Zinc Tin Chalcogenide Complexes and Their Evaluation as Molecular Precursors for Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe)

Article abstract of DOI:10.1021/acs.inorgchem.7b01697

A series of five heteronuclear zinc tin chalcogenide complexes with the general formula [(tmeda)Zn(SnR₂)₂E₃] (1-R, E = S; R = Me, Ph, tBu; 2-R, E = Se; R = Ph, tBu) have been synthesized and characterized by X-ray crystal structure analysis. In all cases, the six-membered ZnSn₂E₃ rings exhibit twist boat conformation. The presence of the molecular structures in solution is confirmed by ¹¹⁹Sn and ⁷⁷Se NMR spectroscopy. Cothermolysis experiments using a mixture of complexes 1-R or 2-R and [(iPr₃PCu)₂(EC₂H₄E)]₂ as a copper source were monitored by thermogravimetry and temperature dependent X-ray powder diffraction to examine the thermolysis reaction. According to Rietveld refinement, the solid residue consists of Cu₂ZnSnS₄ (up to 78 wt %) or Cu₂ZnSnSe₄ (up to 43 wt %) as the main product, respectively.

Full text of DOI:10.1021/acs.inorgchem.7b01697

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Zinc Tin Chalcogenide Complexes and Their Evaluation as Molecular
Precursors for Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe)
Daniel Fuhrmann, Stefan Dietrich, and Harald Krautscheid*
Fakultat fur Chemie und Mineralogie, Universitat Leipzig, Johannisallee 29, D-04103 Leipzig, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: A series of five heteronuclear zinc tin chalcogenide complexes with the
general formula [(tmeda)Zn(SnR2)2E3] (1-R, E = S; R = Me, Ph, tBu; 2-R, E = Se; R =
Ph, tBu) have been synthesized and characterized by X-ray crystal structure analysis. In
all cases, the six-membered ZnSn2E3 rings exhibit twist boat conformation. The presence
of the molecular structures in solution is confirmed by 119Sn and 77Se NMR
spectroscopy. Cothermolysis experiments using a mixture of complexes 1-R or 2-R and
[(iPr3PCu)2(EC2H4E)]2 as a copper source were monitored by thermogravimetry and
temperature dependent X-ray powder diffraction to examine the thermolysis reaction.
According to Rietveld refinement, the solid residue consists of Cu2ZnSnS4 (up to 78 wt
%) or Cu2ZnSnSe4 (up to 43 wt %) as the main product, respectively.
have been recently developed. By deposition from hydrazine
INTRODUCTION
■
based solutions of Cu2S, Zn, SnSe, S, and Se, conversion
efficiencies above 12% were reported.31 To replace toxic and
explosive hydrazine, a solvent system combining alkyl amines
and alkyl thiols has been established that dissolves metal
chalcogenides as well as pure metals at room temperature and is
successfully applied for the deposition of CZTS from
solution.32,33
Cu2ZnSnS4 (CZTS) is a direct band gap semiconductor with a
band gap near 1.45 eV and a high absorption coefficient above
104 cm−1. Therefore, Cu2ZnSnS4 is one of the most promising
materials for solar cell applications, especially in thin film solar
cells.1 CZTS is found as a mineral in the earth’s crust known as
kesterite in which Zn2+ can be replaced by Fe2+. In comparison
to common absorber materials for thin film solar cells like
CdTe, GaAs, or CuInSe2, CZTS does not contain any toxic
elements, and all elements are highly abundant in the earth’s
crust. The first CZTS based solar cell was introduced by H.
Katagiri2 and co-workers in 1997 who used electron beam
evaporation of the elements to form a mixed layer of copper,
zinc, and tin on a molybdenum coated glass substrate.
Sulfurization in an H2S atmosphere resulted in a thin film of
CZTS as p-type absorber layer. The p−n-junction was
generated by a thin layer of n-type cadmium sulfide; aluminum
doped zinc oxide was used as an electric contact. In this
photovoltaic cell a conversion efficiency of 0.66% was achieved.
The general setup of a CZTS solar cell was preserved until
today, but for the deposition of the CZTS layer several
methods were suggested. Besides expensive high vacuum based
methods for deposition of mixed metal layers such as electron
beam evaporation,3−7 thermal evaporation8−11 and sputter-
ing,12−18 also nonvacuum based methods have been reported.
Electrochemical deposition19−25 or sol−gel26−29 methods were
used to deposit the metals or metal salts on a substrate followed
by heating in sulfur vapor or H2S atmosphere to form CZTS.
First spray pyrolysis experiments for depositing a CZTS layer
were performed by Nakayama and Ito starting from a solution
of CuCl, ZnCl2, SnCl4, and thiourea in water/ethanol in 1996.
