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Effects of chlorine and carbon on Cu2ZnSnS4 thin film solar cells prepared by spray pyrolysis deposition
Journal of Alloys and Compounds 616 (2014) 492–497
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Effects of chlorine and carbon on Cu2ZnSnS4 thin film solar cells prepared by spray pyrolysis deposition Kunihiko Tanaka ⇑, Minoru Kato, Hisao Uchiki Department of Electrical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka-machi, Nagaoka, Niigata 940-2188, Japan
a r t i c l e
i n f o
Article history: Received 23 March 2014 Received in revised form 25 June 2014 Accepted 13 July 2014 Available online 27 July 2014 Keywords: Cu2ZnSnS4 Thin-film solar cell Spray pyrolysis deposition Chlorine Carbon
a b s t r a c t Cu2ZnSnS4 (CZTS) thin film solar cells were fabricated under non-vacuum conditions, wherein the absorption layer was prepared by the spray pyrolysis deposition method. By using a chlorine (Cl)-free solution instead of a Cl-containing solution, the solar cell efficiency was improved from 0.18% to 0.35%. On the CZTS surface deposited from the Cl-containing solution, Cl-containing precipitates with size of 10 lm were present and were found to degrade the solar cell efficiency. The still low efficiency of 0.35% was caused by a layer containing small grains that existed between the CZTS layer and the Mo electrode, which included a significant amount of carbon. By optimizing the pre-annealing process, the smallgrained layer became thinner and the efficiency was improved to 0.63%. Finally, by adjusting the chemical composition of the Cl-free solution and the CdS buffer layer deposition conditions, the cell efficiency was improved to 0.94%. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Cu2ZnSnS4 (CZTS) is one of the most interesting materials for the absorption layer in thin-film solar cells. This is because its optical properties are suitable for a single junction solar cell. The absorption coefficient of the CZTS is larger than 104 cm1 in the visible region and the band gap energy is 1.5 eV. Moreover, all the elements contained in CZTS are abundant in the crust of the Earth and are non-toxic. Ito et al. reported a CZTS thin-film solar cell for the first time in 1988 [1]. They deposited CZTS thin films by atomic beam sputtering and fabricated solar cells with the structure tin-oxide transparent conductive layer/CZTS absorption layer/stainless-steel substrate. These solar cells showed an open circuit voltage (V oc ) of 165 mV under irradiation of AM 1.5. Katagiri and coworkers reported several studies on CZTS thinfilm solar cells [2–5]. They reported a CZTS solar cell with an efficiency of almost 7% for the first time [4]. This report motivated other researchers to explore CZTS solar cells. The structure of the cell reported by Katagiri et al. was Al/ZnO:Al/CdS/CZTS/Mo/soda lime glass (SLG) substrate. The CZTS absorber layer was deposited by RF co-sputtering followed by vapor phase sulfurization. In 2014, the highest ever efficiency of 9.2% was reported for a solar cell in which the CZTS absorber layer was deposited by sputtering [6]. ⇑ Corresponding author. E-mail address: [email protected] (K. Tanaka). http://dx.doi.org/10.1016/j.jallcom.2014.07.101 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
All CZTS thin films mentioned above were deposited by vacuum processes. However, vacuum processes require expensive equipment and complex operation. Therefore, many researchers have studied CZTS preparation methods under non-vacuum conditions. We recently reported a CZTS thin film solar cell in which the CZTS absorption layer was deposited under non-vacuum conditions for the first time, to the best of our knowledge [7]. In our previous report, not only the CZTS absorption layer but also the CdS buffer and ZnO:Al window layers were deposited under non-vacuum conditions [7–9]. The CZTS absorption, CdS buffer and ZnO:Al window layers were deposited by a sol–gel sulfurization method, chemical bath deposition (CBD), and a sol–gel method, respectively. The highest efficiency of this solar cell was 3.6% [9]. Spray pyrolysis deposition (SPD) is a non-vacuum deposition process that has been used for the fabrication of CZTS thin films [10–13]. Nakayama and Ito deposited CZTS thin films under nonvacuum conditions for the first time using SPD [10]. The starting materials of the SPD solution were CuCl, ZnCl2, SnCl4 and thiourea. After coating the solution onto a heated substrate, the coated substrate, named the precursor, was annealed in a H2S + Ar atmosphere to obtain a CZTS thin film. The film exhibited an X-ray diffraction (XRD) peak attributed to CZTS (1 1 2) planes and a band gap energy of 1.46 eV. However, Nakayama and Ito did not fabricate solar cells with the CZTS film. There are few reports on CZTS thin film solar cells in which the absorption layer was deposited by SPD. M. Espindola-Rodriguez et al. used CuCl2, Zn acetate dihydrate, SnCl4 hydrate and thiourea as starting materials for the SPD
K. Tanaka et al. / Journal of Alloys and Compounds 616 (2014) 492–497
coating solution [11]. The precursor was annealed in S and a Sncontaining atmosphere to obtain CZTS thin films. They used these films to fabricate ZnO:Al/ZnO/CdS/CZTS/Mo solar cells. The ZnO and ZnO:Al layers were deposited by DC sputtering under a vacuum. The highest efficiency obtained for the fabricated cells was 0.49%. Rajeshmon et al. used CuCl, Zn acetate, SnCl2 or SnCl4, and thiourea as starting materials for the SPD coating solution [12]. They deposited CZTS thin films without annealing in an S-containing atmosphere. They fabricated ITO/In2S3/CZTS/Ag solar cells, and the cell with the highest current density showed V oc ¼ 380 mV and a short circuit current density ðJ sc Þ ¼ 2:40 mA=cm2 . M. Espindola-Rodriguez used CuCl2, Zn acetate, SnCl4, and thiourea as starting materials for their SPD coating solution [13]. They used Ar or Ar–H2 (5%) as carrier gas to avoid oxygen-contamination. The precursor was annealed with S and Sn powders to obtain a CZTS thin film. They then fabricated CZTS solar cells, wherein the ZnO:Al window layer was deposited by sputtering under vacuum conditions, and the efficiency of the cell was 1.4%. Typically, these CZTS thin films prepared by SPD have several disadvantages. One is the annealing process in S vapor or a H2Scontaining atmosphere, because S vapor is corrosive and H2S is toxic. Another is using chlorides as metal sources, because chlorine (Cl) atoms remain in the CZTS thin film and act as impurities. In our previous report, CZTS thin films were prepared by SPD without annealing in S or a H2S-containing atmosphere [14]. The solution for SPD was prepared from Cu(II) acetate monohydrate, Zn(II) acetate dihydrate, Sn(II) chloride dihydrate and pure S powder. The precursors deposited by SPD were annealed in an N2 atmosphere, with another precursor placed face-to-face as shown in Fig. 1. In this face-to-face arrangement, evaporation of S and Sn was suppressed and CZTS films could be prepared without S vapor or a H2S-containing atmosphere. However, one disadvantage still remained in that Sn(II) chloride was used as the metal source, and a CZTS solar cell was not fabricated. In this report, CZTS thin film solar cells, in which the CZTS absorber layer was deposited by SPD with or without using a chloride, were fabricated. In addition, the effects of Cl and carbon (C), included in the starting material for the SPD solution, on the efficiency of the CZTS solar cell formed were investigated.
