Sulphurization of single-phase Cu11In9 precursors for CuInS2 solar cells

Sulphurization of single-phase Cu11In9 precursors for CuInS2 solar cells

Thin Solid Films 515 (2007) 5921 – 5924 www.elsevier.com/locate/tsf Sulphurization of single-phase Cu11In9 precursors for CuInS2 solar cells A. Joswi...

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Thin Solid Films 515 (2007) 5921 – 5924 www.elsevier.com/locate/tsf

Sulphurization of single-phase Cu11In9 precursors for CuInS2 solar cells A. Joswig, M. Gossla, H. Metzner ⁎, U. Reislöhner, Th. Hahn, W. Witthuhn Institut für Festkörperphysik, Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany Available online 29 January 2007

Abstract Using Rutherford backscattering (RBS), X-ray diffraction (XRD), and scanning electron microscopy (SEM), the sulphurization of single-phase Cu11In9 precursors to be employed as light absorbing CuInS2 (CIS) layers in CIS–CdS heterojunction thin-film solar cells has been investigated. The Cu11In9 precursor films were produced by DC-sputtering from a single-phase Cu11In9 target. The sulphurization at 500 or 300 °C was performed by adding different amounts of elemental sulphur with heating rate and sulphurization time as additional parameters. During sulphurization at 500 °C, up to 50% of the indium initially present in the precursor is lost. We relate the In-loss to the volatile In2S compound, the formation of which is favoured by the phase transition of Cu11In9 to Cu16In9 at 307 °C. Consequently, the In-loss can be suppressed by employing a sulphurization temperature of 300 °C. At this temperature, a prolonged sulphurization time and a large sulphur excess are necessary in order to obtain stoichiometric CIS beneath a CuSx surface phase. © 2006 Elsevier B.V. All rights reserved. Keywords: CuInS2; Sulphurization; Rutherford backscattering

1. Introduction The ternary chalcopyrite semiconductor CuInS2 (CIS) with its direct fundamental band gap of 1.5 eV is an alternative to the analogous Se-based CuInSe2 compound for the production of polycrystalline thin-film-solar cells. The conversion efficiencies are, at present, limited to 12.2% for CuInS2 solar cells [1], while more than 19% for Cu(In,Ga)Se2 photovoltaic devices [2] were achieved. Still, CIS offers some advantages including the optimum bandgap so that no addition of the rare element gallium for a bandgap widening is required. Additionally, CIS absorber layers can be produced in a fairly simple two-stage process consisting in the sulphurization of a (mostly Cu-rich) metallic precursor layer. A fast sulphurization process has led to cells with 11.4% conversion efficiency [3] and this process is currently being scaled up for the industrial production of 120 × 60 cm2 CIS solar modules [4]. The sulphurization process with either H2S or elemental S has already been investigated by Binsma and van der Linden [5], while our research has been focussed on the sulphurization in an H2S containing atmosphere [6–8]. Recently, a real-time study of the Cu–In–S thin film growth during sulphurization with elemental sulphur was per⁎ Corresponding author. Tel.: +49 3641 947 353; fax: +49 3641 947 302. E-mail address: [email protected] (H. Metzner). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.12.053

formed by Rudigier and co-workers [9]. The cited works mainly deal with the phase-formation processes during sulphurization. Here, we investigate the sulphurization behaviour of singlephase Cu11In9 precursors with elemental sulphur and put special emphasis on the absolute material balance. Details about singlephase Cu-In precursors can be found in our previous work [10]. 2. Experimental details Using DC sputtering, Mo back-contact films of about 700 nm thickness and Cu11In9 precursor layers of typically 900 nm thickness were deposited onto cleaned 2-mm-thick float glass. The Cu11In9 sputter target of 3 in. diameter had been produced in house by melting appropriate amounts of Cu and In of 99.9999% purity in a carbon crucible under vacuum. For the sulphurization process, precursor samples were placed together with S powder of 99.9995% purity in a petri dish with a cover (quartz glass). This container was placed in a base-plate vacuum chamber and heated by means of radiant heating under vacuum conditions (base pressure 10− 5 hPa). The heating rate was varied between 5 and 30 °C/min, the sulphurization temperature was 500 °C, and sulphurization times between 5 and 120 min were employed. Also, low-temperature sulphurization was performed at 300 °C for 300 min. The temperature of 300 °C was attained at a heating rate of 5 °C/min up to 275 °C and

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Fig. 1. RBS spectrum of a sputtered single-phase Cu11In9 precursor film on a float glass substrate and a Mo back-contact layer. The RBS yields of the single elements are indicated.

