- Email: [email protected]
Fabrication of Cu2ZnSnS4 thin films by microwave assisted sol-gel method
Superlattices and Microstructures 126 (2019) 83–88
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Fabrication of Cu2ZnSnS4 thin films by microwave assisted sol-gel method
T
Lei Qiu, Jiaxiong Xu∗, Weitong Cai, Zhiwei Xie, Yuanzheng Yang School of Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Cu2ZnSnS4 Thin films Sol-gel Microwave-assisted
CZTS thin films were prepared by adding microwave radiation to the sol-gel method. Microwave treatment was performed after sol-gel drying process. The CZTS thin films were analyzed by XRD, Raman, SEM, and UV–Vis spectrophotometer to investigate the influence of microwave irradiation time. The results showed that the microwave treatment promoted the initial formation of the CZTS phase. The crystallinity of the CZTS thin film was improved during the subsequent annealing. With the increase of microwave treatment time, the crystallinity of the annealed CZTS thin films first enhanced and then decreased. When the microwave treatment time was 25 min, the CZTS thin films reached the highest crystallinity with the maximum particle size of about 500 nm. EDS analysis indicated that the compositions of thin films showed Cu-poor properties. The direct optical band gaps of CZTS thin films were 1.58–1.61 eV. The experimental results revealed that the microwave radiation method could improve the properties of sol-gel processed CZTS thin films.
1. Introduction Recently, Cu2ZnSnS4 (CZTS) thin film has been recognized as a promising absorber material of solar cell due to its suitable band gap of 1.5 eV, high absorption coefficient of more than 1 × 104 cm−1 above the band gap, and earth-abundant, non-toxic, and environmentally friendly composition [1–4]. Until now, the reported vacuum techniques for fabricating CZTS thin films include pulsed laser deposition [5], magnetron sputtering [6] and fast co-evaporation technique [7], which are relatively costly because ultra-high vacuum equipment is required. Also, there are low-cost non-vacuum solution methods for the preparations of CZTS thin films, such as sol-gel [8], electrodeposited [9], and spray pyrolysis [10]. The sol-gel method is favorable for the deposition of thin films. During the sol-gel process, a solute or solvent is agglomerated or hydrolyzed to obtain a sol containing nanoparticles. Then, the sol is polycondensed into a gel under certain conditions. Finally, the post-treatment of the gel results in the required thin film. The sol-gel method can improve the chemical uniformity of the multicomponent film to the molecular and atomic levels compared to other non-vacuum preparation methods. In addition, the doping of the prepared material can be easily realized and the film preparation process can be complete and precise control by sol-gel technique. Although many studies have reported the synthesis of CZTS compounds by sol-gel method [8,11–16], some key issues are still noteworthy. According to related reports, secondary phases such as CuxS, ZnS, and SnS are easily formed because of their low chemical potential and the volatility of the elements in the CZTS compound. Therefore, it is difficult to synthesize single-phase CZTS thin film [8]. In addition, since the sol-gel processed CZTS thin films usually contain small and non-uniform grains, carrier recombination ∗
Corresponding author. E-mail address: [email protected] (J. Xu).
https://doi.org/10.1016/j.spmi.2018.12.020 Received 27 October 2018; Received in revised form 10 December 2018; Accepted 15 December 2018 Available online 17 December 2018 0749-6036/ © 2018 Elsevier Ltd. All rights reserved.
Superlattices and Microstructures 126 (2019) 83–88
L. Qiu et al.
induced by grain boundary is detrimental to the photovoltaic properties of solar cell [17,18]. In order to solve these problems, we introduce microwave treatment after drying in the sol-gel process. Microwave irradiation is the body heating caused by the loss of the medium in the electromagnetic field, which is direct heating of the molecules. By applying microwave radiation to the sol-gel process, hysteresis-free heating and small temperature gradients of microwave radiation can effectively reduce abnormal grain growth [19]. It is expected to increase the size and uniformity of CZTS grains. Similarly, plasma is added to conventional chemical vapor deposition methods to control the structural parameters of the prepared materials by adjusting the temperature and concentration of electrons in the plasma discharge [20–22]. Ultrasound addition to the sol-gel process increases the growth rate of nanoparticles and significantly reduces the reaction temperature [23]. In present work, CZTS thin films were prepared by microwave assisted sol-gel method and post-annealing. The microwave radiation was added after drying in the sol-gel process. The structures, compositions, surface and cross-section morphologies, and optical properties of the prepared thin films were investigated to study the effects of microwave treatment time. 2. Experimental Cu(CH3COO)2·H2O, SnCl2·2H2O, Zn(CH3COO)2·H2O and thiourea were sequentially added to ethylene glycol (EG) with molar ratios of Cu/(Zn + Sn) = 0.8, Zn/Sn = 1.15, and S/Cu = 4. In this experiment, EG was chosen as the solvent due to its high absorption rate of microwave. Meantime, acetylacetone was mixed under stirring. All chemicals were analytical reagents purchased from Sigma-Aldrich without any further purification. After stirring for 60 min at a temperature of 60 °C, a uniform sol was formed. The sol was aged for 4 days at room temperature to increase its viscosity. The fluorine-doped tin oxide (FTO) conductive glasses were used as the substrate of CZTS because incident light from back side could enter into CZTS absorber through the transparent back electrode, and the FTO had high conductivity to satisfy the requirement for a back electrode. A suitable amount of CZTS sol was spincoated on a FTO conductive glass at a rotation speed of 3000 revolutions per minute for 60 s, and then dried at 200 °C for 4 min on a hot plate in air to obtain CZTS precursor films. The coating and drying process were repeated for 10 times. After that, the precursor films were heated by microwave irradiation at 700 W for 0–30 min on domestic microwave generator plate in air. Finally, the CZTS precursor films were annealed at 580 °C for 1 h in a nitrogen atmosphere to obtain CZTS thin films. The structural properties of precursors and annealed samples were detected by X-ray diffractometer (Rigaku D/MAX-Ultima IV, Cu with an index kα radiation with wavelength of 0.154 nm) and Raman scattering analysis (FEX, NOST Company Limited, excitation wavelength of 532 nm). Meanwhile, the surface and cross-section morphologies of samples were observed by field emission scanning electron microscope (FESEM, Hitachi SU8010). The compositions of prepared samples were measured by energy dispersive spectroscopy (EDS, IXRF Systems, SDD3030). The optical properties of thin films were investigated by UV–Vis spectrophotometer (Pgeneral TU1810 and Shimadzu UV-3600 Plus). 