EXPLORATION OF COLLOIDAL NANOCRYSTALS FOR ESTABLISHED AND EMERGING SEMICONDUCTOR MATERIALS
Creators: name: Hayes, Daniel Christian
Publisher: name: Purdue University Graduate School
Year: 2024
Subjects: classificationCode: 400499
subject: Chemical engineering not elsewhere classified
classificationCode: 401603
subject: Compound semiconductors
classificationCode: 401605
subject: Functional materials
classificationCode: 401807
subject: Nanomaterials
classificationCode: 401810
subject: Nanoscale characterisation
classificationCode: 340301
subject: Inorganic materials (incl. nanomaterials)

description: For reliable, facile, and user-friendly, solution-based synthesis of materials, the colloidal nanocrystal route has proven to be the method of choice for so many. The tunability that this process renders its users---from choice of precursors, solvent systems, and reaction conditions including temperature, pressure, and precursor addition order---is truly second to none. In their simplest form, these nanomaterials are usually comprised of an inorganic core of the desired material and an outer layer of surface-stabilizing molecules called ligands. These ligands provide colloidal stability and allow for the solution-processing of these materials for downstream usage in devices such as light-emitting diodes and photovoltaics, for example. In this thesis, the study and use of colloidal nanomaterials of Cu(In,Ga)(S,Se)<sub>2</sub> (CIGSSe), IIA-IVB-S<sub>3</sub> (including BaZrS<sub>3</sub> and SrZrS<sub>3</sub>), alkaline earth polysulfides (IIAS<sub>x</sub>; IIA = Sr, Ba; x = 2, 3), and other materials like Cu<sub>2</sub>GeS<sub>3</sub> and Cu<sub>2</sub>BaSnS<sub>4</sub>, for studies into the formation, colloidal stability, and fabrication into solar cells was performed.More specifically, an experimental protocol was developed to fabricate high-quality CIGSSe nanoparticles with carbonaceous residues that are substantially reduced from traditional pathways. Traditional methods for synthesizing colloidal CIGS NPs often utilize heavy, long-chain organic species to serve as surface ligands which, during annealing in a Se/Ar atmosphere, leave behind an undesirable carbonaceous residue in the film. In an effort to minimize these residues, N-methyl-2-pyrrolidone (NMP) was used as an alternative surface ligand. Through the use of the NMP-based synthesis, a substantial reduction in the number of carbonaceous residues was observed in selenized films. Additionally, the fine-grain layer at the bottom of the film, a common observation of solution-processed films from organic media, was observed to exhibit a larger average grain size and increased chalcopyrite character over those of traditionally prepared films, presumably as a result of the reduced carbon content, allowing for superior growth. As a result, a gallium-free CuIn(S,Se)<sub>2</sub> device was shown to achieve power-conversion efficiencies of over 11% as well as possessing exceptional carrier generation capabilities with a short-circuit current density (J<sub>SC</sub>) of 41.6 mA/cm<sup>2</sup>, which is among the highest for the CIGSSe family of devices fabricated from solution-processed methods. It was shown that pre-selenized films of sulfide nanoparticles instead of selenide nanoparticles performed better as solar cells. While the exact mechanism is still under debate, it appears that the growth phase during selenization, which varies depending on the chalcogen present in the starting material plays an important role.The IIA-IVB-S<sub>3</sub> system is just beginning to emerge as a material system shown to be capable of solution-based synthesis methods. This is primarily due to the extremely high oxophilicity of the IVB elements, Ti, Zr, and Hf, necessitating that extreme care and judicial use of inert environments be used to synthesize these materials via solution-based methods. In the IIA-IVB-S<sub>3</sub> system exists some of the chalcogenide perovskites, including BaZrS<sub>3</sub>, which are expected to have similar electronic properties to the well-known, high-performing halide perovskites, albeit much more stable, making them attractive prospects as novel semiconductor materials for optoelectronic applications. This work builds upon recent studies to show a general synthesis protocol, involving the use of carbon disulfide insertion chemistry to generate highly reactive precursors, that can be used towards the colloidal synthesis of numerous nanomaterials in the IIA-IVB-S<sub>3</sub> system, including BaTiS<sub>3</sub>, BaZrS<sub>3</sub>, BaHfS<sub>3</sub>, α-SrZrS<sub>3</sub> and α-SrHfS<sub>3</sub>. Additionally, we establish a method to reliably control the formation of the BaZrS<sub>3</sub> perovskite, a complication seen in previous literature where BaZrS<sub>3</sub> appears to exist as two different phases when synthesized via colloidal methods. The utility of these nanomaterials is also assessed via the measurement of their absorption properties and in the form of highly stable colloidal inks for the fabrication of homogenous, crack-free thin films of BaZrS<sub>3</sub>. In addition to the chalcogenide perovskites, the IIA-S system was also explored to better understand the solution-based formation of these materials and how the control of IIA polysulfides can be achieved. We show that the synthesis of these materials is strongly correlated to the reaction temperature and that the length of the S<sub>n</sub><sup>2-</sup> oligomer chain is the dependent variable. We also report on the synthesis of a previously unreported polymorph of SrS<sub>2</sub> which appears to take on the <i>C2/c</i> space group, the same as BaS<sub>2</sub>.Finally, some discussion is also provided on the use of transmission electron microscopy (TEM) to analyze the crystal structure of materials. Some tips and techniques used throughout this thesis are summarized in this section.