A further sulfurization step was necessary to obtain a
stoichiometric CZTS layer.30
A possibility to adjust the element ratio in the precursor
solution could be the use of a soluble single-source precursor
compound with the elements incorporated in the required
stoichiometric ratio. The reduction of the four component
system to a three component system was possible by using
thiourea,34 xanthate,35 or dithiocarbamate36 complexes of
copper, zinc, and tin for spray pyrolysis or aerosol assisted
chemical vapor deposition (AACVD). Controlling the
stoichiometric composition of CZTS with this kind of methods
is difficult; therefore, the further reduction to a two component
system has been suggested, but only a few examples for
complexes containing a Zn−S−Sn structural motif are known
in the literature. Most publications concerning Sn/Zn/S
37−39
compounds are about cluster anions [Zn4Sn4S17]10−
or
40
[Zn5Sn5S20]10− with alkali metal counterions. These ionic
species are not suitable as molecular precursors for CZTS since
the cations cannot be eliminated during thermolysis. Examples
for neutral complexes are [(RSn)4(Zn8X8)S10] with X = Cl−,
Br−, I−, and R = organic group,41 [{1,3-C6H4(PhPS)2CSnEt2}-
(μ-ZnS)]2 which could be isolated in low yield and as a mixture
with other products only,42 and the tren stabilized mixed zinc
tin sulfide [{(tren)Zn}2(μ-Sn2S6)] (tren = tris(2-aminoethyl)-
amine) which was synthesized under solvothermal conditions.43
For deposition of CZTS and CZTSe thin films with high
Received: July 28, 2017
power conversion efficiency, low-cost solution based methods
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01697
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
ca. 10 mL. Storing for 3 days at 2 °C gave 400 mg (0.54 mmol, 54%)
1-tBu as colorless plates. 1H NMR (400 MHz, THF-d8, 25 °C, TMS):
δ = 1.29 (36H, s, 3JH117Sn = 81 Hz, 3JH119Sn = 84 Hz, Sn−C−(CH3)3),
2.54 (12H, s, N−CH3), 2.66 (4H, s, N−CH2). 119Sn{1H} NMR (149
Herein we present the synthesis and characterization of a
series of five mixed metal zinc tin chalcogenide complexes
[(tmeda)Zn(SnR2)2S3] (1-R, R = Me, Ph, tBu) and [(tmeda)-
Zn(SnR2)2Se3] (2-R, R = Ph, tBu). Preliminary studies of
cothermolysis reactions of 1-Me, 1-Ph, or 2-tBu together with
[(iPr3PCu)2(SC2H4S)2] or [(iPr3PCu)2(SeC2H4Se)2] yield
Cu2ZnSnS4 or Cu2ZnSnSe4, respectively, as main products.
MHz, THF-d8, 25 °C, TMS): δ = 132 (2J119Sn117Sn = 158 Hz, 1J119SnC
=
412 Hz). Anal. Calcd (%) for C22H52N2S3Sn2Zn: C 35.5, H 7.0, N 3.8.
Found: C 35.2, H 6.9, N 3.7.
[(tmeda)Zn(SnPh2)2Se3] (2-Ph). A 367 mg (2 mmol) portion of
Zn(OAc)2 was suspended in 25 mL of DME. A 0.6 mL (4 mmol)
portion of tmeda and 1.00 mL (4 mmol) of (Me3Si)2Se were added
successively at 0 °C. The colorless solution was cooled to −50 °C, and
781 mg (2 mmol) of Ph2Sn(OAc)2 dissolved in 10 mL of DME was
added at −50 °C. The solution was warmed to room temperature
within 1 h and stirred for 30 min. The solvent was reduced to ca. 10
mL. Storage for 3 days at −25 °C gave 200 mg (0.21 mmol, 21%) of 2-
EXPERIMENTAL SECTION
■
All syntheses were performed by applying a Schlenk technique or in an
MBRAUN UniLab glovebox with N2 as inert gas. All solvents and
tmeda were dried according to common methods and distilled prior to
use. Zn(OAc)2·2H2O was dehydrated with Ac2O, R2Sn(OAc)2 (R =
Me, Ph, tBu) was prepared by the reaction of (R2SnO)3 in a 2:1
mixture of acetic acid and Ac2O44−46 and recrystallized from n-
heptane.47 While (Ph2SnO)3 is commercially available, (Me2SnO)3
and (tBu2SnO)3 were synthesized starting from Me2SnCl2 and
tBu2SnCl2 (obtained by the reaction of SnCl4 with tBuMgCl48) in
aqueous NH3.49,50 (Me3Si)2E (E = S, Se) was prepared by reaction of
Na2E (obtained by the reactions of the elements in liquid NH351) and
1
Ph as colorless plates. H NMR (400 MHz, THF-d8, 25 °C, TMS): δ
= 2.23 (12H, s, N−CH3), 2.55 (4H, s, N−CH2), 7.24 (12H, m, m- +
p-H SnPh2), 7.69 (8H, m, 2JH117/119Sn = 62 Hz, o-H SnPh2). 119Sn{1H}
NMR (149 MHz, THF-d8, 25 °C, TMS): δ = −65 (1J119Sn77Se = 1580
1
2
Hz, J119Sn77Se = 1225 Hz, J119Sn117Sn = 215 Hz). 77Se{1H} NMR (76
MHz, THF-d8, 25 °C, TMS): δ = −531 (1J77Se117Sn = 1511 Hz,
1J77Se119Sn = 1580 Hz, Zn−Se−Sn), −433 (1J77Se117Sn = 1172 Hz,
1J77Se119Sn = 1225 Hz, Sn−Se−Sn). Anal. Calcd (%) for C30H36N2-
Se3Sn2Zn: C 37.4, H 3.8, N 2.9. Found: C 36.8, H 3.3, N 2.9.
