A two-step approach for electrochemical deposition of Cu–Zn–Sn and Se precursors for CZTSe solar cells

Solar Energy Materials & Solar Cells 101 (2012) 277–282

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A two-step approach for electrochemical deposition of Cu–Zn–Sn and Se precursors for CZTSe solar cells _ Zenius Mockus, Vidas Pakˇstas, Remigijus Juˇske_ nas n, Stase_ Kanapeckaite_ , Violeta Karpavicˇiene, Auˇsra Selskiene_ , Raimondas Giraitis, Gediminas Niaura State Research Institute Center for Physical Sciences and Technology, Savanoriu Avenue 231, Vilnius, Lithuania

a r t i c l e i n f o

abstract

Article history: Received 2 February 2011 Received in revised form 9 January 2012 Accepted 6 February 2012 Available online 2 March 2012

A two-step electrochemical route for fabrication of Cu–Zn–Sn (CZT) and Se precursors’ layers for a thin film Cu2ZnSnSe4 (CZTSe) absorber for solar cells is demonstrated. The CZT was electrochemically co-deposited in a citrate solution and after that an appropriate amount of Se was electrodeposited on the top. The CZT þ Se were annealed in Ar atmosphere using a slow or fast increase in temperature up to 500 1C. SEM with EDX, XRD and Raman spectroscopy examinations of precursors and of the manufactured CZTSe thin films were carried out. The XRD measurements indicated that the CZT precursor contained Z-Cu6.26Sn5, Sn and g-CuZn5 phases. The electrodeposited Se was polycrystalline with a hexagonal structure. The manufactured CZTSe in all the cases presented Cu2ZnSnSe4 with kesterite or partially disordered kesterite structure; however a purer CZTSe phase was formed using the fast increase in the annealing temperature. & 2012 Elsevier B.V. All rights reserved.

Keywords: Cu2ZnSnSe4 Kesterite Electrodeposition Photovoltaics

1. Introduction Thin film solar cells have attracted steady increasing attention of many researchers during the recent 20 years. CIGS and CIS having record conversion efficiencies of 19.9% [1] and 14.28% [2], respectively, were the most attractive candidates for production of efficient solar cells. However they contain such elements as In, Ga and Se whose concentrations in the earth’s crust are lower than 0.05 ppm. To decrease the price of solar cells these elements should be replaced by abundant ones or must be used more cost-effectively. This circumstance was one of the main reasons to initiate investigations of CZTS and CZTSe, which have the same tetragonal structure as that of CIGS but having Zn and Sn substituted for In and Ga. For laboratory Cu2ZnSn (Se,S) cells the record conversion efficiency of 9.66% was recently achieved by Todorov et.al. [3] using solutionbased mixing, spin coating and heat treatments. Researchers are looking for another cost-saving process suitable for the solar cells manufacturing. Electrochemical deposition could be one of such processes. Record laboratory efficiency for CZTS cell in which the electrochemically deposited precursor was used achieved efficiency of 3.4% [4,5]. There are much more publications devoted to the electrodeposited CZTS [4–14] as compared to those devoted to CZTSe [15]. Nevertheless while manufacturing both of the absorbers there are two main approaches for the electrochemical deposition of precursor layers: (i) a stacked elemental layer [6–8,10,14,15] and (ii)

n

Corresponding author. Tel.: þ3705 264 8881; fax: þ3705 264 9774. E-mail address: [email protected] (R. Juˇske_ nas).

0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2012.02.007

a layer of Cu–Zn–Sn co-electrodeposited as an alloy [4,5,9,11–13]. There are reports on simultaneous electrodeposition of all the elements of CZTS [11,12]. However it is impossible to perform such one-step electrodeposition in the case of CZTSe, at least in water solutions. Normally, the Cu–Zn–Sn precursor layer electrodeposition is followed by sulfidation or selenisation, which is conducted at elevated temperatures in an atmosphere of S or Se vapours or in a gas mixture of H2S and Ar or N2. Sulphur is an abundant element but the scarcity of selenium is well-known. To cost-effectively use Se an appropriate amount of the latter can be electrodeposited. The goal of the current work was to show the possibility of electrodeposition of Cu–Zn–Sn alloy and Se as two precursor layers for the further annealing process in Ar or N2 atmosphere without Se. XRD and Raman spectroscopy examinations revealed that after the annealing at 500 1C Cu2ZnSnSe4 with kesterite or partially disordered kesterite structure was successfully formed.

