CONTRIBUTED ARTICLE www.pss-c.com The Impact of Na and K on Properties of Cu2ZnSnS4 Thin Films Prepared by Ultrasonic Spray Technique Mouaad Sekkati,* Mhamed Taibi, Guy Schmerber, El Bachir Benamar, Mohammed Regragui, Fouzia Cherkaoui El Moursli, Cedric Leuvrey, Zineb Edfouf, Zouheir Sekkat, Aziz Dinia, Abdelilah Slaoui, and Mohammed Abd-Lefdil The later can be controlled by the addition of Se in the CZTS that it becomes Cu2ZnSn(S,Se)4. In addition, CZTS has similar properties as CIGS which is already marketed. The world record for CZTS and CZTSSe is 9.7 and 12.6% respectively,[1,2] even though it has a theoretical yield of 32%.[3] The performance of CZTS is limited by defects like secondary phases and interfaces states. So it is very important to understand the role of these defects, then control them or reduce their amounts. CZTS has been elaborated by different techniques such as physical way like PLD,[4] sputtering,[5] evaporation[6] or by chemical way like electrodeposition,[7] spray,[8] and sol-gel.[9] Among these methods, the ultrasonic spray technique allows to deposit large area thin-films in an easy way. The effects of Na is still far from being explained in commonly accepted terms, in spite of its benefit effect on CIGS PV efficiency.[10] In this work, we study the effect of the alkali addition (Na, K) in the CZTS, on the structural, optical, and electrical properties of CZTS thin films. For this investigation, we have deposited by ultrasonic spray CZTS thin films on glass and Mo-coated glass substrates using different amounts of the alkali in the sprayed solution. CZTS thin films are deposited onto glass and Mo-coated glass substrates by ultrasonic spray technique, followed by a sulfurization under argon atmosphere at 500 C during 1 h. We have investigated how the addition of the alkali can influence Cu2ZnSnS4 thin films properties. X-ray diffraction and Raman spectroscopy proved that CZTS has the kesterite structure with a preferential orientation along 112. The values of Cu/(ZnþSn) and (Zn/Sn) ratios, determined by energy dispersive spectroscopy, for undoped CZTS and alkali-doped CZTS are observed to be affected by the alkali doping. The electrical resistivity, for CZTS films deposited on glass substrates, decreased after alkali doping from 9.2 101 Ω cm for undoped CZTS to reach 1.8 101 and 4.2 101 Ω cm for 15% content of Na and K, respectively. 1. Introduction Cu2ZnSnS4 (CZTS) kesterite is a quaternary material which has attracted the attention of researches in the thin films photovoltaic field due to its optical and electrical properties characterized by a very high absorption coefficient, more than 104 cm1, and a direct optical gap with an optimal value of 1.5 eV. Dr. M. Sekkati, Dr. E. B. Benamar, Prof. M. Regragui, Prof. F. Cherkaoui El Moursli, Prof. Z. Edfouf, Prof. Z. Sekkat, Prof. M. Abd-Lefdil MANAPSE Faculty of Sciences University of Mohammed V-Rabat, Rabat 10000, Morocco E-mail: mouaad.sekkati@gmail.com 2. Experimental Section Prof. M. Taibi Ecole Normale Superieure, University of Mohammed V, LPCMIN, Rabat, Morocco Dr. G. Schmerber, Dr. C. Leuvrey, Prof. A. Dinia IPCMS, Universite de Strasbourg CNRS UMR 7504, 23 rue du Loess, B. P. 43, 67034 Strasbourg Cedex 2, France Dr. Z. Sekkat Optics and Photonics Center Moroccan Foundation for Advanced Science Innovation and Research (MAScIR), Rabat, Morocco Dr. A. Slaoui ICube, Universite de Strasbourg, CNRS UMR 7357, 23 rue du Loess, B.P. 20 CR, 67037 Strasbourg Cedex 2, France The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/pssc.201700170. DOI: 10.1002/pssc.201700170 Phys. Status Solidi C 2017, 1700170 The spray solution of the CZTS contained CuCl2, SnCl2, ZnCl2 SC(NH2)2, and NaCl or KCl with the ratio Cu/(ZnþSn) ¼ 1, Zn/ Sn ¼ 1, S/(CuþZnþSn) ¼ 2, and (Na, K)/Zn ¼ (0.05; 0.1; 0.15). All the precursors were dissolved in deionized water. The solution was sprayed with ultrasonic spray on glass and Mocoated glass substrates, cleaned as we reported in a previous paper,[7] and heated at 200 C. We choose this temperature to avoid oxidation and