A molecular precursor route to quaternary chalcogenide CFTS (Cu2FeSnS4) powders as potential solar absorber materials

Article abstract of DOI:10.1039/c9ra02926e

In the present work we report on the synthesis of tetragonal stannite Cu₂FeSnS₄ powders using a solvent free melt method using a mixture of Cu, Fe, and Sn(ii)/Sn(iv) O-ethylxanthates heated at different temperatures. The as-synthesized powders were characterized by powder X-ray diffraction (p-XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UV-Vis absorption spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy, which confirm the successful synthesis of stannite CFTS. Optical measurements show that Cu₂FeSnS₄ powders have visible light absorption onsets in the far red with direct band gap energies in the range 1.32-1.39 eV which are suitable for acting as efficient absorber layers in solar cells. Electronic characterisation of these materials deposited as thin films by spin coating show that they are p type semiconductors with respectable carrier mobilities of ca. 60 cm² V^-1 s^-1 with carrier densities on the order of 10¹⁴ cm^-1.

Full text of DOI:10.1039/c9ra02926e

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A molecular precursor route to quaternary
chalcogenide CFTS (Cu2FeSnS4) powders as
Cite this: RSC Adv., 2019, 9, 24146
potential solar absorber materials†
ab
Abdulaziz M. Alanazi,ad Firoz Alam,
Abdelmajid Salhi,c Mohamed Missous,c
ab
b
Andrew G. Thomas,b Paul O'Brien
and David J. Lewis
*
In the present work we report on the synthesis of tetragonal stannite Cu2FeSnS4 powders using a solvent
free melt method using a mixture of Cu, Fe, and Sn(II)/Sn(IV) O-ethylxanthates heated at different
temperatures. The as-synthesized powders were characterized by powder X-ray diffraction (p-XRD),
Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UV-Vis absorption spectroscopy,
scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy, which confirm the
successful synthesis of stannite CFTS. Optical measurements show that Cu2FeSnS4 powders have visible
light absorption onsets in the far red with direct band gap energies in the range 1.32–1.39 eV which are
suitable for acting as efficient absorber layers in solar cells. Electronic characterisation of these materials
deposited as thin films by spin coating show that they are p type semiconductors with respectable
Received 18th April 2019
Accepted 25th July 2019
DOI: 10.1039/c9ra02926e
rsc.li/rsc-advances
carrier mobilities of ca. 60 cm2 Vꢀ1 ꢀ1 with carrier densities on the order of 1014 cmꢀ1
s .
elements which are expensive, rare and toxic such as In, Ga and
Cd.14 Therefore, the development of low-cost, nontoxic and
environmental friendly alternatives are needed to make high-
efficiency solar cells. Copper based quaternary chalcogenides
such as Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) have been
used as solar absorber materials in thin lm solar cells.15,16 One
of the challenges in the synthesis of CZTSSe materials is to
obtain pure and stoichiometric kesteritic materials as the
optoelectronic properties are sensitive to in particular the Cu
and Zn ratios.10,17–22 One of the alternatives to CZTS is to Cu2-
FeSnS4 (CFTS) which has been used as an Pt-free counter elec-
trode in dye-sensitized solar cells (DSSCs) as well as an absorber
material in thin lm solar cells.23
Introduction
Among inorganic semiconductors, quaternary metal chalco-
genides have attracted interest as light absorbers in photovol-
taic applications.1–8 Copper iron tin sulphide (Cu2FeSnS4) has
drawn considerable attention in photovoltaics because of its p-
type conductivity, suitable band-gap 1.2–1.5 eV (Table 1) and
high absorption coefficient (>104 cmꢀ1).9–12 The structure of
Cu2FeSnS4 is similar to the zinc blende structure. Structures are
adopted which depend on the conguration of the tetrahedral
holes which are called stannite (CFTS) and kesterite (CZTS),
respectively.13 Stannite is tetragonal with unit-cell parameters
˚
˚
ꢀ
a ¼ 5.449 A, c ¼ 10.726 A with a space group I42m as shown
CZTS and CFTS have suitable optical band gaps of around
in Fig. 1(a), whilst kesterite is tetragonal with unit-cell
1.4 eV and good absorption coefficients (typically a > 104 cmꢀ1
)
˚
˚
parameters a ¼ 5.434 A, c ¼ 10.856 A (Fig. 1(b)) with a space
group I42m.13 Both phases consist of inexpensive, non-toxic and
ꢀ
in the visible spectral range which is comparable to CIGS
materials, making them favourable candidates for photovoltaic
earth-abundant materials.
