- Email: [email protected]
Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization
Materials Chemistry and Physics xxx (2015) 1e6
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization Sandra Dias*, Banavoth Murali, S.B. Krupanidhi Materials Research Centre, Indian Institute of Science, Bangalore, Karnataka, 560012, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Cu2SnS3 thin films have been synthesized by spin coating of a precursor solution. The CueSn-thiourea complex precursor was analysed. The structural, optical and electrical properties of the thin films were studied. Totally 24 infra-red, 30 optical, 29 Raman and 30 hyper Raman modes are active. Refractive index, extinction coefficient and relative permittivity were determined.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2014 Received in revised form 22 October 2015 Accepted 23 October 2015 Available online xxx
Thin films of Cu2SnS3 (CTS) were deposited by the facile solution processed solegel route followed by a low-temperature annealing. The CueSn-thiourea complex formation was analysed using Fourier Transform Infrared spectrophotometer (FTIR). The various phase transformations and the deposition temperature range for the initial precursor solution was determined using Thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC). X-Ray Diffraction (XRD) studies revealed the tetragonal phase formation of the CTS annealed films. Raman spectroscopy studies further confirmed the tetragonal phase formation and the absence of any deterioratory secondary phases. The morphological investigations and compositional analysis of the films were determined using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) respectively. Atomic Force Microscopy (AFM) was used to estimate the surface roughness of 1.3 nm. The absorption coefficient was found to be 104 cm1 and bandgap 1.3 eV which qualifies CTS to be a potential candidate for photovoltaic applications. The refractive index, extinction coefficient and relative permittivity of the film were measured by Spectroscopic ellipsometry. Hall effect measurements, indicated the p type nature of the films with a hole concentration of 2 1018 cm3, electrical conductivity of 9 S/cm and a hole mobility of 29 cm2/V. The properties of CTS as deduced from the current study, present CTS as a potential absorber layer material for thin film solar cells. © 2015 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Chemical synthesis Thin films Raman spectroscopy
1. Introduction * Corresponding author. E-mail address: [email protected] (S. Dias).
Although the progress of Cu(InGa)Se2 and CdTe solar cells over the last decade showed promising efficiencies of 21.7% and 21%
http://dx.doi.org/10.1016/j.matchemphys.2015.10.049 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Dias, et al., Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.10.049
2
S. Dias et al. / Materials Chemistry and Physics xxx (2015) 1e6
respectively [1,2], the pilot scale commercialization is limited due to the fact that In and Ga are expensive [3], Te being a scarcely available element [4] and Cd being environmentally toxic [5]. Hence most of the research in the recent years is focussed on alternate energy materials for thin film photovoltaics. In the group of Cu based chalcogenides, Cu2ZnSnSxSe4ex (CZTS) based absorbers have reached the photon conversion efficiency of 12.6% [6]. Note that CZTS consisting of earth abundant and non-toxic elements, suffers from the formation of secondary phases such as CTS and ZnS and hence controlling the growth of Cu2ZnSnSxSe4ex is difficult. CTS is a p-type ternary semiconductor, consisting of non-toxic, earth-abundant elements, with a tunable direct bandgap from 0.93 eV to 1.51 eV and a high absorption coefficient of 104 cm1 to 105 cm1 which are optimal for solar cell applications [7e10]. Its high conductivity of 0.5e10 Scm1, hole concentration of 1018 cm3 and a hole mobility of 1e80 cm2 V1s1 is an added advantage to improve the transport properties for photovoltaic applications [10,11]. Various methods have been used for the deposition of CTS films. Kuku and Fakolujo thermally evaporated CTS films and made the first solar cell device of CTS [9]. P.A. Fernandes et al. have deposited CTS films by sulphurizing sputtered stacked metal precursors at temperatures of 350 C and 400 C [8]. CTS thin films deposited by using physical vapour deposition techniques like thermal evaporation and sputtering involve expensive equipment [9]. Alternate chemical routes for deposition have also been studied. Q. Chen et al. fabricated a solar cell device by coating a paste of CTS powders in propylene glycol using doctor blade technique [12]. Avellaneda et al. synthesized CTS films by heating stacked layers of CuS and SnS deposited via chemical bath deposition, at 315 C and 350 C in nitrogen atmosphere [11]. Z. Su et al. synthesized CTS films by the successive ionic layer adsorption and reaction (SILAR) method. The CueSneS precursor film was sulphurized in a nitrogen and sulphur atmosphere at 400 C to yield cubic CTS films [13]. Adelifard et al. deposited triclinic CTS films via spray pyrolysis technique at a temperature of 285 C for different Sn/Cu ratios (0.0e1.0) [14]. Note that the above techniques involve several steps, high-temperature annealing procedure and few approaches may also lead to the formation of secondary phases like binary sulphides of Cu and Sn. Therefore, a single step CTS synthesis is essential for affordable photovoltaics. Herein we demonstrate a simple modified method of directly spin coating the precursor solution of CTS on a substrate followed by a low-temperature heat treatment. Such a method not only ensures the complete phase formation but is also applicable for large area depositions.