stabilization of the binary secondary oxide phases and to minimize the sulfur losses by evaporation during the spray process. The as-deposited samples were submitted to a sulfurization in a tubular furnace under argon and amount of sulfur S8 at 500 C during 1 h. The structural properties of the films were analyzed in the 10–80 range 2θ range by means of a Rigaku Smartlab (9 kW) diffractometer equipped with CuKa1 source (λ ¼ 1.54056 Å ) and a Ge (220) x2-bounce front monochromator. Raman 1700170 (1 of 4) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.pss-c.com Table 1. Chemical composition of CZTS thin films. Sample Cu/(ZnþSn) Zn/Sn S/metal CZTS 0.88 1.06 1.01 CZTS:Na5% 1.04 0.95 0.89 CZTS:Na10% 0.96 1.13 0.93 CZTS:Na15% 1.03 1.02 0.94 CZTS:K5% 0.96 1.20 0.94 CZTS:K10% 1.12 1.00 0.93 CZTS:K15% 0.87 1.42 0.99 spectroscopy was carried out at room temperature using a Horiba LabRam ARAMIS spectrometer equipped with a multichannel CCD detection system in the backscattering configuration. The incident laser wavelength was 532 nm. Absorbance and reflectance of the films were recorded in the wavelength ranging from 300 to 1200 nm using a Perkin-Elmer Lambda 950 spectrophotometer with an integrating sphere of Figure 1. Surface and cross-section images of CZTS thin films with different Na nominal doping concentrations (0, 5, 10, and 15%) and K nominal doping concentrations (5 and 15%). Figure 2. XRD diffractogram of CZTS thin films: (a) CZTS thin films with Na nominal concentration (5, 10, and 15%), (b) CZTS thin films with K nominal concentration (5, 10, and 15%). Phys. Status Solidi C 2017, 1700170 1700170 (2 of 4) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.pss-c.com Table 2. Structural parameters of CZTS thin films. Sample 2θ ( ) FWHM ( ) TC (112) D (nm) a (Å ) 0.001 Å c (Å ) 0.003 Å CZTS 28.53 0.178 1.57 45 5.415 10.82 CZTS:Na5% 28.54 0.150 1.90 55 5.412 10.802 CZTS:Na10% 28.51 0.211 1.72 39 5.418 10.811 CZTS:Na15% 28.53 0.193 1.76 43 5.415 10.825 CZTS:K5% 28.54 0.111 2.06 74 5.412 10.783 CZTS:K10% 28.54 0.178 1.99 47 5.413 10.815 CZTS:K15% 28.56 0.146 1.74 56 5.412 10.789 150 mm diameter. The surface morphology and the cross section were observed using a JEOL JSM-6700F scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) analyzer to determine the chemical composition of the films. The electrical resistivity, the carrier concentration, and the mobility were measured at room temperature by an ECOPIA Hall effect system (HMS 5500). Figure 3. Raman spectra of CZTS thin films: (a) CZTS films with Na nominal concentration (0, 5, 10, and 15%). b) CZTS thin films with K nominal concentration (0, 5, 10, and 15%). Phys. Status Solidi C 2017, 1700170 3. Results and Discussion Surface and cross-section images of CZTS thin films with different doping concentrations deposited onto Mo glass substrates are shown in Figure 1. The surface morphology of the samples presents the same aspect with agglomerated particles, as it is usually noted for samples prepared by spray process. The density of this agglomeration depends on the alkali doping content. The Figure 4. Optical properties of CZTS films: (a) absorbance of CZTS films with Na nominal concentration (5, 10, and 15%). b) Absorbance of CZTS films with K nominal concentration (5, 10, and 15%). 1700170 (3 of 4) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.pss-c.com 4. Conclusion Table 3. Electrical parameters of CZTS thin films at room temperature. ρ (101 Ω cm) μ (101 cm2 V1 s1) N (1019 cm3) CZTS 9.2 120.3 0.2 CZTS:Na5% 3.0 6.5 3.2 CZTS:Na10% 1.8 10.1 3.3 CZTS:Na15% 1.5 21.5 4.3 CZTS:K5% 8.3 7.9 0.3 CZTS:K10% 5.3 4.9 2.1 CZTS:K15% 4.2 4.2 3.4 Sample thickness of the CZTS film, estimated from cross-section view, was found to be between 1 and 3 μm, while the Mo thickness was around 600 nm. The chemical composition of the CZTS films obtained by EDS is illustrated in Table 1. The composition value was calculated, as an average of the compositions, obtained at several points distributed arbitrarily on the surface of the sample. The best compositions were obtained for