Growth and characterization of Cu2ZnSnSe4 thin films by a two-stage process

Solar Energy Materials & Solar Cells 115 (2013) 181–188

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Growth and characterization of Cu2ZnSnSe4 thin films by a two-stage process P. Uday Bhaskar, G. Suresh Babu, Y.B. Kishore Kumar, V. Sundara Raja n Solar Energy Laboratory, Department of Physics, Sri Venkateswara University, Tirupati-517502, Andhra Pradesh, India

art ic l e i nf o

a b s t r a c t

Article history: Received 11 June 2012 Received in revised form 13 March 2013 Accepted 15 March 2013 Available online 5 May 2013

Polycrystalline Cu2ZnSnSe4 (CZTSe) thin films were prepared by a two-stage process namely thermal evaporation of stacked layers Cu/ZnSe/Sn/Se on soda-lime glass substrates held at different substrate temperatures (Ts) in the range 523–723 K followed by annealing the stack in selenium atmosphere at 723 K for an hour. The effect of Ts on the growth and properties of these films were analyzed by studying their structural, microstructural and optical properties. XRD studies revealed the structure to be kesterite with a ¼0.569 nm and c ¼1.139 nm. Raman spectroscopy is used as a complimentary tool to know the presence of possible secondary phases. The crystallinity of the films improved with increase in the substrate temperature. Spectral transmittance studies of these films revealed two optical transitions with direct band gaps of ∼1.0 eV and 1.4 eV which are attributed to CZTSe and CZTSe with minor ZnSe, as the annealed stack might be inhomogeneous. & 2013 Elsevier B.V. All rights reserved.

Keywords: CZTSe Selenization Thin films Structural properties Optical band gap Raman spectroscopy

1. Introduction There is an imperative need to develop solar cells which are environmentally benign and economically viable for sustained supply in the long run. In this direction, Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) have recently drawn the attention of investigators as promising alternate solar cell absorber layers to CuInGaSe2 (CIGS), the cells made of which exhibited a record efficiency of 20.3– 20.4% [1,2] but contains expensive elements In and Ga. CZTS and CZTSe with their suitable band gap, high optical absorption coefficient (4104 cm−1) and p-type electrical conductivity, in addition to their elements being abundant and relatively non-toxic make them attractive options for low-cost thin film solar cells. CZTS, CZTSe and Cu2ZnSn(S,Se)4 (CZTSSe) solar cells with laboratory efficiencies of 8.4%, 9.15% and 10.1% [3–5], respectively have been reported. A variety of techniques like RF sputtering [6,7], pulsed laser deposition [8], co-evaporation [9–13], metallization þ selenization [14–18], solution processing [19] etc., have been used to deposit CZTSe thin films. Altosaar et al. [20] synthesized CZTSe monograins powders and investigated their physical and photovoltaic properties. Barkhouse et al. [5] used a solution based approach to develop CZTSSe films. In metallizationþ selenization approach, the precursors have been deposited by thermal evaporation [14], DC magnetron sputtering [15,16], electrodeposition [17,18] etc., and selenized subsequently in

n

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Se atmosphere. This approach is relatively advantageous because of its scalability and high throughput. However, the reported limitation of this approach is the adhesion problem due to volume expansion on selenization which was addressed by Volobujeva et al. [14] and Salomé et al. [15]. In this paper, we report the growth of CZTSe films by a two-stage process. In the first stage, Cu, ZnSe, Sn and Se are sequentially deposited onto glass substrates held at different substrate temperatures to allow the diffusion/inter-mixing of the precursor layers and formation of CZTSe. In the second stage, annealing at 723 K in Se atmosphere was carried out to improve crystallinity and compensate for selenium deficiency, if any. We have chosen ZnSe instead of Zn at the precursor stage to avoid the formation of intermetallics [14–16] and to minimize Zn-losses at higher substrate temperatures since the vapour pressure of ZnSe is much less than that of Zn [12,21]. We have reported earlier the growth and properties of CZTSe films by co-evaporation [10,22,23]. In this paper, we report the effect of substrate temperature (Ts) on the growth of CZTSe films obtained by annealing the sequentially deposited precursor layers, with an objective to optimize the substrate temperature for the growth of CZTSe films. 2. Experimental Spectroscopically pure Cu, ZnSe, Sn and Se (Sigma-Aldrich, USA), kept in molybdenum boats were sequentially evaporated onto chemically and ultrasonically cleaned soda-lime glass substrates held at different temperatures (523 K, 623 K, 673 K and 723 K). Evaporation was carried out using a 4-source vacuum coating system (Hind High Vacuum Company Pvt. Ltd., India,

