Growth and characterization of chalcostibite CuSbSe2 thin films for photovoltaic application

Accepted Manuscript Title: Growth and Characterization of Chalcostibite CuSbSe2 Thin Films for Photovoltaic Application Authors: Kunal J. Tiwari, Vijay Vinod, A. Subrahmanyam, P. Malar PII: DOI: Reference:

S0169-4332(17)30303-3 http://dx.doi.org/doi:10.1016/j.apsusc.2017.01.279 APSUSC 35074

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15-10-2016 25-1-2017 26-1-2017

Please cite this article as: Kunal J.Tiwari, Vijay Vinod, A.Subrahmanyam, P.Malar, Growth and Characterization of Chalcostibite CuSbSe2Thin Films for Photovoltaic Application, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.01.279 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Growth and Characterization of Chalcostibite CuSbSe2Thin Films for Photovoltaic Application Kunal J. Tiwari1,2, Vijay Vinod2, A. Subrahmanyam3, P.Malar1, 2* 1

3

Research Institute,2Department of Physics and Nanotechnology, SRM University Kattankulathur, 603203, Tamilnadu, India

Semiconductor Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai, Tamilnadu, India Highlights 

Chalcostibite CuSbSe2 bulk was synthesized by mechanical alloying using elemental Cu, Sb and Se.



As Synthesized bulk was used as source for the growth of CuSbSe2 thin films on soda lime glass substrates by e-beam evaporation.



Beam current optimization and annealing was done to obtain the near stoichiometric CuSbSe2 thin films.



To obtain near stoichiometric thin films, beam current of 50 mA and annealing at 380 oC for 60 min was found optimal as revealed by detailed XRD, Raman and optical absorption studies.

Abstract: Bulk copper antimony selenide was synthesized using mechanical alloying from the elemental precursors. Phase formation in milled powders was studied using x-ray diffraction (XRD) and Raman spectroscopy studies. The synthesized bulk source after cold compaction was used as source material for thin film deposition by e-beam evaporation. Thin film deposition was carried out at various e-beam current values (Ib ~ 30, 40 and 50 mA) and at a substrate temperature of 200 oC. Near stoichiometric CuSbSe2 thin films were obtained for Ib values closer to 50 mA and post annealing at a temperature of 380oC for 1 hr. Thin films deposited using above conditions were found to

1

[email protected]

exhibit an absorption coefficient () values of > 105 cm-1 and a band gap value ~ 1.18 eV that is closer to the reported band gap for CuSbSe2 compound. Keywords: Inorganic solar absorber, Thin films, e-beam evaporation, Raman spectroscopy, Band gap

1.

Introduction Renewable energy sources have enormous potential to fulfill the present and growing energy demands of the

world. Technologies related to renewable resources namely solar, the wind, tidal and hydrothermal are continuously being developed and improved. Among the different resources solar energy has been prevalent and has seen major developments as compared to the other renewable energy technologies. Photovoltaic (PV) cells are efficient in harnessing the abundant solar power falling on earth in the form of sunlight. PV cells made from the Si have been used over the years. In view of the future of PV, and to ease the dependency on silicon, identification of materials with similar or better PV properties is critical. Earth-abundant non-toxic constituents and superior optical absorption have been the major selection criteria for an alternate absorber material. Thin film solar cells using inorganic compound semiconductors as absorber layer have been explored widely. Inorganic semiconducting compounds namely CdTe, chalcopyrite CIGS, kesterite CZTS/Se,to name a few have been proposed and subsequently considered as potential absorbers for solar cell device applications[1, 2 ]. Direct band gap nature with band gap (Eg) value in the range ~ 1 – 1.5 eV and high absorption coefficient of the order of > 104 cm-1 makes them interesting for photovoltaic application [3]. Thin film solar cells with CdTe and CIGS as absorber layers have been studied well with reported laboratory efficiencies of ~ 19 and ~ 21 % respectively [4]. Though CdTe and CIGS-based solar cells technologies are commercialized, the toxicity issues related to Cd and the scarcity of In have been major drawbacks associated with these absorber materials. Hence earth abundant and nontoxic compound semiconductors such as CZTS/Se and their alloy CZT(S, Se) are considered as probable successor. An investigation carried out by many groups across the world has led to the maximum efficiency of ~ 12.5 % for these materials until now [4]. Being a quaternary system these materials suffer from the stiff phase formation conditions, making the material synthesis and stabilization process complex. Therefore, earth-abundant materials that are less complicated structurally but exhibiting fair optical as well as electronic properties attracts attention. Copper antimony selenide class of compounds namely Cu3SbSe3, Cu3SbSe4 and CuSbSe2 have been recently considered for PV as well as thermoelectric applications [5,6]. First principle calculations performed by Maeda et.al