3. Results and discussions Fig. 1(a) shows the X-ray diffraction (XRD) patterns of CZTS precursors obtained after microwave treatment for 0–30 min. Obviously, no CZTS peak is found in the sample without microwave treatment. After microwave treatment, the XRD peaks at 18.5°, 28.5° and 47.3° match with the standard XRD peaks of CZTS (PDF#26–0575). The two major peaks at 28.5° and 47.3° are corresponding to the (112) and (220) planes of CZTS, respectively. The full width at half maximum (FWHM) of the (112) peak first decreases and then increases with increasing microwave treatment time, and reaches the minimum value at 25 min. Since the positions of XRD peaks of CZTS and its secondary phases overlap, it is necessary to further analyze the structural properties of CZTS precursors by Raman scattering measurement, and the results are shown in Fig. 1(b). The peaks located at 331–336 cm−1 belong to the kesterite CZTS phase. These results imply that the microwave treatment enables CZTS precursors to first nucleate and initially grow. Fig. 2(a) presents the XRD results of the CZTS thin films obtained after annealing. The results demonstrate that the XRD peaks at 18.5°, 28.5°, 47.3° and 56.1° match with the standard XRD peaks of CZTS (PDF#26–0575). The three major peaks at 28.5°, 47.3°, and 56.1° are attributed to the (112), (220), (312) planes of CZTS, respectively. Compared with the samples before annealing, the crystallinity of all annealed samples increases, especially the three strong peaks of kesterite CZTS significantly enhance. Besides, except for the diffraction peaks of CZTS, the other peaks match with the SnO2 phase (PDF#41–1445), which is original from the FTO. As the microwave treatment time increases from 0 to 25 min, the FWHM of the (112) peak decreases and reaches the minimum, and then increases as microwave treatment time further increases. Therefore, the crystallinity of the CZTS thin films improves first and then decreases with the increase of microwave treatment time. In Fig. 2(b), the Raman peaks located at 288 and 338 cm−1 correspond to the CZTS phase. The peaks at 494 cm−1 belong to the phase of SnO2. The EDS results show that the carbon ratios of all annealed films are zero, indicating the absent ethylene glycol (CH2OH)2 solvent in the final CZTS thin films. In this work, the precursor films were prepared by spin-coating of sol followed by drying in air. The solvent in the precursor films began to evaporate during the drying process because the drying temperature of 200 °C was higher than the boiling point of the solvent (198 °C). The precursor films were further annealed at 580 °C for 1 h to obtain the final films. The high annealing temperature (much higher than the decomposition temperature of 215 °C for the solvent) and long annealing time can ensure full evaporation and decomposition of solvent and complete conversion of organic precursors. The EDS results are given in Table 1 which lists the atomic ratios of the Cu, Zn, Sn and S elements and the ratios of Cu/Zn and S/ Cu of the samples. The measurement results show that the atomic ratio of Cu/Zn is less than 2 and S/Cu is greater than 2. Therefore, the prepared CZTS thin films are Cu-poor according to the stoichiometry of CZTS. This meets the composition requirement for CZTS absorbers [8,12,13]. Since both CZTS thin film and FTO contribute to the measured Sn ratios, the Sn ratios in the CZTS thin film are 84
Superlattices and Microstructures 126 (2019) 83–88
L. Qiu et al.
Fig. 1. (a) XRD patterns and (b) Raman spectra of the CZTS precursors after microwave treatment for different time.
smaller than the measured results in Table 1. Fig. 3 shows the surface and cross-section SEM images of prepared CZTS thin films. It can be found that all the thin films are well crystallized by means of large grains and smooth surface. The grain size of the thin films gradually increases with the increase of microwave treatment time. When the microwave treatment time is 25 min, the grain size reaches the maximum value of about 500 nm, which is significantly larger than the grain size of thin film without microwave treatment. When the microwave treatment time further increases to 30 min, the grain size maintains at about 500 nm. This is due to the fact that the film reaction has no induction period under microwave irradiation. A large number of crystal nuclei are formed at one time and preferentially nucleate at the microwave treatment stage and grow at the annealing stage, resulting in an increase in grain size. The grain size and crystal growth in the absorber layer directly affect the efficiency of the solar cell. Solar cells consisting of absorber with densely packed large grain can reduce carrier recombination, resulting in high short circuit currents [11,12]. However, there are some voids on all the surfaces of CZTS thin films. The number of voids first decreases with the increase of microwave treatment time. But when microwave treatment time increases to 30 min, the voids become larger. From the cross-sectional micrographs, all the prepared CZTS thin films were densely packed with CZTS/FTO/glass stack structures arranged in order from top to bottom. The thicknesses of prepared thin films are about 350–400 nm. In the transmittance measurements, the effect of FTO-coated glass substrates has been deducted to obtain the exact transmittances of the CZTS thin films. The optical transmittance and reflectance spectra for the CZTS thin films are shown in Fig. 4(a) and (b), respectively. The transmittance of the CZTS thin films reduces as the wavelength decreases. During microwave treatment, the films are pre-formed with a large amount of nucleation because the small temperature gradient and fast heating speed of microwave radiation heating. After annealing, the grain size and crystallinity of the films improve, and the voids reduce. Therefore, the transmittance of the thin films with microwave treatment is lower than that of the thin film without microwave treatment. According to Ref. [24], using the values of the transmittance (T), reflectance (R), and thickness (d) of the CZTS film, the 85
