Close spaced vapor transport deposition of Cu2ZnSnS4 thin films: Effect of iodine pressure

Accepted Manuscript Close spaced vapor transport deposition of Cu2ZnSnS4 thin films: Effect of iodine pressure A. Sagna, K. Djessas, C. Sene, K. Medjnoun, S. Grillo PII:

S0925-8388(16)31658-9

DOI:

10.1016/j.jallcom.2016.05.297

Reference:

JALCOM 37817

To appear in:

Journal of Alloys and Compounds

Received Date: 5 February 2016 Revised Date:

24 May 2016

Accepted Date: 27 May 2016

Please cite this article as: A. Sagna, K. Djessas, C. Sene, K. Medjnoun, S. Grillo, Close spaced vapor transport deposition of Cu2ZnSnS4 thin films: Effect of iodine pressure, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.05.297. 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.

ACCEPTED MANUSCRIPT

Close spaced vapor transport deposition of Cu2ZnSnS4 thin films: effect of iodine pressure A. Sagna a,b,c *, K. Djessasa,b, C. Senec, K. Medjnouna,b, S. Grilloa,b a

b

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Laboratoire Procédés, Matériaux et Energie Solaire (PROMES)-CNRS, Tecnosud, Rambla de la thermodynamique, 66100 Perpignan, France Université de Perpignan Via Domitia, 52 avenue Paul Alduy, 68860 Perpignan Cedex 9, France

c

Laboratoire des Semi-conducteurs et d'Energie Solaire (LaSES), Faculté des Sciences et Techniques, Université Cheikh Anta Diop de Dakar, BP 5005 Dakar-Fann, Sénégal

author: A. Sagna, E-mail: [email protected] Tel: 0033754153009

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* Corresponding

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ABSTRACT

High quality Cu2ZnSnS4 (CZTS) ingots obtained by cooling a molten stoichiometric mixture have been deposited as thin films on soda-lime glass by Close Spaced Vapor Transport (CSVT). Iodine pressure is one of the important parameters for the CSVT process. The effect of iodine pressure on compositional, morphological, structural, electrical and

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optical properties of CZTS thin films has been investigated. X-ray diffraction and Raman spectroscopy results revealed the formation of polycrystalline CZTS with a (112) preferred orientation plane and Raman shift of 338 cm-1 respectively. Iodine pressure does not have a significant effect on the composition nor on the electrical properties as measured by Hall

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effect measurements. However, scanning electron microscopy (SEM) and UV-Vis-NIR spectrophotometer data revealed an increase of crystallite size and band gap energy (1.47 to

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1.56 eV) with iodine pressure. The photoluminescence (PL) measurements at 77 K exhibit emission peaks around 1.30 eV. The origin of this luminescence is attributed to band-toimpurity (BI) recombination. Keywords: Cu2ZnSnS4,

ingot, thin films, CSVT, iodine pressure

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ACCEPTED MANUSCRIPT Introduction In recent years, CZTS attracted a lot of attention as an absorber material for photovoltaic devices. This interest is due on the one hand to its abundant, low cost and nontoxic chemical elements (Cu, Zn, Sn, and S) compared to scarce (In, Te) and toxic (Cd) elements in Cu(In,Ga)Se2 (CIGS) and CdTe, and on the other hand to its favorable physical

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properties such as a direct optical band gap energy of 1.4-1.5 eV and a high absorption coefficient of 104 cm-1 [1] in the visible spectrum, which make it a promising absorber material for the fabrication of high efficiency thin film photovoltaic devices. Research undertaken these five past years on CZTS and photovoltaic devices based on this material, has

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given rise, at the laboratory scale, to significant improvements in device efficiencies, from 6.7 % in 2008 [2] to 12.6 % in 2013 using mixed S/Se [3]. Although these achievements are

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very encouraging, they still remain far below the performances of CIGS (η=21.7 % [4]) and CdTe (η=21.5 % [5]) based photovoltaic devices. Hence, further investigations are necessary to determine factors limiting CZTS based device performances. Many studies reported that the presence of secondary phases such as ZnS, SnS, Cu2SnS3, CuxS, etc. is one of the main factors that affect experimental conversion efficiency [6–9]. Various deposition techniques have been used for the synthesis of Cu2ZnSnS4 thin films including thermal evaporation [10],