[(tmeda)Zn(SntBu2)2Se3] (2-tBu). A 367 mg (2 mmol) portion of
Zn(OAc)2 was suspended in 50 mL of DME. A 0.6 mL (4 mmol)
portion of tmeda and 1.00 mL (4 mmol) of (Me3Si)2Se were added
successively at 0 °C. The colorless solution was cooled to −50 °C, and
702 mg (2 mmol) of tBu2Sn(OAc)2 dissolved in 10 mL of DME was
added at −50 °C. A colorless solid precipitated. The suspension was
warmed up to room temperature and stirred for 1 h. The clear solution
was concentrated to ca. 50 mL. Storage for 2 days at −25 °C gave 250
mg of crystals of (tBu2SnSe)2. After filtration the volume of the
colorless solution was reduced to ca. 20 mL. Storing the solution for 2
Me3SiCl.52 [(iPr3PCu)2(SC2H4S)]253 and [(iPr3PCu)2(SeC2H4Se)]2
54
were prepared according to published procedures.
NMR spectra were measured using a Bruker Avance DPX-400
spectrometer with 400 MHz proton frequency in dried and distilled
1
THF-d8 as solvent. H NMR spectra were referenced to TMS as an
internal standard. Heteronucleus NMR spectra were referenced using
the Ξ-scale.55 Elemental analyses (C, H, N) were measured with a
Heraeus Vario El, TG/DTA analysis with a Netzsch STA 449 F1
(argon as carrier gas; flow rate 25 mL/min; heating rate 10 K/min)
coupled to an Aeolos QMS 403 D mass spectrometer. Scanning
̈
electron microscopy was performed on a leo gemini device equipped
with an Oxford Instruments 7426 EDX detector.
1
[(tmeda)Zn(SnMe2)2S3] (1-Me). A 734 mg (4 mmol) portion of
Zn(OAc)2 was suspended in 40 mL of DME. A 1.2 mL (8 mmol)
portion of tmeda and 1.69 mL (8 mmol) of (Me3Si)2S were added
successively. The colorless solution was cooled to 0 °C, and 1.066 g (4
mmol) Me2Sn(OAc)2 dissolved in 20 mL of DME was added. After
the solution was warmed to room temperature within 2 h, the solvent
was reduced to half the volume. Storage for 16 h at −25 °C gave 350
days gave 250 mg (0.28 mmol, 28%) of 2-tBu as yellow plates. H
NMR (400 MHz, THF-d8, 25 °C, TMS): δ = 1.26 (36H, s, 3JH117Sn
=
82 Hz, 2JH119Sn = 84 Hz, Sn−C−(CH3)3), 2.54 (12H, s, N−CH3), 2.66
(4H, s, N−CH2). 119Sn{1H} NMR (149 MHz, THF-d8, 25 °C, TMS):
δ = 102 (1J119Sn77Se = 1597 Hz, 1J119Sn77Se = 1390 Hz, 2J119Sn117Sn = 160
Hz). 77Se{1H} NMR (76 MHz, THF-d8, 25 °C, TMS): δ = −631
1
(1J77Se117Sn = 1527 Hz, J77Se119Sn = 1597 Hz, Zn−Se−Sn), −583
1
1
(1J77Se117Sn = 1327 Hz, J77Se119Sn = 1390 Hz, Sn−Se−Sn). Anal. Calcd
mg (0.61 mmol, 30%) of 1-Me as colorless blocks. H NMR (400
2
MHz, THF-d8, 25 °C, TMS): δ = 0.53 (12H, s, JH117Sn = 57.5 Hz,
(%) for C22H52N2Se3Sn2Zn: C 29.9, H 5.5, N 3.2. Found: C 28.9, H
4.8, N 3.8.
2JH119Sn = 60 Hz, Sn−CH3), 2.51 (12H, s, N−CH3), 2.67 (4H, s, N−
CH2). 119Sn{1H} NMR (149 MHz, THF-d8, 25 °C, TMS): δ = 122
X-ray Crystal Structure Analysis. Crystallographic data are given
in Table S1 (Supporting Information). Measurements were performed
using a STOE IPDS or STOE IPDS 2T image plate diffractometer
system equipped with a sealed Mo X-ray tube and a graphite
monochromator crystal (λ(Mo Kα) = 71.073 pm). Data reduction and
numerical absorption correction were done with STOE X-AREA
software.56 All structures were solved by direct methods using
SHELXS-2014 and refined with SHELXL-2014.57 All non-hydrogen
atoms (besides disordered C atoms of the tmeda ligand in 1-tBu, of
the tmeda ligand and one tBu group in 2-tBu) were refined with
anisotropic thermal parameters; hydrogen atoms were included on
idealized positions applying the riding model. Diamond 3.2k was used
for visualization of the crystal structures.58 CCDC 1555378−1555382
contain the crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
The PXRD measurements were carried out on a STOE STADI-P
diffractometer equipped with a sealed Cu X-ray tube and a
germanium(111) monochromator crystal (λ(Cu Kα1) = 154.060
pm). Samples of the air sensitive complexes were prepared in a
glovebox under N2 atmosphere, filled in glass capillaries (Hilgenberg,
outer diameter 0.5 mm), and measured in Debye−Scherrer mode. The
thermolysis products were measured as flat samples between two
acetate films in transmission mode. Rietveld analyses of the diffraction
patterns were performed with Bruker TOPAS 5 software59 using the
fundamental parameter approach. Crystal structure data for
1
(2J119Sn117Sn = 184 Hz, J119SnC = 387 Hz). 13C{1H} NMR (100 MHz,
1
THF-d8, 25 °C, TMS): δ = 3.1 (1JC117Sn = 370 Hz, JC119Sn = 387 Hz,
Sn−CH3), 47.2 (N−CH3), 56.5 (N−CH2). Anal. Calcd (%) for
C10H28N2S3Sn2Zn: C 20.9, H 4.9, N 4.9. Found: C 20.8, H 4.8, N 4.8.