proposed CuSbSe2as potential earth abundant material for solar cell application [7]. In addition, it has been also explored for the thermoelectric application as a potential p-type leg in thermoelectric devices [8,9]. Spectroscopic limited maximum efficiency (SLME) method proposed by Liping Yu et.al have predicted this material as super absorber with > 105 which is comparable to the existing (ternary analogs such as CuInSe2) inorganic compound semiconductors [5]. Among the above three compounds, CuSbSe2 can be considered as chemically similar but structurally dissimilar to the well-known chalcopyrite compound CuInSe2, in which scarce indium is replaced with earth abundant Sb. CuSbSe2 exist in the orthorhombic layered structure unlike the existing inorganic compound semiconductors like CdTe, CIGS, CZTS/Se etc. that exhibit tetragonal bonding. In the case of CuSbSe2 lone pair of electrons available on the Sb distorts the tetragonal bonding leading to the formation of layered structure. The layered structure leads to the absence of dangling bonds and hence reduces the carrier recombination at the grain boundaries. In addition, theoretical calculations have revealed that CuSbSe2 exhibits very high absorption (> 105 cm-1) in short wavelength region just after the band gap (~1.1 eV). This abrupt absorption onset is attributed to the presence of high density of states in this compound [10]. Synthesis of bulk CuSbSe2 has been achieved through conventional solid state synthesis method where vacuum sealing or inert atmosphere is required for heating to high temperatures in a furnace for the reaction of the constituents and subsequent phase formation [8]. Zhang et.al have synthesized the CuSbSe2 by mechanical alloying and studied the thermoelectric properties of hot-pressed samples [9]. Bulk and single crystals have also been synthesized by solvothermal routes [11].Thin films of the CuSbSe2 compound have been deposited by both PVD and electrochemical deposition methods [12,13]. Mechanical alloying is a technique which can facilitate the desired phase formation along with the particle size reduction of the material [14]. Elemental or binary precursors are used as starting materials to obtain the desired composition of the alloy under inert dry milling or atmospheric wet milling conditions. Unlike conventional solid state synthesis method where vacuum sealing and carefully ramped high temperatures are required to synthesize the bulk, ball milling facilitate the synthesis devoid the above requisites. The energy generated due to the frictional forces during the milling process enable the phase evolution in the materials. The contributing parameters such as milling speed, milling time, ball to powder weight ratio are varied to arrive at optimized set of favorable conditions.

Near stoichiometric bulk synthesis via ball milling of variety of compounds such as Sb2Se3, CZTS/Se etc. have been reported in the recent times [15,16, 17]. Physical vapor deposition techniques such as thermal evaporation, e-beam evaporation, sputtering etc. are the well-studied and established techniques for obtaining the device quality thin films of desired materials. The high quality achieved through these methods make them ideal methods to conduct basic studies of the emerging materials [18]. Electron beam evaporation is one of the reliable techniques for the growth of thin films of materials with high melting point. This technique has also been found useful for multi–component alloys with considerable difference in the vapor pressure values of the constituents [19]. Thin films of the ternary alloys such as CIGSe, has been deposited by making use of single phase bulk source material previously [20, 21]. It is reported that appropriate control of the beam current with constant beam voltage during e-beam evaporation lead to the formation of the near stoichiometric thin films from the bulk source. In addition to the above the substrate holder can be rotated to obtain compositional uniformity over an extended area to avoid the combinatorial mixture of different phases or subsystems. In this paper we report the details of synthesis of bulk CuSbSe2 compound by mechanical alloying from elemental precursors. Subsequently, the cold pressed milled material was used as source material for the thin films growth by e-beam evaporation. 2. Experimental Details

Elemental precursors namely Cu, Sb and Se have been taken in the stoichiometric ratio of 1:1:2. Ball milling was done in planetary ball mill model Fritsch Pulverisette P-6 Classics line. Precursors were mixed with the processing agent; in this case toluene has been taken in the milling jar along with the milling media. Milling jar was made up of hardened steel with inner tungsten carbide (WC) lining and WC balls of 10 mm diameter has been used as milling media; the weight of each ball was ~ 10 gm. The ball to powder weight ratio (BPR) was 15:1. Milling was carried out at 500 rpm for 8 hr duration with pause time of 15 min. after every hour of run. The milled powder was cold pressed and used a source material for the growth of CuSbSe2 thin films using Hind Hivac 12A-4D thermal and e-beam evaporator. Thin films have been deposited on the soda lime glass substrates at a constant substrate temperature of 200 oC for which optimization of beam current values has been done carefully. Vacuum annealing was done at temperatures in the range of 350 - 400 oC to facilitate the grain

