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Fabrication and characterization of Cu2ZnSnS4 thin films by sputtering a single target at different temperature
Author’s Accepted Manuscript Fabrication and characterization of Cu2ZnSnS4 thin films by sputtering a single target at different temperature Qichen Zhao, Ruiting Hao, Sijia Liu, Min Yang, Xinxing Liu, Faran Chang, Yilei Lu, Shurong Wang www.elsevier.com/locate/physb
PII: DOI: Reference:
S0921-4526(17)30521-5 http://dx.doi.org/10.1016/j.physb.2017.08.035 PHYSB310175
To appear in: Physica B: Physics of Condensed Matter Received date: 5 June 2017 Revised date: 13 August 2017 Accepted date: 14 August 2017 Cite this article as: Qichen Zhao, Ruiting Hao, Sijia Liu, Min Yang, Xinxing Liu, Faran Chang, Yilei Lu and Shurong Wang, Fabrication and characterization of Cu2ZnSnS4 thin films by sputtering a single target at different temperature, Physica B: Physics of Condensed Matter, http://dx.doi.org/10.1016/j.physb.2017.08.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication and characterization of Cu2ZnSnS4 thin films by sputtering a single target at different temperature Qichen Zhao, Ruiting Hao*, Sijia Liu, Min Yang, Xinxing Liu, Faran Chang, Yilei Lu, Shurong Wang** Institute of Solar Energy, Key Laboratory of Renewable Energy Advanced Materials and Manufacturing Technology Ministry of Education, Provincial Key Laboratory of Rural Energy Engineering, Yunnan Normal University, Kunming, Yunnan Province, 650092, People's Republic of China
Abstract Cu2ZnSnS4(CZTS) thin films were directly deposited by sputtering with a single target at different temperatures, compared with the sulfurization process after sputtering the metal precursor, which is not only simplified the preparation process also the obtained CZTS thin film had good crystallinity with large grain size and dense morphology. The solar cell fabricated with the CZTS thin film sputtered at an optimized temperature of 500℃ shows a conversion efficiency of 1.87% with Voc = 580 mV, Jsc = 8.47 mA/cm2, and FF = 37.8%, its band gap energy is found to be 1.52 eV. These results show that the process without sulfurization is suitable for the growth of kesterite CZTS solar cell absorbers.
Keywords: Cu2ZnSnS4, Single target, magnetron sputtering，Solar cells
1.Introduction
*
Corresponding author. E-mail address: [email protected](R. Hao), [email protected](S. Wang).
Cu2ZnSnS4 (CZTS) is a quite promising material for thin film solar cells. It is known that CZTS thin film has a direct band gap of 1.4–1.6 eV and a high absorption coefficient above 104 cm-1 [1-3]. The theoretical limit conversion efficiency of CZTS solar cells reaches 32.2% [4]. Besides, CZTS thin film solar cells can be low-cost since all the composite elements are nontoxic and earth-abundant. All of these advantages make it one of the most promising materials for thin film solar cells. At present, great efforts have been devoted to the fabrication of CZTS thin film solar cells. Those approaches can be divided into two categories: vacuum and non-vacuum methods. The vacuum methods include evaporation, sputtering, etc [5]. And the highest efficiency of CZTS thin film solar cells fabricated through vacuum based method reaches 9.6% so far [6]. The non-vacuum methods include electroplating, sol-gel, pulsed laser deposition, etc [7-11]. The record efficiency of 12.6% for kesterite CZTS thin film solar cell was achieved by Todorov et al. through non-vacuum methods [12]. Taking the future industrial production of CZTS thin film solar cells into account, sputtering is considered a more promising method since it has obvious advantages in achieving uniform and stable deposition in large areas. For the magnetron sputtering methods, there can also be three catalogs. The most widely used one is sputtering from metallic targets, where precursors composed 2
of stacked metallic layers were prepared from metal or alloy targets [13,14]. And the precursors were then sulfurized in S-containing atmosphere to obtain CZTS thin films. The second catalog is sputtering from S-containing targets, where binary ZnS, SnS or Cu2S targets were used for precursor preparation [15-17]. A post-sulfurization process was also applied to guarantee enough S content in the CZTS films. The third catalog is direct sputtering from a ceramic quaternary Cu-Zn-Sn-S target. Compared to the former two catalogs, better uniformity and much smoother film surface can be obtained using direct sputtering from a quaternary target. However, research work about fabricating CZTS films using this method is rare so far. Seol et al. successfully fabricated CZTS films through RF magnetron sputtering from a ceramic CZTS target in 2003 [18]. Recently, Katagiri et al. and Xie et al. have reported the photoelectric conversion efficiency of 6.48% and 4.04% through magnetron sputtering from CZTS compound target [19-20]. In this work, CZTS thin films were fabricated by RF magnetron sputtering a single target at different temperatures. This preparation method avoids the high temperature process of sulfurization and simplifies the preparation technology. The prepared CZTS thin film has good crystal quality with large grain size and relatively dense morphology. The champion conversion efficiency of the fabricated CZTS film solar cell was 1.87%, although the photoelectric conversion 3
efficiency needs to be improved, this method can greatly save the preparation time of the solar cell and is beneficial to industrial production.