2.2. Characterization The various phase transformations and deposition temperature of the precursor solution was determined using TGA and DSC (SDT Q600 V8.3) under a nitrogen gas flow and 5 C/min heating rate. The precursor gel for the analysis was prepared by dropcasting the precursor solution onto a watch glass and heating it at 120 C for 10 min followed by overnight drying at room temperature. The CueSn-thiourea complex precursor was analysed using FTIR, (Perkin Elmer-Spectrum BX). The annealed CTS films were further characterized. The phase formation of the films was determined using XRD (PANalytical X'Pert PRO Diffractometer) equipped with a Cu Ka1 source (l ¼ 1.540598 Å). Raman spectroscopy measurement was taken using Raman spectrometer (LabRAM HR) with 514 nm line of Arþ laser. The laser was focussed by a 100x objective lens and the laser power and acquisition time used were 1 mW and 30 s respectively. The structure and morphology of the films as well as the composition were determined from SEM (ULTRA 55, FESEM (Carl Zeiss)) and EDS (Oxford X-Max 50 mm2) respectively. The surface morphology of the films and roughness were determined using non contact mode AFM (A.P.E. Research A100-AFM). The thickness of the deposited films was measured using Veeco Dektak 6M surface profilometer. The absorption spectra of the films were obtained using UVeViseNIR spectrophotometer (Perkin ElmerLambda 750). Spectroscopic ellipsometry measurements were carried out on CTS films on glass substrates at room temperature in the 245.02 nme998.49 nm range using a variable angle of incidence spectroscopic ellipsometer (J.A.Woolam Co., Inc. M-2000U) at an angle of incidence of 65 . The WVASE 32 software was used for computing the optical constants. Hall effect measurements were conducted at room temperature and 0.5 T magnetic field using Ecopia HMS 5000 Hall effect measurement system. Gold dots were thermally evaporated at the corners of 1 cm 1 cm samples and measurements were taken in the Van der Pauw geometry. 3. Results and discussions 3.1. Analysis of the CueSn-thiourea complex precursor To
explain
the
mechanism
of
metal-thiourea
complex
2. Experimental 2.1. Synthesis The CTS precursor solution was prepared by dissolving CuCl2 (1M, 99.999% from Sigma Aldrich), SnCl2 (0.5M, 99.99% from Sigma Aldrich) and thiourea (3M, 99.0% from Sigma Aldrich) in anhydrous 2-methoxyethanol (5 ml, 99.8% from Sigma Aldrich). Thiourea was taken in excess to prevent formation of secondary phases and to compensate for the sulphur loss during annealing. Addition of SnCl2 to anhydrous 2-methoxyethanol gave a clear solution. On adding CuCl2, the solution became white in colour. Finally addition of thiourea gave a yellow and transparent solution. The solution was allowed to stir for 1 h. Then the solution was filtered using an Acrodisc CR 13 PTFE 0.2 mm syringe filter and spin coated onto soda lime glass (SLG) substrates at 2500 rpm for 30s and then dried on a hot plate at 135 C for 10 min. The films were then annealed in air at 250 C in a tube furnace for 1 h at a ramp rate of 14.6 C/min.
Fig. 1. FTIR spectra of (a) CueSn-thiourea complex precursor. (b) thiourea.
Please cite this article in press as: S. Dias, et al., Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.10.049
S. Dias et al. / Materials Chemistry and Physics xxx (2015) 1e6
3
formation, the CueSn-thiourea complex precursor and thiourea were analysed using FTIR. Fig. 1 shows the FTIR spectra of thiourea and the CueSn-thiourea complex precursor. The peak at 730 cm1 in the thiourea spectrum assigned to C]S stretching shifts to 700 cm1 in the CueSn-thiourea complex spectrum. This shift to lower wavenumber is ascribed to the bonding of the Cu2þ and Sn2þ ions to the S atom of thiourea aided by the electron transfer from C to S atom of thiourea. Due to the transfer of electron charge from N to C atom in thiourea the CeN bond is transformed to C]N. As a result the frequencies 1470 cm1 and 1608 cm1in the thiourea spectrum shift to higher values of 1490 cm1 and 1620 cm1 respectively in the CueSn-thiourea complex spectrum which also proves that the metal ions couple with thiourea via S atom. These frequencies correspond to the CeN stretching and NH2 bending coupled vibrations. The electron transfer from N to C creates a partial positive charge on N atom. Thus the NeH stretching frequencies 3166 cm1(symmetric) and 3280 cm1 (asymmetric) in thiourea spectrum shift to higher values of 3185 cm1and 3300 cm1 in CueSn-thiourea complex spectrum [10,15]. Fig. 2. Thermal analysis (a) TGA (b) DSC of the CTS precursor.
3.1.1. Cu2SnS3 formation mechanism The dissolved metal chlorides and thiourea in anhydrous 2methoxyethanol form a metal-thiourea complex [CuSn(CS(NH2)n)]mþ via reaction between the metal ions and thiourea. The metal ions couple with thiourea via the sulphur atom. During heat treatment the CeS bond in the CueSn-thiourea complex precursor gets cut and the metal sulphide i.e. Cu2SnS3 is released [10,15,17]. The reaction can be represented as: Hydrolysis of precursors in presence of anhydrous 2methoxyethanol: CuCl2 / Cu2þ þ 2Cl
(1)
SnCl2 / Sn2þ þ 2Cl
(2)
(3)
The exothermic peak at 260 C indicates the crystallization of the CTS film. The TGA-DSC studies show that the CTS films can be deposited at any temperature from 227 C to 260 C. Deposition at temperatures above 260 C would lead to the formation of oxides and hence is avoided in the present case and a lower temperature of 250 C is chosen for the deposition. 3.2. Structural analysis Fig. 3 shows the XRD pattern of the CTS films deposited at a temperature of 250 C. The XRD peaks at 28.5 , 33 , 47.6 and 56.4 correspond to the tetragonal phase of CTS and match well with the JCPDS 01-089-4714. The obtained lattice parameters were a ¼ 5.4187 Å and c ¼ 10.8404 Å. The average crystallite size L was found to be 4.8 nm using the Scherrer formula.
L ¼ 0:9l=b cos q The ions react to form a complex [CuSn(CS(NH2)n)] upon thermolysis yields Cu2SnS3. The overall reaction can be represented as:
mþ
which
(6)
where l is the Cu Ka X-Ray wavelength ¼ 1.5405 Å, b is the full
CuCl2 þSnCl2 mþ anhydrous2methoxyethanol þCSðNH2 Þ2 ! CuSn CSðNH2 Þn
CuSn CSðNH2 Þn
mþ
(4)
Heat treatment
! Cu2 SnS3 þ gaseous products (5)
Thermal analysis using TGA and DSC were conducted in order to examine the various phase transformations that occur starting from the deposition of the CueSn-thiourea complex precursor until the formation of the CTS film. Also the optimum temperature range for the deposition of the CTS films was also deduced. Fig. 2 shows the TGA-DSC curves for the CTS precursor. From the TGA plot in Fig. 2. (a), it can be seen that the first weight loss of 38% occurs from 177 C to 250 C. This can be attributed to the decomposition of the metal-thiourea complex to form CTS. The second weight loss of 17% starting from 267 C and ending at 493 C can be due to the oxidation of the sulphides [15]. From the DSC curve Fig. 2. (b), the first endothermic peak at 170 C is due to the decomposition of thiourea [15]. The next endothermic peak at 227 C corresponds to the formation of CTS by thermolysis of the metal-thiourea complex.
