View
42
Download
2
Category
Tags:
Preview:
Citation preview
Dynamic Article LinksC<Energy &Environmental Science
Cite this: Energy Environ. Sci., 2012, 5, 5340
www.rsc.org/ees PAPER
Dow
nloa
ded
on 0
9 A
pril
2012
Publ
ishe
d on
31
Oct
ober
201
1 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/C
1EE
0231
4DView Online / Journal Homepage / Table of Contents for this issue
A non-toxic, solution-processed, earth abundant absorbing layer for thin-filmsolar cells†‡
Kyoohee Woo, Youngwoo Kim and Jooho Moon*
Received 8th August 2011, Accepted 4th October 2011
DOI: 10.1039/c1ee02314d
Copper zinc tin sulfide (Cu2ZnSnS4, CZTS) has attracted significant attention in the past few years as
a next generation absorber material for the production of thin film solar cells on large scales due to the
high natural abundance of all constituents, tunable direct band gap energy ranging from 1.0 to 1.5 eV,
and large absorption coefficient. In addition, to address the issue of expensive vacuum-based processes,
non-vacuum solution-based approaches are being developed for CZTS absorber layer deposition.
Here, we demonstrate the fabrication of a high quality CZTS absorber layer with a thickness of 2.8–3.0
mm and micrometre-scaled grains (1–2.5 mm) using air-stable non-toxic solvent-based inks. Our
approach for the fabrication of CZTS absorber, reported here, will be the first step in achieving low-
cost and large area solar cells with high efficiency.
Copper zinc tin sulfide (Cu2ZnSnS4, CZTS) is a very promising
material for use as a low cost absorber alternative to other
chalcopyrite-type semiconductors based on Ga or In, because it
is only composed of abundant and economical elements.1–5 In
addition, CZTS has a direct band gap energy of 1.0–1.5 eV and
a large absorption coefficient of over 104 cm�1, properties similar
to those of Cu(In,Ga)Se2 (CIGS), which is regarded as one of the
best absorber materials for sustainable and highly efficient solar
cells.6–8 Typically, metal chalcogenide films such as CIGS and
CZTS are deposited by evaporation or sputtering techniques that
rely on vacuum environments.9–11 However, this vacuum depo-
Department of Materials Science and Engineering, Yonsei University, 50Yonsei-ro, Seodaemun-gu, Seoul, 120-749, Republic of Korea. E-mail:jmoon@yonsei.ac.kr; Fax: +82 2 312 5375; Tel: +82 2 2123 2855
† Electronic supplementary information (ESI) available: A detaileddescription of the experimental methods, surface SEM image ofas-prepared CZTS film, component depth profile of the CZTS filmwith the Cu-poor and Zn-rich composition for cell fabrication. SeeDOI: 10.1039/c1ee02314d
‡ The paper was presented in part at the International ChemicalCongress of Pacific Basin Societies (Pacifichem 2010), in Honolulu,Hawaii, USA, December 15–20, 2010.
Broader context
Solution processing for chalcogenide absorber materials in thin fi
materials have advantages including suitability for use in large-area
communication, we present a facile route to fabricate a Cu2ZnSnS
which commercially available precursor particles such as Cu2S, Z
efficiency of 5.14% under AM 1.5 illumination, the use of the n
convenient access to fabricate high quality CZTS absorber layers a
film solar cells.
5340 | Energy Environ. Sci., 2012, 5, 5340–5345
sition process suffers from relatively low throughput, low
material utilization, and difficulties associated with large-scale
production.12,13 In this regard, solution-based deposition
methods are being developed because they have advantages
including suitability for use in large-area substrates, high
throughput, and efficient materials usage.14–16 Various solution-
based approaches for the fabrication of CZTS thin films have
been reported including sol–gel17,18 and nanocrystal dispersion
processes,4,19 but they face some limitations. The sol–gel method
is vulnerable to contamination by carbon, oxygen, and other
impurities from precursors or starting solutions, which inevitably
leads to the formation of a porous structure with small grain size
due to significant shrinkage. The nanocrystal dispersion method
requires the complex synthesis of nanocrystals, and it is difficult
to achieve dense organic residue-free thick films from the
dispersion of nanocrystals capped with stabilizing molecules.
