Insight into
CuInS2
thin films by electron microscopy
Authors: Frank Anna , Wochnik Angela , Scheu Christina 
Published: December 2016

Over the last decades the interest in materials for renewable energy applications has increased considerably. One promising material is copper indium disulfide, CuInS
2
, which has a band gap of 1.5 eV and a high absorption coefficient.
[1]
Due to the electronic and optical properties CuInS
2
is not only attractive for solar cells but also for solar‐driven water splitting devices and in photo catalysis to generate H
2
as an energy carrier.
[2, 3]
For these applications, a large surface area and a high crystallinity are beneficial. CuInS
2
thin films with such structural features can be grown by several wet chemical approaches. One is a solvothermal synthesis route
[4, 5]
which uses moderate temperatures of around 150 °C and common chemicals as precursors e.g. CuSO
4
· 5H
2
O and InCl
3
.

In the present work the used sulfur source Thioacetamide, which is toxic, has been replaced with L‐Cysteine, a natural amino acid. The other solvothermal conditions have been kept as reported in literature. The CuInS
2
films were grown on Fluorine‐doped tin oxide (FTO) coated glass substrates and investigated using X‐ray diffraction (XRD) and electron microscopic techniques such as scanning electron microscopy (SEM), (scanning) transmission electron microscopy ((S)TEM), energy‐dispersive X‐ray (EDX) spectroscopy and electron energy loss spectroscopy (EELS). SEM was done on a Zeiss Merlin microscope equipped with a Bruker EDX system. (S)TEM was performed on a FEI Titan Themis operated at 300 kV and equipped with a SuperX EDX system. EELS was done on a FEI Titan 80 – 300 kV and a Gatan Tridiem image filter. The aim of the work is to study the reaction path and to determine the microstructure of the films.

For the sulfur source L‐Cysteine a different surface morphology is obtained compared to films grown with Thioacetamide as sulfur source. Thioacetamide gives an open flower‐like surface topology (Figure 1(c)) whereas L‐Cysteine leads to films consisting of agglomerates of small nanoparticles (Figure 1(a,b)). As visible in the SEM images there are also multiple larger agglomerates built up from nanoparticles on top of the film. For both sulfur sources the Chalcopyrite structure of CuInS
2
is proven by XRD (not shown) and selected area electron diffraction (SAED, Figure 2 (d)). For increasing precursor concentration the film thickness increases. STEM images of CuInS
2
films grown with a medium precursor concentration are shown in Figure 2. EDX maps show that the films contain mainly Cu, In and S. Quantification of several areas gives Cu:In 1:1 with a lack in sulfur. Compared to a scratched TEM sample, where a stoichiometric composition of Cu:In:S 1:1:2 is obtained, this results indicate that sulfur is not stable and leaves the sample during FIB preparation.

Since the Cu
2+
of the precursors CuSO
4
must be reduced to Cu
+
to form CuInS
2
, the oxidation state of Cu was examined with EELS (Figure 3 (c)). The reaction product which forms between CuSO
4
and L‐Cysteine without heating consists of an amorphous matrix in which crystalline nanoparticles with a size of a few nanometers are embedded (Figure 3 (a)). Compared to literature
[6]
the EELS data indicate an oxidation state of +1 for copper. This means that L‐Cysteine is sufficient to reduce Cu
2+
to Cu
+
. Furthermore SAED shows distinct d‐values for Cu
x
S phases (Figure 3 (b)).