In fluence of sulfate residue on Cu 2 ZnSnS 4 thin films prepared by directsolution method Thi Ha Tranc,d, Hyeonsik Cheonga,n a Department of Physics, Sogang University, Seoul 121-742, Kor
Trang 1In fluence of sulfate residue on Cu 2 ZnSnS 4 thin films prepared by direct
solution method
Thi Ha Tranc,d, Hyeonsik Cheonga,n
a Department of Physics, Sogang University, Seoul 121-742, Korea
b Advanced Convergence Research Center, Daegu Gyeongbuk Institute of Science and Technology, Daegu 711-873, Korea
c
Vietnam National University, College of Science, 334 Nguyen Trai, Hanoi, Vietnam
d
Hanoi University of Mining and Geology, Co Nhue, Hanoi, Vietnam
a r t i c l e i n f o
Article history:
Received 11 September 2014
Received in revised form
11 December 2014
Accepted 4 January 2015
Keywords:
Copper–zinc–tin sulfide
Thin films
Raman spectroscopy
Laser beam induced current
Residue
Sulfurization
a b s t r a c t Raman scattering and atomic force microscopy measurements on Cu2ZnSnS4thinfilms prepared by a direct solution method revealed that metal sulfates of various morphologies (dense clusters or separated particles) were partially embedded on the surface of the Cu2ZnSnS4layer This residue was removed during the subsequent chemical bath deposition of the CdS buffer layer However, the removal of the residue led to poor crystallinity and reduced photocurrent near the location of the residue, which suggests that controlling the formation of the sulfates during the fabrication of the absorber layer would
be critical for obtaining high efficiency solar cells by the solution method
& 2015 Elsevier B.V All rights reserved
1 Introduction
Recently, Cu2ZnSnS4 (CZTS) and Cu2ZnSn(S,Se)4 (CZTSSe) have
emerged as a promising candidate to replace CuIn1xGaxSe2(CIGS) in
photovoltaic applications As a p-type semiconductor, CZTS closely
resembles CIGS, the leading material for absorber in thinfilm solar
cells to date CZTS has many advantages for conversion of solar
radiation, such as a near optimum direct band gap energy of
Eg¼(1.41.6) eV, a high absorption coefficient (104сm1), and
p-type conductivity[1–3] Using only low cost, abundant elements
instead of rare and expensive elements such as Ga and In, CZTS is
obviously a good choice to reduce the cost and for sustainable
development of the photovoltaics technology However, since the
cost of absorber is only a part of the total cost of solar cell panels, a
cheap and highly effective preparation process, other than vacuum
methods, is needed in order to realize commercially viable CZTS solar
cells It is encouraging that during the last several years, the
efficiency of CZTS solar cells has significantly increased The record
efficiency achieved for CZTS solar cells prepared by vacuum and
non-vacuum methods are not much different Recently, a record efficiency
of 12.6%[4]for the glass/Mo/CZTSSе/CdS/ZnO/ITO/Ni-Al structure has been achieved by a solution-based process However, in order to achieve efficiency close to 20%, which is necessary for this absorber
to become commercially viable, various factors that adversely affect the solar cell performance must be identified and controlled One common problem for solution based processes is the formation of residues in the absorber layer Carbon is the best studied one among detected residues Many researchers reported that carbon could be formed as layers resulting in serious problems and drop the efficiency of the cell because they not only increase the series resistance but could also give rise to low adhesion between the CZTS layer and the Mo substrate[5–7] This seems to be a serious problem for CZTS layers prepared from solution-based methods where most solvents, binders or stabili-zers are organic and carbon-rich in nature Apart from carbon, special attention is necessary to other residues such as oxygen and sulfur in order to raise the quality of solution based absorber layer because they are also abundant in precursor materials However, unlike carbon, the effects of oxygen or sulfur have not been investigated thoroughly It is not even clear whether these residues are harmful or beneficial for the solar cell performances [8–15] In this article, we study the formation of sulfate (SO4–)
Understanding the transformation of such sulfate residues during
Contents lists available atScienceDirect
Solar Energy Materials & Solar Cells
http://dx.doi.org/10.1016/j.solmat.2015.01.003
0927-0248/& 2015 Elsevier B.V All rights reserved.
n Corresponding author Tel./fax: þ82 2 717 8434.
