Growth and characterization of dual ion beam sputtered Cu2ZnSn(S, Se)4 thin films for cost-effective photovoltaic application

Solar Energy 139 (2016) 1–12

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Growth and characterization of dual ion beam sputtered Cu2ZnSn(S, Se)4 thin films for cost-effective photovoltaic application Brajendra S. Sengar a, Vivek Garg a, Vishnu Awasthi a, Aaryashree a, Shailendra Kumar b, C. Mukherjee c, Mukul Gupta d, Shaibal Mukherjee a,⇑ a

Hybrid Nanodevice Research Group (HNRG), Electrical Engineering, Indian Institute of Technology (IIT), Indore 453552, India Indus Synchrotron Utilization Division, Raja Ramanna Center for Advanced Technology, Indore 452013, India Mechanical and Optical Support Section, Raja Ramanna Center for Advanced Technology, Indore 452013, India d University Grants Commission, Department of Atomic Energy (UGC DAE), Consortium for Scientific Research, Indore, India b c

a r t i c l e

i n f o

Article history: Received 4 May 2016 Received in revised form 22 August 2016 Accepted 17 September 2016

Keywords: CZTSSe DIBS Thin film solar cells Composition Structure Optical properties

a b s t r a c t A systematic growth optimization of Cu2ZnSn(S, Se)4 (CZTSSe) thin films by dual ion beam sputtering system from a single CZTSSe target is presented. It is observed that the ratio of Cu/(Zn + Sn) varies from 0.86 to 1.5 and that of (S + Se)/metal varies between 0.62 and 0.97 when substrate temperature (Tsub) is increased from 100 to 500 °C. The crystal structure of all CZTSSe films are identified to be preferentially (1 1 2)-oriented, polycrystalline in nature, and without the existence of secondary phases such as Cu2(S, Se) or Zn(S, Se). The full-width at half-maximum of (1 1 2) diffraction peak is the minimum with a value of 0.12° and the maximum crystallite size 75.11 nm for CZTSSe grown at 300 °C. Morphological investigation reveals the achievement of the largest grain size at Tsub = 300 °C. The band gap of CZTSSe thin films at room temperature, as determined by spectroscopic ellipsometry, varies from 1.23 to 1.70 eV, depending on Tsub. The optical absorption coefficient of all CZTSSe thin films is >104 cm
1. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The optical band gap of 1.45 eV and high absorption coefficient of P104 cm
1 have made Cu2ZnSnS4 (CZTS) films emanate as one of the most promising absorber materials in thin-film photovoltaics (Babu et al., 2008). The fact that CZTS consists of elements that are abundant and non-toxic in nature makes it even more favorable while realizing cost-effective and environmentfriendly solar cells (SCs). Moreover, CZTS and Cu2ZnSnSe4 (CZTSe) based SCs have been reported to produce photon conversion efficiency (PCE) values of up to 9.2% (Sugimoto et al., 2013) and 11.6% (Lee et al., 2015), respectively, while Cu2ZnSn(S, Se)4 (CZTSSe) based kesterite compounds have been reported to achieve PCE value as high as 12.7% (Kim et al., 2014), making CZTSSe even more competitive to Cu(In, Ga)Se2 as compared to CZTS and CZTSe based thin film cost-effective SCs. A number of growth mechanisms have been deployed to deposit CZTS materials such as thermal evaporation (Tanaka et al., 2006; Weber et al., 2009), sputtering (Jimbo et al., 2007; Liu et al., 2010; Seol et al., 2003), spray pyrolysis (Yoo and Kim, 2011), solution-based synthesis (Barkhouse et al., 2012; Wang ⇑ Corresponding author. E-mail address: [email protected] (S. Mukherjee). http://dx.doi.org/10.1016/j.solener.2016.09.016 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.

et al., 2014), electrodeposition (Chan et al., 2010; Scragg et al., 2009), pulsed laser deposition (He et al., 2012b; Sun et al., 2011), etc. However, anticipating future large-scale production for domestic and commercial purposes, sputtering is acknowledged as a more promising method for CZTS preparation, since it has obvious advantages in achieving uniform deposition on large substrate area (Garg et al., 2016). In case of sputtering mechanism, the typical process consists of a two-stage method: (a) firstly, metallic or S-containing precursors are prepared by sputtering, and (b) annealed in N2 or H2(S, Se) (sulfurization or selenization process) atmosphere (He et al., 2013; Inamdar et al., 2013). Further, according to the type of targets used for precursor preparation, sputtering can be broadly categorized into three: (1) sputtering from metallic targets, where elemental metal or metal-alloy targets are sputtered and metallic precursors are obtained (Fernandes et al., 2010); (2) sputtering from S-containing targets, where binary ZnS, SnS, or Cu2S targets are used for precursor preparation (Hlaing OO et al., 2011); and (3) direct sputtering from a single quaternary Cu-ZnSn-S targets (He et al., 2013). However, as compared to the former two sputtering categories, the third category is expected to produce stoichiometrically more uniform film and the growth process becomes less complicated (Frantz et al., 2011). Nonetheless, research work about fabricating CZTSSe absorber films using a single targets is rare so far.

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In the present work, dual ion beam sputtering (DIBS) system is deployed for the growth of CZTSSe films without post-annealing treatment and from a single target sputtering process. Single target sputtering technique offers numerous other advantages such as uniform composition of thin film, smooth surface, and simple deposition process (Shi et al., 2011). A number of advantages of DIBS system as compared to conventional sputtering technique is that it yields high-quality thin films with comparatively superior compositional stoichiometry, uniformity, and adhesion to the substrate (Awasthi et al., 2014; Pandey et al., 2013). Besides these, other salient features of DIBS system include high quality growth with reduced surface roughness on a larger substrate area and in-situ substrate pre-cleaning before carrying out growth process. Therefore, the study of the DIBS growth of CZTSSe films from a single target without annealing treatment would be quite interesting, especially in terms of realizing CZTSSe thin film for SC applications. To the best of author’s knowledge, the detailed analysis of deposition of CZTSSe thin films from a single target by DIBS system and without post annealing treatment has not been reported yet.