Recently, Todorov et al. reported the fabrication of CZTS thin
film solar cells with 9.6% power conversion efficiency (PCE)
using a hydrazine-based hybrid slurry approach.20,21 However,
hydrazine is a highly toxic and very unstable compound that
requires extreme caution during handling and storage.
lm solar cells is an attractive area of research because these
substrates, high throughput and efficient materials usage. In this
4 (CZTS) absorber layer using non-toxic solvent-based ink in
n, Sn, and S are dispersed. With our first cells exhibiting an
on-toxic precursor ink in a scalable coating process provides
t low cost and contributes to the large-scale deployment of thin
This journal is ª The Royal Society of Chemistry 2012
Dow
nloa
ded
on 0
9 A
pril
2012
Publ
ishe
d on
31
Oct
ober
201
1 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/C
1EE
0231
4D
View Online
Furthermore, due to the reactive nature of this solvent,22 all
processing for slurry and film preparations must be performed
under inert atmospheric conditions, and thus hydrazine would
not be easily adaptable for large-scale solar cell fabrication. With
these considerations, it is highly desirable to develop a robust,
easily scalable and relatively safe solution-based process for the
fabrication of a high quality CZTS absorber layer.
Here, we devise for the first time a non-toxic solvent-based
process for the fabrication of a dense CZTS absorber layer. The
slurry (or ink) employed for CZTS deposition is a commercially
available powder mixture of Cu2S, Zn, Sn, and S dispersed in
ethanol that is safe and easy to use. As an environmentally
benign solvent, ethanol was selected because it can evaporate
quickly and thus may minimize residual carbon- or oxygen-
containing impurities in the film. The slurry composition was
controlled to have the atomic ratio of Cu : Zn : Sn : S ¼2 : 1 : 1 : 4. Our simple slurry approach may encounter phase
segregation and the presence of unreacted species or other
unwanted intermediate compounds in the final film because the
precursor particles of Cu2S, Zn, and Sn are insoluble, unlike
hydrazine. In other words, to achieve our goal, the precursor
powders should be well-dispersed in the solvent and must be
reactive enough to be converted into CZTS granular films during
thermal treatment. We employed a milling process to grind
precursor powders to nanosize particles and to obtain homoge-
neous well-dispersed slurry. The large surface areas of finely
milled precursor particles can induce material transfer and
interparticle densification. In addition, some of the precursor
particles retain the low melting points of Zn (420 �C) and Sn
(231 �C), which will bring about reactive liquid-phase sintering
between constituent particles and/or intermediate compounds at
temperatures above 500 �C, even in the presence of Cu2S with its
high melting point (1130 �C).The thermal behavior of the CZTS precursor ink-containing
powder mixture (Cu2S, Zn, Sn, and S) was analyzed by thermo-
gravimetry coupled with differential scanning calorimetry (TG-
DSC) under a nitrogen atmosphere (150 cm3 min�1) as shown in
Fig. 1a. A weight loss of �2.7% accompanying the exothermic
peak at 200 �C is ascribed to the partial sublimation of sulfur,
while the endothermic peak at 480 �C likely results from the
crystallization of CZTS. The TG-DSC data indicate that the
precursor inks might be converted into the CZTS phase by
annealing at temperatures around 500 �C, lower than the glass
transition temperature (Tg) of the soda lime glass (SLG) that is
typically used to fabricate the CZTS thin film solar cells. The
phase development of the precursor films during annealing is
presented in Fig. 1b. Sharp peaks at 2q ¼ 28.45�, 47.3� and 56.2�
can be attributed to the diffraction of the (112), (220) and (312)
planes of kesterite structure CZTS (JCPDS no. 26-0575),
respectively, suggesting the formation of a CZTS crystalline
phase at temperatures ranging from 500 to 530 �C. Fig. 1c
presents SEM images showing the microstructural evolution of
the precursor films annealed under N2 + H2S (5%) atmosphere in
a tubular furnace at temperatures ranging from 400 to 530 �C for
30 min. The particle size in the as-prepared granular film was
smaller than �150 nm (see ESI, Fig. S1†). As the annealing
temperature increased, the films were gradually densified, while
the grain size increased. When annealed at over 530 �C, a rela-
tively dense structure with large grains (1–2.5 mm) and occasional
This journal is ª The Royal Society of Chemistry 2012
voids developed. Microstructural observations support the use of
particle mixture-based ink for the production of the solution-
processed absorbing layer.