2.5 °C/min between 275 and 300 °C: Chemical composition, thickness, and thickness variations of the thin films were analyzed by means of Rutherford backscattering (RBS) using 3.5-MeV He ions delivered by the Jena Tandetron accelerator JULIA. We note that RBS allows one to determine, on an absolute scale, the number of atoms per film area and thus the film thickness for a given atomic density (atoms per volume). For more details see [11]. X-ray diffraction with Cu–Kα radiation was measured for a phase analysis and the data were analyzed by the RIETVELD method as described in [10]. Scanning electron microscopy (SEM) was employed in order to image the surface topography. Some CIS films were processed to complete solar cells. 3. Results and discussion Fig. 1 shows the RBS spectrum of a typical precursor film. The contributions of the single elements which constitute the

Fig. 3. RBS spectrum (a) of a Cu11In9 precursor film sulphurized with 20 mg sulphur before (closed symbols) and after (open symbols) KCN etching. The difference spectra are plotted as triangles. The corresponding SEM image of the film after etching is shown in (b).

Mo–CuIn double layer are indicated. Compared to the heavy film elements, the yield of the light elements in the glass substrate is negligible. The spectrum indicates that both the Mo and the Cu–In film are smooth with thicknesses of 650 and 900 nm, respectively. The composition of the Cu–In film is determined to be Cu1.2In1.0 which matches the atomic ratio of the Cu11In9 compound within the limits of error. The precursor thickness was chosen in order to obtain a 2100-nm thick CIS layer according to the following sulphurization reaction, which reads: Cu11 In9 þ 10S2 →9CuInS2 þ 2CuS:

Fig. 2. RBS spectrum of the precursor of Fig. 1 after sulphurization at 500 °C and KCN etching. The CIS layer shows an homogeneous element distribution but is very rough, while the Mo film remained unaffected by the sulphurization process. During sulphurization about 50% of the initially present In is lost.

ð1Þ

The particular precursor of Fig. 1 had the standard area of 1 in.2 and was annealed together with a similar one of the same dimensions and 200 mg sulphur for 30 min at 500 °C. The RBS spectrum of the reacted film after the CuS layer had been removed by KCN etching is depicted in Fig. 2. Again the contributions of the single elements of the Mo–CIS double layer are indicated. The RBS spectrum reflects a rough CIS film with a composition of Cu0.92In1.00S2.35 and a mean thickness of 1140 nm. The Mo layer has remained unaffected by the sulphurization reaction. The Mo-related RBS yield is smeared out due to the roughness of the covering CIS layer. Clearly, the CIS film has not much more than half the thickness that is

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Fig. 6. XRD spectrum measured for the etched CIS film which was prepared with 20 mg sulphur [cf. Fig. 3 (a) and (b)]. The raw data (solid circles) were fitted (solid line) using the RIETVELD algorithm. The difference spectrum and the contribution of each phase are shown separately.

Fig. 4. RBS spectrum (a) of a Cu11In9 precursor film sulphurized with 158 mg sulphur before (closed symbols) and after (open symbols) KCN etching. The difference spectra are plotted as triangles. The corresponding SEM image of the film after etching is shown in (b).

expected from the precursor thickness. The shown data are representative for altogether three series of sulphurization at 500 °C in which sulphurization time and heating rate were varied systematically. It turns out that the observed material loss

Fig. 5. Quantitative results of the RBS analysis of the etched films of the lowtemperature series as a function of sulphur supply during sulphurization. An Inloss is not observed in this series. For low sulphur supplies the reaction to CIS remains incomplete and the copper excess survives etching.

