is observed (Table 1).
The spectral transmittance data taken in a spectrophotometer (JASCO Corp., V-570, Rev. 1.00) (Fig. 3) show that
the pure and Cr doped films are 50% transparent in the visible
region, indicating a reasonably good light absorbing
behavior. The thickness (t) of the pure and Cr doped thin
films was estimated from the stylus profilometer to be 130
and 330 nm, respectively. The absorption coefficient (a)
decreases exponentially with decreasing photon energy,
indicating an Urbach characteristic [25]. The direct band gap
of undoped and Cr(2%) doped Cu1–xZn1–ySe2–d thin films,
determined from the (ahn)2 versus hn (Tauc relation) [26]
plot by extrapolating the linear fit given to the linear region to
a ¼ 0 (Fig. 4), are 2.832 and 2.882 eV, respectively.
Figure 2 (online colour at: www.pss-a.com) SEM micrographs
and corresponding elemental mapping (left to right) of undoped (top)
and Cr(2%) doped (bottom) Cu1–xZn1–ySe2–d thin films (150 8C on Si)
indicating the distribution of various elements in the compounds.
Phys. Status Solidi A 208, No. 9 (2011)
Table 1 Compositional analysis of thin films of Cu1–xZn1–ySe2–d
and Cu1–xZn1–ySe2–d:Cr (2%) from EDAX.
Cu1–xZn1–ySe2–d film
Cu1–xZn1–ySe2–d:Cr(2%) film
Figure 4 Band gap of undoped and 2% Cr doped Cu1–xZn1–ySe2–d
thin films.
Figure 3 Transmittance spectra of undoped and 2% Cr doped
Cu1–xZn1–ySe2–d compounds deposited on quartz substrates.
The 3D atomic force micrographs of the surface of FTO,
undoped and Cr(2%) doped Cu1–xZn1–ySe2-d thin films
(Fig. 5) were measured in dynamic mode over a 1 mm 1 mm
m area (AFM, Model SPI 6800N). The particles are not of
the same height and are distributed irregularly within the
measured region indicating polycrystalline nature of the
films. The estimated area root mean square (RMS) roughness
decreases on doping with Cr (Fig. 5).
Room temperature magnetization measurements on
the Cr(2%) doped Cu1–xZn1–ySe2–d thin film, deposited at
150 8C over a Si substrate, was carried out in a Vibrating
Sample Magnetometer (Lakeshore, 7404, USA) with a
maximum applied field of 1.5 T parallel to the film’s surface.
The diamagnetic contribution from the substrate was
subtracted and the ferromagnetic hysteresis loop without
saturation is shown in Fig. 6. The magnetization of bulk
Cr(2%) doped Cu1–xZn1–ySe2–d is large [21]. However, the
overall magnetization is very small owing to the small mass
of the thin film over the substrate. Magnetization in the
similar range was reported for thin films of Mn doped
CdGeP2 at 300 K [15, 27]. The coercivity was found to be
97 Oe. However, the origin of ferromagnetism observed in
Cr doped Cu1–xZn1–ySe2–d deserves much consideration,
as it may also be from the nanoscale clusters of spinodal
decomposition [21, 28–33].
The first DSSC introduced in 1991 by Regan and Gratzel
[34] has a low fabrication cost and a simple preparation
procedure [35]. The DSSC has three main components: (i) a
sensitized photoanode, which is typically a dye-sensitized
nanocrystalline TiO2 (Degussa) film on an FTO (F:SnO2)
conducting glass, (ii) an electrolyte solution containing KI
and I2 as redox couple, (iii) platinized FTO conducting glass
Figure 5 (online colour at: www.pss-a.com) Atomic force micrographs of the surfaces of (A) plain FTO, (B) Cu1–xZn1–ySe2–d and C)
Cu1–xZn1–ySe2–d (Cr2%) compounds depositedover FTO substrates.
The plot shows decreasing trend in the RMS roughness value.
as a counter electrode. The principle of DSSC involves the
photo-excitation of the sensitizer, followed by electron
injection into the conduction band of the semiconductor
oxide (TiO2). The dye molecule is regenerated by the redox
system, which itself is regenerated at the counter electrode by
electrons passing through the load. Here, [cis-dithiocyanoto
– N,N-bis (2,20 -bipyridyl-4,40 -dicarboxylic acid) ruthenium
(II)] dihdrate (N3) dye is used as the photo-sensitizer.
