Formation of copper zinc tin sulfide in cadmium iodide for monograin membrane solar cells

CYSENI 2012, May 24-25, Kaunas, Lithuania

ISSN 1822-7554, www.cyseni.com

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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: [email protected]

R. Traksmaa

Centre for Materials Research, Tallinn University of Technology


T. Kaljuvee

Laboratory of Inorganic Materials, Tallinn University of Technology


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

mailto:[email protected]



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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


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


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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Documents

Formation of copper zinc tin sulfide in cadmium iodide for monograin membrane solar cells