E-mail address: hcheong@sogang.ac.kr (H Cheong).
Solar Energy Materials & Solar Cells 136 (2015) 113–119
Trang 2a solution-based synthesis process is important because it could
provide useful information to avoid the formation of this residue
and hence give us a possibility to further improve the efficiency of
CZTS solar cells
2 Experiment
2.1 Samples
Two sets of CZTS, CZTS/CdS and CZTS/CdS/ZnO/Al full cell samples
were prepared by a solution method on soda-lime glass substrates
coated with molybdenum of thickness 500 nm and 1000 nm Metal
chlorides (CuCl2, ZnCl2, SnCl2) and thiourea were used as source
materials These compounds were completely dissolved in a mixture
of de-ionized water and ethanol to produce a precursor solution The
concentrations of CuCl2, ZnCl2, SnCl2 and thiourea in the solution
were 0.9, 0.7, 0.5, and 4 M, respectively The precursor solution was
subsequently spin-coated onto molybdenum-coated soda-lime glass
substrates at 5000 rpm for 30 s The spin-coated CZTS precursor thin
films were then baked on a hotplate at 350 1C for 5 min for
pre-annealing The spin coating process was repeated several times to
obtain the desired thickness The precursorfilms were then
sulfur-ized in a two-zone tubular quartz furnace In order to maintain
proper sulfur concentration, sulfur was evaporated at 3001C, and the
sample zone was heated to different temperatures in the range of
5201C to 570 1C for 30 min Argon carrier gas with 200 sccm at
atmospheric pressure was used in the quartz furnace to facilitate the
flow of sulfur vapor, and both heating zones were naturally cooled to
room temperature The composition, examined by energy-dispersive
spectroscopy (EDS), was copper-poor (Cu/ZnþSn0.7; Zn/Sn1)
More details on the preparation process can be found elsewhere[16]
A CdS buffer layer (80 nm) was deposited onto CZTS films by
chemical bath deposition (CBD) using CdS and thiourea solution A
solution of CdSO4and thiourea in a mixture of ammonia and
de-ionized water was heated to 651C before dipping the absorber
samples inside During the chemical bath process, the solution was
stirred at a constant rate The CdS thickness was controlled by
changing the deposition time A 80-nm CdS was obtained after
13 min when preparing the solar cells To study the effect of the
CBD process on the residues, we used a shorter deposition time of
8 min for the CZTS/CdS samples After depositing the buffer layer, RF
sputtering was used to sequentially deposit a 50-nm-thick intrinsic
ZnO layer and a 300-nm-thick Al-doped ZnO layer on top of the
buffer layer in order to fabricate solar cells Finally, a 500-nm-thick Al
collection grid was thermally evaporated on top of the device At
each step of the fabrication process a set of samples were kept aside
in order to monitor the effect of the processes
Table 1summarizes the samples used in this study S#-1 refers
to bare absorberfilms, S#-2 films with CdS buffer layers, and S#-3
full cells.Table 2summarizes photovoltaic properties of the solar
cells fabricated on 500-nm Mo-coated substrates, characterized by
using a Keithley 2400 source meter unit and a solar simulator
(Newport 69907) to simulate AM 1.5 solar irradiation Solar cells
fabricated on 1000-nm Mo-coated substrates did not show decent
photovoltaic performances and their data are not presented here
2.2 Measurements
For secondary phase detection, macro-Raman measurements
were carried out in the quasi-backscattering geometry by using
several excitation wavelengths to take advantages of
quasi-resonance conditions as well as the different penetration depths
of the lasers The excitation sources were the 632.8-nm line of a He–
Ne laser, the 514.5-nm line of an Ar ion laser, and the 325.0-nm line
of a He-Cd laser The penetration depth of the laser ranges from
140 nm for the 488-nm line to 170 nm for the 633-nm line[17].The measurements were performed on several different parts of the samples in order to confirm that there is no macroscopic inhomo-geneity The laser power was controlled by a tunable neutral-density filter to 20 mW The excitation laser beam was line-focused with a cylindrical lens to an area of100μm 5 mm The scattered light wasfiltered by a pair of holographic edge filters and then dispersed by an iHR-550 or a TR-550 spectrometer (JY Horiba) Finally, the signals were detected with a liquid nitrogen cooled back-illuminated charge-coupled-device (CCD) detector array Micro-Raman imaging measurements were performed with a
50 microscope objective (0.8 N.A.) using the 632.8-nm line of a
He–Ne laser as the excitation source The size of the laser spot on the sample was estimated to be approximately 1mm in diameter This excitation wavelength was chosen in order to avoid optical absorption in the CdS buffer layer (Eg¼2.42 eV at room tempera-ture)[18]and the power was kept at 0.5mW to prevent the sample from being damaged by laser heating
Laser beam induced current (LBIC) measurements were per-formed on the same system by using the 632.8-nm laser as the excitation source The laser power for the LBIC measurements was
1 nW (1.3 kW/m2) in order to simulate the AM 1.5 condition (1 kW/m2) The photocurrent was measured in the ac mode by using
a chopper to modulate the excitation light at a frequency of 410 Hz The images were obtained by raster-scanning the sample with
a computer-controlled translation stages in 1-μm steps for ima-ging large areas and 0.5-μm steps for small areas The
Table 1 Sample information.
