Formation Pathway of CuInSe2 Nanocrystals for Solar Cells

Copper, indium, and gallium chalcogenide nanocrystals (binary, ternary, and quaternary) have been used to fabricate high-efficiency thin-film solar cells. These solution-based methods are being scaled-up and may serve as the basis for the next generation of low-cost solar cells. However, the formation pathway to reach stoichiometric ternary CuInSe2 or any chalcopyrite phase ternary or quaternary nanocrystal in the system has not been investigated but may be of significant importance to improving nanocrystal growth and discovering new methods of synthesis. Here, we present the results of X-ray diffraction, electron microscopy, compositional analysis, IR absorption, and mass spectrometry that reveal insights into the formation pathway of CuInSe2 nanocrystals. Starting with CuCl, InCl3, and elemental Se all dissolved in oleylamine, the overall reaction that yields CuInSe2 involves the chlorination of the hydrocarbon groups of the solvent. Further, we show that the amine and alkene functional groups in oleylamine are not necessary for the formation of CuInSe2 nanocrystals by conducting successful syntheses in 1-octadecene and octadecane. Hence, the role of oleylamine is not limited to nanocrystal size and morphology control; it also acts as a reactant in the formation pathway. Typically, the formation of copper selenide (CuSe) and indium selenide (InSe) nanocrystals precedes the formation of CuInSe2 nanocrystals in oleylamine. But it was also found that Cu2-xSe (0 < x < 0.5) and In2Se3 were the primary intermediates involved in the formation of CISe in a purely non-coordinating solvent such as 1-octadecene, which points to the surface-stabilization effect of the coordinating solvent on the less thermodynamically stable indium selenide (InSe) nanocrystals. We also show that the yield of the chalcopyrite phase of CuInSe2 (as opposed to the sphalerite phase) can be increased by reacting CuSe nanocrystals with InCl3.