Influence of composition and annealing on the characteristics of Cu2SnS3 thin films grown by cosputtering at room temperature

    Influence of composition and annealing on the characteristics of Cu 2 SnS3 thin films grown by cosputtering at room temperature Romain Bodeux, Julien Leguay, S´ebastien Delbos PII: DOI: Reference:

S0040-6090(14)00901-8 doi: 10.1016/j.tsf.2014.09.023 TSF 33712

To appear in:

Thin Solid Films

Please cite this article as: Romain Bodeux, Julien Leguay, S´ebastien Delbos, Influence of composition and annealing on the characteristics of Cu2 SnS3 thin films grown by cosputtering at room temperature, Thin Solid Films (2014), doi: 10.1016/j.tsf.2014.09.023

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ACCEPTED MANUSCRIPT Cover Page Dr. Romain Bodeux1,2,3 a)

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received his PhD from GREMAN in 2009. After two postdoctoral positions at ICMCB

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in Pessac and at IRDEP in Chatou, he joined EDF R&D in 2013.

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Mr. Julien Leguay1,2,3

Is a master student at the university of Paris Sud & ENS Cachan. He is in traineeship

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at IRDEP in Chatou in 2014.

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Dr. Sébastien Delbos1,2,3

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received his PhD from IRDEP in 2008 and joined EDF R&D.

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1- EDF R&D, Institute of Research and Development on Photovoltaic Energy (IRDEP), 6 quai Watier, 78401 Chatou, France

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2- CNRS (IRDEP), UMR 7174, 78401 Chatou, France

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3- Chimie ParisTech (IRDEP), 75005 Paris, France

ACCEPTED MANUSCRIPT Influence of composition and annealing on the characteristics of Cu2SnS3 thin films grown by cosputtering at room temperature

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Romain Bodeux1,2,3 a) Julien Leguay1,2,3 and Sébastien Delbos1,2,3

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1- EDF R&D, Institute of Research and Development on Photovoltaic Energy

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(IRDEP), 6 quai Watier, 78401 Chatou, France 2- CNRS (IRDEP), UMR 7174, 78401 Chatou, France 3- Chimie ParisTech (IRDEP), 75005 Paris, France

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a) Author to whom correspondence should be addressed. Electronic mail:

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[email protected]

Abstract

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This work aims to study the tolerance of Cu2SnS3 to both Cu-poor and Cu-rich

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stoichiometries. For this purpose, films were synthesized by RF magnetron sputtering of Cu2S and SnS2 targets. Films were crystallized and retained the tetragonal structure at room

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temperature, revealing the high stability of this material. A treatment at higher temperature leads to the formation of the metallic Cu3SnS4 phase. The results of optical and electrical measurements indicated that all the samples are p-type semiconductors with an energy band gap of 1.28 eV. A dependence of the electrical properties of the film with the Cu content and

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the annealing conditions was observed. 1 Introduction

Over the last years, Cu2SnS3 (CTS) thin films have attracted interest due to their potential photovoltaic applications: it is a p-type semiconductor with a high absorption coefficient (> 104 cm⁻³) [1,2]. Few studies reporting solar cells based on this material were published [3-6] with a record efficiency at 2.9 % [7] and 6 % for a solid solution Cu2Sn1-xGexS3 [8]. As for Cu(In,Ga)Se2 and Cu2ZnSnS4 (CZTS), the control of the growth and crystallization is crucial for high efficiency solar cells. Many methods of synthesis were used to deposit Cu2SnS3 thin films, in particular, magnetron sputtering was often chosen for easy transferability to the industry [9-11]. A two-step method, which consists to the deposition of a layer followed by annealing in sulfur atmosphere, was employed in most of the previous studies. The annealing step seems to be

ACCEPTED MANUSCRIPT the crucial step. The annealing usually takes place around 550 °C in a sulfur atmosphere using sulfur powder [11] or in an atmosphere of H2S [12]. At this temperature, crystallogen sulfides (i.e sulfur with element of the carbon group) are quite volatile, as can be seen in the numerous papers dealing with SnS evaporation during CZTS synthesis [13] and in few

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papers dealing with Cu-Sn-S synthesis [9]. The loss of volatile elements could favor the

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formation of secondary phases, i.e. Cu3SnS4 and Cu4SnS4, which are copper rich phases. A

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sufficiently high partial pressure of S could also prevent the decomposition reaction [14] The objective of this study is to understand the influence of Cu content in the precursor and sulfur atmosphere during the annealing on the formation of CTS. Structural, microstructural,

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optical and electrical properties of films were investigated and were correlated with the synthesis conditions.