2. Experimental CZTSSe thin films were deposited on soda-lime glass (SLG) substrates by a single-step route with the help of DIBS system. Prior to the growth process, the substrates (10 mm  10 mm) were ultrasonically cleaned in soap solution, acetone, isopropanol, and deionized water sequentially for 20 min each, in order to eliminate dust particles and numerous organic impurities. The substrates were then purged with 5 N-purity (99.999%) nitrogen gas. During CZTSSe growth processes, in addition to the radio-frequency (RF) primary ion source, direct-current coupled (DC) assist ion source consisting of plasma of Ar+ ion, was also turned on to reduce the columnar growth and thereby boost growth uniformity and film adhesion to the substrate. The discharge voltage and current of the assist ion source were kept constant at 60 V and 600 mA, respectively. The CZTSSe thin film growth was carried out using a 4 N (99.99%) pure and 4-in-diameter CZTSSe target mounted on water-cooled target holder inside the DIBS system chamber. The background pressure inside the process chamber was maintained at 1  10
8 mbar while the working pressure during film growth was kept constant at 1  10
4 mbar. CZTSSe growth temperature was varied from 100 to 500 °C in pure Ar gas ambience with the primary ion source RF power of 45 W. The chemical composition, structural properties, and morphology of these CZTSSe thin films were examined by energy dispersive X-ray spectroscopy (EDX, Oxford Instruments), X-ray diffraction (XRD, Rigaku Cu Ka radiation, k = 0.154 nm) and field emission scanning electron microscope (FESEM, Zeiss Supra 55), respectively. The elemental depth profiling was carried out by Hiden secondary ion mass spectroscopy (SIMS) workstation with oxygen ion gun of energy up to 5 keV. Transmission and optical properties of the films were measured by Spectroscopic Ellipsometry (SE, M2000D from J. A. Woollam Co., Inc) instrument, while the chemical bonding states were examined by X-ray photoelectron spectroscopy (XPS) utilizing a PHOIBOS 100 analyzer with an Al Ka radiation (1486.6 eV) as an excitation source. It should be mentioned here that before carrying out XPS measurements, the sample surface was cleaned using 1 keV Ar+ ion beam in order to remove any contamination on the top layer. The film roughness is measured by using Advance Integrated Scanning Tools for Nano-Technology (AIST-NT) atomic force microscope (AFM). Four-probe hall measurement setup was employed to measure the electrical properties of CZTSSe films. Raman spectroscopy was done using a Raman Microscope (Research India Raman Spec-

troscope RIRM151). All measurements were performed at room temperature. 3. Results and discussion 3.1. Composition analysis 3.1.1. EDX EDX measurement was carried out for the detailed investigation of the elemental ratios of CZTSSe films for different substrate temperatures (Tsub). The measurement data thus obtained from EDX analysis, as in Fig. 1(a), has been tabulated in Table 1. EDX results presented in the table are average values of the EDX spectra measured at different points on the sample. These points were taken over a large area (200 lm  200 lm), since it was essential to have the data at various points in order to achieve an accurate chemical composition of the samples. It is observed that, for the films deposited at different Tsub, the ratio of Cu/(Zn + Sn) varies from 0.86 to 1.5 and that of (S + Se)/ metal varies between 0.62 and 0.97. The ratio of Zn/Sn increases quite significantly from 0.84 to 16.01 with an increase in Tsub from 100 to 500 °C. This is because as Tsub is increased, the loss of Sn is observed which is probably due to the fact that Sn is more accessible to loss by evaporation of SnS phase at higher temperatures (Redinger and Siebentritt, 2010; Salomé et al., 2010). Furthermore, the thin films deposited at Tsub = 200 and 300 °C show Cu-poor and Zn-rich states (i.e. Cu/(Zn + Sn) < 1 and Zn/Sn > 1), which is not the case for films grown at other Tsub. Todorov et al. have reported that the CZTSSe thin films with the highest PCE are typically of Cu-poor and Zn-rich states (Todorov et al., 2010). The photovoltaic performance is supposedly enriched by Cu-poor and Zn-rich conditions because of two reasons: (1) the shallow acceptor (Cu vacancy, VCu) becomes the dominant defect, in place of the relatively deeper acceptor (CuZn antisite), and determines the population of the majority carriers (i.e. holes in the absorber material) (Bourdais et al., 2016; Chen et al., 2013); and (2) it suppresses the occurrence of SnZn defect, which is a deep-level donor defect acting as a recombination center (Liu et al., 2016). Further, as observed in Fig. 1(a), for Tsub = 200 and 300 °C the values of Cu/(Zn + Sn) and Zn/Sn ratio are very close to the optimal values (Cu/(Zn + Sn)  0.85 and Zn/Sn  1.1–1.3) (Liu et al., 2016). However, the film grown at 200 °C is (S + Se) deficient, which is not the case with the film grown at 300 °C. Therefore, CZTSSe grown at Tsub = 300 °C seems to possess nearly ideal stoichiometric composition as compared to others grown at different Tsub. It has been reported that many physical properties ‘P’ of a semiconductor compound ‘AxB1
x’ as a function of x, follow the quadratic rules as presented in Eq. (1) (Chen et al., 2007),

PðxÞ ¼ xPðAÞ þ ð1
xÞPðBÞ
bp xð1
xÞ

ð1Þ

where bp is the so-called bowing parameter. Considering CZTSSe as a solid solution, the band gap of the Cu2ZnSn(Sx, Se1
x)4 thin film deposited at different Tsub can be calculated using Eq. (1). Applying Eq. (1) for band gap (EEDX g ) calculation for CZTSSe, we could derive Eq. (2): CZTS EEDX þ ð1
xÞECZTSe
bb xð1
xÞ g ðxÞ ¼ xEg g

ð2Þ

where bb is the band bowing parameter, which is 0.07 eV for CZTSSe (Walsh et al., 2012) and x = S/(S + Se), as determined by EDX measurements and listed in Table 1. Energy band gap of CZTS (ECZTS ) g ) are 1.5 and 1.0 eV, respectively at room temperand CZTSe (ECZTSe g of CZTSSe thin film deposited at difature (Walsh et al., 2012). EEDX g ferent Tsub is calculated and presented in Table 2 based on the EDX increases with the corresults. It is observed that the value of EEDX g