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sputtering [11], spray pyrolysis [12], SILAR [13], electrodeposition [14]. In all of these techniques, control of the presence of secondary phases is a major issue and usually a sulfurization step is needed to get closer to the stoichiometry. Developing a low cost method that will lead to high quality CZTS thin films free of secondary phases and then high efficient

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devices is therefore a major objective for researchers dealing with this material. CSVT is one of the potential techniques to achieve this challenge. In the CSVT deposition process, the film

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properties depend on several parameters of which substrate temperature and iodine pressure are considered the most critical. In a previous work [15], we reported on the influence of substrate temperature on the properties of CZTS films deposited under iodine pressure at 2 kPa by CSVT. To our knowledge, this was the first time CZTS layers were deposited by this technique. In the present paper, we investigate the synthesis and properties of CZTS ingot and thin films deposited at a substrate temperature of 480 °C with particular focus on the effect of iodine pressure.

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ACCEPTED MANUSCRIPT 2. Experimental details A single phase polycrystalline CZTS ingot was grown by cooling a molten stoichiometric mixture of Cu (99.9999%), Zn (99.9999%), Sn (99.9999%) and S (99.9995%) elements (Alfa Aesar). Chemicals were first weighed in stoichiometric proportions then charged into a quartz tube. After charging, the tube was sealed under 2x10-4 Pa and inserted

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into the vertical furnace. The furnace was heated up to 1080 °C at a rate of 0.5 °C/min from room temperature to 600 °C and 2.4 °C/min from 600 °C to 1080 °C and then kept at this temperature for 8 hours to ensure homogeneity. After this ingot growth, the tube was cooled to room temperature at a rate of 5 °C/min. The CZTS alloy ingot was then reduced to a fine

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powder and used as a material source in the CSVT system designed by Djessas et al. [16] for CZTS thin films growth. The milling conditions have been optimized to develop powder grain

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sizes of a few tens of micrometers. After milling, CZTS powder was compressed to form pellets using a hydraulic press system with a pressure higher than 3x104 kPa. The prepared CZTS pellets were placed in the reaction zone of a graphite holder and then loaded into the quartz reactor of CSVT. The pellets of CZTS were evaporated under a controlled iodine (the transport agent) atmosphere. The reaction zone was heated by a SiC spring resistance. The temperatures were controlled using thermocouples mounted near the substrate and the source.

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Source and substrate were separated by a 1 mm thick space with a temperature difference of the order of 50 °C. In this work, CZTS thin films were grown onto soda lime glass (SLG) substrates at a temperature of 480 °C under iodine pressures of 4 kPa and 25 kPa. The

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deposition time was fixed at 10 min.

Scanning electron microscopy was carried out using a JEOL-JMS 5310 LV scanning electron microscope operating at 15 kV and equipped with an energy dispersive spectroscopy (EDS)

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analytical system. The structural properties of ingot and films were studied by X-Ray Diffraction (XRD) using an Empyrean PANalytical diffractometer with Bragg-Brentano focusing geometry and a CuKα line (λ = 0.154056 nm) at 45 kV and 30 mA as well as by Raman Spectroscopy (HORIBA JOBIN YVON LabRam ARAMIS IR2, blue laser diode) with an excitation wavelength of 473 nm. An Ecopia HMS 3000 Hall measurement system with a magnetic field of 0.56 T was used to study the electrical transport properties. The optical properties of the thin films were investigated by UV-Vis-NIR spectrophotometry and by photoluminescence (PL). Transmission spectra in the 300-2000 nm range were measured using a VARIAN Cary 5000 spectrophotometer. For the PL measurements, samples were

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ACCEPTED MANUSCRIPT excited using a 15 mW laser diode emitting at 650 nm, and the signal was detected with a liquid nitrogen cooled InGaAs photomultiplier at a temperature of 77 K. 3. Results and discussion 3.1 Ingot

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Ingot characterization has been performed on powders obtained by milling ingot grown as described above. Table 1 shows results of the chemical composition analysis determined at three points arbitrarily selected in the powder. Composition values shown in

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this table indicate that the material source is homogeneous and almost stoichiometric.

