- Email: [email protected]
Non-uniform distribution of sulfur vapor and its influence on Cu2ZnSnS4 thin film solar cells
Solar Energy 193 (2019) 6–11
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Non-uniform distribution of sulfur vapor and its influence on Cu2ZnSnS4 thin film solar cells Yanwei Zhang, Shenwei Wang, Miaoling Huang, Kai Ou, Liyuan Bai, Kexin Zhang, Lixin Yi
T
⁎
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Sulfur vapor Non-uniform distribution Diffusion velocity Cu2ZnSnS4 thin films Solar cells
The morphological, compositional and structural properties of the Cu2ZnSnS4 (CZTS) thin films and the device parameters of solar cells are strongly influenced by the sulfur partial pressure during the sulfurization process. To investigate the distribution of sulfur vapors along the sulfurization chamber, the uniformity of the CZTS thin films and solar cells during sulfurization process, several same samples were sequentially placed in the chamber. The device performance of CZTS thin film solar cells shows a strong correlation with the positions in the sulfurization zone and the morphologies of the CZTS thin films. An optimal position was found in the sulfurization furnace where could improve the crystallinity of CZTS thin films, reduce the formation of secondary phases, and improve the efficiency of solar cells. The best solar cell in the optimal position shows an efficiency of 3.49%, which was restrained by the Sn loss. The possible reasons for the non-uniform distribution of sulfur vapor are analyzed.
1. Introduction Cu2ZnSnS4 (CZTS) thin film solar cell has attracted wide interest because of its excellent properties which facilitate the cost-effective, large-scale and eco-friendly production (Katagiri, 2005; Kushwaha, 2017). Up until now, the pure sulfide CZTS solar cells have achieved a record efficiency of 11% by heterojunction heat treatment (Green et al., 2018; Yan et al., 2018). For those CZTS thin film solar cells with efficiency over 8%, a classical two-step method was widely used, which required a precursor first and then processed with sulfurization (Shin et al., 2013; Sun et al., 2016; Yan et al., 2018). The sulfurization process plays an important role in the performance of solar cell devices. Researchers have done a lot of work on the sulfurization process to improve device efficiency. Sulfurization temperature of 550–590 °C was commonly used for the fabrication of high efficiency CZTS thin film solar cells (Emrani et al., 2013; Jiang et al., 2014; Shin et al., 2013). Low pressure during sulfurization process was suggested to improve the device performance (Zhang et al., 2014). A static sulfurization process provided a higher sulfur partial pressure which could suppress the loss of Sn, the generation of secondary phases (Liu et al., 2016). In order to achieve cost-effective and large-scale production, the sulfurization process needs to be carried out in a wide area, therefore, achieving uniform sulfurization is critical to device performance. However, few studies have reported on the relations between the ⁎
diffusion uniformity of sulfur vapor in a closed chamber and the performance of CZTS thin film solar cells. For all we know, sulfur powders was often used as the initial sulfur source in the sulfurization process (He et al., 2015; Jiang et al., 2014; Li et al., 2017; Liu et al., 2016; Sun et al., 2016; Yan et al., 2018; Zhang et al., 2014). Sulfur powers were heated in furnace to transform solid state to gas state. Sulfur vapor consists of different gas molecules such as S2, S3, S4, S5, S6, S7, S8 and so on (Berkowitz and Marquart, 1963). Those sulfur molecules have different molecular masses and different diffusion velocities. When the sulfur vapor diffuses in a sealed chamber, the sulfur vapor and the sulfur partial pressure presents a non-uniform distribution along the chamber which could influence the sulfurization process. Another factor makes the distribution more complex: with a ramping rate of 10 °C/min in the chamber, the division and integration between parent ions and fragments proceed at different temperatures. Reaction (2) symbolically points out one of these reactions (Rau et al., 1973). The dynamic equilibrium of sulfur species is hard to achieve in a temperature ramping zone. To investigate the influence of non-uniform distribution of sulfur vapor on the properties of CZTS thin films and the device performance of CZTS thin film solar cells, the experiment is described and analyzed as below. Fig. 1 shows the schematic diagram of the sulfurization furnace in which the sulfur source is placed in the left side and several CZTS samples are sequentially placed in the right side. The chamber of
Corresponding author. E-mail address: [email protected] (L. Yi).
https://doi.org/10.1016/j.solener.2019.09.040 Received 30 May 2019; Received in revised form 22 August 2019; Accepted 11 September 2019 Available online 19 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
Solar Energy 193 (2019) 6–11
Y. Zhang, et al.
Fig. 1. Schematic diagram of sulfurization process with seven CZTS samples sequentially placed in the sulfurization zone.
the sulfurization process is sealed and kept at a static state. As the furnace heats up, the sulfur begins to melt, and the sulfur vapor starts to diffuse in the chamber. Therefore, reaction (1) will start. CZTS precursors will react with the sulfur molecules. Under different sulfur partial pressure, the CZTS thin films and the solar cells based on the thin films will demonstrate different aspects.
2Cu + SnS2 + ZnS + S → Cu2 ZnSnS4
(1)
i S2 ⇌ Si, (i = 2, 3, 4, 5, 6, 7, 8⋯) 2
(2)
The results have shown that there is an optimal position where could improve the crystallinity, reduce the formation of secondary phases, and improve the efficiency of solar cells. But the best efficiency of solar cells was restrained at 3.49%, because of the Sn loss which was revealed from the EQE result. 2. Experimental details
Fig. 2. Temperature profile of sulfurization process in a sealed chamber.
The CZTS precursor layers were deposited on Mo-coated soda lime glass by co-sputtering of Cu, ZnS and SnS2 targets. The working powers for Cu, ZnS and SnS2 targets were 40 W, 80 W, and 92 W respectively, aiming at a Cu/(Zn + Sn) ratio of 0.85 and Zn/Sn ratio of 1.1. The targets have a diameter of 3 in. and 4 N purity. At the time of sputtering, a quartz crucible containing sulfur powders was heated to 135 °C for evaporation. The thickness of the precursors was fixed at 1 μm by adjusting the sputtering time to 20 min. The CZTS precursor thin films were sulfurized in a tubular furnace with dual zone. The furnace chamber is a quartz tube with one end closed and can be evacuated to low pressure using a rotary pump. Fig. 1 shows the schematic diagram of the sulfurization system. Sulfur powers were placed at the center of the left zone, and CZTS samples were sequentially placed in the right zone with an interval of 2 cm. The seven samples were named S1, S2, S3, S4, S5, S6 and S7, respectively. The distance between sulfur power and sample S1 was 40 cm. The chamber was filled with N2 atmosphere and evacuated to 0.4 bar. As the furnace heated up, the chamber was kept in a sealed state. The heating curves for the two-temperature zone are shown in Fig. 2. After the sulfurization process, 60-nm CdS buffer layer, 50-nm i-ZnO and 220-nm ITO window layer were deposited on the CZTS thin films, successively. Al grids were deposited by using thermal evaporation method to finish the fabrication of solar cells. The solar cells have a total device area of 0.25 cm2 and an active area of 0.235 cm2. The structural properties of CZTS thin films were characterized by X-ray diffraction (Bruker D8 Advance). Raman spectra were measured by Raman-11 with an excitation wavelength of 532 nm. The Raman scattering system was calibrated by a silicon wafer before testing. Surface and cross-sectional morphologies of the CZTS thin films were characterized using scanning electron microscopy (SEM, Hitachi S4800). The composition ratios of the CZTS thin films were analyzed by the attached EDAX-system. External quantum efficiency (EQE) of solar
cells was measured by Zolix Solar Cell Scan 100. Current density–voltage curves (J-V) were measured using Keithley 2410 source meter under 1 Sun illumination (AM 1.5G).