It should be noted that three XRD diffraction peaks at around
2q ¼ 28.6�, 47.5� and 56.3� overlap with those of Cu2S and ZnS,
so the crystallization of CZTS cannot be confirmed solely by
XRD analysis. Therefore, Raman spectroscopy was utilized to
obtain further insight into the phase identification, and the
results of the CZTS films as a function of the annealing
temperatures are shown in Fig. 2. The as-prepared precursor
films exhibited a strong peak at 473 cm�1 as well as small peaks at
around 260 cm�1, which correspond to precursor components
such as Cu2S and ZnS. These undesirable phases disappear
completely when annealed at 530 �C, which is in good agreement
with the XRD results. For the sample annealed at 530 �C, peakswere observed at 251, 287, 338, and 368 cm�1; all of these peaks
can be assigned to kesterite CZTS.23,24 Raman analysis indicates
that the large surface area of the finely milled precursor particles
and the low melting points of Zn (420 �C) and Sn (231 �C)promoted the crystallization of CZTS at the temperature of
530 �C and allowed for the formation of a dense absorbing layer
by a reactive liquid phase sintering.
A cross-sectional image of a film annealed at 530 �C is shown
in Fig. 3a, in which a uniform dense structure without significant
large pores and/or cracks can be observed. It should be noted
that such a relatively thick (�2.9 mm) film can be achieved only
by three consecutive spin-coatings. The surface composition of
the CZTS film was determined by electron probe microanalysis
(EPMA) as shown in Fig. 3b and Table 1. The surface compo-
sition of the film was relatively uniform and the average
composition was close to the starting precursor composition
(25.0 at.% Cu, 12.5 at.% Zn, 12.5 at.% Sn, and 50.0 at.% S). We
also confirmed that the impurity levels of carbon and oxygen in
the film prepared under atmospheric conditions were about 3%,
which is lower than that (>5%) of the chalcogenide films fabri-
cated by other solution-based approaches.25,26 In addition,
considering that the oxygen of the SLG substrate could be
detected by EPMA, these impurity levels may be regarded as
negligible. Fig. 3c shows the compositional depth profile of the
CZTS films annealed at 530 �C for 30 min. No significant
compositional variation can be observed across the film. The
average composition across the films was also similar to the
starting precursor composition. This means that the precursor
particles are homogeneously well-dispersed in the ink, resulting
in CZTS phase formation with uniform composition after heat
treatment even though four individual precursor particles are
involved.
The optical absorption coefficient (a) is obtained from the
measured spectral transmittance (Tl) and reflectance (Rl) data
using the following formula:
a ¼ 1/t ln[(1 � Rl)2/Tl] (1)
where t is the thickness of the film. The nature of the optical
transitions and the optical band gap (Eg) of the film are obtained
from eqn (2):
a ¼ A(hn � Eg)n/hn (2)
Energy Environ. Sci., 2012, 5, 5340–5345 | 5341
Fig. 1 (a) TG-DSC analysis of the ethanol-based CZTS precursor ink. This analysis was performed under a nitrogen atmosphere. (b) XRD analysis of
the CZTS film as a function of the annealing temperatures. Enlarged graphs in the 2q-angle range from 28� to 33� are displayed to show low-intensity
peaks. (c) Microstructure evolution of the CZTS film as a function of the annealing temperatures ranging from 400 to 530 �C. The precursor films were
annealed under N2 + H2S (5%) atmosphere in a tubular furnace.
Dow
nloa
ded
on 0
9 A
pril
2012
Publ
ishe
d on
31
Oct
ober
201
1 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/C
1EE
0231
4D
View Online
where A is a constant. The exponent n can take values of 2, 1/2, 3
or 3/2 for indirect-allowed, direct-allowed, indirect-forbidden or
direct-forbidden transitions, respectively. The values of a are
found to obey eqn (2) for n ¼ 1/2 , indicating that the optical
transitions are direct-allowed in nature. Therefore, the Eg of
CZTS with direct transitions can be determined by applying
eqn (3):
(ahn)2 ¼ A(hn � Eg) (3)
Fig. 2 Raman spectra of the CZTS films as a function of annealing t
5342 | Energy Environ. Sci., 2012, 5, 5340–5345
The optical band gap is determined by extrapolating the linear
region of the plot (ahn)2 versus hn and taking the intercept on the
hn-axis. Fig. 3d presents (ahn)2 versus hn plots of the CZTS films
annealed at temperatures of 500 and 530 �C. The direct optical
band gap energy is found to be 1.44 eV for the CZTS film
annealed at 530 �C. This Eg value measured from our CZTS films
is similar to the band gap of bulk CZTS reported by others. In
contrast, for the sample that was heat treated at 500 �C, the bandgap values of the CZTS films increased to 1.66 eV. This increase
emperatures. Detailed graphs indicate traces of unreacted phases.