2. Materials and methods A Cu–Zn–Sn alloy and a Se layer were electrochemically deposited in a three-compartment glass electrolytic cell. Electrolyte solutions were prepared using deionized water and CuSO4  5H2O, ZnSO4  7H2O, SnSO4, sodium citrate, K2SO4 (purity in all the cases 99%) and SeO2 (purity 99.5%). Hydroquinone was used as an antioxidant in the electrolyte solution for co-deposition of CZT. The solutions’ pH was adjusted with H2SO4 of spectroscopic purity. An ITO coated lime-glass (NANOCS, surface resistivity 8–12 O/&) of dimensions 1  2.5 cm2 and in some cases titanium foil was

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used as the working electrode. Prior to the electrodeposition ITO was ultrasonically washed in ethanol and rinsed in distilled water. Titanium foil was abraded with graded emery paper and rinsed in distilled water. A platinum (purity 99.999%) plate of much larger dimensions than that of the working electrode was used as an anode. The reference electrode Ag/AgCl/KCl(saturated) was placed in vicinity of the working electrode through a Lugin capillary. The electrolyte solutions were bubbled with Ar (purity 99.995%) for 30 min prior to electrodeposition. A constant temperature of solution was maintained at þ20 71 1C during electrodeposition of CZT and at 60 71 1C during electrodeposition of Se. A stirring was not provided during electrodeposition. Potentiostatic conditions were maintained using a programmable potentiostat PI-50-1.1. Annealing of the electrochemically deposited CZT and Se layers was conducted in a tube furnace in Ar (purity 99.995%) atmosphere. A device Omron E5CK-TAA1-500 digitally controlled the temperature regime using an N type (Ni–Cr–Si) thermocouple. An accuracy of temperature maintenance was not worse than 71.0 1C. Two annealing approaches were used: (i) slow (2 1C min  1) temperature increase up to 200 1C and staying at this temperature for 20 min, afterwards fast (10 1C min  1) increase up to a temperature of 500 1C and staying at this temperature for 120 min with subsequent natural quench down to room temperature; (ii) fast (20 1C min  1) increase up to a temperature of 500 1C and staying at this temperature for 120 min with subsequent natural quench down to room temperature. XRD characterisation of precursors and synthesised CZTSe was performed using a diffractometer D8 Advance (Bruker AXS). Cu Ka radiation (wavelength 0.154183 nm) was separated by a multilayer Ni/graphite monochromator, which also formed a parallel beam of X-rays. The XRD patterns were measured in grazing incidence (GID) step scan mode with a step size of 0.041 (in 2Y scale) and counting time of 5–10 s. An incidence angle of the primary beam was 0.51. An appropriate adjustment of the diffractometer was controlled using the certified standard SRM 606 from NIST. Phase identification was performed using the powder diffraction data-base PDF-2 (2003 release). Phase purity of CZTSe was examined by Raman scattering spectroscopy. Raman spectra were recorded using Echelle type spectrometer RamanFlex 400 (PerkinElmer Inc.) equipped with thermoelectrically cooled (  50 1C) CCD camera and fiber-optic cable for excitation and collection of the Raman scattering. The 785 nm beam of the diode laser was used as the excitation source. The 1801 scattering geometry was employed. The laser power was restricted to 20 mW and focused to the 200 mm diameter spot on the sample. Raman scattering wavenumber axis was calibrated using the polystyrene standard (ASTM E 1840). Raman bands’ intensity was calibrated using NIST intensity standard SRM 2241. The integration time was 10 s. Each spectrum was recorded by accumulation of 50 scans yielding overall integration time of 500 s. In some experiments 30 mW laser power was used. In these cases in order to avoid laser induced degradation of the sample linearly moving with respect to the laser beam with the rate of 15–25 mm s  1 sample holder was employed [16]. Electron microscopy studies were carried out in SEM EVO50EP (Carl Zeiss SMT) arranged with an X-ray energy dispersive spectrometer (EDX), from Oxford Instruments.