growth and to achieve the near stoichiometry of 1:1:2 in the CuSbSe2 thin films. E-beam evaporation was carried out at fixed voltage of ~ 5 KV and the beam currents of 40 mA and 50 mA was used. Milled bulk material, as grown and annealed thin films have been subjected to the XRD and Raman spectroscopy studies for the phase confirmation. XRD studies were done on the Panalytical Xpert Diffractometer using Cu K having a wavelength of 1.54 Å. Raman spectroscopy measurements were made using Horiba Jobin Yvon Evolution Spectrophotometer in the backscattering mode using 532 nm excitation wavelength emitted from He-Cd laser. The power used for the experiments was < 5.5 mW. Low power was chosen to avoid any changes that can be induced due to thermal effects. Si was used as standard for the calibration purpose. SEM measurements were performed with FEG Quanta 200 FESEM instrument. Optical transmittance data for thin films samples were collected in the wavelength range of 350-1100 nm using Analaytik Jena UV-Vis spectrophotometer fitted with integrating sphere. Optical transmittance spectra for the as grown and annealed thin films were analyzed to calculate the values of absorption coefficient (α) and the bang gap (Eg). 3. Results and Discussion 3.1 XRD and Raman Spectroscopy Study on as milled bulk material

Fig.1(a) shows the typical XRD pattern obtained for the as milled CuSbSe2 powder. Analysis of the XRD data indicated that milled CuSbSe2 powder exhibit the formation of mixed phases along with the desired CuSbSe2 phase. No peaks corresponding to the elemental precursors Cu and Se were observed in the XRD pattern of the as milled samples except for the traces of unreacted Sb. The pattern was indexed with reference to ICDD card no. 04-0082357 for CuSbSe2, 04-006-2232 for Sb2Se3 and 04-007-2064 for Sb. Raman spectroscopy analysis was done to compare and decide the phases present in the system. Raman spectroscopy is a unique method which can provide information about the phase and structure of the material. Depending upon the position of the peak one can comment about the phases which have formed as well as defects if any is present in the system [22]. The deviation in the standard values can be correlated with the stoichiometry of the synthesized material as each compound has unique Raman finger print. Presence of defects can also lead to the slight shift in the peak position from the standard value. Hence in this manner Raman spectroscopy can provide the in depth information about the material under investigation. We have used Raman spectroscopy to study the phase formation as well as to determine the presence

of the secondary phases present. There are not many reports available on the Raman spectroscopic studies of this compound except the study carried out by Xue et.al [10]. In their work they have theoretically calculated the phonon modes for the CuSbSe2 compound and proposed possible total 22 Raman active modes. Fig. 1(b) shows the typical Raman spectrum obtained for the milled bulk compound. Appearance of the broad peak at ~ 186 cm-1 indicates the presence of Sb2Se3 with other vibrations appearing at ~ 250 and ~ 370 cm-1[23]. Peaks with low intensity corresponding to the CuSbSe2compound was observed at ~ 109 cm-1(B2g), ~ 117 cm-1 (Ag), 141 cm-1 (B3g) and 153 cm-1 (Ag) with small shift compared to the theoretically calculated values. Peak due to B2g mode of CuSbSe2 at 204 cm-1was found to overlap with the maximum intensity peak of Sb2Se3 at 186 cm-1 leading to asymmetrical peak broadening. The small shifts in the peak positions of CuSbSe2 phase indicate possible compositional variation. Raman analysis was found to match with XRD analysis on the formation of Sb2Se3 compound along with CuSbSe2 in the milled powder.

3.2 XRD and Raman Spectroscopy Studies on Thin films XRD studies were performed on the as grown and annealed CuSbSe2 thin films. Fig. 2 (a) and (b) show the typical XRD patterns for the thin film deposited at beam current Ib ~ 50 mA and annealed at 380 oC respectively. Absence of diffraction peaks in the as deposited thin films indicates the amorphous nature. The films annealed at 380 oC showed well resolved diffraction peaks confirming the grain growth. The peaks are indexed and found to match with the standard data for the CuSbSe2 phase. In addition, peaks corresponding to Cu3SbSe3 (ICDD card no. 50-1346) were also observed in the XRD pattern. The inset to the figure shows the SEM image presenting a smooth surface. Thin films deposited at Ib ~ 40 mA and annealed at 380 oC were also subjected to the XRD studies but no diffractions peaks were observed indicating the amorphous nature. A detailed Raman spectroscopy studies of these thin films (grown at Ib~ 40 mA and Ib~ 50 mA and annealed at 380 oC) were performed to analyze the phase formation. Typical Raman spectrum obtained for as deposited CuSbSe2 thin films with Ib ~ 40 mA is shown in the Fig. 3(a). In the spectrum the maximum intensity peak appearing at 188 cm-1 correspond to the Sb2Se3 with other Raman modes appearing at 250, 372 and 450 cm-1 [23]. It is to be noted here that the Ag Raman mode for Cu3SbSe3 occurs at 185 cm-1 , which is close to the observed high intensity mode at 188 cm-1 [25]. This indicates the chances of