2. Experimental details CZTS thin films were deposited on Mo-coated soda-lime glass substrates by RF magnetron sputtering with a CZTS single compound target (3 inch diameter and 4 mm thick) at different temperatures. The non-stoichiometric CZTS pellet was synthesized by the solid state reaction of Cu2S, ZnS, SnS2 and S powders mixed at 2:1.8:1:1.2 mol ratio. The substrates were placed on a rotating heater plate and the distance between target and substrate was fixed as 8 cm. The deposition chamber was evacuated to a background pressure of 5.0×10-4 Pa by using a turbo molecular pump (TMP). The substrate is heated up by rotating heater plate to different temperatures (ranging from 400℃ to 550℃) with a heating rate of 35℃/min. Before as-deposited process, the pre-sputtering process has been done and the pre-sputtering time was 5 min. The Ar working pressure of 0.5 Pa was used and RF power for the CZTS compound target was 70W. The thin films were deposited for 130 min at temperature 400℃, 450℃, 500℃ and 550℃, respectively. Finally, the sample was cooled down to room temperature naturally. Solar cells were fabricated by chemical bath deposition of a n-type CdS buffer (60 nm) on 4
CZTS thin films, subsequently RF-magnetron sputtering an intrinsic ZnO (60 nm) and ZnO:Al (280 nm) window layer, finally, preparing Ni-Al metal grids on the top by thermal evaporation. The crystal structure of the CZTS films was examined by Rigaku Ultima IV multipurpose X-ray diffraction (XRD) and Cu-Kα ( λ=0.15406 nm) line was used as a X-ray source. The morphology of the thin films was characterized by field emission scanning electron microscope (FE-SEM, Zeiss SUPRA 55VP), and the composition of the CZTS absorber layers was estimated using Oxford Instrument X-MaxN Energy Dispersive Spectrometer(EDS). Raman spectroscopy measurements were conducted using the 514.5 nm line of an Ar+ laser with 50 mW as excitation sources and the optical band gap energy of the film was estimated by UV–vis–NIR spectroscopy (UV-3600) at room temperature. Current-voltage characteristics were measured under a simulated air mass 1.5 global spectrum (AM 1.5G) and 100 mW/cm2 illumination using a solar simulator.
3. Results and discussion Fig.1 shows the different temperature curves during sputtering samples. It describes the process that heating samples to the setting temperature at a heating rate of 35℃/min and keeps at the temperature for 130 min. Finally the samples were cooled down to room temperature naturally. 5
Sputtering at temperatures of 400 ℃ ,450 ℃ ,500 ℃ ,and 550 ℃ were utilized in this paper. 600
550℃ 500℃
Temperature(℃)
500
450℃ 400℃
400
300
200
natural cooling 100
0 0
20
40
60
80
100 120 140 160 180 200 220 240
Time(min)
Fig. 1 Sputtering of samples at different temperatures
Structural analysis and phase identification were carried out using XRD in combination with Raman scattering. Fig.2(a) shows XRD patterns of the CZTS films sputtered at various heating temperatures. The target of CZTS has three main diffraction peaks at 28.53°,47.33° and 56.18°, which can be assigned to (112), (220) and (312) crystal planes of kesterite CZTS, respectively. For samples sputtered at above 400˚C, other main diffraction peaks of the film correspond to kesterite CZTS structure (JCPDS card: 26-0575). However, there are also several diffraction peaks from SnS2 and Sn1-xS2, which locate at 2θ=32.8°, 51.8° (JCPDS card: 01-1010) and 2θ =46.3° (JCPDS card: 22-0950), respectively. As the temperature increases, the intensity of the (112), (220) and (312) peaks becomes stronger and sharper, which indicates an improvement of crystallinity. From the XRD patterns, when the temperature increases to 400, 450, 500 to 550˚C, the FWHM of CZTS samples are 0.412°, 6
0.353 ° , 0.312 ° to 0.308 ° . This result demonstrates that the temperature significantly affects the crystallinity.
▼
▼
▼▼
●
◆
▼
▼
▼ 550℃ 500℃
-1
CZTS:336cm
CZTS:375cm
SnS2:313cm
CZTS:250cm
▼ ▼ ◆
Intensity(a.u)
Intensity(a.u)
-1
-1
● Sn1-xS2
-1
-1
（b）
▼ CZTS ◆ SnS2
CZTS:286cm
（a） ▼
550℃ 500℃ 450℃
450℃
400℃
400℃ 20
25
30
35
40
45
50
55
60
65
70
2θ/(°)
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Raman Shift(cm-1)
500
Fig. 2 (a)XRD patterns and (b) Raman scattering spectra of the CZTS films Sputtered at different temperatures from 400 to 550℃.
In order to confirm the XRD analysis, Raman measurements were carried out. Fig. 2(b) shows the Raman spectra of these samples. When the temperature is above 400℃, the Raman spectrum has several peaks located at 250, 286, 336 and 375cm−1, corresponding to the CZTS phase. In addition, fingerprint of SnS2 was observed by the presence of a minor peak located at 313cm−1 for the film at temperature of 550 ˚C, which is consistent with XRD analysis [21]. As shown in Fig. 2(b), by increasing the temperature from 400 to 550°C with an interval of 150°C, Raman peaks become sharper and intensity becomes stronger. Moreover, The main Raman peak at 332 cm-1 gradually shifts to 335, 336 and 336 cm-1 with the increase of temperature. The shift of the main Raman peak towards a higher wavenumber agrees well with the result reported by Sheng et al [22], who claimed that, with the increase of the substrate temperature from 400 to 550°C, the main Raman peak of CZTS shifts 7
from 334 to 340cm−1. The Raman spectrum confirms that a good quality single-phase CZTS film forms at a temperature of 500°C.