3. Results and discussion Other authors already reported studies of the electrodeposition of a triple alloy Cu–Zn–Sn [4,5,11,12]. We used an electrolyte solution similar to that presented in [11,12] (mmol dm  3): Cu2 þ 20, Zn2 þ 10, Sn2 þ 10 and 0.1 mol dm  3 of sodium citrate. The pH

of the solution was in the range 5.5–6.0, cathodic potential, Ec ¼  1.3 V with reference to Ag/AgCl/KCl(saturated) electrode. The average thickness of the CZT layer was approximately 1.270.2 mm. Fig. 1 shows a SEM image of a representative CZT precursor layer electrodeposited on ITO under conditions mentioned above. The layer consisted of spherical grains of approximately 0.5 mm in diameter. The solution was not stirred during electrodeposition so it could be expected that agitation or usage of a rotating disc electrode could make the surface even smoother. The elemental composition of the layer depending on solution pH is depicted in Table 1. The values of ratios Cu/(ZnþSn) and Zn/Sn are close to these characteristics of stoichiometric Cu2ZnSnSe4. The quantities of Zn and Sn in the layer can be easily varied by changing solution pH (see Table 1). Other authors [17] achieved best conversion efficiency for CZTS using copper-poor (Cu/ (ZnþSn)¼ 0.85) and zinc-rich (Zn/Sn¼1.25) precursors. Copperpoor and zinc-rich precursors are preferable in the case of CZTSe as well [24]. The XRD patterns measured in the GID mode are shown in Fig. 2. The lower one is for the copper-rich layer, the chemical composition of which is depicted in the 1st row in Table 1 and the upper is for that of Cu-poor and Zn-rich (the 2nd row in Table 1). In both the cases CZT layers presented phases: Z-Cu6.25Sn5 (PDF no. 47-1575), Sn (PDF no. 65-2631) and hexagonal e-CuZn5 (a ¼0.2702 and c¼0.4293 nm). The amount of the latter phase increased with an increase in Zn quantity in the alloy. Schurr et al. [4] claimed that the CuZn phase was present in the copper-poor electroplated CZT precursor. Zoppi et al. [18] have found Cu5Zn8 in the magnetron sputtered metallic CZT precursor layers instead of e-CuZn5. They also supposed the presence of Zn phase in those CZT precursor layers. Heat-treatment of the electrodeposited CZT precursor layer in a slightly reducing atmosphere (Ar 90%, H2 10%) at temperatures

Fig. 1. A SEM image of electrodeposited CZT precursor with the following ratios of elemental composition: Cu/(Znþ Sn)¼ 0.89, and Zn/Sn¼ 1.15.

Table 1 CZT layer elemental composition (in at%) determined by EDX as a function of electrolyte solutions pH. pH of solution

Cu

Zn

Sn

Cu/(Znþ Sn)

Zn/Sn

6.0 5.75 5.5

52.08 47.21 44.41

22.29 28.20 32.69

25.63 24.59 22.90

1.09 0.89 0.80

0.87 1.15 1.43

R. Juˇske_ nas et al. / Solar Energy Materials & Solar Cells 101 (2012) 277–282

Se 201

ITO 440

γ-Cu5Zn8 330

γ-Cu5Zn8 222

Sn 101

CuZnSn+Se

Se 111

Se 101

η-Cu6.26Sn5 101

CuZnSn

Intensity

η-Cu6.26Sn5 202

η-Cu6.26Sn5 103

η-Cu6.26Sn5 201 Sn 301 η-Cu6.26Sn5 112

γ-Cu5Zn8 510

γ-Cu5Zn8 422

γ-Cu5Zn8 332

annealed

Sn 220 Sn 211

γ-Cu5Zn8 321 Ti 002 Ti 101 ε-CuZn5 002 η-Cu6.26Sn5 102

γ-Cu5Zn8 222

Sn 101

400

η-Cu6.26Sn5 002

800

Sn 200

Intensity (cps)

