COMPARISON OF COPPER ZINC TIN SELENIDE FORMATION IN MOLTEN POTASSIUM IODIDE AND SODIUM IODIDE AS FLUX MATERIALS

Abstract

The aim of the present study is to describe the formation of Cu 2 ZnSnSe 4 in molten sodium iodide. The study deals with the possible chemical reactions associated between the binary precursor compounds CuSe, SnSe, ZnSe and NaI. Differential thermal analysis (DTA) runs were used to determine the thermal effects. The phase composition in mixtures of binary precursors and flux materials were studied by X-ray diffraction (XRD) and Raman spectroscopy. It is found that despite the fact that Cu 2 SnSe 3 and Cu 2 ZnSnSe 4 were detectable by Raman in the mixtures of precursors already at temperatures lower than 400ºC the extensive formation process of Cu 2 ZnSnSe 4 starts close to the melting point of flux (KI or NaI). It is found that NaI can be used as a flux material for the synthesis of Cu 2 ZnSnSe 4 .

Full text

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COMPARISON OF COPPER ZINC TIN SELENIDE FORMATION IN
MOLTEN POTASSIUM IODIDE AND SODIUM IODIDE AS FLUX
MATERIALS
I. Leinemann, J. Raudoja, M. Grossberg, M. Altosaar, D. Meissner
Institute of Materials Science, Tallinn University of Technology,
Ehitajate tee 5, 19086 Tallinn, Estonia
Tel. +3726203362
Email: ingaklav@inbox.lv
R. Traksmaa
Centre for Materials Research, Tallinn University of Technology,
Ehitajate tee 5, 19086 Tallinn, Estonia
Tel. +3726203150
Email: rainer@staff.ttu.ee
T. Kaljuvee
Laboratory of Inorganic Materials, Tallinn University of Technology,
Ehitajate tee 5, 19086 Tallinn, Estonia
Tel. +3726203362
Email: tiidu@staff.ttu.ee
ABSTRACT
The aim of the present study is to describe the formation of Cu
2
ZnSnSe
4
in molten sodium iodide. The
study deals with the possible chemical reactions associated between the binary precursor compounds
CuSe, SnSe, ZnSe and NaI. Differential thermal analysis (DTA) runs were used to determine the
thermal effects. The phase composition in mixtures of binary precursors and flux materials were
studied by X-ray diffraction (XRD) and Raman spectroscopy. It is found that despite the fact that
Cu
2
SnSe
3
and Cu
2
ZnSnSe
4
were detectable by Raman in the mixtures of precursors already at
temperatures lower than 400
º
C the extensive formation process of Cu
2
ZnSnSe
4
starts close to the
melting point of flux (KI or NaI). It is found that NaI can be used as a flux material for the synthesis
of Cu
2
ZnSnSe
4
.
Keywords: Cu
2
ZnSnSe
4
, solution growth, XRD, Raman and DTA.
1. INTRODUCTION
A molten phase between the solid particles can act as a contracting or repelling agent
depending on its amount. It is well known that the presence of a liquid phase in an amount
that forms an adhesive attraction assists the growth of big crystallites and even the formation
of continuous compact thin films. On the other hand, an isothermal recrystallization of
semiconductor polycrystalline powders in the presence of a liquid phase of a suitable solvent
material such as KI or NaI, called flux, in an amount sufficient for repelling the initial
crystallites, leads to the formation and growth of semiconductor powder materials with single-
crystalline grain structure and narrow-disperse granularity, so-called monograin powders. The
driving force in this growth process is the differences in surface energies of crystals of
different sizes. The growth of single-crystalline powder grains takes place at temperarures