[(tmeda)Zn(SnPh2)2S3] (1-Ph). A 367 mg (2 mmol) portion of
Zn(OAc)2 was suspended in 20 mL of DME. A 0.6 mL (4 mmol)
portion of tmeda and 0.85 mL (4 mmol) of (Me3Si)2S were added
successively. The colorless solution was cooled to −70 °C, and 781 mg
(2 mmol) of Ph2Sn(OAc)2 dissolved in 10 mL DME was added at
−70 °C. After the solution was warmed to room temperature within 2
h, the solvent was reduced to ca. 10 mL. Storage for 2 days at 2 °C
1
gave 600 mg (0.73 mmol, 73%) of 1-Ph as colorless plates. H NMR
(400 MHz, THF-d8, 25 °C, TMS): δ = 2.18 (12H, s, N−CH3), 2.53
(4H, s, N−CH2), 7.23 (12H, m, m- + p-H SnPh2), 7.68 (8H, m,
2JH117/119Sn = 62 Hz, o-H SnPh2). 119Sn{1H} NMR (149 MHz, THF-d8,
1
25 °C, TMS): δ = 24 (2J119Sn117Sn = 178 Hz, J119SnC = 595 Hz). Anal.
Calcd (%) for C30H36N2S3Sn2Zn: C 43.8, H 4.4, N 3.4. Found: C 43.6,
H 4.2, N 3.7.
[(tmeda)Zn(SntBu2)2S3] (1-tBu). A 367 mg (2 mmol) portion of
Zn(OAc)2 was suspended in 25 mL of DME. A 0.6 mL (4 mmol)
portion of tmeda and 0.85 mL (4 mmol) of (Me3Si)2S were added
successively. The colorless solution was cooled to −40 °C, and 702 mg
(2 mmol) of tBu2Sn(OAc)2 dissolved in 10 mL of DME was added at
−40 °C. After warming the solution to room temperature within 3 h
and stirring for 1 h at room temperature, the solvent was reduced to
B
DOI: 10.1021/acs.inorgchem.7b01697
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Cu2ZnSnS4,60 ZnS (sphalerite),61 SnS,62 Cu2S (tetragonal63 and
c (z = 4). In 2-Ph, Zn1 and Se1 are located on the 2-fold axis:
Half of the molecule forms the asymmetric unit, whereas in 1-
Ph both SnPh2 groups differ in the orientation of the phenyl
groups binding to Sn1 and Sn2. Also, in 1-Me which crystallizes
in the orthorhombic space group C2221, the molecule contains
a crystallographic 2-fold axis; the other complexes are only
approximately C2 symmetric. Both complexes containing
SntBu2 units, 1-tBu and 2-tBu, form isomorphous crystals
with space group P21/n (z = 4).
monoclinic64), Cu2ZnSnSe4,65 ZnSe,66 SnSe,67 Cu2Se,68 and
69
Cu2SnSe4 were taken from literature reports.
RESULTS AND DISCUSSION
■
Synthesis. The heteronuclear complexes 1-R and 2-R (R =
Me, Ph, tBu) are formed in one-pot reactions starting from
Zn(OAc)2, tmeda, E(SiMe3)2 (E = S, Se), and R2Sn(OAc)2
(Scheme 1). Intermediates are tmeda stabilized zinc chalcoge-
On the basis of the torsion angles, in all complexes the six-
membered ZnSn2E3 rings are in twist boat conformation. The
larger selenium atoms in 2-Ph and 2-tBu widen the bond
Scheme 1. Syntheses of the Zinc Tin Chalcogenide
Complexes 1-R and 2-R
lengths (Table 1) to the metal atoms by 12
1 pm in
agreement with the difference in covalent radii (S 104 pm; Se
117 pm).74 The sterically more demanding tert-butyl groups in
1-tBu and 2-tBu cause a distortion of the ring. The Zn−E−Sn
angles are widened by 8° (E = S) or 5° (E = Se) compared to
the methyl and phenyl derivatives; also, the distance between
zinc and the tmeda ligands increases to avoid steric interactions
with the tert-butyl groups. Bond lengths and angles of the
{(tmeda)ZnS2}2− units are similar to those in the reported
crystal structure of the intermediate [(tmeda)Zn(ESiMe3)2].70
The Sn−E bond lengths of the chalcogen atom between the tin
atoms are longer than of those bridging tin and zinc atoms.