3. Results and discussion Fig. 3 shows the XRD patterns of CZTS thin films sulfurized at different positions from S1 to S7. From S1 to S7, peaks located at 28.53°, 29.75°, 33.11°, 47.42° and 56.19° are corresponding to (1 1 2), (1 0 3), (2 0 0), (2 2 0) and (3 1 2) planes of kesterite CZTS (JCPDS 26–0515)
Fig. 3. XRD patterns of CZTS thin films sulfurized at different positions. 7
Solar Energy 193 (2019) 6–11
Y. Zhang, et al.
respectively. Meanwhile, the intensity of these peaks gets stronger and the full width at half maximum of (1 1 2) become narrower, indicating that the crystallinity of CZTS thin films is improved. What can be confirmed from the phenomenon was that the distribution of sulfur gas molecules (S2, S4, S6, S7, S8……) or the sulfur pressure are non-uniform in the tube. Higher sulfur pressure which also means higher concentration of active sulfur gas molecule (S2 is more active than S8 in the reaction) is benefit for the formation of CZTS crystal. Besides the planes of kesterite structure, secondary phases such as ZnS and SnS2 are also found in the XRD patterns. Relatively weak peaks located at 30.3° and 46.12° can be attribute to (0 0 2) and (0 0 3) planes of SnS2 (JCPDS 230677). SnS2 phases only appeared from S5 to S7. A wide peak located at 40.1° can be ascribed to the diffraction from Mo substrate. The diffraction peak of Mo substrate is smaller than 40.51° which is the diffraction peak of the crystal Mo (JCPDS 42-1120), indicating the variation in the crystalline quality of the Mo layer. SEM image and XRD pattern of another piece of Mo-coated soda lime glass are shown in Fig. S1 and Fig. S2 (supporting information). Since kesterite CZTS and ZnS phase share the similar diffraction peaks (2θ), Raman spectroscopy analysis is necessary for further investigation to distinguish CZTS phase from other secondary phases. Fig. 4 shows the Raman spectra measured on the surface of CZTS thin films from sample S1, S2, S3, S4, S5, S6 to S7 with the excitation wavelength of 532 nm. A strong peak at 338.45 cm−1 accompanied by two weak peaks at 287.42 cm−1, 371.2 cm−1 can be attributed to kesterite CZTS which is also identified in the XRD patterns (Wang et al., 2010). The intensity of peak at 287.42 cm−1, 338.45 cm−1 and 371.2 cm−1 for the sample S1, S2 is relatively lower than others, indicating the poor quality of CZTS thin films placed at the left side of the quartz tube. This trend is consistent with the result shown in XRD patterns. As reported in the literature, ZnS shows three Raman peaks at 273 cm−1, 276 cm−1, 351 cm−1 (Cheng et al., 2009; Schneider and Kirby, 1972), and SnS2 shows peak at 314 cm−1 (Alvarez et al., 2016; Panda et al., 2007). But these peaks of secondary phases are not shown in the Raman spectra. Considering the existence of ZnS, SnS2 detected in the XRD patterns, this phenomenon could result from several possibale reasons. Firstly, the particles of secondary phases are non-uniform distributed on the surface of the CZTS thin films and outside of the detection area in the Raman analysis. Secondly, the grain size of secondary phases is quite small. The third reason is that the 532 nm laser light was absorbed in the top 100–200 nm of the CZTS thin films (Ge et al., 2014). ZnS particles were embedded in the inner region of the CZTS thin films, which was revealed in literature (Li et al., 2015; Wang et al., 2011). So futher investigation of Raman spectroscopy using the equipped microscope on the surface of CZTS thin films is performed. Fig. 5(a), (d) and (g) show the surface morphologies of sample S2, S4 and S7, characterized by the optical microscope attached to the Raman scattering system. Fig. 5(b), (e) and (h) show the partial enlarged details in Fig. 5(a), (d) and (g) respectively, and the selected areas in the figures are marked as S2-a, S2-b, S4-a, S4-b, S7-a, S7-b. The surface is smooth and uniform for the S2 while the roughness for the surface of the sample S4, S7 increases, suggesting the crystallization of CZTS thin film increases. For the sample S4, white particles with a diameter of 3–4 μm are found on the surface of the thin film. Furthermore, larger particles with diameter of about 10 μm are found on the surface of sample S7. Raman analysis is performed on the selected areas including the bare surface of thin films and the crystalline particles, and the associated Raman spectra were demonstrated in Fig. 5(c), (f) and (i), respectively. Raman spectra acquired from the area of S2-a, S4-a and S7-a show peaks at 287, 338, 371 cm−1 contributing to CZTS phase, which is consistent with the results from the Fig. 3 (XRD) and Fig. 4 (Raman). For the area S4-b, S7-b focused on the segregated particles, the Raman peak at 314 cm−1 corresponding to SnS2 phase implies the segregation of SnS2 after sulfurization process. Scragg et al. revealed that SnS2 can be stabilized at a higher sulfurization pressure than CZTS in theory (Scragg et al., 2011). Alejandro et al. pointed out
Fig. 4. Raman spectra of CZTS thin films for the sample S1-S7.