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higher than the melting point of the used flux material – much lower than the melting point of
the semiconductor compound [1]. The liquid phase of a solvent material is also usable for the
synthesis of complicated multicomponent semiconductor compounds such as Cu
2
ZnSnS
4
and
Cu
2
ZnSnSe
4
used as absorber materials in solar cells [2]. As a rule, the synthesis of
Cu
2
ZnSnSe
4
(CZTSe) monograin materials in molten fluxes results in homogeneous material
[3]. Conventionally, the flux materials should have low melting temperatures and high
solubilities in water, making the separation of powders from the flux easier. There are several
suitable flux materials, such as KI, NaI and CdI
2
, available for the synthesis and monograin
growth or recrystallization of these Cu
2
ZnSnS
4
and Cu
2
ZnSnSe
4
absorber materials. In the
previous papers [4, 5], we presented the results of studies of the formation of Cu
2
ZnSnSe
4
in
molten CdI
2
and KI. The present study deals with the possible chemical reactions between the
binary precursor compounds – CuSe, SnSe, ZnSe – in molten NaI
comparing them with the
similar processes in KI. Differential thermal analysis (DTA) runs were used for determining
the thermal effects. The phase compositions in mixtures of binary precursors and flux
materials were determined by X-ray diffraction (XRD) and Raman spectroscopy. In this
paper, we present the results of the Cu
2
ZnSnSe
4
formation in NaI and compare them with
findings obtained in the previous investigations using KI as flux. The aim is to find suitable
preparation conditions for the synthesis of Cu
2
ZnSnSe
4
, starting from binary chalcogenides.
2. METHODOLOGY
The Cu
2
ZnSnSe
4
powder materials were synthesized from CuSe, ZnSe and SnSe
precursors in molten KI or NaI in sealed quartz vacuum ampoules. For determining the
temperatures of phase changes and the interactions between the initial binaries and the flux
material, DTA setups were used. According to the thermal effects found in DTA curves, the
samples of mixtures (NaI+CuSe+ZnSe+SnSe and KI+CuSe+ZnSe+SnSe) were prepared. The
samples were kept for prolonged periods at slightly higher temperatures than the thermal
changes observed in the DTA curves, and then steeped in water. The mass ratio of the binary
precursor compounds and the flux material was kept equal to 1:1. The precursors were mixed
by grinding in a mortar and sealed into degassed quartz ampoules. As a reference for DTA, an
identical empty quartz ampoule was used. The applied heating/cooling rate was 5ºC per
minute.
Room temperature Raman spectra were recorded for phase analysis using a Horiba’s
LabRam HR high-resolution spectrometer equipped with a multichannel CCD detection
system in the backscattering configuration. In micro Raman measurements, the incident laser
light with a wavelength of 532 nm can be focused on a 1 µm diameter spot of the studied
sample and the composition of the surface can be analysed. XRD measurements were
performed using a Bruker D5005 diffractometer (Bragg-Brentano geometry) Cu Kα1
radiation with λ=1.5406 Å at 40 kV, 40 mA and graphite monochromator. For composition
determination, the ICDD PDF-4 + 2009 database was used.
3. RESULTS AND DISCUSSIONS
3.1. Differential thermal analysis (DTA)
The DTA heating/cooling curves of a KI/CuSe/ZnSe/SnSe and a NaI/CuSe/ZnSe/SnSe
mixtures are presented in Fig. 1 (parts a and b). As a reference, the DTA heating/cooling
curves of a KI/empty ampoule and a NaI/empty ampoule are also presented (c and d parts of
Fig. 1). The endothermic peak around 377ºC occurring in both heating curves (and

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corresponding exothermic peak at 410ºC in the cooling curve of NaI/CuSe/ZnSe/SnSe) was
seen also in quasi-binary system of CuSe/KI [5]. Therefore, we attribute this peak to the
peritectic temperature of 377ºC – phase transformation of CuSe to Cu
2-x
Se+Se – as shown in
the phase diagram of Cu-Se [6]. The peak at 522ºC in the cooling curve of
KI/ZnSe/CuSe/SnSe can correspond to the monotectic point of Cu-Se phase diagram ( 51.2
at.% Se by T. Gödecke et al.)) [7]. In the vicinity of the melting temperatures of pure KI
(686ºC) and NaI (658ºC) [5], but at a slightly lower temperatures, endothermic effects at 676
and 652ºC (melting processes) can be seen followed by intense exothermic effects at 685 and
661ºC. In the same temperature region, the cooling curves of both systems studied show
endothermic peaks (662 and 639ºC). The peaks of cooling curves at 649 and 626ºC are
attributable to the solidification processes of the molten phases of KI and NaI with the other
compounds dissolved in them.
It was established in [8] that Cu
2
ZnSnSe
4
melts incongruently at 788ºC leaving
ZnSe:Cu:Sn in the solid phase. Therefore, the peaks at 788ºC in the heating curve and at
777ºC in the cooling curve of KI/ZnSe/CuSe/SnSe can be attribeted to the
melting/solidification of CZTSe and are in good agreenment with reference data [8]. Similar
peaks in the heating/cooling curves of NaI/ZnSe/CuSe/SnSe – at 778ºC/770ºC respectively
are slightly shifted in comparision with KI/ZnSe/CuSe/SnSe curves. This shift could be
connected with the formation of a sodium containing quaternary compound similar to CZTSe
(Na
2
SnSe
3
as precursor for it was found in the XRD pattern of the NaI/ZnSe/CuSe/SnSe
mixture heated at and quenched at 790ºC (see Fig. 3).
The peak at 604ºC in the cooling curve of NaI/ZnSe/CuSe/SnSe may belong to the
melting point of CuI (T
m
= 606ºC) [9] found by XRD studies (see Fig. 3).
Fig. 1. DTA curves of KI, NaI and the mixtures for synthesis of quaternary CZTSe.
Heating/cooling rate was 5ºC/min (heating corresponds to the red plot; cooling – blue). Using
arrows on the x axes, the temperatures of prolonged heating of the samples for Raman and
XRD analyses are marked

