Mechanosynthesis, deposition and characterization of CZTS and CZTSe materials for solar cell applications

Author's Accepted Manuscript

Mechanosynthesis, Deposition and Characterization of CZTS and CZTSe Materials for Solar Cell Applications T.S. Shyju, S. Anandhi, R. Suriakarthick, R. Gopalakrishnan, P. Kuppusami

www.elsevier.com/locate/jssc

PII: DOI: Reference:

S0022-4596(15)00123-1 http://dx.doi.org/10.1016/j.jssc.2015.03.033 YJSSC18850

To appear in:

Journal of Solid State Chemistry

Received date: 25 September 2014 Revised date: 25 March 2015 Accepted date: 29 March 2015 Cite this article as: T.S. Shyju, S. Anandhi, R. Suriakarthick, R. Gopalakrishnan, P. Kuppusami, Mechanosynthesis, Deposition and Characterization of CZTS and CZTSe Materials for Solar Cell Applications, Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j.jssc.2015.03.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Mechanosynthesis, Deposition and Characterization of CZTS and CZTSe Materials for Solar Cell Applications T.S.Shyju1&2, S.Anandhi3, R. Suriakarthick4, R.Gopalakrishnan4 and P.Kuppusami1&2 1

Centre for Nanoscience and Nanotechnology, Sathyabama University Chennai-600 119, Tamilnadu, India 2 Centre of Excellence for Energy Research, Sathyabama University Chennai-600 119, Tamilnadu, India 3 Department of Physics, Maamallan Institute of Technology, Sriperumpudur, 602 105, Tamilnadu, India 4 Department of Physics, Anna University, Chennai-600 025, Tamilnadu, India

Abstract Mechanosynthesis of nanocrystalline powders of CZTS and CZTSe by ball milling technique and the physical properties of thermally evaporated CZTS and CZTSe thin films as a function of substrate temperature are investigated. Nanocrystalline Cu-Zn-Tin-Sulphide (CZTS) and Cu-Zn-Tin-Selenide (CZTSe) powders synthesized by ball milling at different milling time using the source materials of Cu,Zn, Sn, S and Cu, Zn, Sn, Se in the ratio 2:1:1:4 are investigated.The above synthesized powder was thermally evaporated on glass substrate kept at room temperature and 673 K under a vacuum of 10-6 mbar to prepare quaternary compound semiconducting thin films in a single step process. The synthesized powder and deposited CZTS and CZTSe thin films belong to tetragonal crystal system. Raman spectra reveal that the synthesized nanocrystals are pure without any secondary phases. A gradual reduction in optical bandgap of films was observed with increasing substrate temperature due to increased crystallinity of the films. The changes in surface morphology of the films with respect to substrate temperature were studied by scanning electron microscopy and atomic force microscopy. Electrical studies indicate that the deposited films have p-type conductivity. Key words: Inorganic materials, Semiconducting materials, Nanomaterials, Ball milling, Solar cells, CZTS and CZTSe *Corresponding author, Fax: 044-2450-3814, Phone : +91-44-2450-3065 E-mail address: [email protected]

1

1. Introduction In energy production, solar energy market is presently dominated by siliconbased material. This is due to a well-established silicon industry, attractive device stability and power conversion efficiencies offered by single crystalline silicon [1-2]. On the other hand, thin-film photovoltaic technology is mainly based on direct-bandgap materials like CdTe, CuIn(SSe)2 and Cu(InGa)(SSe)2 (CIGSSe). Solar energy conversion efficiencies of ∼19.9% have been reported using these materials [3-5].These materials are direct bandgap in the range 1-2 eV which are capable of providing a maximum absorption in the solar spectrum. Currently, the key issue in this technology is to identify an appropriate absorber layer which has properties similar to that of CdTe and CIGSSe. To achieve the cost-effective photovoltaic technology, it is necessary to investigate new materials like quaternary chalcopyrite semiconductors. Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) are promising materials with a bandgap energy of 1.5 eV and crystal structure is similar to that of copper indium gallium sulphide (CIGS) [6–8].The first report on the growth of single-crystal CZTS by Nistcheet et al.,[9] used iodine vapour transfer method for single crystal and provided structural data which helped them to conclude that the structure is stannite type.The first thin films of CZTS were prepared by Ito and Nakazawa in 1988 using ion beam sputtering method [10]. Aron Walsh et al [11] reported first principle calculations relating to the crystal and electronic structure and phase stability of CZTS and related compounds. Over the past five years, there are several studies on the quaternary chalcogenide CZTS for solar cell [10-12]. It has a direct bandgap and optical absorption coefficient of 104 cm-1 in visible light region. Recently,CZTSe solar cell has shown a conversion efficiency12.6% by Wei Wang et al [13]. According to photon balance calculations of Shockley–Queisser,CZTS and CZTSe are expected to have theoretical efficiency better than 30% [14]. Up to now, experimentally measured efficiency of the solar cell is quite low compared with theoretical limit and therefore it is necessary to carry out more systematic studies on these materials. From the material point of view, CZTS and CZTSe based solar cell has still several potential advantages. Combination of these materials could make a cost effective solar cells and enhance the cell

