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Annealing Induced Shape Transformation of CZTS Nanorods Based Thin Films Govind Rajesh, Natarajan Muthukumarasamy, Dhayalan Velauthapillai, and Sudip Kumar K. Batabyal Langmuir, Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 24, 2017

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Annealing Induced Shape Transformation of CZTS Nanorods Based Thin Films G.Rajesh,a N. Muthukumarasamy,a Dhayalan Velauthapillai,b Sudip K. Batabyalc*

-----------------------------------------------------------------------------------------------------------------------------------------a

b

Department of Physics, Coimbatore Institute of Technology, Coimbatore, 641014, India.

Faculty of Engineering and Business Administration, Western Norway University of Applied Sciences, 5063 Bergen, Norway. c

Amrita Centre for Industrial Research & Innovation (ACIRI), Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, Amrita University, Coimbatore, Tamil Nadu 641112, India. email: [email protected]; [email protected]

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Annealing Induced Shape Transformation of CZTS Nanorods Based Thin Films G.Rajesh,a N. Muthukumarasamy,a Dhayalan Velauthapillai,b Sudip K. Batabyalc* a

Department of Physics, Coimbatore Institute of Technology, Coimbatore, India.

b

Faculty of Engineering and Business Administration, Western Norway University of Applied Sciences, 5063 Bergen, Norway. c Amrita Centre for Industrial Research & Innovation (ACIRI), Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, Amrita University, Coimbatore, Tamil Nadu 641112, India, email: [email protected]; [email protected]

Abstract: Here we studied the annealing induced shape transformation of 1-D Cu2ZnSnS4 (CZTS) nanorods from nanospheres and nanocubes by simple sol-gel method without using any toxic chemicals or complicated vacuum based technology. X-ray diffraction pattern and Raman Spectra reveals the formation of Kesterite structure CZTS thin films. The energy dispersive Xray analysis results indicate the presence of Cu, Zn, Sn and S. The elemental distribution of all the CZTS samples was studied using elemental mapping. The Hall effect studies of the CZTS thin film composed of nanorods exhibits the lowest resistivity values which indicates efficient charge transfer for unidirectional structure. The obtained optical band gap energy of CZTS thin film is in the range of 1.46 to 1.54 eV which is quite close to the optimum theoretical value required for solar cell applications.


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INTRODUCTION Recent research focus on the synthesis of device quality semiconductor nanocrystals has lead to the alternative processing of the materials for high efficiency cost effective solution processed photovoltaic devices [1]. Among the various types of photoactive semiconductor nanocrystals such as Cu2S, PbSe, Pb(Sx,Se1-x), CdTe and ternary and quaternary chalcogenide CuInSe2 (CIS), CuInGaS/Se, (CIGS) are the widely studied materials. Also because of the highest reported power conversion efficiency among the thin film PV devices, CIGS has been used in commercial market [2]. However, the high cost of the rare element indium and gallium is the main obstacle for the wide spread use of CIGS PV. Another potential candidate for the thin film PV device, CdTe also contains toxic heavy metal cadmium and tellurium which raises health and environmental issues. Quaternary photo voltaic material Cu2ZnSnS4 (CZTS) having a structure similar to CIGS has emerged out as a potential candidate for photovoltaic application. Abundant availability and non-toxic nature of the components, adjustable band gap (1.0 eV to 1.5 eV) and high optical absorption coefficient of about 104 cm-1 makes CZTS as the research focus in the search for alternative materials for photovoltaic absorber [3]. Therefore it is required to develop a simple preparation technique to synthesize CZTS nanostructures with the required shape and size. In CZTS synthesis, the properties of surfactant or stabilizer, nature of reaction and the annealing conditions play a critical role in determining the shape and size of the nanostructured CZTS. Selective binding of different surfactant to different facet of the crystals actually stabilizes the growth in that direction resulting in an anisotropic growth. Similarly the reaction time can control the growth from kinetically favourable regime to thermodynamically favourable one [4], e.g. from one shape to other shape is possible as the reaction and annealing time changes. Morphology control in CZTS nanoparticle synthesis has high impact on its role to form ideal building blocks for device quality film formation. Among the various shapes of CZTS nanostructures, CZTS nanorods have recently attracted great attention because of its hierarchical assembly, optimal crystal structure and the tuneable geometric parameters, which allows tuning the optical absorption by changing the diameter and length of the