Fig. 3. X-ray diffraction pattern of the CTS thin film on SLG.
Please cite this article in press as: S. Dias, et al., Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.10.049
4
S. Dias et al. / Materials Chemistry and Physics xxx (2015) 1e6
width at half maximum and q is the angle of diffraction. Fig. 4 shows the Raman spectrum of the CTS films deposited at a temperature of 250 C. The peaks are obtained at 297 cm1, 336 cm1 and 351 cm1 which correspond to the tetragonal phase of CTS [16]. Absence of any other peaks implies the absence of secondary phases like Cu2-xS (475 cm1) and SnS (230 cm1 and 330 cm1). CTS belongs to the tetragonal crystal system having space group I-42m (No. 121), point group D2d (42m) and possesses one formula unit per unit cell. Totally 36 vibrational modes are present in this system out of which there are 24 infra-red active modes, 3 acoustic modes, 30 optical modes, 30 hyper Raman active modes and 29 Raman modes. The mechanical representation can be given by M ¼ 2A1 þ A2 þ 3B1 þ 7B2 þ 10E. The acoustic and optic modes are given as Gacoustic ¼ B2 þ E, Goptic ¼ 2A1 þ A2 þ 3B1 þ 6B2 þ 9E respectively. The infra-red (IR) and Raman active and hyper Raman (HR) active modes excluding the acoustic modes are given by GIR ¼ 6B2 þ 9E, GRaman ¼ 2A1 þ 3B1 þ 6B2 þ 9E, GHR ¼ 2A1 þ A2 þ 3B1 þ 6B2 þ 9E respectively. Elemental oxidation states and Wyckoff symbols are tabulated below. The irreducible representations of each element are as follows:
GSn1 ¼ 1B1 þ 1B2 þ 2E, GSn2 ¼ 1B2 þ 1E,
3.3. Morphology The morphology of the CTS thin film deposited at 250 C was observed using SEM and AFM. Fig. 5 shows the SEM image of the CTS film. The film was found to be uniform and homogeneous with the presence of densely packed spherical grains. Table 2 shows the elemental composition of the films obtained using EDS. The elemental ratios were found to be Cu/Sn ¼ 2.27 and S/ (Cu þ Sn) ¼ 0.85. Fig. 6 shows the 2D and 3D AFM micrographs of the CTS films. The image shows homogeneous distribution of grains. The rms roughness was around 1.3 nm indicating formation of smooth films. From surface profilometry measurements the thickness of one layer of the deposited film was found to be 0.6 mm. 3.4. Optical properties Optical properties are of prime importance for a photovoltaic or any optoelectronic material. Fig. 7(a) shows the absorption coefficient a versus the photon energy E. The absorption coefficient has a high value of 104 cm1. The bandgap Eg was calculated from the plot of (ahn)2 versus the photon energy E as shown in Fig. 6(b), using the Tauc's relation [19],
aðlÞ ¼
GCu1 ¼ 1B1 þ 1B2 þ 2E, GCu2 ¼ 1B2 þ 1E, GCu3 ¼ 1B2 þ 1E GS ¼ 2A1 þ 1A2 þ 1B1 þ 2B2 þ 3E. A2 mode is both Raman and IR inactive whereas it is HyperRaman active. The modes were assigned using Bilbao crystallographic server [18] (see Table 1).
Fig. 4. Raman spectrum of the CTS film.
A hv Eg hv
n (7)
where A is a constant and n ¼ 1/2 for direct bandgap. By extrapolating the linear part of the plot to cut the energy axis, the bandgap is found to be 1.3 eV. The high absorption coefficient of 104 cm1 and a direct, near infrared, optimum bandgap of 1.3 eV enable the maximum absorption of the solar spectrum. Hence CTS serves as a potential absorber material for thin film solar cells [8,10]. Spectroscopic ellipsometry is a non-destructive technique for measuring the optical constants. The optical constants viz. the refractive index n, extinction coefficient k and relative permittivity εr were obtained using the WVASE 32 software interfaced with the spectroscopic ellipsometer. The values were obtained by fitting the experimental data with the air/CTS/SLG three layers structural model and Lorentz Oscillator model. Fig. 8(aec) show the variation of the refractive index, extinction coefficient and relative permittivity of the CTS film versus wavelength respectively. The refractive
Fig. 5. Scanning electron micrograph of the CTS thin film showing the surface morphology.
Please cite this article in press as: S. Dias, et al., Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.10.049
S. Dias et al. / Materials Chemistry and Physics xxx (2015) 1e6 Table 1 Crystal structure data of CTS. Element
Labels
Oxidation states
Wyckoff symbols
Cu Cu Cu S Sn Sn
1 2 3 1 2 1
þ1.00 þ1.00 þ1.00 2.00 þ4.00 þ4.00
4d 2b 2a 8i 2b 4d
5
mobility are beneficial for the faster charge transport to the electrodes, thus preventing recombination of the charge carriers and in improving the efficiency of the CTS solar cell.