2. Experimental procedure Both Cl-containing and Cl-free solutions were prepared for SPD. For both solutions, the Cu and Zn sources were Cu(II) acetate monohydrate and Zn(II) acetate dihydrate, respectively. As Sn sources, Sn(II) chloride dihydrate and Sn octylate were used for Cl-containing and Cl-free solutions, respectively. These metal sources together with pure sulfur powder were dissolved in N,N-dimethylformamide, and monoethanolamine was used as the stabilizer. The composition ratios were Cu=ðZn þ SnÞ ¼ 0:30—0:80; Zn=Sn ¼ 1:15 and S=metal ¼ 4:0. The concentration of
Mo/SLG CZTS precursor [12] Sn-S coated (this work) CZTS precursor Mo/SLG Fig. 1. Annealing configuration: annealing with another coated film placed face-toface. The other coated film was the CZTS precursor in our previous work [12] and an Sn–S solution-coated substrate in this work.
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the metal ions in Cl-containing solution was 0.5 M. For Cl-free solution, since Sn octylate has high viscosity, the concentration of the metal ions was reduced to 0.25 M. Precursors of CZTS were prepared by coating the solution on Mo-coated SLG substrates by SPD in air. Spray and drying processes were repeated 7 or 14 times. The distance between the substrate and the spray nozzle was 170 mm and the N2 carrier gas pressure was 0.6 MPa. During the deposition, the Mo-coated SLG substrates were heated to 160 °C. After deposition, the precursors were annealed in an N2 atmosphere with an infrared gold image furnace (ULVAC CP610CP), with Sn- and S-containing-solution-coated substrates (Sn–S coated substrate) placed face-to-face as shown in Fig. 1. The Sn–S coated substrate was deposited by SPD at 160 °C without using Cu(II) acetate monohydrate and Zn(II) acetate dihydrate. The Sn–S coated substrate was an amorphous-like thin film and showed no Raman scattering peaks. Chemical composition ratio of the Sn–S cover was S=Sn ¼ 2:0. Following an optional pre-anneal, the precursors were annealed at 520 °C for 60 min in the case of Cl-containing solution, and 120 min in the case of Cl-free solution. The annealing duration time was optimized for each solution. For Cl-containing solution, long annealing duration caused peeling off of CZTS thin films. Therefore the annealing duration for Cl-containing solution was shorter than that of Cl-free solution. The temperature increase rate for both the pre-anneal and main anneal was 10 °C/min. The structure of the prepared solar cell was Al/ZnO:Al/CdS/CZTS/Mo/SLG. All semiconductor layers were deposited under non-vacuum conditions. The ZnO:Al window layer was deposited by a sol–gel method. Zinc(II) acetate dihydrate, aluminum chloride hexahydrate, 2-methoxyethanol, and monoethanolamine were used in the sol–gel solution. This sol–gel solution was coated 5 times and dried at 350 °C. A CdS buffer layer was deposited by CBD from an aqueous solution of CdI2, thiourea, and ammonia for 10–40 min. Additional details of the solar cell preparation process are presented in Ref. [7] and Table 1. Microstructural information for the deposited films was obtained by using XRD (Rigaku RAD3). The chemical composition and wavelength-dispersive X-ray spectroscopy (WDX) spectrum of the films were obtained by electron probe microanalysis (EPMA; Shimadzu EPMA 1600). The depth profile of the chemical composition was examined by Auger electron spectroscopy (AES; JEOL JAMP-9500F). The surface morphology of the films was ascertained using scanning electron microscopy (SEM; Hitachi S4000). The current density–voltage (J–V) characteristics of the solar cells were measured under irradiation of AM 1.5G at 100 mW/cm2. The system used for this measurement was composed of a USHIO UXL-500SX2 Xe lamp and a Keithley 2400 digital sourcemeter. The J–V characteristics under irradiation were measured using the four-probe method.
3. Results and discussion 3.1. Effect of chlorine on efficiency of CZTS solar cells CZTS thin films and CZTS solar cells were prepared using the Cl-containing solution (Cell 1) or the Cl-free solution (Cell 2). The CZTS thin film and solar cell preparation conditions are shown in Table 1. Fig. 2 shows XRD patterns for CZTS thin films prepared from the Cl-containing and Cl-free solutions. As can be seen, both films showed peaks attributed to (1 1 2), (2 2 0) and (2 0 0) planes of CZTS. As shown in Fig. 2 noise level in the CZTS thin film prepared from the Cl-containing solution was less than that of the Cl-free solution. Film thickness of the CZTS films prepared from the Cl-containing solution and the Cl-free solution was 1470 and 500 nm, respectively. The thicker film caused less noise. Fig. 3 and Table 2 show the J–V characteristic curve under irradiation and the solar cell properties of the CZTS solar cells, respectively. In Table 2, J sc ; V oc ; F:F., g; Rs and Rsh are the short circuit current density, open circuit voltage, fill factor, efficiency, series resistance and shunt resistance, respectively. Rsh and Rs are the effective values estimated from the slopes of the J–V curve with J and V axes, respectively. As shown in Table 2, the efficiency of Cell 1 (g ¼ 0:18%) is almost half that of Cell 2 (g ¼ 0:35%). To clarify the cause of the lower efficiency of 0.18%, the surface morphology of the CZTS thin films was observed. Fig. 4(a) and (b) show surface images of the CZTS thin films formed from the Cl-containing and Cl-free solutions, respectively. As seen in Fig. 4(a), precipitates with size of about 10 lm are observed on the CZTS thin film, although Fig. 4(b) shows no precipitates.
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Table 1 Solar cell preparation conditions used in this work. Cell no.
Solution
CZTS coat repetition
CZTS pre-anneal
Cu/(Zn + Sn)
CdS deposition time (min)
1 2 3 4 5
(a) (b) (b) (b) (b)
7 7 14 14 14
250 °C, 30 min 160 °C, 120 min 160 °C, 120 min Non Non
0.80 0.80 0.80 0.80 0.65
40 40 40 40 30
Diffraction intensity (a.u.)
Mo
(312)
(112)
(220)
(a) Cl-contained solutoin
(b) Cl-free solution 20
40
60
80
Diffraction angle, 2θ (deg) Fig. 2. XRD patterns of CZTS thin films prepared from (a) a Cl-containing solution and (b) a Cl-free solution.
7 No. 1
Current density (mA/cm2)
6
No. 2 No. 3
5
No. 4 4
No. 5
3
3.2. Effect of carbon on efficiency of CZTS solar cell
2 1
0
the precipitate contains Cl, whereas there is no Cl around the precipitate. Since the electronegativity of Cl is very high, Cl attracts elements around itself and forms chlorides easily, and the attraction of other elements causes a distribution in the chemical composition. The compositions of the precipitate and around the precipitate estimated from WDX spectra are Cu:Zn:Sn:S = 18.2:25.4:13.5:42.9 and Cu:Zn:Sn:S = 26.1:14.3:9.9:49.7 respectively. These results clearly indicate a composition distribution in the CZTS films deposited from the Cl-containing solution. The spatial distribution of the composition is one of the causes of low efficiency of solar cells prepared from the Cl-containing solution. It is well known that the efficiency of CZTS solar cells strongly depends on the composition [5,15]. Cu vacancies are the origin of shallow holes that contribute to electric current generation. Therefore, a Cu-poor composition is very important [16]. As mentioned above, the precipitates contain significant amounts of Zn. Therefore, the regions around the precipitates are relatively poor in Zn and rich in Cu. The relatively Cu-rich regions are one of the causes of the low efficiency. The CZTS surface with the precipitates is very rough, which results in low efficiency [9]. If there are large precipitates, the CdS buffer and ZnO:Al window layers cannot cover enough of the CZTS surface and this causes a short between the Al electrode and the CZTS absorption layer. Chlorine itself is also one of the causes of low efficiency of solar cells because Cl is an impurity and forms carrier trap levels. There are no reports on trap levels due to Cl for CZTS, and further investigation is necessary.
100
200
300
400
500
Voltage (mV) Fig. 3. J–V curves for solar cells with CZTS absorption layer deposited by SPD. The cell preparation conditions are shown in Table 1.