1200

η-Cu6.26Sn5 101

200 cps

as deposited

Sn 211

Fig. 2. XRD patterns for CZT precursors on the ITO/glass substrate. The elemental composition of precursor layers is presented in the figure.

η-Cu6.26Sn5 110

60

Se 110 ITO 332

50

ITO 411

Sn 301

η-C6.26Sn5u

ITO 440

ITO 521

40 2Θ (degrees)

ε-CuZn5102

Zn/Sn=0.87

γ-Cu5Zn8 222

η-Cu6.26Sn5

Zn/Sn=1.15 Cu/(Zn+Sn)=1.09;

Sn 220 ε-CuZn5101 Sn 211

ε-CuZn5002 ITO 411

30

Cu/(Zn+Sn)=0.89;

ITO 420

Sn 101

Sn 220 η-Cu6.26Sn5

η-Cu6.26Sn5

Sn 200

Intensity

100 cps

two phases: Z-Cu6.25Sn5 and g-Cu5Zn8 (PDF no. 25-1228). The same phases were determined during the selenisation at a temperature of 250 1C in Ref. [19] where CZT precursor layers were sequentially evaporated from Sn, Zn and Cu sources. Se was electrodeposited onto the CZT precursor layer in the solution containing 20 mmol dm  3 of SeO2 and 0.2 mol dm  3 of K2SO4, at pH 2.8, Ec ¼ 0.7 V, T¼ þ60 1C, and deposition time 20–30 min. Fig. 4A shows SEM images of the Se electrodeposited on the top of as-deposited CZT layer. The electrodeposited Se formed spherical grains of different diameters (0.5–5 mm). Sometimes their surface morphology was indicative of a crystalline state and others look like droplets of mercury. Se electrodeposited onto the annealed CZT looked a little bit different (Fig. 4B): thin needle-like crystals were seen between significantly large spherical grains. That could be caused by a faster electrocrystallization of Se on the annealed CZT. The quantity of selenium in the separate grains or droplets measured by EDX was in the range from 98.0 to 99.5 at% thus bringing the evidence that there was elemental selenium. Fig. 5 presents XRD patterns of the annealed CZT layer and of the same CZT with Se electrodeposited on the top. The additional XRD peaks apparently coincide with those of the hexagonal Se (PDF no. 6-362) so the spherical grains and ‘‘droplets’’ are polycrystalline Se electrodeposited on the CZT precursor layer. Fig. 6A shows a SEM image of the electrodeposited CZT þSe annealed according to the first approach (slow increase in temperature) and Fig. 6B shows an image of analogous CZT þSe layers annealed according to the second method (fast increase in temperature). The images look rather similar although in the case

η-Cu6.26Sn5 002

of 180–200 1C could improve homogeneity of the alloy, reduce oxides and consequently yield better conditions for further electrodeposition of Se onto the CZT. Fig. 3 shows the XRD patterns of the as-deposited CZT layer (the thicker curve) and after the CZT was annealed in the slightly reducing atmosphere at a temperature of þ180 1C for 2 h. The depicted pattern is for the CZT electrodeposited at pH 5.75 (chemical composition is about the same as that presented in 2nd row in Table 1) on a mechanically polished titanium foil. After the annealing the CZT layer it presented only

279

0 30

40

50 2Θ (degrees)

60

Fig. 3. XRD patterns for CZT precursors on titanium: as-deposited (slightly upper and thicker curve); annealed for 2 h at þ 180 1C in atmosphere of 90% Ar þ10% H2.