Current commercialized thin lm solar cells technologies
such as CdTe and Cu2InGaS4 (CIGS) have commonly used
Table 1 Reported band gaps of CFTS nanomaterials prepared by
different methods
aSchool of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL,
UK
bSchool of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL,
Method
Band gap (eV)
Reference
UK. E-mail: david.lewis-4@manchester.ac.uk
Hot-injection
Solvothermal
Microwave irradiation
Electrospinning
Liquid reux
1.28
1.33
1.71
1.24
1.32
1.46
28
9
32
36
37
38
cSchool of Electrical and Electronic Engineering, The University of Manchester,
Sackville Street, Manchester, M13 9PL, UK
dSchool of Chemistry, Islamic University, Prince Naif Ibn Abdulaziz Rd, Madinah,
42351, Kingdom of Saudi Arabia
† Electronic supplementary information (ESI) available: Experimental section,
crystallographic data in CIF, NMR data and IR spectrum peak. See DOI:
10.1039/c9ra02926e
Solution-based
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Thus, it is necessary to design inexpensive approaches for
synthesis of CFTS materials. In this paper, we produce CFTS
from direct thermal decomposition of metal xanthate precur-
sors. To the best of our knowledge, the synthesis of CFTS
powders using solvent free thermolysis has not been reported so
far. The method which we propose has been used to prepare
CFTS powders in large quantities. The technique is straight
forward, solvent free, inexpensive and single step utilizing
single source precursors (SSPs).33–35 Here, we use metal xanthate
precursors because their decomposition happens at a lower
temperature and the by-products are gaseous.34,35,39
Experimental
Materials
Fig. 1 Unit cell representations of Cu2FeSnS4; (a) the Stannite type
structure a ¼ 5.449 A˚; c ¼ 10.726 A˚, a, b and g ¼ 90ꢁ, ICDD: 0005838
(b) Kesterite type structure a ¼ 5.434 A˚; c ¼ 10.856 A˚, a, b and g ¼ 90ꢁ
ICDD: 0005843.23
Tin(II) chloride (99.9%), tin(IV) chloride (98%), carbon disul-
phide (99.9%), iron(III) chloride (97%), copper(II) sulphate
(98%), chloroform (99.8%), hexane (97%), toluene (99.7%) and
ethanol (99.8%) were purchased from Sigma-Aldrich or Alfa
Aesar and used as received.
applications.24,25 Hence, CZTS and CFTS thin lm solar cells
have reached power conversion efficiencies of 12.6% and
8.03%, respectively, where these materials are used as part of
the absorber layer.26 In addition CZTS and CFTS show p-type
conductivities which can be useful for pairing to n-type mate-
rials in cell architectures.27
Up to now range of methods have been reported for the
synthesis of CFTS materials of different shapes and sizes.28–30
Some reports focus on developing solution based processes as
an alternate to vacuum deposition. This offers the advantage
of high productivity and low processing temperatures.31 Other
methods such as solvothermal processing,29 hot injection28
and microwave irradiation32 have been used for solution based
synthesis of CFTS. However, the solvothermal and hot injec-
tion processes have restrictive conditions, oen give low
yields, use toxic chemicals (ethylenediamine and oleylamine)
and require heat treatment for 18–24 h, centrifugation and
vacuum drying.
A Phillips X-PERT PRO with Cu Ka incident beam (l ¼
˚
1.54059 A) was used to record X-ray diffraction patterns. The
samples were scanned in the 2q range of 10ꢁ to 80ꢁ for a period
of 1 h. Scanning electron microscopy (SEM) was carried out
using a Philips XL 30FEG. The voltage used was 40 kV. Carbon
coating was carried out using an Edwards E306A coating unit.
