Preparation and characterization of CuInSe2 nanoparticles elaborated by novel solvothermal protocol using DMF as a solvent

Accepted Manuscript Preparation and characterization of CuInSe2 nanoparticles elaborated by novel solvothermal protocol using DMF as a solvent A. Ben Marai, K. Djessas, Z. Ben Ayadi, S. Alaya PII:

S0925-8388(15)30499-0

DOI:

10.1016/j.jallcom.2015.07.102

Reference:

JALCOM 34793

To appear in:

Journal of Alloys and Compounds

Received Date: 15 April 2015 Revised Date:

1 July 2015

Accepted Date: 13 July 2015

Please cite this article as: A. Ben Marai, K. Djessas, Z. Ben Ayadi, S. Alaya, Preparation and characterization of CuInSe2 nanoparticles elaborated by novel solvothermal protocol using DMF as a solvent, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.07.102. 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.

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ACCEPTED MANUSCRIPT Preparation and characterization of CuInSe2 nanoparticles elaborated by novel solvothermal protocol using DMF as a solvent. A. Ben Maraia,c,*, K. Djessasa,b, Z. Ben AyadicandS. Alayac Laboratoire Procédés, Matériaux et Energie Solaire, PROMES-CNRS,

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a

Tecnosud, Rambla de la thermodynamique, 66100 Perpignan, France. b

Université de Perpignan Via Domitia, 52 avenue Paul Alduy, 68860, Perpignan

Laboratoire de Physique des Matériaux et des Nanomatériaux appliquée à

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Cedex9, France.

l'Environnement, LaPhyMNE, Université de Gabès, Faculté des Sciences de Gabès, Cité Erriadh Manara Zrig, 6072 Gabès, Tunisia.

* Corresponding author: Achraf BEN MARAI, Tel: +216 97 05 55 15.

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E-mail: [email protected]

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ACCEPTED MANUSCRIPT Abstract In this study, high purity and near stoichiometric CuInSe2 (CIS) nanoparticles have been successfully synthesized using solvothermal route. The goal of this paper is to improve

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the crystal quality while reducing production cost and limiting the toxicity of solvothermal reaction, as compared to processes including selenization step. Therefore, the starting solution solvothermal is constituted by the following precursors

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(CuSO4.5H2O, InCl3.xH2O and Se powder) which were dissolved in N, N Dimethylformamide (DMF) as solvent. A reasonable possible mechanism for the

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growth of CIS nanoparticles is proposed. The effect of process parameters on the synthesis and characterization of CIS nanoparticles were examined including reaction temperature (165-240 °C), process time (12-24 hours) and the drying route. The asobtained CIS nanoparticles are analyzed using diverse techniques such as x-ray (XRD),

energy

dispersive

spectrometer

(EDS),

transmission

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diffraction

electron microscopy (TEM), Raman spectroscopy and UV-vis-IR spectrophotometer. All results demonstrate that the optimal conditions for preparing a single-phase CIS

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obtained at 220 °C for 24 hours and followed by annealing at 400 °C for 30 minutes under a nitrogen atmosphere. In addition, XRD results showed that the CIS

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nanoparticles crystallize in the chalcopyrite structure, with grain size in the order of 25 nm, which is also confirmed by TEM images. Raman spectra show the intense peak at 171 cm-1, which correspond to the chalcopyrite structure, no signature of secondary phases. Optical measurements revealed strong absorption in the entire visible light to near-infrared region and band gap (≈1.04 eV) is very close to those of absorbent materials in thin film solar cells. Keywords: CuInSe2, Nanoparticles, Solvothermal, Chalcopyrite, DMF. 2

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1. Introduction The ternary CuInSe2 (CIS) compound, which belongs to the I-III-VI2 family, has

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emerged as a promising candidate for high efficiency and radiation-hard solar cell applications. It has many remarkable advantages such as its excellent optical and electrical properties, with an absorption coefficient as high as105 cm-1 [1], and direct

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band gap with an energy of 1.04 eV [2], well matching the solar spectrum. Several

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methods have been reported for the preparation of CIS nanoparticles, such as thermal decomposition [3], Ball milling [4], polyol reflux method [5] and one-pot method [6]. However, most of the above-mentioned techniques are difficult to scale up because the precursors used are toxic and expensive and the processing process is complicated. Furthermore, it is difficult to control the crystallinity or purity of the obtained

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nanoparticles and they are not well adapted to mass production. CIS nanoparticles were also synthesized by solvothermal route [7,9], which features easy handling and high reproducibility. Many reported solvothermal preparations