The counter electrode of DSSC is generally made of
platinum due to its enhanced conductivity and stability,
but it is more expensive. Many research groups tried
to replace the platinum electrode, for example with
porous carbon materials [36]. Wu et al. explored polypyrrole
(Ppy), one of the most intensively studied conducting
polymers. The overall energy conversion efficiency of
the DSSC with the Pt counter electrode reaches 7.6% [37].
Hard carbon spherules used as a counter electrode for
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
D. Paul Joseph et al.: Fabrication of dye sensitized solar cell
Figure 6 Hysteresis loop of Cu1–xZn1–ySe2–d:Cr (2%) film at
300 K. Inset shows the coercivity.
Figure 7 I–V curves obtained for the counter electrodes of
undoped and 2% Cr doped Cu1–xZn1–ySe2–d thin films at 1.5 AM
under 40 mW/cm2.
the DSSC show an efficiency of 5.7% under 100 mW/cm2
[38]. Chen et al. reported poly (3,4-ethylene dioxythiophene): poly(styrene sulfonate) coating on a conducting
glass (FTO) as a counter electrode [39]. In our studies, the
p-type undoped and Cr doped Cu1–xZn1–ySe2–d thin films
deposited on FTO glass plates were explored as counter
electrodes in DSSC.
Surface derivatization of nanoporous TiO2 coating [40]
was achieved by immersing the thin film electrode overnight,
in a 5 105 M ethanolic solution of the N3 dye at room
temperature. The above dye-sensitized coating was further
coated with an electrolyte solution containing KI and I2
(redox couple), 0.6 M of tetra butyl ammonium iodide, 0.1 M
LiI, 0.5 M 4-tert-butyl pyridine in acetonitrile solvent and it
was placed in between two electrodes without any special
The current–voltage (I–V) characteristics of our solar
cells were measured under illumination [overall area of
1 sq. cm with a tungsten halogen lamp (OSRAM, Germany)
of intensity 40 mW/cm2 using an EXTECH–33 model lightmeter with 1.5 AM by masking the remaining area with
teflon] by means of a BAS 100 A electrochemical analyzer.
The active area of the cell was 1 cm2. From the photovoltaic
measurement of the fabricated DSSC, the fill-factor (FF) and
the overall light to current energy conversion efficiency (h)
of the DSSC were calculated from,
FF ¼ ðVmax Jmax Þ=ðVoc Jsc Þ;
h ¼ ðVoc Jsc FF=Pin Þ100;
where Jsc is the short-circuit current density, Voc the opencircuit voltage, Pin the intensity of light power, and Jmax and
Vmax are the current density and voltage at the point of
maximum power output on the I–V curves, respectively. The
I–V curves (Fig. 7) of the DSSC with undoped and 2% Cr
doped Cu1–xZn1–ySe2–d thin films as couter electrodes were
measured under illumination of 40 mW/cm2. The photoelectric parameters of DSSCs such as Jsc, Voc, FF, and the overall
conversion efficiency (h) are listed in Table 2. The conversion
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Table 2 I–V characteristics of the DSSC using undoped and 2%
Cr doped Cu1–xZn1–ySe2–d thin film counter electrodes at 1.5 AM
under 40 mW/cm2.
(mV) (mA/cm2)
pure Cu1–xZn1–ySe2–d
TiO2/N3 dye/electrolyte/
2% Cr doped Cu1–xZn1–ySe2–d
TiO2/N3/electrolyte/Pt (standard) 620
0.51 2.2
0.51 4.2
efficiencies of undoped and 2% Cr doped Cu1–xZn1–ySe2–d
thin films deposited over FTO glass plates (as counter
electrodes) are 1.5 and 2.2%, respectively. Compared to
undoped Cu1–xZn1–ySe2–d, the efficiency of 2% Cr doped
Cu1–xZn1–ySe2–d is higher, indicating slight improvement on
doping with Cr. Though these values are lower than those of
the Pt counter electrodes, these new combination of
compounds are worth to explore for better efficiency.