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rods. Also because of 1D confinement the charge transfer characteristics will be modified in case of nanorods [5]. There are very few reports available on the synthesis of CZTS with tuneable shapes, synthesized by thermolysis method [5], solvothermal [6], hot injection [7] and microwave [8] assisted methods. These methods are either expensive or require harsh organic solvents. Though there are some reports available on the formation of CZTS nanorods by hot injection [5,9-10] or template directed method [11-12] but most of these nanostructures are of wurtzite phase [5, 9-11]. More over all these nanorods were reported in powder form, which needs sulfurization or selenization for the formation of absorber layer which can be used for thin film device fabrication. The crystal orientation of the nanorods will be in random direction during the time of selenization and sulfurization. There is only limited reports available about the growth of thin film CZTS with oriented nanorods [11, 12] where the growth of nanorods were controlled in 1dimension using 1D template. In the present work CZTS nanorods are grown without using any template in sol-gel based method. Different deposition techniques been used to deposit CZTS based thin films and some of them are thermal spray pyrolysis [13], sulfurization of sol-gel deposited film [14], sputtering [15-16], PLD [17] and e-beam evaporation [18]. Highest efficiency CZTS based device were fabricated by dissolving individual metal chalcogenides in anhydrous hydrazine which is highly toxic and explosive, so all the step of device fabrication were done inside the glove box. Also selenization or sulfurization is an important step for the existing methods, where the absorber layer is annealed at high temperature (~ 500 °C) in the presence of hydrogen sulphide or hydrogen selenide for the grain growth in the absorber layer. Sulfurization process also needs specially designed set up for the annealing in the presence of toxic H2S gas. Over all, physical deposition of the absorber layer needs high vacuum and sophisticated instrumentation which increases the cost of the PV devices. Sol-gel method of thin film deposition is always superior to the other methods as the technology is very simple and toxic chemicals are not used, which have high potential for large scale and low cost production of CZTS layer [18].

Here we report for the first time about shape transformation of kesterite CZTS nanostructures from nanosphere to nanocube and then to nanorods in sol-gel based CZTS thin films, synthesized ACS Paragon Plus Environment

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by simple and cost effective technique by dip coating the substrate in the precursor solution without using any toxic chemicals or complex vacuum technology. The formation of nanorods were studied and the transformation of nanospheres to nanocubes and then to nanorods have been investigated in detail.

■ EXPERIMENTAL SECTION Materials and Methods Analytical reagent grade of Cu(NO3)2, Zn(NO3)2, SnCl2, thiourea, 2-methoxy ethanol and monoethanolamine were purchased from Merck. All the chemicals and solvents were used as received without any further purification.

Fabrication of precursor solution The Cu2ZnSnS4 thin films have been deposited on glass substrate using cost effective simple solgel dip coating technique. Cu(NO3)2, Zn(NO3)2, SnCl2 and thiourea were dissolved in 2-methoxyethanol. Monoethanolamine (MEA) was used here to stabilize the precursor from the precipitate formation [20]. A magnetic stirrer was used to stir the solution for 1h at room temperature to obtain a transparent clear solution.

Thin film deposition and formation of CZTS nanorods The glass plates were ultrasonically cleaned using acetone, ethanol and de-ionised water to acquire a proper adhesion of the films with the substrate. The cleaned glass plates were dipped into the prepared solution for 20 seconds and then it was dried for 25 seconds in air. The sol-gel dip coating was repeated 50 times to obtain films with required thickness and uniform coating. After completing the deposition cycle, the substrates were taken out from the solution and were washed carefully with deionised water for the surface cleaning of the substrate from the residual salt. The deposited films were dried at 70°C to remove the excess solvent and organic residues present in it and for film densification. This was followed by a post annealing treatment at 250°C in air. Finally we got a uniform film with good adherence to the substrate with average thickness in the range of 180 nm to 220 nm (as measured from the FESEM).