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3.2. Raman analysis
In order to get the true average results for every sample, at least 5 Raman spectra were
recorded. The experimental data are presented in Table 1. We also measured the Raman
spectra of NaI·2H
2
O powder. Experimentally recorded Raman peaks of NaI·2H
2
O were found
at 95, 107, 122, 160, 167, 189, 194 and 225 cm
-1
. NaI·2H
2
O was detected in every
CuSe/ZnSe/SnSe/NaI sample since NaI is very hygroscopic and NaI·2H
2
O is easily formed.
KI peaks were detected at 90, 95, 102, 106 cm
-1
.
From the experimental Raman data, it is observed that only the binary compounds CuI,
ZnSe, CuSe, SnSe and SnSe
2
can be found in the mixtures heated at low temperatures (250-
270ºC), see in Table 1. The formation of CuI indicates the chemical interaction of KI and NaI
with CuSe. In Raman spectra of samples heated up to 380–400ºC, the most intensive Raman
peak is characteristic to the ternary compound Cu
2
SnSe
3
at 180 cm
-1
together with Raman
peaks of CZTSe at 171 and 195 cm
-1
[3]. In NaI CZTSe can be formed already at 380ºC. The
appearance of elemental Se indicates the decomposition of CuSe. CuI and elemental Se can be
found by their characteristic peaks at 139 [11] and 240 cm
-1
[12], correspondingly.
The
samples quenched at 790ºC (slightly higher than the melting temperature of CZTSe) show
again Raman spectra of different binary compounds. Besides the spectra of CZTSe with peaks
at 195 and 171 cm
-1
, there are spectra with intensive peaks at 178, 240, 251 and 262 cm
-1
belonging to Cu
2
SnSe
3,
Se, ZnSe and CuSe, respectively (see Fig. 2). It was found in [8] that
Cu
2
ZnSnSe
4
melts incongruently at 788ºC leaving ZnSe in the solid phase. Due to the non-
equilibrium cooling of the mixture (quenching), the formation of CZTSe is incomplete in the
cooling process and therefore the Raman spectra show the characteristic peaks of Cu
2
SnSe
3,
ZnSe, CuSe and Se. Overpressure of Se avoids the decomposition of CuSe [6].
Fig. 2. Raman spectra taken from different places of the CuSe/ZnSe/SnSe/NaI sample

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quenched at 790ºC
Table 1. The phases detected by Raman analyses in the mixtures of CuSe/ZnSe/SnSe/KI and
CuSe/ZnSe/SnSe/NaI annealed at different temperatures (selected annealing temperatures are
based on the DTA results)
CeSe/ZnSe/SnSe/KI CuSe/ZnSe/SnSe/NaI
Annealing
temp.,
ºC
Raman
peaks,
cm
-1
Compon. Reference Annealing
temp.,
ºC
Raman
peaks,
cm
-1
Compon. Reference
263 CuSe Experim.
and [3] 263 CuSe Experim.
and [3]
187 SnSe
2
[10] 181 Cu
2
SnSe
3
[3]
139 CuI [11] 139 CuI [11]
254 ZnSe
Experim.
and
[3, 14] 204, 252 ZnSe
Experim.
and
[3, 14] 240 Elemental
Se [12]
250, 270
380
81, 170,
191, 231 CZTSe [3]
263 CuSe Experim.
and [3]
180 Cu
2
SnSe
3
[3] 186 SnSe
2
[10, 13]
171,
195, 234 CZTSe [3] 171,
194, 236 CZTSe [3]
400
205, 251 ZnSe
Experim.
and
[3, 14]
615
251 ZnSe
Experim.
and
[3, 14]
205, 250 ZnSe
Experim.
and
[3, 14]
249 ZnSe
Experim.
and
[3, 14]
520 171,
195,
220, 234
CZTSe [3]
650
171,
194, 232 CZTSe [3]
107,
127, 149 SnSe [10] 139 CuI [11]
187 SnSe
2
[9, 13] 178 Cu
2
SnSe
3
[3]
81, 171,
195, 233 CZTSe Experim.
and [3] 171, 195 CZTSe [3]
240 Elemental
Se [12]
250 ZnSe
Experim.
and
[3, 14] 251 ZnSe
Experim.
and
[3, 14]
680
790
262 CuSe Experim.
and [3]
3.3. XRD analysis