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B.S. Sengar et al. / Solar Energy 139 (2016) 1–12

(b)

Zn/Sn

Cu/(Zn+Sn)

12 1.2

0.8 6 0.7

1.0 0 100

200

300

400

Intensity (counts/sec )

0.9

(S+Se)/Metals

(a)

Cu/(Zn+Sn) Zn/Sn (S+Se)/Metals

1.4

0.8

Sn

18 1.0

1.6

Cu Zn

S Se

0.6

10

500

20

Tsub ( C)

(c)

40

50

(d) Sn

Sn

Cu

Intensity (counts/sec)

Cu

Intensity (counts/sec)

30

Time (minutes)

Zn S Se

Zn

Se S

10

20

30

40

10

50

20

30

(e)

40

50

Time (minutes)

Time (minutes)

Sn

(f)

Zn

Intensity (counts/sec )

Intensity (counts/sec)

Zn Cu

S

Cu Sn

S

Se 10

Se 20

30

40

50

10

Time (minutes)

20

30

40

50

Time (minutes)

Fig. 1. (a) Composition and SIMS profile of CZTSSe films grown at Tsub = (b) 100 °C, (c) 200 °C, (d) 300 °C, (e) 400 °C, (f) 500 °C.

Table 1 Chemical composition, as measured by EDX, of CZTSSe grown at different Tsub from 100 to 500 °C. Tsub (°C)

CZTSSe Target 100 200 300 400 500

Atomic composition (%)

Ratio

Cu

Zn

Sn

S

Se

S + Se

Cu/(Zn + Sn)

Zn/Sn

(S + Se)/Metals

S/(S + Se)

22.25 29.03 28.68 23.62 29.66 31.23

15.25 13.87 16.74 14.21 12.63 18.9

12.5 16.5 16.14 13 10.56 1.18

40 33.7 32.31 41.47 39.05 41.59

10 6.9 6.12 7.7 8.1 7.1

50 40.6 38.43 49.17 47.15 48.69

0.81 0.96 0.87 0.86 1.2 1.5

1.22 0.84 1.04 1.1 1.19 16.01

1 0.68 0.62 0.97 0.89 0.94

0.8 0.83 0.841 0.843 0.828 0.854

responding rise in S/(S + Se). This is because the location of p core level in S is at lower position than that in Se, thus the position of valence band maximum of the sulfides is lower than that of the

selenides and the shorter bond length of Sn-S in comparison to that of Sn-Se makes the level repulsion stronger and moves its conduction band minimum upwards (Chen et al., 2011).

B.S. Sengar et al. / Solar Energy 139 (2016) 1–12

Table 2 Room temperature band gap (Eg ) ratio from EDX and SE analysis. Tsub (°C)

S/(S + Se) as calculated from EDX

EEDX (eV) g

ESE g (eV)

100 200 300 400 500

0.83 0.841 0.843 0.828 0.854

1.405 1.411 1.412 1.404 1.420

1.35 1.43 1.49 1.23 1.70

3.1.2. SIMS The elemental distribution in the films as a function of depth was probed by SIMS measurement and the depth profile of the constituent elements such as Cu, Zn, Sn, S, and Se, as obtained, are shown in Fig. 1(b)–(f). The qualitative evaluation indicates a uniform distribution of the elements throughout the CZTSSe grown at Tsub = 100 and 200 °C. For thin films grown at 300, 400, and 500 °C, the ion intensity of Cu is increased slightly at the surface, since Cu remains at the surface even after the volatilization of S,

Se, Zn and Sn; while that of Zn decreased considerably near the surface, as Zn is volatilized at high Tsub (Byeon et al., 2013). Hence, in comparison to the bulk, the surface of the film grown at Tsub = 300, 400, and 500 °C has higher Cu and lower Zn. Lower Zn at the surface inhibits the formation of Zn related secondary compounds i.e. Zn(S, Se), which plays a role of a current-blocking entity in SCs. The film grown at Tsub = 300, 400, and 500 °C, show a strong reduction of Cu intensity with a relatively higher Zn intensity close to the surface of SLG substrate. This higher intensity may be related to the presence of a minor phase containing Zn such as Zn(S, Se). Noticeable loss of Sn is also observed in the thin film grown at Tsub = 500 °C in agreement with the EDX study. 3.2. Structural properties XRD patterns of CZTSSe thin films deposited at various Tsub are depicted in Fig. 2(a). From the figure the diffraction peaks can be indexed to (1 1 2) and (2 2 0/2 0 4) lattice planes at 2h value of 28°

100

FWHM Crystallite size

(b)

74

0.4

58

0.3

42

0.2

26

b(

C)

200 300 400 500

FWHM (degree)

Intensity (a.u)

(112)

(220/224)

0.5

(a)

0.1

T su

20

25

30

35

40

45

10 100

50

200

2 (degree) 28.28

(c)

300

400

500

Tsub ( C)

2 S/(S+Se)

0.83

Intensity (a.u.)

28.20

S/(S+Se)

0.84

CZTS

(d)

0.85

28.24

2 (degree)

Crystallite size (nm)

4

CZTSe ZnS

28.16 0.82 100

200

300

400

200

500

250

300

350

400

-1

Tsub ( C)

Raman shift (cm )

(e) Intensity (a.u.)

338 cm-1(A) 333 cm-1 (A )

310

320

330

340

350

360

370

-1

Raman shift (cm ) Fig. 2. (a) XRD patterns, (b) FWHM and crystallite size of (1 1 2) peak, (c) variation of S/(S + Se) and position of (1 1 2) peaks with the change in Tsub from 100 to 500 °C, (d) Raman spectra of the CZTS thin films at Tsub = 300 °C, and (e) Raman spectra of CZTS with the peak fitting with Lorentzian curves.