Characterization of the Complexes by PXRD and NMR
Spectroscopy. Complexes 1-R and 2-Ph crystallized phase
pure as confirmed by X-ray powder diffraction (PXRD)
whereas 2-tBu crystallized in at least two phases (Supporting
Information, Figures S1−S5). Triclinic crystals of 2-tBu could
be isolated which have the same composition as the described
crystal structure. However, the quality of the crystals was too
poor for a complete crystal structure analysis.75 Obviously
different orientations of the tert-butyl groups lead to different
packing in the crystal structures. NMR spectra confirm
composition and connectivity in solution for all complexes,
although in solution all complexes decompose slowly under
formation of ZnS or ZnSe and organotin species; however, after
1 week the original product is still the main compound.
(tBu2SnS)2 and (tBu2SnSe)2 could be identified as decom-
position products of 1-tBu and 2-tBu. 1-Ph decomposes under
formation of (PhSn)4S6. Phenylstannanes are known for
metathesis reactions by exchange of phenyl groups.76 As
nide complexes [(tmeda)Zn(ESiMe3)2] (E = S, Se) reported by
DeGroot and Corrigan.70 These complexes were already used
as starting materials for the formation of ternary nanoclusters
like [ZnxCd10−xE4(EPh)12(PnPr3)4]71 (E = Se, Te) or
[(tmeda)6Zn14−xMnxS13Cl2] (E = S, Se).72,73 The reaction of
[(tmeda)Zn(ESiMe3)2] with stoichiometric amounts (ratio
SiMe3/OAc = 1/1) of diorganotin acetates Me2Sn(OAc)2,
Ph2Sn(OAc)2, and tBu2SnOAc2 (Scheme 1) yields trinuclear
zinc tin sulfide and selenide complexes 1-R and 2-R. These
compounds are stable in solution for some time but decompose
slowly under formation of organotin chalcogenides and ZnS or
ZnSe, respectively.
Single Crystal Structure Analysis. All complexes 1-R and
2-R could be characterized by single crystal X-ray structure
analysis (Table S1). They are isostructural with a six-membered
ring consisting of one zinc, two tin, and three chalcogen atoms.
The tmeda ligand chelating the zinc atoms and the organic
groups R binding to tin atoms complete the tetrahedral
coordination of the metal atoms.
As examples for the structure motif the structures of both
complexes containing SnPh2 units, 1-Ph and 2-Ph, are shown in
Figure 1. Whereas 1-Ph crystallizes in the triclinic space group
P1 (z = 2), 2-Ph crystallizes in the monoclinic space group C2/
̅
Figure 1. Molecular structures of 1-Ph (left) and 2-Ph (right). Atoms are drawn as 50% probability ellipsoids. Hydrogen atoms are omitted for
clarity. Symmetry code (′): −x, y, 0.5 − z.
C
DOI: 10.1021/acs.inorgchem.7b01697
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Inorganic Chemistry
Article
Table 1. Bond Lengths/pm and Angles/deg in the Molecular Structures of 1-R and 2-R
1-Me
1-Ph
1-tBu
2-Ph
2-tBu
Zn1−N1
Zn1−N2
Zn1−E2
Zn1−E3
Sn1−E2
Sn1−E1
Sn2−E1
Sn2−E3
N1−Zn1−N1′/N2
E2−Zn1−E2′/E3
E1−Sn1−E2
E1−Sn2−E3
Sn1−E1−Sn1′/Sn2
Zn1−E2−Sn1
Zn1−E3−Sn2
213.5(3)
213.4(2)
215.1(2)
226.54(7)
226.98(7)
236.32(7)
241.35(7)
241.48(7)
235.20(7)
85.79(9)
126.86(3)
115.13(2)
113.54(2)
103.10(3)
98.87(2)
99.51(2)
216.7(3)
213.2(3)
218.6(3)
217.6(3)
217.9(3)
227.7(1)
224.85(8)
226.19(8)
236.78(7)
240.90(8)
241.72(8)
238.17(8)
84.5(1)
238.44(4)
236.38(5)
238.01(5)
249.53(4)
253.98(5)
254.16(5)
250.80(4)
84.5(1)
236.8(1)
247.27(4)
253.23(4)
242.49(9)
86.2(2)
121.99(6)
112.21(3)
85.3(2)
126.49(3)
118.53(1)
128.61(3)
116.27(3)
118.67(3)
110.08(3)
106.84(3)
107.10(3)
128.74(2)
117.07(2)
119.79(2)
106.51(2)
104.57(2)
104.79(2)
100.65(5)
99.04(4)
98.85(2)
99.44(2)
1
Figure 2. H, 119Sn{1H}, and 77Se{1H} NMR spectra of 2-Ph in THF-d8.
Scheme 2. Proposed Thermolysis Reaction of a Mixture of 1-Me and [(iPr3PCu)2(SC2H4S)]2 Forming CZTS and Volatile
a
Thermolysis Products (Red)
a
The percentage numbers correspond to the expected mass loss according to the given overall reaction equation.
1
2
representative examples, H, 119Sn{1H}, and 77Se{1H} NMR
difference between 117Sn and 119Sn isotopes, JSnSn coupling is
spectra of 2-Ph in THF-d8 are shown in Figure 2; NMR spectra
of the other compounds are reported in the Supporting
Information (Figures S6−S9).
visible between 119Sn and the chemically identical, symmetry
equivalent tin atom, if their two positions in the six-membered
ring are occupied by one 117Sn and one 119Sn atom each.