A typical XRD pattern of the CZTS powder is shown in Fig. 1. The CZTS powder has multiple reflection peaks observed at 2θ values of 24.42°, 28.40°, 32.90°, 47.23°, 56.11°,

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58.83° and 69.02° and corresponding respectively to the (110), (112), (200), (220), (312), (224) and (008) planes of the kesterite structure of CZTS semiconductor (JCPDS card no. 026-0575). However due to overlapping of CZTS diffraction peaks with those of secondary phases such as Cu2SnS3 and ZnS [17], X-ray diffraction characterization is insufficient to precisely determine whether both CZTS ingot and films are pure or not. We therefore carried out Raman spectroscopy experiments to study the phase purity. The Raman spectrum

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exhibited in Fig. 2 confirms that the material crystallizes in the kesterite type structure since intense peaks located at 287 cm-1, 338 cm-1 and 368 cm-1 are characteristic of the kesterite CZTS [18,19]. These results indicate that the ingot is of good quality, even though they

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cannot confirm the complete absence of the ZnS phase, whose detection by Raman spectroscopy is only possible by using a UV resonant excitation (λ = 325 nm) [20]. However, ZnS phases generally exist in Cu-poor and Zn-rich compositions [21,22], and this was not the

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case for our ingot. 3.2 Thin films.

Based on our previous work [15], substrate temperatures of 460 and 500 °C have been

found to be suitable to obtain nearly stoichiometric CZTS thin films free of secondary phases. In this work substrate temperature was maintained constant at 480 °C while iodine pressure was varied in order to investigate this parameter’s effect. Table 2 shows the films atomic composition as well as the following ratios: Cu/(Zn+Sn), Cu/Zn and Zn/Sn. These results indicate that the films composition depends slightly on the iodine pressure, in agreement with results already reported by Colombara et al [23]. Nevertheless, one can note that nearly 4

ACCEPTED MANUSCRIPT stoichiometric CZTS thin films with a slightly Cu-poor and Zn-rich composition are obtained at low as well as at high pressure. Such metal ratios are known to be beneficial for photovoltaic device performance. However, the S concentration is below stoichiometry, implying that the films are probably rich of chalcogen vacancies, which is known to be detrimental for device performance. Furthermore, the little quantity of iodine observed at high

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iodine pressure, can affect the thin films properties. Fig. 3 shows XRD patterns of CZTS thin films deposited under low and high iodine pressure. As it can be seen, both films appear polycrystalline with a (112) preferred orientation plane. The diffraction peaks can be assigned to the CZTS kesterite structure, except for the reflection

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peak at 2θ = 25.5° observed at high pressure, corresponding to a CuI phase [24]. Its presence can be due to the high iodine pressure used during film growth. These results are to be

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correlated with the EDS data showing the presence of iodine in the thin film. The XRD patterns also show that CZTS deposited at higher pressure have a pronounced shift of the lattice parameters to smaller sizes. Our calculations reveal that the lattice parameters of the films deposited under low iodine pressure are a = b = 5.427 Å, and c = 10.880 Å, with a volume of unit cell of 320.441 Å3. The lattice parameters of the films deposited under high iodine pressure are instead a = b = 5.411 Å, and c = 10.809 Å, with a volume of unit cell of

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316.476 Å3. These values are close to the CZTS powder data from JCPDS card no. 026-0575 (a = b = 5.427 Å, c = 10.848 Å v = 319.50 Å3). It can be clearly seen that the volume of unit cell of the CZTS film deposited under low iodine pressure is larger than that of the CZTS film deposited under high iodine pressure. This phenomenon has been observed by Colombara et

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al (the unit cell is compressed along the [001] crystallographic direction) [23]. This behavior probably implies that a compressive internal stress exists in the CZTS film deposited under

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high iodine pressure.

Fig. 4 presents room temperature Raman scattering results of CZTS layers deposited under different iodine pressures. The existence of kesterite CZTS at both low and high iodine pressures is confirmed by the Raman shift at 338 cm-1 attributed to the CZTS A1 mode and the shoulder at 288 cm-1, which is consistent with results reported in the literature [18,19]. These results are in good agreement with those obtained for CZTS ingot characterization. As for the ingot, the presence of the ZnS phase could only be totally ruled out by Raman spectroscopy using a UV resonant excitation. At high iodine pressure, a little diffraction peak of CuI was detected in the XRD pattern of the thin film. However, no CuI was observed by Raman analysis. This may be due to the fact that the Raman shift of the CuI phase is localized 5

ACCEPTED MANUSCRIPT between 100 and 150 cm-1 [25], while Raman spectra of the CZTS samples were recorded in the range of 200-500 cm-1. Observations of the surface morphology (Fig. 5.a1,a2 and Fig. 5.b1,b2) and cross section (Fig. 5.a3 and Fig. 5.b3) of the thin films deposited at two different iodine pressures have been carried out. The low magnification images indicate homogenous deposits (Fig. 5.a2 and Fig.