4. Conclusions
Table 2 Elemental Composition of the CTS films. Element
Weight%
Atomic%
SK Cu K Sn L Totals
25.49 40.94 33.57 100.00
46.16 37.41 16.43
index varied from 1.7 to 2.3, the extinction coefficient varied from 0.1 to 0.76 and the relative permittivity varied from 2.9 to 5.4 over a wavelength range of 245.02 nme998.49 nm. 3.5. Electrical properties The Hall effect measurements conducted on the CTS film, revealed the p type nature of the CTS films with a hole concentration of 2 1018 cm3, electrical conductivity of 9 S/cm and a hole mobility of 29 cm2/V. The high electrical conductivity and hole
Thin films of CTS were synthesized by the solution processible route of spin coating of the CTS precursor solution followed by a low-temperature annealing. The mechanism of formation of the CueSn-thiourea complex was deduced using FTIR. The phase transformations and deposition temperature were determined by thermal analysis of the CTS precursor. The annealed CTS films were further characterized. The phase formation and tetragonal crystal structure was inferred from XRD. The formation of tetragonal phase of CTS and the absence of secondary phases was confirmed using Raman spectroscopy. The films were found to be homogeneous and stoichiometric. Using AFM the surface roughness was found to be 1.3 nm. The thickness of the deposited films was measured to be 0.6 mm. The films were found to possess desirable optical properties for an absorber layer such as a high absorption coefficient of 104 cm1 and a bandgap of 1.3 eV. The refractive index, extinction coefficient and relative permittivity values of the film were determined by spectroscopic ellipsometry. The films were found to be p type with a hole concentration of 2 1018 cm3, electrical conductivity of 9 S/cm and a hole mobility of 29 cm2/V. The superior properties of CTS evaluated from the above characterization techniques corroborate the application of CTS as a potential absorber in solar cells and other optoelectronic devices.
Fig. 6. Atomic force microscope images showing (a) 2-Dimensional surface morphology and (b) 3-Dimensional topography of the CTS films.
Fig. 7. Optical properties of the CTS films (a) Absorption coefficient a versus photon energy E (b) Tauc plot of (ahn)2 versus E for the CTS film.
Please cite this article in press as: S. Dias, et al., Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.10.049
6
S. Dias et al. / Materials Chemistry and Physics xxx (2015) 1e6
Fig. 8. (a) Refractive index n (b) Extinction coefficient k (c) Relative Permittivity εr of the CTS film obtained from spectroscopic ellipsometry measurements.
References [1] P. Jackson, D. Hariskos, R. Wuerz, O. Kiowski, A. Bauer, T.M. Friedlmeier, M. Powalla, Properties of Cu(In,Ga)Se2 solar cells with new record efficiencies up to 21.7%, Phys. Status Solidi Rapid Res. Lett. 9 (1) (2015) 28e31. [2] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (Version 45), Prog. Photovoltaics Res. Appl. 23 (1) (2015) 1e9. [3] G. Phipps, C. Mikolajczak, T. Guckes, Indium and Gallium: long-term supply, Renew. Energy Focus 9 (2008) 56e59. [4] B.L. Cohen, Anomalous behavior of tellurium abundances, Geochim. Cosmochim. Acta 48 (1984) 203e205. [5] P. Das, S. Samantaray, G.R. Rout, Studies on cadmium toxicity in plants: a review, Environ. Pollut. 98 (1997) 29e36. [6] W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, D.B. Mitzi, Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency, Adv. Energy Mater. 4 (7) (2014) 1301465. [7] D.M. Berg, R. Djemour, L. Gütay, G. Zoppi, S. Siebentritt, P.J. Dale, Thin film solar cells based on the ternary compound Cu2SnS3, Thin Solid Films 520 (2012) 6291e6294. , A.F. da Cunha, A study of ternary Cu2SnS3 and [8] P.A. Fernandes, P.M.P. Salome Cu3SnS4 thin films prepared by sulfurizing stacked metal precursors, J. Phys. D. Appl. Phys. 43 (2010) 215403. [9] T. Kuku, O. Fakolujo, Photovoltaic characteristics of thin films of Cu2SnS3, Sol. Energy Mater 16 (1987) 199e204. [10] D. Tiwari, T.K. Chaudhuri, T. Shripathi, U. Deshpande, R. Rawat, Non-toxic, earth-abundant 2% efficient Cu2SnS3 solar cell based on tetragonal films direct-coated from single metal-organic precursor solution, Sol. Energy Mater.
Sol. Cells 113 (2013) 165e170. [11] D. Avellaneda, M.T.S. Nair, P.K. Nair, Cu2SnS3 and Cu4SnS4 thin films via chemical deposition for photovoltaic application, J. Electrochem. Soc. 157 (6) (2010) D346eD352. [12] Q. Chen, X. Dou, Y. Ni, S. Cheng, S. Zhuang, Study and enhance the photovoltaic properties of narrow-band gap Cu2SnS3 solar cell by pen junction interface modification, J. Colloid Interface Sci. 376 (2012) 327e330. [13] Z. Su, K. Sun, Z. Han, F. Liu, Y. Lai, J. Li, Y. Liu, Fabrication of ternary Cu-Sn-S sulfides by a modified successive ionic layer adsorption and reaction (SILAR) method, J. Mater. Chem. 22 (2012) 16346e16352. [14] M. Adelifard, M.M.B. Mohagheghi, H. Eshghi, Preparation and characterization of Cu2SnS3 ternary semiconductor nanostructures via the spray pyrolysis technique for photovoltaic applications, Phys. Scr. 85 (2012) 035603e035608. [15] T.K. Chaudhuri, D. Tiwari, Earth-abundant non-toxic Cu2ZnSnS4 thin films by direct liquid coating from metalethiourea precursor solution, Sol. Energy Mater. Sol. Cells 101 (2012) 46e50. [16] H. Guan, H. Shen, C. Gao, X. He, Structural and optical properties of Cu2SnS3 and Cu3SnS4 thin films by successive ionic layer adsorption and reaction, J. Mater Sci. Mater Electron 24 (5) (2013) 1490e1494. [17] G. Rajesh, N. Muthukumarasamy, E.P. Subramaniam, S. Agilan, Dhayalan Velauthapillai, Synthesis of Cu2ZnSnS4 thin films by dip-coating method without sulphurization, J. Sol-Gel Sci. Technol. 66 (2013) 288e292. [18] M.I. Aroyo, J.M. Perez-Mato, D. Orobengoa, E. Tasci, G. de la Flor, A. Kirov, Crystallography online: bilbao crystallographic server, Bulg. Chem. Commun. 43 (2) (2011) 183e197. [19] M. Banavoth, S. Dias, S.B. Krupanidhi, Near-infrared photoactive Cu2ZnSnS4 thin films by co-sputtering, AIP Adv. 3 (8) (2013) 082132.