is independent of sulphurization time and increases with heating rate. In the example shown in Fig. 2, a heating rate of 20 °C/min was employed corresponding to the maximum material loss. We assume the material loss to be related to the volatile In compound In2S and propose the following reaction scheme which considers the stability limit of Cu11In9 at 307 °C [10]. This reads: 32Cu11 In9 →22Cu16 In9 þ 90ðInÞliquid ;

ð2Þ

90ðInÞliquid þ 45S→45ðIn2 SÞgas ;

ð3aÞ

22Cu16 In9 þ 275S2 →198CuInS2 þ 154CuS:

ð3bÞ

Hence, if all the liquid In that is released in the phase transition of Eq. (2) is transformed into the volatile In compound as described by Eq. (3a), then, by combining Eqs. (2) and (3b), the CIS material loss can be calculated to be 1 − 198 / (9 × 32) = 31% which is still less than the observed 50%. In order to suppress the material loss, we performed sulphurization experiments at 300 °C, i.e., below the phase transition described in Eq. (2). In these experiments, the amount of sulphur per precursor of 1 in.2 was varied between 5 and 158 mg. In Figs. 3(a) and 4(a) we show RBS spectra of films sulphurized with 20 and 158 mg sulphur, respectively, before (closed symbols) and after (open symbols) KCN etching. Also, the difference spectra are plotted as triangles. The corresponding SEM images of the films after etching are depicted in Figs. 3(b) and 4(b), respectively. Under sulphurpoor conditions (20 mg S), the RBS spectrum [Fig. 3(a)] indicates an In-enrichment (and a Cu-depletion) at the film surface. Moreover, as demonstrated by the difference spectrum, the KCN etching has practically no effect on film composition, roughness and thickness. The slight differences below 2.0 MeV reflect a certain lateral thickness variation of the film. The

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corresponding SEM image [Fig. 3(b)] shows a film with a grainy surface structure. Under high resolution [inset in Fig. 3 (b)], these surface grains look somewhat like sea urchins. Similar structures have already been observed by Binsma under In-rich conditions and were identified as indiumsulphide spheres [5]. Our results then suggest that also under Cu-rich conditions (Cu / In = 1.2) and with sulphur deficiency the formation of indiumsulphide at the surface occurs. Under sulphur excess (158 mg S), the formation of quite different films is observed. Now the RBS spectra [Fig. 4 (b)] indicate an In-depletion (and a Cu-enrichment) at the surface in the asdeposited state. This inhomogeneity is removed completely by KCN etching and a stoichiometric CIS film remains. The SEM image [Fig. 4(b)] indicates a film consisting of quite densely packed rocket-shaped grains of typically 1 μm diameter. During the preparation of the SEM sample, small flakes of the CIS film blistered off the Mo film, and the rear side of such a flake is also seen in the SEM image. The flake has a very smooth surface and shows a laterally homogeneous distribution of small voids. The quantitative results of the RBS analysis of the etched films of the low-temperature series are shown in Fig. 5 as a function of sulphur supply during sulphurization. As the most important result, we note that an In-loss is not observed in this series. The atomic area-density (atoms per film area) of the In atoms in the precursors is conserved during sulphurization and etching. The Cu-excess of the precursors (Cu / In = 1.2) gradually decreases with increasing sulphur supply. In parallel, the sulphur incorporation into the films increases and reaches the stoichiometric value according to the CIS 1–1–2 phase at 80 mg S where also the Cu-excess completely vanishes. Hence, a sulphur supply of 80 mg is needed for the complete sulphurization at 300 °C which means thirty times as much sulphur as is finally incorporated into the CIS film. The fact that the sulphurization is incomplete at lower sulphur supplies is confirmed by the XRD data shown in Fig. 6. The Figure shows the XRD spectrum measured for the etched CIS film which was prepared with 20 mg sulphur [cf. Fig. 3 (a) and (b)]. The raw data (solid circles) were fitted (solid line) using the RIETVELD algorithm. The difference spectrum indicates the high quality of the fit, in which the phases CIS, Cu11In9, In203 and Mo are assumed. The contribution of each of these phases to the fit curve is also depicted in Fig. 6. The prominent precursor signal (Cu11In9) shows that the sulphurization was incomplete. The main reaction path leads directly from Cu11In9 to CIS since binary sulphides are not seen in XRD. Obviously, the assumed indiumsulphide phase of this film (see discussion above) has a too low volume fraction and/or a too short coherence length to be visible here. The appearance of In203 has also been observed in CuIn precursors [10] and its formation can not be suppressed completely even under the best vacuum conditions [12]. Hence, a complete sulphurization with elemental sulphur can also be obtained at comparatively low temperatures (300 °C) when large sulphur excess and long sulphurization times are employed. Some of the Mo–CIS layers out of the lowtemperature sulphurization were processed to form complete CIS–CdS heterojunction solar cells. Their efficiencies were limited to 3% mainly due to shunts. Using these cells, the