2

efficiency. The bandgap energy of CZTS is optimal for the usage as an absorber layer in solar cell devices in the visible region of electromagnetic spectrum. To reduce the fabrication costs and to enhance the energy conversion efficiency of the solar cells, many research groups have employed chemical methods to deposit thin films such as solution process [15], sol-gel [16], chemical bath deposition [17-19], photochemical

deposition

[20],

spray

pyrolysis

[21-22],

spin

coating

[23],

electrodeposition [24-25], SILAR [26], screen printing [27], and physical vapour deposition (PVD) methods such as thermal evaporation [28-29], electron beam evaporation [30], sputtering [10, 31-33], and pulsed laser deposition [34-37]. Among the PVD methods, thermal evaporation is simple, cost effective and also meets device based qualities like optimum stoichiometry, morphology and crystalline nature, which are the key factors on deciding the performance of the films for their suitability in developing special devices. However, only a few reports are available [28-29] on the deposition of CZTS thin films by thermal evaporation technique using the commercially available Cu, Zn and Sn evaporated in sulfur atmosphereor by sulphurization process.To the best of our knowledge, no attempt has been made to grow CZTS and CZTSe films by thermal evaporation method using synthesized CZTS and CZTSe nanocrystalline powder.Guo et al [38] first reported the synthesis of CZTS nanoparticles using ahot injection method. Cao et al., [39] synthesized CZTS using solvothermal method, in which they observed small amount of compositional variation in the synthesized powder. In addition to that Zaberca et al., [40] and Wang et al., [41] described the synthesis of CZTS and CZTSe using a chemical route. Such a study will be useful to clearly identify the particle size dependent physical properties of CZTS and CZTSe for technological applications. The present work reports an alternative route to synthesize nanocrystalline CZTS and CZTSe materials in a single step process using ball milling. The synthesized nanocrystalline powders of CZTS and CZTSe were used to deposit thin films by thermal evaporation technique at room temperature (RT) and substrate temperature of 673K. The substrate temperature induced changes in the physical properties of CZTS and CZTSe films were studied. The deposited films were analyzed to study the structural,

3

optical, morphological and electrical properties using XRD, Raman, UV-Vis, SEM, AFM and Hall effect measurement system. 2. Experimental details 2.1 Synthesis of bulk and thin films of CZTS and CZTSe The mechanosynthesis of CZTS and CZTSe nanoparticle were carried out using SPEX Sample Prep 8000D Dual ball mill. The compound formation depends on many parameters such as milling speed, milling time, ball-to-powder weight ratio, milling type, particle size and material of the grinding balls. In the present work, elemental copper (Cu), zinc (Zn), tin (Sn) and sulfur (S) or selenium (Se) powders with the grain size in the range 10-50 µm were used to synthesize CZTS and CZTSe nanoparticles. Fine powders of Cu, Zn, Sn and S or Se were mixed in the ratio of 2:1:1:4 and ball to powder ratio was kept at 2:1, and the powders were milled in a stainless steel vial with stainless steel balls [42] at the milling speed of 2750 rotations per minute (RPM).The experiments were performed for different time durations ranging from 1 to 60 h to synthesize the CZTS and CZTSe compound semiconductors. It was observed that the CZTS ball milled for duration less than 30 h doesn’t crystallize so easily while compared to that of CZTSe. Therefore the results obtained on CZTS ball milled for the duration from 15 to 60 h are only reported in the present work. In the case of CZTSe, the structural properties were investigated for the powder ball milled for the duration in the range 2-14 h. The CZTS powders ball milled for 60 h and CZTSe powders ball milled for 14 h are used as source material for the deposition of thin films.The thermal evaporation was carried out using a

custom designed coating unit (HINDHIVAC , Model: 12 A4D,

Bangalore) under a chamber vacuum of 10-6 mbar on chemically and ultrasonically cleaned [43] glass substrates kept at room temperature (RT) and

at 673 K. The

substrate cleaning plays an important role in the preparation of thin films. The substrates were mounted in a designed substrate holder fitted with a substrate heater. The optimized distance between the substrate and source was found to be 15 cm. 4

Shyju et al [44] have reported on the various parameters involved in the growth of thin films by thermal evaporation. 2.2 Characterization The structural properties of the CZTS and CZTSe thin films deposited at different substrate temperature have been analyzed by glancing angle X-ray diffraction technique, with angle of incidence 0.3 by X-ray diffraction in the . 2θ scan range of 2080º using CuKα radiation ( λ= 1.5418 Å).The crystallite size of the material was evaluated using Williamson-Hall plot (W-H). W-H analysis is a simplified integral breadth method where both (size and strain) induced broadening are deconvoluted by considering the peak width as a function of 2θ. W–H plots are drawn with 4sinθ along x-axis and βcosθ along y-axis. From the linear fit to the data, the lattice strain (ε) was determined from the slope and the intercept determines the crystallite size (D) as shown in the eq. (1)

β cos θ =

Kλ + 4ε Sinθ D

(1)

where, D is the crystallite size, K is the shape factor, β is the full width at half maximum of a strong peak. The value of β was deduced after subtracting the instrumental broadening(b) from the FWHM of the film (B) using the relation: β = B 2 − b 2

.

The value of b determined from the Si standard is found to be 0.0716, 0.066 and 0.065. The dislocation density (δ) was evaluated from Williamson and Smallman’s formula [45] given in Eq. (2). The values are presented in Table 1.

δ =

1 D2

(2)

In thin films, the strain is due to imperfections within the crystalline lattice, including vacancies, dislocations and antisite defects. The strain (ε) was obtained using the relation given in Eq. (3).

ε = ( β s cot θ ) / 2

(3)

5

where βs is the integral breadth of the (hkl) reflection, θ is the Bragg angle [46-47]. Raman Measurement was performed using RENISHAW inVia Raman Microscope coupled with metallographic microscope with the excitation wavelength of 514 nm by focusing laser spot onto the surface. Substrate temperature induced changes in the surface morphology of CZTS and CZTSe films were examined using SUPRA 55 – (CARL ZEISS, GERMANY) field emission scanning electron microscopy (FESEM) and Atomic Force Microscopy (AFM-NTMDT, Ireland). The thickness of the deposited films were measured from the cross sectional view using SEM. Optical properties of the films were studied by UV-Vis-NIR spectrophotometer. Electrical properties such as resistivity, Hall mobility and carrier concentration of thermally evaporated CZTS and CZTSe films were studied by Hall effect measurement system (van-der-Pauw method) at room temperature. 3 Results and discussion 3.1 Structural properties of CZTS &CZTSe The X-ray diffraction patterns of ball milled CZTS and CZTSe powder are shown in Fig.1 [A]&[B]. The observed diffraction data are in good agreement with the standard 2θ values of kesterite CZTS (JCPDS-26-0575) [48] and stannite CZTSe [49] (JCPDS-520868). A progressive improvement in the compound formation, composition and crystallinity is observed with increasing ball milling time and is clearly shown in Fig. 1(A) and ( B). The crystallite size and tensile strain of the ball milled powders of CZTS and CZTSe are found to be about 10-50 nm and 0.04 respectively. Fig.2 shows the Raman spectra of CZTS & CZTSe synthesized materials. Fig 3(a) clearly shows a strong peak at 337 cm-1 which corresponds to the main features of the vibrational A1 symmetry mode of CZTS; in addition, we observed a broad peak at 285 and 219 cm-1 which are close to the values reported for CZTS [50]. Fig.3 (b) shows a strong peak at 191

cm-1

corresponding

to

the

A1

symmetry -1

mode

of

CZTSe

material.