Table 2 Solar cell properties with CZTS absorber layer deposited by SPD. Cell preparation conditions are shown in Table 1. Cell no.
Jsc (mA/cm2)
V oc (mV)
F:F:
g (%)
Rs (X cm2)
Rsh (X cm2)
1 2 3 4 5
1.90 3.34 0.94 6.19 6.71
437 392 397 318 451
0.21 0.27 0.21 0.32 0.31
0.18 0.35 0.08 0.63 0.94
344 92 629 30 43
189 133 311 87 102
Fig. 5 shows WDX spectra obtained from a precipitate and from the region around it, where the solid circle, delta and square symbols denote Sn, Cl and Mo, respectively. The observation positions are shown in the inset in Fig. 5(i) corresponding to the precipitate and (ii) corresponding to around the precipitate. As shown in Fig. 5,
By using the Cl-free solution instead of the Cl-containing solution, the cell efficiency was improved from 0.18% to 0.35%, as shown in Section 3.1. Therefore Cl is one of the causes of the low efficiency. However the efficiency of 0.35% for the Cl-free solution is lower than the reported value of 1.4% for a cell prepared by SPD using a Cl-containing solution [13]. We assumed that one of the causes of the low efficiency is the thinner absorption layer. Since the absorption coefficient of CZTS is on the order of 104 cm1, the optimal thickness of the CZTS absorption layer is 1–2 lm. In Ref. [13], the thickness of the CZTS absorber layer was 1.0 lm. Fig. 6(a) shows a cross-sectional SEM image of the CZTS thin film which is the same as the absorption layer of the Cl-free CZTS solar cell (Cell 1) described in Section 3.1. The CZTS film thickness is 0.68 lm. To improve the efficiency, a thick CZTS absorption layer was deposited by increasing the coating repetition from 7 times to 14 times. Fig. 6(b) shows a cross-sectional SEM image of the CZTS thin film obtained by coating 14 times. The film thickness is larger at about 1.3 lm. However, the efficiency of a CZTS solar cell with 14 times coating was 0.08% (see Cell 3 in Tables 1 and 2). Therefore the thinner CZTS absorption layer was not the cause of the low efficiency. As seen in Fig. 6(a) and (b), there is a CZTS small-grained layer between the CZTS large-grained layer and the Mo. This
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(b)
(a)
(b)
(a)
20 μm
20 μm
Fig. 4. Surface images of the CZTS thin film (a) deposited with the Cl-containing solution and (b) deposited with the Cl-free solution.
WDX intensity (a.u.)
(i) a precipitate
Sn Mo Cl
(ii) around a precipitate
(i) (ii) 20 μm
3
4
5
Wavelength ( ) Fig. 5. WDX at (i) a precipitate and (ii) around a precipitate.
small-grained layer becomes thicker as the coating repetitions increase. We assumed that the cause of the low efficiency is the small-grained layer, and therefore the composition depth profile for the CZTS thin film was investigated. Fig. 7 shows the depth profile for the CZTS thin film with 14 times coating. Any treatment was not carried out before AES-measurement. Therefore at beginning of the measurement, the results of the AES were affected from surface conditions. As shown in Fig. 7, at surface the sample was Cu poor and Zn- and Sn-rich. However, near the Mo, the chemical composition of the sample became Cu-rich and Zn- and Sn poor. The CZTS and Mo layers seem to be interdiffused. The region near the Mo, which corresponds to the small-grained layer, contains a large amount of carbon. There have been some reports on a carbon-containing layer between the Mo electrode and a CuInSe2 (CIS) or a CuIn1xGaxSe2 (CIGS) thin film deposited by a solution-based process [17–20]. In Refs.
(a) coating 7 times
(b) coating 14 times
CZTS large grain CZTS small grain Mo
[17–19] it was reported that the resistivity of the carbon-containing layer is relatively high and the carbon-containing layer may be one of the causes of low efficiency of the solar cell. On the other hand, Ref. [20] reported that the carbon-containing layer becomes the current flow path. In the present work, as shown in Table 2 (Cell 3), since the solar cell with a thick carbon-containing layer shows a low current density and a high series resistance, the resistivity of the carbon-containing layer is high and it causes low efficiency. To reduce the thickness of the carbon-containing layer, the preannealing conditions were investigated. Fig. 8 shows film thickness ratio of the carbon-containing layer as a function of pre-annealing temperature. The film thickness of each CZTS film was shown in Fig. 8. As shown in Fig. 8 the thickness of the carbon-containing layer decreases as the pre-annealing temperature increases from 160 to 200 °C. However, at 220 °C, it increases again. The thinnest carbon-containing layer was obtained for the case of no pre-annealing. From this result, we assumed the thickness reduction mechanism shown in Fig. 9 for the carbon-containing layer during the pre-annealing process. In the early stage of the pre-annealing process, the organic solvent containing a large amount of carbon evaporates from the precursor. In the middle stage, a Cu–Zn–Sn–S sulfide grows and covers the surface. The sulfide compound prevents evaporation of the organic solvents. Therefore, the carboncontaining layer without pre-annealing is thinner than that with pre-annealing. From 160 to 200 °C, the organic solvent evaporates earlier as the pre-annealing temperature increases. Therefore, a higher temperature results in a thinner carbon-containing layer. However, at the high pre-annealing temperature of 220 °C, a sulfide cap forms immediately and prevents evaporation. Therefore, a higher pre-annealing temperature results in a thicker carboncontaining layer. A CZTS thin film solar cell whose CZTS absorption layer was deposited without pre-annealing was also prepared (Cell 4). Except for the fact that no pre-annealing was performed, the preparation
CZTS large grain CZTS small grain 1 μm
Mo
1 μm
Fig. 6. Cross-sectional image of the CZTS thin film with (a) coating 7 times and (b) coating 14 times.
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250
Organic solvents are evaporated
Cu
200
Intensity (cps)
Zn Sn 150
Mo
precursor
C 100
Mo layer
(a) early stage
50
Mo
CZTS 0
0
500
1000
CZTS large grain layer
1500
Sputtering time (sec) Fig. 7. Depth profile of chemical composition in the CZTS thin film.
Mo layer
(b) middle stage
CZTS large grain layer Carbon-containing layer Mo layer
(c) final stage Fig. 9. Mechanism for carbon reduction during pre-annealing process.
Fig. 8. Thickness of the carbon-containing layer as a function of pre-annealing temperature.
conditions were the same as for Cell 3. The J–V curve under irradiation and the cell properties of cell 4 are shown in Fig. 3 and Table 2, respectively. As seen in Table 2, the efficiency was improved from 0.08% to 0.63%.