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involve the reaction of constituent elements (Cu, In, Ga and Se) or their salts in heated

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solution for several hours even several days [10,12]. Ethylenediamine (en) (C2H8N2) has often been employed as a solvent in the preparation of CIGS, and the synthesized CIGS powders were formed with a mixture of different phases [13,15]. However, we could mention that the low boiling point of ethylenediamine (116°C) is a drawback in potential solvothermal preparations of CIGS. Also, Olejnicek et al [16] have synthesized a single phase CIGS using triethylenetetramine as a solvent, but this process might be dangerous because the reaction temperature needs to be increased to 267 °C (boiling point of triethylenetetramine). The use of another organic solvent with a boiling 3

ACCEPTED MANUSCRIPT point higher than that of the ethylenediamine and lower than that of the triethylenetetramine may show greater promise. In this work, we optimized the solvothermal pathway to prepare CuInSe2

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nanoparticles using for the first time the combination CuSO4.5H2O, InCl3.xH2O and Se as reagents and N, N Dimethylformamide (DMF, boiling point =156 °C) as a solvent. The complexing properties of the solvent can lead to the intermediate formation of

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stable complex systems [Cu(DMF)n]+ (n= 1 or 2) [17]. We investigated the effect of reaction time and reaction temperature on phase formation. Furthermore, with this

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protocol, an annealing process at different temperature and atmosphere might equilibrate the different compounds toward CIS. 2. Experimental

First, the synthesis of CuInSe2 nanoparticles proceeded through a simple

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solvothermal route. In a typical process, stoichiometric amounts of CuSO4.5H2O (15 mmol, 99.995% purity), InCl3.xH2O (15 mmol, 99.99% purity) and Selenium powder (30 mmol, 99.999% purity) were dissolved in 500 ml of N,N Dimethylformamide

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(DMF) (99.5% purity) to form a homogeneous solution under magnetic stirring. The

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resulting solution was then transferred into 1L Teflon lined stainless steel autoclave and maintained at a reaction temperature in the range of 165-240 °C for different reaction times between 12 and 24 hours, then cooled to room temperature naturally. Nanoparticles existing in DMF solution with black color were collected by filtration in a Buchner linked to a primary pump, and washed several times with deionized water and absolute ethanol. Finally, different ways of drying were performed on the obtained powder under an inert atmosphere at different temperatures and for definite time.

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ACCEPTED MANUSCRIPT The structural properties of CIS nanoparticles were analyzed by X-ray diffraction (XRD) using a Philips X’Pert PRO diffractometer with a Cu-Kα radiation (λ=1.5405 Å, 45 kV, 30 mA). Typically, θ-2θ diffractograms were collected between

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2θ=10° and 70° in 0.02° steps. For Raman spectroscopy measurements, excitation was done at 632.8 nm emission line of a He-Ne laser, the power excitation was fixed at 1.1mW using neutral density filters. Nanoparticles morphology and sizes were

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investigated by a JEM-200CX type Transmission Electron Microscope (TEM). Elemental composition of CIS nanoparticles was analyzed by Energy Dispersive X-ray

Optical

properties

were

also

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Spectroscopy (EDS), using an EDS attachment to transmission electron microscope. investigated

using

a

Shimadzu–UV3101PC

Spectrophotometer in the wavelength range from 200 to 800 nm. 3. Results and discussion

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N, N dimethylformamide is a polar (hydrophilic) aprotic solvent with a high boiling point (156 °C) [18]. In the solvothermal process, a nucleophilic SN2 attack by DMF can activate elemental selenium to form Se2- ions [19]. It is well known that our precursors

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are soluble in DMF at room temperature, supported by the change in the solution color. Selenium ions Se2- are reacted with the precursors of indium salt to form unstable In2Se3

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molecules. With the increasing of synthesis temperature, these molecules make call to other selenium ions for became ionized and formed (InSe2)-. With the excess of DMF molecules in the starting solution, in this electron transfer reaction, DMF can reduce Cu2+ to Cu+ ion [20]. The gas-phase studies showed that DMF is a highly preferred ligand for Cu+ and promotes the formation of stable complex [Cu(DMF)n]+ [17]. The mechanism of preparation of CuInSe2 by solvothermal reaction is detailed in the following equations:

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ACCEPTED MANUSCRIPT 2 InCl3 + 3Se 2− ⇒ In2 Se3 + 6Cl − In2 Se3 + Se 2− ⇒ 2( InSe2 ) −

Cu + + nDMF ⇔ [Cu ( DMF ) n ]

+

[Cu ( DMF ) n ]+ + ( InSe2 ) − ⇒ CuInSe2 + nDMF

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To understand the effect of reaction temperature, time process and the ways of annealing on CIS nanoparticles, EDX and XRD analyses are carried out on the samples prepared at different reaction parameters, in order to determine the optimal synthesis

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conditions.

