2
4
Xiaolei Liu, Hongtao Cui, Charlie Kong, Xiaojing Hao, Yidan Huang, Fangyang Liu, Ning
Song, Gavin Conibeer and Martin Green
School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia
Supplementary Material
Characterization
FIG. S1 shows the AFM images of as-deposited and RTA treated Mo back contact. The surface roughness is 2.39 nm and 2.25 nm in the average root-mean-square (RMS) value for asdeposited and RTA treated Mo films. Therefore the RTA treatment does not modify the surface morphology. The cross section SEM micrographs of CZTS devices are shown in FIG. S2. The
CZTS device made on the RTA treated Mo back contact shows smoother surface. The higher
Na concentration in the CZTS films due to the RTA treatment of Mo back contact results in the smoother surface morphology. The CZTS films grown on the RTA treated Mo back contact exhibit smaller grain size. The correlation between the grain size and Na concentration still remains an open issue. The effects of Na on grain size depend on the absorber growth method and the Na-doping process. A quasi-resonant Raman scattering of ZnS happens under the excitation wavelength of 325 nm because the band gap energy of ZnS (~3.7 eV) is very close to the laser photon energy (~3.8 eV). Therefore, a 325 nm laser excitation was used to determine the presence of ZnS on the CZTS surface. The 325 nm Raman spectra are shown in
FIG. S3, which demonstrate the ZnS modes at 345, 695 and 1045 cm -1 . The ZnS was observed on the CZTS films grown on both as-deposited and RTA treated Mo back contact. The XRD results of as-deposited and RTA treated Mo films are tabulated in TABLE SI. The TABLE SII shows the XRD results of the sulfurized CZTS films grown on as-deposited and RTA treated
Mo back contact.

(a) (b)
FIG. S1 AFM images of (a) as-deposited Mo film (b) RTA treated Mo film at 600 °C.
FIG. S2 Cross section SEM images of CZTS cells (a) without RTA treated Mo back contact
(b) with RTA treated Mo back contact.

FIG. S3 325 nm Raman spectra of the sulfurized CZTS films.

TABLE SI XRD results on RTA treated and as-deposited Mo films. as-deposited Mo
RTA treated Mo orientation (110)
Position (°) FWHM (°) Position (°) FWHM (°)
40.45
40.34
0.25
0.23 orientation (211)
73.7
73.45
0.75
0.68
TABLE SII XRD results of the sulfurized CZTS films on as-deposited and RTA treated Mo back contact.
CZTS on asdeposited Mo
CZTS on RTA treated Mo orientation (101)
Position
(°)
18.24
18.23
FWHM
(°)
0.25
0.15 orientation (112)
Position
(°)
FWHM
(°)
28.45
28.45
0.22
0.19 orientation (200)
Position
(°)
FWHM
(°)
33
32.99
0.5
0.35 orientation (220)
Position
(°)
FWHM
(°) orientation (312)
Position
(°)
FWHM
(°)
47.33
47.33
0.4
0.39
56.17
56.16
0.34
0.47