Superlattices and Microstructures 126 (2019) 83–88
L. Qiu et al.
Fig. 2. (a) XRD patterns and (b) Raman spectra of the final CZTS thin films with different microwave treatment time. Table 1 The EDS results of the samples. Microwave treatment time (min)
Cu (at.%)
Zn (at.%)
Sn (at.%)
S (at.%)
Cu/Zn
S/Cu
0 5 10 15 20 25 30
17.91 17.11 20.16 19.32 18.58 19.57 20.93
11.10 10.30 12.50 10.65 10.20 10.43 10.50
24.21 23.22 19.87 24.69 24.08 19.29 19.07
46.78 49.37 47.47 45.34 47.14 50.71 49.50
1.61 1.66 1.61 1.81 1.82 1.88 1.99
2.61 2.89 2.35 2.35 2.54 2.59 2.37
absorption coefficient (α) of the film can be obtained by the formula
α= −
1 ⎛ T 2 − (1 − R)2 + ln d ⎜ ⎝
4T 2 + ((1 − R)2 − T 2)2 ⎞ ⎟ 2T ⎠
The absorption coefficients of the CZTS thin films are shown in Fig. 4(c). The absorption coefficients of CZTS thin films with microwave treatment are larger than those of thin film without microwave treatment. After obtaining the absorption coefficient, the direct optical band gap (Eg) of the thin film can be deduced from (αh ν)2 = C (h ν− Eg ) , where h, ν , and C are Planck constant, the frequency of photon, and a constant, respectively. Fig. 4(d) shows the relation between (αh ν )2 and hν . Eg is determined by extrapolating the straight line region of the (αh ν )2- hν curves to the intercept on the hν axis [25]. Fig. 4(e) shows the variation of band gap 86
Superlattices and Microstructures 126 (2019) 83–88
L. Qiu et al.
Fig. 3. The surface and cross-sectional SEM images of the CZTS thin films (a) without microwave treatment and with microwave treatment time of (b) 5 min, (c) 10 min, (d) 15 min, (e) 20 min, (f) 25 min, and (g) 30 min.
Fig. 4. The (a) transmittance, (b) reflectance, (c) absorption coefficient, (d) (αhν )2- hν curves, and (e) band gaps of the CZTS thin films with different microwave treatment time.
of the CZTS thin films as the microwave treatment time increases. The optical band gaps of prepared CZTS thin films are 1.58–1.61 eV, which are suitable for absorber of single junction solar cell.
4. Conclusion In this study, microwave assisted sol-gel method and post-annealing were used for depositions of CZTS thin films on FTO-coated glass substrates. The effect of microwave treatment was analyzed. The microwave-treated sample before annealing is preferentially nucleated, and a CZTS structure having a (112) plane preferred orientation is formed on the substrate after annealing. The crystallinity of the annealed sample increases first and then decreases with the extension of the microwave treatment time. Microwavetreated CZTS thin films exhibit Cu-poor properties and have high absorption coefficients. The CZTS thin film with microwave treatment for 25 min has the largest grains, the highest crystallinity, and the direct optical band gap of 1.59 eV. 87
Superlattices and Microstructures 126 (2019) 83–88
L. Qiu et al.
Acknowledgements This work was supported by National Natural Science Foundation of China (No. 61504029) and Science and Technology Planning Project of Guangdong Province, China (No. 2017A010104017). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.spmi.2018.12.020. References [1] H. Katagiri, N. Sasaguchi, S. Hando, S. Hoshino, J. Ohashi, T. Yokota, Preparation and evaluation of Cu2ZnSnS4 thin sulfurization of E-B evaporated precursors, Sol. Energy Mater. Sol. Cells 49 (1997) 407–414. [2] M. Kumar, A. Dubey, N. Adhikari, S. Venkatesan, Q. Qiao, Strategic review of secondary phases, defects and defect-complexes in kesterite CZTS–Se solar cells, Energy Environ. Sci. 8 (2016) 3134–3159. [3] S.Y. Li, C. Hägglund, Y. Ren, J.J.S. Scragg, J.K. Larsen, C. Frisk, Optical properties of reactively sputtered Cu2ZnSnS4 solar absorbers determined by spectroscopic ellipsometry and spectrophotometry, Sol. Energy Mater. Sol. Cells 149 (2016) 170–178. [4] S.A. Vanalakar, G.L. Agawane, S.W. Shin, M.P. Suryawanshi, K.V. Gurav, K.S. Jeon, P.S. Patil, C.W. Jeong, J.Y. Kim, J.H. Kim, A review on pulsed laser deposited CZTS thin films for solar cell applications, J. Alloy. Comp. 619 (2015) 109–121. [5] A.V. Moholkar, S.S. Shinde, A.R. Babar, K.U. Sim, H.K. Lee, K.Y. Rajpure, P.S. Patil, C.H. Bhosale, J.H. Kim, Synthesis and characterization of Cu2ZnSnS4 thin films grown by PLD: solar cells, J. Alloy. Comp. 