3.3. Improvement of cell efficiency In Section 3.2, it was seen that by skipping the pre-annealing process, the conversion efficiency improved to 0.63%. However, this is still lower than the efficiency reported in Ref. [13]. As mentioned in Section 3.1, the conversion efficiency of the CZTS solar cell strongly depends on the chemical composition [5,15]. Therefore, the dependence of cell properties on chemical composition was investigated. The chemical composition ratio Zn=Sn in the solution was fixed at 1.15 and the ratio Cu=ðZn þ SnÞ in solution was varied from 0.30 to 0.80. Other cell preparation conditions were the same as for Cell 4. Table 3 shows that the chemical composition of the deposited CZTS thin film depends on that of the coating solution. For Cu/(Zn + Sn) in a solution of 0.80 to 0.60, Cu/(Zn + Sn) and Zn/Sn in the deposited CZTS thin film were almost constant and the chemical composition became almost stoichiometric. For Cu/(Zn + Sn) less than 0.50 in the solution, Cu/(Zn + Sn) in the
deposited film decreased as the ratio in the solution decreased. From these results, in order to control the chemical composition in the deposited films by adjusting the chemical composition in the solution, the ratio in the solution needs to be varied drastically. Table 4 shows the dependence of the CZTS cell properties on the chemical composition in the solution. The efficiency clearly does not depend on the Cu/(Zn + Sn) ratio in the solution. The highest efficiency of 0.72% was obtained for Cu/(Zn + Sn) of 0.65 in the solution. The chemical composition in the highest efficiency cell was Cu/(Zn + Sn) = 0.90 and Zn/Sn = 0.96. Katagiri et al. reported that the ideal chemical composition ratios for high efficiency were Cu/(Zn + Sn)=0.8–0.9 and Zn/Sn = 1.1–1.3 [15]. The chemical composition of the 0.72% cell was relatively close to the ideal chemical composition ratio. Therefore, the efficiency was probably high. Further study is necessary for controlling chemical composition to improve the efficiency. Finally, the CdS deposition time was adjusted, since it affects the cell efficiency [7]. A CZTS solar cell was prepared from a solution with Cu/(Zn + Sn) = 0.65 and Zn/Sn = 1.15 from which the highest efficiency cell was fabricated. The CdS deposition time was varied from 10 to 40 min. The highest efficiency was obtained for the 30 min deposition. The cell properties and J–V curve under irradiation are shown in Table 2 and Fig. 3, respectively, labeled as Cell 5. As seen in Table 2, the efficiency was improved to 0.94% and the other parameters were V oc ¼ 451 mV, J sc ¼ 6:71 mA=cm2 and F:F: ¼ 0:31. In the present study, the ZnO:Al window layer was deposited by a sol–gel method while usually for CZTS solar cells ZnO:Al window layer was deposited by DC or RF-magnetron sputtering. The resistivity of the ZnO:Al in the present report is 101 X cm [9]. This value is three or four orders of magnitude larger than that of the ZnO:Al deposited by sputtering. The high resistivity of the window layer degrades cell performance.
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K. Tanaka et al. / Journal of Alloys and Compounds 616 (2014) 492–497 Table 3 Chemical composition of CZTS thin films deposited from solution with various chemical compositions. Zn/Sn was fixed at 1.15. Ratio of solution
Cu
Zn
Sn
S
Cu/(Zn + Sn)
Zn/Sn
S/metal
Cu/Sn
Cu=ðZn þ SnÞ ¼ 0:30 Cu=ðZn þ SnÞ ¼ 0:50 Cu=ðZn þ SnÞ ¼ 0:60 Cu=ðZn þ SnÞ ¼ 0:65 Cu=ðZn þ SnÞ ¼ 0:70 Cu=ðZn þ SnÞ ¼ 0:80
14.2 24.3 24.8 24.4 27.0 26.9
23.8 18.0 13.4 13.3 13.2 13.8
9.6 12.1 13.1 13.8 13.6 13.4
52.4 45.6 48.7 48.5 47.2 45.9
0.42 0.81 0.94 0.90 0.97 0.99
2.48 1.48 1.02 0.96 0.96 1.03
1.10 0.84 0.95 0.94 0.89 0.85
1.47 2.01 1.89 1.77 1.90 2.01
Table 4 CZTS thin film solar cell properties prepared from solution with various chemical compositions. Zn/Sn was fixed at 1.15. Sample
Jsc (mA/cm2)
V oc (mV)
F:F:
g (%)
Rs (X cm2)
Rsh (X cm2)
Cu=ðZn þ SnÞ ¼ 0:30 Cu=ðZn þ SnÞ ¼ 0:50 Cu=ðZn þ SnÞ ¼ 0:60 Cu=ðZn þ SnÞ ¼ 0:65 Cu=ðZn þ SnÞ ¼ 0:70 Cu=ðZn þ SnÞ ¼ 0:80
3.34 4.47 3.59 6.98 4.15 6.19
393 409 259 326 300 318
0.28 0.27 0.25 0.31 0.31 0.32
0.37 0.49 0.24 0.72 0.38 0.63
93 79 69 29 49 30
147 114 78 78 117 87
4. Conclusion CZTS thin film solar cells with CZTS absorption layers deposited by SPD were fabricated. For SPD, two types of coating solutions, Cl-containing and Cl-free solutions, were prepared. On the surface of the CZTS thin films deposited from the Cl-containing solution, precipitates with a size of about 10 lm and containing significant Cl were observed. The efficiency of the solar cells prepared from Cl-containing and Cl-free solutions were 0.18% and 0.35%, respectively. Thus, without using a chloride as the starting material, the efficiency of the solar cell was improved. However the efficiency of 0.35% is still low compared to other reports wherein the CZTS absorption layer was prepared by SPD. From the results of the film thickness dependence of efficiency and cross-sectional SEM images, a small-grained CZTS layer was present between the Mo layer and the large-grained CZTS layer, and this small-grained layer resulted in lower efficiency. Since the small-grained layer contained a large amount of carbon atom, the pre-annealing conditions were adjusted to reduce the carbon. By skipping the pre-annealing process, the carbon-containing small-grained layer became thinner and the conversion efficiency was improved from 0.35% to 0.63%. Finally, the chemical composition and CdS buffer layer deposition conditions were adjusted to improve the efficiency, and the properties of the cell with the highest efficiency in this work were g ¼ 0:94%; V oc ¼ 451 mV, J sc ¼ 6:71 mA=cm2 and F:F: ¼ 0:31.
