Elsevier

Thin Solid Films

Volumes 480–481, 1 June 2005, Pages 433-438
Thin Solid Films

Formation and characterisation of MoSe2 for Cu(In,Ga)Se2 based solar cells

https://doi.org/10.1016/j.tsf.2004.11.098Get rights and content

Abstract

The reaction kinetics of the MoSe2 formation have been investigated by selenizing Mo layers in Se vapor at different temperatures and for different durations. The samples were characterized by means of Rutherford backscattering spectrometry, X-ray diffraction (XRD), electron diffraction, and by bright-field and high-resolution transmission electron microscopy. It was found that in all samples, a homogeneous MoSe2 layer is formed on top of the Mo layer. The temperature dependence shows that the MoSe2 layer thickness increases strongly for temperatures higher than ca. 550 °C. At a substrate temperature of 450 °C, no difference in the MoSe2 thickness was detected for samples selenized for different durations. The diffusion constant of Se in MoSe2 is estimated from the selenization duration dependence of MoSe2 layer thicknesses at 580 °C. Finally, Cu(In,Ga)Se2 (CIGS) solar cells in substrate configuration were developed on indium tin oxide (ITO) transparent back contacts. An intentionally grown MoSe2 intermediate layer on ITO, prior to CIGS deposition, causes a significant efficiency improvement, suggesting that MoSe2 can facilitate a quasi-ohmic contact. Solar cell efficiencies of up to 11.8% are obtained using an ITO/MoSe2 back contact.

Introduction

Molybdenum is widely used as back contact material in Cu(In,Ga)Se2 (CIGS) based thin-film solar cells in “substrate configuration”. First results of Matson et al. [1] and Russell et al. [2] suggested that Mo back contacts form a Schottky-type barrier to the CuInSe2 (CIS) absorber, while later Shafarman et al. [3] showed the contact between Mo and CIS to be ohmic, owing to the formation of an intermediate MoSe2 layer during CI(G)S deposition [4], [5]. It is now believed that the formation of a MoSe2 layer is essential to facilitate a quasi-ohmic electrical contact across the CIGS–Mo interface. Electron microscopy and X-ray diffraction measurements have revealed that the formation and properties of MoSe2 layer may depend on the CIGS deposition method and the growth recipes [4], [6]. An understanding of the reaction kinetics of the MoSe2 formation is vital for the development of low resistance electrical back contacts for high efficiency CIGS solar cells, since thickness and crystallographic orientation of MoSe2 layer determine the adhesion and electrical properties of the CIGS–Mo interface.

Recent interest in CIGS photovoltaics is aimed at developing multi-junction tandem solar cells which require suitable semi-transparent “bi-facial” cells grown on transparent conducting oxide (TCO) coated glass substrates [7], [8], [9]. For an efficient semi-transparent CIGS solar cell in substrate configuration, the TCO layer should form a low resistance ohmic back contact with the CIGS layer. Haug et al. [7] assessed various TCOs and suggested that indium tin oxide (ITO) and SnOx:F can be used to form a quasi-ohmic contact with CIGS, since the CIGS layers grown on these TCOs showed somewhat non-rectifying IV characteristics.

Thus, one approach is to apply a TCO layer as a back contact directly to the CIGS absorber: efficiencies of up to 14.9% have been reported for solar cells with ITO back contact [10]. However, it was shown that the efficiency depends crucially on the CIGS deposition recipe [10], and also that Ga2O3 formation between CIGS and ITO deteriorates the current transport [11]. In addition, Terheggen et al. [12] identified the formation of Ga2O3 between ZnO:Al and CIGS.

A second approach is to find a suitable buffer layer between CIGS and TCO which gives flexibility to use any CIGS growth recipe. To achieve this, the capability of the MoSe2 layer of providing low resistance quasi-ohmic contact on alternative back contact materials can be exploited. A very thin MoSe2 layer may be grown on the TCO back contact material, e.g. on an ITO layer. Thus, the ohmic contact formation is independent of the CIGS deposition recipe, and in addition, the MoSe2 layer acts as a buffer between ITO and CIGS, preventing gallium oxide formation at the ITO/CIGS interface.

In this paper, we describe an investigation of the reaction kinetics, and chemical and structural properties of MoSe2 layers obtained by direct selenization of Mo with Se vapor. Also, first results showing improvements in cell efficiencies by incorporating intentionally grown MoSe2 layer on ITO prior to the CIGS deposition will be reported.

Section snippets

Experimental

Reaction kinetics of MoSe2 was investigated by selenizing the Mo layers in a two-temperature zone quartz tubular reactor where temperatures of the Se source and substrate for selenization could be independently controlled. Mo layers of different thicknesses were deposited by dc-magnetron sputtering at 3×10−3 mbar on Si(001) wafers. For an easier evaluation of the MoSe2 layer and to avoid substrate-related ambiguities, Si wafers were used as substrates, instead of the soda-lime glass (SLG).

After

Characterization of MoSe2 layers

Since separate XRD, ED, HRTEM or EDX measurements delivered ambiguous results on the identification of the Mo–Se compound as MoSe2, all of the mentioned measurements were performed for the same selenized Mo layer and a non-selenized Mo layer as a reference.

The thickness of the MoSe2 layer formed on Mo was determined by means of RBS. Preliminary measurements showed that when a 45 nm thick Mo layer on a Si substrate is selenized at 580 °C for 60 min, the whole Mo layer as well as a part of the Si

Conclusion

Mo layers on Si wafer substrates were selenized at different temperatures and for different durations, and the formation kinetics and structural properties of the formed MoSe2 layers were investigated. A MoSe2 layer formed at 580 °C showed a homogeneous composition and a hexagonal layered structure with the c-axis parallel to the Mo substrate. For substrate temperatures lower than ca. 550 °C, the obtained MoSe2 layer thickness remains rather small, whereas for substrate temperatures higher than

Acknowledgements

The authors are grateful to David Sager, Nonmetallic Inorganic Materials, ETH Zurich, Switzerland for the XRD measurements. This work was supported in part by the Swiss National Science Foundation, and also by the Swiss Federal Office of Science and Education for the EU project PROCIS.

References (17)

  • R.J. Matson et al.

    Sol. Cells

    (1984)
  • T. Wada et al.

    Thin Solid Films

    (2001)
  • R. Würz et al.

    Thin Solid Films

    (2003)
  • M. Terheggen et al.

    Thin Solid Films

    (2003)
  • L.R. Doolittle

    Nucl. Instrum. Methods

    (1986)
  • M.K. Agarwal et al.

    Mater. Res. Bull.

    (1985)
  • A. Mallouky et al.

    Thin Solid Films

    (1988)
  • P.E. Russell et al.

    Appl. Phys. Lett.

    (1982)
There are more references available in the full text version of this article.

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Also at: Centre for Renewable Energy Systems and Technology, Department of Electronic and Electrical Engineering, Loughborough University, Leicestershire, LE11 3TU, UK.

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