509 (2011) 7439–7446. [6] J.X. Xu, Z.M. Cao, Y.Z. Yang, Z.W. Xie, Fabrication of Cu2ZnSnS4 thin films on flexible polyimide substrates by sputtering and post-sulfurization, J. Renew. Sustain. Energy 6 (2014) 5564–5576. [7] B.A. Schubert, B. Marsen, S. Cinque, T. Unold, R. Klenk, Cu2ZnSnS4 thin film solar cells by fast coevaporation, Prog. Photovoltaics Res. Appl. 19 (2011) 93–96. [8] Z.H. Su, K.W. Sun, Z.L. Han, H.T. Cui, F.Y. Liu, Y.Q. Lai, J. Li, X.J. Hao, Y.X. Liu, M.A. Green, Fabrication of Cu2ZnSnS4 solar cells with 5.1% efficiency via thermal decomposition and reaction using a non-toxic sol–gel route, J. Mater. Chem. A. 2 (2013) 500–509. [9] M.I. Khalil, O. Atici, A. Lucotti, S. Binetti, A.L. Donne, L. Magagnin, CZTS absorber layer for thin film solar cells from electrodeposited metallic stacked precursors (Zn/Cu-Sn), Appl. Surf. Sci. 379 (2016) 91–97. [10] V.G. Rajeshmon, C.S. Kartha, K.P. Vijayakumar, C. Sanjeeviraja, T. Abe, Y. Kashiwaba, Role of precursor solution in controlling the optoelectronic properties of spray pyrolysed Cu2ZnSnS4 thin films, Sol. Energy 85 (2011) 249–255. [11] P.K. Sarswat, M.L. Free, Demonstration of a sol-gel synthesized bifacial CZTS photo-electrochemical cell, Phys. Status Solidi 12 (2011) 2861–2864. [12] L.M. Dong, S.Y. Cheng, Y.F. Lai, H. Zhang, H.J. Jia, Sol-gel processed CZTS thin film solar cell on flexible molybdenum foil, Thin Solid Films 626 (2017) 168–172. [13] K.Z. Zhang, J.H. Tao, J.F. Liu, J. He, Y.C. Dong, L. Sun, P.X. Yang, J.H. Chu, Compact Cu2ZnSn(S,Se)4 thin films fabricated by a simple sol-gel technique, J. Inorg. Mater. 29 (2014) 781–784. [14] H. Park, Y.H. Hwang, B.S. Bae, Sol-gel processed Cu2ZnSnS4 thin films for a photovoltaic absorber layer without sulfurization, J. Sol. Gel Sci. Technol. 65 (2013) 23–27. [15] K. Tanaka, Y. Fukui, N. Moritake, H. Uchiki, Chemical composition dependence of morphological and optical properties of Cu2ZnSnS4 thin films deposited by solgel sulfurization and Cu2ZnSnS4 thin film solar cell efficiency, Sol. Energy Mater. Sol. Cells 95 (2011) 838–842. [16] Z.H. Su, J.M.R. Tan, X.L. Li, X. Zeng, S.K. Batabyal, L.H. Wong, Cation substitution of solution-processed Cu2ZnSnS4 thin film solar cell with over 9% efficiency, Adv. Energy Mater. 19 (2015) 5. [17] J.H. Tao, K.Z. Zhang, C.J. Zhang, L.L. Chen, H.Y. Cao, J.F. Liu, J.C. Jiang, L. Sun, P.X. Yang, J.H. Chu, A sputtered CdS buffer layer for co-electrodeposited Cu2ZnSnS4 solar cells with 6.6% efficiency, Chem. Commun. 51 (2015) 10337–10340. [18] J.H. Tao, J.F. Liu, L.L. Chen, H.Y. Cao, X.K. Meng, Y.B. Zhang, C.J. Zhang, L. Sun, P.X. Yang, J.H. Chu, 7.1% efficient co-electroplated Cu2ZnSnS4 thin film solar cells with sputtered CdS buffer layers, Green Chem. 18 (2016) 550–557. [19] A. Loupy, Microwaves in Organic Synthesis, second ed., (2008). [20] L. Mochalov, D. Dorosz, A. Nezhdanov, M. Kudryashov, S. Zelentsov, D. Usanov, A. Logunov, A. Mashin, D. Gogova, Investigation of the composition-structureproperty relationship of AsxTe100-x films prepared by plasma deposition, Spectrochim. Acta, Part A 191 (2018) 211–216. [21] L. Mochalov, A. Nezhdanov, A. Logunov, M. Kudryashov, I. Krivenkov, A. Vorotyntsev, D. Gogova, A. Mashin, Optical emission of two-dimensional arsenic sulfide prepared by plasma, Superlattice. Microst. 114 (2018) 305–313. [22] L. Mochalov, M. Kudryashov, A. Logunov, S. Zelentsov, A. Nezhdanov, A. Mashin, D. Gogova, G. Chidichimo, G.D. Filpo, Structural and optical properties of arsenic sulfide films synthesized by a novel PECVD-based approach, Superlattice. Microst. 111 (2017) 1104–1112. [23] T. Chandel, V. Thakur, M.B. Zaman, S.K. Dwivedi, R. Poolla, Ultrasonically assisted sol-gel synthesis of nanocrystalline Cu2ZnSnS4 particles for solar cell applications, Mater. Lett. 212 (2018) 279–282. [24] Y.S. Kuzyutkina, E.A. Romanova, V.I. Kochubei, V.S. Shiryaev, Specific features of linear and nonlinear optical responses of chalcogenide glasses in the As-S-Se and As-Se-Te systems, Optic Spectrosc. 117 (2014) 53–59. [25] A.J. Leenheer, J.D. Perkins, M.F.A.M. Van Hest, J.J. Berry, R.P. O'Hayre, D.S. Ginley, General mobility and Carrier concentration relationship in transparent amorphous indium zinc oxide films, Phys. Rev. B 11 (2008) 115215.
