Electronic Supplementary Material (ESI) for Physical Chemistry Chemical Physics.
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Supporting Information
Growth mechanism of Ge doped CZTSSe thin film by sputtering method and
solar cells
Jinze Li, Honglie Shen,* Jieyi Chen, Yufang Li and Jiale Yang
Table S1 presented composition of precursors with different thickness Ge layer and related Ge
doped CZTSSe thin films. The composition of all the precursors and as-obtained thin films were
Cu-poor, which was a critical condition to prepare high efficiency solar cells.S1 And the
composition of precursors did not change much compared to that of Ge doped CZTSSe thin films.
That means that the usually reported Zn and Sn evaporation problem during selenization was not
serious in our experiment.S2 Ge’s ratio slightly increased with thicker Ge layer, which accounted
for 10-15% of the total amount of Ge and Sn.
Table S1 Element composition of precursors with different thick Ge layer and as-obtained Ge
doped CZTSSe thin films
Types
Cu/%
Zn/%
Sn/%
Ge/%
S/%
Se/%
CZTS precursor
CZTS precursor with 5nm Ge layer
CZTS precursor with 10nm Ge layer
CZTS precursor with 15nm Ge layer
Sample 0nm
Sample 5nm
Sample 10nm
Sample 15nm
20.40
19.81
20.14
19.35
19.42
20.39
20.85
19.69
12.80
12.95
12.47
12.89
13.01
12.62
13.09
12.85
13.01
13.02
12.64
13.16
14.53
12.67
13.43
13.11
1.40
2.01
2.58
1.38
2.04
2.32
53.79
52.82
52.74
52.02
19.05
18.54
16.53
17.80
33.99
34.40
34.06
34.23
Fig. S1 presented the vertical images of Ge doped CZTSSe solar cells. All the Ge doped CZTSSe
thin films had a mono-grain layer structure in vertical direction. However, there were some holes
around the interface between Mo layer and CZTSSe thin film in sample 5nm (Fig. S1(b)). This
may be another reason for the low fill factor of solar cell based on sample 5nm. The enlarged
inner stress of sample 5nm confirmed by XRD test may cause the formation of these holes.
Several rod-shaped structures were found in the upper region in sample 5nm. Considering the
view angle of the image and surface images, we thought these structures were on the surface of
CZTSSe thin film. No obvious rod-shaped structure was found in other three samples, which was
in accordence with surface SEM image results. The greatly decreased number of rod-shaped
structure in sample 10nm and sample 15nm led to a smaller probability of them to be found in
vertical images.
Fig. S1 vertical SEM images of Ge doped CZTSSe solar cells: (a) 0nm; (b) 5nm; (c) 10nm; (d)
15nm
To find out where Ge is located, EDS line scan along vertical direction of Ge doped CZTSSe thin
films was done. Considering the small amount of Ge deposited in our experiment, sample 15nm
was chosen as its Ge amount was the highest among four samples, which may bring an obvious
Ge signal. Sample 0nm was also used for EDS line scan test in order to check the noise level of
the equipment. The results of element distribution in sample 0nm and sample 15nm were
presented in Fig. S2. Ge atomic ratio of sample 0nm was below 1% in the whole range of vertical
direction, indicating the noise level was around that number. While in the first 400nm depth of
sample 15nm, Ge atomic ratio was about 2-3%. After that, Ge signal decreased to noise level.
Thus it could be concluded that Ge mainly distributed in the top half of CZTSSe thin film. And
Ge’s introduction did not affect the other elements’ distribution.
Fig. S2 EDS line scan results along vertical direction of (a) sample 0nm and (b) sample 15nm
Fig. S3(a) and (b) presented vertical morphology of precursors with 5nm and 10nm thick Ge layer,
respectively. Ge layer was the light grey layer above the CZTS precursor. It was hard to see a
clear Ge layer in an around 500nm width area in precursor with 5nm thick Ge in Fig. S3(a), while
10nm thick Ge layer almost covered the whole surface of CZTS precursor in Fig. S3(b). To be
more precise, EDS mapping scan of Ge element in a 12×12μm area was done for the two samples,
of which the results were shown in Fig. S3(c) and (d), respectively. After 10min scanning time, Ge
signal in precursor with 5nm thick Ge layer distributed sparsely. It distributed as clusters in some
extent, indicating Ge layer formed islands in this situation. For precursor with 10nm thick Ge
layer, the number of Ge signal was much greater and it spread more uniformly. It should be
pointed out that as Ge layer was very thin, the electron beam of EDS had a large chance to
penetrate it. Thus Ge signal could not fill all the areas of the tested surface. Nevertheless,
combined with vertical mophology analysis, such a dense density of Ge signal could illustrate that
10nm thick Ge layer formed a continous film.
Fig. S3 cross-section SEM images of the precursors with (a) 5nm thick Ge overlayer and (b) 10nm
thick Ge overlayer; EDS mapping results of Ge in the surface of the precursors with (c) 5nm thick
Ge overlayer and (d) 10nm thick Ge overlayer
Additional References
S1 J. Kim, H. Hiroi, T. K. Todorov, O. Gunawan, M. Kuwahara, T. Gokmen, D. Nair, M.
Hopstaken, B. Shin and Y. S. Lee, Adv. Mater., 2014, 26, 7427–7431.
S2 J. J. Scragg, T. Ericson, T. Kubart, M. Edoff, C. Platzer-Bjorkman, Chem. Mater., 2011, 23(20),
4625–4633.