FORMATION OF COPPER ZINC TIN SULFIDE IN CADMIUM IODIDE FOR MONOGRAIN MEMBRANE SOLAR CELLS

Abstract

The formation process of the quaternary Cu2ZnSnS4 compound in CdI2 is studied. Focus is on chemical reactions between the binary precursor compounds involved in the formation process of CZTS and reactions of the precursor compounds with molten CdI2 as a flux material. The aim was to describe conditions for the synthesis of CZTS as an absorber material and to determine the presence of cadmium and secondary phases in the final product. Differential thermal analysis (DTA) was used to show the thermal effects, including the melting points, the various phase transitions and possible reactions in the samples. Closed quartz vacuum ampoules were used for the heating/cooling process of the mixtures. An empty ampoule was used as a reference. Various mixtures of the individual precursors with CdI2 as well as the mixtures used for CZTS synthesis in CdI2 were annealed and quenched from different temperatures. The phase composition of the mixtures was determined by X-Ray diffraction (XRD), Energy Dispersive X-ray (EDX), and Raman Spectroscopy. A possible chemical route of the CZTS formation is discussed. It was found that CZTS forms from Cu2SnS3 and ZnS if sufficient elemental S is added into the precursor mixtures.

Full text

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FORMATION OF COPPER ZINC TIN SULFIDE IN CADMIUM IODIDE
FOR MONOGRAIN MEMBRANE SOLAR CELLS
G. Nkwusi, I. Leinemann, J. Raudoja, M. Grossberg, M. Altosaar, D. Meissner
Institute of Materials Science, Tallinn University of Technology
Ehitajate tee 5, 19086 Tallinn – Estonia
Phone: +3726203362
Email: godswillnkwusi@yahoo.com
R. Traksmaa
Centre for Materials Research, Tallinn University of Technology
Ehitajate tee 5, 19086 Tallinn – Estonia
T. Kaljuvee
Laboratory of Inorganic Materials, Tallinn University of Technology
Ehitajate tee 5, 19086 Tallinn – Estonia
ABSTRACT
The formation process of the quaternary Cu2ZnSnS4 compound in CdI2 is studied. Focus is on
chemical reactions between the binary precursor compounds involved in the formation process of
CZTS and reactions of the precursor compounds with molten CdI2 as a flux material. The aim was to
describe conditions for the synthesis of CZTS as an absorber material and to determine the presence of
cadmium and secondary phases in the final product. Differential thermal analysis (DTA) was used to
show the thermal effects, including the melting points, the various phase transitions and possible
reactions in the samples. Closed quartz vacuum ampoules were used for the heating/cooling process of
the mixtures. An empty ampoule was used as a reference. Various mixtures of the individual
precursors with CdI2 as well as the mixtures used for CZTS synthesis in CdI2 were annealed and
quenched from different temperatures. The phase composition of the mixtures was determined by X-
Ray diffraction (XRD), Energy Dispersive X-ray (EDX), and Raman Spectroscopy. A possible
chemical route of the CZTS formation is discussed. It was found that CZTS forms from Cu2SnS3 and
ZnS if sufficient elemental S is added into the precursor mixtures.
Keywords : Cu2ZnSnS4, XRD, EDX, Raman, DTA
1. INTRODUCTION
Resulting from the world’s fast population growth rate, the increasing energy demand in
the near future forces us to seek environmentally clean and economically viable alternative
energy resources that could replace those we have currently without any fear of further
significant environmental impact. Among the various options available to alternative sources
of energy, solar energy has been proven a viable alternative to meet our energy demands. But
despite being clean and inexhaustible, the energy produced from solar radiation has only
contributed a very minimal percentage of the total energy demand. During the past decades,
considerable work has been done in order to achieve the aim of taking solar derived energy to
a significant level in the energy sector and substantial progress has been made. Followed from
proper considerations, it is expedient to develop a solar panel from environmentally friendly
and readily available materials at low cost. So far the most efficient semiconductor
compounds used as solar absorber materials in large scale production are CuInGaSe2 (CIGS)
and CdTe (company First Solar) with record conversion efficiencies of 20.2 and 17.3 %,
respectively [1-5]. The market for thin-film PV grew at a 60% annual rate from 2002 to 2007