Fig. 3 Surface morphologies of the CZTS films sputtered at different temperatures
The surface morphologies and cross-section images of CZTS thin films are shown in Fig.3 and 4. The CZTS grains with size of several hundred nanometers can be easily observed. Fig.3 shows the grain size increases as the temperatures increased from 400℃ to 550℃. Besides, for films sputtered at 400℃ and 450℃, the grains seem to be sparsely scattered and obvious holes (marked with circles) exist between the grains. And densely packed CZTS films can be obtained at the temperature higher than 500℃. Such trend reveals that increasing the temperature can greatly improve the crystallinity and CZTS films with dense surface can be obtained. 8
Fig. 4 Cross-section morphologies of the CZTS films sputtered at different temperatures
Especially it can be seen in Fig.4 that the thin films have a good adhesion to the substrate with the gradual increase of temperature, which is beneficial for the CZTS device because when depositing CdS buffer layer[23]. A intensely rough surface of absorber layer may cause an uneven CdS film and a bad CZTS/CdS interface, especially some regions were not covered by CdS film that are easy to form leakage paths and deteriorate the open circuit voltage. 60
1.2
（a）
（b） Cu/(Zn+Sn) Zn/Sn S/M
Atomic Percentage(%)
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Cu Zn Sn S
40
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20
10 400
450
500
400
550
450
500
550
Temperature(℃)
Temperature(℃)
Fig. 5(a, b) Chemical compositions of the CZTS films sputtered at different temperatures 9
EDS measurement was performed to clarify the compositional and elemental distributions in the CZTS films. Fig.5 (a) and (b) display the chemical compositions of the CZTS films sputtered at different heating temperatures from 400 to 550 ˚C (the EDS data were taken point by point in different areas for these samples). In addition, comparing these samples with the film obtained at temperature of 400℃,the composition of S in the films increases about 1-3% in atomic percent, but the compositions of Zn and Sn decrease slightly. According to the report of Tanaka et al. [24], the loss of Zn and Sn may result from the evaporation of Zn or Sn compounds, which rise to voids in the synthesized films. As shown in Fig. 5(b), the ratios of Cu/(Zn + Sn), Zn/Sn and S/(Cu + Zn + Sn) of the CZTS thin films are in the range of 0.93-0.84, 0.85-1.07 and 0.96-1.14, respectively. After sputtering at different temperatures, CZTS films exhibit Cu-poor and Sn-rich, which is in a good agreement with the existence of SnS2 impurity as revealed by XRD and Raman analyses previously. 6x1010
10
(h)2(eV/cm)2
5x10
400℃ 450℃ 500℃ 550℃
1.59ev 1.56ev 1.52ev 1.47ev
4x1010
3x1010
2x1010
1x1010
0 1.0
1.5
2.0
h(eV)
Fig. 6 The plot of (αhν)2 versus hν for CZTS films sputtered at different temperatures 10
Fig. 6 shows the plot of (αhν)2 versus the photon energy (hν) for these samples. The optical band gap was obtained by Tuac's relation [25]: (1) where α is absorption coefficient, A is constant, Eg is the band gap energy, n=1/2 for direct transition. The optical band gap is obtained by extrapolating the linear region of the plot (αhν)2 vs. hν, α is determined from the measured transmittance (T) and reflectance (R) using the formula [26]: (2) As shown in the inset of Fig. 6, these determined Eg values of four CZTS films (i.e.400℃, 450℃, 500℃, and 550℃) are 1.59 eV, 1.56 eV, 1.52 eV and 1.47eV, respectively. With increase in the growth temperatures, the band gap values shift to lower energies. The band gap energy of the CZTS thin film is dependent on the composition. Jun He et al [27]. Investigated the band gap energy of CZTS thin film is as a function of the chemical composition of Cu/(Zn + Sn). They found that the band gap energy of CZTS shifts to lower energies as the ratio of Cu/(Zn + Sn) increases. Therefore, when the temperature is at 500℃, the CZTS thin film not only has good crystal quality and large grain size, but also the proper band gap energy (i.e. 1.52eV) that is very close to the optimum band gap energy of semiconductor used for photovoltaic device. Thus, in 11
the future work, the CZTS thin film solar cells will be prepared with CZTS thin films sputtered at 500℃.