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5.b2) with an average thickness of 2 µm (Fig. 5.a3 and fig. 5.b3). Furthermore, the effect of the iodine pressure can be observed on the CZTS grain size. At high iodine pressure the CZTS films have a columnar structure with larger grain sizes (Fig. 5.b3).

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The electrical properties were determined by Hall effect measurements, using the Van der Pauw method at room temperature. Both thin films exhibit p-type conductivity. The electrical properties depend slightly of the iodine pressure. The carrier concentration, Hall mobility and

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resistivity were approximately 1017 cm-3, 7 cm2V-1 s-1 and 11 Ω.cm, respectively. These values are consistent with data in the literature for CZTS [26,27].

The optical transmittance of the CZTS thin films was measured by UV-Vis-NIR spectrophotometry to evaluate their optical band gaps. Fig. 6 shows the optical transmission spectra obtained for the CZTS thin layers. The transmittance of both films exhibits a falls at

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the band gap edge and gets close to zero in the visible region. This phenomenon reveals promising absorption properties of the CZTS layers in this region. One can furthermore note from the relative difference in the two samples, that the CZTS film grown at low iodine pressure exhibits an onset of absorption at lower photon wavelengths than does the film

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grown at high iodine pressure.

Neglecting the reflectivity [28,29], the optical absorption coefficient (α) can be estimated

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from the measured transmission (T) using the following formula: ଵ

ଵ

ߙ = ௗ ݈݊ ቀ்ቁ

where d is the thickness and T the transmittance of the film. Calculated absorption coefficients are of the order of 104 cm-1 for both samples. As CZTS is a direct band gap semiconductor, the optical band gap (Eg) can be evaluated using the following equation [30]: ଵ/ଶ

ߙሺℎߥሻ = ‫ܣ‬൫ℎߥ − ‫ܧ‬௚ ൯

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ACCEPTED MANUSCRIPT where hν is the photon energy (eV) and A a constant related to the effective masses of electrons and holes in the bands of the material. The optical band gap energy of the thin films are obtained by extrapolating the linear part of the plot of (αhν)2 versus photon energy (hν) (presented in Fig. 7) to the (hν) axis. Estimated values of Eg are 1.47 eV and 1.56 eV for the thin films deposited at low and high iodine pressure respectively. These values are in good

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agreement with results reported for CZTS thin films obtained by other deposition methods [28,31]. The optical band gap of the thin films clearly increases with iodine pressure. As mentioned above, our XRD results show that the lattice parameters are significantly lower for the film deposited under higher iodine pressure. This shrink may be at the origin of the band

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gap increase [32–34].

Optical absorption is significantly affected by defects in CZTS thins films and this can in turn

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lead to lower efficiencies than expected. Such defects have been shown to exist in near stoichiometric films. In order to improve understanding of the defect structures in CZTS thin films we did photoluminescence measurements at low temperature on the samples grown at different iodine pressures. Fig. 8 shows spectra obtained at 77 K. A PL peak is observed for both CZTS films at around 1.3 eV. This broad PL peak was observed by several authors in the literature [35–37]. The electrostatic fluctuating potential model is currently used to explain the

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photoluminescence emission in CZTS. In our case, we have chosen this model due the clear asymmetry of our PL peaks. At high iodine pressure, we observed that the PL peak intensity of the sample is lower. This phenomenon can be explained by the presence of surface defects in the thin films, caused by the CuI phase, observed in the XRD spectrum. As reported by

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Levanyuk et al. [38] and Tanaka et al [11], the luminescence spectrum of a semiconductor with fluctuation potentials is usually dominated by three recombination channels: band-to-

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impurity (BI), band-to-tail (BT) and band-to-band (BB). The PL peak observed at about 1.3 eV is smaller than the optical band gaps determined above, therefore eliminating the possibility of BB transition. According to Tanaka et al. [11] and Grossberg et al. [39], the energetic difference between BB and BT is small, approximately 130 meV. Thus, the PL peak at around 1.3 eV is likely to be due to BI because the energy difference between the optical band gap and the PL peak is significantly larger than 130 meV (it is approximately 170 meV for the sample deposited at low iodine pressure and 260 meV for the one deposited at high iodine pressure). Also, according to Tanaka et al. [11], for the samples with Cu/Sn ≤ 2, as is the case of our samples, the BI transitions are dominant. Since there is no significant peak

