Influence of Na on the properties of Cu-rich prepared CuInS2 thin films and the performance of corresponding CuInS2/CdS/ZnO solar cells

https://doi.org/10.1016/S0927-0248(00)00275-0Get rights and content

Abstract

Using different glass substrate types the Na content in sequentially and Cu-rich prepared CuInS2 films and corresponding CuInS2/CdS/ZnO thin-film solar cells is varied. The purpose was to investigate the influence of different Na concentrations on absorbers and devices. While the morphology of the absorbers seems not to be affected by this variation, corresponding PL spectra differ significantly. The properties of the solar cells, however, show no dependence on the Na concentration. This implies that even though the defect chemistry of CuInS2, sequentially prepared under Cu excess, is changed by the presence of Na this influence has no impact on properties of corresponding solar cells.

Introduction

Cu(In,Ga)Se2 and CuInS2 are regarded to be the two most promising materials for high-efficiency single-junction chalcopyrite thin-film solar cells [1], [2]. In the case of Cu(In,Ga)Se2-based solar cells, the presence of Na in the absorber proved to be necessary for good performance [3], [4]. Film morphology as well as lateral conductivity and solar cell characteristics improve [5], [6], [7]. For CuInS2, where in contrast to Cu(In,Ga)Se2 high-efficiency solar cells are still prepared under Cu-rich conditions [8], systematic investigations on this subject are rare [9]. The purpose of this work was to elucidate whether the presence of Na in the absorber is as crucial for CuInS2-based cells as it is for Cu(In,Ga)Se2-based cells.

Section snippets

Sample preparation

CuInS2 films were prepared under Cu-rich conditions on different glass substrate types using a sequential process. The various substrate types were alkali free AF 45, Corning 0317, float glass (HMI_standard, Vegla_float) and float glass with Si3N4 diffusion barrier for Na (Vegla_float_db). Their chemical composition is given in Table 1. Several sets of samples were processed. Na was not intentionally added but was supposed to diffuse from the substrate into the CuInS2 absorber during film

Results

In Fig. 1, the Na profiles recorded by SIMS are presented. Due to the extremely low Na concentration in the substrate the samples on AF 45 have not been measured. For all samples the highest Na amount is found at the surface with a decreasing concentration gradient towards the back contact. The SIMS experiments confirmed that the Na content in the CuInS2 absorbers varies with the substrate type exhibiting a maximum concentration of ca. 1019 cm−3. The highest overall Na concentration is observed

Discussion and conclusion

The maximum of the incorporated Na concentration in the investigated CuInS2 samples was found to be about 1019 cm−3. This is at least one order of magnitude less than for CuInSe2 [3], [15] and might be a consequence of the here applied Cu-rich preparation of CuInS2. The maximum Na concentration in CuInSe2 is found for stoichiometric, In-rich grown samples [16] . An improvement of the crystal size of Cu-rich prepared CuInS2 thin films by the presence of Na was not observed as for CuInSe2. In the

Acknowledgements

The authors would like to thank E. Müller, C. Kelch, M. Kirsch and T. Münchenberg for the solar cell preparation. This work has been partially funded by the Joule III Programme of the European Commission (Contract JOR3-CT98-0297).

References (21)

  • R. Scheer et al.

    Thin Solid Films

    (1995)
  • K. Töpper et al.

    Solar Energy Mater

    (1997)
  • M. Contreras et al.

    Prog. Photovolt.

    (1999)
  • J. Klaer et al.

    Semicond. Sci. Technol.

    (1998)
  • V. Probst et al.

    Mater Res. Soc. Symp. Proc.

    (1996)
  • B.M. Basol, V.K. Kapur, C.R. Leidholm, A. Minnick, A. Halani, proceedings of the First World Conference on Photovoltaic...
  • M. Bodegard, L. Stolt, J. Hedström, 12th European Photovoltaic Solar Energy Conference, Amsterdam,...
  • J.H. Scofield, S. Asher, D. Albin, J. Tuttle, M. Contreras, D. Niles, R. Reedy, A. Tennant, R. Noufi, proceedings of...
  • S.H. Wei et al.

    J. Appl. Phys.

    (1999)
  • T. Watanabe et al.

    Jpn. J. Appl. Phys.

    (1998)
There are more references available in the full text version of this article.

Cited by (0)

View full text