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2. Experimental Details

Cu-Sn-S films were grown on soda lime glass (SLG) substrates by RF magnetron cosputtering (Plassys MP500S) of Cu2S and SnS2 targets. The 3 inches targets have 99.99 %

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purity. Thin films were deposited at room temperature at a constant Ar gas pressure of

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0.3 Pa. The possible ternary phases located along the Cu2S-SnS2 join in the Gibbs phase triangle for the system Cu–Sn–S are Cu2Sn3S7, Cu2SnS3, and Cu4SnS4 [15]. As both

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Cu2Sn3S7 and Cu4SnS4 compounds have poor electrical and optical properties compared to Cu2SnS3 we focus on the deposition of films with the ratio [Cu] / [Sn] around 2.0 [2]. The [Cu] / [Sn] ratio was controlled by varying the power applied to the targets. The films with [Cu] / [Sn] = 1.9, [Cu] / [Sn] = 2.0 and [Cu] / [Sn] = 2.2 were noted Cu-poor, Cu-stoich. and Cu-rich,

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respectively. A deposition time of 1 h was used for the deposition of 1.3 μm thick films. A rapid thermal annealing process was developed on an as-deposited film with a ratio [Cu] / [Sn] = 1.9. Films were placed in the furnace inside a graphite box with 120 mg of sulfur powder. The box was heated during 1 h at 400°C with a 5 °C/s heating ramp. The chemical analysis of precursors was carried out by means of X-Ray fluorescence (XRF, Fischerscope X-ray XDV-SDD, 50 kV). X-ray diffraction (XRD) data were recorded on a Panalytical Empyrean with Cu K1 radiation in the -2 Bragg Brentano geometry (10° < 2 < 90° and step = 0.02°). Raman spectra at 532.5 nm wavelength using an Olympus microscope x50 were performed on a Horiba-Jobin-Yvon Labram 900 where back-scattering measurements were made by a CCD camera. Surface and cross sections of films were examined by Scanning Electron Microscope - Field Emission Gun (SEM-FEG, Leo Supra 35, 15 kV). The optical properties of the CTS films were determined from transmission and reflection measurements in the range of 500-2000 nm (Perkin-Elmer Lambda 900

ACCEPTED MANUSCRIPT spectrophotometer). For electrical characterizations of films a standard Hall (Ecopia-HMS3000) and 4 probes measurements (Keithley) were used. 3 Results and discussion

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Fig. 1 shows [Cu] / [Sn] and [S] / ([Cu]+[Sn]) composition ratios of the precursors

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deposited on SLG substrates. The [Cu] / [Sn] ratio was varied between 1.8 and 2.4. The

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deposition conditions were set to keep the S / (Cu+Sn) ratio close to unity, but it decreased slightly from 1 to 0.93 when [Cu] / [Sn] increased. It suggests a difference of sputtering yield between sulfur and the metals when the applied power is varied.

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Fig. 2 shows the X-Ray diffraction patterns of films with different ratios [Cu] / [Sn] = 1.9, [Cu] / [Sn] = 2.0 and [Cu] / [Sn] = 2.2. The films exhibited two diffraction peaks. The first stays at 28.32° regardless the Cu content. The second shifts slightly from 47.12° for the Cu-

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poor sample to 47.26° for the Cu-rich sample, indicating a decrease of the interplanar distance (d-spacing) and the lattice parameters when the Cu content increases. We especially note an inversion of the intensity ratios when the Cu content increases, indicating

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a preferential growth orientation along the orientation at 28.32° to the detriment of the other.