CZTS: Na (10%) and CZTS: K (5%) where the films are copper-poor and zinc-rich, as it is desired for solar cells. Figure 2 exhibits the X-ray diffraction diffractograms of CZTS thin films. All the films have a polycrystalline structure of kesterite Cu2ZnSnS4 (JCPDS card No. 00-026-0575 or No. 04015-7542) with a preferential orientation according to (112) crystallographic plane. No binary or ternary secondary phases were observed in the detection limit of X-ray diffraction measurement. The Scherrer formula was used to determine the average crystallite size of our samples. We used the most important crystallographic orientation (112). For undoped CZTS, D was found around 45 nm (Table 2). After alkali doping, D reached 55 and 74 nm for CZTS:Na (5%) and CZTS:K (5%), respectively. For larger content, the crystallite size decreased, in agreement with Gershon et al.[11] results. However, the crystalline structures of ZnS and Cu2SnS3 compounds can not be distinguished from the kesterite CZTS by only the X-ray diffraction. Therefore, we use the Raman spectroscopy to better identify the crystal structure. The Raman spectra exhibited in Figure 3 show three peaks at 144, 287, and 338 cm1 corresponding to the vibration modes B/E and A, respectively, of kesterite CZTS structure for all the samples. The red shift of Raman peak obtained using Na and K doping is attributed to the tensile stress of the crystal lattice.[12] Absorbance spectra of CZTS films are shown in Figure 4. We note an increase in absorbance after increasing Na concentration. The same observation is also valid for K doping. Table 3 shows the electrical parameter values of undoped and alkali-doped CZTS deposited on glass substrates at room temperature. The best electrical resistivity values were 1.5 101 and 4.2 101 Ω cm for Na (15%) and K (15%) doping, respectively. Phys. Status Solidi C 2017, 1700170 XRD and Raman spectroscopy, for CZTS thin films, undoped and doped with alkali metals (Na, K), prepared by ultrasonic spray technique on Mo-coated glass substrate, showed that all the samples had the kesterite structure with a best improvement of the preferential orientation along 112 axis for 5% content for both metals. Alkali doping was benefit for CZTS crystallite size and also for its electrical resistivity. Acknowledgments The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 608593 (EUROSUNMED Project). Conflict of Interest The authors declare no conflict of interest. Keywords alkali, CZTS, sulfurization, ultrasonic spray Received: June 10, 2017 Published online: [1] W. Wang, M. T. Winkler, O. Gunawan, T. Gokmen, T. K. Todorov, Y. Zhu, D. B. Mitzi, Adv. Energy Mater. 2014, 4, 1301465-1-1301465-5. [2] H. S. Takuya Kato, H. Hiroi, N. Sakai, S. Muraoka, 27th European Photovoltaic Solar Energy Conference and Exhibition, 2009, pp. 2236–2239. [3] W. Shockley, H. J. Queisser, J. Appl. Phys. 1961, 32, 510. [4] S. M. Pawar, A. V. Moholkar, I. K. Kim, S. W. Shin, J. H. Moon, J. I. Rhee, J. H. Kim, Curr. Appl. Phys. 2010, 10, 565. [5] J. Seol, S. Lee, J. Lee, H. Nam, K. Kim, Sol. Energy Mater. Sol. Cells 2003, 75, 155. [6] K. Wang, O. Gunawan, T. Todorov, B. Shin, S. J. Chey, N. A. Bojarczuk, D. Mitzi, S. Guha, Appl. Phys. Lett. 2010, 97, 143508. [7] T. SlimaniTlemSc ani, E. B. Benamar, F. Cherkaoui El Moursli, F. Hajji, Z. Edfouf, M. Taibi, H. Labrim, B. Belhorma, S. Aazou, G. Schmerber, K. Bouras, Z. Sekkat, A. Dinia, A. Ulyashin, A. Slaoui, M. Abd-Lefdil, Energy Procedia 2015, 84, 127. [8] N. Kamoun, H. Bouzouita, B. Rezig, Thin Solid Films 2007, 515, 5949. [9] K. Tanaka, N. Moritake, H. Uchiki, Sol. Energy Mater. Sol. Cells 2007, 91, 1199. [10] Y. M. Shin, D. H. Shin, J. H. Kim, B. T. Ahn, Curr. Appl. Phys. 2011, 11, S59. [11] T. Gershon, B. Shin, N. Bojarczuk, M. Hopstaken, D. B. Mitzi, S. Guha, Adv. Energy Mater. 2015, 5, 1400849. [12] Z. Tong, C. Yan, Z. Su, F. Zeng, J. Yang, Y. Li, L. Jiang, Y. Lai, F. Liu, Appl. Phys. Lett. 2014, 105, 223903. 1700170 (4 of 4) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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