external quantum efficiency (EQE) was measured (data not shown). The EQE data yield a bandgap of 1.42 eV for the CIS films out of the low-temperature sulphurization. 4. Conclusions The sulphurization of single-phase Cu11In9 precursors to be employed as light absorbing CuInS2 (CIS) layers in CIS–CdS heterojunction thin-film solar cells has been investigated. The Cu11In9 precursor films were produced by DC-sputtering from a single-phase Cu11In9 target onto glass substrates covered with a Mo layer as back-contact. The sulphurization at 500 or 300 °C was performed in an evacuated reactor by adding different amounts of elemental sulphur with heating rate and sulphurization time as additional parameters. During sulphurization at 500 °C, up to 50% of the indium initially present in the precursor is lost and this In-loss is shown to increase with heating rate. In a model, we relate the In-loss to the volatile In2S compound, the formation of which is favoured by the phase transition of Cu11In9 to Cu16In9 at 307 °C. Consequently, the In-loss can be suppressed by employing a sulphurization temperature of 300 °C. At this temperature, a prolonged sulphurization time and a large sulphur excess are necessary in order to obtain stoichiometric CIS beneath a CuSx surface phase. Acknowledgment The authors thank the Deutsche Forschungsgemeinschaft for the generous financial support. References [1] D. Braunger, Th. Dürr, D. Hariskos, Ch. Köble, Th. Walter, N. Wieser, H.W. Schock, Proc. 25 IEEE Photovoltaic Specialists Conf. IEEE, New York, 1996, p. 1001. [2] K. Ramanathan, M.A. Contreras, C.L. Perkins, S. Asher, F.S. Hasoon, J. Keane, D. Young, M. Romero, W. Metzger, R. Noufi, J. Ward, A. Duda, Thin Film Solar Cells,Prog. Photovolt. Res. Appl. 11 (2003) 225. [3] K. Siemer, J. Klaer, I. Luck, J. Bruns, R. Klenk, D. Bräunig, Sol. Energy Mater. Sol. Cells 49 (1997) 375. [4] R. Klenk, J. Klaer, R. Scheer, M.Ch. Lux-Steiner, I. Luck, N. Meyer, U. Rühle, Thin Solid Films 480–481 (2005) 509. [5] J.J.M. Binsma, H.A. van der Linden, Thin Solid Films 97 (1982) 237. [6] C. Dzionk, H. Metzner, S. Hessler, H.-E. Mahnke, Thin Solid Films 299 (1997) 38. [7] M. Gossla, H.-E. Mahnke, H. Metzner, Thin Solid Films 361–362 (2000) 56. [8] M. Gossla, H. Metzner, H.-E. Mahnke, Thin Solid Films 387 (2001) 77. [9] E. Rudigier, B. Barcones, I. Luck, T. Jawhari-Colin, A. Perez-Rodriguez, R. Scheer, J. Appl. Phys. 95 (2004) 5153. [10] M. Gossla, H. Metzner, H.-E. Mahnke, J. Appl. Phys. 86 (1999) 3624. [11] H. Metzner, Th. Hahn, M. Gossla, J. Conrad, J.-H. Bremer, Nucl. Instrum. Methods Phys. Res., B Beam Interact. 134 (1998) 249 and references cited therein. [12] J. Grzanna, H. Migge, J. Mater. Res. 12 (1997) 355.