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Characterization Structural properties of the fabricated films were studied by X-ray diffractometer Bruker D8. Raman spectrometer (Horiba-Jobin Labram 800 HR) was used to record the Raman spectra. FESEM analysis has been carried out by CARL ZEISS-SIGMA instrument. JEOL, JEM-ARM200F was used to reveals the High-resolution TEM (HRTEM) images of the samples along with the selected area diffraction pattern (SAED). Optical properties of the CZTS thin films were studied in the UV-Vis–NIR range using Jasco V570 spectrophotometer. Hall measurement values were measured by ECOPIA HMS-2000 Hall measurement system.

■ RESULTS AND DISCUSSION Formation of CZTS Kesterite type CZTS nanospheres, nanocubes and nanorods based thin films have been synthesized by simple sol -gel dip coating technique. The CZTS films were formed by following possible reaction mechanism [21].

The over all ionic reaction is

So finally the metal ions and the thiourea complex formed the precursor solution, where the complex interacts with the metal ions through the sulphur atom. Thus the Cu2ZnSnS4 formation is according to the equation


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S 2Cu (NO 3)2

+

Zn (NO3)2

+

SnCl2

Cu2 Zn Sn S4 + 6 NH4NO3

+ 4 NH2-C-NH2

+ 2NH4Cl

+

+

4CO2

8H2O

(6)

Figure 1 Schematic diagram of the formation of CZTS nanosphere, nanocubes and nanorods The as deposited CZTS films were annealed in air ambience at 250°C for different time period. As shown in figure 1, the prepared CZTS is found to have different nanostructures depending on the annealing time. When the prepared CZTS films were annealed for 1 hour at 250°C, nanospheres are found inside film. As the annealing time was increased it is observed that there is a shape transformation in nanostructures. When the annealing time is increased to 2 hours the nanosphere got transformed to nano cubes and as the time further increased to 3 hours CZTS nanorods has been formed.

Structural Properties The crystalline nature of the synthesized CZTS films annealed for a period of 1 hour, 2 hours and 3 hours at a temperature of 250° C has been studied using x-ray diffraction method


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and the obtained diffraction pattern is shown in figure 2. The X-ray diffraction patterns of the nanospheres and nanocubes exhibit broad peaks revealing that the crystallite size of the CZTS nanospheres and nanocubes are small. The observed peak at the 2θ positions 28.64°, 47.58° and 55.4° are indexed as the corresponding to the (112), (220), (312) planes of tetragonal phase of kesterite CZTS. The lattice parameters have been calculated and are a = 5.431 Ǻ and c = 10.847Å. These resulted values are quite close with the reported values (a = 5.427Å and c = 10.848 Å) in the JCPDF card (JCPDS card no. 26-0575).


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Figure 2. X-ray diffraction pattern of the CZTS nanosphere, and nanocube and nanorod based thin films The XRD pattern of CZTS nanorods shows sharp, high intensity and well defined peak corresponding to (112) plane of kesterite CZTS. The sharp peaks indicate the crystallinity of the sample. The diffraction peak of (112) plane is dominant than (220) and (312) diffraction peaks, and this indicates that the CZTS nanorods have almost aligned along the c-axis and the growth direction is nearly perpendicular to the base surface. The presence of relatively high intensity peaks in the 3 hours annealed films shows that the small size grains have started to grow with increasing annealing time. The presence of very small peaks in the 1 hour annealed film shows that nucleation of grains has started. The presence of relatively high intensity peaks in the 2 hours and 3 hours annealed films shows that the small size grains have started to grow with increasing annealing time.