88
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Fabrication of Cu2ZnSnS4 thin films by microwave assisted sol-gel method
T
Lei Qiu, Jiaxiong Xu∗, Weitong Cai, Zhiwei Xie, Yuanzheng Yang School of Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Cu2ZnSnS4 Thin films Sol-gel Microwave-assisted
CZTS thin films were prepared by adding microwave radiation to the sol-gel method. Microwave treatment was performed after sol-gel drying process. The CZTS thin films were analyzed by XRD, Raman, SEM, and UV–Vis spectrophotometer to investigate the influence of microwave irradiation time. The results showed that the microwave treatment promoted the initial formation of the CZTS phase. The crystallinity of the CZTS thin film was improved during the subsequent annealing. With the increase of microwave treatment time, the crystallinity of the annealed CZTS thin films first enhanced and then decreased. When the microwave treatment time was 25 min, the CZTS thin films reached the highest crystallinity with the maximum particle size of about 500 nm. EDS analysis indicated that the compositions of thin films showed Cu-poor properties. The direct optical band gaps of CZTS thin films were 1.58–1.61 eV. The experimental results revealed that the microwave radiation method could improve the properties of sol-gel processed CZTS thin films.
1. Introduction Recently, Cu2ZnSnS4 (CZTS) thin film has been recognized as a promising absorber material of solar cell due to its suitable band gap of 1.5 eV, high absorption coefficient of more than 1 × 104 cm−1 above the band gap, and earth-abundant, non-toxic, and environmentally friendly composition [1–4]. Until now, the reported vacuum techniques for fabricating CZTS thin films include pulsed laser deposition [5], magnetron sputtering [6] and fast co-evaporation technique [7], which are relatively costly because ultra-high vacuum equipment is required. Also, there are low-cost non-vacuum solution methods for the preparations of CZTS thin films, such as sol-gel [8], electrodeposited [9], and spray pyrolysis [10]. The sol-gel method is favorable for the deposition of thin films. During the sol-gel process, a solute or solvent is agglomerated or hydrolyzed to obtain a sol containing nanoparticles. Then, the sol is polycondensed into a gel under certain conditions. Finally, the post-treatment of the gel results in the required thin film. The sol-gel method can improve the chemical uniformity of the multicomponent film to the molecular and atomic levels compared to other non-vacuum preparation methods. In addition, the doping of the prepared material can be easily realized and the film preparation process can be complete and precise control by sol-gel technique. Although many studies have reported the synthesis of CZTS compounds by sol-gel method [8,11–16], some key issues are still noteworthy. According to related reports, secondary phases such as CuxS, ZnS, and SnS are easily formed because of their low chemical potential and the volatility of the elements in the CZTS compound. Therefore, it is difficult to synthesize single-phase CZTS thin film [8]. In addition, since the sol-gel processed CZTS thin films usually contain small and non-uniform grains, carrier recombination ∗
Corresponding author. E-mail address: [email protected] (J. Xu).
https://doi.org/10.1016/j.spmi.2018.12.020 Received 27 October 2018; Received in revised form 10 December 2018; Accepted 15 December 2018 Available online 17 December 2018 0749-6036/ © 2018 Elsevier Ltd. All rights reserved.
Superlattices and Microstructures 126 (2019) 83–88
L. Qiu et al.
induced by grain boundary is detrimental to the photovoltaic properties of solar cell [17,18]. In order to solve these problems, we introduce microwave treatment after drying in the sol-gel process. Microwave irradiation is the body heating caused by the loss of the medium in the electromagnetic field, which is direct heating of the molecules. By applying microwave radiation to the sol-gel process, hysteresis-free heating and small temperature gradients of microwave radiation can effectively reduce abnormal grain growth [19]. It is expected to increase the size and uniformity of CZTS grains. Similarly, plasma is added to conventional chemical vapor deposition methods to control the structural parameters of the prepared materials by adjusting the temperature and concentration of electrons in the plasma discharge [20–22]. Ultrasound addition to the sol-gel process increases the growth rate of nanoparticles and significantly reduces the reaction temperature [23]. In present work, CZTS thin films were prepared by microwave assisted sol-gel method and post-annealing. The microwave radiation was added after drying in the sol-gel process. The structures, compositions, surface and cross-section morphologies, and optical properties of the prepared thin films were investigated to study the effects of microwave treatment time. 2. Experimental Cu(CH3COO)2·H2O, SnCl2·2H2O, Zn(CH3COO)2·H2O and thiourea were sequentially added to ethylene glycol (EG) with molar ratios of Cu/(Zn + Sn) = 0.8, Zn/Sn = 1.15, and S/Cu = 4. In this experiment, EG was chosen as the solvent due to its high absorption rate of microwave. Meantime, acetylacetone was mixed under stirring. All chemicals were analytical reagents purchased from Sigma-Aldrich without any further purification. After stirring for 60 min at a temperature of 60 °C, a uniform sol was formed. The sol was aged for 4 days at room temperature to increase its viscosity. The fluorine-doped tin oxide (FTO) conductive glasses were used as the substrate of CZTS because incident light from back side could enter into CZTS absorber through the transparent back electrode, and the FTO had high conductivity to satisfy the requirement for a back electrode. A suitable amount of CZTS sol was spincoated on a FTO conductive glass at a rotation speed of 3000 revolutions per minute for 60 s, and then dried at 200 °C for 4 min on a hot plate in air to obtain CZTS precursor films. The coating and drying process were repeated for 10 times. After that, the precursor films were heated by microwave irradiation at 700 W for 0–30 min on domestic microwave generator plate in air. Finally, the CZTS precursor films were annealed at 580 °C for 1 h in a nitrogen atmosphere to obtain CZTS thin films. The structural properties of precursors and annealed samples were detected by X-ray diffractometer (Rigaku D/MAX-Ultima IV, Cu with an index kα radiation with wavelength of 0.154 nm) and Raman scattering analysis (FEX, NOST Company Limited, excitation wavelength of 532 nm). Meanwhile, the surface and cross-section morphologies of samples were observed by field emission scanning electron microscope (FESEM, Hitachi SU8010). The compositions of prepared samples were measured by energy dispersive spectroscopy (EDS, IXRF Systems, SDD3030). The optical properties of thin films were investigated by UV–Vis spectrophotometer (Pgeneral TU1810 and Shimadzu UV-3600 Plus). 