Thermal Behavior and Thermolysis Studies. Complexes
1-R and 2-R are potential sources of Zn, Sn, and S or Se,
respectively, in thermolysis reactions. Although their Zn/Sn
ratio of 1/2 is different from that in the kesterite type
semiconductors Cu2ZnSnE4, cothermolysis reactions together
with [(iPr3PCu)2(EC2H4E)]2 as a CuI source were performed.
Both phosphine stabilized copper ethane dichalcogenolate
complexes thermally decompose under elimination of ethylene
sulfide or ethene and selenium to form copper sulfide or copper
selenide, respectively.53,54 Zinc tin complexes 1-R and 2-R,
respectively, were homogeneously mixed with the copper
sulfide/selenide source and thermally treated in a simultaneous
thermogravimetric and difference thermoanalysis experiment
(TG/DTA) up to 600 °C. An elimination of the tin excess
during thermolysis is possible since exchange reactions during
1
The H NMR spectrum of 2-Ph shows two signals for the
methylene and methyl protons of the tmeda ligand as well as
two signal groups for the SnPh2 ortho, meta, and para protons.
2
The ortho-H atoms exhibit two satellite pairs resulting from J
coupling to 117Sn and 119Sn nuclei. Consistent with the C2
symmetry (crystallographic symmetry in 1-Me and 2-Ph) of the
complex molecules, both tin atoms are chemically and
magnetically identical and give one signal in the 119Sn{1H}
NMR spectrum at −65 ppm. The selenium atoms binding to
two tin atoms and the selenium atom bridging tin and zinc
atoms result in two signals in the 77Se NMR spectrum at −433
ppm for Se1 and −531 ppm for Se2. The Sn−Se coupling
1
constants are JSnSe = 1225 Hz for the longer Sn1−Se1 bonds
and 1580 Hz for the shorter Sn1−Se2 bonds; the satellite pairs
are well-separated in the 119Sn spectrum. Due to the magnetic
D
DOI: 10.1021/acs.inorgchem.7b01697
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. Cothermolysis of a 2:1 mixture of 1-Me and [(iPr3PCu)2(SC2H4S)]2 at temperatures up to 600 °C. The TG curve (black), the DTA curve
(olive), and selected ion current curves are shown.
Figure 4. Rietveld refinement (wRp = 4.58%, Rp = 3.50%, Rexp = 2.86%) of the powder diffraction pattern (λ = 154.060 pm) obtained from the
cothermolysis residue of 1-Me with 0.5 equiv of [(iPr3PCu)2(SC2H4S)]2 at temperatures up to 600 °C. Measured, calculated intensities, background
curve, and difference curve as well as reflection positions of CZTS (78.2(7) wt %, first row), ZnS (8.4(2) wt %), SnS (2.7(1) wt %), tetragonal Cu2S
(1.9(2) wt %), and monoclinic Cu2S (8.8(5) wt %).
the thermolysis can result in volatile tin compounds such as
SnR4. The proposed reaction equation for the cothermolysis of
a mixture of 1-Me with 0.5 equiv of [(iPr3Cu)2(SC2H4S)]2 is
shown in Scheme 2; the results of the simultaneous thermal
analysis are presented in Figure 3. Related thermal analysis
results for mixtures of 1-Ph, 1-tBu, 2-Ph, and 2-tBu with
[(iPr3Cu)2(EC2H4E)]2 are shown in Figures S10−S13. All
expected decomposition products besides CZTS are volatile
and observed in the mass spectra of the volatile thermolysis
products. Thermolysis studies of 1-R and 2-R without a copper
source reveal decomposition at similar temperatures as in the
cothermolysis experiments by evaporation of tmeda and volatile
organic thermolysis products. The experimentally observed
weight loss is significantly higher than expected for the
stoichiometric formation of zinc and tin chalcogenides.
According to the weight loss for 1-Me (observed, 52.4 wt %;
calculated for the quantitative formation of ZnS and SnS, 30.7
wt %), volatile organotin compounds are formed; for 1-tBu and
2-tBu, iso-butene is detected by mass spectrometry (Figures
S24−S26).
Thermolysis starts at 169 °C (onset temperature) with
elimination of the tmeda and iPr3P ligands: tmeda (Tb = 120
°C) is detected by the increase of the mass signal m/z = 58
+
((CH3)2N = CH2 ); iPr3P (Tb = 170 °C) is detected at m/z =
43 (iPr+). The methyl groups of the SnMe2 units are detected
+
at m/z = 15 (CH3 ). Before the end of decomposition at 300
°C, the decomposition of ethane dithiolate forms ethylene
sulfide (m/z = 60, C2H4S+) and ethene (m/z = 28, C2H4 ). The
+
E
DOI: 10.1021/acs.inorgchem.7b01697
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 2. Details for Cothermolysis Experiments of 1-R and 2-R Together with [(iPr3PCu)2(SC2H4S)]2 or
a
[(iPr3PCu)2(SeC2H4Se)]2
1-Me
1-Ph
1-tBu
2-Ph
2-tBu
100 °C
260 °C
58.7%
49.7%
TG/DTA
PXRD
start
80 °C
100 °C
100 °C
124 °C
end
300 °C
60.5%
63.6%
350 °C
67.8%
63.8%
280 °C
65.7%
65.3%
340 °C
60.7%
59.1%
calcd mass loss
exp mass loss
CZTS|CZTSe
ZnS|ZnSe
SnS|SnSe
Cu2S|Cu2Se
Cu2SnSe4
Cu
78.2(4) wt %
8.4(2) wt %
2.7(1) wt %
10.7(5) wt %
72.2(3) wt %
9.0(2) wt %
3.8(1) wt %
15.0(3) wt %
24.9(2) wt %
20.3(2) wt %
28.7(2) wt %
26.1(2) wt %
11.1(2) wt %
27.7(2) wt %
32.1(2) wt %
29.1(2) wt %
43.4(3) wt %
18.6(5) wt %
11.2(2) wt %
12.4(2) wt %
14.5(6) wt %
24.2(5) mol %
15.4(4) mol %
11.8(3) mol %
48.6(9) mol %
26.7(5) mol %
14.4(2) mol %
10.7(1) mol %
48.2(6) mol %
28.3(3) mol %
14.0(2) mol %
10.3(1) mol %
47.4(4) mol %
25.3(2) mol %
15.2(2) mol %
14.2(1) mol %
45.3(6) mol %
24.0(4) mol %
17.1(2) mol %
14.2(2) mol %
44.7(5) mol %
Zn
Sn
S
a
The Cu, Zn, Sn, and S percentages are calculated from the results of phase analysis by PXRD.
