ORIGINAL PAPER

A water- and sulfurization-free solution route to Cu2-xZn1+xSnS4

Renato D’Angelo • Cristy Leonor Azanza Ricardo •

Alberto Mittiga • Paolo Scardi • Matteo Leoni

Received: 24 April 2014 / Accepted: 28 July 2014

� Springer Science+Business Media New York 2014

Abstract Zinc-rich/copper-poor Cu2-xZn1?xSnS4 (x = 0.2,

CZTS) has been successfully produced in film and powder

form using two non-aqueous solutions (of metal salts and

thiourea) without the need for sulfurization during the

annealing phase. A reaction route is proposed and the

choices of the solvents (water, ethyleneglycol, ethanol,

methanol) and of the tin source (tin chloride pentahydrate

or anhydrous) discussed and justified. A pure and coarse-

grained material is obtained with a mix of metal salts in

methanol and thiourea in ethylene glycol. The tin penta-

hydrate salt seems a better alternative to the commonly

used anhydrous chloride.

Keywords CZTS � Non-vacuum � Dip-coating method �Sol–gel � Kesterite

1 Introduction

Thin film solar cells aim to become competitive with tra-

ditional silicon-based devices, as a low cost per kW of the

base materials is coupled to the possibility of building the

cell on flexible and non-flat substrates. Energy harvesting

in a thin film solar cell is obtained via electron–hole pair

formation in a suitable absorber layer. The best absorbers

so far contain cadmium (e.g. CdTe, max laboratory effi-

ciency of 18.3 % [1]) or indium (CIGS, CuInxGa1-xSe2,

max efficiency 20 % [2]), the first one being notoriously

toxic and the other quite expensive.

The research is thus fostered towards finding alternative

absorbersnot containing toxic or expensive elements. The

most interesting results are currently shown by a synthetic

analog of the mineral kesterite with formula Cu2ZnSn

(S, Se)4 (CZTS). An efficiency of 11 % has been already

achieved in laboratory using a partly selenized CZTS [3].

Many processes have been proposed for the production of

CZTS thin films and powders (for solar inks), most of them

based on vacuum deposition, complex synthetic routes or

requiring a sulfurization step, all too expensive for a mass

production. Limiting to cheap production routes, the best

stoichiometry, morphology and band gap (1.44–1.51 eV)

have been obtained with absorbers synthesized from liquid

phase [4]. Conventional aqueous solutions processes entail

the possibility of forming M–O–M bonds in the precursor

solution or during the annealing step, which can lead to the

formation of unwanted extra phases if the wrong solvent is

employed. Recent studies [5, 6] have shown the effects of

water, ethanol, ethylene glycol and methanol as solvents:

near stoichiometric CZTS films were obtained up to

500–550 �C. Park et al. [7] tested nitrogen to replace H2S

for heat treatment and confirm the optimal stoichiometry of

sulfur in the material. Here we report a simple process to

obtain CZTS without vacuum and without sulfurization.

A Zn-rich/Cu-poor CZTS is obtained, beneficial e.g. for

solar energy applications: previous reports [8, 9] suggested

that a Cu-poor material leads to the formation of Cu

vacancies, which are shallow acceptors in CZTS, while a

Zn-rich condition suppresses Cu substitution on the Zn

sites, and thus decreases the quantity of deep acceptors.

R. D’Angelo � C. L. Azanza Ricardo � P. Scardi � M. Leoni (&)

Department of Civil, Environmental and Mechanical

Engineering, University of Trento, via Mesiano, 77,

38123 Trento, TN, Italy

e-mail: matteo.leoni@unitn.it

A. Mittiga

ENEA, Casaccia Research Center, Via Anguillarese 301,

00123 Rome, Italy

123

J Sol-Gel Sci Technol

DOI 10.1007/s10971-014-3462-x

2 Experimental

A solution route, similar to the one proposed in [5–7] is

employed. Copper(II) chloride (Cu(II)Cl2�2H2O, Sigma-

Aldrich, 99.0 % purity), zinc chloride (Zn(II)Cl2, Sigma-

Aldrich, 98.0 % purity) and tin(IV) chloride were chosen

as metal sources. Both the anhydrous and the pentahy-

drate tin (IV) chloride (Sn(IV)Cl4 and Sn(IV)Cl4�5H2O,

respectively, Sigma-Aldrich, 99.0 %) were tested. Thio-

urea (SC(NH2)2, Sigma-Aldrich, 98.0 %) was chosen as

sulfur source.