This journal is ª The Royal Society of Chemistry 2012
Fig. 3 (a) Cross-sectional image of the film annealed at 530 �C. (b) Composition mapping at the surface of the CZTS films annealed at 530 �C by
EPMA. (c) Component depth profile by Auger electron spectroscopy. (d) Band gap energy of the CZTS films annealed at 500 and 530 �C.
Dow
nloa
ded
on 0
9 A
pril
2012
Publ
ishe
d on
31
Oct
ober
201
1 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/C
1EE
0231
4D
View Online
might be due to the presence of Cu2S, which has the direct optical
band gap in the range 1.7–2.16 eV.27,28
We fabricated thin film solar cells using non-toxic solvent-
based ink. The CZTS precursor layer of �2.9 mm in thickness
was formed by three spin coatings. The precursor film was dried
at 80 �C followed by annealing under N2 + H2S (5%) atmosphere
at 530 �C for 30 min. Chen et al. reported that Cu-poor and Zn-
rich conditions improve the efficiency of the CZTS solar cells
because a Cu-poor composition enhances the formation of Cu
vacancies, which gives rise to shallow acceptors in the CZTS,
while a Zn-rich condition suppresses the substitution of Cu at Zn
sites, which results in relatively deep acceptors.29 Therefore, our
film composition for the cell performance measurement was
selected to include Cu-poor and Zn-rich compositions (approx-
imately Cu/(Zn + Sn) ¼ 0.8 and Zn/Sn ¼ 1.2) (see ESI, Fig. S2†).
Table 1 Composition ratios at the surface of the CZTS films annealed at 53
Composition ratio (before heat treatment) Composition ratio
Cu/(Zn + Sn) Zn/Sn S/metal Cu/(Zn + Sn)
1.06 0.84 1.0 0.99
This journal is ª The Royal Society of Chemistry 2012
The sintered CZTS absorber films were processed into photo-
voltaic devices following standard procedures, including the
chemical bath deposition of CdS (�50 nm), DC sputtering of
i-ZnO (�50 nm), RF sputtering of ITO (�250 nm), and thermal
evaporation of a patterned Ni/Al grid as the top electrode, as
shown in Fig. 4a. Finally, the samples (2 � 2.5 cm2) were
mechanically scribed into the cells with a total area of 0.25 cm2.
The current–voltage (I–V) characteristics for our best performing
CZTS solar cell measured in the dark and under AM 1.5 illu-
mination are shown in Fig. 4b. All device performance param-
eters were reported based on the cell area, excluding the shaded
areas (�11% of the total device area) by the Ni/Al finger elec-
trode. The as-fabricated device exhibited a total area efficiency of
5.14% [open-circuit voltage (Voc)¼ 0.517 V, short-circuit current
density (Jsc) ¼ 18.86 mA cm�2, fill factor (FF) ¼ 52.8%]. Fig. 4c
0 �C for 30 min by EPMA
(after heat treatment at 530 �C under N2 + 5% H2S)
Zn/Sn S/metal Atomic ratio % (Cu/Zn/Sn/S/O/C)
0.93 0.98 23.5 : 11.5 : 12.3 : 46.2 : 3.5 : 2.9
Energy Environ. Sci., 2012, 5, 5340–5345 | 5343
Fig. 4 (a) Cross-sectional image of the CZTS thin film solar cell. Inset
shows a photograph of the as-fabricated solar cell. The samples (2 � 2.5
cm2) were mechanically scribed into the cells with a total area of 0.25 cm2.