Figure 3. Raman spectra of the CZTS nanosphere, nanocube and nanorod based thin films


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Raman spectroscopy is a tool used to analyse the structure and phase purity of ternary and quaternary materials. The quaternary semiconductor CZTS often contains secondary and ternary phases with similar crystal structure may coexist with the peak position of CZTS and this makes hard to identify phase identification of CZTS using X-ray diffractogram. To more exact identification of secondary phases in quaternary materials, The Raman scattering measurements has been extensively performed. The Raman spectra of CZTS nanospheres, nanocubes and nanorods based thin films are shown in figure 3. The maximum intensity peak observed at 331 cm-1 corresponds to the vibrational A1 symmetry mode of CZTS. This peak appears to be shifted to lower wave number [22-24] when compared to reported value 338 cm-1 [25]. This shift to the lower wavenumber is probably due to the gradient in strain in nanostructures [25]. The Raman peak is observed to be broad. The broadening of the Raman peak is due to the phonon confinement within the nanocrystal and is in agreement with the reported results [26]. Also peak position for nanorods is shifted slightly lower wave number than the nanospheres. Probably this shifting is because of the strain and the crystal quality in nanorods [25]. It is observed that with the excitation of 532 nm there are no additional peaks corresponding to other phases such as, SnS, and Cu2S.

(a)

(b)

200 nm

200 nm

2 μm

2 μm

(c)

200 nm


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Figure 4. FESEM images of (a) nanosphere (b) nanocube and (c) nanorods based CZTS thin films. Figure 4(a,b,c) shows the surface morphology of CZTS thin films annealed for 1 hour, 2 hours and 3 hours at 250°C respectively. The CZTS thin film prepared using 1 hour annealing time is shown in figure 4(a). The FESEM image reveals that the constituent particles of CZTS have agglomerated and has lead to the formation of particles in spherical shape and the image also suggests that the samples have uniform grain distribution. When the annealing time is increased to 2 hours the aggregates are formed from small particles covering the substrate. The FESEM image Figure 4(b) shows the formation of cubes all over the surface and they exhibit a smooth surface morphology without any cracks. The CZTS thin film annealed for a period of 3 hours shown in Figure 3(c). The FESEM images exhibits the formation of uniformly distributed nanorods all over the surface.


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Figure 5: EDAX spectra and Elemental mapping (inset) of CZTS nanosphere (a), nanocube (b) and nanorods(c) based thin films.

The EDAX analysis of the samples reveals that the constituent elements Cu, Zn Sn and S are present in the samples and it shows the formation of CZTS with almost stoichiometric


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composition. Obtained compositions of different samples are tabulated in Table 1. Composition tuning is considered to be important in the pathway towards the high efficiency solar cell. Elemental distributions in all the sample and identification of elemental distribution of the CZTS thin films were carried out by elemental mapping. Figure 5(a) reveals the EDAX spectrum and the elemental mapping of the CZTS nanospheres based thin film. EDAX peak from excess chlorine confirms the presence of excess thiourea SnCl2 complex. The EDAX spectrum along with the elemental mapping of nanocubes based CZTS films were shown in figure5 (b). Finally the EDAX spectrum of nanorods based CZTS thin film reveals the formation Cu poor CZTS, which probably ideal for the PV devices (figure 5c). The elemental mapping of the nanorods based CZTS film shows the uniform distribution of the elements along the whole surface of the films. Table 1 compositional ratio of CZTS nanosphere, nanocube and nanorod CZTS

Cu (at%)

Zn (at%)

Sn (at%)

S (at%)

Cl (at%)