Solutions were prepared by dissolving 0.45 mol of the

copper source, 0.30 mol of the zinc source and 0.25 mol of

the chosen tin source in 20 mL of solvent using a magnetic

stirrer. Subsequently, 20 mL of a 0.60 M solution of

thiourea were added: this results in a brown sol that turns

white after short stirring and then reverts to a transparent

and limpid yellow liquid after ca. 5 min continuous stir-

ring. The resulting solution can be employed for the pro-

duction of both powders and films. Several combinations of

solvents (water, ethanol, ethylene glycol and methanol)

were tested: a detailed list of synthesis conditions is

reported in Table 1.

Dip-coating was chosen for film deposition: the sub-

strates were dip and retracted at 5 m/s with no yield time in

the beaker. Bare and Mo-sputtered (0.6 lm) soda lime

glass slides (Corning 2947) were used as substrates: 5

specimens were produced with each solution.

We employed a post-deposition annealing in two steps.

The first step, performed at 220 �C for 10 min in air

(T0-air) or in Ar (T0-Ar), respectively for the set A and set

B of specimens, is aimed at removing the process solvents.

The second step at 550 �C in Ar promotes the crystalliza-

tion of the film and, if necessary, completes the sulfuriza-

tion. Two alternative were employed: with 400 mg sulfur

(in the furnace) for 1 h (T1-S-1h) and without sulfur for 3 h

(T1-noS-3h). For more details see Table 1. The sulfur

powder was positioned inside the tubular furnace on an

alumina boat, close to the specimen to be sulfurized.

All samples were characterized using grazing incidence

X-ray diffraction with a fixed h = 0.8�. Diffraction data

were collected on a PANalytical X’Pert MRD diffractom-

eter operated with acobalt source (45 kV, 30 mA) and

equipped with a polycapillary primary optics and a sec-

ondary flat graphite analyzer.

A Jeol JMS 7401F Field-Emission SEM equipped with a

Bruker EDX detector was employed for the compositional/

morphological characterization. The microscope was

operated at 10 kV for the imaging and at 25 kV for the

quantification, using external standards.

Infrared spectra were collected on a PerkinElmer

SPECTRUM ONE FT-IR spectrometer (633 nm Class 1

laser) on liquid droplets using an ATR (Attenuated Total

Reflection) attachment (ZnSe crystal). Raman spectra were

collected on a LabRAM Aramis confocal microRaman

system in backscattering configuration at 532 nm using a

spectrometer with a grating of 1,800 grooves/mm coupled

to an air cooled 1,0249256 VIS CCD.

3 Results and discussion

The solution route presented here allows for a wide flexi-

bility in the type of final specimens that can be produced.

Both films and powders can in fact be produced starting

from the same precursor. As an example, the SEM

micrographs of Fig. 1 show a thin compact film produced

by dip coating and a porous solid produced by sintering a

powder obtained from the same sol. The chemical com-

position of the final specimen is the same, whereas the

porosity (and thus the apparent density) differs enor-

mously. Unless otherwise stated, the data for the films (that

might have more immediate applications) will be shown in

the following.

The X-ray diffraction patterns taken in grazing inci-

dence on the films prepared from the proposed solutions

are shown in Fig. 2. Kesterite (ICDD PDF2 card #26-0575)

is present in all cases: impurities in some of the patterns are

Table 1 Details of the solutions and their corresponding ID’s. EG stands for ethylene glycol

Specimen Solvent for the salts ratio Solvent for thiourea Tin source Thermal treatment Other phases

S1A Ethanol 1:1 H2O Sn(IV)Cl2 T0-air ? T1-S-1 h SnO2??, CuxS?

S2A Methanol 1:1 EG Sn(IV)Cl2 T0-air ? T1-S-1 h SnO2?

S3A Ethanol 1:1 EG Sn(IV)Cl2 T0-air ? T1-S-1 h SnO2?, CuxS??

S4A Ethanol 1:2 EG Sn(IV)Cl2 T0-air ? T1-S-1 h SnO2??, CuxS??

S1B Ethanol 1:1 H2O Sn(IV)Cl2�5H2O T0-Ar ? T1-noS-3 h SnO2??

S2B Methanol 1:1 EG Sn(IV)Cl2�5H2O T0-Ar ? T1-noS-3 h

S3B Ethanol 1:1 EG Sn(IV)Cl2�5H2O T0-Ar ? T1-noS-3 h CuxS?

S4B Ethanol 1:2 EG Sn(IV)Cl2�5H2O T0-Ar ? T1-noS-3 h SnO2??

J Sol-Gel Sci Technol

123

due to the presence of cassiterite (SnO2, ICDD PDF2 card

#41-1445) and copper sulfide (digenite CuxS, e.g. ICDD

PDF2 card 24-0061). Other sulfides cannot be directly

identified as their diffraction patterns can hide under that of

CZTS, due to its similarity to the zinc blende crystal

structure [5, 10].