experimental mass loss of 63.6% is 3.1% higher than expected.
The reason is that more than 50% of the tin atoms, as expected
in the idealized reaction equation in Scheme 2, are eliminated
during thermolysis as volatile compounds.
Thermolysis of 2-Ph with [(iPr3PCu)2(SeC2H4Se)]2 at
temperatures up to 600 °C yields only 11.1 wt % CZTSe.
Selenide ions are easily oxidized; thus, reduction of SnIV to
SnIISe is preferred. Due to β-hydride elimination, the
thermolysis of 2-tBu is already complete at 260 °C. Under
these conditions the reduction of SnIV is limited, and the
content of CZTSe increases to 43.4(3) wt %. However, exact
determination of the CZTSe content by analysis of the X-ray
diffraction pattern is difficult since formation of other phases
cannot be ruled out: Cu2SnSe4 crystallizes in cubic space group
In order to check the phase composition of the solid
thermolysis residue, the X-ray powder diffraction pattern
(Figure 4) has been analyzed. According to the Rietveld
refinement of the diffraction pattern, 78.2 wt % CZTS, 8.4 wt %
ZnS, 10.7 wt % Cu2S, and 2.7 wt % SnS were formed. ZnS and
Cu2S were formed due to the loss of tin during thermolysis.
SnS probably arises from the decomposition of CZTS under
evolution of gaseous sulfur.77 The molar percentages of copper,
zinc, tin, and sulfur calculated on the basis of the phase
composition data are in agreement with SEM-EDX analysis
results of the thermolysis residue of 1-Me. EDX signal
intensities measured in an area of 160 μm × 230 μm
correspond to 28 mol % copper, 16 mol % zinc, 11 mol %
tin, and 45 mol % sulfur (Figure S23). Table 2 also summarizes
the phase compositions after cothermolysis of 1-R and 2-R (R
= Me, tBu) together with [(iPr3PCu)2(EC2H4E)]2 as
determined by Rietveld analysis (Figures S14−S17).
Thermolysis of 1-Ph with [(iPr3PCu)2(SC2H4S)]2 needs a
50 K higher temperature compared to that for 1-Me due to the
lower volatility of the phenylstannanes. Rietveld analysis
(Figure S14) confirms the elimination of more than 50% of
the tin atoms of 1-Ph. The yield of CZTS decreases to 72.2 wt
%. The quantitative determination of CZTS and ZnS by PXRD
is difficult since Cu2SnS3 could be formed as a side product.
Cu2SnS3 crystallizes with lattice parameters almost identical to
those of ZnS and very close to those of CZTS.61,78,79
F43m with lattice parameters very close to those of CZTSe.
̅
According to these results, both selenium complexes, 2-Ph and
2-tBu, together with [(iPr3PCu)2(SeC2H4Se)]2 form
Cu2ZnSnSe4, however, not as a pure product.
SUMMARY AND CONCLUSIONS
■
We successfully synthesized a series of five isostructural neutral
zinc tin chalcogenide complexes [(tmeda)Zn(SnR2)2E3] (R =
Me, Ph, tBu; E = S, Se). All complexes are characterized by X-
ray single crystal structure analysis. They consist of six-
membered ZnSn2E3 rings in twist boat conformation; 1H,
119Sn{1H}, and 77Se{1H} NMR spectra confirm the presence of
the molecular structures in solution. The molecular complexes
1-R and 2-R contain three out of four elements that are
required for the formation of Cu2ZnSnS4 or Cu2ZnSnSe4.
Although the formation of volatile tin compounds and
reduction to SnII prevent quantitative formation of Cu2ZnSnE4
(E = S, Se) in cothermolysis experiments together with the
copper chalcogenolate complex [(iPr3PCu)2(EC2H4E)]2, ther-
molysis yields are up to 78 wt % CZTS and 43 wt % CZTSe.