EDX spectroscopy (Philips EDAX DX4 X-ray micro-analyser SEM)
was used to determine elemental composition as well used for
elemental mapping in order to know the spatial distribution of
elements in the sample. The optical properties of the CFTS
powders were characterized by UV-Vis-NIR absorption spec-
troscopy recorded on a Shimadzu UV-1800. Raman spectra were
collected using a Renishaw 1000 Micro-Raman system equipped
with a 50ꢂ objective and a 514 nm laser. X-ray photoelectron
spectroscopy (XPS) measurements were performed using either
a Kratos Axis Ultra or SPECS XPS instrument. Both facilities are
equipped with monochromated Al Ka X-ray sources with
a photon energy of 1486.6 eV. Emitted photoelectrons were
collected using either a 165 mm hemispherical energy analyser
(Kratos) or a 150 mm hemispherical energy analyser (Phoibos
150 SPECS), respectively. The peaks were calibrated through
referencing C 1s to 284.8 eV. Infrared spectra were recorded on
a Specac single reectance ATR instrument (4000–400 cmꢀ1
,
resolution 4 cmꢀ1). Melting points were determined using
a Barloworld SMP10 device and the elemental analyses of
complexes were done using a Flash 2000 Thermo Scientic
elemental analyser. Thermogravimetric analysis (TGA) was
performed using a Mettler Toledo TGA/DSC 1 system under an
atmosphere of dry nitrogen.
Synthesis of metal xanthate complexes
Synthesis of potassium ethylxanthate. The synthesis of the
potassium ethylxanthate was conducted in the following way.
Potassium hydroxide (11.29 g, 0.2 mmol) was dissolved in
ethanol (75 ml) and cooled in an ice bath. Carbon disulphide
(15.32 g, 12.16 ml, 0.2 mmol) was added dropwise while stirring.
Fig.
2 Thermogravimetric analysis of [Cu(S2COEt)2] (red colour),
[Fe(S2COEt)3] (blue colour), [Sn(S2COEt)2] (green colour) and [Sn(S2-
COEt)4] (black colour) precursors.
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(1.596 g, 9.9 mmol) and CuSO4$5H2O (1.242 g, 4.9 mmol) mixed
at room temperature while stirring and the stirring was
continue for 60 minutes. Orange precipitate was obtained and
washed with deionised water. The precipitate was ltered, and
then product was nally dried in a vacuum oven overnight at
room temperature. Yield: 87%. Melting point: 187 ꢁC. Elemental
analysis: calc. (%): C, 23.58; H, 3.30; S, 41.85; Cu, 20.80. Found
(%): C, 23.67; H, 3.21; S, 41.39; Cu, 21.09. IR (nmax/cmꢀ1):
2979.63–2932.92 (w), 1462.34–1367.20 (s), 1239.20 (s), 1119.11
(s), 846 (w).
Synthesis of (O-ethylxanthato) copper(I) triphenylphosphine.
The synthesis of [(Ph3P)2CuS2COEt] was carried out by following
the literature.41 A mixture of triphenylphosphine (2.09 g, 0.008
mol) and CuCl (0.40 g, 0.0040 mol) was dissolved in 40 ml of
chloroform and later it was added to the potassium ethyl-
xanthate (0.641 g, 0.0040 mol) that was dissolved in 40 ml of
chloroform. Aer stirring for 1 h at room temperature a white
precipitate was obtained. The precipitate was ltered to obtain
ꢁ
a clear yellow solution. At ꢀ20 C the yellow crystals of O-eth-
ylxanthato copper(I) triphenylphosphine was obtained. Yield:
ꢁ
85%. Melting point: 185–191 C. Elemental analysis: calc. (%):
C, 66.1; H, 4.97; S, 9.02; P, 8.74; Cu, 8.96. Found (%): C, 65.7; H,
5.08; S, 8.77; P, 8.44; Cu, 8.74. IR (nmax/cmꢀ1): 3048 (w), 2992 (w),
1478 (m) 1433 (m), 1290 (s), 1142 (m), 1041 (m), 1009 (s), 849.5
(s), 740.8 (m), 617.7 (s), 559.2 (s).