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and is still growing rapidly. Cu2ZnSnS4 (CZTS) with an absorption coefficient ˃ 10-4 cm-1 as
an absorber material has received increasing attention in the past few years due to the
enormous advantages embedded in it in terms of material availability, product efficiency
relative to the cost of production and ease of handling. It may become a perfect replacement
for CIGS, CdTe and other absorber materials that have been developed. Currently CZTS has
had quite an enormous improvement in terms of performance proven already to yield solar
efficiencies of more than 10 %, as reached by the IBM company using hydrazine solutions
[4]. In the monograin technology a solvent material is used for absorber material synthesis, a
so-called flux material (CdI2). A molten phase between the solid precursor particles acts as a
contracting or repelling agent depending on its amount. An isothermal recrystallization of
semiconductor polycrystalline powders in the presence of a liquid phase of a suitable solvent
material 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, a so-called monograin powders. In thin film technologies, flux materials
are also often used in the form of some low melting precursor that is consumed in the film
growth process – molten phase between particles enables sintering and crystal growth (for
example, CuSe in thin film deposition of CIGS). The role of flux material makes the main
difference between monograin growth and thin film synthesis methods, such as physical
vapour deposition, chemical vapour deposition (CVD) or chemical baths deposition (CBD),
widely used for thin film synthesis. The driving force in the isothermal crystalline growth
process of monograin materials is in the differences in the surface energies of crystals of
different sizes. The growth of single-crystalline powder grains takes place at temperatures
higher than the melting point of the used flux material – much lower than the melting point of
the semiconductor compound. An optimal amount of the used flux material is observed if the
volume of the liquid phase is around 0.7 of the volume of the solid phase. The advantage of
using a flux is that it allows powder materials to be produced where every grain is single-
crystalline and has uniform composition [6, 7]. In monograin layer (MGL) solar cells each
single crystal is working as an individual solar cell.
A production process of MGL solar cells has been developed in our spin-off company
Crystalsol for the first production line for flexible CZTS solar cell modules. A roll-to-roll
process was designed for the large scale device preparation. Cu2ZnSnS4 (CZTS) monograin
powders synthesized in KI have been used as absorber materials in monograin membrane
solar cells with efficiencies around 8 %.
The monograin powder growth of semiconductor compounds in molten salts started in
the Philips Company. Ties Siebold te Velde from the Philips Company filed the first patent in
1964 on LED based ZnS or CdS films, using already a p/n junction and during the next year
he filed the first patent on monograin membrane devices [8].
The monograin powder granulometry is characterized by sieving analysis. Due to the
large grain size, the XRD measurements are not used to measure the grain size and the Touc
plots cannot be used to characterize the crystalline material. Narrow granulometric fractions
(in between 32-100 µm) of the grown monograin powder grains of CZTS are separated by
sieving and are used as an absorber material in MGL solar cell structures:
graphite/CZTSe/CdS/ZnO. Powder crystals are covered with CdS thin layer by chemical bath
deposition. A monolayer of nearly unisize grains is embedded into a thin layer of epoxy resin,
so that the contamination of upper surfaces of crystals with epoxy is avoided. The polymer
film thickness was adjusted to half of the grain size. Since the grains sink into the polymer
and reach the underneath rubber glue layer, after washing completely off the rubber glue, the
lower part of each grain sticks out of the polymer film. After polymerization of epoxy, ZnO
window layer is deposited onto the front side of the monograin layer by RF-sputtering. Solar
cell structure is completed by the vacuum evaporation of 1-2 µm thick In grid contacts onto