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Model BC-300). To achieve the desired evaporation rate and thickness for each of the precursor layer, each source was precalibrated using quartz crystal digital thickness monitor. The thicknesses of Cu, ZnSe and Sn were chosen in accordance with the stoichiometry while Se precursor thickness was a little higher than the stoichiometric requirement to compensate for the loss of selenium from the substrate due to re-evaporation. The base pressure in the vacuum coating unit was 5  10−6 mbar and

Table 1 Elemental composition of annealed stacked layers deposited at different Ts. Ts (K)

523 623 673 723

TA (K)

723 723 723 723

Elemental composition (at%) Cu

Zn

Sn

Se

22.8 17.4 24.2 19.6

11.2 13.6 9.9 13.1

13.5 15.5 13.1 11.7

52.5 53.5 52.8 55.7

Cu/(Znþ Sn)

Zn/Sn

0.92 0.60 1.05 0.79

0.83 0.88 0.76 1.17

working pressure was 5  10−5 mbar. Prior to evaporation, the substrates were subjected to ion bombardment. Immediately after the deposition of the precursor layers, the substrate temperature was slowly increased from Ts to 723 K at the rate of 10 K/min using a PID controller. They were held at 723 K for an hour to ensure proper inter-diffusion of precursor layers and compound formation. During this process, Se was continuously evaporated at a very slow rate to compensate for the loss of Se from the substrates. The substrates were then slowly cooled to room temperature at the rate of 5 K/min. Selenium evaporation was continued during this period also until Ts reached 573 K. The films were analyzed by studying their composition, structural and optical properties. The film thickness was determined from its deposited mass measured using METTLER microbalance (Model: AE240) and bulk density. Spectral transmittance and reflectance of the films were recorded using UV–Vis-NIR double beam spectrophotometer (PerkinElmer, Model: LAMDA 950) in the wavelength region 300–2500 nm. X-Ray diffraction (XRD) patterns of the films were taken using X-ray diffractometer (SEIFERT, Model: 3003TT) with Cu Kα radiation (λ¼0.15406 nm). Microstructure of

Fig. 1. XRD patterns of annealed stacked layers (glass/Cu/ZnSe/Sn/Se) deposited at different Ts.

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the films was recorded using a Carl Zeiss scanning electron microscope (SEM) (Model: EVO MA15) and elemental composition was determined using Oxford Instruments, U.K energy dispersive spectrometer (Model: INCA 250) attached to the SEM. Raman spectra were recorded using Horiba Jobin Yvon confocal Raman spectrometer. The spectra were recorded in back scattering mode using Ar þ laser source (λ¼514.3 nm).

3. Results and discussion 3.1. Compositional analysis Table 1 shows the elemental composition of CZTSe films obtained by annealing of the stack Cu/ZnSe/Sn/Se deposited at different substrate temperatures (Ts). It is observed that the films are slightly copper-poor and selenium-rich. Only the films deposited at Ts ¼ 673 K and annealed at 723 K, to some extent, are nearstoichiometric. The uncertainty in the determination of elemental composition is 75 at%. The composition of each element shown in the table is the cumulative result that includes contribution of that element from CZTSe as well as the secondary phases, if any, that might be present in the film. Control of final film composition is vital for achieving good device performance. It is now established [3,4,24–26] that to achieve reasonably high efficiencies in the case of kesterite based