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Table 1 shows the Chemical composition of the CuInSe2 nanoparticles prepared by solvothermal route. For the samples (1- 4), prepared at temperatures ranging from 165 to 240 °C for a fixed reaction time of 24 hours, we found that the atomic ratio of copper decreases from 39 to 28% as the reaction temperature is increased. In addition, the Cu composition becomes constant from T= 220 °C. This could be explained by the

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effect of the increase of vapor pressure until 30 bars at 220 °C ( 5 bars at 165 °C) in the autoclave, in such conditions the organic solvent DMF activates the formation of [Cu(DMF)n]+ complexes, which will react with (InSe2)- to form CIS nanoparticles.

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From EDS data summarized in table 1, it is observed that for a reaction temperature of

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220 °C and a reaction time greater than 20 hours (samples 3, 5 and 6) the atomic ratio of copper, indium and selenium was very near to stoichiometric CIS (25: 25: 50). Fig. 1 shows XRD patterns of CuInSe2 nanoparticles prepared at various

synthesis conditions. For all the synthesized powders, the main XRD peaks (112), (204/220) and (116/312) can be well assigned to CuInSe2 chalcopyrite phase according to JCPDS card no. 40-1487. Fig. 1 represented the effects of process temperature and time on crystallization of CIS nanoparticles obtained through solvothermal method. It

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ACCEPTED MANUSCRIPT could be seen that when the synthesis process is carried out by increasing either temperature reaction (from 165 °C to 240 °C, at fixed reaction time of 24 H) or process time (from 12H to 24H at fixed reaction temperature of 220 °C) residues are eliminated,

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leading to CIS single phase. The two weak XRD peaks of residues can be observed in samples n° 1 and 2 were attributed to the Se related structures of (102) and (201) crystal planes (JCPDS card no. 42-1425). For these samples, the process temperature (165 and

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190 °C) is lower than the melting point of selenium (220 °C), which causes the appearance of impurities such as crystalline selenium [21]. In addition, samples 1, 2 and

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6, show many XRD peaks corresponding to Cu2Se secondary phase (JCPDS card no. 02-1275).

If we consider an error of 3 % in the EDS analysis, the atomic ratio of selenium does not vary by the change of time and temperature reaction. To get a near stoichiometric

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CIS nanoparticles, solvothermal process was followed by a heat treatment under inert atmosphere. Accordingly, sample 3 was prepared at 220 °C for 24 hours and treated at high temperature for 30 minutes. Consequently, It can be observed that the diffraction

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peaks from Se and Cu2Se are practically eliminated and converted to pure chalcopyrite and all peaks observed indicates the perfect chalcopyrite structure (fig. 2). Hence, the

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best stoichiometry (26.44: 25.04: 48.52 atomic percent of Cu, In and Se respectively) is obtained with the CIS nanoparticles dried at 400°C for 30 min under nitrogen atmosphere. In these conditions of elaboration, the lattice constant were calculated to be a= 5.752 Å and c= 11.536 Å with u=c/a ratio almost equal to the reported values for the chalcopyrite α-phase CIS (u=2) [22]. According to the Scherrer’s formula, the average particle size G of the CIS was approximately between 20 and 30 nm.

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ACCEPTED MANUSCRIPT Fig. 3 shows a typical EDS spectrum for sample synthetized at 220 °C for 24 H and followed by heat treatment at 400 °C for 30 minutes. It confirms the existence of only the three elemental components in our CIS nanoparticles. The TEM images of

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CuInSe2 (annealed at 400 °C) are shown in Fig. 4. It was observed that the individual particles were nearly spherical in the size range 20–30 nm. However, slight agglomeration was also noticed. The estimated values, obtained using TEM photograph,

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are in close agreement with those obtained from the XRD data.

Fig. 5 depicts the Raman spectrum of CIS nanoparticles It exhibits a single and

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intense scattering peak at 171 cm−1 corresponding to the A1 optical phonon mode, which represents the vibration of the Se anions in the x-y plane with the cations at rest [22]. This result is characteristic of the chalcopyrite crystal structure [23]. Moreover, the two weak and broad peaks observed around 220 cm−1 should be regarded as a combination

together [24].