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B.S. Sengar et al. / Solar Energy 139 (2016) 1–12

and 47°, respectively. The crystal structure of all CZTSSe films, as observed in Fig. 2(a), are identified to be preferentially (1 1 2)oriented, polycrystalline in nature, and without secondary phases such as Cu2(S, Se) or Zn(S, Se) (Salomé et al., 2012b). As Tsub is increased from 100 to 300 °C, the intensity of (1 1 2) peak increased consistently, indicating an improvement of (1 1 2) crystal orientation. However, further increment in Tsub resulted in a reduction of (1 1 2) peak intensity. The crystallite size (D) in these films can be evaluated using Scherrer’s formula (Cohen, 1956):

D¼

sk f cos h

ð3Þ

where s is Scherrer’s constant, k is X-ray wavelength, f is full-width at half-maximum (FWHM) of (1 1 2) diffraction peak and h is the corresponding Bragg’s angle. In Fig. 2(b), the crystallite size is seen to increase from 32 to 75 nm with increasing Tsub from 100 to 300 °C. Supposedly, when Tsub increases from 100 to 300 °C, the atoms gain enough kinetic energy and surface mobility to occupy stable lattice positions inside CZTSSe crystal lattice structure. It gives rise to the stable and highly c-axis oriented structure with improved crystalline quality of film. However, with further increase in Tsub up to 500 °C, the crystallite size is observed to decline to as low as 18 nm, indicating a subsequent degradation in the crystalline quality. In this phase, the increase in Tsub results in the breaking of CZTSSe bonds and consequent re-sputtering of the deposited film rather than enabling the atoms to move to their stable lattice sites, producing defects in the film, and thus leading to degradation in the crystalline property. CZTSSe grown at 500 °C has a very poor crystal quality, in comparison to other films, with the lowest (1 1 2) peak intensity with a very high value of FWHM (0.49°) and the corresponding crystallite size of 18 nm. This can be attributed to the low concentration of Sn at this temperature, as verified from the EDX analysis. Hence, from the above investigation it is observed that the FWHM of (1 1 2) diffraction peak is the minimum with a value of 0.12° and the maximum crystallite size 75.11 nm for CZTSSe film grown at Tsub = 300 °C, establishing 300 °C as the optimized Tsub for DIBS to achieve the best CZTSSe crystalline quality. The lattice constants of CZTSSe, having a tetragonal crystalline structure, have been calculated using Eq. (4) (Cohen, 1956): 2

4sin h k

2

¼

p2 þ q2 r 2 þ 2 a2 c

ð4Þ

where p, q and r are the Miller indices of the plane. For an instance, to calculate the lattice constant of CZTSSe at Tsub = 300 °C, the values of 2h, as obtained from Fig. 2(a) are 28.24° and 47.20° corresponding to (1 1 2) and (2 2 0) crystal planes, respectively. The lattice constants thus obtained using Eq. (4) are a = 5.436 Å and c = 11.0 Å. The values of lattice constant of CZTSSe at different Tsub are populated in Table 3. These values are in good agreement with the data available in literature on single crystal of CZTS and CZTSe (Bonazzi et al., 2003; Olekseyuk et al., 2002). The crystal structure of CZTSSe is commonly reported to be  stannite (with space group I42m) (Ji et al., 2013; Kim and Hong,  (Repins et al., 2012). In fact, the stannite 2015) or kesterite (I4)

Table 3 Calculated lattice parameters and the XRD peak intensity ratio for CZTSSe.

(Cu2FeSnS4) and kesterite (Cu2ZnSnS4) structures are the derivatives of chalcopyrite (CuFeS2) structure. They mainly differ in the arrangement of cations. In case of kesterite structure, c  2a, while in the stannite structure c – 2a. Hence, the diffraction peaks (2 2 0) and (2 0 4) are not experimentally resolvable in the kesterite structure since c  2a. Additionally, from the single crystal data of Cu2FeSnS4, it is found that the values of intensity ratio of (1 1 2) to (2 2 0)/(2 0 4) [I(1 1 2)/I(2 2 0/2 0 4)] is approximately 2.5 and 4 in case of kesterite and stannite structures, respectively (Bernardini et al., 2000). From the presented work, it is observed that c  2a and the values of I(1 1 2)/I(2 2 0/2 0 4), for the most part, in the neighborhood of 2.5 (from Fig. 2(a)), as populated in Table 3. Thus, from the obtained data we can conclude that the DIBS-grown CZTSSe films  are polycrystalline and exhibit kesterite structure (I4). The variation in (1 1 2) peak positions with different Tsub is presented in Fig. 2(c). It is observed that the diffraction peak moves to higher angle quite consistently when the ratio of S to (S + Se) increases, since the replacement of smaller S for relatively larger Se atoms results in lattice shrinkage (He et al., 2012a). From the obtained values of 2h as in Fig. 2(a), we have calculated S to (S + Se) ratio by using Eq. (5) (Salomé et al., 2012a),

S ð2h
27:16Þ ¼ S þ Se M

ð5Þ

where M is a linear factor and takes the value 1.28, which is actually the difference between CZTS and CZTSe (1 1 2) peak positions (28.44° and 27.16°) (Salomé et al., 2012a). It is found that the values of S/(S + Se) as obtained from XRD measurements shows discrepancy with those obtained from EDX measurements, as shown in Table 4. EDX is a direct method to obtain S/(S + Se) ratio and the method allows for the acquisition of the S/(S + Se) ratio from direct calculations of the atomic concentration of each element, while XRD analysis to calculate the elemental ratio is an indirect method. One may deploy Vegard’s law applied to the lattice constant with composition (Xie et al., 2015) to evaluate S/(S + Se) ratio from XRD analysis. To further confirm the deposition of CZTSSe, Raman scattering of thin film grown at Tsub = 300 °C has been included as in Fig. 2 (d), in the range of 200–400 cm
1. From the figure, it can be clearly observed that the main Raman peaks of CZTS is detected at 336 cm
1, while those of CZTSe appear at 206 cm
1. This experimental observation is consistent with the published data in literature (Salomé et al., 2012a). In addition, a minute Raman peak of ZnS has been observed in the Raman spectra at 349 cm
1. As can be seen in Fig. 2(e), the peak of CZTS at 336 cm
1 is constituted by a combination of two peaks: (1) peak at 333 cm–1, which is associated with ‘‘A⁄” mode, and (2) another peak at 338 cm–1, which is associated with ‘‘A” mode, with the relative dominance of ‘‘A” mode. The larger linewidth of peak at 333 cm
1 indicates that A⁄ mode corresponds to structural disorder of kesterite due to numerous antisite defects (Valakh et al., 2013). Therefore, we can conclude that the film deposited at Tsub = 300 °C may be disordered kesterite.