Thermolysis of 1-tBu together with [(iPr3PCu)2(SC2H4S)]2
results in 24.9 wt % CZTS. We assume a different thermolysis
mechanism. While a radical cleavage is likely for the Sn−C
bonds in 1-Me and 1-Ph, 1-tBu can undergo β-hydride
elimination under formation of iso-butene. iso-Butene was
detected during thermolysis by the mass signals m/z = 56
(C4H8 ) and 41 (C3H5 ). The hydride ion is transferred to tin,
and the reducing conditions favor the formation of SnS (28.7
wt %) in the thermolysis product. This alternative thermolysis
mechanism also explains the lower thermolysis end temperature
of 280 °C which is even lower than that of 1.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01697.
+
+
Detailed information on analytical studies, including
crystal structure data, X-ray powder diffraction patterns,
and NMR spectra of 1-R and 2-R; thermogravimetric
data of the cothermolyses of 1-R together with
[(iPr3PCu)2(SC2H4S)]2 and 2-R together with
F
DOI: 10.1021/acs.inorgchem.7b01697
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
[(iPr3PCu)2(SeC2H4Se)]2; X-ray powder diffraction
Article
(10) Schubert, B.-A.; Marsen, B.; Cinque, S.; Unold, T.; Klenk, R.;
Schorr, S.; Schock, H.-W. Cu2ZnSnS4 thin film solar cells by fast
coevaporation. Prog. Photovoltaics 2011, 19, 93−96.
(11) Shin, B.; Gunawan, O.; Zhu, Y.; Bojarczuk, N. A.; Chey, S. J.;
Guha, S. Thin film solar cell with 8.4% power conversion efficiency
using an earth-abundant Cu2ZnSnS4 absorber. Prog. Photovoltaics
2013, 21, 72−76.
(12) Katagiri, H.; Jimbo, K.; Yamada, S.; Kamimura, T.; Maw, W. S.;
Fukano, T.; Ito, T.; Motohiro, T. Enhanced Conversion Efficiencies of
Cu2ZnSnS4-Based Thin Film Solar Cells by Using Preferential Etching
Technique. Appl. Phys. Express 2008, 1, 041201.
patterns of the thermolysis products; temperature
dependent X-ray powder diffraction patterns of the
cothermolyses; SEM and EDX analysis; thermogravi-
metric data of 1-Me, 1-tBu, and 2-tBu (PDF)
Accession Codes
CCDC 1555378−1555382 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
́
(13) Fernandes, P. A.; Salome, P.; da Cunha, A. F. Growth and
Raman scattering characterization of Cu2ZnSnS4 thin films. Thin Solid
Films 2009, 517, 2519−2523.
́
(14) Fernandes, P. A.; Salome, P.; da Cunha, A. F.; Schubert, B.-A.
Cu2ZnSnS4 solar cells prepared with sulphurized dc-sputtered stacked
metallic precursors. Thin Solid Films 2011, 519, 7382−7385.
(15) Chalapathy, R.; Jung, G. S.; Ahn, B. T. Fabrication of
Cu2ZnSnS4 films by sulfurization of Cu/ZnSn/Cu precursor layers
in sulfur atmosphere for solar cells. Sol. Energy Mater. Sol. Cells 2011,
95, 3216−3221.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: Krautscheid@rz.uni-leipzig.de.
ORCID
Harald Krautscheid: 0000-0002-5931-5440
Notes
The authors declare no competing financial interest.
(16) Grenet, L.; Bernardi, S.; Kohen, D.; Lepoittevin, C.; Noel, S.;
̈
Karst, N.; Brioude, A.; Perraud, S.; Mariette, H. Cu2ZnSn(S1−xSex)4
based solar cell produced by selenization of vacuum deposited
precursors. Sol. Energy Mater. Sol. Cells 2012, 101, 11−14.
(17) Platzer-Bjorkman, C.; Scragg, J.; Flammersberger, H.; Kubart,
̈
ACKNOWLEDGMENTS
T.; Edoff, M. Influence of precursor sulfur content on film formation
and compositional changes in Cu2ZnSnS4 films and solar cells. Sol.
Energy Mater. Sol. Cells 2012, 98, 110−117.
■
We thank Regina Zabe for measuring NMR spectra; Merten
̈
Kobalz, Jens Bergmann, and Oliver Erhart for TG/DTA
measurements; and Manuela Roßberg for CHN analyses. We
(18) Feng, Y.; Lau, T.-K.; Cheng, G.; Yin, L.; Li, Z.; Luo, H.; Liu, Z.;
Lu, X.; Yang, C.; Xiao, X. A low-temperature formation path toward
highly efficient Se-free Cu2ZnSnS4 solar cells fabricated through
sputtering and sulfurization. CrystEngComm 2016, 18, 1070−1077.
(19) Scragg, J. J.; Dale, P. J.; Peter, L. M. Towards sustainable
materials for solar energy conversion: Preparation and photo-
electrochemical characterization of Cu2ZnSnS4. Electrochem. Commun.
2008, 10, 639−642.
gratefully acknowledge financial support by Universitat Leipzig
(PbF-1) and ESF. This work was funded by the European
Union and the Free State of Saxony.
̈
DEDICATION
■
This work is dedicated to Prof. Dr. Dieter Fenske on the
occasion of his 75th birthday.
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characterization of Cu2ZnSnS4 absorber layers by an electro-
deposition-annealing route. Thin Solid Films 2009, 517, 2481−2484.
(21) Ennaoui, A.; Lux-Steiner, M.; Weber, A.; Abou-Ras, D.;
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