presence of Cu3SbSe3 and Sb2Se3 as secondary phases in the as grown CuSbSe2 film with Ib ~ 40 mA. The low intensity peaks observed at 82 (Ag mode), 117 (Ag mode) and 144 (B2g) are due to the CuSbSe2 phase [10]. The higher peak intensities observed for the Sb2Se3 modes in comparison to the modes of CuSbSe2 suggest the possibility that the surface of the film (as grown and annealed) consists of Sb2Se3 phase. Raman spectroscopy is surface technique which has depth limitation. The depth (d) that can be probed with laser of particular wavelength is inversely proportional to the absorption coefficient () of the material (d 1/2) [24]. Assuming a value of ~ 15 x 105 cm-1 (for the annealed film in this study) for the absorption coefficient, a green laser having a wavelength of 532 nm can probe a depth of approximately ~ 350 nm from the surface to extract the phase information. Hence the observed results lead to an understanding that the CuSbSe2 films present surface compositions that is predominantly Sb2Se3.Work reported by Zakyutev et.al shows that for Sb rich growth conditions CuSbSe2 and Sb2Se3 phases coexist if the processing temperature is below the sublimation temperature ( 350 oC) of Sb2Se3 [12]. In our case, the growth temperature was less (200 oC) leading to the formation of Sb2Se3 on the surface. In order to remove the Sb2Se3 from the surface thin films were annealed at 380 oC for 60 mins. The Raman spectrum for the annealed thin films is shown in the Fig. 3b. The spectrum was deconvlouted using the Lorentizan function to resolve the contribution from the different co existing phases. The observed Raman modes indicated the formation of CuSbSe2 with Cu3SbSe3 phase when compared with the available literature [10, 25, 26]. Raman spectrum of annealed thin films at 380 oC (Fig. 3b) showed the formation of the CuSbSe2 phase along with the Cu3SbSe. The decomposed Raman spectrum shows the peaks corresponding to both CuSbSe2 phase and Cu3SbSe3. In Fig. 3b the red colored peaks in decomposed Raman spectrum indicates the vibration modes belonging to the CuSbSe2 phase whereas those in green corresponds to the Cu3SbSe3. All the observed Raman modes are found to match very well with the reported Raman spectrum for CuSbSe2 [10]. Raman spectra obtained for the as deposited and annealed thin films grown using e-beam current of 50 mA is shown in figs. 4 (a) and (b). The as grown thin film in fig. 4(a) shows the formation of CuSbSe2 compound along with Cu3SbSe3. Unlike the as grown films using Ib ~ 40 mA, Raman modes corresponding to Sb2Se3 was not observed in this case. Raman spectrum for the annealed thin films is shown in Fig. 4(b). The maximum intensity vibrational mode was found to consist of the contribution from the vibration mode of CuSbSe2 and Cu3SbSe3 compound. It is also observed that in the annealed sample the Ag mode for CuSbSe2 compound at 210 cm-1 shows

increase in the intensity. This observation indicates the possibility of achieving films with near stochiometry. The observed Raman modes in the as grown and annealed films are tabulated in Table 1a and 1b.

3.3 Band gap measurement

Optical transmittance spectra for the as deposited and annealed thin films deposited using beam current value of Ib ~ 40 and 50 mA were recorded at room temperature. Absorption coefficient and hence the band gap value were calculated from the optical transmittance data. Band gap values for as grown and annealed films were calculated from the plots of (hυ)2 vs. hυ. Extrapolation of the linear region of the (hυ)2 vs. hυ plot on the x-axis gives the band gap.(hυ)2 vs. hυ for the as deposited and annealed thin films is shown in fig. 4. The band gap value obtained for the as deposited and annealed thin films using beam current value of Ib ~ 40 mA was found to be 1.26 and 1.29 eV respectively. The observed values were higher compared to the expected value for CuSbSe2 confirming the presence of Cu3SbSe3 (Eg~1.6 eV) along with Sb2Se3 (Eg~1.1 eV). Band gap values of Eg~ 1.41 and 1.18 eV was obtained for the as deposited and annealed thin film grown at Ib ~ 50 mA. A higher band gap value of E g~ 1.41 eV for the films deposited using e-beam current of 50 mA agree well with the Raman observation where Cu3SbSe3 was found to co-exist with CuSbSe2. This observation also leads to conclude that for as deposited thin films the amount of Cu3SbSe3 phase present along with CuSbSe2 was more which was reduced after annealing at 380 oC and hence the band gap was found to reduce to 1.18 eV. The band gap value obtained for the annealed thin film is close and in good agreement with the reported band gap value for stoichiometric CuSbSe2 compound [5, 10].

