Soluble precursors for CuInSe ₂, CuIn _1-xGa _xSe ₂, and Cu ₂ZnSn(S,Se) ₄ based on colloidal nanocrystals and molecular metal chalcogenide surface ligands

We report a new platform for design of soluble precursors for CuInSe ₂ (CIS), Cu(In _1-xGa _x)Se ₂ (CIGS), and Cu ₂ZnSn(S,Se) ₄ (CZTS) phases for thin-film potovoltaics. To form these complex phases, we used colloidal nanocrystals (NCs) with metal chalcogenide complexes (MCCs) as surface ligands. The MCC ligands both provided colloidal stability and represented essential components of target phase. To obtain soluble precursors for CuInSe ₂, we used Cu _2-xSe NCs capped with In ₂Se ₄ ^2- MCC surface ligands or CuInSe ₂ NCs capped with {In ₂Cu ₂Se ₄S ₃} ^3- MCCs. A mixture of Cu _2-xSe and ZnS NCs, both capped with Sn ₂S ₆ ^4- or Sn ₂Se ₆ ^4- ligands was used for solution deposition of CZTS films. Upon thermal annealing, the inorganic ligands reacted with NC cores forming well-crystallized pure ternary and quaternary phases. Solution-processed CIS and CZTS films featured large grain size and high phase purity, confirming the prospects of this approach for practical applications.