the SnS2 segregation of CZTS thin films at high sulfurization pressure (Alvarez et al., 2016). Consequently, the appearance of SnS2 proves that higher sulfur pressure exists at the right side of the sulfurization zone and higher sulfur pressure contributes to the crystallization of CZTS thin films. Fig. 6(a–g) and (h–n) show planes and cross-sectional SEM images of CZTS thin films sulfurized at different positions. From the top view, all the CZTS thin films show a compact and uniform surface. The grain size of CZTS thin films increases from about 30 nm (S1) to about 1 μm (S7), indicating the sulfurization atmosphere at the right side of the quartz tube becomes beneficial for the growth of CZTS crystal. The grain size of CZTS thin films from S3 to S4 changes sharply, and after that the grain size increases slowly.It demonstrates the phase evolution process of CZTS thin films. Accompanied with the phase evolution, several white particles with a diameter of 100–200 nm appeared on the surface of S4-S7. The white particle is highlighted in red rectangle. As shown in 1red rectangle of Fig. 6(d), one of the particles was measured by EDS to determine the composition ratio of the particle. The result is shown in Table 1. As the grain size of the selected white particle could be less than the penetration depth of X-ray, the signal from CZTS also can be detected. The atomic ratio of is Zn is 25.47%, which is Zn rich in the overall compositional ratio, suggesting the white particle is ZnS. From the cross-sectional view of CZTS thin films sulfurized at different positions, the variation trend of grain size is consistent with the plane view. No voids which are considered as a shunting path of solar cells are found in the cross-sectional view SEM images (Katagiri et al., 2008; Wang et al., 2011). After sample S4, several small grains begin to aggregate into a big crystal and the complete crystals start to occupy the film along the depth profile. A whole CZTS crystal shows fewer grain boundaries and is beneficial for the transportation and collection of photogenerated carriers. No MoS2 layer can be identified in the cross-sectional view, which could stem from the limitation of measuring instrument or the short sulfurization time. To investigate influence of the distance between the sulfur source and the CZTS thin film on the device performance, the CZTS thin film solar cells were fabricated in the classical method mentioned before. Fig. 7 shows the current density-voltage curves of the CZTS thin film solar cells sulfurized at different positions. The associated device parameters are presented in Table 2. The device parameters of solar cells varied with the positions, showing the influence of non-uniform
1 For interpretation of color in Fig. 6, the reader is referred to the web version of this article.
8
Solar Energy 193 (2019) 6–11
Y. Zhang, et al.
Fig. 5. Micrographs (a, d and g) and the enlarged micrographs (b, e and h) of the Sample S2, S4 and S7. Fig. 5(c), (f) and (i): Raman spectra of the selected area in sample S2,S4 and S7.
that accounts for the PCE decline is the generated secondary phases observed in XRD and Raman analysis of CZTS thin films. Accompanied with generation of the secondary phases, the fill factor (FF) decreases and the series resistance (Rs) increases. The CZTS thin film solar cell with the best performance is the sample S4, showing a PCE of 3.49%, Voc of 649 mV, Jsc of 11.53 mA/cm2 and FF of 46.7%. Compared to the
sulfurization atmosphere on devices. From S1 to S4, the power conversion efficiency (PCE), the open-circuit voltage (Voc) and the short current density (Jsc) increase significantly. The variation trend is similar to that of the grain size shown in SEM images, which proved the larger grains in CZTS thin films are beneficial to the device performance again. After that, the PCE decreases slightly from S4 to S7. The reason 9
Solar Energy 193 (2019) 6–11
Y. Zhang, et al.
Table 1 Compositional ratio of the selected area in Fig. 6(d). Sample
Cu
Zn
Sn
S
Cu/(Zn + Sn)
Zn/Sn
Zn/Cu
White particle
7.49
25.47
6.57
60.46
0.23
3.87
3.4
Fig. 7. J-V curves of CZTS thin film solar cells sulfurized at different positions. Table 2 Device parameters of CZTS solar cell sulfurized at different positions. Sample
Voc (mV)
Jsc (mA/cm2)
FF (%)
Eff (%)
Rs (Ω cm2)
Rsh (kΩ cm2)
S1 S2 S3 S4 S5 S6 S7
134 387 532 649 625 617 496
2.29 4.75 6.22 11.53 10.52 9.73 10.34
35.8 48.8 54.6 46.7 46.3 44.7 39.5
0.11 0.89 1.81 3.49 3.04 2.68 2.03
143 19 16 22.3 58 52 174
1.33 0.95 0.89 0.83 0.8 0.77 0.98
Fig. 8. EQE response of CZTS thin film solar cells. Table 3 The energy band gap caclulated from the peak of the dIQE/dE curve.
Fig. 6. Plane and cross-sectional SEM images of CZTS thin films sulfurized at different positions.
10
Sample
S1
S2
S3
S4
S5
S6
S7
Eg (eV)
1.55
1.72
1.72
1.68
1.68
1.68
1.68
Solar Energy 193 (2019) 6–11
Y. Zhang, et al.
Foundation of China (Grant no. 61275058, 51772019). It was also supported by the Key Laboratory of Luminescence and Optical Information of China in Beijing Jiaotong University.
reported devices with efficiencies higher than 8% (Shin et al., 2013; Su et al., 2015; Yan et al., 2018), Voc of ~650 mV, Jsc of ~20 mA/cm2 and FF of ~65%, the main limitation on the fabricated device performance is the lower Jsc and lower FF. The reason for the low short-circuit current may be due to the high series resistance and the recombination losses. Another factor that is detrimental to device efficiency is Sn loss in CZTS thin films, which can be evidenced by the formation of the SnS2. Sn loss has been confirmed as a detrimental factor for the device performance of CZTS thin films (Redinger et al., 2011). To explain the relatively low Jsc of these solar cells, the external quantum efficiency (EQE) spectra were measured. Fig. 8 shows EQE spectra of the CZTS thin film solar cells sulfurized at different positions. For the champion solar cell S4 with the highest short circuit current, the EQE presents the highest data in the most region. The maximum EQE of S4 is 61.3% at 670 nm, which is quite lower than the reported solar cell with maximum EQE of ~90% (Shin et al., 2013; Yan et al., 2018). This indicates the recombination of photogenerated carriers in the entire spectral wavelength region is the first reason for the limited short circuit current. Another reason that can’t be ignored is the bad photoresponse in the long wavelength region (800–1000 nm). To understand why the EQE starts to increase at 800 nm, the energy band gap of the CZTS absorber in devices was extracted from the EQE data. Combining reflectance spectra with EQE data, the internal quantum efficiency (IQE) data was calculated and shown in supplementary information Fig. S3(a–g). Subsequently, the energy band gap was calculated according to the peak of the dIQE/dE curve. The calculated values are shown in Table 3. It should be noted that the EQE value of S1 in the near-infrared region is small (lower than 5%), so the calculated band gap value of S1 may incorrect. The energy band gap of all samples are larger than the theoretical value of 1.5 eV (Walsh et al., 2012), which leads to a decrease in absorption in the near-infrared spectrum. The only one reason for the increase of the band gap is the deviation of compositional ratio. Therefore, Sn loss which has been reported in literatures could account for the deviation. Reducing the Sn loss or providing a Sn rich atmosphere may be a good solution for the sulfurization process to improve device performance.