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The assignment of the phases was done using the International Centre for Diffraction Data (ICDD) database. The full width of half maximum (FWHM) for the peaks at 28.30° and 47.17°

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is  ~ 0.4°. The crystallite size was calculated from the FWHM by the Scherrer relationship and was estimated to 60 nm. The analysis did not allow a distinction between the cubic-CTS, the tetragonal-CTS, the monoclinic-CTS and the tetragonal Cu3SnS4 phases because of their relatively similar lattice parameters and the large FWHM of peaks [9,16,17].

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However, a deviation from the reference codes 01-089-2877, 01-089-8714, 04-010-5719 and 00-033-050 for respectively the cubic, the tetragonal, the monoclinic -CTS and the tetragonal-Cu3SnS4 phases is clearly visible. This deviation could be related to the presence of strain in the sample which can be explained by the high energy of species in the discharge at low working pressure of 0.3 Pa during the deposition. The structural properties of films annealed were further analyzed by Raman scattering. The spectrum of Cu-poor compound is showed in Fig. 3. The vibrational modes of the tetragonal CTS phase can be clearly identified. The main peaks of the tetragonal CTS phase were observed by other teams at 297 cm-1, 337 cm-1 and 352 cm-1 [18]. Here, the spectrum presents a weak intensity of peaks which could be due to the crystalline quality of films in agreement with the small size of crystallites. To improve the crystallinity and the sulfur deficiency, we performed a sulfurization at 400°C as described in section 2. After annealing, the same peaks as before annealing were observed, denoting the presence of the CTS tetragonal phase. Well-defined peaks attest to a better crystalline quality. However, an

ACCEPTED MANUSCRIPT additional and intense peak is visible at 312 cm-1. This peak was assigned to the orthorhombic Cu3SnS4 phase, whose the main peak is usually observed at 318 cm-1 [9,18]. In S-rich conditions, the neighboring compound of Cu2SnS3 in the Gibbs phase triangle is Cu3SnS4 [15]. Cu3SnS4 is relatively stable and can be grown at room temperature [19]. In

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fact, it was showed on the calculated chemical potential phase space of Cu-Sn-S system that

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ternary could also occur in spite of low Cu content compound.

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Cu3SnS4 has a wide thermodynamic window of stability [2]. Thus, the formation of that

The micrographs in Fig. 4a, Fig. 4b and Fig. 4c show the surface of as-deposited films Cu-poor, Cu-stoich and Cu-rich, respectively. We observed that the films presented a

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similar morphology whatever the Cu content. The surfaces are smooth, dense and the grains size is about 100 nm. It is interesting to note that the grain size is of the same order of magnitude than that of crystallites deduced by XRD measurements. The micrographs in Fig.

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4d, corresponding to the annealed film, shows a rougher surface. Moreover both small grains of 100 nm and large grains of 300 nm were visible. It could be expected that this bimodal repartition is due to the presence of both Cu2SnS3 and Cu3SnS4 in the films. Investigations

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are needed to assess the mechanisms involved in the growth of its phases.

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In Fig. 5a and in Fig. 5b are shown the optical transmittance T() and reflectance R() spectra, respectively. We point to the presence of interference fringes on spectra resulting

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from multiple reflections between the air/film and the film/glass interfaces. The reflectance is almost constant in this wavelength range and is around 20 %. The transmittance reached 40 % at a wavelength of 1900 nm for as-the deposited films. It decreased slightly to 30 % for the

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Cu-rich compound. On the contrary, the transmittance reached 60 % after annealing. This seems to be due to the improvement of crystalline quality of the Cu2SnS3 phase [20]. The transmittance decreases smoothly by decreasing of the wavelength. The films contain absorption centers in the gap that significantly contribute to the absorption of photons. This behavior is caused by the presence of defects in the films, i.e. vacancies and interstitial substitutions [21]. The optical absorption coefficient α was estimated using the data presented in Fig. 5, i.e. the transmittance and reflectance measurements. It was calculated using the formula [9] (1) where t is the thickness of films. The optical absorption coefficients were greater than 104 cm1 for both as-deposited films and annealed-films. These values are in agreement with the literature [9]. The band gap energy value Eg was calculated using the following expression [9]