Sample name Stack sequences
Sulfurization temperature
Mo-thickness (nm)
Absorber-thickness (μm)
Duration of CBD (min)
S1-1 CZTS 520 1C 1000 1.60 S2-1 CZTS 540 1C 1000 1.55 S3-1 CZTS 560 1C 1000 1.52 S4-1 CZTS 570 1C 1000 1.43 S5-1 CZTS 520 1C 500 2.22 S6-1 CZTS 540 1C 500 1.94 S7-1 CZTS 560 1C 500 1.81 S8-1 CZTS 570 1C 500 1.67 S1-2 CZTS/CdS 520 1C 1000 1.60 8 S2-2 CZTS/CdS 540 1C 1000 1.55 8 S3-2 CZTS/CdS 560 1C 1000 1.52 8 S4-2 CZTS/CdS 570 1C 1000 1.43 8 S5-2 CZTS/CdS 520 1C 500 2.22 8 S6-2 CZTS/CdS 540 1C 500 1.94 8 S7-2 CZTS/CdS 560 1C 500 1.81 8 S8-2 CZTS/CdS 570 1C 500 1.67 8 S5-3 CZTS/CdS/
ZnO/Al
520 1C 500 2.22 13 S6-3 CZTS/CdS/
ZnO/Al
540 1C 500 1.94 13 S7-3 CZTS/CdS/
ZnO/Al
560 1C 500 1.81 13 S8-3 CZTS/CdS/
ZnO/Al
570 1C 500 1.67 13
Table 2 Photovoltaic properties of the solar cells with absorber layer sulfurized at different temperatures.
Sample name V OC (V) J SC (mA/cm 2
) FF (%) Efficiency (%) S5-3 0.42 13.35 54.34 3.08 S6-3 0.45 16.29 52.95 3.95 S7-3 0.48 1.36 27.67 0.18 S8-3 0.34 1.33 32.22 0.15
Trang 3measurements on the same sample, and from the same area All
measurements were carried out in an ambient condition
3 Results and discussion
It is well known that the secondary phases in kesterite
com-pounds are difficult to distinguish by the X-ray diffraction technique
[17] Raman scattering with different excitation wavelengths to take
advantages of the quasi-resonant conditions could help resolve this
problem For CZTS, two most likely secondary phases are Cu2SnS3
(CTS) and ZnS Whereas the former has a small band gap (0.9 eV) and can be detected by using long-wavelength excitations such as
633 or 785 nm[19], the latter can be detected easily with ultraviolet excitation due to the resonance with the ZnS band gap (Eg¼3.84 eV) with the 325-nm line (3.82 eV) of a He-Cd laser[20]
Firstly, we took macro Raman spectra of the samples with the 514.5 nm excitation wavelength which is far from the resonant conditions of CTS and ZnS in order to study the influence of the processing conditions on the development of the main CZTS phase Fig 1(a) shows that at all sulfurization temperature, the main peaks of CZTS at 287 and 337 cm1and their second order
Fig 1 Macro Raman spectra of samples prepared on 500 nm-thick Mo substrates (S5-1 to S8-1) measured with different excitation wavelengths: (a) 514.5 nm, (b) 632.8 nm, and (c) 325 nm.