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[5] H. Katagiri, K. Jimbo, W.S. Maw, K. Oishi, M. Yamazaki, H. Araki, A. Takeuchi, Development of CZTS-based thin film solar cells, Thin Solid Films 517 (2009) 2455–2460. [6] T. Kato, H. Hiroi, N. Sakai, S. Muraoka, H. Sugimoto, Characterization of front and back interfaces on Cu2ZnSnS4 thin-film solar cells, in: Proceedings of the 27th European Photovoltaic Solar Energy Conference and Exhibition (EUPVSEC), 2012, pp. 2236–2239. [7] K. Tanaka, M. Oonuki, N. Moritake, H. Uchiki, Cu2ZnSnS4 thin film solar cells prepared by non-vacuum processing, Sol. Energy Mater. Sol. Cells 93 (2009) 583–587. [8] K. Tanaka, N. Moritake, H. Uchiki, Preparation of Cu2ZnSnS4 thin films by sulfurizing sol–gel deposited precursors, Sol. Energy Mater. Sol. Cells 91 (2007) 1199–1201. [9] T. Aizawa, K. Tanaka, K. Tagami, H. Uchiki, Investigation of ZnO:Al window layer of Cu2ZnSnS4 thin film solar cells prepared by non-vacuum processing, Phys. Stat. Solidi C 10 (2013) 1050–1054. [10] N. Nakayama, K. Ito, Sprayed films of stannite Cu2ZnSnS4, Appl. Surf. Sci. 92 (1996) 171–175. [11] M. Espindola-Rodriguez, M. Placidi, O. Vigil-Galán, V. Izquierdo-Roca, X. Fontané, A. Fairbrother, D. Sylla, E. Saucedo, A. Pérez-Rodríguez, Compositional optimization of photovoltaic grade Cu2ZnSnS4 films grown by pneumatic spray pyrolysis, Thin Solid Films 535 (2013) 67–72. [12] V.G. Rajeshmon, C. Sudha Kartha, K.P. Vijayakumar, C. Sanjeeviraja, T. Abe, Y. Kashiwaba, Role of precursor solution in controlling the opto-electronic properties of spray pyrolysed Cu2ZnSnS4 thin films, Sol. Energy 85 (2011) 249– 255. [13] M. Espindola-Rodriguez, D. Sylla, Y. Sanchez, S. López-Marino, X. Fontané, J. López-García, M. Placidi, A. Pérez-Rodríguez, O. Vigil-Galán, E. Saucedo, Pneumatically sprayed Cu2ZnSnS4 films under Ar and Ar–H2 atmosphere, J. Phys. D: Appl. Phys. 47 (2014) 245101. [14] K. Tanaka, M. Kato, K. Goto, Y. Nakano, H. Uchiki, Face-to-face annealing process of Cu2ZnSnS4 thin films deposited by spray pyrolysis method, Jpn. J. Appl. Phys. 51 (2012) 10NC26 (4pages). [15] H. Katagiri, K. Jimbom, M. Takahara, H. Aaraki, K. Oishi, The influence of the composition ratio on CZTS-based thin film solar cells, Mater. Res. Soc. Symp. Proc. 116 (2009) 125–136. [16] S. Chen, J.H. Yang, X.G. Gong, A. Walsh, S.H. Wei, Intrinsic point defects and complexes in the quaternary kesterite semiconductor Cu2ZnSnS4, Phys. Rev. B 81 (2010) 245204 (10 pages). [17] M. Kaelin, D. Rudmann, F. Kurdesau, H. Zogg, T. Meyer, A.N. Tiwari, Low-cost CIGS solar cells by paste coating and selenization, Thin Solid Films 480–481 (2005) 486–490. [18] S.J. Ahn, C.W. Kim, J.H. Yun, J.H. Gwak, S.H. Jeong, B.H. Ryu, K.H. Yoon, CuInSe2 (CIS) thin film solar cells by direct coating and selenization of solution precursors, J. Phys. Chem. C114 (2010) 8108–8113. [19] M.G. Park, S.J. Ahn, J.H. Yun, J.H. Gwak, A. Cho, S.K. Ahn, K. Shin, D. Nam, H. Cheong, K.H. Yoon, Characteristics of Cu(InGa)Se2 thin films deposited by a direct solution coating process, J. All. Comp. 513 (2012) 68–74. [20] A. Cho, S.J. Ahn, J.H. Yun, Y.J. Eo, H. Song, K. Yoon, Carbon layer reduction via a hybrid ink of binary nanoparticles in non-vacuum-processed CuInSe2 thin films, Sol. Energy Mater. Sol. Cells 110 (2013) 126–132.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effects of chlorine and carbon on Cu2ZnSnS4 thin film solar cells prepared by spray pyrolysis deposition Kunihiko Tanaka ⇑, Minoru Kato, Hisao Uchiki Department of Electrical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka-machi, Nagaoka, Niigata 940-2188, Japan
a r t i c l e
i n f o
Article history: Received 23 March 2014 Received in revised form 25 June 2014 Accepted 13 July 2014 Available online 27 July 2014 Keywords: Cu2ZnSnS4 Thin-film solar cell Spray pyrolysis deposition Chlorine Carbon
a b s t r a c t Cu2ZnSnS4 (CZTS) thin film solar cells were fabricated under non-vacuum conditions, wherein the absorption layer was prepared by the spray pyrolysis deposition method. By using a chlorine (Cl)-free solution instead of a Cl-containing solution, the solar cell efficiency was improved from 0.18% to 0.35%. On the CZTS surface deposited from the Cl-containing solution, Cl-containing precipitates with size of 10 lm were present and were found to degrade the solar cell efficiency. The still low efficiency of 0.35% was caused by a layer containing small grains that existed between the CZTS layer and the Mo electrode, which included a significant amount of carbon. By optimizing the pre-annealing process, the smallgrained layer became thinner and the efficiency was improved to 0.63%. Finally, by adjusting the chemical composition of the Cl-free solution and the CdS buffer layer deposition conditions, the cell efficiency was improved to 0.94%. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Cu2ZnSnS4 (CZTS) is one of the most interesting materials for the absorption layer in thin-film solar cells. This is because its optical properties are suitable for a single junction solar cell. The absorption coefficient of the CZTS is larger than 104 cm1 in the visible region and the band gap energy is 1.5 eV. Moreover, all the elements contained in CZTS are abundant in the crust of the Earth and are non-toxic. Ito et al. reported a CZTS thin-film solar cell for the first time in 1988 [1]. They deposited CZTS thin films by atomic beam sputtering and fabricated solar cells with the structure tin-oxide transparent conductive layer/CZTS absorption layer/stainless-steel substrate. These solar cells showed an open circuit voltage (V oc ) of 165 mV under irradiation of AM 1.5. Katagiri and coworkers reported several studies on CZTS thinfilm solar cells [2–5]. They reported a CZTS solar cell with an efficiency of almost 7% for the first time [4]. This report motivated other researchers to explore CZTS solar cells. The structure of the cell reported by Katagiri et al. was Al/ZnO:Al/CdS/CZTS/Mo/soda lime glass (SLG) substrate. The CZTS absorber layer was deposited by RF co-sputtering followed by vapor phase sulfurization. In 2014, the highest ever efficiency of 9.2% was reported for a solar cell in which the CZTS absorber layer was deposited by sputtering [6]. ⇑ Corresponding author. E-mail address: [email protected] (K. Tanaka). http://dx.doi.org/10.1016/j.jallcom.2014.07.101 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
All CZTS thin films mentioned above were deposited by vacuum processes. However, vacuum processes require expensive equipment and complex operation. Therefore, many researchers have studied CZTS preparation methods under non-vacuum conditions. We recently reported a CZTS thin film solar cell in which the CZTS absorption layer was deposited under non-vacuum conditions for the first time, to the best of our knowledge [7]. In our previous report, not only the CZTS absorption layer but also the CdS buffer and ZnO:Al window layers were deposited under non-vacuum conditions [7–9]. The CZTS absorption, CdS buffer and ZnO:Al window layers were deposited by a sol–gel sulfurization method, chemical bath deposition (CBD), and a sol–gel method, respectively. The highest efficiency of this solar cell was 3.6% [9]. Spray pyrolysis deposition (SPD) is a non-vacuum deposition process that has been used for the fabrication of CZTS thin films [10–13]. Nakayama and Ito deposited CZTS thin films under nonvacuum conditions for the first time using SPD [10]. The starting materials of the SPD solution were CuCl, ZnCl2, SnCl4 and thiourea. After coating the solution onto a heated substrate, the coated substrate, named the precursor, was annealed in a H2S + Ar atmosphere to obtain a CZTS thin film. The film exhibited an X-ray diffraction (XRD) peak attributed to CZTS (1 1 2) planes and a band gap energy of 1.46 eV. However, Nakayama and Ito did not fabricate solar cells with the CZTS film. There are few reports on CZTS thin film solar cells in which the absorption layer was deposited by SPD. M. Espindola-Rodriguez et al. used CuCl2, Zn acetate dihydrate, SnCl4 hydrate and thiourea as starting materials for the SPD
K. Tanaka et al. / Journal of Alloys and Compounds 616 (2014) 492–497
coating solution [11]. The precursor was annealed in S and a Sncontaining atmosphere to obtain CZTS thin films. They used these films to fabricate ZnO:Al/ZnO/CdS/CZTS/Mo solar cells. The ZnO and ZnO:Al layers were deposited by DC sputtering under a vacuum. The highest efficiency obtained for the fabricated cells was 0.49%. Rajeshmon et al. used CuCl, Zn acetate, SnCl2 or SnCl4, and thiourea as starting materials for the SPD coating solution [12]. They deposited CZTS thin films without annealing in an S-containing atmosphere. They fabricated ITO/In2S3/CZTS/Ag solar cells, and the cell with the highest current density showed V oc ¼ 380 mV and a short circuit current density ðJ sc Þ ¼ 2:40 mA=cm2 . M. Espindola-Rodriguez used CuCl2, Zn acetate, SnCl4, and thiourea as starting materials for their SPD coating solution [13]. They used Ar or Ar–H2 (5%) as carrier gas to avoid oxygen-contamination. The precursor was annealed with S and Sn powders to obtain a CZTS thin film. They then fabricated CZTS solar cells, wherein the ZnO:Al window layer was deposited by sputtering under vacuum conditions, and the efficiency of the cell was 1.4%. Typically, these CZTS thin films prepared by SPD have several disadvantages. One is the annealing process in S vapor or a H2Scontaining atmosphere, because S vapor is corrosive and H2S is toxic. Another is using chlorides as metal sources, because chlorine (Cl) atoms remain in the CZTS thin film and act as impurities. In our previous report, CZTS thin films were prepared by SPD without annealing in S or a H2S-containing atmosphere [14]. The solution for SPD was prepared from Cu(II) acetate monohydrate, Zn(II) acetate dihydrate, Sn(II) chloride dihydrate and pure S powder. The precursors deposited by SPD were annealed in an N2 atmosphere, with another precursor placed face-to-face as shown in Fig. 1. In this face-to-face arrangement, evaporation of S and Sn was suppressed and CZTS films could be prepared without S vapor or a H2S-containing atmosphere. However, one disadvantage still remained in that Sn(II) chloride was used as the metal source, and a CZTS solar cell was not fabricated. In this report, CZTS thin film solar cells, in which the CZTS absorber layer was deposited by SPD with or without using a chloride, were fabricated. In addition, the effects of Cl and carbon (C), included in the starting material for the SPD solution, on the efficiency of the CZTS solar cell formed were investigated.