Current density(mA/cm2)
12
10
450℃ 500℃ 550℃
400℃ 450℃ 500℃ 550℃
8
η （%） — 0.98 1.87 1.54
Voc （mV） 6 526 580 549
Jsc （mA/cm2） 0.72 6.23 8.47 7.27
FF — 0.313 0.378 0.383
6
4
2
0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage(V)
Fig. 7 Illuminated J-V characteristics (AM1.5,100mA/cm2,300K)of CZTS film solar cells
In order to examine the quality of the CZTS samples used in the appliance,
CZTS
thin
film
solar
cells
with
structure
of
glass/Mo/CZTS/CdS/i-ZnO/AZO) were fabricated. The current-voltage (J-V) characteristics for solar cells measured under AM 1.5 illumination are presented in Fig. 7. As shown in Table, the CZTS-based solar cell sputtered at 500˚C exhibits the highest efficiency of 1.87% (open-circuit voltage (Voc) = 580mV, short-circuit current density (Jsc) = 8.47mA/cm2, fill factor (FF) = 37.8%, series resistance (Rs) = 32.17Ωcm2, shunt resistant (Rsh) = 127.64Ωcm2 among all these fabricated solar cells. The high Rs can reduce Jsc and the low Rsh leads to decrease of Voc, then the fill factor of solar cells is reduced. In terms of high efficiency CZTS thin film solar cell, the Rs of it is expected to be less than 1Ωcm2 and Rsh needs to be over 103Ωcm2[28]. Bad ohmic contact or poor carrier transportation 12
in the absorber layers can result in high Rs and the existence of pinholes or the insufficient thickness of the absorber layer can result in low Rsh. Note that there is no metallic grid electrode prepared on the top of the cell, which may result in high Rs. As shown in the inset of Fig.4, the thickness of CZTS absorber is about 1.2μm, thickness of the absorber layer is insufficient compared to high efficiency solar cell, therefore, a low Rsh can be induced. In the further work, the thickness of CZTS absorber will be enhanced by appropriately increasing the sputtering time. In addition, the solar cells with CZTS thin film sputtered at temperatures of 450°C and above show rectification properties, while the cell without rectification properties is considered to be due to the low crystallinity of the CZTS thin film sputtered at low temperature [29]. The cross-sectional images of the fabricated CZTS solar cells are shown in Figs. 4. When increasing temperature，the CZTS grain size becomes large, and the surface is with densely packed granular. However, when the temperature is 550℃, the conversion efficiency of the solar cell is lower than that of prepared at 500℃. This may be due to the high temperature leads to the increase of grain boundaries, which increases the recombination of electrons and holes at grain boundaries and reduces the mobility and diffusion coefficient of the photo-generated carriers. Therefore, single sputtering with the CZTS compound target at 500℃ is suitable for the 13
fabrication of CZTS thin films.
4. Conclusions In this work, CZTS thin films were deposited by RF magnetron sputtering with a single CZTS compound target, and the effects of heating on CZTS thin films properties were investigated. The CZTS thin film solar cell with an open-circuit voltage of 580mV, a short-circuit current density of 8.47mA/cm2 and a power conversion efficiency of 1.87% was obtained by sputtering a single target at 500℃. The experimental results confirmed that CZTS thin film can be obtained by direct heating substrate during sputtering instead of sulfurization process. However, we found that the grain size of the prepared CZTS films is unhomogeneous and the pin holes exist between the films, this may be the reason why the short circuit current and the filling factor were low. Our future effort would be towards promoting the crystal quality of films and improving the electrical properties of solar cells to increase power conversion efficiency.
Acknowledgements This work was supported by National Natural Science Foundation of China (No.61176127, 11474248, 61006085), Key Program for International S&T cooperation Projects of China (No. 2011DFA62380), and Ph.D. Programs Foundation of Ministry of Education of China (No. 20105303120002).
14