30

40 2Θ (degrees)

50

Fig. 5. XRD patterns for CZT annealed in Arþ H2 atmosphere (lower and thinner curve) and for the same CZT with Se electrodeposited on the top (thicker curve).

Fig. 4. SEM images of Se electrodeposited on CZT (A) and on CZT previously annealed in Ar þH2 atmosphere (B).

R. Juˇske_ nas et al. / Solar Energy Materials & Solar Cells 101 (2012) 277–282

280

Fig. 6. SEM images of CZTSe films produced with a slow (A) and a fast (B) increase in annealing temperature.

Table 2 Elemental composition (in at%) of CZTSe layers after the annealing.

10 25

1.13 0.93 1.13

30

35

SnSe 311

1.03 1.11 1.22

40 2Θ (degrees)

Se/CZT

1.02 1.09 0.99

45

116 312

Cu2Se 090 SnSe 102 105 SnSe 411 Cu2Se 012 Cu2-xSe 220

Slow annealing Fast annealing

Zn/Sn

50

SnSe 601

11.47 11.75 10.62

Cu/(ZnþSn)

114

100

50.45 52.10 49.66

112 103 SnSe 011 η-Cu6.26Sn5 101 SnSe 111 SnSe 400 ITO 222 202 211

SnSe 201

Intensity (cps)

1000

11.81 13.06 12.99

Sn

SnSe 420

26.27 23.10 26.74

Se

204 220

6.0 5.75 5.5

Zn

SnSe 302 214 301 SnSe 511 ITO 440

Cu

Cu2Se 060

CZT electro deposited at pH

Table 3 Calculated and experimental 2Y values for the XRD peaks of Cu2ZnSnSe4 with lattice parameters: a ¼0.56945; c¼ 1.138 nm.

55

Fig. 7. XRD patterns for CZTSe films produced with a slow (upper) and a fast (lower) increase in annealing temperature.

of the slow annealing the grains are slightly larger. The surface of CZTSe looks rough, has a lot of voids. The grain size is o1 mm and the thickness of the CZTSe layer is about 2–3 mm. It is evident that a layer with such a surface morphology does not meet requirements for a good quality solar cell absorber. It is known that the surface should be smooth, the grain size of about 1 mm, i.e. of the same dimensions as the thickness of the absorber layer [20]. Nevertheless, the elemental composition of CZTSe layers depicted in Fig. 6 complies with that of Cu2ZnSnSe4 as can be seen in Table 2. Fig. 7 shows XRD patterns of the CZTSe layers depicted in Fig. 6: the upper was annealed according to the first approach and the lower according to the second one. The logarithmic intensity scale was used to show better the low intensity peaks. In both the cases nearly all the XRD peaks presented in PDF card no. 52-868 for Cu2ZnSnSe4 are seen. The rest of the low intensity peaks of the upper pattern in Fig. 7 can be attributed to selenides: Cu2Se (PDF nos. 65-2982, 46-1129 (both fcc) and 37-1187 (orthorhombic)) and SnSe (PDF no. 48-1224). There were also some peaks belonging to the phases Z- Cu6.25Sn5 and g-Cu5Zn8. A lesser amount of the latter phases and no evidence of Cu2Se and SnSe were found

hkl

2Y Calc.

2Y Exp.

D2Y

110 112 103 200 202 211 114 105 220 214 006 301 312 116

22.058 27.108 28.245 31.393 35.218 36.113 38.722 42.731 45.009 47.902 47.924 48.588 53.317 53.351

22.058 27.109 28.258 31.399 35.217 36.11 38.699 42.74 45.007 47.881 47.881 48.578 53.331 53.331