Synthesis of tris (O-ethylxanthato) iron(III). The synthesis of
[Fe(S2COEt)3] was carried out by following the literature.42
Briey, an aqueous solution of potassium ethylxanthate
(1.596 g, 9.9 mmol) and an aqueous solution of FeCl3 (0.538 g,
3.3 mmol) mixed at room temperature while stirring and the
stirring was continue for 60 minutes. The black precipitates was
obtained and washed with deionised water. The precipitate was
ltered using Whatman paper, and the product was nally dried
in a vacuum oven overnight at room temperature. Yield: 85%.
Melting point: 118 ꢁC. Elemental analysis: calc. (%): C, 25.79; H,
3.61; S, 45.81; Fe, 13.34. Found (%): C, 25.59; H, 3.35; S, 45.36;
Fe, 12.70. IR (nmax/cmꢀ1): 2987.63–2979.92 (w), 1458.55–1425.36
(s), 1233.18 (s), 1059.25 (s), 856 (w).
Fig. 3 The p-XRD patterns of Cu2FeSnS4 powders (1) and (2) syn-
thesised at (a) 250 ꢁC; (b) 350 ꢁC and (c) 450 ꢁC for 1 h.
The ethanol was evaporated at room temperature to obtain the
product, 71.8% yield.
Synthesis of bis (O-ethylxanthato) copper(II). The synthesis
of [Cu(S2COEt)2] was carried out according to the literature.40
Briey, an aqueous solution of potassium ethylxanthate
Synthesis of bis (O-ethylxanthato) tin(II). The synthesis of
[Sn(S2COEt)2] was carried out by following the literature.43
Briey, tin(II) ethylxanthate was produced by adding an aqueous
solution of K(S2COEt) (5 g, 31.1 mmol) into an aqueous solution
of tin(II) chloride (2.95 g, 15.5 mmol) in 50 ml deionised water
while stirring and the stirring was continue for 60 minutes that
results in a black precipitates. The precipitate was ltered using
Whatman paper, and the product was nally dried in a vacuum
oven overnight at room temperature. Yield: 87.5%. Melting
ꢁ
point: 47 C. Elemental analysis: calc. (%): C, 19.98; H, 2.79; S,
35.45; Sn, 32.91. Found (%): C, 20.04; H, 2.72; S, 35.19; Sn, 32.87
IR (nmax/cmꢀ1): 2977.24–2935.84 (w), 1462.82–1364.91 (s),
1272.76 (s), 1113.75 (s), 860 (w).
Synthesis of tetrakis (O-ethylxanthato) tin(IV). The synthesis
of [Sn(S2COEt)4] was carried out using a technique that was
modied from literature.44 Briey, SnCl4 (1 g, 3.8 mmol) was
dissolved in 50 ml of toluene and added drop by drop to the
potassium ethylxanthate (2.5 g, 15.3 mmol) in toluene at room
temperature. The reaction mixture was stirred for 60 minutes
Fig. 4 The Raman spectra of Cu2FeSnS4 powders (1) and (2) synthe-
sized at a temperature of 450 ꢁC for 1 h.
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from the solution. Yield: 91.3%. Melting point ¼ 73 ꢁC.
Elemental analysis: calc. (%): C, 23.91; H, 3.34; S, 42.43; Sn,
19.69. Found (%): C, 23.98; H, 3.29; S, 42.01; Sn, 20.11. IR
(nmax/cmꢀ1): 2987.29 (w), 1462.48–1365.84 (s), 1247.83 (s),
1025.47 (s), 860 (w).
Synthesis of CFTS powders
For the synthesis of CFTS powders, 2 mmol copper(II) eth-
ylxanthate, 1 mmol iron(III) ethylxanthate and 1 mmol tin(II)
or tin(IV) ethylxanthate were mixed together. Then the
mixture was heated in a furnace at either 250 ꢁC, 350 ꢁC and
450 ꢁC for 1 h under a nitrogen atmosphere. The CFTS
powders produced were allowed to cool-down to room
temperature in the inert atmosphere. The CFTS powder
synthesised using Sn(II) and Sn(IV) are named as (1) and (2),
respectively.