3. Results and discussions Fig. 1(a) shows the X-ray diffraction (XRD) patterns of CZTS precursors obtained after microwave treatment for 0–30 min. Obviously, no CZTS peak is found in the sample without microwave treatment. After microwave treatment, the XRD peaks at 18.5°, 28.5° and 47.3° match with the standard XRD peaks of CZTS (PDF#26–0575). The two major peaks at 28.5° and 47.3° are corresponding to the (112) and (220) planes of CZTS, respectively. The full width at half maximum (FWHM) of the (112) peak first decreases and then increases with increasing microwave treatment time, and reaches the minimum value at 25 min. Since the positions of XRD peaks of CZTS and its secondary phases overlap, it is necessary to further analyze the structural properties of CZTS precursors by Raman scattering measurement, and the results are shown in Fig. 1(b). The peaks located at 331–336 cm−1 belong to the kesterite CZTS phase. These results imply that the microwave treatment enables CZTS precursors to first nucleate and initially grow. Fig. 2(a) presents the XRD results of the CZTS thin films obtained after annealing. The results demonstrate that the XRD peaks at 18.5°, 28.5°, 47.3° and 56.1° match with the standard XRD peaks of CZTS (PDF#26–0575). The three major peaks at 28.5°, 47.3°, and 56.1° are attributed to the (112), (220), (312) planes of CZTS, respectively. Compared with the samples before annealing, the crystallinity of all annealed samples increases, especially the three strong peaks of kesterite CZTS significantly enhance. Besides, except for the diffraction peaks of CZTS, the other peaks match with the SnO2 phase (PDF#41–1445), which is original from the FTO. As the microwave treatment time increases from 0 to 25 min, the FWHM of the (112) peak decreases and reaches the minimum, and then increases as microwave treatment time further increases. Therefore, the crystallinity of the CZTS thin films improves first and then decreases with the increase of microwave treatment time. In Fig. 2(b), the Raman peaks located at 288 and 338 cm−1 correspond to the CZTS phase. The peaks at 494 cm−1 belong to the phase of SnO2. The EDS results show that the carbon ratios of all annealed films are zero, indicating the absent ethylene glycol (CH2OH)2 solvent in the final CZTS thin films. In this work, the precursor films were prepared by spin-coating of sol followed by drying in air. The solvent in the precursor films began to evaporate during the drying process because the drying temperature of 200 °C was higher than the boiling point of the solvent (198 °C). The precursor films were further annealed at 580 °C for 1 h to obtain the final films. The high annealing temperature (much higher than the decomposition temperature of 215 °C for the solvent) and long annealing time can ensure full evaporation and decomposition of solvent and complete conversion of organic precursors. The EDS results are given in Table 1 which lists the atomic ratios of the Cu, Zn, Sn and S elements and the ratios of Cu/Zn and S/ Cu of the samples. The measurement results show that the atomic ratio of Cu/Zn is less than 2 and S/Cu is greater than 2. Therefore, the prepared CZTS thin films are Cu-poor according to the stoichiometry of CZTS. This meets the composition requirement for CZTS absorbers [8,12,13]. Since both CZTS thin film and FTO contribute to the measured Sn ratios, the Sn ratios in the CZTS thin film are 84
Superlattices and Microstructures 126 (2019) 83–88
L. Qiu et al.
Fig. 1. (a) XRD patterns and (b) Raman spectra of the CZTS precursors after microwave treatment for different time.
smaller than the measured results in Table 1. Fig. 3 shows the surface and cross-section SEM images of prepared CZTS thin films. It can be found that all the thin films are well crystallized by means of large grains and smooth surface. The grain size of the thin films gradually increases with the increase of microwave treatment time. When the microwave treatment time is 25 min, the grain size reaches the maximum value of about 500 nm, which is significantly larger than the grain size of thin film without microwave treatment. When the microwave treatment time further increases to 30 min, the grain size maintains at about 500 nm. This is due to the fact that the film reaction has no induction period under microwave irradiation. A large number of crystal nuclei are formed at one time and preferentially nucleate at the microwave treatment stage and grow at the annealing stage, resulting in an increase in grain size. The grain size and crystal growth in the absorber layer directly affect the efficiency of the solar cell. Solar cells consisting of absorber with densely packed large grain can reduce carrier recombination, resulting in high short circuit currents [11,12]. However, there are some voids on all the surfaces of CZTS thin films. The number of voids first decreases with the increase of microwave treatment time. But when microwave treatment time increases to 30 min, the voids become larger. From the cross-sectional micrographs, all the prepared CZTS thin films were densely packed with CZTS/FTO/glass stack structures arranged in order from top to bottom. The thicknesses of prepared thin films are about 350–400 nm. In the transmittance measurements, the effect of FTO-coated glass substrates has been deducted to obtain the exact transmittances of the CZTS thin films. The optical transmittance and reflectance spectra for the CZTS thin films are shown in Fig. 4(a) and (b), respectively. The transmittance of the CZTS thin films reduces as the wavelength decreases. During microwave treatment, the films are pre-formed with a large amount of nucleation because the small temperature gradient and fast heating speed of microwave radiation heating. After annealing, the grain size and crystallinity of the films improve, and the voids reduce. Therefore, the transmittance of the thin films with microwave treatment is lower than that of the thin film without microwave treatment. According to Ref. [24], using the values of the transmittance (T), reflectance (R), and thickness (d) of the CZTS film, the 85