References [1] Ito K, Nakazawa T. Jpn J Appl Phys 1988; 27(11):2094-7. [2] Katagiri H, Sasaguchi N, Hando S, Hoshino S, Ohashi J, Yokota T. Sol Energy Mat Sol C 1997;49(1-4):407-14. [3] Nakayama N, Ito K. Appl Surf Sci 1996; 92:171-5. [4] Shockley William, Queisser Hans J. Jpn J Appl Phys 1961; 32(3):510-9 [5] Repins I, Beall C, Vora N, Dehart C, Kuciauskas D, Dippo P, et al. Sol Energ Mat Sol C 2012; 101:154-9. [6] Chawla V, Clemens B. 38th IEEE PVSC 2012; 2012. p. 2990-2. [7] Moriya K, Tanaka K, Uchiki H. Jpn J Appl Phys 2007; 46(9A):5780-1. [8] Tanaka K, Moritake N, Uchiki H. Sol Energ Mat Sol C 2007; 91(13):1199-201. [9] Todorov T, Mitzi DB. Eur J Inorg Chem 2010; 1:17-28. [10] Zhou ZH, Wang YY, Xu D, Zhang YF. Sol Energ Mat Sol C 2010; 94(12):2042-5. [11] Ennaoui A, Lux-Steiner M, Weber A, Abou-Ras D, Kotschau I, Schock HW, et al.Thin Solid Films 2009; 517(7):2511-4. [12] Todorov TK, Tang J, Bag S, Gunawan O, Gokmen T, Zhu Y, et al. Adv Energy Mater 2012; 3(1):34-8. [13] Chalapathy RBV, Jung Gwang Sun, Ahn Byung Tae. Sol Energ Mat Sol C 2011; 95(12):3216-21. [14] Katagiri H, Jimbo K, Moriya K, Tsuchida K, Kurokawa K, Kazmerski LL, et al. In: Proceedings of 3rd world conference on photovoltaic energy conversion 2003.p. 2874-9. [15] Hlaing WM, Johnson JL, Bhatia A, Lund EA, Nowell MM, Scarpulla MA. J Electron Mater 2011;40(11):2214-21. [16] Platzer-Bjorkman C, Scragg J, Flammersberger H, Kubart T, Edoff M. Sol Energ Mater Sol C 2012;98:110-7. [17] Katagiri H, Ishigaki N, Ishida T, Saito K. Jpn J Appl Phys 2001; 40(2A):500-4. [18] Seol JS, Lee SY, Lee JC, Nam HD, Kim KH. Sol Energ Mater Sol C 2003; 75(1e2):155-62. 15
[19] H. Katagiri and K. Jimbo, Proceeding of the 37th IEEE Photovoltaic Specialists Confrence, 2011, pp. 3516–3521. [20] M. Xie, D. Zhuang, M. Zhao, Z. Zhuang, L. Ouyang, X. Li and J. Song, Int. J. Photoenergy, 2013, 929454. [21] M. Xie, D. Zhuang, M. Zhao, B. Li, M. Cao, J. Song, Vacuum, 101 (2014) 146-150. [22] H. Sheng, L. Wenjun, Z. Zhigang, J. Phys. D: Appl. Phys, 46 (2013) 235108 [23] Min Yang,Xun Ma,Zhi Jing, et al. The Cu2ZnSnS4 solar cell with high open circuit voltage [J]. PhySica B, 509(2017)50-54. [24] K. Tanaka, Y. Fukui, N. Moritake, H. Uchiki, Sol. Energy Mater. Sol. Cells,95(2011) 838-842 [25] U . Chalapathi, Y . Jayasree. S. Uthana, V. Sundara Raja. Effect of annealing on the struc ral microstural and optical properties of co-evaporated CTS Thin films, J, Vac.117(2015) 121-126. [26] Min Xie, Daming Zhuang, Ming Zhao, et al. Fabriction of Cu2ZnSnS4 thin films using a ceramic quaternary target, J, Vacuum. 101(2014)146-150. [27] Jun He, Lin Sun, Ye Chen, et al. Cu2ZnSnS4 thin film solar cell utilizing rapid thermal process of precursors sputtered from a quaternary target: a promising application in industrial processes, J, RSC Advances. 2014,4,43080. [28] B. Shin, O. Gunawan, Y. Zhu, N. A. Bojarczuk, S. J. Chey and S. Guha, Prog. Photovolt. Res. Appl. 2011, 21, 72–76. [29] Ryota Nakamura, Kunihiko Tanaka, Hisao Uchiki, et al.Cu2ZnSnS4 thin film deposited by sputtering with Cu2ZnSnS4 compound target. J Japanese Journal of Applied Physics 53,02BC10(2014).
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PII: DOI: Reference:
S0921-4526(17)30521-5 http://dx.doi.org/10.1016/j.physb.2017.08.035 PHYSB310175
To appear in: Physica B: Physics of Condensed Matter Received date: 5 June 2017 Revised date: 13 August 2017 Accepted date: 14 August 2017 Cite this article as: Qichen Zhao, Ruiting Hao, Sijia Liu, Min Yang, Xinxing Liu, Faran Chang, Yilei Lu and Shurong Wang, Fabrication and characterization of Cu2ZnSnS4 thin films by sputtering a single target at different temperature, Physica B: Physics of Condensed Matter, http://dx.doi.org/10.1016/j.physb.2017.08.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication and characterization of Cu2ZnSnS4 thin films by sputtering a single target at different temperature Qichen Zhao, Ruiting Hao*, Sijia Liu, Min Yang, Xinxing Liu, Faran Chang, Yilei Lu, Shurong Wang** Institute of Solar Energy, Key Laboratory of Renewable Energy Advanced Materials and Manufacturing Technology Ministry of Education, Provincial Key Laboratory of Rural Energy Engineering, Yunnan Normal University, Kunming, Yunnan Province, 650092, People's Republic of China
Abstract Cu2ZnSnS4(CZTS) thin films were directly deposited by sputtering with a single target at different temperatures, compared with the sulfurization process after sputtering the metal precursor, which is not only simplified the preparation process also the obtained CZTS thin film had good crystallinity with large grain size and dense morphology. The solar cell fabricated with the CZTS thin film sputtered at an optimized temperature of 500℃ shows a conversion efficiency of 1.87% with Voc = 580 mV, Jsc = 8.47 mA/cm2, and FF = 37.8%, its band gap energy is found to be 1.52 eV. These results show that the process without sulfurization is suitable for the growth of kesterite CZTS solar cell absorbers.
Keywords: Cu2ZnSnS4, Single target, magnetron sputtering，Solar cells
1.Introduction
*
Corresponding author. E-mail address: [email protected](R. Hao), [email protected](S. Wang).