A qualitative evaluation of the phase content is proposed

in Table 1. The synthesis in methanol/ethylene glycol leads

to the formation of pure kesterite, as witnessed by the

absence of peaks from spurious phases and confirmed also

from EDXS analysis and Raman spectroscopy measure-

ments (Fig. 3). The ratio between Cu and Zn is slightly

unbalanced towards Zn (EDXS estimated stoichiometry

Cu1.8Zn1.2SnS4), a condition that can be advantageous for

solar energy harvesting applications.

Comparing the various specimens, the sulfurization step

seems to be the responsible for the formation of the extra

copper sulfide. The choice of the solvent has also an effect

on the stabilization of the tin ion, leading to the formation

of tin oxide together with kesterite. On the other hand, the

tin source affects the quality and microstructure of the

resulting material. The soluble but volatile anhydrous tin

chloride (SnCl4) is always employed in the literature. The

less dangerous pentahydrate (SnCl4�5H2O) [11] gives less

contamination and a reduced tendency, for the films, to

island growth. Further, tin(IV) pentahydrate does not

evaporate and can thus guarantee a better stoichiometry

control.

In order to understand the sequence of reactions leading

to the formation of kesterite, we performed some FTIR and

Raman spectroscopy experiments (Figs. 4, 5). The analysis

was done on the S2B solution as well as on the metal and

the thiourea solutions leading to it (named, respectively,

S2B-metals and S2B-thiourea). The methanol solution of

the chlorides can be quite a complex system: at the given

low molarity, the salt is totally decomposed and just

coordination complexes are expected in S2B-metals. Some

water should also be present due to the atmospheric

moisture, to residual water in the alcohol, and to the

structural water released by the solids. Under those con-

ditions, some alkoxides may form, but their condensation

(thus the formation of a proper gel) is unlikely, at least for

short processing times: the metal ions will be therefore

coordinated by the alcohol and by the water molecules. A

green color is observed, typical of coordination compounds

e.g. of Cu. The FTIR spectrum of S2B-metals confirms this

picture, being very similar to the methanol one, but with

the extra presence of a 1,635 cm-1 band compatible with

the m2 bend of liquid water (cf. Fig. 4).

Thiourea, in an analogous way, dissolves completely in

the diol. The alcoholic dissolution of thiourea has already

been studied in the literature [12]. The FTIR spectrum of

S2B-thiourea shows all features of ethylene glycol plus

some extra bands related to the nitrogens of thiourea. The

double band observed around 3,300 cm-1 is in fact the

combination of the OH signal from the diol with the dou-

blet coming from the stretching of the NH2 of thiourea

[13].

When the two solutions are mixed, the lone pairs of

sulphur in thiourea are attracted by the metal cations and

will likely substitute at least some of the existing species

coordinating the metal (more presumably the alcohol and

water molecules, less bound to it). Considering literature

data [12, 13], the cations will therefore be octahedrally

coordinated by the original anions (chlorine) plus a vari-

able number of thiourea, methanol and water molecules.

The number largely depends on the cation valence and on

the steric hindrance. This coordination of the metal

Fig. 1 SEM micrographs of a film (a) and of a highly porous solid

(b) obtained from the same sol. The film was produced by dip coating

whereas the porous solid was obtained by sintering a powder with no

compaction. Operating conditions for the imaging (voltage and

working distance WD are reported in the micrographs)

J Sol-Gel Sci Technol

123

ion(s) with sulfur is the responsible for the change of color

of the S2B solution just after S2B-metals and S2B-thiourea

are mixed. This coordination lowers the strength of the S–C

bond in thiourea that becomes more prone to a nucleophilic

attack and conversion into a O–C one. The change of color

and the consequent weakening of the bond are entirely

compatible with crystal field theory [14]. At this stage of

the reaction, a plausible type of attack involves the trans-

formation of the complex into a thiol and therefore the

conversion of thiourea into urea mediated by hydroxide

ions. This second reaction step is suggested also by the

solution turning white (opaque), creating a metastable sol.

