(576b) Reaction Mechanism for the Formation of CuInSe2 Nanocrystals for Low-Cost Solar Cells

The development of suitable colloidal nanocrystal inks are a key step in the development of low-cost solar cells since they enable the use of fast and inexpensive coating processes such as spray coating and roll coating to form a thin film photo-absorbing layer. Guo et al. recently reported the solution-phase synthesis of chalcopyrite structured CuInSe2 (CISe) nanocrystals from elemental and halide reagents via a simple batch reaction in a single component coordinating solvent [1]. The use of these nanocrystals for solar cells has been demonstrated by fabricating devices, with the first solar cells having an efficiency of 2.8%. Insights into the reaction mechanism for the formation of the nanocrystals are a key step to gain a better control over the electronic properties of the ternary semiconductor. A reaction mechanism for the formation of CISe nanocrystals is proposed using powder x-ray diffraction, electron microscopy, compositional analysis, FT-IR spectroscopy, mass spectrometry and thermodynamic calculations. The formation of CISe nanocrystals from metal-chloride precursors and elemental selenium in oleylamine is preceded by the formation of copper selenide (CuSe) and indium selenide (InSe) as the solid-phase intermediates. And the reaction takes place via the chlorination of the methylene groups in oleylamine with hydrochloric acid being the other liquid-phase by-product. However, it has been observed that the reaction of CuSe with indium chloride (InCl3) dissolved in oleylamine leads to the formation of the ordered chalcopyrite structure of CISe. In support of the proposed reaction mechanism and contrary to previous reports in the literature, chalcopyrite CISe nanocrystals have also been synthesized in solvents without any metal-coordinating groups.

References:

1. Guo, Q., S.J. Kim, M. Kar, W.N. Shafarman, R.W. Birkmire, E.A. Stach, R. Agrawal, and H.W. Hillhouse, Development of CuInSe2 nanocrystal and nanoring inks for low-cost solar cells. Nano Letters, 2008. 8(9): p. 2982-2987.