Lateral phase separation in Cu-In-Ga precursor and Cu(In,Ga)Se2 absorber thin films

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Highlights

  • Lateral phase separation and dewetting of Mo during precursor heating is observed.

  • Dewetting is strongly decreased by applying faster heating rates.

  • NaF deposited on top/underneath the precursor de-/increases the dewetting of Mo.

  • Reduced dewetting during heating enables delayed Se supply during selenization.

Abstract

Sequential Cu(In,Ga)Se2 fabrication with a thermally activated reaction of Cu-In-Ga metal precursor layers in chalcogen atmosphere is an industrially attractive route for preparation of Cu(In,Ga)Se2 absorber based thin film solar cells. Recent results show that controlling the selenium supply during rapid thermal processing has a huge impact on absorber growth. Especially a two stage process applying a first annealing step with or without Se at temperatures up to 400 °C was shown to have a positive effect on the elemental in-depth distribution. However, during this annealing, lateral phase separation, dewetting and coarsening may occur in the metal phase, leading to lateral non-uniformity of the absorber. In this study we show how the dewetting can be strongly decreased by adjusting the precursor architecture, applying faster heating rates and NaF addition on top of a precursor. In contrast, NaF deposited underneath the precursor increases the dewetting effect. Further we show that lateral phase separation during annealing increases with temperature and leads to phase domain sizes of several micrometers at 580 °C.

Introduction

Recently, energy conversion efficiency for Cu(In,Ga)(S,Se)2 (CIGS) thin film solar cells reached a record value of 22.3% [1] when produced by the sequential process. In the sequential process, CIGS forms when metallic Cu-In-Ga precursors are heated up in a chalcogen containing atmosphere (chalcogenization), i.e. selenium and/or sulfur. Elemental Se/S or the toxic gases H2Se or H2S may be used. A promising approach towards a cost effective and industrially scalable production is the chalcogenization within a few minutes by rapid thermal processing (RTP) in N2 atmosphere at ambient pressure [2], [3]. However, with RTP the Ga in-depth distribution cannot be adjusted easily and Ga accumulation at the back contact is often observed, leading to a non-ideal band-gap grading [4], [5], [6]. A first annealing step at 350–400 °C either with [7] or without [8] Se followed by selenization at 530 °C with a controlled Se flux has been shown to lead to a more uniform Ga in-depth distribution. In addition, the amount of Se supplied in different temperature regimes affects not only the Ga grading, but also the film roughness [2].

For Cu-In-Ga precursor layers as used for the sequential absorber preparation, the predicted ternary phase diagram from Muzzillo et al. and Kim et al. [9], [10] together with experimental data by Purwins et al. [11] show a mixture of phases, including Cu16(In,Ga)9, Cu9(In,Ga)4 and an In-rich liquid phase or pure solid In, depending on the temperature. On the one hand, since the melting point of pure In is 157 °C, lateral phase separation and dewetting seems plausible during precursor annealing above that temperature. Such a separation of elements, dewetting and coarsening in Cu-In-Ga precursor films can lead to solar cells with low shunt resistance due to pinhole formation and to a locally varying Ga concentration and in-depth distribution. On the other hand, the intermixing of Cu16(In,Ga)9 and Cu9(In,Ga)4 as well as the solubility of In in these phases increases with increasing temperature, as Hölzing et al.[12] have shown. This may also counteract a possible phase separation.

The addition of alkali metals is known to have an effect on the growth and properties of Cu(In,Ga)Se2 (CIGSe): Na can influence the grain size [13] and crystal orientation [14]. Further, Na may impede the In/Ga-interdiffusion in CIGSe films [15]. Nonetheless, the influence of Na on a chalcogen-free annealing step, where phase separation may occur, remains unclear.

The possibility to reach temperatures of up to 600 °C in an annealing step prior to the chalcogenization without phase separation and dewetting, would widen the parameter space for selenization and possibly enable process designs leading to optimized Ga depth profiles. In this study we investigate the lateral phase separation and dewetting during annealing of Cu-In-Ga metal precursors with different layer stacks as well as the role of Na therein. Moreover, the effect of phase separation in the precursor on the lateral homogeneity of selenized films is investigated.

Section snippets

Experimental

The samples consist of a glass substrate, coated with SiOxNy/Mo/Mo:Na/Mo on top of which the precursor stack is deposited. All layers are deposited on a 3.1 mm thick soda lime glass using a Leybold Optics Dresden A600V7 DC magnetron sputtering system. The Na diffusion barrier (SiOxNy) is 150 nm thick, while the back contact (Mo/Mo:Na/Mo) contains a 70 nm thick Mo:Na (with 5 wt% Na) layer and is in total 850 nm thick. The 700 nm thick precursor stacks are sputtered from an In and a Cu-Ga target (75%

Influence of precursor architecture on roughness and homogeneity

Before performing annealing experiments with metal precursor layers, it is useful to know as much as possible about their initial properties, such as morphology, element distribution, etc. Fig. 1 shows SEM and elemental distribution images of two precursor layers exhibiting a different architecture: Fig. 1a shows the triple layer precursor (see Experimental) which exhibits droplets at the surface. Fig. 1b shows the multilayer precursor, which does not exhibit such droplets. The RMS of the

Discussion

Fig. 5 and Table 2 show that NaF affects the dewetting of precursor metals on Mo during annealing up to 580 °C differently, depending on whether the NaF layer was deposited on top or underneath the precursor stack. This suggests that the effect of NaF on the precursor during annealing is more than simply entering the solution of In or Cux(In,Ga)y and changing the respective chemical and physical properties, since the effect should be similar if NaF is deposited on top or underneath the

Conclusion

Lateral phase separation and dewetting during selenization of Cu-In-Ga precursors is an unwanted effect for CIGSe production and may become significant, if the Se supply during selenization is adjusted such that the precursor is in part or fully heated up without Se. We studied the impact of heating rate and maximum temperature during annealing of metal precursors in Se free environment. A suitable starting point was a smoother and more homogeneous multilayer precursor in comparison with a

Acknowledgments

The authors would like to thank C. Klimm for the measurement of Fig. 6 and A. Scheu for measuring XRD. This work was supported by the Federal Ministry of Education and Research (BMBF) and the state government of Berlin (SENBWF) in the framework of the program ”Spitzenforschung und Innovation in den Neuen Ländern” [grant no. 03IS2151]. The authors further gratefully acknowledge financial support by the Federal Ministry for Economic Affairs and Energy in the framework of the ACCESS-CIGS project [

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