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thin film solar cells, the elemental compositional ratios of the absorber layer need to fall in the range 0.80–0.85 for Cu/[Zn þSn] and in the range 1.15–1.30 for Zn/Sn. Further Studies are under progress to obtain CZTSe films with above criteria. 3.2. Structural properties Fig. 1 shows XRD patterns of stacked layers (glass/Cu/ZnSe/Sn/ Se) deposited at different substrate temperatures (Ts) and annealed at 723 K for one hour in Se atmosphere. The intense diffraction peaks agree well with those of CZTSe (JCPDS-010-0708930). However, it is difficult to rule out the presence of secondary phases like Cu2SnSe3 (CTSe) and ZnSe since the intense diffraction lines of these phases are also close to that of CZTSe. The diffraction pattern is most intense for films deposited at Ts ¼673 K. CZTSe films were reported to exhibit either kesterite structure (space group I4) or stannite structure (space group I42m). In stannite structure, the lattice parameter co 2a and the peaks (2 2 0) and (2 0 4) as well as (3 1 2) and (1 1 6) get resolved. In the present case, the lattice parameters were found to be a ¼0.569 nm and c¼ 1.139 nm. Here, c∼2a, the doublets (2 2 0)/(2 0 4) as well as (3 1 2)/(1 1 6) are not resolved and thus CZTSe film structure is kesterite. Schorr et al. [27] showed from neutron diffraction studies, a partially disordered kesterite structure can exist. Chen et al. [28] reported from theoretical studies that kesterite is more stable and stannite ordering may co-exist in synthesized samples.

Fig. 2. Raman spectra of annealed stacked layers (glass/Cu/ZnSe/Sn/Se) deposited at different Ts.

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Persson [29] also reported from theoretical studies that most stable structure is kesterite structure for both CZTSe and CZTS but suggested that it is possible to grow in stannite structure. 3.3. Micro-Raman studies Micro-Raman spectroscopy is used as a complementary technique to distinguish different phases formed during the growth of CZTSe films. Fig. 2 shows the micro-Raman spectra of annealed stacked layers (Cu/ZnSe/Sn/Se) deposited at different substrate temperatures (Ts) recorded using Ar þ laser (514 nm). The intense Raman line of these films fall in the range 195–197 cm−1, with a satellite peak at 172/173 cm−1 and a weak peak at 231/234 cm−1. These are close to the reported values of 196 cm−1, 173 cm−1 by Altosaar et al. [20] and 197 cm−1, 172 cm−1 by Salomé et al. [15] for CZTSe films. Also other modes of much lower intensity were reported by Redinger et al. [30] for CZTSe films at 78 cm−1 and 244 cm−1. The intense Raman modes of possible secondary phases Cu2SnSe3 (CTSe) and ZnSe were reported to occur at 178/180 cm−1 [20,31] and 251/252–256 cm−1[32–34], respectively. This clearly indicates that these secondary phases are not present in the film deposited at different Ts within the probe depth of Ar þ laser (514 nm). We have also made an attempt to elicit some information depth-wise by recording Raman spectra at two or three depths by moving the stage in the z-direction manually. Fig. 3 shows typical depth-wise Raman spectra of annealed stacks deposited at Ts ¼523 K and Ts ¼ 673 K. These spectra show intense Raman modes due to CZTSe. There is a slight shift in the peak position of observed Raman lines from surface to lower regions which is clearly noticeable in films deposited at Ts ¼ 523 K. This might be due to depth-wise non-uniformity/inhomogeneity in the film. The decrease of the full width at half maximum (FWHM) in the spectra from lower region to surface indicates improvement in crystallinity. A low intense peak at 65 cm−1 is also observed in the spectra of films at Ts ¼523 K taken at different depths. This feature is absent in the spectra of the film deposited at Ts ¼673 K. Probably this might be due to some impure phase which we are unable to precisely assign. A broad feature succeeds the Raman signature at 231/232 cm−1, in film deposited at Ts ¼ 673 K and its extent decreases from lower depths to surface. It might be the broad Raman mode reported to occur at 244 cm−1[30] or due to minor impure ZnSe phase.

