Phosphine-Free Synthesis of p-Type Copper(I) Selenide Nanocrystals in Hot Coordinating Solvents

The fabrication of low-cost, efficient solar cells made of green materials is a main goal of energy-related research.1 Several copper- based materials, such as Cu2S, CuInS2, CuInSe2, CuInxGa1-xSe2, and Cu2ZnSnS4, have been explored to date in photovoltaics, mainly as thin films,2 but recently, colloidal nanocrystals of these materials have also been developed and used to make solar cells.3 Today "all-nanocrystal" or organic-inorganic nanocomposite films can be prepared over large areas using various deposition techniques. Also, copper selenide, a superionic conductor, has been studied in thin-film applications in photovoltaics, optical filters, and dry galvanic cells (as a solid electrolyte).1e,2c It can form in many stoichiometries (Cu2Se, Cu2-xSe, CuSe, Cu2Se3) and phases. Cop- per(I) selenide (Cu2Se, Cu2-xSe) crystallizes generally in the face- centered cubic berzelianite phase.4 Cu2-xSe has both a direct band gap of 2.2 eV and an indirect band gap of 1.4 eV (at the limit for solar cell applications) and shows p-type conductivity. Nanocrystals of Cu2-xSe have been prepared via various routes, including colloidal synthesis methods in hot surfactants.4 In contrast, CuSe has a hexagonal phase at room temperature, and it is often found as impurity in copper(I) selenide.5