In addition to the synthesis of CFTS powders, we also
deposited the CFTS thin lms using spin coating technique
from Sn(II) and Sn(IV), which are named as (3) and (4),
respectively. Full details on the synthesis and character-
isation of these thin lm samples can be found in the ESI.†
Results and discussion
Thermogravimetric analysis (TGA) of precursors
The synthesis of [Cu(S2COEt)2], [Fe(S2COEt)3], [Sn(S2COEt)2]
and [Sn(S2COEt)4] complexes were performed and their
suitability for melt reactions was measured through thermal
stability measurement in a nitrogen atmosphere. Fig. 2
shows the TGA proles of [Sn(S2COEt)2], [Cu(S2COEt)2],
[Sn(S2COEt)4] and [Fe(S2COEt)3], respectively. The [Cu(S2-
COEt)2] and [Sn(S2COEt)4] complexes display a two-step
decomposition pattern. In the case of the [Cu(S2COEt)2]
precursor, the mass residue obtained from the TGA proles
Fig. 5 XPS spectra of Cu2FeSnS4 powders (1) and (2) synthesized at
a temperature of 450 ꢁC for 1 h: (a) Fe 2p, (b) Cu 2p, (c) Sn 3d and (d) S
2p.
then precursor solution was evaporated under reduced pres-
sure and then oily residue was shaken by adding 50 ml of
hexane. The yellow crystals of [Sn(S2COEt)4] were extracted
Fig. 6 SEM images of Cu2FeSnS4 powders (1) and (2) synthesised at 450 ꢁC for 1 h taken at different magnifications.
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for the rst decomposition stage (58%) agreed with the
theoretical value calculated for the removal of one molecule
of xanthate and half from another one (58%). While in the
second step, there is a mass loss of 31% in the temperature
range of 200 to 450 ꢁC that agrees with theoretical value (31%)
for production of CuS. In the case of [Sn(S2COEt)4] the rst
step involves a degradation of the mass loss 59% in the
temperatures range of 45 to 120 ꢁC obtained from the TGA
prole, which corresponds and agreed with the theoretical
value calculated for the removal of three molecules of
xanthate (60%), and the nal decomposition residue ob-
Bulk structural characterisation of CFTS powders
The powder XRD patterns of CFTS synthesized at different
temperatures using Sn(II) and Sn(IV) precursors are shown in
Fig. 3. The diffraction peaks observed at 2q values of 28.50ꢁ,
32.85ꢁ, 33.36ꢁ, 36.97ꢁ, 47.15ꢁ, 47.50ꢁ, 50.93ꢁ, 56.66ꢁ, 70.04ꢁ and
76.69ꢁ correspond to the (112), (200), (004), (202), (220), (204),
(301), (116), (008) and (316) planes of tetragonal stannite,
respectively.
The calculated lattice parameters for powder (1) and (2) are
˚
˚
˚
a ¼ 5.4501 A, c ¼ 10.7468 and a ¼ 5.4467 A, c ¼ 10.7510 A,
respectively which are in good agreement with the reported
literature values for CFTS.29 The XRD peak intensities increased
with increasing the temperature without affecting the phase of
the powder. The average domain size of both powder (1) and (2)
are approximately 13 nm calculated using Scherrer's equation.