are relatively thin. Absorber layer of thickness 1–2 μm is required from the device point of view. Increase in the absorber layer thickness to some extent would lead to improvement in the grain size. However, the focus in the present study was to use ZnSe as a precursor layer instead of Zn to minimize Zn re-evaporation losses as well as to understand the effect of deposition temperature on the growth and properties of CZTSe films obtained by annealing the stacked layers. Thinner precursor layers have been chosen in the present investigation to promote easy intermixing of the layers expecting homogeneous film formation. 3.5. Optical properties The spectral transmittance (Tλ) and reflectance (Rλ) curves of in-situ annealed stacked layers deposited at different Ts are shown Figs. 5 and 6, respectively. The spectral transmittance curves of annealed stack deposited at Ts ¼523 K, 623 K and 673 K

3.4. Microstructure Fig. 4 shows scanning electron micrographs of in-situ annealed stacked layers deposited at different Ts. Micrograph of the films deposited at Ts ¼523 K shows uniform surface morphology with grains about ∼0.3 μm size. Micrograph of films deposited at Ts ¼623 K shows that the surface is smeary with a few agglomerated grains and a few void spaces. These voids might be due to loss of volatile species like SnSe or ZnSe on annealing. The micrograph of annealed film deposited at Ts ¼673 K shows distinct large grains with small grains in the interspaces. The average grain size is ∼0.5 μm. The morphology of films deposited at Ts ¼723 K shows more or less uniform grains with a few clusters of grains. From the cell performance point of view, an absorber layer with an average grain size 4 1 μm, columnar and free from voids is essential to achieve reasonable conversion efficiency. Large grain size results in lower grain boundary recombination losses thus leading to higher photovoltage. Annealing at temperatures in the range 773–823 K under controlled pressure would result in CZTSe film with larger grain growth than observed in the present investigation. Unfortunately due to the limitations of vacuum coating system, annealing at temperatures 4723 K could not be performed. Secondly, the films grown in the present investigation

Fig. 3. Typical Raman spectra of annealed stacked layers deposited at Ts ¼523 K and Ts ¼ 73 K, taken at different depths.

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Fig. 4. SEM images of annealed stacked layers (glass/Cu/ZnSe/Sn/Se) deposited at different Ts.

80

(a) 523 K (b) 623 K (c) 673 K (d) 723 K

70 60

Transmittance (%)

(Fig. 5(a)–(c)) clearly show the onset of fundamental absorption edge (λg) at ∼1260 nm. The slight difference in the onset of absorption edge among them might be due to difference in film thickness, crystallinity and/or in composition. But the onset of absorption edge in the case of annealed stack deposited at Ts ¼ 723 K is not as sharp as in the former cases. But clearly there is a shift towards shorter wavelengths. Similar trend is observed in the spectral reflectance curves shown in Fig. 6. The slight difference in the onset of peaks in the transmittance and reflectance curves at each Ts might be due to the film inhomogeneity. The spectral transmittance curve of film deposited at Ts ¼523 K shows two distinct transitions: (i) in the wavelength range ∼1260 nm to 900 nm (R-I in Fig. 5a) and (ii) in the wavelength regiono900 nm (R-II in Fig. 5a). Below 600 nm, the spectral transmittance is almost zero. But in the corresponding region in the reflectance curve (Fig. 6a), a reflectance feature with a fall below 600 nm is observed indicating absorption due to a phase with λg at ∼600 nm. This could not be observed in the transmittance curve probably due to the too low spectral intensity. This might be due to minor ZnSe phase with inclusion of copper in it. The reported optical band gap of ZnSe: Cu (2.48 eV) [35] is less than that of pure ZnSe (2.68 eV). Transmittance spectra of films deposited at Ts ¼ 623 K and 673 K also exhibited two regions R-I and R-II, which are more clearly seen in the latter case. The spectral width of R-I decreased for film deposited at Ts ¼673 K compared to film deposited at Ts ¼ 523 K. With regard to the reflectance spectra shown in Fig. 6, the spectral features observed below 600 nm in the reflectance curve of films deposited at Ts ¼523 K (Fig. 6a) reduced to a considerable extent in the reflectance spectra of films deposited at Ts ¼623 K and 673 K (Fig. 6b and c), especially in the latter case. This might be due to the reduction of the phase responsible for absorption below 600 nm. The spectral reflectance curve of annealed stack deposited at Ts ¼723 K shows a peak ∼1020 nm with a clear shift towards shorter wavelengths compared to films deposited at lower Ts. This is in concurrence with the trend observed in the transmittance spectra.