0.000  0.001  0.013  0.006 0.001 0.003 0.023  0.009 0.002 0.021 0.043 0.010  0.014 0.020

in the CZTSe annealed according to the 2nd approach. The latter CZTSe film was nearly single-phase. However, still there were a couple of low intensity peaks which can be attributed to any of the suitable phases. These are peaks at 2Y angles: 28.2, 35.2, 38.7, 42.7, 47.9 and 48.61. These peaks could be attributable to peaks of tetragonal structure with hkl indexes such as 103, 202, 114, 105, 214 and 301. Table 3 presents all possible hkl indexes for tetragonal Cu2ZnSnSe4, the calculated values of corresponding 2Y angles and 2Y angles of the peaks observed on the lower XRD pattern presented in Fig. 7. Rather good coincidence of the calculated and experimental 2Y values is apparent. The calculated 2Y values were obtained using following lattice parameters: a ¼0.56945, c¼1.138 nm and Z ¼c/2a¼1.00005. These lattice constants were determined from XRD data for Cu2ZnSnSe4 synthesised in the current work. The constants’ values determined are larger than those presented by other authors [21]. One of the reasons could be ITO substrate used instead of Mo in the current work. It is impossible to reliably determine stannite or kesterite structure was formed on the basis of the powder XRD. The powder XRD studies of Cu2ZnSnSe4 single crystal revealed it was stannite structure with lattice parameters a ¼0.56882, c¼1.13378 nm and Z ¼1.0034 [21]. According to Ref. [22] CZTSe can crystallise forming both of the structures, but in the case of kesterite the a lattice parameter should be slightly larger and tetragonal distortion parameter Z smaller as compared with those of stannite. And one more structure that CZTSe can take is a partially disordered kesterite as it was shown by the recent neutron diffraction studies [23]. The disorder in the Cuþ Zn layer causes a volume expansion of 0.3% [22]. Taking into account the above mentioned features of stannite and kesterite structures and the results obtained, it may be inferred that electrochemically deposited precursors crystallised in kesterite or partially disordered kesterite structure.

R. Juˇske_ nas et al. / Solar Energy Materials & Solar Cells 101 (2012) 277–282

25 193

30

CuxSe

20

150

200 250 Wavenumber (cm-1)

300

Fig. 8. Raman scattering spectra of CZTSe films. The upper curve: CZT layer was annealed in reducing atmosphere prior to electrodeposition of Se; the lower curve: Se was electrodeposited on the as-deposited CZT layer.

However the XRD studies can tell little not only about in what structure CZTSe crystallises but also about the presence of cubic ZnSe and Cu2SnSe3 in the synthesised CZTSe as the most intensive XRD peaks of these compounds coincide with those of Cu2ZnSnSe4 phase. The Raman spectra can be helpful in these cases. Dashed vertical lines in Fig. 8 indicate positions of Raman lines corresponding to various selenides, which might be present in the film [24–34]. Previously it was demonstrated that CZTSe exhibits characteristic Raman spectrum with major peaks at 194–197, 172–173 and 231–235 cm  1 [24–27]. Intense peak near 192 cm  1 along with the shoulder near 172 cm  1 in the Raman spectra shown in Fig. 8 confirmed the presence of CZTSe phase in the electrochemically synthesised films. However, spectrum of the sample CZT precursor, which was annealed in Arþ10% H2 at 180 1C prior to electrodeposition of Se, differed considerably from that with Se electrodeposited on as-deposited CZT. Presence of relatively large amounts of SnSe2 and Cu2SnSe3 compounds was manifested by considerable broadening of the 192 cm  1 band in the former CZTSe. In addition, due to ZnSe, elemental Se and Cu2SnSe3 phases relatively strong and broad peak was visible near 243 cm  1. Thus electrodeposition of Se on as-deposited CZT resulted in more pure CZTSe phase as compared to electrodeposition of Se on CZT layer annealed in reducing atmosphere. More detailed spectral analysis was performed with the sample which was annealed by fast rise in temperature and copper selenides were anodically dissolved thereafter (Fig. 9). Two components due to CZTSe phase located at 193 and 173 cm  1 dominated in the Raman spectrum, indicating that the major phase was Cu2ZnSnSe4. The major component (193 cm  1) was relatively narrow; the full width at half maximum (FWHM) value was found to be 7.6 cm  1, indicating relatively defects-free Cu2ZnSnSe4 phase. The lower intensity component at 234 cm  1 could be attributed to CZTSe as well. Based on the Raman data it could be concluded that compound CuxSe was absent in the studied material, because characteristic band was not visible in the vicinity of 256–263 cm  1 [29,30]. The low intensity band in the higher frequency side (246 cm  1) most probably belonged to cubic ZnSe phase. Bulk ZnSe exhibits intensive longitudinal (LO) mode at slightly higher wavenumbers (252–256 cm  1) [30,32,34]. However, this mode shifts to 248 cm  1 for nanoparticles [34]. Thus ZnSe phase in the studied sample must be defected or in the form of nanoparticles. The relatively narrow peak at 186 cm  1 (FWHM¼11 cm  1) has been attributed to Cu2SnSe3 compound. Previously it was demonstrated that this compound exhibits intense and narrow bands near 180 cm  1 [27]. It should be noted that 186 cm  1 band may have a contribution from the SnSe2 compound,