In addition a shoulder peak (broad peak) at 238 cm which corresponds to the CZTSe material [51].

6

Fig. 3(A) illustrates the XRD patterns obtained for CZTS thin films deposited at RT and substrate temperature of 673 K. It is clear that the peaks are less intense and broader for the films deposited at RT than the films deposited at 673 K.The CZTS films deposited at 673 K show improvement in the intensity of the diffraction peaks of kesterite structure and the peaks are well matched with JCPDS data. In contrast, the CZTSe films deposited at RT and 673 K are

more crystalline than that of CZTS.

Determination of texture coefficient of these films qualitatively shows a preferred orientation of (112) plane for the CZTSe, while there is a random orientation for CZTS. XRD also confirms that the tetragonal CZTSe crystallizes in stannite structure in accordance with the literature [48-49]. We do not observe any major structural changes in the films at 673 K. In Fig.3 the intensity of (112) oriented peak increases with increase in the substrate temperature.The improvement in the crystallinity of film occurs by coalescence of the neighboring islands driven by the increased mobility of adatoms due to thermal energy.Table 1 clearly shows the increase in crystallite size (D) and the decreasing dislocation density (δ) for all the reflections of CZTS with increasing substrate temperature.The diffraction data of CZTS was given as an input for PDXL software to match with the standard data of CZTS and also viewed the unit cell to confirm kesterite structure in the deposited materials. Estimation of crystallite size in the CZTS and CZTSe films indicated that the size of crystallites increase by about two times with the increase in the substrate temperature from

RT to 673 K, while the

tensile strain decreases from 0.1 to 0.03. 3.2 Surface Morphology of CZTS and CZTSe Powders SEM in combination with EDAX was used to study the surface morphology and composition of the synthesized powder and deposited thin films. The surface morphology and composition of synthesized CZTS and CZTSe particles are shown in Fig. 4 and 5, respectively.The morphology of the powders are shown in Fig.4(a) and5(a), while the composition of the ball milled CZTS and CZTSe analyzed by EDAX are shown in Table 3 & Fig. 4(b) and 5(b).

7

Figure 4 (a) shows that the particles are agglomerated to form clusters and composition is slightly deviated from theoretical value. This may be due to the difference in the solubility and ionic radius of the starting metal precursors [52]. (i)

CZTS Thin Films

Figs.6-7 show the surface morphology of CZTS thin films deposited at substrate temperature of RT and 673 K and it is inferred that the closely packed granular crystallites are uniformly arranged on the surface of the substrate with an average size of 50-100 nm. Where as, the film morphology becomes smoother and the nano grains tend to form grow on the surface with the increasing substrate temperature from RT to 673 K. The inset image (Fig. 6) shows the cross sectional view of CZTS thin film indicating the film thickness of about 500 nm.The slight compositional variation was observed in the stoichiometry of CZTS; this may be due to the difference in the evaporation rates of different metal precursors. Also CZTS thin films showed a significant amount of copper deficiency for the films deposited at room temperature due to insufficient energy available to form a compound. On the other hand, at the substrate temperature of 673K, the stoichiometry ratio is close 2:1:1:4 for CZTS. The fact that no additional peaks are seen in XRD and Raman also supports the formation of single phase of CZTS. (ii)

CZTSe

Figs. 8 and 9 illustrate the shape, size and composition of the CZTSe films deposited at RT and 673 K. The inset of Fig.9 show the cross sectional view of CZTSe indicates

thickness

of

about 500 nm EDAX analysis reveals that the thermally

evaporated thin films maintain stoichiometry and the values are presented in Table 3. It is well known that the efficiency of polycrystalline thin film solar cell increases with increasing grain size of the absorber layer. SEM studies also reveal that the grain size or the agglomeration of particles is significant for the films deposited at 673 K due increased mobility and reactivity of the constituent elements of the CZTSe.

8

Fig.10-13 show the AFM images of CZTS and CZTSe films deposited at substrate temperature of RT and 673 K. AFM image of room temperature deposited CZTS thin film is shown in Fig.10 which indicates the growth of granular particles on the surface of the film with small cavities at low temperature. Fig.11 shows the surface profiles of CZTS deposited at 673K, revealing that films are very dense, well crystallized and uniformly covered on the surface of substrate without any cracks and pin holes.The film surface looked more compact with relatively larger grains along with group of islands. The crystallite size also has significantly increased for the film deposited at 673K. It is noted that film composed of isolated nanoparticles throughout the surface with an average size of 30 and 150 nm for the films deposited at RT and 673K respectively. Fig. 12 and 13 show the AFM images of CZTSe thin films deposited at RT and 673K, respectively. It is noticed that the particles are uniformly distributed and well crystallized on the substrate surface at higher substrate temperature. A statistical analysis of the roughness data obtained for the 5 × 5 µm scan area highlighted that there is a significant difference between the roughness of the CZTS and CZTSe thin films with increasing substrate temperatures. Average surface roughness, root mean square (RMS) roughness, averageparticle size, average particle height obtained for the films deposited at RT and 673 K are presented in Table 4. The roughness and particle sizes obtained for the films deposited at the substrate temperature of 673 K are higher than those obtained for the films deposited at room temperature. It is also noticed that the increase in roughness and particle sizes are very significant in CZTS thin films when compared with CZTSe thin films. 3.3 Optical properties of CZTS and CZTSe Optical (transmittance and absorption) spectra recorded in the visible and near infrared regions are related to electronic transitions and are also useful in understanding the electronic band structure of the semiconducting films [53]. The optical transmittance and absorbance spectra of the CZTS and CZTSe films were recorded using UV-Vis spectrophotometer at room temperature in the wavelength range 300-1100 nm and are shown in Fig. 14 [A] & [B]. From the optical transmittance spectra, it is observed that the transmittance of the film decreases with increasing substrate temperature, which is 9