Superlattices and Microstructures 126 (2019) 83–88
L. Qiu et al.
Fig. 2. (a) XRD patterns and (b) Raman spectra of the final CZTS thin films with different microwave treatment time. Table 1 The EDS results of the samples. Microwave treatment time (min)
Cu (at.%)
Zn (at.%)
Sn (at.%)
S (at.%)
Cu/Zn
S/Cu
0 5 10 15 20 25 30
17.91 17.11 20.16 19.32 18.58 19.57 20.93
11.10 10.30 12.50 10.65 10.20 10.43 10.50
24.21 23.22 19.87 24.69 24.08 19.29 19.07
46.78 49.37 47.47 45.34 47.14 50.71 49.50
1.61 1.66 1.61 1.81 1.82 1.88 1.99
2.61 2.89 2.35 2.35 2.54 2.59 2.37
absorption coefficient (α) of the film can be obtained by the formula
α= −
1 ⎛ T 2 − (1 − R)2 + ln d ⎜ ⎝
4T 2 + ((1 − R)2 − T 2)2 ⎞ ⎟ 2T ⎠
The absorption coefficients of the CZTS thin films are shown in Fig. 4(c). The absorption coefficients of CZTS thin films with microwave treatment are larger than those of thin film without microwave treatment. After obtaining the absorption coefficient, the direct optical band gap (Eg) of the thin film can be deduced from (αh ν)2 = C (h ν− Eg ) , where h, ν , and C are Planck constant, the frequency of photon, and a constant, respectively. Fig. 4(d) shows the relation between (αh ν )2 and hν . Eg is determined by extrapolating the straight line region of the (αh ν )2- hν curves to the intercept on the hν axis [25]. Fig. 4(e) shows the variation of band gap 86
Superlattices and Microstructures 126 (2019) 83–88
L. Qiu et al.
Fig. 3. The surface and cross-sectional SEM images of the CZTS thin films (a) without microwave treatment and with microwave treatment time of (b) 5 min, (c) 10 min, (d) 15 min, (e) 20 min, (f) 25 min, and (g) 30 min.
Fig. 4. The (a) transmittance, (b) reflectance, (c) absorption coefficient, (d) (αhν )2- hν curves, and (e) band gaps of the CZTS thin films with different microwave treatment time.
of the CZTS thin films as the microwave treatment time increases. The optical band gaps of prepared CZTS thin films are 1.58–1.61 eV, which are suitable for absorber of single junction solar cell.
4. Conclusion In this study, microwave assisted sol-gel method and post-annealing were used for depositions of CZTS thin films on FTO-coated glass substrates. The effect of microwave treatment was analyzed. The microwave-treated sample before annealing is preferentially nucleated, and a CZTS structure having a (112) plane preferred orientation is formed on the substrate after annealing. The crystallinity of the annealed sample increases first and then decreases with the extension of the microwave treatment time. Microwavetreated CZTS thin films exhibit Cu-poor properties and have high absorption coefficients. The CZTS thin film with microwave treatment for 25 min has the largest grains, the highest crystallinity, and the direct optical band gap of 1.59 eV. 87
Superlattices and Microstructures 126 (2019) 83–88
L. Qiu et al.
Acknowledgements This work was supported by National Natural Science Foundation of China (No. 61504029) and Science and Technology Planning Project of Guangdong Province, China (No. 2017A010104017). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.spmi.2018.12.020. References [1] H. Katagiri, N. Sasaguchi, S. Hando, S. Hoshino, J. Ohashi, T. Yokota, Preparation and evaluation of Cu2ZnSnS4 thin sulfurization of E-B evaporated precursors, Sol. Energy Mater. Sol. Cells 49 (1997) 407–414. [2] M. Kumar, A. Dubey, N. Adhikari, S. Venkatesan, Q. Qiao, Strategic review of secondary phases, defects and defect-complexes in kesterite CZTS–Se solar cells, Energy Environ. Sci. 8 (2016) 3134–3159. [3] S.Y. Li, C. Hägglund, Y. Ren, J.J.S. Scragg, J.K. Larsen, C. Frisk, Optical properties of reactively sputtered Cu2ZnSnS4 solar absorbers determined by spectroscopic ellipsometry and spectrophotometry, Sol. Energy Mater. Sol. Cells 149 (2016) 170–178. [4] S.A. Vanalakar, G.L. Agawane, S.W. Shin, M.P. Suryawanshi, K.V. Gurav, K.S. Jeon, P.S. Patil, C.W. Jeong, J.Y. Kim, J.H. Kim, A review on pulsed laser deposited CZTS thin films for solar cell applications, J. Alloy. Comp. 619 (2015) 109–121. [5] A.V. Moholkar, S.S. Shinde, A.R. Babar, K.U. Sim, H.K. Lee, K.Y. Rajpure, P.S. Patil, C.H. Bhosale, J.H. Kim, Synthesis and characterization of Cu2ZnSnS4 thin films grown by PLD: solar cells, J. Alloy. Comp. 