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the ZnO window layer. After gluing the structures on glass, the back contact area of crystals is
opened by etching epoxy with H2SO4, followed by an additional abrasive treatment. The back
contact is made using graphite paste [9].
One of the peculiarities of the materials synthesized and recrystallized in molten salts is
the high level of contamination of the semiconductor compound with constituent elements of
the used salt. Therefore, the replacement of KI with some other salt not containing K as a
foreign element for the CZTS compound, for example, with CdI2 is of great importance. In
our previous report [10] we studied the formation process of Cu2ZnSnSe4 in CdI2. It is known
that Cd from CdI2 incorporates to the crystals of CZTSe forming a solid solution of
Cu2ZnxCd1-xSnSe4. In Cu2ZnxCd1-xSnSe4 solid solutions the direct band gap of material is
shifting to the lower energy side, as shown in [11]. The band gap value of Cu2CdSnS4 is 1.39
eV by Matsushita et al. [12]. However, the formation of Cu2ZnSnS4 in CdI2 has not yet been
studied. The incorporation of Cd to the crystal lattice of Cu2ZnSnS4 could shift the band gap
energy of the solar absorber material from 1.43 eV (CZTS) [11, 13] to the lower energy side,
enabling better fitting with solar spectrum. CdI2 as a flux was chosen due to the low melting
temperature in comparison with KI and NaI, which have also been used as flux materials for
the synthesis of Cu2ZnSnSe4. Our previous research showed that the formation of single-
crystalline powder grains takes place at temperatures just above the melting point of the flux
material, but much lower than the melting point of the semiconductor compound itself (990
°C) [12].
In the present work various mixtures of the individual precursors with CdI2 as well as
the mixtures used for CZTS synthesis in CdI2 were annealed and quenched at different
temperatures and the phase compositions of the mixtures were analysed by X-Ray diffraction
(XRD), Energy Dispersive X-ray (EDX) and Raman Spectroscopy. The possible chemical
route of the CZTS formation is discussed.
2. METHODOLOGY
The synthesis of quaternary Cu2ZnSnS4 powders from the binary precursors Cu2S, SnS,
and ZnS with additional S were carried out in molten CdI2 using sealed quatz vacuum
ampoules. The precursors and the flux material were mixed by grinding in an agate mortar in
a mass ratio of 1:1, and sealed in a degassed quartz vacuum ampoule. The precursors for
CZTS synthesis were used in their stoichiometric ratio. A DTA set up was used to determine
the temperatures of phase changes and chemical interactions between the initial binaries and
the flux material. As a reference point, an empty degassed and sealed quartz ampoule of equal
mass was used. The heating rate was 5 °C/min and two heating cycles were recorded starting
from room temperatures up to 800 °C. The temperatures of the peak positions in the DTA
curves are determined. To identify thermal effects found in the DTA curves, probe mixtures
with identical proportions as in the DTA samples (using larger amounts than for DTA
samples) were prepared and heated separate individual probes for every heating and every
cooling. The samples were either heated up or cooled down and then quenched at the
specified temperatures. The phases formed in these samples were determined by Raman and
XRD analysis after opening the ampoules. The number of samples prepared for Raman and
XRD measurements corresponds to the number of peaks in the DTA curves. The Raman
spectra were recorded using a Horiba LabRam HR high resolution spectrometer equipped
with a multichannel CCD detection system in backscattering configuration. Incident laser
light of 532 nm was focused on different 1 μm2 spots of the studied sample and an average of
five readings were taken for every sample to obtain an average result of the sample. The XRD
measurements were performed using a Bruker D5005 diffractometer. For the analysis, the