caused by free-carrier absorption corresponding to increase in the conductivity. The transmission thresholds shift towards higher wavelength region indicating a systematic reduction in the optical bandgap of the films with substrate temperature. Optical investigations of films reveal that there is a band to band direct transition. The absorption data were analyzed using the following classical relation for near edge optical absorption of semiconductors:

α hν = A(hν − Eg )n

(4)

where, hν is photon energy, Eg is bandgap and A is constant,n can have values 1/2, 2, 3/2 and 3 for allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively. The bandgap energy (Eg) was determined by plotting (Tauc plot) a graph of hν versus (αhν)2 (Fig. 15 [A]) for the direct bandgap. The bandgap 'Eg' was determined by extrapolating the straight line portion to the energy axis, whose intercept to the X-axis gives the optical bandgap [54]. The optical bandgap is found to be 2.5 and 2 eV for CZTS thin films deposited at room temperature and 673K respectively. A gradual reduction in optical bandgap of films was observed with increasing substrate temperature, due to the increased crystallization of films. In the present work, CZTS and CZTSe thin films showed an increase in Eg values compared to the bulk value. This may be attributed to the presence of nanocrystallites,

minor

compositional variation and defects in the films and is also in agreement with the literature [55].The extinction coefficient (k) was calculated from the optical absorption spectra using the relation (5 ) and the variation in k with respect to the wavelength is presented in Fig.15 [B]. This clearly shows that the extinction coefficient is significant near the absorption edge and low at higher wavelength regions.The increase in k with the substrate temperature may be due to the increase in the surface roughness of the film which enhances the scattering losses and reduction in the transmittance. Optical transmittance and absorbance of the CZTSe film deposited at RT and 673 K are shown in Fig. 16 [A] and [B] respectively. The optical bandgap was determined by plotting a graph of hν vs (αhν)2 (Fig. 17 [A and B]) and it is found to be 1.7 and 1.6 eV for the film deposited at RT and 673K, respectively.The extinction coefficient (k) with respect to the substrate temperature is presented in Fig. 17[B].

10

k=

λα 4π

(5)

The bandgap of the film was quite close to the optimum bandgap for solar cell. These optical characteristics indicate that the CZTS and CZTSe are very promising materials for thin film solar cells.

3.4 Electrical Properties of CZTS and CZTSe Hall effect measurements were carried out to determine the sheet resistivity, carrier concentration and Hall mobility of CZTS and CZTSe thin films and the values are tabulated in Table 5. All the films were found to have p-type conductivity with an average sheet carrier concentration of the order of 1014-1015 cm2.The values reported by Sanjay Kumar Swami et al [56] for the carrier concentration of 7.9x1019 cm3 for CZTS thin films prepared by spin coating technique are in agreement with our data.The electrical properties of the films meet the properties of the absorber layer for the thin film solar cells. The favorable electrical properties of the films were attributed to the enhanced grain size. The large grains in the material would have reduced the grain boundaries and thus effectively reducing the recombination of the charge carriers [5657]. 4 Conclusions Single phase CZTS and CZTSe nanocrystalline powders were synthesized by simple ball milling method for different time duration. The ball milled CZTS and CZTSe powders were used as source material to deposit thin films by thermal evaporation at room temperature and at the substrate temperature of 673 K. The following are some of the important conclusions drawn: (i)The substrate temperature has profound influence on physical properties of the thermally evaporated CZTS andCZTSe thin films. XRD and Raman measurements confirm that the deposited films belong to tetragonal crystal system with kesterite and stannite structure of CZTS and CZTSerespectively. CZTS doesn’t show any preferred orientation, while CZTSe thinfilms exhibited preferred orientation along (112) plane for the films deposited at 673 K. XRD analysis also reveals that the higher substrate 11

temperature leads to an increase in the crystallite size and reduction in the dislocation density. (ii) SEM and AFM micrographs illustrate the granular type of growth and the roughness and particle sizes obtained for the films deposited at the substrate temperature of 673 K are higher than those obtained for

the room temperature

deposited films because of the higher mobility of adatoms at higher substrate temperatures. (iii) A gradual reduction in optical bandgap was observed with increasing substrate temperature, which is associated with the enhanced crystallinity of the films. (iv)The positive Hall coefficient obtained for the film confirms the p-type conductivity in the films. Therefore, it is suggested

that the deposited CZTS andCZTSe have

optimal properties as absorber materials for for thin film solar cells.

5 Acknowledgment One of the authors (T.S.Shyju) thank MHRD, Govt. of India for funding Centre of Excellence for Energy Research at Sathyabama University. 6 References [1] Ignacio Tobıas,CarlosdelCanizo and Jesus Alonso, Handbook of Photovoltaic Science and Engineering. Edited by A. Luque and S. Hegedus 2003 John Wiley & Sons, Ltd [2] Stefaan De Wolf, Antoine Descoeudres, Zachary C. Holman and Christophe Ballif, High-efficiency silicon heterojunction solar cells: A review, Green, 2 (2012) 7– 24. [3] Ingrid Repins, Miguel Contreras, Manuel Romero, Yanfa Yan, Wyatt Metzger, Jian Li, Steve Johnston, Brian Egaas, Clay DeHart, John Scharf, Brian E. McCandless, and Rommel Noufi, 33rd IEEE Photovoltaic Specialists Conference San Diego, California, (2008) 11–16. [4] Wei Liu,David B. Mitzi,Min Yuan,Andrew J. Kellock,S. Jay Chey,and Oki Gunawan, 12% Efficiency CuIn(Se,S)2 Photovoltaic Device Prepared Using a Hydrazine Solution Process, Chem. Mater., 22 (2010) 1010–1014.