Table 4 S/(S + Se) ratio from EDX and XRD analysis and Thickness from SE analysis.

Tsub (°C)

a (Å)

c (Å)

c/2a

I(1 1 2)/I(2 2 0/2 0 4)

Tsub (°C)

S/(S + Se) as calculated from EDX

S/(S + Se) as calculated from XRD

Thickness (nm)

100 200 300 400 500

5.439 5.438 5.436 5.439 5.434

11.1 11.0 11.0 11.1 11.0

1.02 1.01 1.01 1.02 1.02

2.23 2.76 2.54 2.39 2.94

100 200 300 400 500

0.83 0.841 0.843 0.828 0.854

0.796 0.805 0.844 0.773 0.867

396.9 397.3 399.6 402.2 404.6

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B.S. Sengar et al. / Solar Energy 139 (2016) 1–12

3.3. Morphological properties Fig. 3(a)–(e) shows FESEM micrographs of CZTSSe films deposited at various Tsub. As Tsub increases, there is a noticeable improvement in the morphology and dimension of grains. The microstructure of the film deposited at Tsub = 100 °C shows a fuzzy appearance and the grains are not distinct. Moreover, well-defined blisters are also observed in CZTSSe grown at this temperature, which are hindrances to achieve SCs with high values of PCE (Bras et al., 2015). At 200 °C, grain agglomeration seems to occur and hence surface uniformity to a large extent is observed to appear. Further, at 300 °C, the grains are found to be distinct and larger as compared to that at lower Tsub. The grain size is found to decrease for film deposited at Tsub = 400 °C. At Tsub = 500 °C numerous void formation takes place. As a result of this void formation, the grain size reduces significantly. A larger value of grain size may be advantageous depending on the electronic structure of the grain boundaries. The large grain size in the absorber layer maximizes both the minority carrier diffusion length and the built-in potential in a polycrystalline thin film solar cell (Son et al., 2015). The variation of grain size with Tsub, as calculated from

FESEM, is shown in Fig. 3(k). In our case, largest grain size is obtained at Tsub = 300 °C, and therefore, 300 °C can be considered to be the optimum temperature to grow uniform CZTSSe crystal with the largest grains. Further, AFM measurement has been done and corresponding surface images are shown in Fig. 3(f)–(j). The grain size is evaluated from AFM measurement, and the values match well with that obtained FESEM measurement. The values of root-mean-square (RMS) roughness of films are plotted in Fig. 3(k). It is observed that the surface RMS roughness of CZTSSe thin films increases with increasing Tsub from 100 to 200 °C and then it reduces somewhat till Tsub reaches 400 °C, and again it increases further for Tsub = 500 °C as shown in Fig. 3(k). 3.4. Optoelectronic properties SE is a non-invasive and non-destructive technique to determine optical constants through the analysis of the complex dielectric function, e. SE determines the complex ratio (q) of polarized plane wave components. Traditionally, this ratio is measured in reflection or transmission mode for light polarized parallel (p), and perpendicular (s) to the plane of incidence. The measurement

Fig. 3. FESEM images of surfaces of CZTSSe thin films at Tsub = (a) 100 °C, (b) 200 °C, (c) 300 °C, (d) 400 °C, (e) 500 °C. AFM topography images of CZTSSe thin films in 10  10 lm2 scan area at Tsub = (f) 100 °C, (g) 200 °C, (h) 300 °C, (i) 400 °C; and (j) 500 °C; and (k) variation of grain size, RMS surface roughness vs Tsub.

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B.S. Sengar et al. / Solar Energy 139 (2016) 1–12

(h)

(g)

(j)

600

Grain Size (nm)

(k)

Grain Size Roughness

500

20 16

400 12 300 8 200 4 100 100

200

300

400

RMS Roughness (nm)

(i)

500

Tsub ( C) Fig. 3 (continued)

gives the spectral variation of the two ellipsometric angels, W and D, as presented in Eq. (6):

q¼

rp ¼ tan WeiD rs

ð6Þ

where rp and rs are the amplitude reflection coefficients for the pand s-polarized light waves, respectively, W is the ratio of the amplitude reflection coefficients, and D is the phase difference between s- and p-polarized light waves. It is well known that the SE technique is sensitive to the surface roughness of a thin film Therefore, in order to account for this surface roughness, the Bruggemann effective medium approximation (EMA) (Postava et al., 2000) is added on top of the stack layer as observed in Fig. 4(a). In this report, the SE analysis is based on a three-phase model i.e., air/surface-roughness layer/CZTSSe layer/ SLG substrate, as shown in Fig. 4(a). The EMA model, in general, is based on Eq. (7) (Postava et al., 2000):

eCZTSSe
ei eair
ei f þ ð1
f v Þ ¼ 0 eCZTSSe þ 2ei v eair þ 2ei

ð7Þ

where ei is the effective complex dielectric function of the surfaceroughness layer, f v is the volume fraction of CZTSSe in the surfaceroughness layer and the value is fixed to 50%, eair is the dielectric constant of air (eair = 1 assumed in all SE analysis), and eCZTSSe is

the complex dielectric function of CZTSSe. The optical functions of the film and EMA layer are both parameterized with a TaucLorentz expression but are kept independent from each other (Crovetto et al., 2015). All ellipsometric measurements are performed with an incidence angle of 70° in the wavelength range of 250–1000 nm. The measured data is fitted by Tauc-Lorentz model (Crovetto et al., 2015) using Complete EASE software (J. A. Wollam Co. Inc, 2011) and the model shows good agreement with the experimentally obtained data in the entire spectral range. The real, e1 and imaginary components, e2 of complex dielectric function of CZTSSe thin films, as determined from ellipsometry measurements, are plotted in Fig. 4(b) and (c), respectively. The value of e1 is observed to increase initially with photon energy, approaches its peak value, and then decreases with further increase in photon energy. The values of e1 of CZTSSe grown at 500 °C are the lowest and that indicates that this film has the poorest crystalline quality which has already been verified by XRD (Daranfed et al., 2012). e2 is sensitive to the presence of surface overlayer, and thus its value reduces from the corresponding intrinsic value (ei2 ) (Choi et al., 2012). The magnitude of difference, ei2
e2 , is roughly proportional to the thickness of the overlayer (Aspnes and Studna, 1983). The critical point (CP) features at 2.02, 1.67, and 1.98 eV appear distinctly in the thin films grown at Tsub = 300, 400, and 500 °C, whereas no evident CP feature is observed for CZTSSe grown at 100 and 200 °C which is perhaps