50 40 30

R-I

R-I

b

20 10

R-II

0 300

600

900

1200 1500 1800 Wavelength (nm)

2100

2400

Fig. 5. Spectral transmittance curves of annealed stacked layers (glass/Cu/ZnSe/Sn/ Se) deposited at different Ts.

The optical absorption coefficient (α) of these films was determined from the measured spectral transmittance (Tλ) and reflectance (Rλ) data using the formula [36]. " # 1 ð1−Rλ Þ2 α ¼ ln ð1Þ t Tλ where ‘t’ is the thickness of the film. The nature of the optical transition and optical band gap (Eg) of the films were determined using the equation αðhνÞ ¼ Aðhν−Eg Þn

ð2Þ

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where A is constant. The exponent ‘n’ can take values 1/2, 3/2 or 2 based on whether the optical transition is direct-allowed, direct-forbidden or indirect-allowed, respectively. In the present

Fig. 6. Spectral reflectance curves of annealed stacked layers (glass/Cu/ZnSe/Sn/Se) deposited at different Ts.

investigation, Eq. (2) is satisfied for n ¼1/2 indicating that the optical transitions are direct-allowed in nature. The optical band gap of the film was determined by extrapolating the linear portion of (αhν)2 versus hν plot and taking the intercept on hν-axis. Fig. 7(a) shows (αhν)2 versus hν plots corresponding to two regions (R-I and R-II) observed in Fig. 5(a). The direct optical band gaps are found to be 1.08 eV and 1.46 eV. The uncertainty in the determination of Eg is 70.02 eV. The band gap of 1.08 eV is close to the reported band gap of∼1.0 eV [11,16,29] in the case of Cu/ [ZnþSn]- poor and [Zn/Sn]-rich CZTSe films targeted for device fabrication and to the theoretical value estimated by Chen et al. [28] and Persson [29]. Chen et al. [28] reported that the band gap of kesterite CZTSe is slightly (by about 0.04 eV) greater than that of stannite CZTSe. Further, the predicated Eg would be less by about 0.1 eV at room temperature, if one considers temperature effect [28]. Persson [29] estimated the band gap of kesterite and stannite CZTSe phases to be 1.05 eV and 0.89 eV, respectively. The other observed optical band gap value, 1.46 eV, corresponding to the spectral region R-II in the present case, is attributed to fractional CZTSe film with inclusion of minor ZnSe [11]. This might be due to slight compositional non-uniformity/inhomogeneity in the CZTSe film. Since the films were prepared by annealing the stack glass/ Cu/ZnSe/Sn/Se, such a possibility is there based on the deposition and annealing temperatures. AES or SIMS depth profile studies would have helped us to confirm such a compositional gradient. Unfortunately, these sophisticated analytical facilities are not available in our university and getting a slot on these facilities at other advanced institutes is a difficult task and time consuming.

Fig. 7. (αhν)2 versus hν plots of films deposited at different Ts.