12

[5] Leonid Kosyachenkoa, Toshihiko Toyama, Current–voltage characteristics and quantum efficiency spectra of efficient thin-film CdS/CdTe solar cells, Sol. Energy Mater. Sol. Cells, 120 (2014) 512–520. [6] A. Lafond, L. Choubrac, C. Guillot-Deudon, P. Fertey, M. Evain and S. Jobic, Xray resonant single-crystal diffraction technique, a powerful tool to investigate the kesterite structure of the photovoltaic Cu2ZnSnS4 compound, Acta Cryst. B70 (2014) 390-394. [7] Hironori Katagiri, Kotoe Saitoh, Tsukasa Washio, Hiroyuki Shinohara, Tomomi Kurumadani, Shinsuke Miyajima, Development of thin film solar cell based on Cu2ZnSnS4 thin films, Sol. Energy Mater. Sol. Cells,65 (2001) 141–148. [8] H. Matsushita, T. Ichikawa and A. Katsui, Structural, thermodynamical and optical properties of Cu2-II-IV-VI4 quaternary compounds,J. Mater. Sci., 40 (2005) 20032005. [9] R. Nitsche, D. F. Sargent and P. Wild,Crystal growth of quaternary 122464 chalcogenides by iodine vapor transport,J. Cryst. Growth, 1 (1967) 52-53. [10] K. Ito and T. Nakazawa,Electrical and Optical Properties of Stannite-Type Quaternary Semiconductor Thin Films,Jpn. J. Appl. Phys., 27 (1988) 2094 [11] Aron Walsh, shiyouchen, Su-Huai Wei, and Xin-Gao Gong,Kesterite Thin-Film Solar Cells: Advances in Materials Modelling of Cu2ZnSnS4, Adv. Energy Mater., 2 (2012) 400-409 [12] H. Katagiri , K. Jimbo, M. Tahara, H. Araki, K. Oishi,, in : A Yamada, C. Heske, M. Contreras, M. Igalson, S. J.C. Irvine (Eds.), “Thin-Film Compound Semiconductor Photovoltaics” 2009, San Francisco, U. S. A., April 13-7,2009, Mater. Res. Soc. Symp, Proc., 1165 (2009) 1165-M04-01. [13] Wei Wang, Mark T. Winkler , Oki Gunawan , TayfunGokmen , Teodor K. Todorov , Yu Zhu , and David B. Mitzi, Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency,Adv. Energy Mater. (2013) 1-5. [14] W. Shockley, H. J. Queisser,Detailed Balance Limit of Efficiency of p‐n Junction Solar Cells, J. Appl. Phys., 32(1961) 510-519. [15] T. Todorov, M. Kita, J. Carda and P. Escribano,Cu2ZnSnS4 films deposited by a soft-chemistry method, Thin Solid Films, 517 (2009) 2541-2544. [16] K. Tanaka, M. Oonuki, N. Moritake and H. Uchiki,Cu2ZnSnS4 thin film solar cells prepared by non-vacuum processing, Sol. Energy Mater. Sol. Cells, 93 (2009) 583587. 13

[17] T. K. Todorov, K. B. Reyter and D. B. Mitzi,High-Efficiency Solar Cell with EarthAbundant Liquid-Processed Absorber,Adv. Mater., 22 (2010) E156-E159. [18] M. Farinella, R. Inguanta, T. Spanò, P. Livreri, S. Piazza, C. Sunseri, Electrochemical Deposition of CZTS Thin Films on Flexible Substrate,Energy Procedia, 44 (2014) 105-110. [19] A. Wangperawong, J.S. King, S.M. Herron, B.P. Tran, K. Pangan-Okimoto, S.F. Bent, High-Efficiency Solar Cell with Earth-Abundant Liquid-Processed Absorber,Thin Solid Films, 519 (2011) 2488–2492. [20] K. Moriya, J. Watabe, K. Tanaka and H. Uchiki,Characterization of Cu2ZnSnS4 thin films prepared by photo-chemical deposition,Phys. Status Solidi C, 3 (2006) 2848-2852. [21] N.M. Shinde, R.J. Deokate, C.D. Lokhande, Properties of spray deposited Cu2ZnSnS4 (CZTS) thin films,J. of Anal.Appl Pyrolysis, 100 (2013) 12–16. [22] Zeineb Seboui, Yvan Cuminal, and Najoua Kamoun-Turki, Physical properties of Cu2ZnSnS4 thin films deposited by spray pyrolysis technique, J. Renewable Sustainable Energy 5 (2013) 023113. [23] Sanjay Kumar Swami, Anuj Kumar, Viresh Dutta, Deposition of Kesterite Cu2ZnSnS4 (CZTS) Thin Films by Spin Coating Technique for Solar Cell Application, Energy Procedia, 33 (2013) 198-202. [24] Ji Li, Tuteng Ma, Ming Wei, Weifeng Liu, Guoshun Jiang, Changfei Zhu, The Cu2ZnSnSe4 thin films solar cells synthesized by electrodeposition route, Appl. Surf Sci., 258 (2012)6261-6265. [25] SeulGi Lee, JongMin Kim, HuynSuk Woo, YongChul Jo, A.I. Inamdar, S.M. Pawar, Hyung Sang Kim, Woong Jung, HyunSikImH, Structural, morphological, compositional, and optical properties of single step electrodeposited Cu2ZnSnS4 (CZTS) thin films for solar cell application,Curr. Phys., 14 (2014) 254–258. [26] K. Wang, O. Gunawan, T. Todorov, B. Shin, S. J. Chey, Thermally evaporated Cu2ZnSnS4 solar cells,ApplPhyslett.97 (2010) 143508-3. [27] Z. Zhou, Y. Wang, D. Xu and Y. Zhang,Fabrication of Cu2ZnSnS4 screen printed layers for solar cells,Sol. Energy Mater. Sol. Cells, 94 (2010) 2042-2045. [28] B. Shin, O. Gunawan, Y. Zhu, N. A. Bojarczuk, S. J. Chey and S. Guha, Thin film solar cell with 8.4% power conversion efficiency using an earth-abundant Cu2ZnSnS4 absorber, Prog. Photovoltaics, 21 (2013) 72–76. 14