4 Conclusions Undoped and 2% Cr doped Cu1–xZn1–y
Se2–d thin films with (112) texture were successfully
grown using the polycrystalline compound as charge. The
Cu1–xZn1–ySe2–d compound also has considerable band gap
with high absorption coefficient suitable for absorbing wide
range of wavelengths from the solar radiation. The Cr doped
Cu1–xZn1–ySe2–d film presents hysteresis behavior at room
temperature. The I–V characteristics indicate the potential of
Cu–Zn–Se chalcopyrite as counter electrodes for DSSC.
Though the synthesis procedure for controlling the stoichiometry has not yet been achieved, these preliminary
experimental observations underline the inherent versatility
of the new combination of the chalcopyrite, making it a
candidate worth exploring for both for ferromagnetic
engineering and solar cell applications.
Acknowledgements The author C. V. thanks the UGCDAE CSR, Indore centre for the CRS project. The authors are
Phys. Status Solidi A 208, No. 9 (2011)
grateful to Dr. Ajay Gupta, Dr. V. Ganesan, and Dr. V. R. Reddy,
UGC-DAE CSR, Indore for their help. The authors C. V. and D. P. J.
thank Prof. N. Victor Jaya of Anna University for his help. The
author D. P. J. thanks the CSIR, Govt. of India, for the award of
Senior Research Fellowship (2007) and Dr. A. Manigandan, Mr. V.
Anbarasan of Anna University and Mr. A. Sendilkumar (HCU) for
their help in various aspects.
[1] J. Schmidt, H. H. Roschar, and R. Labusch, Thin Solid Films
251, 116–120 (1994).
[2] A. M. Gabor, J. R. Tuttle, M. Contreras, D. S. Albin, A. Franz,
D. W. Niles, and R. Noufi, Proceedings of the 12th European
Photovoltaic Solar Energy Conference (H.S. Stephens and
Assoc., Amsterdam, 1994), pp. 939–943.
[3] A. Rockett and R. W. Birkmire, J. Appl. Phys. 70(7), R81–
R97 (1991).
[4] R. R. Philip and B. Pradeep, Thin Solid Films 472, 136–143
[5] Y. Tani, K. Sato, and H. Katayama-Yoshida, Appl. Phys.
Express 3, 101201 (2010).
[6] K. Rode, A. Anane, R. Mattana, J. P. Contour, O. Durand, and
R. Le Bourgeois, J. Appl. Phys. 93, 7676–7678 (2003).
[7] P. V. Radovanovic and D. R. Gamelin, Phys. Rev. Lett. 91,
157202 (2003).
[8] D. P. Joseph, G. Senthil Kumar, and C. Venkateswaran,
Mater. Lett. 59, 2720–2724 (2005).
[9] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa,
T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow,
S. Koshihara, and H. Koinuma, Science 291, 854–856
[10] S. B. Ogale, R. J. Choudhary, J. P. Buban, S. E. Lofland,
S. R. Shinde, S. N. Kale, V. N. Kulkarni, J. Higgins, C. Lanci,
J. R. Simpson, N. D. Browning, S. Das Sarma, H. D. Drew,
R. L. Greene, and T. Venkatesan, Phys. Rev. Lett. 91, 077205
[11] T. S. Herng, S. P. Lau, S. F. Yu, H. Y. Yang, K. S. Teng, and
J. S. Chen, J. Phys.: Condens. Matter 19, 356214 (2007).
[12] S. Deka and P. A. Joy, Solid State Commun. 142, 190–194
[13] G. A. Medvedkin, T. Ishibashi, T. Nishi, K. Hayata,
Y. Hasegawa, and K. Sato, Jpn. J. Appl. Phys. 39, L949–
L951 (2000).
[14] G. A. Medvedkin, K. Hirose, T. Ishibashi, T. Nishi, V. G.
Voevodin, and K. Sato, J. Cryst. Growth 236, 609–612
[15] K. Sato, G. A. Medvedkin, T. Ishibashi, S. Mitani,
K. Takanashi, Y. Ishida, D. D. Sarma, J. Okabayashi,
A. Fujimori, T. Kamatani, and H. Akai, J. Phys. Chem. Solids
64, 1461–1468 (2003).
