Heterogeneous reaction route to CuInS2 thin films

Article abstract of DOI:10.1039/b203156f

High quality thin films of CuInS₂ have been deposited on a copper coated silicon substrate using vapors obtained from (Et₃NH)[In(SC(O)Ph)₄].

Full text of DOI:10.1039/b203156f

Heterogeneous reaction route to CuInS2 thin films
Ming Lin, Kian Ping Loh,* Theivanayagam C. Deivaraj and Jagadese J. Vittal*
Department of Chemistry, 3 Science Drive 3, National University of Singapore, Singapore 117543.
E-mail: chmlohkp@nus.edu.sg; chmjjv@nus.edu.sg
Received (in Cambridge, UK) 3rd April 2002, Accepted 17th May 2002
First published as an Advance Article on the web 30th May 2002
High quality thin films of CuInS2 have been deposited on a
copper coated silicon substrate using vapors obtained from
(Et3NH)[In(SC(O)Ph)4].
decomposition of the compound (130–400 °C). X-Ray powder
diffraction measurement on the residue obtained from thermol-
ysis of the compound, at 300 °C and under a dynamic vacuum
of 0.5 Torr, indicated pure tetragonal In2S3 (JCPDS No.
25–390).
CuInS2, a ternary I–III–VI2 material with chalcopyrite like
structure, has a high absorption co-efficient and its band gap
( ~ 1.5 eV) is suitable for solar energy absorption. Hence thin
films of CuInS2 have been regarded as a potential candidates for
photovoltaic applications.1,2 Thin films of CuInS2 have been
obtained using various techniques including evaporation,
sputtering, spray pyrolysis, multi-source and single source
precursor CVD.3–12 Thiolate, thiocarbamate and thiocarbox-
ylate ligands have been used to extensively synthesize single
source precursors for various metal sulfide materials.13–16
Hampden-Smith and his coworkers have reported that thin films
of b-In2S3 could be obtained through aerosol assisted chemical
vapor deposition from thf solutions of the compound [HL][In-
Compound 1, which resembles [HL][In(SC(O)Me)4] re-
ported by Hampden-Smith and coworkers,17 is expected to
sublime at low pressure and hence was thought to be a suitable
candidate to produce thin films of In2S3 using the CVD
technique.§ CVD experiments conducted on Ni coated silicon
substrates using the precursor 1 show the deposition of
crystalline In2S3 thin films. The SEM in Fig. 2 shows an image
of crystalline facets with good coverage attained after 10 min of
growth time. The formation of stoichiometric In2S3 has been
substantiated by XRD and XPS of the thin films. XPS show
peaks at 444.9 and 161.9 eV corresponding to the In 3d and S 2p
binding energies of In2S3. The grazing angle thin film XRD
pattern of b-In2S3 is shown in Fig. 3(a). All of the peaks could
be indexed to a body centred tetragonal lattice with cell
constants a = b = 7.555 Å, c = 32.053 Å, which are close to
the reported data for In2S3. Preferred orientation along the (109)
plane could be observed.
(SC(O)Me)4] (L
= 3,5-dimethylpyridine) over a silicon
substrate.17 Here in this paper, we report an unprecedented
method to produce thin films of CuInS2 by employing a copper
coated silicon substrate and a precursor compound (Et3NH)-
[In(SC(O)Ph)4]·H2O (1).
Compound 1 was synthesized by reacting InCl3, NaSC(O)Ph
and (Et3NH)(SC(O)Ph) in the ratio 1+3+1 in appropriate
solvents.† The structure of 1 has been unequivocally charac-
terized using various techniques including single crystal X-ray
diffraction techniques.‡ The structure of 1 is illustrated in Fig 1.
The compound crystallized in a chiral space group P212121, is
similar to the structure reported for the anhydrous compound.18
The N–H proton of the Et3NH+ group is involved in N–H…O
bonding to the lattice water. The In–S distances are normal and
In(1) has distorted tetrahedral symmetry as revealed by the S–
In–S angles.
However if copper-coated silicon was used as the substrate
during growth using 1, stoichiometric CuInS2 was found to be
deposited instead of In2S3. Fig. 4(a) shows the crystalline facets
Thermal decomposition and pyrolysis studies show that the
compound decomposes to In2S3 in mainly two steps. The first
involves the removal of the lattice water (42–94 °C) followed by
Fig. 2 SEM image of In2S3 film on a Ni–Si substrate.
Fig. 1 Structure of 1. Selected bond distances (Å) and angles (°): In(1)–S(1)
2.470(2), In(1)–S(2) 2.481(2), In(1)–S(3) 2.475(1), In(1)–S(4) 2.465(2);
S(1)–In(1)–S(2) 104.29(7), S(1)–In(1)–S(3) 104.70(6), S(1)–In(1)–S(4)
117.99(6), S(2)–In(1)–S(3) 110.97(5), S(2)–In(1)–S(4) 103.81(5), S(3)–
In(1)–S(4) 114.65(6). H-bond parameters: O…H 1.97 Å, O…N 2.860(8) Å;
N–H…O 164°.
Fig. 3 Grazing angle X-ray diffraction of (a) In2S3 thin film on a Ni-Si
substrate and (b) CuInS2 thin film on a Cu–Si substrate.
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This journal is © The Royal Society of Chemistry 2002