Please cite this article in press as: S. Dias, et al., Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.10.049
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization Sandra Dias*, Banavoth Murali, S.B. Krupanidhi Materials Research Centre, Indian Institute of Science, Bangalore, Karnataka, 560012, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Cu2SnS3 thin films have been synthesized by spin coating of a precursor solution. The CueSn-thiourea complex precursor was analysed. The structural, optical and electrical properties of the thin films were studied. Totally 24 infra-red, 30 optical, 29 Raman and 30 hyper Raman modes are active. Refractive index, extinction coefficient and relative permittivity were determined.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2014 Received in revised form 22 October 2015 Accepted 23 October 2015 Available online xxx
Thin films of Cu2SnS3 (CTS) were deposited by the facile solution processed solegel route followed by a low-temperature annealing. The CueSn-thiourea complex formation was analysed using Fourier Transform Infrared spectrophotometer (FTIR). The various phase transformations and the deposition temperature range for the initial precursor solution was determined using Thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC). X-Ray Diffraction (XRD) studies revealed the tetragonal phase formation of the CTS annealed films. Raman spectroscopy studies further confirmed the tetragonal phase formation and the absence of any deterioratory secondary phases. The morphological investigations and compositional analysis of the films were determined using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) respectively. Atomic Force Microscopy (AFM) was used to estimate the surface roughness of 1.3 nm. The absorption coefficient was found to be 104 cm1 and bandgap 1.3 eV which qualifies CTS to be a potential candidate for photovoltaic applications. The refractive index, extinction coefficient and relative permittivity of the film were measured by Spectroscopic ellipsometry. Hall effect measurements, indicated the p type nature of the films with a hole concentration of 2 1018 cm3, electrical conductivity of 9 S/cm and a hole mobility of 29 cm2/V. The properties of CTS as deduced from the current study, present CTS as a potential absorber layer material for thin film solar cells. © 2015 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Chemical synthesis Thin films Raman spectroscopy
1. Introduction * Corresponding author. E-mail address: [email protected] (S. Dias).
Although the progress of Cu(InGa)Se2 and CdTe solar cells over the last decade showed promising efficiencies of 21.7% and 21%
http://dx.doi.org/10.1016/j.matchemphys.2015.10.049 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Dias, et al., Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.10.049
2
S. Dias et al. / Materials Chemistry and Physics xxx (2015) 1e6
respectively [1,2], the pilot scale commercialization is limited due to the fact that In and Ga are expensive [3], Te being a scarcely available element [4] and Cd being environmentally toxic [5]. Hence most of the research in the recent years is focussed on alternate energy materials for thin film photovoltaics. In the group of Cu based chalcogenides, Cu2ZnSnSxSe4ex (CZTS) based absorbers have reached the photon conversion efficiency of 12.6% [6]. Note that CZTS consisting of earth abundant and non-toxic elements, suffers from the formation of secondary phases such as CTS and ZnS and hence controlling the growth of Cu2ZnSnSxSe4ex is difficult. CTS is a p-type ternary semiconductor, consisting of non-toxic, earth-abundant elements, with a tunable direct bandgap from 0.93 eV to 1.51 eV and a high absorption coefficient of 104 cm1 to 105 cm1 which are optimal for solar cell applications [7e10]. Its high conductivity of 0.5e10 Scm1, hole concentration of 1018 cm3 and a hole mobility of 1e80 cm2 V1s1 is an added advantage to improve the transport properties for photovoltaic applications [10,11]. Various methods have been used for the deposition of CTS films. Kuku and Fakolujo thermally evaporated CTS films and made the first solar cell device of CTS [9]. P.A. Fernandes et al. have deposited CTS films by sulphurizing sputtered stacked metal precursors at temperatures of 350 C and 400 C [8]. CTS thin films deposited by using physical vapour deposition techniques like thermal evaporation and sputtering involve expensive equipment [9]. Alternate chemical routes for deposition have also been studied. Q. Chen et al. fabricated a solar cell device by coating a paste of CTS powders in propylene glycol using doctor blade technique [12]. Avellaneda et al. synthesized CTS films by heating stacked layers of CuS and SnS deposited via chemical bath deposition, at 315 C and 350 C in nitrogen atmosphere [11]. Z. Su et al. synthesized CTS films by the successive ionic layer adsorption and reaction (SILAR) method. The CueSneS precursor film was sulphurized in a nitrogen and sulphur atmosphere at 400 C to yield cubic CTS films [13]. Adelifard et al. deposited triclinic CTS films via spray pyrolysis technique at a temperature of 285 C for different Sn/Cu ratios (0.0e1.0) [14]. Note that the above techniques involve several steps, high-temperature annealing procedure and few approaches may also lead to the formation of secondary phases like binary sulphides of Cu and Sn. Therefore, a single step CTS synthesis is essential for affordable photovoltaics. Herein we demonstrate a simple modified method of directly spin coating the precursor solution of CTS on a substrate followed by a low-temperature heat treatment. Such a method not only ensures the complete phase formation but is also applicable for large area depositions.