[16] S. C. Erwin and I. Zutic, Nature Mater. 3, 410–414
[17] B. R. Pamplin, T. Kiyosawa, and K. Masumoto, Prog. Cryst.
Growth Charact. 1, 331–387 (1979).
[18] R. C. Weast and M. J. Astle, Handbook of Chemistry and
Physics, 63rd ed. (Chemical Rubber Co., Boca Raton Fla.,
1982), p. B-121.
[19] Z. Ali, Fabrication of II-VI semiconductor thin films
and a study of structural, optical and electrical properties,
Ph. D. Thesis, Dept. of Physics, Quaid-i-Azam University,
Islamabad, Pakistan (2005).
[20] http://www.,wipo.,int/pctdb/en/wo.,jsp?IA=EP2001007461,
[21] D. P. Joseph and C. Venkateswaran, Phys. Status Solidi A
207(11), 2549–2552 (2010).
[22] T. Wada, H. Kinoshita, and S. Kawata, Thin Solid Films 431/
432, 11–15 (2003).
[23] C. R. Abernathy, C. W. Bates, A. A. Anani, B. Haba, and
G. Smestad, Appl. Phys. Lett. 45(8), 890–892 (1984).
[24] B. D. Cullity, Elements of X-ray Diffraction, second ed.
(Addison-Wesley, Ontario, 1977).
[25] A. K. Abbas and M. T. Mohammed, Phys. Status Solidi A
100, 633–637 (1987).
[26] J. Tauc, Amorphous and Liquid Semiconductors (Plenum
Press, New Yok, 1974).
[27] K. Sato, G. A. Medvedkin, T. Nishi, Y. Hasegawa,
R. Misawa, K. Hirose, and T. Ishibashi, J. Appl. Phys.
89(11), 7027–7029 (2001).
[28] T. Fukushima, K. Sato, H. Katayama-Yoshida, and P. H.
Dederichs, Jpn. J. Appl. Phys. Part 2 45(12–16), L416–L418
[29] H. Katayama-Yoshida, K. Sato, T. Fukushima, M. Toyoda,
H. Kizaki, V. A. Dinh, and P. H. Dederichs, Phys. Status
Solidi A 204(1), 15–32 (2007).
[30] K. Sato, H. Katayama-Yoshida, and P. H. Dederichs,
Jpn. J. Appl. Phys. 44, Part 2 (28-32), L948–L951
[31] H. Katayama-Yoshida, K. Sato, T. Fukushima, M. Toyoda,
H. Kizaki, V. A. Dinh, and P. H. Dederichs, J. Magn. Magn.
Mater. 310(2), 2070–2077 (2007).
[32] K. Sato, T. Fukushima, and H. Katayama-Yoshida, Jpn. J.
Appl. Phys. 46, Part 2 (25-28), L682–L684 (2007).
[33] K. Sato, W. Schweika, P. H. Dederichs, and H. KatayamaYoshida, Phys. Rev. B 70(20), 201202 (2004).
[34] B. O(. Regan and M. Gratzel, Nature 353, 737–740 (1991).
[35] M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker,
E. Muller, P. Liska, N. Vlachopoulos, and M. Gratzel, J. Am.
Chem. Soc. 115, 6382–6390 (1993).
[36] A. Kay and M. Gratzel, Sol. Energy Mater. Sol. Cells 44, 99–
117 (1996).
[37] J. Wu, Q. Li, L. Fan, Z. Lan, P. Li, J. Lin, and S. Hao, J. Power
Sources 181, 172–176 (2008).
[38] Z. Haung, X. Liu, D. Li, Y. Luo, H. Li, W. Song,
L. Chen, and Q. Meng, Electrochem. Commun. 9,
596–598 (2007).
[39] J. G. Chen, H. Y. Wei, and K. C. Ho, Sol. Energy Mater. Sol.
Cells 91, 1472–1477 (2007).
[40] P. M. Sirimanne, T. Shirata, T. Soga, and J. Jimbo, Solid State
Chem. 166, 142–147 (2002).
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