Absorption coefficient values obtained for the as deposited thin films at 50 mA as well as annealed at 380 oC is compared in fig. 5. It can be seen from the figure that very sharp absorption onset appears just after the band gap. This behavior is usually observed for direct band gap semiconductors implying the direct band gap nature of the ebeam grown CuSbSe2 thin films. The absorption coefficient () reaches a maximum value of 6.6 x 106 cm-1 just after absorption onset and remains constant for the lower wavelengths suggesting CuSbSe2 as a superior absorber material. Liping Yu et.al has termed the compounds exhibiting absorption coefficient > 105 cm-1 as super absorbers. The large absorption coefficient can be attributed to the peculiar band structure of these compounds [5].

4

Conclusion:

Chemically similar and structurally dissimilar ternary chalcostibite near stoichiometric CuSbSe2 films were grown by E-beam evaporation from the pre milled source material. Thin films grown using this source material at different e-beam currents and post annealed were studied in detail using the XRD, Raman and optical analysis. The comprehensive analysis of results of these studies suggest that a careful optimization of e-beam deposition current and post annealing is required to obtain near stoichiometric CuSbSe2 films with superior optical absorption coefficient as high as ~ 6 x 106 cm-1. The observed results of this study reveal the promising characteristics of CuSbSe2 as a futuristic solar absorber for thin film solar cell applications.

Acknowledgements: Authors acknowledge financial support provided through DST-SERB (YSS) funding for carrying out this research work (Grant No. YSS/2015/00957).

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Sl. No

1.

2.

Samples

40 mA As Deposited

40 mA Annealed

Raman Shift (cm-1)

Assigned Symmetry

82

Ag

117

Ag

144

B2g

206

Ag

188

Se-Sb-Se bending

251

Sb – Se Stretching

372

Sb – Se Stretching

450

Sb – Se Stretching

117

Ag

129

B2g

140

B3g

147

B2g

153

Ag

158

B2g

192

Ag

200

B2g

208

B2g

167

B1g

174

B2g

180

B2g

186

Ag

Corresponding Phase

CuSbSe2

Sb2Se3

CuSbSe2

Cu3SbSe3

Table 1. (a) Observed Raman Modes for CuSbSe2 depsoited at 40 mA and Annealed at 380 oC.

Sl. No

1.

2.

Samples

50 mA As Deposited

50 mA Annealed

Raman Shift (cm-1)

Assigned Symmetry

116

Ag

126

B2g

152

Ag

194

Ag

209

Ag

136

B2g

168

B1g

180

B2g

186

Ag

117

Ag

127

B2g

149

Ag

194

Ag

210

Ag

135

B2g

163

B1g

179

B2g

183

B2g

189

Ag

Corresponding Phase

CuSbSe2

Cu3SbSe3

CuSbSe2

Cu3SbSe3

Table 1. (b) Observed Raman Modes for CuSbSe2 depsoited at 50 mA and Annealed at 380 oC.

Fig 1. (a) Typical XRD pattern for the as milled CuSbSe2 powder. (b) Typical Raman spectrum for the as milled CuSbSe2 powder.

Fig. 2 Typical XRD pattern for the CuSbSe2 thin film (a) As grown at Ib ~ 50 mA; (b) Deposited at 50 mA and annealed. (c) Inset shows typical the SEM image of the annealed thin film

Fig. 3 Typical Raman spectrum for the CuSbSe2 thin film: (a) As grown at 40 mA; (b) Annealed at 380 oC

Fig. 4 Typical Raman spectrum for the CuSbSe2 thin film: (a) As grown at Ib ~ 50 mA; (b) Annealed at 380 oC

Fig 4.Calculated band gap values for the as grown and annealed CuSbSe2 thin films.

Fig 5. Absorption coefficient for the CuSbSe2 thin film grown at 50 mA and annealed at 380 oC. Inset image shows the fall of the absorption edge.