2.2. Characterization The various phase transformations and deposition temperature of the precursor solution was determined using TGA and DSC (SDT Q600 V8.3) under a nitrogen gas flow and 5 C/min heating rate. The precursor gel for the analysis was prepared by dropcasting the precursor solution onto a watch glass and heating it at 120 C for 10 min followed by overnight drying at room temperature. The CueSn-thiourea complex precursor was analysed using FTIR, (Perkin Elmer-Spectrum BX). The annealed CTS films were further characterized. The phase formation of the films was determined using XRD (PANalytical X'Pert PRO Diffractometer) equipped with a Cu Ka1 source (l ¼ 1.540598 Å). Raman spectroscopy measurement was taken using Raman spectrometer (LabRAM HR) with 514 nm line of Arþ laser. The laser was focussed by a 100x objective lens and the laser power and acquisition time used were 1 mW and 30 s respectively. The structure and morphology of the films as well as the composition were determined from SEM (ULTRA 55, FESEM (Carl Zeiss)) and EDS (Oxford X-Max 50 mm2) respectively. The surface morphology of the films and roughness were determined using non contact mode AFM (A.P.E. Research A100-AFM). The thickness of the deposited films was measured using Veeco Dektak 6M surface profilometer. The absorption spectra of the films were obtained using UVeViseNIR spectrophotometer (Perkin ElmerLambda 750). Spectroscopic ellipsometry measurements were carried out on CTS films on glass substrates at room temperature in the 245.02 nme998.49 nm range using a variable angle of incidence spectroscopic ellipsometer (J.A.Woolam Co., Inc. M-2000U) at an angle of incidence of 65 . The WVASE 32 software was used for computing the optical constants. Hall effect measurements were conducted at room temperature and 0.5 T magnetic field using Ecopia HMS 5000 Hall effect measurement system. Gold dots were thermally evaporated at the corners of 1 cm 1 cm samples and measurements were taken in the Van der Pauw geometry. 3. Results and discussions 3.1. Analysis of the CueSn-thiourea complex precursor To
explain
the
mechanism
of
metal-thiourea
complex
2. Experimental 2.1. Synthesis The CTS precursor solution was prepared by dissolving CuCl2 (1M, 99.999% from Sigma Aldrich), SnCl2 (0.5M, 99.99% from Sigma Aldrich) and thiourea (3M, 99.0% from Sigma Aldrich) in anhydrous 2-methoxyethanol (5 ml, 99.8% from Sigma Aldrich). Thiourea was taken in excess to prevent formation of secondary phases and to compensate for the sulphur loss during annealing. Addition of SnCl2 to anhydrous 2-methoxyethanol gave a clear solution. On adding CuCl2, the solution became white in colour. Finally addition of thiourea gave a yellow and transparent solution. The solution was allowed to stir for 1 h. Then the solution was filtered using an Acrodisc CR 13 PTFE 0.2 mm syringe filter and spin coated onto soda lime glass (SLG) substrates at 2500 rpm for 30s and then dried on a hot plate at 135 C for 10 min. The films were then annealed in air at 250 C in a tube furnace for 1 h at a ramp rate of 14.6 C/min.
Fig. 1. FTIR spectra of (a) CueSn-thiourea complex precursor. (b) thiourea.
Please cite this article in press as: S. Dias, et al., Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.10.049
S. Dias et al. / Materials Chemistry and Physics xxx (2015) 1e6
3
formation, the CueSn-thiourea complex precursor and thiourea were analysed using FTIR. Fig. 1 shows the FTIR spectra of thiourea and the CueSn-thiourea complex precursor. The peak at 730 cm1 in the thiourea spectrum assigned to C]S stretching shifts to 700 cm1 in the CueSn-thiourea complex spectrum. This shift to lower wavenumber is ascribed to the bonding of the Cu2þ and Sn2þ ions to the S atom of thiourea aided by the electron transfer from C to S atom of thiourea. Due to the transfer of electron charge from N to C atom in thiourea the CeN bond is transformed to C]N. As a result the frequencies 1470 cm1 and 1608 cm1in the thiourea spectrum shift to higher values of 1490 cm1 and 1620 cm1 respectively in the CueSn-thiourea complex spectrum which also proves that the metal ions couple with thiourea via S atom. These frequencies correspond to the CeN stretching and NH2 bending coupled vibrations. The electron transfer from N to C creates a partial positive charge on N atom. Thus the NeH stretching frequencies 3166 cm1(symmetric) and 3280 cm1 (asymmetric) in thiourea spectrum shift to higher values of 3185 cm1and 3300 cm1 in CueSn-thiourea complex spectrum [10,15]. Fig. 2. Thermal analysis (a) TGA (b) DSC of the CTS precursor.
3.1.1. Cu2SnS3 formation mechanism The dissolved metal chlorides and thiourea in anhydrous 2methoxyethanol form a metal-thiourea complex [CuSn(CS(NH2)n)]mþ via reaction between the metal ions and thiourea. The metal ions couple with thiourea via the sulphur atom. During heat treatment the CeS bond in the CueSn-thiourea complex precursor gets cut and the metal sulphide i.e. Cu2SnS3 is released [10,15,17]. The reaction can be represented as: Hydrolysis of precursors in presence of anhydrous 2methoxyethanol: CuCl2 / Cu2þ þ 2Cl
(1)
SnCl2 / Sn2þ þ 2Cl
(2)
(3)
The exothermic peak at 260 C indicates the crystallization of the CTS film. The TGA-DSC studies show that the CTS films can be deposited at any temperature from 227 C to 260 C. Deposition at temperatures above 260 C would lead to the formation of oxides and hence is avoided in the present case and a lower temperature of 250 C is chosen for the deposition. 3.2. Structural analysis Fig. 3 shows the XRD pattern of the CTS films deposited at a temperature of 250 C. The XRD peaks at 28.5 , 33 , 47.6 and 56.4 correspond to the tetragonal phase of CTS and match well with the JCPDS 01-089-4714. The obtained lattice parameters were a ¼ 5.4187 Å and c ¼ 10.8404 Å. The average crystallite size L was found to be 4.8 nm using the Scherrer formula.
L ¼ 0:9l=b cos q The ions react to form a complex [CuSn(CS(NH2)n)] upon thermolysis yields Cu2SnS3. The overall reaction can be represented as:
mþ
which
(6)
where l is the Cu Ka X-Ray wavelength ¼ 1.5405 Å, b is the full
CuCl2 þSnCl2 mþ anhydrous2methoxyethanol þCSðNH2 Þ2 ! CuSn CSðNH2 Þn
CuSn CSðNH2 Þn
mþ
(4)
Heat treatment
! Cu2 SnS3 þ gaseous products (5)
Thermal analysis using TGA and DSC were conducted in order to examine the various phase transformations that occur starting from the deposition of the CueSn-thiourea complex precursor until the formation of the CTS film. Also the optimum temperature range for the deposition of the CTS films was also deduced. Fig. 2 shows the TGA-DSC curves for the CTS precursor. From the TGA plot in Fig. 2. (a), it can be seen that the first weight loss of 38% occurs from 177 C to 250 C. This can be attributed to the decomposition of the metal-thiourea complex to form CTS. The second weight loss of 17% starting from 267 C and ending at 493 C can be due to the oxidation of the sulphides [15]. From the DSC curve Fig. 2. (b), the first endothermic peak at 170 C is due to the decomposition of thiourea [15]. The next endothermic peak at 227 C corresponds to the formation of CTS by thermolysis of the metal-thiourea complex.