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.09.040. References Alvarez, A., Exarhos, S., Mangolini, L., 2016. Tin disulfide segregation on CZTS films sulfurized at high pressure. Mater. Lett. 165, 41–44. Berkowitz, J., Marquart, J., 1963. Equilibrium composition of sulfur vapor. J. Chem. Phys. 39 (2), 275–283. Cheng, Y., Jin, C., Gao, F., Wu, X., Zhong, W., Li, S., Chu, P.K., 2009. Raman scattering study of zinc blende and wurtzite ZnS. J. Appl. Phys. 106 (12), 123505. Emrani, A., Vasekar, P., Westgate, C.R., 2013. Effects of sulfurization temperature on CZTS thin film solar cell performances. Sol. Energy 98, 335–340. Ge, J., Jiang, J., Yang, P., Peng, C., Huang, Z., Zuo, S., Yang, L., Chu, J., 2014. A 5.5% efficient co-electrodeposited ZnO/CdS/Cu2ZnSnS4/Mo thin film solar cell. Sol. Energy Mater. Sol. Cells 125, 20–26. Green, M.A., Hishikawa, Y., Dunlop, E.D., Levi, D.H., Hohl-Ebinger, J., Ho-Baillie, A.W., 2018. Solar cell efficiency tables (version 52). Prog. Photovoltaics Res. Appl. 26 (7), 427–436. He, J., Sun, L., Chen, Y., Jiang, J., Yang, P., Chu, J., 2015. Influence of sulfurization pressure on Cu2ZnSnS4 thin films and solar cells prepared by sulfurization of metallic precursors. J. Power Sour. 273, 600–607. Jiang, F., Ikeda, S., Harada, T., Matsumura, M., 2014. Pure sulfide Cu2ZnSnS4 thin film solar cells fabricated by preheating an electrodeposited metallic stack. Adv. Energy Mater. 4 (7), 1301381. Katagiri, H., 2005. Cu2ZnSnS4 thin film solar cells. Thin Solid Films 480, 426–432. Katagiri, H., Jimbo, K., Yamada, S., Kamimura, T., Maw, W.S., Fukano, T., Ito, T., Motohiro, T., 2008. Enhanced conversion efficiencies of Cu2ZnSnS4-based thin film solar cells by using preferential etching technique. Appl. Phys Exp. 1 (4), 041201. Kushwaha, A.K., 2017. Critical review on sputter-deposited Cu2ZnSnS4 (CZTS) based thin film photovoltaic technology focusing on device architecture and absorber quality on the solar cells performance. Solar Energy Mater. Solar Cells. Li, W., Chen, J., Yan, C., Hao, X., 2015. The effect of ZnS segregation on Zn-rich CZTS thin film solar cells. J. Alloy. Compd. 632, 178–184. Li, X., Cao, H., Dong, Y., Yue, F., Chen, Y., Xiang, P., Sun, L., Yang, P., Chu, J., 2017. Investigation of Cu2ZnSnS4 thin films with controllable Cu composition and its influence on photovoltaic properties for solar cells. J. Alloy. Compd. 694, 833–840. Liu, R., Tan, M., Xu, L., Zhang, X., Chen, J., Tang, X., 2016. Preparation of high-quality Cu2ZnSnS4 thin films for solar cells via the improvement of sulfur partial pressure using a static annealing sulfurization approach. Sol. Energy Mater. Sol. Cells 157, 221–228. Panda, S., Antonakos, A., Liarokapis, E., Bhattacharya, S., Chaudhuri, S., 2007. Optical properties of nanocrystalline SnS2 thin films. Mater. Res. Bull. 42 (3), 576–583. Rau, H., Kutty, T., De Carvalho, J.G., 1973. Thermodynamics of sulphur vapour. J. Chem. Thermodyn. 5 (6), 833–844. Redinger, A., Berg, D.M., Dale, P.J., Siebentritt, S., 2011. The consequences of kesterite equilibria for efficient solar cells. J. Am. Chem. Soc. 133 (10), 3320–3323. Schneider, J., Kirby, R.D., 1972. Raman scattering from ZnS polytypes. Phys. Rev. B 6 (4), 1290. Scragg, J.J., Ericson, T., Kubart, T., Edoff, M., Platzer-Björkman, C., 2011. Chemical insights into the instability of Cu2ZnSnS4 films during annealing. Chem. Mater. 23 (20), 4625–4633. Shin, B., Gunawan, O., Zhu, Y., Bojarczuk, N.A., Chey, S.J., Guha, S., 2013. Thin film solar cell with 8.4% power conversion efficiency using an earth-abundant Cu2ZnSnS4 absorber. Prog. Photovoltaics Res. Appl. 21 (1), 72–76. Su, Z., Tan, J.M.R., Li, X., Zeng, X., Batabyal, S.K., Wong, L.H., 2015. Cation substitution of solution-processed Cu2ZnSnS4 thin film solar cell with over 9% efficiency. Adv. Energy Mater. 5 (19), 1500682. Sun, K., Yan, C., Liu, F., Huang, J., Zhou, F., Stride, J.A., Green, M., Hao, X., 2016. Over 9% efficient kesterite Cu2ZnSnS4 solar cell fabricated by using Zn1–xCdxS buffer layer. Adv. Energy Mater. 6 (12), 1600046. Walsh, A., Chen, S.Y., Wei, S.H., Gong, X.G., 2012. Kesterite thin-film solar cells: advances in materials modelling of Cu2ZnSnS4. Adv. Energy Mater. 2 (4), 400–409. Wang, K., Gunawan, O., Todorov, T., Shin, B., Chey, S., Bojarczuk, N., Mitzi, D., Guha, S., 2010. Thermally evaporated Cu2 ZnSnS4 solar cells. Appl. Phys. Lett. 97 (14), 143508. Wang, K., Shin, B., Reuter, K.B., Todorov, T., Mitzi, D.B., Guha, S., 2011. Structural and elemental characterization of high efficiency Cu2 ZnSnS4 solar cells. Appl. Phys. Lett. 98 (5), 051912. Yan, C., Huang, J., Sun, K., Johnston, S., Zhang, Y., Sun, H., Pu, A., He, M., Liu, F., Eder, K., 2018. Cu2ZnSnS4 solar cells with over 10% power conversion efficiency enabled by heterojunction heat treatment. Nat. Energy 3 (9), 764. Zhang, K., Su, Z., Zhao, L., Yan, C., Liu, F., Cui, H., Hao, X., Liu, Y., 2014. Improving the conversion efficiency of Cu2ZnSnS4 solar cell by low pressure sulfurization. Appl. Phys. Lett. 104 (14), 141101.
