The behaviour of Na implanted into Mo thin films during annealing

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Abstract

Na implants have been used to study diffusion of Na in rf diode sputtered Mo thin films used as back contacts for Cu(In,Ga)Se2 solar cells. The samples were analysed with secondary ion mass spectrometry before and after vacuum anneals at 420°C and 550°C. In addition, X-ray photoelectron spectroscopy has been used for surface studies. The diffusion of Na within the Mo grains was found to be very slow as indicated by the unchanged shape and position of the implant peak after the anneal. An increased level of Na in the bulk of the Mo layer strongly suggests diffusion of Na out of the soda lime glass substrate into the Mo film. The oxygen content of the rf diode sputtered Mo films was 8 at% as found by Rutherford backscattering spectroscopy. It is suggested that Mo oxide phases are present in the grain boundaries and that these oxides, being intercalation hosts for Na, are responsible for the rapid diffusion and high solubility of Na in the sputter-deposited Mo films.

Introduction

The fabrication of high-efficiency thin film solar cells based on Cu(In,Ga)Se2 and related materials includes, as a crucial step, Cu(In,Ga)Se2 deposition on Mo-coated soda lime glass substrates. The nucleation and growth of Cu(In,Ga)Se2 grains are directly influenced by the thermal, morphological and chemical properties of the Mo back contact layer. It has been found that the use of soda lime glass as the substrate material for Cu(In,Ga)Se2-based solar cells leads to high concentrations of Na both in the Mo and in Cu(In,Ga)Se2 1, 2, 3, 4, 5, 6. The substrate temperature during deposition of Cu(In,Ga)Se2 reaches up to 550°C. During this high temperature step, Na diffuses out of the glass and through the Mo and affects the growth of Cu(In,Ga)Se2 in a favourable way, resulting in large grains and high film density [3]. Secondary ion mass spectrometry (SIMS) analysis of Cu(In,Ga)Se2 samples deposited on both Mo coated and uncoated soda lime glass shows no significant difference in the Na concentration of the Cu(In,Ga)Se2 as seen in Fig. 1, which indicates that the Mo back contact layer can be a very efficient medium for Na diffusion. However, variations of the Mo properties, and hence the Na transport, may explain unintentional variations in Cu(In,Ga)Se2 film properties. As the best solar cell results have been achieved for Cu(In,Ga)Se2 grown in the presence of Na, the role of Mo as a medium for Na transport is an important issue. Of importance for the use of substrates other than soda lime glass and adding the Na by an Na precursor layer as e.g. Na2S is the ability of Mo to absorb Na. Solubility of Na in Mo has been observed by Ruckh et al. [5], where an alkali-free sample with a Mo layer was coated with an Na precursor layer, then heated and analysed with X-ray photo electron spectroscopy (XPS) in situ and after a few days of air exposure. Na disappeared from the surface at 600°C (approximately the same temperature as the Cu(In,Ga)Se2 deposition temperature) but reappeared after the air exposure.

In this study, we report on the diffusion of Na from ion implants in Mo films used as back contacts for Cu(In,Ga)Se2 solar cells, and make comparisons with the sodium coming from the soda lime glass.

Section snippets

Experimental procedure

The Mo was rf diode sputtered on soda lime glass substrates from a high-purity Mo target in 2 Pa of ultrahigh purity Ar at 650 W. The background pressure of residual gases was ∼5×10−5 Pa and the deposition time 15–20 min depending on the thickness. The Mo films used for Na implants were 0.5 μm thick; Mo films used for back contacts to solar cells are typically 0.3–0.5 μm. The lateral grain size was ∼0.1 μm and the vertical grain size was equal to the film thickness. The as-deposited films were in a

Results

As-deposited Mo samples were analysed by XPS, SIMS, and RBS. The Mo films were found by all the methods to contain significant amounts of oxygen. Mo was found in the +4 and +6 valence states corresponding to MoO2 and MoO3 in addition to metallic, +0 valence, Mo. The Mo 3d XPS spectrum of an as-deposited sample is shown in Fig. 3. There is a clear distortion of the 3d pair caused by the presence of Mo oxides. The peak positions of the oxides and of the elemental Mo are marked with arrows. The O

Na diffusion

The diffusivity of the Na in the Mo layers cannot be calculated directly from the above results because a well-defined change in the implant did not occur. We explain the unchanged shape of the implant as follows. The majority of the implanted Na does not diffuse significantly. However, Na diffused completely through the layer sufficiently rapidly to bring the background concentration to a constant value. Since the implant will be distributed in both grains and grain boundaries we interpret

Conclusions

The findings in this work are important contributions to the understanding of how Na is supplied during growth of Cu(In,Ga)Se2, a supply which is critical in order to achieve best device performance. Our normal deposition process for Cu(In,Ga)Se2 on soda lime glass leads to Na concentrations of about 0.1% in the Mo layer, which is enough to give solar cells with high conversion efficiencies.

Na diffuses rapidly in sputtered Mo-containing oxide phases, i.e. in the grain boundaries of the Mo

Acknowledgements

We gratefully acknowledge the support of the Swedish Board for Industrial and Technical Development (NUTEK). Dr Angus Rockett also acknowledges the support of the Electric Power Research Institute and the National Renewable Energy Laboratory. Use of the Centre for Microanalysis of the Materials at the Materials Research Laboratory at the University of Illinois, supported by the Department of Energy under contract DEFG02-91-ER45439, is greatly appreciated. We wish to thank Prof. L. Allen for

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