Fig. 3. X-ray diffraction pattern of the CTS thin film on SLG.
Please cite this article in press as: S. Dias, et al., Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.10.049
4
S. Dias et al. / Materials Chemistry and Physics xxx (2015) 1e6
width at half maximum and q is the angle of diffraction. Fig. 4 shows the Raman spectrum of the CTS films deposited at a temperature of 250 C. The peaks are obtained at 297 cm1, 336 cm1 and 351 cm1 which correspond to the tetragonal phase of CTS [16]. Absence of any other peaks implies the absence of secondary phases like Cu2-xS (475 cm1) and SnS (230 cm1 and 330 cm1). CTS belongs to the tetragonal crystal system having space group I-42m (No. 121), point group D2d (42m) and possesses one formula unit per unit cell. Totally 36 vibrational modes are present in this system out of which there are 24 infra-red active modes, 3 acoustic modes, 30 optical modes, 30 hyper Raman active modes and 29 Raman modes. The mechanical representation can be given by M ¼ 2A1 þ A2 þ 3B1 þ 7B2 þ 10E. The acoustic and optic modes are given as Gacoustic ¼ B2 þ E, Goptic ¼ 2A1 þ A2 þ 3B1 þ 6B2 þ 9E respectively. The infra-red (IR) and Raman active and hyper Raman (HR) active modes excluding the acoustic modes are given by GIR ¼ 6B2 þ 9E, GRaman ¼ 2A1 þ 3B1 þ 6B2 þ 9E, GHR ¼ 2A1 þ A2 þ 3B1 þ 6B2 þ 9E respectively. Elemental oxidation states and Wyckoff symbols are tabulated below. The irreducible representations of each element are as follows:
GSn1 ¼ 1B1 þ 1B2 þ 2E, GSn2 ¼ 1B2 þ 1E,
3.3. Morphology The morphology of the CTS thin film deposited at 250 C was observed using SEM and AFM. Fig. 5 shows the SEM image of the CTS film. The film was found to be uniform and homogeneous with the presence of densely packed spherical grains. Table 2 shows the elemental composition of the films obtained using EDS. The elemental ratios were found to be Cu/Sn ¼ 2.27 and S/ (Cu þ Sn) ¼ 0.85. Fig. 6 shows the 2D and 3D AFM micrographs of the CTS films. The image shows homogeneous distribution of grains. The rms roughness was around 1.3 nm indicating formation of smooth films. From surface profilometry measurements the thickness of one layer of the deposited film was found to be 0.6 mm. 3.4. Optical properties Optical properties are of prime importance for a photovoltaic or any optoelectronic material. Fig. 7(a) shows the absorption coefficient a versus the photon energy E. The absorption coefficient has a high value of 104 cm1. The bandgap Eg was calculated from the plot of (ahn)2 versus the photon energy E as shown in Fig. 6(b), using the Tauc's relation [19],
aðlÞ ¼
GCu1 ¼ 1B1 þ 1B2 þ 2E, GCu2 ¼ 1B2 þ 1E, GCu3 ¼ 1B2 þ 1E GS ¼ 2A1 þ 1A2 þ 1B1 þ 2B2 þ 3E. A2 mode is both Raman and IR inactive whereas it is HyperRaman active. The modes were assigned using Bilbao crystallographic server [18] (see Table 1).
Fig. 4. Raman spectrum of the CTS film.
A hv Eg hv
n (7)
where A is a constant and n ¼ 1/2 for direct bandgap. By extrapolating the linear part of the plot to cut the energy axis, the bandgap is found to be 1.3 eV. The high absorption coefficient of 104 cm1 and a direct, near infrared, optimum bandgap of 1.3 eV enable the maximum absorption of the solar spectrum. Hence CTS serves as a potential absorber material for thin film solar cells [8,10]. Spectroscopic ellipsometry is a non-destructive technique for measuring the optical constants. The optical constants viz. the refractive index n, extinction coefficient k and relative permittivity εr were obtained using the WVASE 32 software interfaced with the spectroscopic ellipsometer. The values were obtained by fitting the experimental data with the air/CTS/SLG three layers structural model and Lorentz Oscillator model. Fig. 8(aec) show the variation of the refractive index, extinction coefficient and relative permittivity of the CTS film versus wavelength respectively. The refractive
Fig. 5. Scanning electron micrograph of the CTS thin film showing the surface morphology.
Please cite this article in press as: S. Dias, et al., Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.10.049
S. Dias et al. / Materials Chemistry and Physics xxx (2015) 1e6 Table 1 Crystal structure data of CTS. Element
Labels
Oxidation states
Wyckoff symbols
Cu Cu Cu S Sn Sn
1 2 3 1 2 1
þ1.00 þ1.00 þ1.00 2.00 þ4.00 þ4.00
4d 2b 2a 8i 2b 4d
5
mobility are beneficial for the faster charge transport to the electrodes, thus preventing recombination of the charge carriers and in improving the efficiency of the CTS solar cell.