4. Conclusion For the sulfurization in a long diffusion range, the sulfur vapor showed a non-uniform distribution which had a significant influence on the properties of the CZTS thin film solar cells and the device performance of solar cells. In the position close to the sulfur source the grain size of CZTS thin films was quite small and the associated PCE of solar cells was limited. With the extension of distance between sulfur source and CZTS sample, the grain size increased and the device efficiency of solar cells improved. In the position far away from the sulfur source, the secondary phases such as SnS2 and ZnS began to appear. The appearance of SnS2 was a signal of higher sulfur partial pressure, indicating the higher sulfur partial pressure existed at the far end of the chamber (Alvarez et al., 2016; Scragg et al., 2011). There is an optimal position where could promote the growth of CZTS crystal, reduce the formation of secondary phases, and improve the efficiency of solar cells. In addition, there are several reasons to explain why the higher sulfur partial pressure exists at the far end of the tube. Firstly, the saturation pressure of sulfur vapor is lower in lower temperature and the sample S1 is more close to the lower temperature zone. Secondly, the S2 which is more active have a faster diffusion speed than the gas molecule S8. Lastly, the sulfur vapor may accumulate at the far end of the quartz tube. A higher energy band gap of ~1.68 eV calculated from the EQE data indicated the Sn loss which should be optimized in further experiment. Acknowledgments This work was financially supported by the National Science 11
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Non-uniform distribution of sulfur vapor and its influence on Cu2ZnSnS4 thin film solar cells Yanwei Zhang, Shenwei Wang, Miaoling Huang, Kai Ou, Liyuan Bai, Kexin Zhang, Lixin Yi
T
⁎
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Sulfur vapor Non-uniform distribution Diffusion velocity Cu2ZnSnS4 thin films Solar cells
The morphological, compositional and structural properties of the Cu2ZnSnS4 (CZTS) thin films and the device parameters of solar cells are strongly influenced by the sulfur partial pressure during the sulfurization process. To investigate the distribution of sulfur vapors along the sulfurization chamber, the uniformity of the CZTS thin films and solar cells during sulfurization process, several same samples were sequentially placed in the chamber. The device performance of CZTS thin film solar cells shows a strong correlation with the positions in the sulfurization zone and the morphologies of the CZTS thin films. An optimal position was found in the sulfurization furnace where could improve the crystallinity of CZTS thin films, reduce the formation of secondary phases, and improve the efficiency of solar cells. The best solar cell in the optimal position shows an efficiency of 3.49%, which was restrained by the Sn loss. The possible reasons for the non-uniform distribution of sulfur vapor are analyzed.
1. Introduction Cu2ZnSnS4 (CZTS) thin film solar cell has attracted wide interest because of its excellent properties which facilitate the cost-effective, large-scale and eco-friendly production (Katagiri, 2005; Kushwaha, 2017). Up until now, the pure sulfide CZTS solar cells have achieved a record efficiency of 11% by heterojunction heat treatment (Green et al., 2018; Yan et al., 2018). For those CZTS thin film solar cells with efficiency over 8%, a classical two-step method was widely used, which required a precursor first and then processed with sulfurization (Shin et al., 2013; Sun et al., 2016; Yan et al., 2018). The sulfurization process plays an important role in the performance of solar cell devices. Researchers have done a lot of work on the sulfurization process to improve device efficiency. Sulfurization temperature of 550–590 °C was commonly used for the fabrication of high efficiency CZTS thin film solar cells (Emrani et al., 2013; Jiang et al., 2014; Shin et al., 2013). Low pressure during sulfurization process was suggested to improve the device performance (Zhang et al., 2014). A static sulfurization process provided a higher sulfur partial pressure which could suppress the loss of Sn, the generation of secondary phases (Liu et al., 2016). In order to achieve cost-effective and large-scale production, the sulfurization process needs to be carried out in a wide area, therefore, achieving uniform sulfurization is critical to device performance. However, few studies have reported on the relations between the ⁎
diffusion uniformity of sulfur vapor in a closed chamber and the performance of CZTS thin film solar cells. For all we know, sulfur powders was often used as the initial sulfur source in the sulfurization process (He et al., 2015; Jiang et al., 2014; Li et al., 2017; Liu et al., 2016; Sun et al., 2016; Yan et al., 2018; Zhang et al., 2014). Sulfur powers were heated in furnace to transform solid state to gas state. Sulfur vapor consists of different gas molecules such as S2, S3, S4, S5, S6, S7, S8 and so on (Berkowitz and Marquart, 1963). Those sulfur molecules have different molecular masses and different diffusion velocities. When the sulfur vapor diffuses in a sealed chamber, the sulfur vapor and the sulfur partial pressure presents a non-uniform distribution along the chamber which could influence the sulfurization process. Another factor makes the distribution more complex: with a ramping rate of 10 °C/min in the chamber, the division and integration between parent ions and fragments proceed at different temperatures. Reaction (2) symbolically points out one of these reactions (Rau et al., 1973). The dynamic equilibrium of sulfur species is hard to achieve in a temperature ramping zone. To investigate the influence of non-uniform distribution of sulfur vapor on the properties of CZTS thin films and the device performance of CZTS thin film solar cells, the experiment is described and analyzed as below. Fig. 1 shows the schematic diagram of the sulfurization furnace in which the sulfur source is placed in the left side and several CZTS samples are sequentially placed in the right side. The chamber of
Corresponding author. E-mail address: [email protected] (L. Yi).
https://doi.org/10.1016/j.solener.2019.09.040 Received 30 May 2019; Received in revised form 22 August 2019; Accepted 11 September 2019 Available online 19 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
Solar Energy 193 (2019) 6–11
Y. Zhang, et al.
Fig. 1. Schematic diagram of sulfurization process with seven CZTS samples sequentially placed in the sulfurization zone.
the sulfurization process is sealed and kept at a static state. As the furnace heats up, the sulfur begins to melt, and the sulfur vapor starts to diffuse in the chamber. Therefore, reaction (1) will start. CZTS precursors will react with the sulfur molecules. Under different sulfur partial pressure, the CZTS thin films and the solar cells based on the thin films will demonstrate different aspects.
2Cu + SnS2 + ZnS + S → Cu2 ZnSnS4
(1)
i S2 ⇌ Si, (i = 2, 3, 4, 5, 6, 7, 8⋯) 2
(2)
The results have shown that there is an optimal position where could improve the crystallinity, reduce the formation of secondary phases, and improve the efficiency of solar cells. But the best efficiency of solar cells was restrained at 3.49%, because of the Sn loss which was revealed from the EQE result. 2. Experimental details
Fig. 2. Temperature profile of sulfurization process in a sealed chamber.
The CZTS precursor layers were deposited on Mo-coated soda lime glass by co-sputtering of Cu, ZnS and SnS2 targets. The working powers for Cu, ZnS and SnS2 targets were 40 W, 80 W, and 92 W respectively, aiming at a Cu/(Zn + Sn) ratio of 0.85 and Zn/Sn ratio of 1.1. The targets have a diameter of 3 in. and 4 N purity. At the time of sputtering, a quartz crucible containing sulfur powders was heated to 135 °C for evaporation. The thickness of the precursors was fixed at 1 μm by adjusting the sputtering time to 20 min. The CZTS precursor thin films were sulfurized in a tubular furnace with dual zone. The furnace chamber is a quartz tube with one end closed and can be evacuated to low pressure using a rotary pump. Fig. 1 shows the schematic diagram of the sulfurization system. Sulfur powers were placed at the center of the left zone, and CZTS samples were sequentially placed in the right zone with an interval of 2 cm. The seven samples were named S1, S2, S3, S4, S5, S6 and S7, respectively. The distance between sulfur power and sample S1 was 40 cm. The chamber was filled with N2 atmosphere and evacuated to 0.4 bar. As the furnace heated up, the chamber was kept in a sealed state. The heating curves for the two-temperature zone are shown in Fig. 2. After the sulfurization process, 60-nm CdS buffer layer, 50-nm i-ZnO and 220-nm ITO window layer were deposited on the CZTS thin films, successively. Al grids were deposited by using thermal evaporation method to finish the fabrication of solar cells. The solar cells have a total device area of 0.25 cm2 and an active area of 0.235 cm2. The structural properties of CZTS thin films were characterized by X-ray diffraction (Bruker D8 Advance). Raman spectra were measured by Raman-11 with an excitation wavelength of 532 nm. The Raman scattering system was calibrated by a silicon wafer before testing. Surface and cross-sectional morphologies of the CZTS thin films were characterized using scanning electron microscopy (SEM, Hitachi S4800). The composition ratios of the CZTS thin films were analyzed by the attached EDAX-system. External quantum efficiency (EQE) of solar
cells was measured by Zolix Solar Cell Scan 100. Current density–voltage curves (J-V) were measured using Keithley 2410 source meter under 1 Sun illumination (AM 1.5G).