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We have investigated earlier [10,22,23], the properties of co-evaporated CZTSe films and reported band gap values in the range 1.42 eV to 1.65 eV depending on the deposition temperature, annealing conditions and Cu/[ZnþSn] ratio. In our earlier studies on in-situ annealed CZTSe films [22], the onset of fundamental absorption in the spectral transmittance curve (Fig. 6 Ref. [22]) occurred at ∼900 nm for films deposited at Ts ¼ 573 K and 623 K and at ∼1050 nm for films deposited at other Ts. In the present studies, the onset of fundamental absorption edge mostly occurred at ∼1260 nm. The films in the present case are Cu-poor and Se-rich, while in our earlier studies, the films were near-stoichiometric. Even in the case of spectral transmittance curves reported by Ahn et al. [11], the onset of fundamental absorption edge varied from ∼1400 nm to ∼1280 nm for co-evaporated CZTSe films as Ts varied from 320 1C to 400 1C. Table 2 shows the Eg values (obtained from (αhν)2 versus hν plots of CZTSe films reported by Ahn et al. [11] ) at different Ts along with Cu/[Znþ Sn], [Zn/Sn] ratios (read from the graphs in the supplementary material of their paper). This clearly shows that Cu/[Znþ Sn] being almost same, Eg increases from 0.99 eV to 1.30 eV (Ts ¼320 1C to 400 1C) with increase in [Zn/Sn]. In spite of large variation in the Eg with [Zn/Sn] ratio, these films were found to be apparently phase pure from XRD analysis [11].The above observations reveal that the onset of the fundamental absorption edge and hence optical band gap of CZTSe films is critically dependent on the composition. Wada et al. [37], based on electronic structure studies on CZTSe and CTSe, concluded that the difference in the band gap between these compounds arise due to shifts in valence band maximum (VBM) and conduction band minimum (CBM) levels which in turn depend on Cu–Se and Sn–Se distance, respectively. Thus the inclusion of ZnSe in Cu2SnSe3 lattice affects the Cu–Se and Sn–Se distances and hence VBM and CBM levels. Thus band gap of CZTSe might depend on the extent of inclusion of ZnSe in CTSe. One of the formation reaction routes suggested by Hergert and Hock [38] is Cu2SnSe3 þ3C−ZnSe-Cu2ZnSnSe4. Botti et al. [39], from their band structural studies, observed a strong variation in the band gap of CZTSe from 1.0 eV to 1.75 eV with anion displacement parameter. Persson [29] also observed a bending in the optical absorption spectrum of kesterite CZTSe and stannite CZTSe at around 1.6 eV which was attributed to broadening effect and/or defect related transition. Thus a great deal of further study is required to clearly understand and explain the spread in the band gap values of CZTSe, though it is largely agreed that ZnSe minor phase might be the reason for the observed higher values [11]. Fig. 7(b) shows the (αhν)2 versus hν plots of films deposited at Ts ¼623 K for the two spectral regions R-I and R-II observed in Fig. 5b. The direct optical band gaps are found to be 1.14 eV and 1.48 eV which are attributed to two compositionally different CZTSe regions in the film. Fig. 7(c) shows the (αhν)2 versus hν plots of films deposited at Ts ¼673 K for the two spectral regions R-I and R-II observed in Fig. 5c. The transition R-II is attributed earlier to CZTSe with inclusion of ZnSe and the increase in the spectral width of this transition compared to that of the films deposited at Ts ¼523 K indicates increase in the fraction of CZTSe with ZnSe in these films. The optical band gap values 1.00 eV and 1.42 eV obtained from Fig. 7(c) are attributed to CZTSe and CZTSe with ZnSe, respectively, as in the earlier case. Fig. 7(d) shows the (αhν)2 versus hν plot for annealed stacked layers deposited at T s ¼723 K. Only one transition is observed in the spectral transmission curve in this case (Fig. 5d). The direct optical band gap is found to be 1.55 eV which is attributed to CZTSe with ZnSe inclusion. As stated earlier, with increase in T s, the fraction of CZTSe with ZnSe in the entire film increased, probably due to higher extent of ZnSe inclusion.

187

Table 2 Eg values of CZTSe films deposited at different Ts from the reported data [11]. S. no.