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ICDD-4 + 2009 data base was used. EDX was used to determine the elemental composition of
the precursors and the formed compounds.
3. RESULTS AND DISCUSSION
3.1. Interactions of individual precursor compounds with CdI2 as a flux material
Our previous report showed that pure CdI2 has an endothermic effect in the DTA
heating curve at 385 °C attributed to its melting process [10]. When elemental S was added to
CdI2 in the present study, there was no chemical reaction observed, but a decrease of the
melting point from 385 to 370 °C was noticeable in the DTA heating curve and to 382 °C in
the cooling curve. Also, DTA curves of ZnS mixture with CdI2 show melting/solidification of
the mixture at 379/351 °C, no chemical reaction was observed by XRD.
CdI2 mixed with SnS melted at 347 °C. CdS and different iodides (SnI4, Sn2SI2) formed
at the same time. SnS+CdI2 freezed at 300 °C.
The Cu2S+CdI2 mixture melted at 353 °C. CuI, Cu2Cd3I4S2 formed and Cu2S
transformed to Cu1.96S in the same temperature range of melting. Cu1.96S retransformed to
Cu2S by cooling down in accordance with the report in [14]. An endothermic peak around 400
°C in the DTA heating curve (Fig. 1, 1a) of the Cu2S+CdI2 mixture corresponds to the
formation of CdS and Cu4Cd3 intermetallic compound. The mixture of Cu2S+CdI2 freezed at
333 °C. The phases formed during heating and cooling are given in Table 1.
3.2. Interactions between mixtures of precursors and CdI2 as a flux material
Fig. 1. DTA heating (red) and cooling (blue) curves of the quasi-binary system (1a), ternary
systems (1b, 1c), and the sample of the mixture used for the quaternary CZTS synthesis (1d)

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The peak of low intensity at 105 °C in the Fig. 1 (1a, 1b, 1c) DTA heating curves can be
attributed to the phase transformation of monoclinic to low-chalcocite (αCh) Cu2S , which is
stable up to 103.5 ± 0.5 °C for a stoichiometric composition of Cu2S, and up to 90 ± 2 °C for
33.41 at. % S [15].
The S-deficient (considering the possible formation of the stable compounds Cu2SnS3
and Cu2ZnSnS4) mixtures of Cu2S+SnS+CdI2 and Cu2S+SnS+ZnS+CdI2 (see fig. 1b and 1c)
melt at 320 °C and did not form any new compounds even if heated up to 800 °C, in the
cooling cycle both effects – at 632 and 502 °C – correspond to the formation of CdS, CuI and
Sn2SI2 [16] as detected by XRD presented, see in Table 1. The existance of Cu1.96S was found
between 250-320 °C. In other regions of temperatures Cu2S existed. The mixtures solidified at
273 and 284 °C. The peak at 264 °C involves not just the solidification process of the
Cu2S+SnS+ZnS+CdI2 mixture, but it also involves the incorporation of Cd into the ZnS
structure, the formation of (Zn1-xCdx)S and ZnI2, while the released S reacts with Cu2S and
SnS forming the ternary compounds Cu2Sn3S7 [17] and Cu4SnS6 [18]. In the second heating
cycle the ternary compounds decompose forming Cu81Sn22 and CuS phases (Table 1).
Table 1. Overview of the phases detected by Raman and XRD. EDX elemental composition
data were used for an additional confirmation of the formed phases. The samples were
prepared und quenched at slightly higher temperatures than the observed effects in the DTA
heating curves, and at slightly lower temperatures than the effects in the cooling curves.
Studied
mixture
Annealing
temperature OC
Phases by XRD
Phases by Raman
Compound
Raman
shift,
cm-1
ZnS+CdI2
Heated up to 800 oC
ZnS, CdI2
-
-
SnS+CdI2
Heated up to 800 oC
CdI2, CdS, Sn2SI2,
SnI4, SnS
CdS
301 [5]
Heated up to 800 oC
and cooled down to 250 oC
CdI2, CdS, Sn2SI2,
SnI4, SnS
-
-
Cu2S+CdI2
Heated up to 370 oC
Cu1.96S, CuI,
Cu2Cd3I4S2
CdI2
CuI
CdS
110 [10]
148 [6, 7]
294 [5]
Heated up to 400 oC
CdI2, CdS, CuI,
Cu4Cd3
CdI2
111 [10]
Heat up to 800 oC and
cooled down to 330 oC
CdI2, CdS, Cu2S,
Cu2Cd3I4S2
CdI2
CuI
CdS
111 [10]
150 [6, 7]
298 [5]
Cu2S+SnS+CdI2
Sulphur
deficient
composition for
Cu2SnS3
formation
Heated up to 800 oC and
cooled down to 270 oC
CdI2, CdS, Cu2S,
Cu1.96S, CuI, Sn2SI2
Raman spectra are very
difficult to analyze due to the
overlapping of peaks of
multi-component systems.
Heated up to 800 oC and
cooled down to 150 oC,
then heated up to 290 oC
CdI2, CdS, Cu2S,
CuI, SnS, Sn2SI2