2. Experimental procedure Both Cl-containing and Cl-free solutions were prepared for SPD. For both solutions, the Cu and Zn sources were Cu(II) acetate monohydrate and Zn(II) acetate dihydrate, respectively. As Sn sources, Sn(II) chloride dihydrate and Sn octylate were used for Cl-containing and Cl-free solutions, respectively. These metal sources together with pure sulfur powder were dissolved in N,N-dimethylformamide, and monoethanolamine was used as the stabilizer. The composition ratios were Cu=ðZn þ SnÞ ¼ 0:30—0:80; Zn=Sn ¼ 1:15 and S=metal ¼ 4:0. The concentration of
Mo/SLG CZTS precursor [12] Sn-S coated (this work) CZTS precursor Mo/SLG Fig. 1. Annealing configuration: annealing with another coated film placed face-toface. The other coated film was the CZTS precursor in our previous work [12] and an Sn–S solution-coated substrate in this work.
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the metal ions in Cl-containing solution was 0.5 M. For Cl-free solution, since Sn octylate has high viscosity, the concentration of the metal ions was reduced to 0.25 M. Precursors of CZTS were prepared by coating the solution on Mo-coated SLG substrates by SPD in air. Spray and drying processes were repeated 7 or 14 times. The distance between the substrate and the spray nozzle was 170 mm and the N2 carrier gas pressure was 0.6 MPa. During the deposition, the Mo-coated SLG substrates were heated to 160 °C. After deposition, the precursors were annealed in an N2 atmosphere with an infrared gold image furnace (ULVAC CP610CP), with Sn- and S-containing-solution-coated substrates (Sn–S coated substrate) placed face-to-face as shown in Fig. 1. The Sn–S coated substrate was deposited by SPD at 160 °C without using Cu(II) acetate monohydrate and Zn(II) acetate dihydrate. The Sn–S coated substrate was an amorphous-like thin film and showed no Raman scattering peaks. Chemical composition ratio of the Sn–S cover was S=Sn ¼ 2:0. Following an optional pre-anneal, the precursors were annealed at 520 °C for 60 min in the case of Cl-containing solution, and 120 min in the case of Cl-free solution. The annealing duration time was optimized for each solution. For Cl-containing solution, long annealing duration caused peeling off of CZTS thin films. Therefore the annealing duration for Cl-containing solution was shorter than that of Cl-free solution. The temperature increase rate for both the pre-anneal and main anneal was 10 °C/min. The structure of the prepared solar cell was Al/ZnO:Al/CdS/CZTS/Mo/SLG. All semiconductor layers were deposited under non-vacuum conditions. The ZnO:Al window layer was deposited by a sol–gel method. Zinc(II) acetate dihydrate, aluminum chloride hexahydrate, 2-methoxyethanol, and monoethanolamine were used in the sol–gel solution. This sol–gel solution was coated 5 times and dried at 350 °C. A CdS buffer layer was deposited by CBD from an aqueous solution of CdI2, thiourea, and ammonia for 10–40 min. Additional details of the solar cell preparation process are presented in Ref. [7] and Table 1. Microstructural information for the deposited films was obtained by using XRD (Rigaku RAD3). The chemical composition and wavelength-dispersive X-ray spectroscopy (WDX) spectrum of the films were obtained by electron probe microanalysis (EPMA; Shimadzu EPMA 1600). The depth profile of the chemical composition was examined by Auger electron spectroscopy (AES; JEOL JAMP-9500F). The surface morphology of the films was ascertained using scanning electron microscopy (SEM; Hitachi S4000). The current density–voltage (J–V) characteristics of the solar cells were measured under irradiation of AM 1.5G at 100 mW/cm2. The system used for this measurement was composed of a USHIO UXL-500SX2 Xe lamp and a Keithley 2400 digital sourcemeter. The J–V characteristics under irradiation were measured using the four-probe method.
3. Results and discussion 3.1. Effect of chlorine on efficiency of CZTS solar cells CZTS thin films and CZTS solar cells were prepared using the Cl-containing solution (Cell 1) or the Cl-free solution (Cell 2). The CZTS thin film and solar cell preparation conditions are shown in Table 1. Fig. 2 shows XRD patterns for CZTS thin films prepared from the Cl-containing and Cl-free solutions. As can be seen, both films showed peaks attributed to (1 1 2), (2 2 0) and (2 0 0) planes of CZTS. As shown in Fig. 2 noise level in the CZTS thin film prepared from the Cl-containing solution was less than that of the Cl-free solution. Film thickness of the CZTS films prepared from the Cl-containing solution and the Cl-free solution was 1470 and 500 nm, respectively. The thicker film caused less noise. Fig. 3 and Table 2 show the J–V characteristic curve under irradiation and the solar cell properties of the CZTS solar cells, respectively. In Table 2, J sc ; V oc ; F:F., g; Rs and Rsh are the short circuit current density, open circuit voltage, fill factor, efficiency, series resistance and shunt resistance, respectively. Rsh and Rs are the effective values estimated from the slopes of the J–V curve with J and V axes, respectively. As shown in Table 2, the efficiency of Cell 1 (g ¼ 0:18%) is almost half that of Cell 2 (g ¼ 0:35%). To clarify the cause of the lower efficiency of 0.18%, the surface morphology of the CZTS thin films was observed. Fig. 4(a) and (b) show surface images of the CZTS thin films formed from the Cl-containing and Cl-free solutions, respectively. As seen in Fig. 4(a), precipitates with size of about 10 lm are observed on the CZTS thin film, although Fig. 4(b) shows no precipitates.
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Table 1 Solar cell preparation conditions used in this work. Cell no.
Solution
CZTS coat repetition
CZTS pre-anneal
Cu/(Zn + Sn)
CdS deposition time (min)
1 2 3 4 5
(a) (b) (b) (b) (b)
7 7 14 14 14
250 °C, 30 min 160 °C, 120 min 160 °C, 120 min Non Non
0.80 0.80 0.80 0.80 0.65
40 40 40 40 30
Diffraction intensity (a.u.)