8

B.S. Sengar et al. / Solar Energy 139 (2016) 1–12

8

(b)

(a)

7 6

1

5 TSub=100 C TSub=200 C TSub=300 C

4 3

TSub=400 C

2

TSub=500 C

1

2

3

4

5

E (eV) 5

3

(c)

(d)

E2

E1

4

2

3 2

TSub= TSub= TSub=

2

n

TSub=100 C TSub=200 C TSub=300 C TSub=400 C

TSub= TSub=

TSub=500 C

1 0

1 2

3

4

2

5

3

4

5

E (eV)

E (eV) 8

1.5

(f) -1

(e)

Absorption Coeff. 10 (cm )

TSub= TSub=

1.0

TSub= 100 C TSub= 200 C

6

5

TSub= TSub=

k

TSub=

0.5

TSub= 300 C TSub= 400 C

4

TSub= 500 C

2

0.0 0 2

3

4

400

5

600

800

1000

Wavelength (nm)

E (eV)

Fig. 4. (a) Schematic stack-layer structure for ellipsometry modelling, (b) e1, (c) e2, (d) n, (e) k variation of CZTSSe thin films for different Tsub ranging from 100 to 500 °C, (f) variation of optical absorption coefficient with wavelength, (g) (a h m)2 vs hm, (h) variation of band gap and Cu/(Zn + Sn) with Tsub, (i) second linear fitting of (a h m)2 vs hm for CZTSSe at Tsub = 500 °C, and (j) band gap from EDX and SE vs. S/(S + Se) ratio.

hidden by the thick surface overlayers (Choi et al., 2012). In addition, significant enhancement of the CP structures at 3.11 eV for Tsub = 300 °C in e2 spectra indicates the successful reduction of surface overlayers in comparison to other films grown at different Tsub. Generally, CPs in CZTSSe material occur at the Z-point of the Brillouin-zone (Choi et al., 2012). In our case, two CPs can be observed as in Fig. 4(c): (1) E1 CP structure, which comprises of the net effect of Cu(3d) electrons excited to Sn(5 s) orbit, and (2) E2 CP structure, (evident only for thin film grown at Tsub = 300 °C), that consists of the net transition of Cu(3d) ? Sn(5p) (Choi et al., 2012). The spectral variation of the optical constants, n and k, can be realized using Eq. (8)

ðn þ ikÞ ¼ e1 þ ie2 2

ð8Þ

where n is the refractive index, and k is the extinction coefficient, as shown in Fig. 4(d) and (e). Hence, a, the optical absorption coefficient, can be calculated using Eq. (9):

a¼

4pk kp

ð9Þ

where kp is the wavelength of photon. Fig. 4(f) shows the optical absorption coefficient (a) of CZTSSe thin films. It is observed that the absorption coefficient of all thin films are >104 cm
1 in the visible region, as reported earlier (Chan et al., 2010; Kumar et al., 2009) and therefore, the film

9

B.S. Sengar et al. / Solar Energy 139 (2016) 1–12

1.8

1.6

(g)

(h)

-2

10 (eV cm )

4

Cu/(Zn+Sn) Band gap

1.4

1.6

TSub=100 C TSub=200 C TSub=300 C TSub=400 C

1.70 eV

1.49 eV

1.0

1.2

TSub=500 C

1.6

2.0

1.4

0.8 100

2.4

200

h (eV)

(j)

2

Band gap, Eg (eV)

3

2

1

2.54 eV 2.5

h (eV)

400

1.6

2

11

500

500

TSub= 500 C

100

1.7

-2

10 (eV cm )

(i)

( h )

400

Tsub ( C)

4

0 2.0

300

Tsub ( C)

300

-1 1.2

1.35 eV 1.43 eV

0

1.23 eV

( h )

2

1

1.2

200

10

2

Cu/(Zn+Sn)

2

3

Band gap (eV)

5

1.5 1.4

Band gap from EDX Band gap from SE

1.3 1.2

3.0

3.5

0.82

0.83

0.84

0.85

0.86

S/(S+Se) Fig. 4 (continued)

material can be adjudged suitable for photovoltaic solar energy conversion. According to Davis and Mott (1970), Tauc et al. (1966), the opti(ESE g )

cal band gap is calculated using the well-known Tauc plot. The plot is derived from Eq. (10):

a h v ¼ Aðh v
ESE g Þ

m

ð10Þ

where A is a constant, m is power index whose values can be 1/2, 2, 3/2, and 3 for allowed direct, allowed indirect, forbidden direct, and forbidden indirect band gap respectively. As CZTSSe is a direct band gap semiconductor, hence the value of m is considered ½ (Babu et al., 2010). (a h m)2 vs. hm graphs are drawn for CZTSSe films, as shown in Fig. 4(g). The value of ESE g has been determined by extrapolating the linear region of the plot (a h m)2 versus photon energy