The comparison between kesterite and stannite with experi-
mentally determined and calculated values is represented by
the tetragonal distortion (deviation of the c/2a ratio from 1,
where, c and a are the lattice parameters). The tetragonal
distortion parameter is important for the resulting electronic
structure of the material,46 for example, strong deviations away
from the ideal structure caused by a changes in crystal eld can
lead to non-degenerate valence band maxima.47–49
ꢁ
tained aer 150 C was found to be SnS2 which is almost in
conformity with the mass loss data obtained from the TGA
prole (32%) and the theoretical value (33%). In contrast, the
[Sn(S2COEt)2] and [Fe(S2COEt)3] precursor complexes have
a single step decomposition. The single step decomposition
of [Sn(S2COEt)2] occurred in the temperature range of 304–
ꢁ
396 C with a mass loss of 42% and for [Fe(S2COEt)3] is 27%
in the temperature range of 73–400 ꢁC which is in good
agreement with the theoretical values of SnS (42%) and FeS2
(29%), respectively. A number of other authors have observed
these typical weight loss steps with metal xanthates in their
decomposition to corresponding metal suldes.35,39 For
instance, Almanqur et al., have successfully synthesised
a series of iron alkyl xanthate complexes to deposit iron
sulphide thin lms and nanostructures using the spin
coating and the solvent free pyrolysis methods. The TGA
proles of these complexes showed approximately the same
with a rapid residue loss within the temperature range of 120
In kesterite, the c/2a ratio has been reported to be greater
than 1 in a neutron diffraction study done on powder samples.46
However, in stannite this ratio has been reported to be less than
1, as estimated using XRD studies. In our study, the ratio of
stannite CFTS was determined to be 0.99, which is slightly less
than 1 and thus this value is in good agreement with the values
in the literature.13,46
In order to prove the pristine nature of the synthesized
powders and to rule out the existence of secondary phases that
were not distinguished by the XRD, Raman spectroscopy was
performed. Fig. 4 shows the Raman spectra of CFTS, which
exhibits a large peak at 312.22 cmꢀ1 and 317.25 cmꢀ1 corre-
sponding to tetragonal CFTS in both (1) and (2), respectively. It
is reported in the literature that this is the A1 symmetric
vibrational motion of sulphur atoms in CFTS.50–52
ꢁ
ꢁ
to 300 C, and nal step occurred between 320 to 500 C. All
complexes showed the nal solid residue amounts that
matched with the calculated values for FeS2 or FeS.34
Al-Shakban et al., have synthesised SnS thin lms from
diphenyltin bis(iso-butylxanthate) complexes using aerosol-
assisted chemical vapor deposition (AACVD). The TGA prole
of this complex showed two-step decomposition, the rst step of
which involves elimination of the alkyl groups, followed by
carbonyl sulde (SCO). Then, the nal step may involve the loss
of another carbonyl sulde.45
Fig. 7 EDX elemental mapping of Cu2FeSnS4 powders (1) and (2) synthesised at 450 ꢁC for 1 h showing the spatial distribution of Cu, Fe, Sn and S.
Scale bars represent 5 mm in all cases. A secondary electron SEM image of the mapped area is included in each case, labelled as SE.
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Fig. 8 Tauc plot (ahn)2 vs. hn showing the direct bandgaps of Cu2FeSnS4 powders (1) and (2) and their energies.
We also note the absence of Raman peaks corresponding to satellite and the 711.7 eV peaks are much more pronounced for
FeS (214 and 282 cmꢀ1) and Cu2SnS3 (267, 303 and 356 cmꢀ1
)
the material synthesized from the Sn(IV) precursor, but the
which are common contaminants of CFTS,53 and is consistent reasons for this are unclear. Unfortunately, a lack of high
with the XRD patterns of the powders being a single crystalline quality XPS spectra from FeCuSnS2 standards means it is diffi-
phase (Fig. 3).
cult to determine contributions from this material.
Fig. 5(a–d) show the Fe 2p, Cu 2p, Sn 3d and S 2p X-ray
photoelectron spectra recorded from powders prepared using
a Sn(II) and Sn(IV) precursor. The Sn 3d spectra show no differ-
ence between the two samples, with peaks at 486.9 eV and
Microscopic characterisation of CFTS powders
Scanning electron microscopy (SEM) images of the CFTS
powders at different magnications are shown in Fig. 6. Images
of CFTS (2) show that the crystallites were largely agglomerated
with variation in their size, while agglomerated but less poly-
disperse particles are obtained in CFTS (1). In both cases
agglomeration is random with no ordering of particles
observed. The compositional data and EDS spectra of CFTS
powder synthesized at 450 ꢁC are shown in Fig. S1.† The atomic
% of Cu, Fe, Sn2+ and S were 27.76, 13.19, 13.73 and 45.32,
respectively in (1), while in (2) the atomic % of Cu, Fe, Sn4+ and S
were 25.41, 14.09, 15.40 and 45.10, respectively which indicates
that both Cu2FeSnS4 powders have the required stoichiometry.