4. Conclusions
Table 2 Elemental Composition of the CTS films. Element
Weight%
Atomic%
SK Cu K Sn L Totals
25.49 40.94 33.57 100.00
46.16 37.41 16.43
index varied from 1.7 to 2.3, the extinction coefficient varied from 0.1 to 0.76 and the relative permittivity varied from 2.9 to 5.4 over a wavelength range of 245.02 nme998.49 nm. 3.5. Electrical properties The Hall effect measurements conducted on the CTS film, revealed the p type nature of the CTS films with a hole concentration of 2 1018 cm3, electrical conductivity of 9 S/cm and a hole mobility of 29 cm2/V. The high electrical conductivity and hole
Thin films of CTS were synthesized by the solution processible route of spin coating of the CTS precursor solution followed by a low-temperature annealing. The mechanism of formation of the CueSn-thiourea complex was deduced using FTIR. The phase transformations and deposition temperature were determined by thermal analysis of the CTS precursor. The annealed CTS films were further characterized. The phase formation and tetragonal crystal structure was inferred from XRD. The formation of tetragonal phase of CTS and the absence of secondary phases was confirmed using Raman spectroscopy. The films were found to be homogeneous and stoichiometric. Using AFM the surface roughness was found to be 1.3 nm. The thickness of the deposited films was measured to be 0.6 mm. The films were found to possess desirable optical properties for an absorber layer such as a high absorption coefficient of 104 cm1 and a bandgap of 1.3 eV. The refractive index, extinction coefficient and relative permittivity values of the film were determined by spectroscopic ellipsometry. The films were found to be p type with a hole concentration of 2 1018 cm3, electrical conductivity of 9 S/cm and a hole mobility of 29 cm2/V. The superior properties of CTS evaluated from the above characterization techniques corroborate the application of CTS as a potential absorber in solar cells and other optoelectronic devices.
Fig. 6. Atomic force microscope images showing (a) 2-Dimensional surface morphology and (b) 3-Dimensional topography of the CTS films.
Fig. 7. Optical properties of the CTS films (a) Absorption coefficient a versus photon energy E (b) Tauc plot of (ahn)2 versus E for the CTS film.
Please cite this article in press as: S. Dias, et al., Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.10.049
6
S. Dias et al. / Materials Chemistry and Physics xxx (2015) 1e6
Fig. 8. (a) Refractive index n (b) Extinction coefficient k (c) Relative Permittivity εr of the CTS film obtained from spectroscopic ellipsometry measurements.
References [1] P. Jackson, D. Hariskos, R. Wuerz, O. Kiowski, A. Bauer, T.M. Friedlmeier, M. Powalla, Properties of Cu(In,Ga)Se2 solar cells with new record efficiencies up to 21.7%, Phys. Status Solidi Rapid Res. Lett. 9 (1) (2015) 28e31. [2] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (Version 45), Prog. Photovoltaics Res. Appl. 23 (1) (2015) 1e9. [3] G. Phipps, C. Mikolajczak, T. Guckes, Indium and Gallium: long-term supply, Renew. Energy Focus 9 (2008) 56e59. [4] B.L. Cohen, Anomalous behavior of tellurium abundances, Geochim. Cosmochim. Acta 48 (1984) 203e205. [5] P. Das, S. Samantaray, G.R. Rout, Studies on cadmium toxicity in plants: a review, Environ. Pollut. 98 (1997) 29e36. [6] W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, D.B. Mitzi, Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency, Adv. Energy Mater. 4 (7) (2014) 1301465. [7] D.M. Berg, R. Djemour, L. Gütay, G. Zoppi, S. Siebentritt, P.J. Dale, Thin film solar cells based on the ternary compound Cu2SnS3, Thin Solid Films 520 (2012) 6291e6294. , A.F. da Cunha, A study of ternary Cu2SnS3 and [8] P.A. Fernandes, P.M.P. Salome Cu3SnS4 thin films prepared by sulfurizing stacked metal precursors, J. Phys. D. Appl. Phys. 43 (2010) 215403. [9] T. Kuku, O. Fakolujo, Photovoltaic characteristics of thin films of Cu2SnS3, Sol. Energy Mater 16 (1987) 199e204. [10] D. Tiwari, T.K. Chaudhuri, T. Shripathi, U. Deshpande, R. Rawat, Non-toxic, earth-abundant 2% efficient Cu2SnS3 solar cell based on tetragonal films direct-coated from single metal-organic precursor solution, Sol. Energy Mater.
Sol. Cells 113 (2013) 165e170. [11] D. Avellaneda, M.T.S. Nair, P.K. Nair, Cu2SnS3 and Cu4SnS4 thin films via chemical deposition for photovoltaic application, J. Electrochem. Soc. 157 (6) (2010) D346eD352. [12] Q. Chen, X. Dou, Y. Ni, S. Cheng, S. Zhuang, Study and enhance the photovoltaic properties of narrow-band gap Cu2SnS3 solar cell by pen junction interface modification, J. Colloid Interface Sci. 376 (2012) 327e330. [13] Z. Su, K. Sun, Z. Han, F. Liu, Y. Lai, J. Li, Y. Liu, Fabrication of ternary Cu-Sn-S sulfides by a modified successive ionic layer adsorption and reaction (SILAR) method, J. Mater. Chem. 22 (2012) 16346e16352. [14] M. Adelifard, M.M.B. Mohagheghi, H. Eshghi, Preparation and characterization of Cu2SnS3 ternary semiconductor nanostructures via the spray pyrolysis technique for photovoltaic applications, Phys. Scr. 85 (2012) 035603e035608. [15] T.K. Chaudhuri, D. Tiwari, Earth-abundant non-toxic Cu2ZnSnS4 thin films by direct liquid coating from metalethiourea precursor solution, Sol. Energy Mater. Sol. Cells 101 (2012) 46e50. [16] H. Guan, H. Shen, C. Gao, X. He, Structural and optical properties of Cu2SnS3 and Cu3SnS4 thin films by successive ionic layer adsorption and reaction, J. Mater Sci. Mater Electron 24 (5) (2013) 1490e1494. [17] G. Rajesh, N. Muthukumarasamy, E.P. Subramaniam, S. Agilan, Dhayalan Velauthapillai, Synthesis of Cu2ZnSnS4 thin films by dip-coating method without sulphurization, J. Sol-Gel Sci. Technol. 66 (2013) 288e292. [18] M.I. Aroyo, J.M. Perez-Mato, D. Orobengoa, E. Tasci, G. de la Flor, A. Kirov, Crystallography online: bilbao crystallographic server, Bulg. Chem. Commun. 43 (2) (2011) 183e197. [19] M. Banavoth, S. Dias, S.B. Krupanidhi, Near-infrared photoactive Cu2ZnSnS4 thin films by co-sputtering, AIP Adv. 3 (8) (2013) 082132.
Please cite this article in press as: S. Dias, et al., Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.10.049