Cu2ZnSnS4 (CZTS) is a quite promising material for thin film solar cells. It is known that CZTS thin film has a direct band gap of 1.4–1.6 eV and a high absorption coefficient above 104 cm-1 [1-3]. The theoretical limit conversion efficiency of CZTS solar cells reaches 32.2% [4]. Besides, CZTS thin film solar cells can be low-cost since all the composite elements are nontoxic and earth-abundant. All of these advantages make it one of the most promising materials for thin film solar cells. At present, great efforts have been devoted to the fabrication of CZTS thin film solar cells. Those approaches can be divided into two categories: vacuum and non-vacuum methods. The vacuum methods include evaporation, sputtering, etc [5]. And the highest efficiency of CZTS thin film solar cells fabricated through vacuum based method reaches 9.6% so far [6]. The non-vacuum methods include electroplating, sol-gel, pulsed laser deposition, etc [7-11]. The record efficiency of 12.6% for kesterite CZTS thin film solar cell was achieved by Todorov et al. through non-vacuum methods [12]. Taking the future industrial production of CZTS thin film solar cells into account, sputtering is considered a more promising method since it has obvious advantages in achieving uniform and stable deposition in large areas. For the magnetron sputtering methods, there can also be three catalogs. The most widely used one is sputtering from metallic targets, where precursors composed 2
of stacked metallic layers were prepared from metal or alloy targets [13,14]. And the precursors were then sulfurized in S-containing atmosphere to obtain CZTS thin films. The second catalog is sputtering from S-containing targets, where binary ZnS, SnS or Cu2S targets were used for precursor preparation [15-17]. A post-sulfurization process was also applied to guarantee enough S content in the CZTS films. The third catalog is direct sputtering from a ceramic quaternary Cu-Zn-Sn-S target. Compared to the former two catalogs, better uniformity and much smoother film surface can be obtained using direct sputtering from a quaternary target. However, research work about fabricating CZTS films using this method is rare so far. Seol et al. successfully fabricated CZTS films through RF magnetron sputtering from a ceramic CZTS target in 2003 [18]. Recently, Katagiri et al. and Xie et al. have reported the photoelectric conversion efficiency of 6.48% and 4.04% through magnetron sputtering from CZTS compound target [19-20]. In this work, CZTS thin films were fabricated by RF magnetron sputtering a single target at different temperatures. This preparation method avoids the high temperature process of sulfurization and simplifies the preparation technology. The prepared CZTS thin film has good crystal quality with large grain size and relatively dense morphology. The champion conversion efficiency of the fabricated CZTS film solar cell was 1.87%, although the photoelectric conversion 3
efficiency needs to be improved, this method can greatly save the preparation time of the solar cell and is beneficial to industrial production.
2. Experimental details CZTS thin films were deposited on Mo-coated soda-lime glass substrates by RF magnetron sputtering with a CZTS single compound target (3 inch diameter and 4 mm thick) at different temperatures. The non-stoichiometric CZTS pellet was synthesized by the solid state reaction of Cu2S, ZnS, SnS2 and S powders mixed at 2:1.8:1:1.2 mol ratio. The substrates were placed on a rotating heater plate and the distance between target and substrate was fixed as 8 cm. The deposition chamber was evacuated to a background pressure of 5.0×10-4 Pa by using a turbo molecular pump (TMP). The substrate is heated up by rotating heater plate to different temperatures (ranging from 400℃ to 550℃) with a heating rate of 35℃/min. Before as-deposited process, the pre-sputtering process has been done and the pre-sputtering time was 5 min. The Ar working pressure of 0.5 Pa was used and RF power for the CZTS compound target was 70W. The thin films were deposited for 130 min at temperature 400℃, 450℃, 500℃ and 550℃, respectively. Finally, the sample was cooled down to room temperature naturally. Solar cells were fabricated by chemical bath deposition of a n-type CdS buffer (60 nm) on 4
CZTS thin films, subsequently RF-magnetron sputtering an intrinsic ZnO (60 nm) and ZnO:Al (280 nm) window layer, finally, preparing Ni-Al metal grids on the top by thermal evaporation. The crystal structure of the CZTS films was examined by Rigaku Ultima IV multipurpose X-ray diffraction (XRD) and Cu-Kα ( λ=0.15406 nm) line was used as a X-ray source. The morphology of the thin films was characterized by field emission scanning electron microscope (FE-SEM, Zeiss SUPRA 55VP), and the composition of the CZTS absorber layers was estimated using Oxford Instrument X-MaxN Energy Dispersive Spectrometer(EDS). Raman spectroscopy measurements were conducted using the 514.5 nm line of an Ar+ laser with 50 mW as excitation sources and the optical band gap energy of the film was estimated by UV–vis–NIR spectroscopy (UV-3600) at room temperature. Current-voltage characteristics were measured under a simulated air mass 1.5 global spectrum (AM 1.5G) and 100 mW/cm2 illumination using a solar simulator.
3. Results and discussion Fig.1 shows the different temperature curves during sputtering samples. It describes the process that heating samples to the setting temperature at a heating rate of 35℃/min and keeps at the temperature for 130 min. Finally the samples were cooled down to room temperature naturally. 5
Sputtering at temperatures of 400 ℃ ,450 ℃ ,500 ℃ ,and 550 ℃ were utilized in this paper. 600
550℃ 500℃
Temperature(℃)
500
450℃ 400℃
400
300
200
natural cooling 100
0 0
20
40
60
80
100 120 140 160 180 200 220 240
Time(min)
Fig. 1 Sputtering of samples at different temperatures
Structural analysis and phase identification were carried out using XRD in combination with Raman scattering. Fig.2(a) shows XRD patterns of the CZTS films sputtered at various heating temperatures. The target of CZTS has three main diffraction peaks at 28.53°,47.33° and 56.18°, which can be assigned to (112), (220) and (312) crystal planes of kesterite CZTS, respectively. For samples sputtered at above 400˚C, other main diffraction peaks of the film correspond to kesterite CZTS structure (JCPDS card: 26-0575). However, there are also several diffraction peaks from SnS2 and Sn1-xS2, which locate at 2θ=32.8°, 51.8° (JCPDS card: 01-1010) and 2θ =46.3° (JCPDS card: 22-0950), respectively. As the temperature increases, the intensity of the (112), (220) and (312) peaks becomes stronger and sharper, which indicates an improvement of crystallinity. From the XRD patterns, when the temperature increases to 400, 450, 500 to 550˚C, the FWHM of CZTS samples are 0.412°, 6
0.353 ° , 0.312 ° to 0.308 ° . This result demonstrates that the temperature significantly affects the crystallinity.