Mo
(312)
(112)
(220)
(a) Cl-contained solutoin
(b) Cl-free solution 20
40
60
80
Diffraction angle, 2θ (deg) Fig. 2. XRD patterns of CZTS thin films prepared from (a) a Cl-containing solution and (b) a Cl-free solution.
7 No. 1
Current density (mA/cm2)
6
No. 2 No. 3
5
No. 4 4
No. 5
3
3.2. Effect of carbon on efficiency of CZTS solar cell
2 1
0
the precipitate contains Cl, whereas there is no Cl around the precipitate. Since the electronegativity of Cl is very high, Cl attracts elements around itself and forms chlorides easily, and the attraction of other elements causes a distribution in the chemical composition. The compositions of the precipitate and around the precipitate estimated from WDX spectra are Cu:Zn:Sn:S = 18.2:25.4:13.5:42.9 and Cu:Zn:Sn:S = 26.1:14.3:9.9:49.7 respectively. These results clearly indicate a composition distribution in the CZTS films deposited from the Cl-containing solution. The spatial distribution of the composition is one of the causes of low efficiency of solar cells prepared from the Cl-containing solution. It is well known that the efficiency of CZTS solar cells strongly depends on the composition [5,15]. Cu vacancies are the origin of shallow holes that contribute to electric current generation. Therefore, a Cu-poor composition is very important [16]. As mentioned above, the precipitates contain significant amounts of Zn. Therefore, the regions around the precipitates are relatively poor in Zn and rich in Cu. The relatively Cu-rich regions are one of the causes of the low efficiency. The CZTS surface with the precipitates is very rough, which results in low efficiency [9]. If there are large precipitates, the CdS buffer and ZnO:Al window layers cannot cover enough of the CZTS surface and this causes a short between the Al electrode and the CZTS absorption layer. Chlorine itself is also one of the causes of low efficiency of solar cells because Cl is an impurity and forms carrier trap levels. There are no reports on trap levels due to Cl for CZTS, and further investigation is necessary.
100
200
300
400
500
Voltage (mV) Fig. 3. J–V curves for solar cells with CZTS absorption layer deposited by SPD. The cell preparation conditions are shown in Table 1.
Table 2 Solar cell properties with CZTS absorber layer deposited by SPD. Cell preparation conditions are shown in Table 1. Cell no.
Jsc (mA/cm2)
V oc (mV)
F:F:
g (%)
Rs (X cm2)
Rsh (X cm2)
1 2 3 4 5
1.90 3.34 0.94 6.19 6.71
437 392 397 318 451
0.21 0.27 0.21 0.32 0.31
0.18 0.35 0.08 0.63 0.94
344 92 629 30 43
189 133 311 87 102
Fig. 5 shows WDX spectra obtained from a precipitate and from the region around it, where the solid circle, delta and square symbols denote Sn, Cl and Mo, respectively. The observation positions are shown in the inset in Fig. 5(i) corresponding to the precipitate and (ii) corresponding to around the precipitate. As shown in Fig. 5,
By using the Cl-free solution instead of the Cl-containing solution, the cell efficiency was improved from 0.18% to 0.35%, as shown in Section 3.1. Therefore Cl is one of the causes of the low efficiency. However the efficiency of 0.35% for the Cl-free solution is lower than the reported value of 1.4% for a cell prepared by SPD using a Cl-containing solution [13]. We assumed that one of the causes of the low efficiency is the thinner absorption layer. Since the absorption coefficient of CZTS is on the order of 104 cm1, the optimal thickness of the CZTS absorption layer is 1–2 lm. In Ref. [13], the thickness of the CZTS absorber layer was 1.0 lm. Fig. 6(a) shows a cross-sectional SEM image of the CZTS thin film which is the same as the absorption layer of the Cl-free CZTS solar cell (Cell 1) described in Section 3.1. The CZTS film thickness is 0.68 lm. To improve the efficiency, a thick CZTS absorption layer was deposited by increasing the coating repetition from 7 times to 14 times. Fig. 6(b) shows a cross-sectional SEM image of the CZTS thin film obtained by coating 14 times. The film thickness is larger at about 1.3 lm. However, the efficiency of a CZTS solar cell with 14 times coating was 0.08% (see Cell 3 in Tables 1 and 2). Therefore the thinner CZTS absorption layer was not the cause of the low efficiency. As seen in Fig. 6(a) and (b), there is a CZTS small-grained layer between the CZTS large-grained layer and the Mo. This
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(b)
(a)
(b)
(a)
20 μm
20 μm
Fig. 4. Surface images of the CZTS thin film (a) deposited with the Cl-containing solution and (b) deposited with the Cl-free solution.
WDX intensity (a.u.)
(i) a precipitate
Sn Mo Cl
(ii) around a precipitate
(i) (ii) 20 μm
3
4
5
Wavelength ( ) Fig. 5. WDX at (i) a precipitate and (ii) around a precipitate.
small-grained layer becomes thicker as the coating repetitions increase. We assumed that the cause of the low efficiency is the small-grained layer, and therefore the composition depth profile for the CZTS thin film was investigated. Fig. 7 shows the depth profile for the CZTS thin film with 14 times coating. Any treatment was not carried out before AES-measurement. Therefore at beginning of the measurement, the results of the AES were affected from surface conditions. As shown in Fig. 7, at surface the sample was Cu poor and Zn- and Sn-rich. However, near the Mo, the chemical composition of the sample became Cu-rich and Zn- and Sn poor. The CZTS and Mo layers seem to be interdiffused. The region near the Mo, which corresponds to the small-grained layer, contains a large amount of carbon. There have been some reports on a carbon-containing layer between the Mo electrode and a CuInSe2 (CIS) or a CuIn1xGaxSe2 (CIGS) thin film deposited by a solution-based process [17–20]. In Refs.
(a) coating 7 times
(b) coating 14 times
CZTS large grain CZTS small grain Mo
[17–19] it was reported that the resistivity of the carbon-containing layer is relatively high and the carbon-containing layer may be one of the causes of low efficiency of the solar cell. On the other hand, Ref. [20] reported that the carbon-containing layer becomes the current flow path. In the present work, as shown in Table 2 (Cell 3), since the solar cell with a thick carbon-containing layer shows a low current density and a high series resistance, the resistivity of the carbon-containing layer is high and it causes low efficiency. To reduce the thickness of the carbon-containing layer, the preannealing conditions were investigated. Fig. 8 shows film thickness ratio of the carbon-containing layer as a function of pre-annealing temperature. The film thickness of each CZTS film was shown in Fig. 8. As shown in Fig. 8 the thickness of the carbon-containing layer decreases as the pre-annealing temperature increases from 160 to 200 °C. However, at 220 °C, it increases again. The thinnest carbon-containing layer was obtained for the case of no pre-annealing. From this result, we assumed the thickness reduction mechanism shown in Fig. 9 for the carbon-containing layer during the pre-annealing process. In the early stage of the pre-annealing process, the organic solvent containing a large amount of carbon evaporates from the precursor. In the middle stage, a Cu–Zn–Sn–S sulfide grows and covers the surface. The sulfide compound prevents evaporation of the organic solvents. Therefore, the carboncontaining layer without pre-annealing is thinner than that with pre-annealing. From 160 to 200 °C, the organic solvent evaporates earlier as the pre-annealing temperature increases. Therefore, a higher temperature results in a thinner carbon-containing layer. However, at the high pre-annealing temperature of 220 °C, a sulfide cap forms immediately and prevents evaporation. Therefore, a higher pre-annealing temperature results in a thicker carboncontaining layer. A CZTS thin film solar cell whose CZTS absorption layer was deposited without pre-annealing was also prepared (Cell 4). Except for the fact that no pre-annealing was performed, the preparation
CZTS large grain CZTS small grain 1 μm
Mo
1 μm
Fig. 6. Cross-sectional image of the CZTS thin film with (a) coating 7 times and (b) coating 14 times.