agents.19 However, the reported procedure involves the usage of
toxic substances (e.g. H2Se) and leads to formation of various
crystalline phases along with the formation of thin films of
CuInSe2 (e.g. In6Se7, Cu2 2 xSe, InSe, Cu11In9).
This research work is supported by the National University of
Singapore through research grants to J. J. V. (Grant No. R-
143-000-084-112) and K. P. L. (Grant No. R-
143-000-1060-112).
Notes and references
† A creamy white precipitate was formed when InCl3 (0.20 g, 0.90 mmol)
was allowed to react with NaSC(O)Ph, formed in situ by reacting NaOH
(0.11 g, 2.70 mmol) and PhC(O)SH (319 mL, 2.70 mmol), in 30 mL of
water. The solution was stirred for about 30 min and then [Et3NH]+[Ph-
C(O)S]2 in CH2Cl2 (15 mL) (prepared by reacting 106 mL of PhC(O)SH
and 126 mL of NEt3) was added. The yellow CH2Cl2 layer was separated and
layered with light petroleum (bp 35–60 °C) to obtain a cream colored
crystalline precipitate almost immediately. The compound was filtered off,
washed with cold MeOH and Et2O and dried under vacuum. Yield 0.62 g
(89%). Anal. Calc. for C34H36O4S4InN·H2O: C, 52.10; H, 4.89; N, 1.79.
Found: C, 51.78; H, 4.69; N, 2.28%. 1H NMR (CDCl3), d 1.20 (t, 9H,
CH3CH2, J 7.6 Hz), 3.22 (q, 6H, CH3CH2, J 10.9 Hz), 7.30–8.09 (m, 20H,
C6H5). 13C NMR (CDCl3), d 8.61 (CH3CH2), 46.84 (CH3CH2), 127.84 (C2/3
of Ph), 128.68 (C2/3 of Ph), 132.28 (C4 of Ph), 139.41 (C1 of Ph), 202.63
(PhCOS).
Fig. 4 SEM image of CuInS2 thin films grown on a Cu–Si substrate: top
view (top) and side view (bottom).
observed in SEM while the thickness of the film was measured
to be ~ 1 mm (Fig. 4, side view). The grazing angle thin film
XRD in Fig. 3(b) shows only the peaks characteristic of the
CuInS2 phase with no evidence of segregation of Cu2S.
TEM diffraction of the CuInS2 film observed along the [110]
zone axis showing strong Bragg reflections at 004, 220, 112
which is typical of perfectly ordered chalcopyrite structure, as
shown in Fig. 5. Rutherford backscattering measurements¶
show that the composition of the film is homogeneous within 1
‡ Single crystals of the compound 1 were grown by slow diffusion of light
petroleum (bp 35–60 °C) in to a CHCl3 solution of the compound. Crystal
data for 1: orthorhombic, space group, P212121; a
= 12.9124(6), b
= 12.9187(7), c = 21.561(1) Å, Z = 4, V = 3596.6(3) Å3; Dc = 1.447 g
cm23; R1 = 0.0434; wR2 = 0.1029, c = 20.01(3). CCDC reference
number 183392. See http://www.rsc.org/suppdata/cc/b2/b203156f/ for
crystallographic data in CIF or other electronic format.
§ CVD was carried out by thermally evaporating the single source precursor
1 from a boron nitride cup in a vacuum reactor with a dynamic vacuum of
1 3 1025 Torr. The boron nitride cup was maintained at 100 °C. The
substrates were prepared by sputtering 1000 Å of nickel or copper films on
silicon. The substrate temperature was maintained at 400 °C during
growth.
¶ RBS meaurements were carried out using 2 MeV protons with Singletron
accelerator at the Research Center for Nuclear Microscopy. The composi-
tion of the films were simulated using the SIMNRA code.
mm and stoichiometric for CuInS2 (Cu+In+S
=
0.24+0.26+0.50). Therefore the result clearly indicates that the
heterogeneous reaction of 1 with copper affords a more efficient
route for the growth of stoichiometric, highly ordered CuInS2
films.
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The implications of this unprecedented heterogenous reac-
tion are as follows. This new route may open the door for
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40.
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