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of the B2 and E modes of the CIS phase; they represent vibrations of anions and cations

The optical absorption properties of the CIS nanoparticles were investigated by

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the optical absorption spectroscopy. Fig. 6 shows the absorbance spectrum of sample 3.

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A broad and strong absorption peak is observed at 700 nm. The band-gap energy is related to the incident photon energy hν by: (αhν)n = B(hν-Eg)

where α is the absorption coefficient, B is a specific constant of the material, Eg is band gap, and n is either 1/2 for an indirect transition or 2 for a direct transition.

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ACCEPTED MANUSCRIPT Eg can be deduced from the plot of (αhν)2 versus photon energy hν from the intercept of the extrapolated linear portion of the curve with the energy axis (the onset of fig. 6). The Eg value of CIS is determined to be 1.062 eV, a slightly higher value

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than that of CIS single crystals (Eg = 1.04 eV) [8]. 4. Conclusion

In the present study, Chalcopyrite CIS nanoparticles for solar cell materials are

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successfully synthesized by using a relatively simple and convenient solvothermal

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route. The method proposed is suitable for mass production. The DMF solvent acts as both solvent and complexing agent. The effects of process parameters including process time, reaction temperature and heat treatment on the properties of the CuInSe2 nanoparticles were investigated. According to the X-ray diffraction and Raman results, it was concluded that for synthesis of CIS nanoparticles, the appropriate reaction

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temperature was higher to 220°C (melting point of Se), synthesis time does not exceed one day (24 hours) and followed by heat treatment at high temperature (> 400°C) under an inert atmosphere. The synthesized particles show high crystallinity, low impurity

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phases, and size of about 30 nm.

Acknowledgment

This work was financially supported by the European Commission under the Averroes 4 Program. (Erasmus Mundus, Action 2, Europe-Maghreb).

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ACCEPTED MANUSCRIPT [23] C. Rincón and F. J. Ramírez. Lattice vibrations of CuInSe2 and CuGaSe2 by Raman microspectrometry. J. Appl. Phys. 72 (1992) 4321. [24] I.-H. Choi. Raman spectroscopy of CuIn1−xGaxSe2 for in-situ monitoring of the

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composition ratio. Thin Solid Films.519 (2011) 4390–4393. [25] S. Roy, P. Guha, S. N. Kundu, H. Hanzawa, S. Chaudhuri, and A. K. Pal.

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Characterization of Cu(In,Ga)Se2 films by Raman scattering. 73 (2002) 24–30.

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ACCEPTED MANUSCRIPT Table captions Table 1. Chemical composition of CIS nanoparticles prepared at various synthesis

Figure captions

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conditions.

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Figure 1. XRD patterns of CuInSe2 nanoparticles obtained at various reaction

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temperature and time.

Figure 2. XRD patterns of CuInSe2 prepared by solvothermal route at 220°C for 24H (a) wet powder (b) annealed under nitrogen atmosphere at 400°C/30min and (c) 550°C/30min.

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Figure 3. TEM images of CuInSe2 nanoparticles prepared by solvothermal route at 220°C for 24H and annealed under nitrogen atmosphere at 400°C/30min. Figure 4.EDS spectrum of CIS nanoparticles prepared by solvothermal route at 220°C

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for 24H and dried at 400°C /30 min under nitrogen atmosphere.

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Figure 5. Raman spectra of CuInSe2 nanoparticles prepared by solvothermal route at 220°C for 24H and dried at 400°C/30min under nitrogen atmosphere. Figure 6. UV/Vis absorbance spectra of the CuInSe2 nanoparticles prepared at 220°C for 24H and annealed at 400°C /30 min under nitrogen atmosphere. The inset shows the plot of (αhν)2 =f(hν) curve.

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ACCEPTED MANUSCRIPT Table 1. Chemical composition of CIS nanoparticles prepared at various synthesis conditions. Reaction time (hours)

Sample N°

Reaction temperature (°C)

1

165

35.98 : 21.36 : 42.66

2

190

32.05 : 27.60 : 40.35

3

220

4

240

5

220

20

30.69 : 26.82 : 42.48

6

220

12

38.85 : 18.07 : 43.08

28.37 : 24.27 : 47.36

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Atomic ratio (%) (Cu:In:Se)

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28.78 : 24.76 : 46.76

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Highlights

Highly dispersed chalcopyrite CuInSe2 nanoparticles were successfully synthesized.

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The DMF solvent act as both the solvent and the complexing agent.

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Band gap was calculated to be 1.04 eV according to the UV–vis absorption spectrum.

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•