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1. CZTSSe synthesis section.
1.1 Reagent and apparatus.
All commercially available chemicals (99.995% CuCl from Sigma Aldrich, 99.99% ZnCl2
from Sigma Aldrich, 99.99% SnCl4 from Alfa Aesar, 99.999% Selenium pill from Alfa Aesar, 99%
1-butylamine from Alfa Aesar, 99.99% Cd(NO3)2 and 99.99% Na2S from Alfa Aesar, AR grade 99%
ethanol and AR grade CS2 from Sinopharm Chemical Reagent Co., Ltd) were used as received.
Mo-coated glass substrates were prepared by PVD75 (Kurt J. Lesker Company) with square
resistance of 118mΩ/ and 1μm thickness. The morphology of the films was characterized by
scanning electron microscopy (SEM JOEL 6701, JEOL Ltd, Tokyo, Japan). X-ray diffraction (XRD)
data were measured under a Regaku D/Max-2500 diffractometer equipped with a Cu Kα1 radiation
(λ= 1.54056 Å, Rigaku Corporation, Tokyo, Japan). Raman
spectra were recorded using a
Thermo system with an objective lens of X50 and a laser of 532nm.
1.2 Metal-organic precursor synthesis method
In a typical synthesis, CuCl (0.1633 g, 1.65 mmol), ZnCl2 (0.1400 g, 1.03 mmol) and 3 ml of
ethanol were mixed in a 25 ml three-necked flask. Kept the mixture in constant stirring, 6 ml of 1butylamine and 4 ml of CS2 were added after 5 min. Another 5 min later, 95 μL (0.81 mmol) of
anhydrous tin tetrachloride was injected into the flask and formed a buff solution. Turned the argon
flow down to 0.4 sccm and waited for another twenty minutes. Centrifuged the as synthesized
solution at 10,000 rpm for 15 min and collected the upper layer into a serum bottle. The clearly
yellow solution was marked as precursor S1.
1.3 Spin coating
Spin-coated the precursor solution at 2600 rpm for five minutes and subsequently annealed on
a 340 ℃ hot plate for 3 min. Repeated the above procures for 7 times and the CZTS thin film with
final thickness of 1.0-1.2 μm was prepared. All these procedures were conducted in a glove box,
with water and oxygen all below 1 ppm.
1.4 Selenization
Put the as-synthesized film into a φ15x150 mm glass tube with 50 mg selenium pill. The tubes
were sealed under vacuum using hydroxide flame. The selenization was processed under 520 ℃
for 30 min.
2. Characterization
Figure 1 (a) XRD profiles of the as-synthesized films fabricated from different precursor S1. (b) Raman
Spectra of the as-synthesized films fabricated from precursor S1. (c) XRD profiles of the selenized films
fabricated from precursor S1. (d) Raman Spectra of the selenized films fabricated from precursor S1
Figure 1a shows the diffraction pattern of the CZTS thin films fabricated by precursor prepared
in air (S1). In both case, three strong diffraction peaks (2θ=28.86°, 47.74°, 56.58°) are identical with
the kesterite CZTS (JCPDS# 26-0575). There is no impurity peaks observed in the diffraction
patterns except Mo from the substrate and a MoSe2 interfacial phase. However, the film fabricated
by precursor synthesized in argon has a smaller FWHM at 2θ=28.86°. According to the Scherrer
equation, a smaller FWHM indicates larger grains. We can conclude that the as-synthesized film
fabricated by the precursor synthesized is composed of larger grains. Since the binary and ternary
chalcogenides (e.g., ZnS, Cu2SnS3) X-ray diffractions overlap with CZTS pattern, it is impossible
to demonstrate the film is single phase simply by XRD pattern. According to previous reports, the
domain Raman Shift of CZTS can be posited at 331-338cm-1, with two additional peaks at 287 and
368cm-1.Figure 1b shows that the major peak of films prepared by S1 is located in 332 cm-1, 287
cm-1 and 368 cm-1. Figure 1c shows the XRD profiles of the selenized film (CZTSSe）made from
S1.
Compared with standard XRD data of CZTS (JCPDS#26-0575) and CZTSe (JCPDS#52-
0868), the selenized film diffraction peak located between them. As we know, the S/Se ration could
affect the diffraction peak position. There is no impurity peaks observed, other than the Mo from
substrate and MoSe2 from interfacial. This has proved that the selenized films were CZTSSe. Figure
1d gives the Raman Spectra of the CZTSSe thin films. A major peak located at 195 cm-1 with two
additional peaks at 173 and 235 cm-1 and the S-S vibration shifted to 327cm-1 is in accordance with
previous report. Figure 2 shows the SEM of CZTSSe thin films.
Figure 2 The SEM of CZTSSe thin film.
3. The growth of CdS on the CZTSSe film
The CZTSSe film was successively immersed into 1 M aqueous Cd(NO3)2 and 1 M aqueous
Na2S for 1 min each. Following each immersion, the film was rinsed by deionized water for 2 min
to remove excess precursors before the next dipping. This immersion cycle was repeated three
times.