(hm) and taking the intercept on hm-axis at a = 0. The values of in the fundamental absorption region of CZTSSe are found to be in the range of 1.23–1.70 eV depending upon the Cu/(Zn + Sn) ratio. It is also observed that there is a decline in optical band gap with the increase in Cu/(Zn + Sn) ratio (evaluated from EDX measurement) as shown in Fig. 4(h). Cu-poor films are found to have higher optical band gap than Cu-rich films (Babu et al., 2010). Jaffe and Zunger (Jaffe and Zunger, 1984) have predicted change in band gap due to p-d hybridization between Cu d-levels and Se p-levels on the basis of band structural calculations in the case of ternary chalcopyrite compounds. In a similar manner, the decrease in the band gap with increase in Cu content in case of CZTSSe films may be attributed to changes in the extent of p-d hybridization between Cu d-levels and (S, Se) p-levels. However, at Tsub = 500 °C, the reduction in band gap energy is not observed. This can be because of two probable reasons: (1) at this Tsub, there is a significant Sn loss as verESE g

ified from EDX analysis, and (2) it can be noticed that the film fabricated at 500 °C demonstrates two linear regions, one in Fig. 4(g) SE (ESE g = 1.70 eV) and the other in Fig. 4(i) (Eg = 2.54 eV). The band gap at 2.54 eV hints towards the formation of compound similar to Zn(S, Se) and this could be the reason for the occurrence of reduced and broad (1 1 2) diffraction peak for CZTSSe grown at 500 °C, as observed in Fig. 2(a). The values of energy band gap as evaluated from SE fitting are observed to match closely with those obtained from transmission measurements, as shown in Table 5.

It is also observed that the values of EEDX and ESE g g increase with and ESE increasing S/(S + Se). However, the values of EEDX g g show some difference and this discrepancy is the highest at Tsub = 400 and 500 °C, as shown in Fig. 4(j) as well as in Table 2. This may be due to the fact that EEDX is calculated taking influence of only g S/(S + Se) with respect to standard band gap values of CZTS and CZTSe and somewhat ignoring the elemental variation of Cu, Zn, and Sn with different Tsub. At Tsub = 400 °C, Cu/(Zn + Sn) is comparatively higher while at Tsub = 500 °C, Sn is very less, as observed in

Table 5 Room temperature energy band gap (Eg ), as calculated from SE analysis and transmission measurements. Tsub (°C)

Band gap from ellipsometry fitting, ESE g (eV)

Band gap by transmission (eV)

100 200 300 400 500

1.35 1.43 1.49 1.23 1.70

1.38 1.45 1.53 1.24 –

B.S. Sengar et al. / Solar Energy 139 (2016) 1–12

930

Zn 2p3/2 940

Cu 2p1/2

Cu 2p3/2

Intensity (counts/s)

(a)

Zn 2p1/2

10

950

1020

1040

Binding energy (eV)

(b)

485

490

495

505

Se3p

S2p 500

Binding energy (eV)

Se3d

Intensity (counts/s)

Sn 3d3/2

Intensity (counts/s)

Sn 3d5/2

(c)

54

57

160

162

164

166

168

Binding energy (eV)

Fig. 5. XPS spectra of (a) Cu 2p, Zn 2p, (b) Sn 3d, (c) Se 3d, S 2p, Se 3p core levels of CZTSSe grown at 300 °C.

Table 1. However, ESE g is evaluated from experimental SE measurement, which is sensitive to all constituent elements in CZTSSe. As we notice that the optical bandgap (ESE g ) are higher than expected ) at Tsub = 200, 300 and 500 °C, the reason may be due value (EEDX g to a slight difference in the film composition (Babu et al., 2008) or the formation of ZnS phase (peak at 349 cm
1), which one may observe in Raman measurement result as described in Fig. 2 (d). Further p-type conductivity of the thin films was verified by Hall measurement, which utilizes van der Pauw geometry. The resistivity, carrier concentration, and mobility for the CZTSSe film grown at Tsub = 300 °C are 0.93 X cm, 4.25  1017 cm
3 and 15.7 cm2 v
1 s
1, respectively. 3.5. Elemental properties The detailed afore-stated experimental investigation clearly affirms that 300 oC is the best temperature to grow CZTSSe material by DIBS system. Hence, this film has been preferred for XPS measurement. High-resolution XPS analysis has been used to confirm the presence of all five elements, Cu, Zn, Sn, S and Se in their expected chemical states. Fig. 5(a) shows XPS spectra of Cu 2p core level for CZTSSe thin film. There is an obtrusive splitting of the Cu 2p spectral line into 2p3/2 and 2p1/2 core levels at 932.6 and 952.3 eV, respectively with a peak separation of 19.7 eV, confirming the presence of Cu1+ ion (Tsega and Kuo, 2013; Wang and Gong, 2011). Fig. 5(a) also shows Zn 2p core level spectra of for CZTSSe thin film. A pair of peaks at 1021.8 eV (Zn 2p3/2) and 1045.1 eV (Zn 2p1/2) is observed with an energy separation of 23.2 eV, which in Zn 2p is attributed to the presence of Zn2+ state. Also, Zn 2p3/2 line is observed to be shifted by DEZn =
0.2 eV from

the reported average binding energy position of 1021.6 eV for elemental Zn (Shinde and Rajpure, 2011). The sign of the chemical shift indicates electron transfer during the bonding process leading to a net change in the charges (Avalle et al., 1992). The narrow-scan XPS spectra of Sn 3d core states of CZTSSe thin film can be seen in Fig. 5(b). The splitting of the Sn 3d spectral line into the 3d5/2 and 3d3/2 core levels is observed. The corresponding values of binding energy are observed at 486.3 eV (Sn 3d5/2) and 494.7 eV (Sn 3d3/2) with a peak separation of 8.4 eV. The peak separation of 8.3–8.4 eV in Sn 3d can be attributed to Sn4+, and is in a conformity with previous reports (Tsega and Kuo, 2013; Zhang et al., 2012). In Fig. 5(c), two peaks appear at binding energy of 161.7 and 166.2 eV, respectively. While the former can be assigned to the S 2p core (S2
), which is consistent with the 160–164 eV range expected for S in sulfide phases (Riha et al., 2009); the latter is assigned to Se 3p core (Se2
). Se2
is also confirmed by the presence of Se 3d peak located at a binding energy of 54.2 eV as in Fig. 5(c), which is often observed for Se in selenide phases (Sun et al., 2013).