▼
▼
▼▼
●
◆
▼
▼
▼ 550℃ 500℃
-1
CZTS:336cm
CZTS:375cm
SnS2:313cm
CZTS:250cm
▼ ▼ ◆
Intensity(a.u)
Intensity(a.u)
-1
-1
● Sn1-xS2
-1
-1
（b）
▼ CZTS ◆ SnS2
CZTS:286cm
（a） ▼
550℃ 500℃ 450℃
450℃
400℃
400℃ 20
25
30
35
40
45
50
55
60
65
70
2θ/(°)
200
300
400
Raman Shift(cm-1)
500
Fig. 2 (a)XRD patterns and (b) Raman scattering spectra of the CZTS films Sputtered at different temperatures from 400 to 550℃.
In order to confirm the XRD analysis, Raman measurements were carried out. Fig. 2(b) shows the Raman spectra of these samples. When the temperature is above 400℃, the Raman spectrum has several peaks located at 250, 286, 336 and 375cm−1, corresponding to the CZTS phase. In addition, fingerprint of SnS2 was observed by the presence of a minor peak located at 313cm−1 for the film at temperature of 550 ˚C, which is consistent with XRD analysis [21]. As shown in Fig. 2(b), by increasing the temperature from 400 to 550°C with an interval of 150°C, Raman peaks become sharper and intensity becomes stronger. Moreover, The main Raman peak at 332 cm-1 gradually shifts to 335, 336 and 336 cm-1 with the increase of temperature. The shift of the main Raman peak towards a higher wavenumber agrees well with the result reported by Sheng et al [22], who claimed that, with the increase of the substrate temperature from 400 to 550°C, the main Raman peak of CZTS shifts 7
from 334 to 340cm−1. The Raman spectrum confirms that a good quality single-phase CZTS film forms at a temperature of 500°C.
Fig. 3 Surface morphologies of the CZTS films sputtered at different temperatures
The surface morphologies and cross-section images of CZTS thin films are shown in Fig.3 and 4. The CZTS grains with size of several hundred nanometers can be easily observed. Fig.3 shows the grain size increases as the temperatures increased from 400℃ to 550℃. Besides, for films sputtered at 400℃ and 450℃, the grains seem to be sparsely scattered and obvious holes (marked with circles) exist between the grains. And densely packed CZTS films can be obtained at the temperature higher than 500℃. Such trend reveals that increasing the temperature can greatly improve the crystallinity and CZTS films with dense surface can be obtained. 8
Fig. 4 Cross-section morphologies of the CZTS films sputtered at different temperatures
Especially it can be seen in Fig.4 that the thin films have a good adhesion to the substrate with the gradual increase of temperature, which is beneficial for the CZTS device because when depositing CdS buffer layer[23]. A intensely rough surface of absorber layer may cause an uneven CdS film and a bad CZTS/CdS interface, especially some regions were not covered by CdS film that are easy to form leakage paths and deteriorate the open circuit voltage. 60
1.2
（a）
（b） Cu/(Zn+Sn) Zn/Sn S/M
Atomic Percentage(%)
50 1.1
Cu Zn Sn S
40
1.0 30
0.9
20
10 400
450
500
400
550
450
500
550
Temperature(℃)
Temperature(℃)
Fig. 5(a, b) Chemical compositions of the CZTS films sputtered at different temperatures 9
EDS measurement was performed to clarify the compositional and elemental distributions in the CZTS films. Fig.5 (a) and (b) display the chemical compositions of the CZTS films sputtered at different heating temperatures from 400 to 550 ˚C (the EDS data were taken point by point in different areas for these samples). In addition, comparing these samples with the film obtained at temperature of 400℃,the composition of S in the films increases about 1-3% in atomic percent, but the compositions of Zn and Sn decrease slightly. According to the report of Tanaka et al. [24], the loss of Zn and Sn may result from the evaporation of Zn or Sn compounds, which rise to voids in the synthesized films. As shown in Fig. 5(b), the ratios of Cu/(Zn + Sn), Zn/Sn and S/(Cu + Zn + Sn) of the CZTS thin films are in the range of 0.93-0.84, 0.85-1.07 and 0.96-1.14, respectively. After sputtering at different temperatures, CZTS films exhibit Cu-poor and Sn-rich, which is in a good agreement with the existence of SnS2 impurity as revealed by XRD and Raman analyses previously. 6x1010
10
(h)2(eV/cm)2
5x10
400℃ 450℃ 500℃ 550℃
1.59ev 1.56ev 1.52ev 1.47ev
4x1010
3x1010
2x1010
1x1010
0 1.0
1.5
2.0
h(eV)
Fig. 6 The plot of (αhν)2 versus hν for CZTS films sputtered at different temperatures 10
Fig. 6 shows the plot of (αhν)2 versus the photon energy (hν) for these samples. The optical band gap was obtained by Tuac's relation [25]: (1) where α is absorption coefficient, A is constant, Eg is the band gap energy, n=1/2 for direct transition. The optical band gap is obtained by extrapolating the linear region of the plot (αhν)2 vs. hν, α is determined from the measured transmittance (T) and reflectance (R) using the formula [26]: (2) As shown in the inset of Fig. 6, these determined Eg values of four CZTS films (i.e.400℃, 450℃, 500℃, and 550℃) are 1.59 eV, 1.56 eV, 1.52 eV and 1.47eV, respectively. With increase in the growth temperatures, the band gap values shift to lower energies. The band gap energy of the CZTS thin film is dependent on the composition. Jun He et al [27]. Investigated the band gap energy of CZTS thin film is as a function of the chemical composition of Cu/(Zn + Sn). They found that the band gap energy of CZTS shifts to lower energies as the ratio of Cu/(Zn + Sn) increases. Therefore, when the temperature is at 500℃, the CZTS thin film not only has good crystal quality and large grain size, but also the proper band gap energy (i.e. 1.52eV) that is very close to the optimum band gap energy of semiconductor used for photovoltaic device. Thus, in 11
the future work, the CZTS thin film solar cells will be prepared with CZTS thin films sputtered at 500℃.
