Investigation of doped barium stannate nanocrystals for transparent electrode applications
Creators: name: Shepherd, William
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340303
subject: Nanochemistry
classificationCode: 340305
subject: Physical properties of materials
classificationCode: 401805
subject: Nanofabrication, growth and self assembly

description: The global warming climate emergency driven by greenhouse gas emissions is undoubtably one of the predominant existential crises that faces humanity today. As a result, governments and global industry leaders are now invoking a transition to renewable energy production as a means to reduce dependence on fossil fuels in order to mitigate future greenhouse gas emissions. Along with widespread rollout of renewable energy systems such as solar and wind power plants, a major part of the transition involves electrification of the energy grid. This means a significant portion of the consumer market that traditionally operated on fossil fuels, such as internal combustion engine vehicles and gas stove tops (to name a few examples), are to be replaced with electric alternatives. Consequently, energy in the form of electricity, will need to supplement the deficit that abandoning fossil fuels will leave behind and so the demand for renewably generated electricity is forecast only to increase in demand and in urgency. Of the renewable technologies available, solar photovoltaics and wind turbines provide the cheapest sources of electricity generation with prices of ~$68 and ~$53 per MWh (as of 2019).

Another global trend in consumer markets is a constant demand for smarter, more efficient and cheaper electronic devices such as televisions, computer monitors, smartphones and tablets. This demand is only every increasing in accordance with Moore’s law. Devices such as the aforementioned and including photovoltaic solar cells, fall into a category of electronics known as optoelectronics. Optoelectronics are devices that convert light, usually in the visible spectrum, to electricity (in the case of solar panels) or vice versa (display based devices, phones televisions etc) as part of their function. Optoelectronics do so by letting light in or out of the device via the use of a common piece of componentry known as the transparent electrode.

The transparent electrode is ubiquitous in all optoelectronics and subsequently one can see that an increase in demand for more solar power and better smart devices directly results in increased demand for more transparent electrodes. The current material used for manufacturing the transparent electrode is a semiconducting metal oxide known as indium tin oxide (ITO), more formally known as tin doped indium oxide. ITO is typically manufactured with compositions of approximately 74% In, 18% Sn, and 8% O by weight. With indium being one of the rarest elements to exist within the earth’s crust, making up only 2.5×10−5 % [1] it start to become obvious that indium supply will not reach the increasing demand for transparent electrodes at a cost efficient basis. As a result, a new material with higher elemental abundance and similar (if not better) performance needs to be developed. Barium stannate poses itself as such a material.

Barium stannate (BaSnO3, BSO) is a type of perovskite metal oxide that has recently garnered attention as a substitute for ITO in optoelectronic devices. In addition to higher elemental abundance of its components, BSO offers several advantages. Firstly, it possesses a wide band gap (~3.1 eV) in the near ultraviolet (UV) range, ensuring transparency to visible and near-infrared (NIR) wavelengths. Additionally, its electron mobility is exceptionally high, with experimental demonstrations reaching up to 320 cm2V-1s-1. This characteristic makes it desirable for improved electrical conductivity, particularly when BSO is doped with aliovalent cations like lanthanum. 

Therefore, it is the goal of this thesis to improve the understanding of the material in terms of wet chemical synthesis, the effects of aliovalent doping, manufacturing of stable colloidal BSO nanoparticle suspensions and finally low cost production methods for BSO nanoparticle transparent electrode development. As a result, the experimental work of this thesis was split into three chapters. 

The initial project focuses on the synthesis of BSO nanoparticles. Within this chapter, a comprehensive study is conducted by manipulating the parameters of an aqueous-based synthesis method for BSO. The primary objective was to gain further insights, including the innovative utilization of acid-mediated control to achieve precise stoichiometric composition control, encompassing the synthesis of off-stoichiometric BSO. Moreover, the chapter explores novel research involving La doping and the thermal activation of these dopants. A thorough investigation into the aqueous synthesis of BaSnO3 nanoparticles is presented, specifically addressing various issues of irreproducibility that have not been observed previously. By resolving these challenges, a robust and reproducible method to synthesize BSO nanoparticles with adjustable stoichiometry and controllable levels of doping is provided.

The second project investigates the impacts of co-doping at both metal sites within the BSO structure. The study examines the effects of dopant strain, specifically through single doping of the Ba and Sn sites, as well as simultaneous co-doping of both sites using La and Sb, respectively. The project focuses on assessing the influence of these doping combinations on crystallography, morphology, and optical properties of BSO powders. The validity of doping is confirmed through EDX, XPS, and absorption spectroscopy, which cover the visible and near-infrared range. By employing high-resolution XRD and TEM, I evaluate how doping affects crystal size, strain, and the plasmonic properties of BSO nanoparticles at varying dopant concentrations and annealing temperatures. These findings indicate that co-doping across both sites reduces lattice strain and the occurrence of defects, leading to larger particles overall. Through a systematic investigation, design guidelines for synthesizing high-quality BSO nanomaterials with a range of doping profiles are established.

Chapter 5 revolves around the conversion of BSO nanoparticles into a transparent electrode. The initial segment of the chapter focuses on achieving a stable colloidal suspension of BSO by addressing crucial aspects of nanoparticle synthesis. This includes optimizing the particle washing procedure, determining the appropriate amount of surfactant (citric acid), and evaluating the stability of different solvent-particle combinations. Through qualitative observations and dynamic light scattering (DLS), a highly concentrated and shelf-stable colloidal suspension was successfully developed, utilizing tetraethylene glycol as the preferred solvent. The second part of the chapter investigates the deposition of the nanoparticle ink using spray coating and spin coating methods. Furthermore, post-deposition treatments such as annealing environments are explored. The films produced were characterized using profilometry to assess surface morphology, X-ray diffraction to analyse crystallography, UV-Vis-NIR spectroscopy to examine optical properties, and four-point probe measurements to evaluate electrical properties. It was found that, in the case of our colloidal suspensions, spin coating followed by a post-annealing treatment in a forming gas (4% H2/Ar) environment yielded the best film performance.