3. Results and discussion Fig. 3 shows the XRD patterns of CZTS thin films sulfurized at different positions from S1 to S7. From S1 to S7, peaks located at 28.53°, 29.75°, 33.11°, 47.42° and 56.19° are corresponding to (1 1 2), (1 0 3), (2 0 0), (2 2 0) and (3 1 2) planes of kesterite CZTS (JCPDS 26–0515)
Fig. 3. XRD patterns of CZTS thin films sulfurized at different positions. 7
Solar Energy 193 (2019) 6–11
Y. Zhang, et al.
respectively. Meanwhile, the intensity of these peaks gets stronger and the full width at half maximum of (1 1 2) become narrower, indicating that the crystallinity of CZTS thin films is improved. What can be confirmed from the phenomenon was that the distribution of sulfur gas molecules (S2, S4, S6, S7, S8……) or the sulfur pressure are non-uniform in the tube. Higher sulfur pressure which also means higher concentration of active sulfur gas molecule (S2 is more active than S8 in the reaction) is benefit for the formation of CZTS crystal. Besides the planes of kesterite structure, secondary phases such as ZnS and SnS2 are also found in the XRD patterns. Relatively weak peaks located at 30.3° and 46.12° can be attribute to (0 0 2) and (0 0 3) planes of SnS2 (JCPDS 230677). SnS2 phases only appeared from S5 to S7. A wide peak located at 40.1° can be ascribed to the diffraction from Mo substrate. The diffraction peak of Mo substrate is smaller than 40.51° which is the diffraction peak of the crystal Mo (JCPDS 42-1120), indicating the variation in the crystalline quality of the Mo layer. SEM image and XRD pattern of another piece of Mo-coated soda lime glass are shown in Fig. S1 and Fig. S2 (supporting information). Since kesterite CZTS and ZnS phase share the similar diffraction peaks (2θ), Raman spectroscopy analysis is necessary for further investigation to distinguish CZTS phase from other secondary phases. Fig. 4 shows the Raman spectra measured on the surface of CZTS thin films from sample S1, S2, S3, S4, S5, S6 to S7 with the excitation wavelength of 532 nm. A strong peak at 338.45 cm−1 accompanied by two weak peaks at 287.42 cm−1, 371.2 cm−1 can be attributed to kesterite CZTS which is also identified in the XRD patterns (Wang et al., 2010). The intensity of peak at 287.42 cm−1, 338.45 cm−1 and 371.2 cm−1 for the sample S1, S2 is relatively lower than others, indicating the poor quality of CZTS thin films placed at the left side of the quartz tube. This trend is consistent with the result shown in XRD patterns. As reported in the literature, ZnS shows three Raman peaks at 273 cm−1, 276 cm−1, 351 cm−1 (Cheng et al., 2009; Schneider and Kirby, 1972), and SnS2 shows peak at 314 cm−1 (Alvarez et al., 2016; Panda et al., 2007). But these peaks of secondary phases are not shown in the Raman spectra. Considering the existence of ZnS, SnS2 detected in the XRD patterns, this phenomenon could result from several possibale reasons. Firstly, the particles of secondary phases are non-uniform distributed on the surface of the CZTS thin films and outside of the detection area in the Raman analysis. Secondly, the grain size of secondary phases is quite small. The third reason is that the 532 nm laser light was absorbed in the top 100–200 nm of the CZTS thin films (Ge et al., 2014). ZnS particles were embedded in the inner region of the CZTS thin films, which was revealed in literature (Li et al., 2015; Wang et al., 2011). So futher investigation of Raman spectroscopy using the equipped microscope on the surface of CZTS thin films is performed. Fig. 5(a), (d) and (g) show the surface morphologies of sample S2, S4 and S7, characterized by the optical microscope attached to the Raman scattering system. Fig. 5(b), (e) and (h) show the partial enlarged details in Fig. 5(a), (d) and (g) respectively, and the selected areas in the figures are marked as S2-a, S2-b, S4-a, S4-b, S7-a, S7-b. The surface is smooth and uniform for the S2 while the roughness for the surface of the sample S4, S7 increases, suggesting the crystallization of CZTS thin film increases. For the sample S4, white particles with a diameter of 3–4 μm are found on the surface of the thin film. Furthermore, larger particles with diameter of about 10 μm are found on the surface of sample S7. Raman analysis is performed on the selected areas including the bare surface of thin films and the crystalline particles, and the associated Raman spectra were demonstrated in Fig. 5(c), (f) and (i), respectively. Raman spectra acquired from the area of S2-a, S4-a and S7-a show peaks at 287, 338, 371 cm−1 contributing to CZTS phase, which is consistent with the results from the Fig. 3 (XRD) and Fig. 4 (Raman). For the area S4-b, S7-b focused on the segregated particles, the Raman peak at 314 cm−1 corresponding to SnS2 phase implies the segregation of SnS2 after sulfurization process. Scragg et al. revealed that SnS2 can be stabilized at a higher sulfurization pressure than CZTS in theory (Scragg et al., 2011). Alejandro et al. pointed out
Fig. 4. Raman spectra of CZTS thin films for the sample S1-S7.
the SnS2 segregation of CZTS thin films at high sulfurization pressure (Alvarez et al., 2016). Consequently, the appearance of SnS2 proves that higher sulfur pressure exists at the right side of the sulfurization zone and higher sulfur pressure contributes to the crystallization of CZTS thin films. Fig. 6(a–g) and (h–n) show planes and cross-sectional SEM images of CZTS thin films sulfurized at different positions. From the top view, all the CZTS thin films show a compact and uniform surface. The grain size of CZTS thin films increases from about 30 nm (S1) to about 1 μm (S7), indicating the sulfurization atmosphere at the right side of the quartz tube becomes beneficial for the growth of CZTS crystal. The grain size of CZTS thin films from S3 to S4 changes sharply, and after that the grain size increases slowly.It demonstrates the phase evolution process of CZTS thin films. Accompanied with the phase evolution, several white particles with a diameter of 100–200 nm appeared on the surface of S4-S7. The white particle is highlighted in red rectangle. As shown in 1red rectangle of Fig. 6(d), one of the particles was measured by EDS to determine the composition ratio of the particle. The result is shown in Table 1. As the grain size of the selected white particle could be less than the penetration depth of X-ray, the signal from CZTS also can be detected. The atomic ratio of is Zn is 25.47%, which is Zn rich in the overall compositional ratio, suggesting the white particle is ZnS. From the cross-sectional view of CZTS thin films sulfurized at different positions, the variation trend of grain size is consistent with the plane view. No voids which are considered as a shunting path of solar cells are found in the cross-sectional view SEM images (Katagiri et al., 2008; Wang et al., 2011). After sample S4, several small grains begin to aggregate into a big crystal and the complete crystals start to occupy the film along the depth profile. A whole CZTS crystal shows fewer grain boundaries and is beneficial for the transportation and collection of photogenerated carriers. No MoS2 layer can be identified in the cross-sectional view, which could stem from the limitation of measuring instrument or the short sulfurization time. To investigate influence of the distance between the sulfur source and the CZTS thin film on the device performance, the CZTS thin film solar cells were fabricated in the classical method mentioned before. Fig. 7 shows the current density-voltage curves of the CZTS thin film solar cells sulfurized at different positions. The associated device parameters are presented in Table 2. The device parameters of solar cells varied with the positions, showing the influence of non-uniform