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Where A is a constant and n = 1/2 corresponds to a direct optical transition. The value of

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band gap was determined by extrapolating the linear region of the curve (αhν)2 vs hν to the

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axis hν. The plots (αhν)2 vs hν are shown for the as-deposited films and annealed films in Fig. 6. An estimated value Eg = 1.28 eV (error bar ± 0.08 eV) was obtained for the films. This

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value is similar to the band gap energy reported in the literature for the tetragonal CTS structure, Eg = 1.35 eV [9, 22].

In Table 1 are reported the electrical properties of as-deposited and annealed films.

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All the films showed a p-type conductivity. We can see a dependence of the electrical properties on the ratio [Cu] / [Sn], i.e. conductivity, carrier concentration and mobility increase with a higher Cu content, in agreement with the literature [7, 23, 24]. In addition, the

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conductivity of films increased after annealing. The higher conductivity of films could be due to the presence of the metallic Cu3SnS4, which appears during the annealing process.

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4 Conclusion

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Smooth and dense CTS films were deposited on glass substrates using RF magnetron cosputtering from Cu2S and SnS2 targets. The films retained the Cu2SnS3 tetragonal structure

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at room temperature. The films grew along two preferential orientations. We observed that the intensity ratio between these two orientations varied with the Cu content. The band gap energy of films was estimated to be 1.28 eV. The Hall measurements displayed a dependence of the conductivity, carrier concentration and mobility with the Cu content. After

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a treatment at 400°C under sulfur atmosphere the electrical measurements showed a high value of conductivity which was associated with the presence of the metallic Cu3SnS4 phase.

Acknowledgements This work was supported by the French Agence Nationale de la Recherche (ANR) under grant NovACEZ (ANR-10-HABISOL-008).

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[Cu] / [Sn] [S] / ([Cu]+[Sn])

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ratios

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0.5 0.0 2

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6 samples

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Fig. 1 Composition ratios of as-deposited films on glass. Red and black dash lines illustrate respectively the [Cu]/[Sn] and [S]/([Cu]+[Sn]) ratios of the Cu2SnS3 phase

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+ Cu2SnS3 ; Cu3SnS4

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Cu-rich

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I0 (arb. u.)

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Cu-poor

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Cubic-CTS Tetragonal-CTS Monoclinic-CTS Tetragonal-Cu3SnS4

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Fig. 2 XRD patterns of as-deposited films with different [Cu] / [Sn] ratios

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ACCEPTED MANUSCRIPT x + Cu2SnS3

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++

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+

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x Cu3SnS4

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250 300 350 400 Raman shift (cm-1)

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150

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as-deposited 450

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Fig. 3 : Raman scattering spectra for as-deposited and annealed films

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Fig. 4 Surface SEM image of as deposited CTS films a) Cu-poor, b) Cu-stoich, c) Cu-rich and d) annealed films

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80 Transmittance (%)

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20 10 0 80

Reflectance (%)

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b)

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annealed Cu-poor Cu-rich Cu-stoich Cu-poor

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R (%)

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annealed Cu-poor Cu-rich Cu-stoich Cu-poor

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Figure 5 a) Transmittance and b) reflectance measurements of as-deposited and annealed films

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(h )(cm-1eV)2

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Fig. 6 (h )2 vs h and band gap energy estimation of as-deposited and annealed films

ACCEPTED MANUSCRIPT Table 1 Dependence of the conductivity (S), the carrier concentration N and the mobility µ with the Cu content in the CTS films Cu-stoich

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N (cm-3)

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µ (cm2/Vs)

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Annealed Cupoor

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ACCEPTED MANUSCRIPT Highlights

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Solar cells based on earth abundant elements // Synthesis of Cu2SnS3 for photovoltaic applications // Co-sputtering and reactive annealing atmosphere // P-type semiconductors with a band gap energy 1.25 eV and high absorption coefficient > 104//