Note that the stoichiometry of the CZTS film was selected to yield Cu-
poor and Zn-rich compositions (approximately Cu/(Zn + Sn) ¼ 0.8 and
Zn/Sn¼ 1.2). (b) Current–voltage (I–V) characteristics of the CZTS solar
cell annealed at 530 �C for 30 min. The efficiency of the cell is 5.14%
under standard AM 1.5 illumination. (c) External quantum efficiency
(EQE) curve of the corresponding cell. The band gap of the absorber
layer is determined to be 1.51 eV by a plot of [E ln (1 � EQE)]2 vs. E, as
shown in the inset.
Dow
nloa
ded
on 0
9 A
pril
2012
Publ
ishe
d on
31
Oct
ober
201
1 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/C
1EE
0231
4D
View Online
shows the external quantum efficiency (EQE) of the corre-
sponding solar cell as a function of photon wavelength. The
maximum quantum efficiency of 74% is obtained for a photon
wavelength of 540 nm. The band gap of the absorber layer is
determined to be 1.51 eV by fitting a plot of [E ln(1 � EQE)]2 vs.
5344 | Energy Environ. Sci., 2012, 5, 5340–5345
E near the band edge, as shown in the inset of Fig. 4c. The band
gap energy of a CZTS film with the Cu-poor and Zn-rich
composition is larger than the stoichiometric film (1.44 eV). The
observed value is reasonable since the band gap energy of CZTS
shifts to higher energies as Cu/(Zn + Sn) decreases.30
Although the initial efficiency of our cell is lower than those of
the other solution-processed cells such as those produced by the
hybrid slurry method (9.6%) and nanocrystal dispersion
(7.2%),4,20 improvements in device performance are expected
with further studies to resolve several issues. A possible reason
for the low efficiency is the relatively thick CZTS absorber layer.
Although the devices made with thicker CZTS layers absorb
more light, the high intrinsic resistivity of the p-type CZTS
absorber layer itself and the charge carrier trap density inherent
in the thick layer can contribute to the increase in high series
resistance and low short circuit current that lead to losses in
efficiency,4,31 suggesting the fabrication process must be opti-
mized for the thinner film. In addition, fine-tuning the band gap
of the CZTS film through the replacement of S by Se and MgF2
antireflection coating on top of the device are currently underway
to improve the cell efficiency. We believe that resolving these
issues will allow us to produce large and low cost CZTS solar
cells with much higher efficiencies, which is highly desirable for
photovoltaic applications.
Conclusions
In summary, our simple solution-based deposition approach
employs a non-toxic solvent (ethanol)-based ink composed of
commercially available precursor particles. Our readily achiev-
able air-stable precursor ink, without the involvement of
complex particle synthesis, high toxic solvents, or organic addi-
tives, facilitates a convenient method to fabricate a high quality
CZTS absorber layer with uniform composition at the surface
and across the thin depth. Well-dispersed ink containing four
different finely milled precursor particles of low melting points
allows for the CZTS crystallization when annealed at 530 �C and
forms dense films with large grains (1–2.5 mm), possibly by
a reactive liquid-phase sintering between the constituent parti-
cles. The preliminary conversion efficiency and fill factor for the
non-toxic ink based solar cells are 5.14% and 52.8%, respectively,
although the processing details are not yet optimized. Our simple
and safe approach reported here represents the first step toward
realizing low-cost, large-area, high efficiency solar cells.
Acknowledgements
This research was financially supported by the Basic Research
Laboratory (BRL) Program through an NRF grant funded by
the MEST (No. 2011-8-2048). It was also partially supported by
the Second Stage of the Brain Korea 21 Project.
References
1 J. Paier, R. Asahi, A. Nagoya and G. Kresse, Phys. Rev. B: Condens.Matter Mater. Phys., 2009, 79, 115126.
2 J. Scragg, P. Dale, L. Peter, G. Zoppi and I. Forbes, Phys. StatusSolidi B, 2008, 245, 1772.
3 H. Katagiri, K. Jimbo, W. Maw, K. Oishi, M. Yamazaki, H. Arakiand A. Takeuchi, Thin Solid Films, 2009, 517, 2455.
This journal is ª The Royal Society of Chemistry 2012
Dow
nloa
ded
on 0
9 A
pril
2012
Publ
ishe
d on
31
Oct
ober
201
1 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/C
1EE
0231
4D
View Online
4 Q. Guo, G. M. Ford, W. Yang, B. C. Walker, E. A. Stach,H. W. Hillhouse and R. Agrawal, J. Am. Chem. Soc., 2010, 132,17384.