As this situation is not found, as is, in the final sol, we can

infer that it just represents the starting step towards the

production of the sulfide. An increase in the temperature is

necessary to promote the elimination of HCl and the

20 30 40 50 60 70 80

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

*

S4A

S3A

S2AIn

tens

ity (

a.u)

2θ (degrees)

(112)

S1A

(101)(200) (220)

(312)

*

**

*°

20 30 40 50 60 70 80

0

500

1000

1500

2000

2500

3000

3500

*

S4BS3BS2B

***

(101)

(312)

(220)

(200)Inte

nsity

(a.

u)

2θ (°)

(112)

S1B

*

°

Fig. 2 Grazing-incidence XRD

patterns of CZTS films. The

major (indexed) peaks

correspond to CZTS. Signatures

of the minor phases are

identified, respectively, by

*(SnO2) and �(CuxS)

150 200 250 300 350 400 450 500 550 600

0

500

1000

1500CZTSZnSSn

xS

y

Cu2S

Cu2SnS

3

Inte

nsity

(ar

b. u

nits

)

Raman shift (cm-1)

Fig. 3 Raman spectrum of the S2B specimen deposited on glass

J Sol-Gel Sci Technol

123

formation of sulfide bonds. The FTIR spectrum is unable to

give a full confirmation to this process: thiourea is in

excess, the solvents mask all other signals and sulfide

bonds cannot be seen in the accessible range. The Raman

spectrum, however, shows a downshift of the m(CS) at ca.

700 cm-1 in S2B with respect to S2B-thiourea, compatible

with a coordination of thiourea with the metal ion(s) [13].

We can also observe, at lower wave numbers, a simulta-

neous reduction of the signal related to the coordination of

the metal chlorides with the alcohol/water and an increase

in the signal from the metal-sulfur bond. All of those

observations are compatible with the proposed reaction

scheme.

The increase in steric hindrance when moving from

ethanol to methanol to ethylene glycol is the responsible

for the slowing down of the reactions in the corresponding

cases and, ultimately, for the observed increase in stoi-

chiometry control. The high density and polarity of the diol

seems also the key for a better solubilization and stabil-

ization of thiourea with respect to the water-based and

conventional sol–gel approaches usually proposed in the

literature.

4 Conclusion

Complex sulfides can be created using a facile reaction

involving mixing a solution of metal salts with a thiourea

solution. By treating the resulting material in inert atmo-

sphere, Zn-rich/Cu-poor CZTS can be created without the

need for vacuum or for a sulfurization step. A possible

reaction scheme leading to the final product is proposed,

4000 3500 3000 2500 2000 1500 1000 500

20

30

40

50

60

70

80

90

100

ν H-N-C=Sν NH

2

1080 cm-1

1400 cm-1

2937-- 2874 cm-1

1623 cm-1

Tra

nsm

ittan

ce (

%)

wavenumber (cm-1)

Ethyleneglycol Thiourea Thiourea - S2B

3300 cm-13266 cm-1

1602 cm-1

ν O-H

(a)

(b)

2825 cm-1

νs CH

2

νas CH

2

ν2 bend H

2O

1650 cm-1

1080 cm-1

3300 cm-1

ν O-H

Methanol -S2B Methanol

2950 cm-1

(c)

νas CH2

νs CH2

2825 cm-1

2950 cm-1

1080 cm-1

3200 cm-1

S2B1 (Metals - Methanol) S2B2 (Thiourea - Ethyleneglycol)S2B (S2B1+S2B2)

3300 cm-11635 cm-1

ν2 bend H2Oν O -H

ν NH2

20

30

40

50

60

70

80

90

100

Tra

nsm

ittan

ce (

%)

20

30

40

50

60

70

80

90

100

Tra

nsm

ittan

ce (

%)

4000 3500 3000 2500 2000 1500 1000 500

wavenumber (cm-1)

4000 3500 3000 2500 2000 1500 1000 500

wavenumber (cm-1)

Fig. 4 FTIR spectra. In a, thiourea, ethylene glycol, and

thiourea in ethylene glycol (S2B-thiourea). In b, methanol

and metal salts in methanol (S2B-metals). In c the final

solution (S2B) together with the precursors S2B-thiourea and

S2B-metals

0 200 400 600 800 1000 1200 1400 16000

2000

4000

6000

8000

10000

12000

Inte

nsity

(ar

b. u

nits

)

Raman shift (cm-1)

νM-S

δNCN

νCSδNCN

Methanol

S2B S2B-metals S2B thiourea

δCS

νMCly methanol

complexesM= Cu,Zn,Sn

νCS

Ethyleneglycol

δM-S

Fig. 5 Raman spectra of the CZTS precursors and of the final sol

J Sol-Gel Sci Technol

123

involving the coordination of the metal ions by thiourea

and the intermediate formation of thiols. A control of this

reaction could be the key for a greener route to the pro-

duction of complex metal sulfides.

Acknowledgments The authors acknowledge the financial support

of the Italian Government through the project FIRB Futuro in Ricerca

RBFR10CWDA and the Italian Ministry of Economic Development

in the framework of the Operating Agreement with ENEA for the

Research on the Electric System.

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