509 (2011) 7439–7446. [6] J.X. Xu, Z.M. Cao, Y.Z. Yang, Z.W. Xie, Fabrication of Cu2ZnSnS4 thin films on flexible polyimide substrates by sputtering and post-sulfurization, J. Renew. Sustain. Energy 6 (2014) 5564–5576. [7] B.A. Schubert, B. Marsen, S. Cinque, T. Unold, R. Klenk, Cu2ZnSnS4 thin film solar cells by fast coevaporation, Prog. Photovoltaics Res. Appl. 19 (2011) 93–96. [8] Z.H. Su, K.W. Sun, Z.L. Han, H.T. Cui, F.Y. Liu, Y.Q. Lai, J. Li, X.J. Hao, Y.X. Liu, M.A. Green, Fabrication of Cu2ZnSnS4 solar cells with 5.1% efficiency via thermal decomposition and reaction using a non-toxic sol–gel route, J. Mater. Chem. A. 2 (2013) 500–509. [9] M.I. Khalil, O. Atici, A. Lucotti, S. Binetti, A.L. Donne, L. Magagnin, CZTS absorber layer for thin film solar cells from electrodeposited metallic stacked precursors (Zn/Cu-Sn), Appl. Surf. Sci. 379 (2016) 91–97. [10] V.G. Rajeshmon, C.S. Kartha, K.P. Vijayakumar, C. Sanjeeviraja, T. Abe, Y. Kashiwaba, Role of precursor solution in controlling the optoelectronic properties of spray pyrolysed Cu2ZnSnS4 thin films, Sol. Energy 85 (2011) 249–255. [11] P.K. Sarswat, M.L. Free, Demonstration of a sol-gel synthesized bifacial CZTS photo-electrochemical cell, Phys. Status Solidi 12 (2011) 2861–2864. [12] L.M. Dong, S.Y. Cheng, Y.F. Lai, H. Zhang, H.J. Jia, Sol-gel processed CZTS thin film solar cell on flexible molybdenum foil, Thin Solid Films 626 (2017) 168–172. [13] K.Z. Zhang, J.H. Tao, J.F. Liu, J. He, Y.C. Dong, L. Sun, P.X. Yang, J.H. Chu, Compact Cu2ZnSn(S,Se)4 thin films fabricated by a simple sol-gel technique, J. Inorg. Mater. 29 (2014) 781–784. [14] H. Park, Y.H. Hwang, B.S. Bae, Sol-gel processed Cu2ZnSnS4 thin films for a photovoltaic absorber layer without sulfurization, J. Sol. Gel Sci. Technol. 65 (2013) 23–27. [15] K. Tanaka, Y. Fukui, N. Moritake, H. Uchiki, Chemical composition dependence of morphological and optical properties of Cu2ZnSnS4 thin films deposited by solgel sulfurization and Cu2ZnSnS4 thin film solar cell efficiency, Sol. Energy Mater. Sol. Cells 95 (2011) 838–842. [16] Z.H. Su, J.M.R. Tan, X.L. Li, X. Zeng, S.K. Batabyal, L.H. Wong, Cation substitution of solution-processed Cu2ZnSnS4 thin film solar cell with over 9% efficiency, Adv. Energy Mater. 19 (2015) 5. [17] J.H. Tao, K.Z. Zhang, C.J. Zhang, L.L. Chen, H.Y. Cao, J.F. Liu, J.C. Jiang, L. Sun, P.X. Yang, J.H. Chu, A sputtered CdS buffer layer for co-electrodeposited Cu2ZnSnS4 solar cells with 6.6% efficiency, Chem. Commun. 51 (2015) 10337–10340. [18] J.H. Tao, J.F. Liu, L.L. Chen, H.Y. Cao, X.K. Meng, Y.B. Zhang, C.J. Zhang, L. Sun, P.X. Yang, J.H. Chu, 7.1% efficient co-electroplated Cu2ZnSnS4 thin film solar cells with sputtered CdS buffer layers, Green Chem. 18 (2016) 550–557. [19] A. Loupy, Microwaves in Organic Synthesis, second ed., (2008). [20] L. Mochalov, D. Dorosz, A. Nezhdanov, M. Kudryashov, S. Zelentsov, D. Usanov, A. Logunov, A. Mashin, D. Gogova, Investigation of the composition-structureproperty relationship of AsxTe100-x films prepared by plasma deposition, Spectrochim. Acta, Part A 191 (2018) 211–216. [21] L. Mochalov, A. Nezhdanov, A. Logunov, M. Kudryashov, I. Krivenkov, A. Vorotyntsev, D. Gogova, A. Mashin, Optical emission of two-dimensional arsenic sulfide prepared by plasma, Superlattice. Microst. 114 (2018) 305–313. [22] L. Mochalov, M. Kudryashov, A. Logunov, S. Zelentsov, A. Nezhdanov, A. Mashin, D. Gogova, G. Chidichimo, G.D. Filpo, Structural and optical properties of arsenic sulfide films synthesized by a novel PECVD-based approach, Superlattice. Microst. 111 (2017) 1104–1112. [23] T. Chandel, V. Thakur, M.B. Zaman, S.K. Dwivedi, R. Poolla, Ultrasonically assisted sol-gel synthesis of nanocrystalline Cu2ZnSnS4 particles for solar cell applications, Mater. Lett. 212 (2018) 279–282. [24] Y.S. Kuzyutkina, E.A. Romanova, V.I. Kochubei, V.S. Shiryaev, Specific features of linear and nonlinear optical responses of chalcogenide glasses in the As-S-Se and As-Se-Te systems, Optic Spectrosc. 117 (2014) 53–59. [25] A.J. Leenheer, J.D. Perkins, M.F.A.M. Van Hest, J.J. Berry, R.P. O'Hayre, D.S. Ginley, General mobility and Carrier concentration relationship in transparent amorphous indium zinc oxide films, Phys. Rev. B 11 (2008) 115215.
88