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Cu2S+ZnS+
SnS +CdI2
Sulphur
deficient
composition for
CZTS
Heated up to 760 oC
CdI2, Cu2S, SnS,
ZnS
Raman spectra are very
difficult to analyze due to the
overlapping of peaks of
multi-component systems.
Heated up to 800 oC
and cooled down to
270 oC
CdI2, CdS, Cu2S,
SnS, Sn2SI2, ZnS,
CuI,
Heated up to 800 oC
and cooled down to
250 oC
CdI2, CdS, Cu2S,
Cu1.96S, CuI,
Cu2Sn3S7, Cu4SnS6,
Sn2SI2, ZnS, ZnI2,
(Zn1-xCdx)S
Heated up to 800 oC
and cool down to
150 oC, then heated up
to 290 oC
CdI2, CdS,Cu2S,
CuI, SnS, Sn2SI2
Heated up to 800 oC
and cooled down to
150 oC, then heated up
to 320 oC
CdI2, CdS, Cu2S,
Cu1.96S, CuS, CuI,
Cu81Sn22, Sn2SI2,
ZnI2,
Cu2S+ZnS+
SnS+S+CdI2
Heated up to 500 oC
CdI2, Cu2SnS3,
Cu2ZnSnS4**, ZnI2
CdI2
CuI
Cu2ZnxCd1-xSnS4
110 [10]
145
[6, 7]
336*
Heated up to 800 oC
CdI2, CuI, Cu2SnS3,
Cu2ZnSnS4**
CdI2
CuI
Cu2ZnxCd1-xSnS4
110 [10]
145
[6, 7]
336*
Heated up to 800 oC
and cooled down to
600 oC
CdI2, Cu8S5,
Cu2SnS3, SnI4,
Cu2ZnSnS4**
CdI2
CuI
Cu2ZnxCd1-xSnS4
110 [10]
146
[6, 7]
335*
Heated up to 800C and
cooled down to 350 oC
CdI2, CuI, SnI4,
Cu2ZnSnS4**
CdI2
CuI
CZTS
Cu2ZnxCd1-xSnS4
110 [10]
145
[6, 7]
166,
250,
286,
336*,
374 [19]
* Raman peak position of Cu2ZnSnS4 is shifted from 338 cm-1 to 336 cm-1 due to the Cd
incorporation into Cu2ZnSnS4 and formation of solid solution of Cu2ZnxCd1-xSnS4 [11].
** Close lattice parameters do not allow to identify Cu2ZnxCd1-xSnS4 phase from Cu2ZnSnS4
phase by XRD patterns [12], EDX analyses show about 3 at% of Cd in CZTS monograins
synthesized in CdI2.
3.3. Synthesis of CZTS
The mixture of the binary precursors corresponding to the required stoichiometric
compositon for the formation of pure Cu2ZnSnS4 in CdI2 (Cu2S+SnS+ZnS+S+CdI2) melts and
solidificates at 366 and 353 °C, showing endo/exo-thermic peaks in DTA curves. Besides the
formation of CZTS, a Cu2SnS3 ternary compound [17, 18] and ZnI2 were found in the sample,
when heated and quenched at 500 °C.