Sphere

7.08

15.75

22.18

36.95

18.04

Cube

16.81

13.65

16.02

38.52

15.00

Rods

16.28

16.01

16.77

36.22

14.72


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Figure 6. HRTEM images of CZTS nanosphere, nanocube and nanorod based film. High-resolution transmission electron microscopy (HRTEM) has been used to investigate the nanostructure of CZTS thin films. A portion of the thin film was peeled out and dispersed in solvent for TEM analysis. Figure 6 shows the HRTEM images of different CZTS samples which exhibits spherical (Figure 6a,) or cube-like (Figure 6b), or rod-like (Figure 6c) shapes as obtained depending on the annealing conditions. Figure 6 (a) shows the HRTEM image of spherical CZTS nanocrystals synthesized by 1hour annealing. The average size of the spherical CZTS nanocrystals is ~ 11 nm. When the annealing time is increased from 1 hour to 2 hours spherical nanocrystals of CZTS transform to cube-like CZTS (figure 6(b)) with average size of ~16. nm. Rod-like CZTS nanocrystals (Figure 6(c)) with an average length of ~400 nm and diameter of ~50 nm are obtained when the annealing time is increased to 3 hours. High-resolution (HRTEM) images of single CZTS nanocrystals of different shapes exhibits clear lattice fringe patterns, which establish the single crystalline nature of the samples. The d spacing value of the 1D rod-like nanostructure measured from SAED pattern is 0.318 nm corresponding to the (112) lattice plane of kesterite CZTS (figure 6(d)). The electrical properties of the materials lead the charge carrier dynamics, and ultimately efficiency of the PV devices depends largely on the electrical parameter of the absorber material. Table 2 shows the Hall measurement values obtained for the CZTS nanospheres, nanocubes and nanorods based thin films of size 1cm X 1cm on glass substrate. Four metal circular dots were deposited on four corner of the film for better contact with the probe. The CZTS thin film composed of rods exhibits the lowest resistivity values which may be due to the crystalline nature of the sample and efficient charge transfer in the 1-D structure. The carrier concentration is found to be higher in the rods shaped film when compared to the spherical and cube-like CZTS thin films, which is beneficial for photovoltaic application. The obtained resistivity and the carrier concentration of these CZTS nanorods are of same order as reported for similar 1-D structures of CZTS nanocrystals [5]. The obtained Hall mobility of the samples truly depends on the grain size and the texture, the orientation of the grain and the crystallinity. Hall measurement confirmed the p-type conductivity of the CZTS thin films.


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Table 2. Hall effect measurement values for the CZTS thin films. Carrier concentration CZTS

Resistivity(Ωcm)

-3

Mobility(cm2/Vs)

Type

(cm ) nanosphere

12.13 x103

2.17 x1011

1.91

P

nanocube

8.24 x103

1.66 x1012

2.52

p

nanorods

4.14 x103

4.76 x1012

2.86

P

The optical properties of CZTS films have been investigated by measuring the absorbance spectra in UVVis –NIR region. The light absorptions by the CZTS films annealed at different temperatures at different wavelength are shown in figure 7. The absorption spectra of the CZTS films shows that the absorption edge is slightly shifted towards longer wavelength with increase in annealing time and the red shift of absorption edge can be attributed to the increase in grain size of CZTS films on annealing. This shift towards longer wavelength indicates that there is decrease in the optical band gap.

Absorbance(a.u)

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3h

2h 1h

Figure

7.

Absorbance spectra of the 400

600

CZTS nanosphere, nanocube and

800

1000

Wavelength (nm)

1200

1400

nanorod based thin film


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Optical absorption arises from the electronic transition is very much useful to understand the nature and the value of the optical band gap of the materials. The optical band gap Eg of the thin films were calculated using the formula,

(αhυ ) = A(hυ − E g )

1 2

Where A is a constant, α is the absorption coefficient, and hυ` is the incident photon energy. A Plot of (αhυ)2 versus (hυ) is drawn and using the plot the band gap energy of the CZTS films has been determined. The graph of (αhυ)2 versus (hυ) of CZTS films is shown in figure 8. The straight line portion of the (αhυ)2 vs (hυ) plots is extrapolated to hυ axis and optical band gap were determined from the crossing point. It is found that the band gap energy of the film decrease with increase in annealing time. The obtained band gap values of the CZTS films at different annealing time are in good agreement with the literature reported values [5] as given in Table 2.