[29] H. Katagiri, N. Sasaguchi, S. Hando, S. Hoshino, J. Ohoshi and T. Yokota,Preparation and evaluation of Cu2ZnSnS4thin films by sulfurization of E-B evaporated precursors Sol. Energy Mater. Sol. Cells, 49 (1997) 407-414. [30] K. Jimbo, R. Kimura, T. Kamimura, S. Yamada, W. S. Maw, H. Araki, K. Oishi and H. Katagiri,Cu2ZnSnS4-type thin film solar cells using abundant materials,Thin Solid Films, 515 (2007) 5997-5999. [31] H. Katagiri, K. Jimbo, S. Yamada, T. Kamimura, W. S. Maw, T. Fukano, T. Ito and T. Motohiro,Enhanced Conversion Efficiencies of Cu2ZnSnS4-Based Thin Film Solar Cells by Using Preferential Etching Technique,Appl. Phys. Express, 1 (2008) 041201 [32] F. Liu, Y. Li, K. Zhang, B. Wang, C. Yan, Y. Lai, Z. Zhang, J. Li and Y. Liu,In situ growth of Cu2ZnSnS4 thin films by reactive magnetron co-sputtering,Sol. Energy Mater. Sol. Cells, 94 (2010) 2431-2434. [33] Tara P. Dhakal, Chien–Yi Peng, R. Reid Tobias, Ramesh Dasharathy, Charles R. Westgate, Characterization of a CZTS thin film solar cell grown by sputtering method,Sol. Energy, 100 (2014)23–30. [34] K. Moriya, K. Tanaka and H. Uchiki,Fabrication of Cu2ZnSnS4 Thin-Film Solar Cell Prepared by Pulsed Laser Deposition,Jpn. J. Appl. Phys., 46 (2007) 5780. [35] S. M. Pawar, A. V. Moholkar, I. K. Kim, S. W. Shin, J. H. Moon, J. I. Rhee and J. H. Kim,Effect of laser incident energy on the structural, morphological and optical properties of Cu2ZnSnS4 (CZTS) thin films Curr. Appl. Phys., 10 (2010) 565-569. [36] A. V. Moholkar, S. S. Shinde, A. R. Babar, K. U. Sim, H. K. Lee, K. Y. Rajpure, P. S. Patil, C. H. Bhosale and J. H. Kim,Synthesis and characterization of Cu2ZnSnS4 thin films grown by PLD: Solar cells, J. Alloys Compd., 509 (2011) 74397446. [37] Q. Guo, H. W. Hillhouse and R. Agrawal,Synthesis of Cu2ZnSnS4 Nanocrystal Ink and Its Use for Solar Cells, J. Am. Chem. Soc., 131 (2009) 11672. [38] A.V. Moholkar, S.S. Shinde, G.L. Agawane, S.H. Jo, K.Y. Rajpure, P.S. Patil, C.H. Bhosale and J.H. Kim, Studies of compositional dependent CZTS thin film solar cells by pulsed laser deposition technique: An attempt to improve the efficiency,J. Alloys Compd., 544 (2012) 145–151. [39] M. Cao and Y. Shen,A mild solvothermal route to kesterite quaternary Cu2ZnSnS4 nanoparticles,J. Cryst. Growth, 318 (2011) 1117-1120.

15

[40] O. Zaberca, A. Gillorin, B. Durand and J. Y. Chane-Ching, A general route to the synthesis of surfactant-free, solvent-dispersible ternary and quaternary chalcogenide nanocrystals,J. Mater. Chem., 21 (2011) 6483. [41] X.Wang,Z.Sun,C.Shao,D.M.BoyeandJ.Zhao,A facile and general approach to polynary semiconductor nanocrystals via a modified two-phase method,Nanotechnol., 22 (2011) 245605. [42] F.Padella, C.Alvani, A.La Barbera, G.Ennas, R.Liberatore and F.Varsano, Mechanosynthesis and process characterization of nanostructured manganese ferrite,Mater. Chem. Phys. 90 (2005) 172-177, [43] T.S. Shyju, S. Anandhi, R. Indirajith, R. Gopalakrishnan, Effects of annealing on cadmium selenide nanocrystalline thin films prepared bychemical bath deposition,J. Alloys Compd.,506 (2010) 892–897. [44] T.S.Shyju, S.Anandhi, R.Sivakumar, R.Gopalakrishanan, Studies on Lead Sulphide (PbS) SemiconductingThin Films Deposited from Nanoparticlesand Its NLO Applications,Int. J Nanosci., 13 (2014) 1450001 [45] G K Williamson, R E Smallman, Dislocation densities in some annealed and cold-worked metals frommeasurements on the X-ray debye-scherrer spectrum,Philos. Mag.,1 (1956) 34. [46] A. R. Stokes, A. J. C. Wilson, The Diffraction of X Rays By DistortedCrystal Aggregates Proc. Phys. Soc., 56 (1944) 174. [47] G. K. Williamson and W. H. Hall, X-Ray Line Broadening from Filed Aluminium and Wolfram Acta Metall., 1 (1953)22. [48] JCPDS Card Numner : 26-0575 [49] JCPDS Card Number : 52-0868 [50] P.A. Fernandes, P.M.P. Salome, and A.F. da Cunha, Study of polycrystalline Cu2ZnSnS4 films by Raman scattering,J. Alloys Compd., 509 (2011) 7600– 7606 [51] Xianzhong Lin,Jaison Kavalakkatt, Kai Kornhuber, Daniel Abou-Ras, Susan Schorr, Martha Ch. Lux-Steiner and Ahmed Ennaoui, Synthesis of Cu2ZnxSnySe1+x+2y nanocrystals with wurtzite-derived structure,RSC Adv., 2012, 2, 9894–9898. [52]Wen-Hui Zhou, Yan-Li Zhou, Jun Feng, Ji-wei Zhang, Si-Xin Wu, Xiu-chun Guo, and Xuan Cao, Chem. Phys.Lett.,546 (2012) 115-119