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The endothermic effects at 432 °C and 495 °C correspond to the border crossing of the
equilibrium between stoichiometrical Cu2S (Cu-rich) and Cu-deficient areas to form the
defect compound Cu2-xS [15].
Samples quenched at 800 °C show that the ternary compound reacts with ZnS to form
CZTS as a final product. The secondary phases CuI, Cu2-xS, and SnI4 remain in the cooling
process. The detected phases are presented with their characteristic peaks in the Raman
spectra (Fig. 1) and in the XRD pattern (Fig. 2), taken from the sample that were heated up to
800 °C and cooled down to 350 °C. CZTS is the prevailing phase with its characterestic
Raman peaks at 166, 250, 286, 336, 374 cm-1. CdI2 at 110 cm-1 and CuI at 145 cm-1 was
detected. CuI is dissolvable in KI or NaI solutions, as also reported in our previous report,
allowing to separate single phase CZTS [6].
Fig. 1. Raman spectrum of the mixture of Cu2S+SnS+ZnS+S+CdI2 heated up to 800 °C,
cooled to 350 °C and quenched. Raman peak position of CZTS is shifted from 338 cm-1 to 336
cm-1 due to the Cd incorporation into CZTS and formation of solid solution of
Cu2ZnxCd1-xSnS4 [11].

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Fig. 2. XRD pattern of the mixture of Cu2S+SnS+ZnS+S+CdI2 heated up to 800 °C, cooled to
350 °C and quenched. The marked pattern area was used for determination of CZTS (dark
blue in figure a) and CZTS co-existing with Cu2SnS3 (light blue in b) phases
It is seen from Table 1, that ZnS reacts with CdI2 only if other binaries are present,
forming ZnI2 and Cd-containing ternary sulphides Zn1-xCdxS, detectable by XRD and EDS. In
CdI2 forms solid solution of Cu2ZnxCd1-xSnS4 at T≥500 °C. The chemical pathway of CZTS
synthesis in CdI2 can be described as follows:
SnS + S → SnS2 [17, 18] (1)
Cu2S + SnS2 → Cu2SnS3 [17, 18] (2)
ZnS + x CdI2 → Zn1-xCdxS + x ZnI2 (only in the presence of other binaries) (3)
Cu2SnS3 + Zn1-xCdxS → Cu2Zn1-xCdxSnS4 (4)
or Cu2S + SnS + Zn1-xCdxS + S → Cu2Zn1-xCdxSnS4 (5)
CZTS formation can be described as two stage process: first Cu2SnS3 forms from Cu2S,
SnS and S (1, 2); secondly Cu2SnS3 reacts with Zn1-xCdxS forming Cu2Zn1-xCdxSnS4 (3, 4).
However, Cu2Zn1-xCdxSnS4 can be directly formed from the binaries Cu2S, SnS, Zn1-xCdxS in
the presence of sufficient amount of elemental sulphur (5).
4. CONCLUSIONS
CdI2 mixed with ZnS, Cu2S or SnS melts at temperatures much lower than pure CdI2
due to the freezing-point depression effect. CdS and different iodine-containing compounds
CdI2
Cu2ZnSnS4
SnI4
CuI

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(SnI4, Sn2SI2, CuI, and Cu2Cd3I4S2) form in CdI2+Cu2S and CdI2+SnS. ZnS reacts with CdI2
only if other binaries are present forming ZnI2 and Cd-containing ternary sulphides Zn1-xCdxS.
In the S-deficient mixtures (considering the stoichiometry of Cu2SnS3 and
Cu2ZnSnS4), the ternary compound Cu2SnS3 and quaternary CZTS do not form.
In CdI2 the mixture of precursors with stoichiometric composition results in solid
solution of Cu2ZnxCd1-xSnS4 at T≥500°C. Additional sulphur leads to successful CZTS
synthesis.
CZTS formation can be discribed as two stage process: first forms Cu2SnS3; secondly
Cu2SnS3 reacts with Zn1-xCdxS, forming Cu2ZnxCd1-xSnS4. Also, Cu2ZnxCd1-xSnS4 can be
directly formed from the binaries in the presence of sufficient amounts of elemental sulphur.
A single phase Cu2ZnxCd1-xSnS4 can be separated by washing away the flux and removing
secondary phases by etching.
5. 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
Contracts No. SF0140099s08, TK117, AR10128 and by grants (No 8964, ETF 9425).
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