Discovery & Design of Complex Chalcogenide Semiconductors for Optical & Energy Conversion Applications

Abstract

Multinary chalcogenide semiconductors have long been a mainstay within optoelectronics industries. Chalcogenide materials consisting of at least four elements—i.e., quaternaries—have tunable structural, optical, and electronic properties, allowing the semiconductors to be tailored for specific applications. Recently, the I2-II-IV-X4 (I = Li, Cu, Ag; II = Ba, Sr, Pb, Eu; IV = Si, Ge, Sn; X = S, Se) family of materials has emerged as a source of promising semiconductors with applications in nonlinear optics and optoelectronics. A major challenge for these complex compounds is maintaining the ability to predictably control the desired properties. In this dissertation, solid-state chemistry methods are used to tackle three major goals: to investigate known I2-II-IV-X4-type compounds that have not been thoroughly explored; to develop predictable property trends within the wider family of materials; and to predict and make brand new semiconductors. The study of the quaternary semiconductor Cu2BaGeSe4 and the mixing of Ge and Sn within this compound (to make Cu2BaGe1-xSnxSe4) serves as the platform for branching into new compounds in the I2-II-IV-X4 family. Using the structural analysis established in the work on Cu2BaGe1-xSnxSe4, a structural tolerance factor is developed to predict the probable crystal structure of hypothetical compounds that fit into the family of the I2-II-IV-X4-type materials. Four new semiconductors (Cu2PbGeS4, Cu2SrSiS4, Ag2SrSiS4, and Ag2SrGeS4) were made and found to conform to the anticipated crystal structures based on the structural tolerance factor. The newly synthesized Cu2SrSiS4, Ag2SrSiS4, and Ag2SrGeS4 are potential nonlinear optical materials, while each of the four semiconductors may be used as buffer or n-type layers in thin film solar cells. In the pursuit of new I2-II-IV-X4-type compounds, a family of cubic compounds is found to either co-exist and compete with the synthesized materials (Ag2Sr3Si2S8 & Ag2Sr3Ge2S8) or to be more stable than the hypothetical I2-II-IV-X4 materials (Ag2Pb3Si2S8 & Ag2Sr3Sn2S8) with the same elemental makeup. Of these four cubic semiconductors, Ag2Sr3Si2S8 and Ag2Pb3Si2S8 have not been reported by others. The compounds within this family have predictable trends of optical properties and have applications as nonlinear optical materials if large single crystals can be synthesized.Finally, the solvothermal synthesis and properties of Ag2(NH4)AsS4 are explored, extending the scope of this dissertation from the I2-II-IV-X4 materials to those of the form Ag2-I’-V-X4 (I’ = NH4, K, Rb, Cs; V = As, Sb, Nb, Ta, V, P). Similar to the other studied Ag-based materials, Ag2(NH4)AsS4 has applications as a nonlinear optical material and as a buffer layer in solar cells. Understanding the Ag2(NH4)AsS4 synthesis technique allows future researchers to synthesize new Ag2-I’-V-X4-type semiconductors with the same methods or apply these principles to the fabrication of Ag2(NH4)AsS4 thin films. The work presented in this dissertation furthers the understanding of the synthesis and prediction of quaternary chalcogenide semiconductors and lays the foundation for future device and thin film studies using the semiconductors studied here.