Fig 2 (a) Optical microscope and (b) AFM images of sample S2-1; and (c) optical microscope and (d) AFM images of sample S6-1 The red (normal region) and blue (dark
V.T Nguyen et al / Solar Energy Materials & Solar Cells 136 (2015) 113–119 115
Trang 4peaks in the region from 600 to 750 cm1are detected clearly As
the annealing temperature is increased, the intensity of the main
CZTS peaks at 287 and 337 cm1(A1peak)[21]increases
promi-nently; indicating that the crystallinity of the samples further
improved at higher sulfurization temperatures For samples S1-1
through S4-1, the same trend was observed (not shown here)
From the Raman spectra taken with red laser as an excitation
source (Fig 1b), it is noted that at higher sulfurization
tempera-tures (560 and 5701C), the signal due to the CTS secondary phase
becomes clear at 264, 302, and 366 cm1[17] Even though the
crystal quality increases at higher temperatures, the detrimental
effects of secondary phases seem to be more significant and so the
efficiency of the cell drops abruptly at temperatures higher than
5401C Since sample S6-3, sulfurized at 540 1C, showed the
high-est efficiency, we will focus on the samples sulfurized at this
temperature in the following
Optical microscope images show that on all the samples there
were a number of dark regions of different morphologies ranging
from dense clusters of several tens of micrometers to particles of
several micrometers.Fig 2(a) and (c) show optical images of two
typical morphologies of these dark regions on samples prepared at
5401C The corresponding AFM images [Fig 2(b) and (d)] reveal
that the dark regions are segregated as dense clusters with the
height of several micrometers
Fig 3shows the micro Raman spectra taken from the normal and
dark regions of sample S2-1 sulfurized at 5401C The Raman spectrum
taken from the dark region is clearly different from the one taken from
the normal region Composition analysis by EDS (not shown) revealed
that the dark regions are rich in oxygen Oxygen could be included
into the CZTS absorber layer because some metal precursors could be hydrolyzed in water, which was used as solvent [15] Thermal gravimetric analysis studied by Madarászet al.[22] and M Krunks
et al.[23]show that the reaction of metal chlorides and thiourea at high temperatures could lead to the formation of the sulfate group (SO4 –) In order to check this possibility, we took the Raman spectra of CuSO4and ZnSO4powders and compared them with that from the dark regions in our samples.Fig 3clearly shows that many features of the spectrum taken from the dark region match with the characteristic peaks in the Raman spectra of CuSO4 and ZnSO4, and so it is reasonable to identify the residues on the surface of our samples as metal sulfates X-ray photoemission spectroscopy measurements also confirmed this interpretation In the EDS analysis, some carbon signals were detected But the fact that no carbon-related peaks were observed in the Raman spectra indicates that carbon impurities do not aggregate in the film to form cluster-like residues No other impurities such as chlorine were observed
Fig 4shows the images of the Raman intensity at 988 cm1, which show the distribution of metal sulfates on the surface of the samples S2-1 and S6-1 Note that the image areas match with corresponding optical and AFM images in Fig 2 These images reconfirmed that the morphology of the dark regions match with the distribution of sulfates
In order to study the effects of CBD, small pieces of the samples were cut and a thin CdS layer was deposited by CBD The CdS layer was thinner than what is usually used in full solar cell structures because we wanted to probe the impact of CBD without the interference from the CdS layer After CBD, the optical images in Fig 5(a) and (c) show that the dark regions still remain The AFM images inFig 5(b) and (d) show that the remaining dark regions are pits with depths of several hundreds of nanometers, not clusters Micro Raman measurements were used again to investigate the pit regions formed after CBD of CdS In order to probe the CZTS under the CdS layer, we used the 632.8 nm-line of He-Ne laser for its energy is smaller than the band gap of CdS.Fig 6shows that the characteristic peaks of sulfates disappeared Instead, the MoS2 signals are enhanced in the pit regions We interpret that metal sulfates were removed during CBD, which is justifiable because metal sulfates are polar compounds of ionic bonding and can be easily dissolved and washed away by polar solvent used in the CBD process This was further verified by dipping a sample in DI water: the residue was removed after water-dipping Because the sulfates are partially embedded in the CZTS layer, removal of the sulfates would leave pits in the CZTSfilm Since the CZTS layer is thinner in the pit regions, the laser can reach the MoS2layer underneath This would explain the enhanced MoS2signal from the pit regions.Fig 7
samples and confirm that the morphology of the pits matches with the MoS2Raman signal image
Fig 3 Micro Raman spectra taken from the normal and dark regions on sample
S2-1 as indicated by the red and blue dots in Fig 2 (a); and Raman spectra of CuSO 4 and
ZnSO 4 for reference.