Elemental mapping of CFTS is used to investigate the spatial
homogeneity in terms of elemental distribution at the micro-
scale. Fig. 7 shows the elemental mapping CFTS powders. It
seems likely based on these images that the distributions of Cu,
Fe, Sn and S elements in the sample are uniform at the
microscale.
495.3 eV arising from the spin orbit split 3d5/2 and 3d3/2
,
respectively. It is difficult to determine the Sn oxidation state
from the Sn 3d XPS spectrum since the literature reports both
Sn(II) and Sn(IV) compounds with binding energies in the
region. It is possible there is some surface oxidation for both
synthesis methods. The S 2p spectra in Fig. 5(d) are tted with
three spin orbit split doublets from S 2p3/2 and S 2p1/2. Both
samples show signicant surface oxidation with a substantial
sulphate derived peak with the S 2p3/2 at a binding energy of
168.8 eV. There is a clear sulphide derived doublet with the 2p3/2
at a binding energy of 161.6 eV and some residual contamina-
tion at the surface attributed to S–O, S–H or S–C at the surface.54
We found that the Fe 2p and Cu 2p spectra in Fig. 5(a and b) are
difficult to t. The d-electrons in these transition metals lead to
a range of multiplet split features, and complex shake up
structures.55,56 Simple analysis of the binding energies of the
features in the Cu 2p3/2 region are consistent with the presence
of CuO at the surface. The strong, narrow peak at a binding
energy of 932.2 eV, however, is similar to that found in chal-
copyrite (CuFeS2) and is more intense than would be expected
for CuO.56 The Fe 2p spectra also suggest some oxidation at the
surface. The binding energy of the lowest energy peak of
711.7 eV is oen attributed to the presence of FeSO4 and Fe2O3.
The former is consistent with the binding energy of sulphate
derived S.55 The satellite feature at a binding energy of 725.22 eV
is 2p peak in Fig. 6(d) and the lower energy satellite at 716.5
indicative of the presence of Fe(III) seemingly conrming the
oxidation of Fe to Fe2O3. It is clear that the high binding energy
Optical and electronic properties
The UV-Vis-NIR absorbance spectra of the CFTS powders
dispersed in ethanol in the wavelength range of 400–1100 nm
are shown in Fig. S2.† Fig. 8 shows Tauc plots of (ahn)2 versus hn
with a straight line tting, indicating the direct bands gaps of
1.32 eV and 1.39 eV for (1) and (2), respectively, which are in
good agreement with literature values.29,57 Ideally, the absorber
material of a thin lm solar cell should be a direct bandgap
semiconductor because of the strong optical transitions
between the energy bands and high absorption coefficient (a >
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104 cmꢀ1). The calculated limiting efficiency for a single band
gap solar cell of Eg ¼ 1.3–1.4 eV in a simulated solar spectrum
(AMG 1.5, i.e. xed incident light) is around 30%. Hence, the
optical properties that we measure for these materials suggest
that they may well be useful for applications in the absorber
layers in solar cells. We therefore studied the electronic prop-
erties of these materials as thin lms deposited using spin
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.
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Copper, iron and tin O-ethylxanthate complexes have been
successfully synthesized. The complexes were found to
decompose in the temperature range of 150–450 C to give the
ꢁ
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loss data and were used for the synthesis of CFTS powders. The
CFTS powder (1) and (2) have been successfully synthesised
from both Sn(II) and Sn(IV) precursors respectively using pyrol-
ysis in the temperature range of 250 to 450 ꢁC. The stannite
phase is obtained for both CFTS powders, which was ascer-
tained from a tetragonal distortion parameter c/2a of less than 1
in all cases. Absorption measurements conrm that Cu2FeSnS4
powder (1) and (2) are direct band gap semiconductors having
bandgap energies of 1.32 eV and 1.39 eV, respectively and thus
are suitable for photovoltaic absorber layer applications.
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Conflicts of interest
There are no conicts of interest to declare.
Acknowledgements
A. Al-A. thankful to Ministry of Higher Education in Saudi
Arabia for funding and the University of Islamic, Saudi Arabia
for permission to study in the United Kingdom. DJL and FA are
funded by EPSRC grant EP/R020590/1.
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