1 For interpretation of color in Fig. 6, the reader is referred to the web version of this article.
8
Solar Energy 193 (2019) 6–11
Y. Zhang, et al.
Fig. 5. Micrographs (a, d and g) and the enlarged micrographs (b, e and h) of the Sample S2, S4 and S7. Fig. 5(c), (f) and (i): Raman spectra of the selected area in sample S2,S4 and S7.
that accounts for the PCE decline is the generated secondary phases observed in XRD and Raman analysis of CZTS thin films. Accompanied with generation of the secondary phases, the fill factor (FF) decreases and the series resistance (Rs) increases. The CZTS thin film solar cell with the best performance is the sample S4, showing a PCE of 3.49%, Voc of 649 mV, Jsc of 11.53 mA/cm2 and FF of 46.7%. Compared to the
sulfurization atmosphere on devices. From S1 to S4, the power conversion efficiency (PCE), the open-circuit voltage (Voc) and the short current density (Jsc) increase significantly. The variation trend is similar to that of the grain size shown in SEM images, which proved the larger grains in CZTS thin films are beneficial to the device performance again. After that, the PCE decreases slightly from S4 to S7. The reason 9
Solar Energy 193 (2019) 6–11
Y. Zhang, et al.
Table 1 Compositional ratio of the selected area in Fig. 6(d). Sample
Cu
Zn
Sn
S
Cu/(Zn + Sn)
Zn/Sn
Zn/Cu
White particle
7.49
25.47
6.57
60.46
0.23
3.87
3.4
Fig. 7. J-V curves of CZTS thin film solar cells sulfurized at different positions. Table 2 Device parameters of CZTS solar cell sulfurized at different positions. Sample
Voc (mV)
Jsc (mA/cm2)
FF (%)
Eff (%)
Rs (Ω cm2)
Rsh (kΩ cm2)
S1 S2 S3 S4 S5 S6 S7
134 387 532 649 625 617 496
2.29 4.75 6.22 11.53 10.52 9.73 10.34
35.8 48.8 54.6 46.7 46.3 44.7 39.5
0.11 0.89 1.81 3.49 3.04 2.68 2.03
143 19 16 22.3 58 52 174
1.33 0.95 0.89 0.83 0.8 0.77 0.98
Fig. 8. EQE response of CZTS thin film solar cells. Table 3 The energy band gap caclulated from the peak of the dIQE/dE curve.
Fig. 6. Plane and cross-sectional SEM images of CZTS thin films sulfurized at different positions.
10
Sample
S1
S2
S3
S4
S5
S6
S7
Eg (eV)
1.55
1.72
1.72
1.68
1.68
1.68
1.68
Solar Energy 193 (2019) 6–11
Y. Zhang, et al.
Foundation of China (Grant no. 61275058, 51772019). It was also supported by the Key Laboratory of Luminescence and Optical Information of China in Beijing Jiaotong University.
reported devices with efficiencies higher than 8% (Shin et al., 2013; Su et al., 2015; Yan et al., 2018), Voc of ~650 mV, Jsc of ~20 mA/cm2 and FF of ~65%, the main limitation on the fabricated device performance is the lower Jsc and lower FF. The reason for the low short-circuit current may be due to the high series resistance and the recombination losses. Another factor that is detrimental to device efficiency is Sn loss in CZTS thin films, which can be evidenced by the formation of the SnS2. Sn loss has been confirmed as a detrimental factor for the device performance of CZTS thin films (Redinger et al., 2011). To explain the relatively low Jsc of these solar cells, the external quantum efficiency (EQE) spectra were measured. Fig. 8 shows EQE spectra of the CZTS thin film solar cells sulfurized at different positions. For the champion solar cell S4 with the highest short circuit current, the EQE presents the highest data in the most region. The maximum EQE of S4 is 61.3% at 670 nm, which is quite lower than the reported solar cell with maximum EQE of ~90% (Shin et al., 2013; Yan et al., 2018). This indicates the recombination of photogenerated carriers in the entire spectral wavelength region is the first reason for the limited short circuit current. Another reason that can’t be ignored is the bad photoresponse in the long wavelength region (800–1000 nm). To understand why the EQE starts to increase at 800 nm, the energy band gap of the CZTS absorber in devices was extracted from the EQE data. Combining reflectance spectra with EQE data, the internal quantum efficiency (IQE) data was calculated and shown in supplementary information Fig. S3(a–g). Subsequently, the energy band gap was calculated according to the peak of the dIQE/dE curve. The calculated values are shown in Table 3. It should be noted that the EQE value of S1 in the near-infrared region is small (lower than 5%), so the calculated band gap value of S1 may incorrect. The energy band gap of all samples are larger than the theoretical value of 1.5 eV (Walsh et al., 2012), which leads to a decrease in absorption in the near-infrared spectrum. The only one reason for the increase of the band gap is the deviation of compositional ratio. Therefore, Sn loss which has been reported in literatures could account for the deviation. Reducing the Sn loss or providing a Sn rich atmosphere may be a good solution for the sulfurization process to improve device performance.