Fig 4 Raman images of sulfates (a) sample S2-1and (b) sample S6-1 by tracing the intensity of the sulfate peak at 988 cm1 The scan area of (b) is indicated by a small blue
Trang 5InFig 6, the main CZTS peak at 337 cm1appears broader and
redshifted in the spectrum taken from the pit region The sharpness of
Raman peak could be used as a qualitative indicator for the
crystal-linity The peak position, on the other hand, reflects local stoichiometry
or strain of thefilm.Fig 8shows the images of the position and the
width of the most intense A1mode peak of CZTS after CBD The high
contrast between the pit and normal regions clearly demonstrate the
difference between the two regions It is possible that the CZTSfilm
underneath sulfate clusters has different crystallinity, composition,
and/or strain It is also possible that such difference was induced
during the CBD process During the sulfurization process, the forma-tion of sulfates may result in Cu or Zn vacancies in the CZTSfilm as the metal ions are taken by the sulfates High density of vacancies would degrade the crystallinity and result in broader Raman peaks The chemical composition would also be affected, resulting in relative shift
of the Raman peaks Such effects have been observed in Raman scattering of disordered kesterite phase of CZTS and CZTSe[24–28] Also, local strain distribution could contribute to the redshift of the Raman peak As thefilm is thinner in the pit regions, the strain could
be locally different from the normal region.Fig 9
To demonstrate the negative impact of the sulfate residues on the solar cell efficiency, we conducted an LBIC measurement on sample S6-3 The photocurrent in the dark region in the optical image is much smaller than in the normal region The anti-correlation between the MoS2Raman intensity and the LBIC signal clearly demonstrates the detrimental effect of the sulfate residues even after being removed by the CBD process Pits formed by the removal of metal sulfates during CBD certainly should have some negative impact on the efficiency of the cell As the thickness of the absorber layer is reduced, such regions cannot absorb as many photons as normal regions If the CZTS layer is totally removed in some regions, the cell could be shunted as the front and back contacts are short-circuited Furthermore, the decrease in the crystallinity of CZTS may well contribute to a lower efficiency
4 Conclusion Metal sulfates were found in CZTSfilms prepared by a solution-based method, regardless of the sulfurization temperature The sulfates were removed by the CBD process for the CdS buffer layer, but left pits in the CZTS films The CZTS film in the pit regions seems to have lower
Fig 5 (a) Optical microscope and (b) AFM images of sample S2-2, and (c) optical microscope and (d) AFM images of sample S6-2 after depositing CdS by chemical bath process.
In (a), the red and blue dots indicate the positions where the Raman spectra in Fig 6 were taken The blue square in (d) represents the area for Raman imaging in Fig 7 (b).
Fig 6 Micro Raman spectra taken in normal and pit (dark) regions of sample S2-2
after CBD.
V.T Nguyen et al / Solar Energy Materials & Solar Cells 136 (2015) 113–119 117
Trang 6crystallinity, different stoichiometry and/or local strain After a solar cell is
fabricated, such pit regions give significantly low photocurrents, reducing
the solar cell efficiency Therefore, preventing the formation of such
residues in the CZTSfilms would be critical to achieve high efficiency
CZTS solar cells by solution-based methods The origins of such sulfates
and the mechanism for their formation are subjects of further studies
Acknowledgments This work was supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy (No 20123010010130)
Fig 7 Images of the integrated intensities of the MoS 2 Raman signals from 400 to 460 cm1from (a) S2-2 and (b) S6-2 after CBD.
Fig 8 Images of (a) position and (b) width of the CZTS A 1 mode peak at 337 cm1from S2-2.
Fig 9 (a) Optical microscope, (b) AFM, (c) MoS 2 Raman intensity, and (d) corresponding LBIC images of a CZTS solar cell (S6-3).
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