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CuSe/ZnSe/SnSe/KI: XRD phase analysis data are presented in Fig. 3. All the samples
also contain crystalline KI, not shown in Fig. 3. From the XRD data, the route of the CZTSe
formation can be drawn. In some samples heated at 250ºC, Cu
1.85
Se was found. SnSe
2
was
detected in samples heated at 400ºC. Se, released from CuSe, reacts with SnSe forming SnSe
2.
Reaction of Cu
2
Se with SnSe
2
results in the formation of Cu
2
SnSe
3
, as it was already
proposed by F. Hergert and R. Hock [15]. Cu
2
SnSe
3
was found in samples heated at 520ºC. At
680ºC (a little below the melting point of KI), Cu
2
ZnSnSe
4
is formed from Cu
2
SnSe
3
and
ZnSe. This provides some evidence that the formation of the liquid phase of the flux material
is the main mediator for the synthesis of CZTSe. Micro XRD measurements done from
individual grains of the sample quenched at 790ºC gave the information that in the molten
phase CZTSe decomposes to Cu
2
SnSe
3
and to Cu
2
Se, SnSe
2
and ZnSe binary phases.
CuSe/ZnSe/SnSe/NaI: The XRD results confirm the formation of Cu
2
ZnSnSe
4
in NaI
already at 380ºC. The formation of CZTSe starts also with the release of Se from CuSe and
through the formation of SnSe
2,
as it was the case in the KI flux. No Cu
2
SnSe
3
was found by
XRD. Se-rich phase Cu
0.87
Se coexists with the Cu
1.82
Se at very low temperatures of synthesis,
giving information that CuSe precursor has not been homogeneous single phase material and
more attention has to be payed to the phase composition of precursors got from different
producers. All samples also contain CuI. The particularity of using NaI flux is the formation
of different phases containing Na and oxygen (Na
2
SnSe
3,
Na
2
Cu(OH)
4
, Na
2
SeO
4
), as found by
XRD in all samples investigated. All sodium containing compounds are well soluble in water
and can be separated, except CuI, because CuI is poorly soluble in water (0.00042 g/L at
25ºC), but it dissolves in the presence of NaI or KI to give the linear anion [CuI
2
]
−
. Dilution of
such solutions with water re-precipitates CuI [16]. The by-products such as NaI·2H
2
O,
Na
2
SeO
4
, Na
2
Cu(OH)
4
, CuI and Na
2
SnSe
3
can be separated from CZTSe absorber by washing
it with distilled water. The hydroscopic nature of NaI makes the syntheses process
complicated and requires the preparation of samples in a glove box, because the formation of
NaI·2H
2
O as a result of the NaI hydrolysis process is detected.

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Fig. 3. The formed phases in CuSe/ZnSe/ SnSe/KI (lower part) and CuSe/ZnSe/SnSe/NaI
(upper part) mixtures annealed at different temperatures and detected by XRD
On the basis of the Raman and the XRD results obtained from the mixtures
CuSe/ZnSe/SnSe/NaI and CuSe/ZnSe/SnSe/KI, it can be derived that Se, released in the
decomposition of CuSe to Cu
1.8
Se, reacts with SnSe forming SnSe
2
. However, CuSe does not
completely decomposes due to the formation of Se overpressure in closed ampoules as CuSe
was detected by Raman and XRD measurements in all samples. The formation of CZTSe in
KI involves intermediate reaction steps where the formation of Cu
2
Se, SnSe
2
and the ternary
compound Cu
2
SnSe
3
from the formers can be derived. Raman measurements show that
Cu
2
SnSe
3
and CZTSe already form at temperatures lower than 400ºC. However, they are not
yet detectable in XRD measurements due to the lower sensibility of XRD. At lower
temperatures, CZTSe formation is more intensive in NaI. Most likely, some sodium
containing compound is involved in the chemical route. The extensive formation process of
CZTSe starts close to the melting point of flux (KI or NaI). Comparison of the DTA curves
reveals that melting of the mixtures starts even at lower temperatures than the melting of pure
KI or NaI. The occurrence of an intensive exothermic peak right after an endothermic peak
due to the melting process suggests a strong chemical interaction between the precursors.
3. CONCLUSIONS
By DTA, XRD and Raman spectroscopy data, it can be concluded that in the presence
of solid KI or NaI, the formation of CZTSe is inhibited. The chemical interactions are induced
by the formation of the liquid phase in the mixture of materials for the synthesis of the
quaternary compound. The melting and extensive formation process of CZTSe starts close to
the melting point of flux (KI or NaI). NaI as flux material is preferable, due to the possibility
to reduce the synthesis temperature. The hygroscopic nature of NaI requires the preparation of
samples in a glove box.
The phase in-homogeneity of the used precursors was revealed and has to be carefully
studied.
ACKNOWLEDGEMENTS
This research was supported by the Doctoral Studies and Internationalization Program
DoRa of the European Social Funds, the Estonian Ministry of Education and Research
Contract No. SF0140099s08, and the EAS project EU29713.
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