Temperature dependent conductivity of polycrystalline Cu2ZnSnS4 thin films
V. Kosyak ,1 M. A. Karmarkar,1 and M. A. Scarpulla,1,2 a)
1
Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah
USA, UT 84112-9206, USA
2
Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, Utah
USA, UT 84112-9206, USA
These figures and discussion are provided in support of the main data presented in the
manuscript.
Two types of substrates: one from Saint Gobain consisting of 3 mm soda-lime glass
(SLG) and 275 nm of Mo and the 2nd being 1 mm SLG with 750 nm Mo direct current (DC)
sputtered in-house were used for the composition series and annealing time series set
respectively. Preliminary substrate cleaning was done by rinsing in solvents and ultraviolet (UV)
ozone cleaning and finally plasma cleaning in the sputtering chamber for lower back contact
resistance.1 The load-locked sputtering chamber was equipped with liquid nitrogen cooled coldfinger and pumped by a turbo molecular pump backed by a roughing pump and has base pressure
2x10-7 Torr or lower. Deposition pressure of 2.5 mTorr argon was used for all films with radiofrequency (RF) deposition powers close to 110 W for Cu, 145 W for ZnS, and 48 W for Sn
targets (Plasmaterials Livermore, CA 99.5% purity). The target–substrate distance was 16.5 cm,
and the sample was rotated during deposition with no substrate heating. Annealing of the films
was done in a small volume, closed box, static gas tube furnace as described in the paper text.
Sample surface morphology, grain size, and composition were analyzed in a FEI Quanta
600FEG scanning electron microscope (SEM) equipped with energy dispersive X-ray analysis
a)
Author to whom correspondence should be addressed. Electronic mail: scarpulla@eng.utah.edu
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(EDX). The grain sizes listed in Table I were determined by averaging images taken at multiple
locations using digital image analysis software. Cross-sectional images at cleaved edges of the
films showed equiaxed grains throughout the cross-section for both sample series. All samples
in composition series (Fig. S1) exhibit SnS2 surface precipitates but samples 2C and 4C had
higher density to be detected in Raman. In figure labeled 2C and 4C these precipitates are
encircled to show their position in center and right edge of the micrograph respectively
Figure S1 - SEM images of the composition series samples all annealed for 30 min (clockwise
from upper left): 1C {[Cu]/([Zn]+[Sn])=0.9, [Zn]/[Sn]=1.3}, 2C {[Cu]/([Zn]+[Sn])=0.74,
[Zn]/[Sn]=1.4}, 3C {[Cu]/([Zn]+[Sn])= 0.73, [Zn]/[Sn]=1.8}, 4C ([Cu]/([Zn]+[Sn])= 0.61,
[Zn]/[Sn]=0.71), and 5C ([Cu]/([Zn]+[Sn])= 0.63, [Zn]/[Sn]=1.9). Note the changes of scale
amongst the images. Sn phases are encircled in the 2C and 4C samples.
These SnS2 surface precipitates are highly faceted portions of hexagonal plates which
appear to grow outwards from the CZTS film surface. Additionally at cleaved edges they are
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seen not to penetrate into the films suggesting a vapor-phase growth mechanism. Their absence
in similarly large densities on the temperature series samples (especially 30T with identical
annealing conditions) discounts hypotheses in which they form by condensation/reaction of
excess Sn or SnS vapors during cooling in the graphite box. Thus we propose that they form by
vapor phase epitaxy from the reaction of SnS vapors emerging from the CZTS film and S from
the annealing atmosphere. The areal density and thickness of individual SnS 2 platelets are
greatest for the composition series samples because of their rather exaggerated Cu-poorness
(=0.61-0.74); we assume that some ZnS is probably also present in the films as required from
stoichiometry considerations perhaps near the Mo contact but it was not observed in Raman
characterization probably because of the probe depth. The standard 1 min KCN etch thins and
removes many of these platelets but in the two noted samples they still remain in sufficient
density that areas devoid of them larger than the 2 m Raman probe area could not be located.
For all Cu-poor samples, some mixture of ZnS and SnxSy phases have the potential to
form assuming all Cu incorporates into CZTS. While it is possible that only Cu-poor CZTS
would exist in full thermal equilibrium, it is possible that these samples are not in full
equilibrium and that near-stoichiometric CZTS is formed along with other phases. Although
sample 2C is Zn-rich, [Cu] limits the amount of stoichiometric CZTS that can form and thus both
SnSx and ZnS may be formed along with CZTS. Although somewhat counterintuitive based on
cursory examination of the  and  values, limiting reactant calculations may be critical for
understanding the appearance of 2nd phases in/on CZTS thin films. All of the annealing time
series samples show higher grain size than the composition series due to larger . The annealing
time series samples are close to the compositions optimal for solar cells and in this set (Figure S2)
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we do not find any Sn phases on the surface. Higher [Cu] which makes Zn the limiting reactant
can be the reason why such phases are not seen in the annealing time series.
Figure S2 - SEM images of the annealing time series samples annealed at 10, 30, 75 and 120 min
respectively (clockwise from upper left).
All samples have [Cu]/([Zn]+[Sn])= 0.9 and
[Zn]/[Sn]=1.4 as measured by EDS after annealing.
Current-voltage (I-V) measurements were done on etched samples with sputtered gold
contacts. The linearity of I-V curves up to 0.1 V (indicating Ohmic contact) was verified for
every contact on each sample at multiple temperatures; example data is shown in Fig. S3.
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Figure S3. Example Ohmic I-V behaviour of one of the contacts on sample 4C from data taken at
298 K.
Conductivity value was extracted from these I-V dependencies. A comparison of fitting
the measured data with two exponential function representing Mott-variable range hopping (MVRH) and thermionic emission (TE) versus three exponential functions that also includes nearest
neighbor hoping (NNH) is shown in Figure S4.
Figure S4. Example showing that fitting the conductivity data from sample 5C using a model
including only Mott variable range hopping (M-VRH) and thermionic emission (TE) is
inadequate in the intermediate temperature regime (approximately 50-150 K)
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1
J. Johnson, H. Nukala, E.A. Lund, W.M. Hlaing Oo, A. Bhatia, L.W. Rieth and M.A. Scarpulla,
in Proceedings of the Material Research Society Symposium, San Francisco, USA (MRS, 2010),
pp. EE03-03.
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