4. Conclusion The influence of Tsub on composition, structural, electrical, optical and elemental properties of kesterite CZTSSe thin film grown on SLG substrate using a single target without annealing or selenization by DIBS system has been thoroughly analyzed. CZTSSe thin film grown at Tsub = 300 and 400 °C are Cu poor and Zn rich, as confirmed by EDX measurements. XRD results reveal that CZTSSe thin films are preferentially (1 1 2)-oriented, and without having any secondary phases such as Cu2(S, Se) or Zn(S, Se). The value of FWHM for (1 1 2) diffraction peak is the minimum with a value of 0.12° and the maximum crystallite size 75.11 nm for CZTSSe film

B.S. Sengar et al. / Solar Energy 139 (2016) 1–12

grown at Tsub = 300 °C. FESEM shows the homogeneous and dense surface morphology. At 300 °C, the grains are found to be distinct and larger as compared to that at lower Tsub. Substantial enhancement of the CP structures at 3.11 eV for Tsub = 300 °C in e2 spectra indicates the successful reduction of surface overlayers in comparison to other films grown at different Tsub. Analysis of absorption spectra for CZTSSe thin films confirm that ESE g gradually decreases with increasing Cu to (Zn + Sn) ratio and the larger value of ESE g for CZTSS grown at Tsub = 500 °C is presumably attributed to the presence of Zn(S, Se) in the CZTSSe matrix. The optical absorption coefficient of all CZTSSe thin films are evaluated to be >4  104 cm
1. Therefore, all experimental characterizations determine 300 °C is the optimized substrate temperature to grow highquality DIBS-grown CZTSSe materials, which can be used to realize cost-effective and high-performance photovoltaic applications. Acknowledgement This work is partially supported by Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy (DAE), Government of India and Clean Energy Research Initiative (CERI), Department of Science and Technology (DST), Government of India. We are thankful to DIBS, FESEM, EDX and XRD facility equipped at Sophisticated Instrument Centre (SIC) at IIT Indore. The authors (Brajendra S. Sengar, Vivek Garg) acknowledge CSIR and UGC, India for their fellowships. Prof. Shaibal Mukherjee is thankful to Deity YFRF, Government of India award. We are thankful to Dr. Somaditya Sen, IIT Indore for providing access to Raman measurements. We are thankful to Dr. D.M. Phase, Dr. R.J. Chaudhary and Mr. A. Wadikar for using AIPES beam line Indus facility at RRCAT. References Aspnes, D.E., Studna, A.A., 1983. Dielectric functions and optical parameters of Si, Ge, GaP, GaAs, GaSb, InP, InAs, and InSb from 1.5 to 6.0 eV. Phys. Rev. B 27, 985– 1009. http://dx.doi.org/10.1103/PhysRevB.27.985. Avalle, L., Santos, E., Leiva, E., Macagno, V.A., 1992. Characterization of TiO2 films modified by platinum doping. Thin Solid Films 219, 7–17. http://dx.doi.org/ 10.1016/0040-6090(92)90717-P. Awasthi, V., Pandey, S.K., Pandey, S.K., Verma, S., Gupta, M., Mukherjee, S., 2014. Growth and characterizations of dual ion beam sputtered CIGS thin films for photovoltaic applications. J. Mater. Sci.: Mater. Electron. 25, 3069–3076. http:// dx.doi.org/10.1007/s10854-014-1985-0. Babu, G.S., Kishore Kumar, Y.B., Uday Bhaskar, P., Raja Vanjari, S., 2010. Effect of Cu/ (Zn + Sn) ratio on the properties of co-evaporated Cu2ZnSnSe4 thin films. Sol. Energy Mater. Sol. Cells 94, 221–226. http://dx.doi.org/10.1016/ j.solmat.2009.09.005. Babu, G.S., Kishore Kumar, Y.B., Uday Bhaskar, P., Raja Vanjari, S., 2008. Growth and characterization of co-evaporated Cu2ZnSnSe4 thin films for photovoltaic applications. J. Phys. D Appl. Phys. 41, 205305. http://dx.doi.org/10.1088/ 0022-3727/41/20/205305. Barkhouse, D.A.R., Gunawan, O., Gokmen, T., Todorov, T.K., Mitzi, D.B., 2012. Device characteristics of a 10.1% hydrazine-processed Cu2ZnSn(Se, S)4 solar cell. Prog. Photovoltaics Res. Appl. 20, 6–11. http://dx.doi.org/10.1002/pip. Bernardini, G.P., Borrini, D., Caneschi, a., Di Benedetto, F., Gatteschi, D., Ristori, S., Romanelli, M., 2000. EPR and SQUID magnetometry study of Cu2FeSnS4 (stannite) and Cu2ZnSnS4 (kesterite). Phys. Chem. Miner. 27, 453–461. http:// dx.doi.org/10.1007/s002690000086. Bonazzi, P., Bindi, L., Bernardini, P., Menchetti, S., 2003. A model for the mechanism of incorporation of Cu, Fe and Zn in the Stannite – Kesterite series, Cu2FeSnS4– Cu2ZnSnS4 41, 639–647. Bourdais, S., Chon, C., Delatouche, B., Jacob, A., Larramona, G., Moisan, C., Lafond, A., Donatini, F., Rey, G., Siebentritt, S., Walsh, A., Dennler, G., 2016. Is the Cu/Zn disorder the main culprit for the voltage deficit in kesterite solar cells? Adv. Energy Mater. http://dx.doi.org/10.1002/aenm.201502276. Bras, P., Sterner, J., Platzer-björkman, C., Sterner, J., 2015. Investigation of blister formation in sputtered Cu2ZnSnS4 absorbers for thin film solar cells Investigation of blister formation in sputtered Cu2ZnSnS4 absorbers for thin film solar cells 061201. http://dx.doi.org/10.1116/1.4926754. Byeon, M.R., Chung, E.H., Kim, J.P., Hong, T.E., Jin, J.S., Jeong, E.D., Bae, J.S., Kim, Y.D., Park, S., Oh, W.T., Huh, Y.S., Chang, S.J., Lee, S.B., Jung, I.H., Hwang, J., 2013. The effects for the deposition temperature onto the structural, compositional and optical properties of pulsed laser ablated Cu2ZnSnS4 thin films grown on soda lime glass substrates. Thin Solid Films 546, 387–392. http://dx.doi.org/10.1016/ j.tsf.2013.05.032.

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