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, CuSbS<sub>2</sub> (Chalcostibite), Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub> (Tetrahedrite), Cu<sub>3</sub>SbS<sub>3</sub> (Skinnerite), and Cu<sub>3</sub>SbS<sub>4</sub> (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 10<sup>5</sup> cm<sup>–1</sup>. We have for the first time developed facile colloidal hot-injection methods for the phase-pure synthesis of nanocrystals of all four phases. Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub> and Cu<sub>3</sub>SbS<sub>3</sub> 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 CuSbS<sub>2</sub> and Cu<sub>3</sub>SbS<sub>4</sub>, respectively). The synthesis methods yield nanocrystals with distinct morphology for the different phases. CuSbS<sub>2</sub> is synthesized as nanoplates, and Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub> is isolated as hollow structures, while uniform spherical Cu<sub>3</sub>SbS<sub>3</sub> and oblate spheroid nanocrystals of Cu<sub>3</sub>SbS<sub>4</sub> 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 CuSbS<sub>2</sub> and Cu<sub>3</sub>SbS<sub>4</sub> are indirect band gap materials but with somewhat higher band gap values of 1.6 and 2.5 eV, respectively. Similarly, Cu<sub>3</sub>SbS<sub>3</sub> 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 Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub> 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 10<sup>4</sup> and 10<sup>5</sup> cm<sup>–1</sup>, confirming their potential for solar energy conversion applications.