5 C. Steinhagen, M. G. Panthani, V. Akhavan, B. Goodfellow, B. Kooand B. A. Korgel, J. Am. Chem. Soc., 2009, 131, 12554.
6 K. Ito and T. Nakazawa, Jpn. J. Appl. Phys., 1988, 27, 2094.7 T. Tanaka, T. Nagatomo, D. Kawasaki, M. Nishio, Q. Guo,A. Wakahara, A. Yoshida and H. Ogawa, J. Phys. Chem. Solids,2005, 66, 1978.
8 S. C. Riha, B. A. Parkinson and A. L. Prieto, J. Am. Chem. Soc., 2009,131, 12054.
9 F. Liu, K. Zhang, Y. Lai, J. Li, Z. Zhang and Y. Liu, Electrochem.Solid-State Lett., 2010, 13, H379.
10 H. Katagiri, K. Jimbo, S. Yamada, T. Kamimura, W. S. Maw,T. Fukano, T. Ito and T. Motohiro, Appl. Phys. Express, 2008, 1,041201.
11 K. Jimbo, R. Kimura, T. Kamimura, S. Yamada, W. Maw, H. Araki,K. Oishi and H. Katagiri, Thin Solid Films, 2007, 515, 5997.
12 A. Weber, R. Mainz and H. W. Schock, J. Appl. Phys., 2010, 107,013516.
13 S. E. Habas, H. A. S. Platt, M. F. A. M. van Hest and D. S. Ginley,Chem. Rev., 2010, 110, 6571.
14 T. K. Todorov and D. B. Mitzi, Eur. J. Inorg. Chem., 2010, 2010, 17.15 F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2009, 93, 394.16 Z. Zhou, Y. Wang, D. Xu and Y. Zhang, Sol. Energy Mater. Sol.
Cells, 2010, 94, 2042.17 M. Yeh, C. Lee and D. Wuu, J. Sol-Gel Sci. Technol., 2009, 52, 65.
This journal is ª The Royal Society of Chemistry 2012
18 K. Tanaka, N.Moritake andH. Uchiki, Sol. EnergyMater. Sol. Cells,2007, 91, 1199.
19 T. Kameyama, T. Osaki, K. Okazaki, T. Shibayama, A. Kudo,S. Kuwabata and T. Torimoto, J. Mater. Chem., 2010, 20,5319.
20 T. K. Todorov, K. B. Reuter and D. B. Mitzi, Adv. Mater., 2010, 22,E156.
21 O. Gunawan, T. K. Todorov and D. B. Mitzi, Appl. Phys. Lett., 2010,97, 233506.
22 L. F. Audrieth and B. Ackerson Ogg, The Chemistry of Hydrazine,John Wiley & Sons, New York, 1951.
23 H. Yoo and J. Kim, Sol. Energy Mater. Sol. Cells, 2011, 95, 239.24 P. A. Fernandes, P. M. P. Salom�e and A. F. da Cunha, Thin Solid
Films, 2009, 517, 2519.25 M. Kemell, M. Ritala andM. Leskela, J.Mater. Chem., 2001, 11, 668.26 M. Krunks, O. Kijatkina, H. Rebane, I. Oja, V. Mikli and A. Mere,
Thin Solid Films, 2002, 403–404, 71.27 Y. B. Kishore Kumar, P. Uday Bhaskar, G. Suresh Babu and
V. Sundara Raja, Phys. Status Solidi A, 2010, 207, 149.28 A. C. Rastogi and S. Salkalachen, Thin Solid Films, 1982, 97, 191.29 S. Chen, X. G. Gong, A. Walsh and S. Wei, Appl. Phys. Lett., 2010,
96, 021902.30 G. S. Babu, Y. B. K. Kumar, P. U. Bhaskar and V. S. Raja, Sol.
Energy Mater. Sol. Cells, 2010, 94, 221.31 V. A. Akhavan, B. W. Goodfellow, M. G. Panthani, D. K. Reid,
D. J. Hellebusch, T. Adachi and B. A. Korgel, Energy Environ. Sci.,2010, 3, 1600.
Energy Environ. Sci., 2012, 5, 5340–5345 | 5345
Recommended