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250
Organic solvents are evaporated
Cu
200
Intensity (cps)
Zn Sn 150
Mo
precursor
C 100
Mo layer
(a) early stage
50
Mo
CZTS 0
0
500
1000
CZTS large grain layer
1500
Sputtering time (sec) Fig. 7. Depth profile of chemical composition in the CZTS thin film.
Mo layer
(b) middle stage
CZTS large grain layer Carbon-containing layer Mo layer
(c) final stage Fig. 9. Mechanism for carbon reduction during pre-annealing process.
Fig. 8. Thickness of the carbon-containing layer as a function of pre-annealing temperature.
conditions were the same as for Cell 3. The J–V curve under irradiation and the cell properties of cell 4 are shown in Fig. 3 and Table 2, respectively. As seen in Table 2, the efficiency was improved from 0.08% to 0.63%.
3.3. Improvement of cell efficiency In Section 3.2, it was seen that by skipping the pre-annealing process, the conversion efficiency improved to 0.63%. However, this is still lower than the efficiency reported in Ref. [13]. As mentioned in Section 3.1, the conversion efficiency of the CZTS solar cell strongly depends on the chemical composition [5,15]. Therefore, the dependence of cell properties on chemical composition was investigated. The chemical composition ratio Zn=Sn in the solution was fixed at 1.15 and the ratio Cu=ðZn þ SnÞ in solution was varied from 0.30 to 0.80. Other cell preparation conditions were the same as for Cell 4. Table 3 shows that the chemical composition of the deposited CZTS thin film depends on that of the coating solution. For Cu/(Zn + Sn) in a solution of 0.80 to 0.60, Cu/(Zn + Sn) and Zn/Sn in the deposited CZTS thin film were almost constant and the chemical composition became almost stoichiometric. For Cu/(Zn + Sn) less than 0.50 in the solution, Cu/(Zn + Sn) in the
deposited film decreased as the ratio in the solution decreased. From these results, in order to control the chemical composition in the deposited films by adjusting the chemical composition in the solution, the ratio in the solution needs to be varied drastically. Table 4 shows the dependence of the CZTS cell properties on the chemical composition in the solution. The efficiency clearly does not depend on the Cu/(Zn + Sn) ratio in the solution. The highest efficiency of 0.72% was obtained for Cu/(Zn + Sn) of 0.65 in the solution. The chemical composition in the highest efficiency cell was Cu/(Zn + Sn) = 0.90 and Zn/Sn = 0.96. Katagiri et al. reported that the ideal chemical composition ratios for high efficiency were Cu/(Zn + Sn)=0.8–0.9 and Zn/Sn = 1.1–1.3 [15]. The chemical composition of the 0.72% cell was relatively close to the ideal chemical composition ratio. Therefore, the efficiency was probably high. Further study is necessary for controlling chemical composition to improve the efficiency. Finally, the CdS deposition time was adjusted, since it affects the cell efficiency [7]. A CZTS solar cell was prepared from a solution with Cu/(Zn + Sn) = 0.65 and Zn/Sn = 1.15 from which the highest efficiency cell was fabricated. The CdS deposition time was varied from 10 to 40 min. The highest efficiency was obtained for the 30 min deposition. The cell properties and J–V curve under irradiation are shown in Table 2 and Fig. 3, respectively, labeled as Cell 5. As seen in Table 2, the efficiency was improved to 0.94% and the other parameters were V oc ¼ 451 mV, J sc ¼ 6:71 mA=cm2 and F:F: ¼ 0:31. In the present study, the ZnO:Al window layer was deposited by a sol–gel method while usually for CZTS solar cells ZnO:Al window layer was deposited by DC or RF-magnetron sputtering. The resistivity of the ZnO:Al in the present report is 101 X cm [9]. This value is three or four orders of magnitude larger than that of the ZnO:Al deposited by sputtering. The high resistivity of the window layer degrades cell performance.
497
K. Tanaka et al. / Journal of Alloys and Compounds 616 (2014) 492–497 Table 3 Chemical composition of CZTS thin films deposited from solution with various chemical compositions. Zn/Sn was fixed at 1.15. Ratio of solution
Cu
Zn
Sn
S
Cu/(Zn + Sn)
Zn/Sn
S/metal
Cu/Sn
Cu=ðZn þ SnÞ ¼ 0:30 Cu=ðZn þ SnÞ ¼ 0:50 Cu=ðZn þ SnÞ ¼ 0:60 Cu=ðZn þ SnÞ ¼ 0:65 Cu=ðZn þ SnÞ ¼ 0:70 Cu=ðZn þ SnÞ ¼ 0:80
14.2 24.3 24.8 24.4 27.0 26.9
23.8 18.0 13.4 13.3 13.2 13.8
9.6 12.1 13.1 13.8 13.6 13.4
52.4 45.6 48.7 48.5 47.2 45.9
0.42 0.81 0.94 0.90 0.97 0.99
2.48 1.48 1.02 0.96 0.96 1.03
1.10 0.84 0.95 0.94 0.89 0.85
1.47 2.01 1.89 1.77 1.90 2.01
Table 4 CZTS thin film solar cell properties prepared from solution with various chemical compositions. Zn/Sn was fixed at 1.15. Sample
Jsc (mA/cm2)
V oc (mV)
F:F:
g (%)
Rs (X cm2)
Rsh (X cm2)
Cu=ðZn þ SnÞ ¼ 0:30 Cu=ðZn þ SnÞ ¼ 0:50 Cu=ðZn þ SnÞ ¼ 0:60 Cu=ðZn þ SnÞ ¼ 0:65 Cu=ðZn þ SnÞ ¼ 0:70 Cu=ðZn þ SnÞ ¼ 0:80
3.34 4.47 3.59 6.98 4.15 6.19
393 409 259 326 300 318
0.28 0.27 0.25 0.31 0.31 0.32
0.37 0.49 0.24 0.72 0.38 0.63
93 79 69 29 49 30
147 114 78 78 117 87
4. Conclusion CZTS thin film solar cells with CZTS absorption layers deposited by SPD were fabricated. For SPD, two types of coating solutions, Cl-containing and Cl-free solutions, were prepared. On the surface of the CZTS thin films deposited from the Cl-containing solution, precipitates with a size of about 10 lm and containing significant Cl were observed. The efficiency of the solar cells prepared from Cl-containing and Cl-free solutions were 0.18% and 0.35%, respectively. Thus, without using a chloride as the starting material, the efficiency of the solar cell was improved. However the efficiency of 0.35% is still low compared to other reports wherein the CZTS absorption layer was prepared by SPD. From the results of the film thickness dependence of efficiency and cross-sectional SEM images, a small-grained CZTS layer was present between the Mo layer and the large-grained CZTS layer, and this small-grained layer resulted in lower efficiency. Since the small-grained layer contained a large amount of carbon atom, the pre-annealing conditions were adjusted to reduce the carbon. By skipping the pre-annealing process, the carbon-containing small-grained layer became thinner and the conversion efficiency was improved from 0.35% to 0.63%. Finally, the chemical composition and CdS buffer layer deposition conditions were adjusted to improve the efficiency, and the properties of the cell with the highest efficiency in this work were g ¼ 0:94%; V oc ¼ 451 mV, J sc ¼ 6:71 mA=cm2 and F:F: ¼ 0:31.
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