16

[53] K. S. Usha, R. Sivakumar and C. Sanjeeviraja, Optical constants and dispersion energy parameters of NiO thin films prepared by radio frequency magnetron sputtering technique, J. Appl. Phys., 114 (2013) 123501. [54]J. Tauc, R. Grigorovici and A. Vancu, Optical Properties and Electronic Structure of Amorphous Germanium, Phys. Status Solidi 15 (1966) 627–637. [55] VipulKheraj, K.K. Patel, S.J. Patel, D.V.Shah, Synthesis and characterisation of Copper Zinc Tin Sulphide (CZTS) compound for absorber material in solar-cells, J. Cryst. Growth 362 (2013) 174-177. [56] Sanjay Kumar Swami, Anuj Kumar and Viresh Dutta,Deposition of Kesterite Cu2ZnSnS4 (CZTS) Thin Films by Spin Coating Technique for Solar Cell Application, Energy Procedia 33, (2013) 198–202. [57] E.M. Mkawi, K. Ibrahim, M. K. M. Ali and Abdussalam Salhin Mohamed, Dependence of Copper Concentration on the Properties of Cu2ZnSnS4 Thin Films Prepared by Electrochemical Method, Int. J. Electrochem. Sci., 8 (2013) 359 - 368

List of Tables Table 1. Structural parameters of CZTS nanoparticles after ball milling for different durations and CZTS films deposited at RT and 673K. Table 2. Structural parameters of CZTSe nanoparticles ball milled for different durations and CZTSe films deposited at RT and 673K. Table 3: Composition of CZTS and CZTSe in at.%. Table 4: Surface properties of CZTS and CZTSe thin films Table 5. Electrical properties of CZTS and CZTSe thin films

17

Table 1. Structural parameters of CZTS nano particles after ball milling for different durations and CZTS films deposited at RT and 673K. Sample details CZTS 30 h CZTS 45 h CZTS 60 h CZTS RT CZTS 673K

2θ exp

cal

28.385 47.454 56.006 28.403 47.313 56.08 28.417 47.315 56.115 28.444 47.637 56.366 28.417 32.592 47.393 56.158

28.53 47.32 56.175 28.53 47.32 56.175 28.53 47.32 56.175 28.53 47.32 56.175 28.53 32.988 47.32 56.175

(hkl)

β

Crystallite size (nm)

112 220 312 112 220 312 112 220 312 112 220 312 112 200 220 312

0.0322 0.0391 0.0390 0.0126 0.0307 0.0545 0.0121 0.0297 0.0513 0.0329 0.0592 0.0533 0.0083 0.0135 0.0126 0.0126

18

5.71

5.93

6.42 11.0

17.7

d-spacing (Å) exp cal

3.141 1.914 1.641 3.139 1.919 1.638 3.138 1.919 1.637 3.135 1.907 1.631 3.138 2.745 1.916 1.636

3.126 1.919 1.636 3.126 1.919 1.636 3.126 1.919 1.636 3.126 1.919 1.636 3.126 2.713 1.919 1.636

Dislocation 15 density×10 2 lines/m

30.7

28.4

24.2

8.25

3.18

Table 2. Structural parameters of CZTSe nano particles ball milled for different durations and CZTSe films deposited at RT and 673K. Sample details CZTSe 2h CZTSe 4h CZTSe 9h CZTSe 14 h CZTSe RT CZTSe 673K

2θ exp

cal

27.28 45.28 53.62 27.20 45.16 53.53 27.18 45.15 53.55 27.15 45.11 53.48 26.78 44.44 52.66 27.06 44.97 53.29

27.14 45.07 53.41 27.14 45.07 53.41 27.14 45.07 53.41 27.14 45.07 53.41 27.14 45.07 53.41 27.14 45.07 53.41

(hkl)

β

Crystallite size (nm)

112 204 312 112 204 312 112 204 312 112 204 312 112 204 312 112 204 312

0.01643 0.02048 0.02713 0.01077 0.01550 0.01787 0.01076 0.01483 0.01742 0.01046 0.01502 0.01786 0.01191 0.01273 0.01600 0.01100 0.01883 0.02580

21.59 35.04 31.46 40.57 17.72 59.3

19

d-spacing (Å) exp cal

3.266 2.001 1.707 3.275 2.002 1.710 3.278 2.006 1.710 3.281 2.008 1.712 3.325 2.036 1.736 3.295 2.014 1.718

3.283 2.010 1.714 3.283 2.010 1.714 3.283 2.010 1.714 3.283 2.010 1.714 3.283 2.010 1.714 3.283 2.010 1.714

Dislocation density×1015lines/m2

2.1 0.8 1.0 0.6 3.1 0.2

Table 3: Composition of CZTS and CZTSe in at.%.