Ts (in 1C)

Cu/(Zn þ Sn)

Zn/Sn

Eg (in eV)

1 2 3

320 370 430

∼0.75 ∼0.80 ∼0.75

∼1.1 ∼1.15 ∼1.50

0.99 1.05 1.30

4. Conclusions CZTSe films with kesterite structure could be obtained by a two stage process of selenization at 723 K of stacked Cu/ZnSe/Sn/Se precursor layer deposited at different Ts. From these studies, based on the crystallinity and the intensity of diffraction pattern, it is concluded that the optimum deposition temperature for the growth of CZTSe films is 673 K. The thickness of the films deposited at Ts ¼ 723 K is relatively lower compared to the thickness of the films deposited at Ts ¼ 673 K. The structure of the films was found to be kesterite with the lattice parameters a¼ 0.569 nm and c¼1.139 nm. The films are non-stoichiometric which might be due to the inhomogeneity in CZTSe film arising either due to insufficient annealing temperature or precursor layers thickness sizing. From optical absorption studies, two direct optical band gaps viz. ∼1.0 eV and ∼1.4 eV were obtained which were attributed to two different regions of CZTSe film formed. Since films were prepared by annealing the stacked layers Cu/ZnSe/Sn/Se, such a possibility is there based on the deposition and annealing temperatures. Annealing the stacked layers in the range 773–823 K, as done by others, and appropriately choosing the thickness of the precursor layers based on Ts would lead to homogenous single phase CZTSe film with relatively large grains. However, due to the limitations of the vacuum coating unit used, annealing of the stack at temperatures 4723 K could not be explored.

Acknowledgements One of the authors Dr. P. Uday Bhaskar gratefully acknowledges the NRE fellowship grant provided by the Ministry of New and Renewable Energy sources, New Delhi. The authors also gratefully acknowledge the help of Dr. P.S.R. Prasad, Scientist-E-II, NGRI, Hyderabad for his help in recording Raman spectra. References [1] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, New world record efficiency for Cu(In,Ga)Se2 thin-film solar cells beyond 20%, Progress in Photovoltaics: Research and Applications 19 (2011) 894–897. [2] 〈http://www.empa.ch/plugin/template/empa/*/131441〉. [3] B. Shin, O. Gunawan, Yu Zhu, Nestor A. Bojarczuk, S. Jay Chey, S Guha, Thin film solar cell with 8.4% power conversion efficiency using an earth-abundant Cu2ZnSnS4 absorber, Progress in Photovoltaics: Research and Applications 21 (2013) 72–76. [4] I. Repins, C. Beall, N. Vora, C. DeHart, D. Kuciauskas, P. Dippo, B. To, J. Mann W.-C. Hsu, A. Goodrich, R. Noufi, Co-evaporated Cu2ZnSnSe4 films and devices, Solar Energy Materials and Solar Cells 101 (2012) 154–159. [5] D.A.R. Barkhouse, O. Gunawan, T. Gokmen, T.K. Todorov, D.B. Mitzi, Device characteristics of a 10.1% hydrazine-processed Cu2ZnSn(Se,S)4 solar cell, Progress in Photovoltaics: Research and Applications 20 (2012) 6–11. [6] R.A. Wibowo, W.S. Kim, B. Munir, K.H. Kim, Growth and properties of stannitequaternary Cu2ZnSnSe4 thin films prepared by selenization of sputtered binary compound precursors, Advances in Materials Research 29 (2007) 79–82. [7] R.A. Wibowo, W.S. Kim, E.S. Lee, B. Munir, K.H. Kim, Single step preparation of quaternary Cu2ZnSnSe4 thin films by RF magnetron sputtering from binary chalcogenide targets, Journal of Physics and Chemistry of Solids 68 (2007) 1908–1913. [8] R.A. Wibowo, E.S. Lee, B. Munir, K.H. Kim, Pulsed laser deposition of quaternary Cu2ZnSnSe4 thin films, Physica Status Solidi A Applications and Material Science 204 (2007) 3373–3379.

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