Synthesis of Novel (111) Facet Cubic Phase SnO2 Thin Films for Gas Sensing and Perovskite Solar Cell Applications

Abstract

The tetragonal phase is a well-known phase of SnO2, which shows high stability under ambient conditions. SnO2 has been used in many applications, such as transparent conductive electrodes (TCEs), gas sensors, photocatalysis, energy storage, photovoltaic, optoelectronic devices, etc. In contrast, the cubic phase SnO2 (C-SnO2) in bulk form exhibits stability under high-pressure conditions (≥17 GPa), whereas (111)-oriented C-SnO2 thin films demonstrate high stability under ambient conditions. The (111) oriented cubic phase, combined with the exceptional stability of C-SnO2 thin films, unlocks distinctive properties, such as a textured surface, a larger band gap, and higher surface polarity, making them highly promising for advanced applications. These characteristics position C-SnO2 thin films as ideal candidates for gas sensors and perovskite solar cells, driving innovation in sensing technologies and renewable energy systems. In Chapter #1, an overview of SnO2 polymorphs and their diverse applications has been discussed. This chapter begins with a detailed discussion on various types of gas sensors. We then briefly discussed metal oxide-based gas sensors, elucidating their working principles as chemical gas sensors. Following this, we discuss solar cells, focusing on perovskite solar cells. The discussion encompasses the properties of perovskite materials, highlighting the impact of cation doping on their performance. Further, we discussed the working principles, characteristics, and architectural designs of perovskite solar cells. In Chapter #2, we present the samples’ synthesis procedures, characterization, and analysis using various experimental techniques. This chapter also briefly discusses the fundamentals and working principles of characterization techniques, offering a comprehensive understanding of the methodologies employed throughout this thesis. In Chapter #3, we have investigated the synthesis of (111) oriented cubic phase SnO2 thin films using the standard Sol-Gel technique, followed by the spin coating method. Various solvents have been used to synthesize the cubic phase SnO2 thin films. From the XRD analysis, it was found that the formation of cubic SnO2 film depends on different parameters such as concentration of SnCl2.2H2O, stirring temperature, RPM, annealing temperature, etc. Additionally, the phase stability of C-SnO2 is confirmed by X-ray diffraction (XRD) across a temperature range of 0 °C to 500 °C. Further XPS analysis provided insights into the oxidation states at different depths, contributing to a deeper understanding of the material’s properties. In Chapter #4, the cubic SnO2 thin film has been optimized for gas sensing properties, and corresponding characterizations have been employed to understand the surface reactivity of gas. Thin films of C-SnO2 demonstrate exceptional gas sensing capabilities, exhibiting high selectivity toward carbon monoxide (CO) at ambient temperature. Density Functional Theory (DFT) simulations elucidate a reduced energy barrier for CO adsorption and desorption in the cubic phase relative to the conventional tetragonal phase, underscoring its potential for practical sensor applications. In Chapter #5, we have introduced the use of (111) facet-engineered cubic phase SnO2 as an electron transport layer (ETL) in triple-cation mixed-halide PSCs. A comprehensive analysis of the tetragonal and cubic phases of the SnO2 electron transport layer has been employed. It was found that the cubic phase of SnO2 significantly enhances charge transfer dynamics and extraction efficiency, resulting in perovskite solar cells (PSCs) achieving a power conversion efficiency (PCE) of 20.34%. Furthermore, these PSCs maintain over 81% of their initial performance after 480 hours of stability testing. In Chapter #6, the preparation method of Ag-Cu-Zn alloy electrodes for perovskite solar cells has been discussed, followed by their characterization. Phase transition engineering of silver (Ag) electrodes through alloying with copper (Cu) and zinc (Zn) has been explored, facilitating a cubic-to-tetragonal phase shift. Degradation analysis over time revealed that incorporating Cu into Ag metal enhances stability, while incorporating Zn improves the resistance to corrosion. Optimized Ag-Cu-Zn electrodes improve perovskite solar cell performance, achieving a PCE of 19.02%, and maintaining operational integrity for 460 hours for the PSCs stored in N2 glove box. Chapter #7 provides a summary of the research work presented in this thesis and outlines potential directions for future research. This research advances the understanding of cubic phase SnO2 and its multifaceted applications, offering novel pathways for developing high-performance gas sensors and stable, efficient PSCs.