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.09.040. References Alvarez, A., Exarhos, S., Mangolini, L., 2016. Tin disulfide segregation on CZTS films sulfurized at high pressure. Mater. Lett. 165, 41–44. Berkowitz, J., Marquart, J., 1963. Equilibrium composition of sulfur vapor. J. Chem. Phys. 39 (2), 275–283. Cheng, Y., Jin, C., Gao, F., Wu, X., Zhong, W., Li, S., Chu, P.K., 2009. Raman scattering study of zinc blende and wurtzite ZnS. J. Appl. Phys. 106 (12), 123505. Emrani, A., Vasekar, P., Westgate, C.R., 2013. Effects of sulfurization temperature on CZTS thin film solar cell performances. Sol. Energy 98, 335–340. Ge, J., Jiang, J., Yang, P., Peng, C., Huang, Z., Zuo, S., Yang, L., Chu, J., 2014. A 5.5% efficient co-electrodeposited ZnO/CdS/Cu2ZnSnS4/Mo thin film solar cell. Sol. Energy Mater. Sol. Cells 125, 20–26. Green, M.A., Hishikawa, Y., Dunlop, E.D., Levi, D.H., Hohl-Ebinger, J., Ho-Baillie, A.W., 2018. Solar cell efficiency tables (version 52). Prog. Photovoltaics Res. Appl. 26 (7), 427–436. He, J., Sun, L., Chen, Y., Jiang, J., Yang, P., Chu, J., 2015. Influence of sulfurization pressure on Cu2ZnSnS4 thin films and solar cells prepared by sulfurization of metallic precursors. J. Power Sour. 273, 600–607. Jiang, F., Ikeda, S., Harada, T., Matsumura, M., 2014. Pure sulfide Cu2ZnSnS4 thin film solar cells fabricated by preheating an electrodeposited metallic stack. Adv. Energy Mater. 4 (7), 1301381. Katagiri, H., 2005. Cu2ZnSnS4 thin film solar cells. Thin Solid Films 480, 426–432. Katagiri, H., Jimbo, K., Yamada, S., Kamimura, T., Maw, W.S., Fukano, T., Ito, T., Motohiro, T., 2008. Enhanced conversion efficiencies of Cu2ZnSnS4-based thin film solar cells by using preferential etching technique. Appl. Phys Exp. 1 (4), 041201. Kushwaha, A.K., 2017. Critical review on sputter-deposited Cu2ZnSnS4 (CZTS) based thin film photovoltaic technology focusing on device architecture and absorber quality on the solar cells performance. Solar Energy Mater. Solar Cells. Li, W., Chen, J., Yan, C., Hao, X., 2015. The effect of ZnS segregation on Zn-rich CZTS thin film solar cells. J. Alloy. Compd. 632, 178–184. Li, X., Cao, H., Dong, Y., Yue, F., Chen, Y., Xiang, P., Sun, L., Yang, P., Chu, J., 2017. Investigation of Cu2ZnSnS4 thin films with controllable Cu composition and its influence on photovoltaic properties for solar cells. J. Alloy. Compd. 694, 833–840. Liu, R., Tan, M., Xu, L., Zhang, X., Chen, J., Tang, X., 2016. Preparation of high-quality Cu2ZnSnS4 thin films for solar cells via the improvement of sulfur partial pressure using a static annealing sulfurization approach. Sol. Energy Mater. Sol. Cells 157, 221–228. Panda, S., Antonakos, A., Liarokapis, E., Bhattacharya, S., Chaudhuri, S., 2007. Optical properties of nanocrystalline SnS2 thin films. Mater. Res. Bull. 42 (3), 576–583. Rau, H., Kutty, T., De Carvalho, J.G., 1973. Thermodynamics of sulphur vapour. J. Chem. Thermodyn. 5 (6), 833–844. Redinger, A., Berg, D.M., Dale, P.J., Siebentritt, S., 2011. The consequences of kesterite equilibria for efficient solar cells. J. Am. Chem. Soc. 133 (10), 3320–3323. Schneider, J., Kirby, R.D., 1972. Raman scattering from ZnS polytypes. Phys. Rev. B 6 (4), 1290. Scragg, J.J., Ericson, T., Kubart, T., Edoff, M., Platzer-Björkman, C., 2011. Chemical insights into the instability of Cu2ZnSnS4 films during annealing. Chem. Mater. 23 (20), 4625–4633. Shin, B., Gunawan, O., Zhu, Y., Bojarczuk, N.A., Chey, S.J., Guha, S., 2013. Thin film solar cell with 8.4% power conversion efficiency using an earth-abundant Cu2ZnSnS4 absorber. Prog. Photovoltaics Res. Appl. 21 (1), 72–76. Su, Z., Tan, J.M.R., Li, X., Zeng, X., Batabyal, S.K., Wong, L.H., 2015. Cation substitution of solution-processed Cu2ZnSnS4 thin film solar cell with over 9% efficiency. Adv. Energy Mater. 5 (19), 1500682. Sun, K., Yan, C., Liu, F., Huang, J., Zhou, F., Stride, J.A., Green, M., Hao, X., 2016. Over 9% efficient kesterite Cu2ZnSnS4 solar cell fabricated by using Zn1–xCdxS buffer layer. Adv. Energy Mater. 6 (12), 1600046. Walsh, A., Chen, S.Y., Wei, S.H., Gong, X.G., 2012. Kesterite thin-film solar cells: advances in materials modelling of Cu2ZnSnS4. Adv. Energy Mater. 2 (4), 400–409. Wang, K., Gunawan, O., Todorov, T., Shin, B., Chey, S., Bojarczuk, N., Mitzi, D., Guha, S., 2010. Thermally evaporated Cu2 ZnSnS4 solar cells. Appl. Phys. Lett. 97 (14), 143508. Wang, K., Shin, B., Reuter, K.B., Todorov, T., Mitzi, D.B., Guha, S., 2011. Structural and elemental characterization of high efficiency Cu2 ZnSnS4 solar cells. Appl. Phys. Lett. 98 (5), 051912. Yan, C., Huang, J., Sun, K., Johnston, S., Zhang, Y., Sun, H., Pu, A., He, M., Liu, F., Eder, K., 2018. Cu2ZnSnS4 solar cells with over 10% power conversion efficiency enabled by heterojunction heat treatment. Nat. Energy 3 (9), 764. Zhang, K., Su, Z., Zhao, L., Yan, C., Liu, F., Cui, H., Hao, X., Liu, Y., 2014. Improving the conversion efficiency of Cu2ZnSnS4 solar cell by low pressure sulfurization. Appl. Phys. Lett. 104 (14), 141101.
4. Conclusion For the sulfurization in a long diffusion range, the sulfur vapor showed a non-uniform distribution which had a significant influence on the properties of the CZTS thin film solar cells and the device performance of solar cells. In the position close to the sulfur source the grain size of CZTS thin films was quite small and the associated PCE of solar cells was limited. With the extension of distance between sulfur source and CZTS sample, the grain size increased and the device efficiency of solar cells improved. In the position far away from the sulfur source, the secondary phases such as SnS2 and ZnS began to appear. The appearance of SnS2 was a signal of higher sulfur partial pressure, indicating the higher sulfur partial pressure existed at the far end of the chamber (Alvarez et al., 2016; Scragg et al., 2011). There is an optimal position where could promote the growth of CZTS crystal, reduce the formation of secondary phases, and improve the efficiency of solar cells. In addition, there are several reasons to explain why the higher sulfur partial pressure exists at the far end of the tube. Firstly, the saturation pressure of sulfur vapor is lower in lower temperature and the sample S1 is more close to the lower temperature zone. Secondly, the S2 which is more active have a faster diffusion speed than the gas molecule S8. Lastly, the sulfur vapor may accumulate at the far end of the quartz tube. A higher energy band gap of ~1.68 eV calculated from the EQE data indicated the Sn loss which should be optimized in further experiment. Acknowledgments This work was financially supported by the National Science 11