Current density(mA/cm2)
12
10
450℃ 500℃ 550℃
400℃ 450℃ 500℃ 550℃
8
η （%） — 0.98 1.87 1.54
Voc （mV） 6 526 580 549
Jsc （mA/cm2） 0.72 6.23 8.47 7.27
FF — 0.313 0.378 0.383
6
4
2
0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage(V)
Fig. 7 Illuminated J-V characteristics (AM1.5,100mA/cm2,300K)of CZTS film solar cells
In order to examine the quality of the CZTS samples used in the appliance,
CZTS
thin
film
solar
cells
with
structure
of
glass/Mo/CZTS/CdS/i-ZnO/AZO) were fabricated. The current-voltage (J-V) characteristics for solar cells measured under AM 1.5 illumination are presented in Fig. 7. As shown in Table, the CZTS-based solar cell sputtered at 500˚C exhibits the highest efficiency of 1.87% (open-circuit voltage (Voc) = 580mV, short-circuit current density (Jsc) = 8.47mA/cm2, fill factor (FF) = 37.8%, series resistance (Rs) = 32.17Ωcm2, shunt resistant (Rsh) = 127.64Ωcm2 among all these fabricated solar cells. The high Rs can reduce Jsc and the low Rsh leads to decrease of Voc, then the fill factor of solar cells is reduced. In terms of high efficiency CZTS thin film solar cell, the Rs of it is expected to be less than 1Ωcm2 and Rsh needs to be over 103Ωcm2[28]. Bad ohmic contact or poor carrier transportation 12
in the absorber layers can result in high Rs and the existence of pinholes or the insufficient thickness of the absorber layer can result in low Rsh. Note that there is no metallic grid electrode prepared on the top of the cell, which may result in high Rs. As shown in the inset of Fig.4, the thickness of CZTS absorber is about 1.2μm, thickness of the absorber layer is insufficient compared to high efficiency solar cell, therefore, a low Rsh can be induced. In the further work, the thickness of CZTS absorber will be enhanced by appropriately increasing the sputtering time. In addition, the solar cells with CZTS thin film sputtered at temperatures of 450°C and above show rectification properties, while the cell without rectification properties is considered to be due to the low crystallinity of the CZTS thin film sputtered at low temperature [29]. The cross-sectional images of the fabricated CZTS solar cells are shown in Figs. 4. When increasing temperature，the CZTS grain size becomes large, and the surface is with densely packed granular. However, when the temperature is 550℃, the conversion efficiency of the solar cell is lower than that of prepared at 500℃. This may be due to the high temperature leads to the increase of grain boundaries, which increases the recombination of electrons and holes at grain boundaries and reduces the mobility and diffusion coefficient of the photo-generated carriers. Therefore, single sputtering with the CZTS compound target at 500℃ is suitable for the 13
fabrication of CZTS thin films.
4. Conclusions In this work, CZTS thin films were deposited by RF magnetron sputtering with a single CZTS compound target, and the effects of heating on CZTS thin films properties were investigated. The CZTS thin film solar cell with an open-circuit voltage of 580mV, a short-circuit current density of 8.47mA/cm2 and a power conversion efficiency of 1.87% was obtained by sputtering a single target at 500℃. The experimental results confirmed that CZTS thin film can be obtained by direct heating substrate during sputtering instead of sulfurization process. However, we found that the grain size of the prepared CZTS films is unhomogeneous and the pin holes exist between the films, this may be the reason why the short circuit current and the filling factor were low. Our future effort would be towards promoting the crystal quality of films and improving the electrical properties of solar cells to increase power conversion efficiency.
Acknowledgements This work was supported by National Natural Science Foundation of China (No.61176127, 11474248, 61006085), Key Program for International S&T cooperation Projects of China (No. 2011DFA62380), and Ph.D. Programs Foundation of Ministry of Education of China (No. 20105303120002).
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