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ACCEPTED MANUSCRIPT shift from one sample to the other, it appears that the luminescence originates from the same type of radiation recombination in the two samples. Conclusion Using a simple and low cost process, i.e. cooling of a molten stoichiometric mixture

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and CSVT, we successfully obtained CZTS thin films free of the secondary phases Cu2SnS3, Cu2S, SnS which lead to degradations in the efficiencies of CZTS-based photovoltaic devices. However, for the film synthesized at high iodine pressure, CuI which is a residual phase of the transportation process, was detected. XRD, Raman and PL measurements confirmed the

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formation of kesterite CZTS. The effect of iodine pressure on composition, morphology as well as on structural and optoelectronic properties of the kesterite CZTS thin films has been investigated. At low or high iodine pressure, EDS data indicated a slightly Cu-poor and Zn-

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rich composition for both thin films. No significant effect of the iodine pressure was observed on the chemical composition of the thin films nor on their electrical properties. However, SEM and UV-Vis analysis respectively revealed an increase in grain size and optical band gap when the iodine pressure increases. The lattice parameters, obtained from XRD patterns, of CZTS deposited under high iodine pressure are smaller than those of CZTS deposited under low iodine pressure. This lattice shrinkage may be at the origin of the band gap enlargement.

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The photoluminescence study of CZTS thin films revealed that the nature of the defects responsible for the luminescence do not change with iodine pressure. Increasing iodine pressure can be beneficial when growing CZTS thin films by CVST. However, very high

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iodine pressure can lead to a high proportion of CuI phases in the CZTS thin films, which should in turn affect significantly the thin film properties.

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Acknowledgements

This work was financially supported by the French Embassy in Senegal (Department for Cooperation and Cultural Action, Grant No 657/SCAC). The authors also wish to thank Mr. J.L. Gauffier, Mrs. S. Reyjal and Mr. S. Cayez from INSA Toulouse (Laboratory of Physics and Chemistry of Nano-objects) as well as E. Hernandez from PROMES-CNRS Laboratory for their technical assistance.

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Table captions Table 1: Atomic composition of CZTS ingot

Figure captions

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Fig. 1. X-ray diffraction patterns of CZTS powder

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Table 2: Atomic composition of CZTS thin films grown at 480°C and under different iodine

Fig. 2. Raman spectrum of CZTS powder

Fig. 3. X-ray diffraction patterns of thin CZTS films grown at 480 °C under different iodine pressures

Fig. 4. Raman spectra of thin CZTS films grown at 480 °C under different iodine pressures

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Fig. 5. Surface and cross section SEM images of CZTS thin films deposited under two different iodine pressures: low iodine pressure (a1, a2, a3) and high pressure (b1, b2, b3).

pressures Fig. 7. (αhν)

vs photon energy for CZTS thin films deposited under two different iodine

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Fig. 6. Transmission spectra of CZTS thin films deposited under two different iodine

Fig. 8. PL spectrum of CZTS samples measured at 77 K

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Chemical composition (at.%) Cu

Zn

Sn

Point 1

26.15

13.72

13.26

Point 2

27.03

13.35

11.82

Point 3

26.00

13.21

14.68

Average

26.39

13.42

S 46.87

47.80

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CZTS

46.92

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Table 2: Atomic composition of CZTS thin films grown at 480 °C and under different iodine

Iodine pressure (kPa)

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Chemical composition (at.%)

47.27

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Cu

Zn

Sn

24.97

15.05

12.71

22.56

16.01

10.81

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46.27

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4.35

Cu/(Zn+Sn)

Cu/Sn

Zn/Sn

0.89

1.96

1.18

0.84

2.09

1.48

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ACCEPTED MANUSCRIPT Highlights 1. High quality Cu2ZnSnS4 ingot was obtained by cooling a molten stoichiometric mixture. 2. CZTS thin films were prepared by Close spaced vapor transport deposition technique. 3. Effect of iodine pressure on the properties CZTS films has been investigated.

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4. XRD and Raman spectra revealed the kesterite phase with a good crystalline quality.

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5. Crystallite size and band gap energy of CZTS films increase with the iodine pressure.