2.00E+014

1h 2h 3h

2

1.60E+014

1.20E+014

2

(α hυ ) (eV/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8.00E+013

4.00E+013

0.00E+000 0.5

1.0

Figure 8. Plot of (αhυ)2 versus (hυ)

1.5

hυ (eV)

2.0

of the CZTS sphere, cube and rod based

thin


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Table 2 Band gap value of the CZTS (sphere, cube rod) based films

CZTS Shapes

Band gap (eV)

sphere

1.53

cube

1.48

rod

1.46

Discussion In initial stage of annealling CZTS nanospheres were formed, which slowly transformed to nanocubes and then to nanorods. The CZTS nanospheres are probably formed by three different steps. Initially the metal thiourea complexes were from when metal ions (Cu2+, Zn2+, and Sn2+) are coordinated with thiourea [27]. Among the three different metal complex Copper complex first decomposed and formed the copper sulphide and slowly other two metal complex formed Zn and Sn sulphide. These three activated sulphides are then reacted to form the CZTS. In the formation of CZTS the binary copper sulfide start the reaction as a nucleation point and growth of CZTS happens when copper sulfide gradually transform into CZTS grains. The small CZTS grains were aggregated to form a spherical of lowest surface energy [28-29]. The formation sequence of 1D CZTS nanorod from nanospheres and nanocubes, consists of various distinct steps nucleation, aggregation, coalescence and subsequent grain growth in liquid flux. At high enough concentration of component most eventually growth happens in to meso phases or in liquid flux. The consequence is that the shape of the nanocrystals can be transformed from one to another by introducing changes in the growth condition such as


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concentration, temperature or the annealing condition [30]. Heat and mass transport phenomena play a crucial role in the transformation of CZTS nanospheres and cubes to nanorods. Surface melting and dynamic viscosity of the liquid flux are the thermophysical properties which significantly influence the shape transformation in nanocrystals. The thermal conductivity and dynamic viscosity of nanofluids are functions of the size, shape of nanoparticles as well as the type and working temperature or melting point of the liquid flux [31]. In this growth process of 1D CZTS nanorods, maybe excess tin chloride thiourea complex in the CZTS thin film act as a liquid flux. Probably SnCl2 and thiourea formed a complex with low melting temperature, which plays a vital role in shape transformation of CZTS which can be achieved by tuning the annealing period at 250°C. During nucleation, as the annealing time increases from 1 hour to 2 hours clusters of spherical CZTS particles likely undergo rapid nucleation to form the cube like nanocrystals. Cube like CZTS nanostructures is formed by coalescence of aggregated spherical particles. Subsequently, cube like CZTS film grows to a certain thickness on the substrate surface by stacking of the particles. Spherical aggregates formed first, nucleate as cube like CZTS nanocrystals and gradually after mass transport through the molten complex, nanocubes grow to nanorods in 2 to 3 hours. In the EDX analysis of nano spheres it has been observed that there are excess of Sn and Cl which act as the liquid flux for the growth of the nanorods from the nanocubes. The FESEM image of nanorods also shows the cross section of these rods are square like which also supports the rods growth from the cube on prolonged annealing. Conclusions:


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We have synthesized CZTS nanorods based thin films by sol-gel mediated solution based method. The growth mechanism of the CZTS nanorods were studied and found that rods were formed from nanospheres which transformed to nanocubes and finally nanorods. The excess Sn and Cl in the preliminary stage of the growth formed the molten flux for the growth of nanorods. The optical and electrical properties of these nanostructures were investigated in detail. The resistivity and the carrier concentration of the CZTS nanorods are found to be more than those of the CZTS nanocubes and nanospheres. Acknowledgments: SKB acknowledges Science and Engineering Research Board (SERB) of Department of Science and Technology (DST) for financial support (research grant -ECR/2015/000208). References 1.

Guo, Q.; Hillhouse, H. W.; Agrawal, R. Synthesis of Cu2ZnSnS4 Nanocrystal Ink and its Use for

Solar Cells. J. Am. Chem. Soc. 2009, 131, 11672–11673. 2.

Jackson, P.; Wuerz, R.; Hariskos, D.; Lotter, E.; Witte, W.; Powalla, M.; Effects of heavy alkali

elements in Cu(In,Ga)Se2solar cells with efficiencies up to 22.6% Phys. Status Solidi Rapid Res. Lett.. 2016, 10, 583-586. 3.

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