CuGaSe₂ and CuIn_xGa_1–xSe₂ Nanocrystals with Sphalerite or Wurtzite Phase for Optoelectronic Applications

There has been significant interest in I–III–VI2 nanocrystals for photonic and optoelectronic applications, especially solar cells. CuInSe2 nanocrystals have been studied extensively as model materials for this class of compounds. By incorporation of Ga to make CuInxGa1–xSe2 (CIGSe), the optical gap can be tuned by composition as well as size for higher efficiency solar cells or other applications. The synthesis of CIGSe nanocrystals, however, has not been studied in much detail. It turns out that the addition of Ga to the typical arrested precipitation reactions for CuInSe2 nanocrystals in oleylamine (OLAm) leads to very slow particle nucleation and growth rates. In order to achieve consistent morphology, reaction yield, and Ga incorporation, a lengthy (∼24 h) low temperature incubation step is needed. Under these slow growth conditions, the crystal structure of the CIGSe nanocrystals is cubic. By addition of diphenylphosphine (DPP) to the reactions, the nucleation and growth rates are significantly increased; however, this leads to CIGSe (and CuGaSe2) nanocrystals with wurtzite phase. In contrast, CuInSe2 nanocrystals made under similar fast-growth conditions are always cubic.