Influence of Na on the properties of Cu-rich prepared CuInS2 thin films and the performance of corresponding CuInS2/CdS/ZnO 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)
- et al.
Thin Solid Films
(1995) - et al.
Solar Energy Mater
(1997) - et al.
Prog. Photovolt.
(1999) - et al.
Semicond. Sci. Technol.
(1998) - 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...
- et al.
J. Appl. Phys.
(1999) - et al.
Jpn. J. Appl. Phys.
(1998)