Sample details powder Thin film deposited at RT Thin film deposited at 673 K

CZTS

CZTSe

Cu

Zn

Sn

S

Cu

Zn

Sn

Se

24.77

14.46

16.03

44.73

22.55

11.29

12.27

53.90

5.41

26.24

34.37

34.04

23.51

10.96

11.36

54.17

23.21

18.89

20.95

36.96

25.11

11.48

11.84

50.57

20

Table 4: Surface properties of CZTS and CZTSe thin films Sample details CZTS /RT CZTS/673 K CZTSe/ RT CZTSe/673 K

Average Surface Roughness (Sa) (nm) 4.99 26.17 3.7 10.8

Surface property by AFM RMS (Sq) (nm) Average Particle roughness Size (nm) 7.13 30.1 33.09 149.7 4.65 16.1 13.63 35.3

21

Average height (nm) 49.72 153.87 18.83 52.91

Table 5. Electrical properties of CZTS and CZTSe thin films

Smaple details

Resitivity (Ω)

Mobility(cm /Vs)

Sheet Carrier concentration (cm2)

CZTS/ RT CZTS/ 673K CZTSe /RT CZTSe/ 673K

1073 204 119 1082

26.26 4.29 56.6 2.08

2.221×1014 7.123×15 9.821×1014 2.77×15

2

22

List of Figures Fig.1. XRD pattern of [A]CZTS materials ball milled at different duration for (a) 15 (b) 30 (c) 45 & (d) 60 h [B] CZTSe ball milled for (a) 2 (b) 4 (c) 9 & (d) 14 h Fig.2. Raman spectra of ball milled powders of (a) CZTS (b) CZTSe Fig.3. XRD pattern of thermally evaporated thin films of [A] CZTS [B] CZTSe at different substrate temperatures (a) RT and (b) 673K. Fig.4. (a) SEM image of ball milled powder of CZTS (b) EDAX of CZTS Fig.5. (a) SEM image of ball milled powder of CZTSe(b) EDAX of CZTSe Fig.6.(a) SEM image (b) EDAX of CZTS thin films deposited at room temperature Fig.7. (a) SEM image of CZTS thin films (b) EDAX of CZTS deposited at 673K Fig.8. (a) SEM image (b) EDAX of CZTSe thin films deposited at roomtemperature Fig.9 (a) SEM image of deposited CZTSe thin films deposited at 673 K (b) EDAX of CZTSe at 673K Fig.10.AFM images of CZTS thin films deposited at RT(a) 2D (b) 3D image Fig.11.AFM images of CZTS thin films deposited at 673 K (a) 2D (b) 3D image Fig.12. AFM images of CZTSe thin films deposited at RT (a) 2D (b) 3D image Fig.13. AFM images of CZTSe thin films deposited at 673K (a) 2D (b) 3D image Fig.14.Transmittance and Absorbance spectra of CZTS thin films (a) RT (b) 673 K Fig.15.[A] Plot of (αhν2) versus hν to determine the optical bandgap of CZTS [B] variation of extinction coefficient (k) with wavelength of CZTS thin films: (a) RT and (b) 673K Fig.16.Transmittance and Absorbance spectra of CZTSe thin films (a) RT (b) 673K Fig.17.[A] Plot of (αhν2) versus hν to determine the optical bandgap of CZTSe[B] variation of extinction coefficient (k) with wavelength of CZTS thin films (a) RT and (b) 673 K

23

Fig.1. XRD pattern of [A] CZTS materials ball milled at different duration for (a) 15 (b) 30 (c) 45 & (d) 60 h [B] CZTSe ball milled for (a) 2 (b) 4 (c) 9 & (d) 14 h

24

Fig.2. Raman spectra of ball milled powders of (a) CZTS (b) CZTSe

25

Fig. 3 XRD pattern of thermally evaporated thin films of [A] CZTS [B] CZTSe at different substrate temperatures (a) RT and (b) 673K

26

Fig.4. (a) SEM image of ball milled powder of CZTS (b) EDAX of CZTS

27

Fig.5. (a) SEM image of ball milled powder of CZTSe(b) EDAX of CZTSe

28

Fig.6. (a) SEM image (b) EDAX of CZTS thin films deposited at room temperature

29

Fig.7. (a) SEM image of CZTS thin films (b) EDAX of CZTS deposited at 673 K

30

Fig.8. (a) SEM image (b) EDAX of CZTSe thin films deposited at roomtemperature

31

Fig.9 (a) SEM image of deposited CZTSe thin films deposited at 673 K (b) EDAX of CZTSe at 673K

32

Fig.10. AFM images of CZTS thin films deposited at RT (a) 2D (b) 3D image

33

Fig.11. AFM images of CZTS thin films deposited at 673 K (a) 2D (b) 3D image

34

Fig.12. AFM images of CZTSe thin films deposited at RT (a) 2D (b) 3D image

35

Fig.13. AFM images of CZTSe thin films deposited at 673K (a) 2D (b) 3D image

36

Fig.14.Transmittance and Absorbance spectra of CZTS thin films (a) RT (b) 673 K

37

hν to determine the optical bandgap of CZTS Fig.15. [A] Plot of (αhν2) versus h [B] variation of extinction coefficient (k) with wavelength of CZTS thin films: (a) RT and (b) 673K

38

Fig.16.Transmittance and Absorbance spectra of CZTSe thin films (a) RT (b) 673K

39

Fig.17.[A] Plot of (αhν2) versus h hν to determine the optical bandgap of CZTSe[B] variation of extinction coefficient (k) with wavelength of CZTS thin films (a) RT and (b) 673 K

40

Graphical Abstract

41

Highlights


Nanocrystalline powders of CZTS and CZTSe are synthesized by ball milling technique.
The ball milled powder was thermally evaporated on glass at room temperature and 673 K to prepare quaternary compound semiconducting thin films of CZTS and CZTSe in a single step process.
The synthesized powder and deposited CZTS and CZTSe thin films belong to tetragonal crystal system. Raman spectroscopy reveals that the synthesized nanocrystals are pure without any secondary phases. XRD analysis also revealed that the higher substrate temperature has caused an increase in crystallite size and reduction in the dislocation density.
SEM and AFM micrographs illustrate the granular type of growth and the roughness and particle sizes obtained at the substrate temperature of 673 K are higher than those obtained in the room temperature deposited films because of the higher mobility of the adatoms at higher substrate temperatures
The positive Hall coefficient obtained for the film confirms the p-type conductivity in these films. Therefore, the present work reveals that the deposited CZTS and CZTSe are promising candidates suitable for the solar cell fabrication as absorber thin film materials
A gradual reduction in optical bandgap was observed with increasing substrate temperature, which is associated with the enhanced crystallinity of the films

42