15 CZTSe

10

CZTSe 246

Raman intensity (cps)

251

ZnSe

260

CZTSe

236 240

Cu2SnSe3

40

20 Se

CZTSe

216

CZTSe

Cu2SnSe3

186

CZT as-deposited

173

192

186

CZT annealed in H2

231

CZTSe

180

Cu2SnSe3 172

Raman intensity (a.u.)

SnSe2

234

60

50

281

ZnSe

5

0 150

200 250 Wavenumber (cm-1)

300

Fig. 9. Raman spectrum of the CZTSe film synthesised with the fast increase in annealing temperature. The thinner lines represent the fitted Gaussian–Lorentzian line shapes. Excitation wavelength is 785 nm (30 mW).

which in amorphous state shows broad band near 182 cm  1, while upon crystallisation to hexagonal phase a narrow Raman peak appears at 186 cm  1 [28]. However, SnSe2 was not uniquely detected by XRD analysis, and possibility for formation of such a compound is questionable. Recently, Raman marker bands for discrimination of kesterite and stannite structures of CZTSe have been suggested [35]. In particular, first principles calculations using a density-functional theory (DFT) revealed the shift of outermost frequency longitudinal vibrational mode from 239 cm  1 in kesterite to 254 cm  1 in the case of stannite. No band was detected in the vicinity of 254 cm  1 in the spectrum of electrochemically prepared sample (Fig. 9), while the peak near 234 cm  1 is visible after decomposition of the experimental contour into the components. In addition, calculated spectrum of CZTSe– kesterite in 150–200 cm  1 spectral region predicts two bands located at 174 and 187 cm  1, which correspond-well with the experimentally observed bands at 173 and 192 cm  1 (Fig. 9). However, only one band is expected to be observed in this spectral region for stannite phase [35]. Let us consider the experimentally observed band at 216 cm  1 (Fig. 9). For CZTSe–kesterite structure calculations suppose vibrational mode near 216 cm  1 (vibrations of Cu, Zn and Sn cations against Se), which might be associated with experimentally observed feature at the same frequency. In the case of stannite similar band is expected to be observed near 222 cm  1 [35]. The above analysis of the Raman data suggests that CZTSe synthesised using the electrochemically deposited precursors resembles the Cu2ZnSnSe4 with kesterite structure.

4. Conclusions The electrochemical deposition of selenium can be successfully used for the cost-effective fabrication of a Se precursor layer while manufacturing CZTSe thin film absorbers for solar cells. The annealing of the CZT precursor layer in the slightly reducing atmosphere (Arþ10% H2) at temperature of 180 1C does not bring advantages during further electrodeposition of Se and overall fabrication of CZTSe. The fast increase in annealing temperature of CZT and Se precursors is preferable over the slow rise in temperature yielding purer Cu2ZnSnSe4 phase. The manufactured CZTSe film presented Cu2ZnSnSe4 with kesterite or partially disordered kesterite structure. Further investigations will be carried out to produce CZT and Se precursor layers with better morphological characteristics on a Mo/sodium lime glass substrate. The suggested electrochemical approach for the fabrication

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