Selective Nanocrystal Synthesis and Calculated Electronic Structure of All Four Phases of Copper–Antimony–Sulfide

A wide variety of copper-based semiconducting chalcogenides have been investigated in recent years to address the need for sustainable solar cell materials. An attractive class of materials consisting of nontoxic and earth abundant elements is the copper–antimony–sulfides. The copper–antimony–sulfide system consists of four major phases, namely, CuSbS₂ (Chalcostibite), Cu₁₂Sb₄S₁₃ (Tetrahedrite), Cu₃SbS₃ (Skinnerite), and Cu₃SbS₄ (Fematinite). All four phases are p-type semiconductors having energy band gaps between 0.5 and 2 eV, with reported large absorption coefficient values over 10⁵ cm^–1. We have for the first time developed facile colloidal hot-injection methods for the phase-pure synthesis of nanocrystals of all four phases. Cu₁₂Sb₄S₁₃ and Cu₃SbS₃ are found to have direct band gaps (1.6 and 1.4 eV, respectively), while the other two phases display indirect band gaps (1.1 and 1.2 eV for CuSbS₂ and Cu₃SbS₄, respectively). The synthesis methods yield nanocrystals with distinct morphology for the different phases. CuSbS₂ is synthesized as nanoplates, and Cu₁₂Sb₄S₁₃ is isolated as hollow structures, while uniform spherical Cu₃SbS₃ and oblate spheroid nanocrystals of Cu₃SbS₄ are obtained. In order to understand the optical and electrical properties, we have calculated the electronic structures of all four phases using the hybrid functional method (HSE 06) and PBE generalized gradient approximation to density functional theory. Consistent with experimental results, the calculations indicate that CuSbS₂ and Cu₃SbS₄ are indirect band gap materials but with somewhat higher band gap values of 1.6 and 2.5 eV, respectively. Similarly, Cu₃SbS₃ is determined to be a direct band gap material with a gap of 1.5 eV. Interestingly, both PBE and HSE06 methods predict metallic behavior in fully stoichiometric Cu₁₂Sb₄S₁₃ phase, with opening up of bands leading to semiconducting or insulating behavior for off-stoichiometric compositions with a varying number of valence electrons. The absorption coefficient values at visible wavelengths for all the phases are estimated to range between 10⁴ and 10⁵ cm^–1, confirming their potential for solar energy conversion applications.