https://scholarworks.unist.ac.kr/handle/201301/120 2026-05-13T00:26:48Z https://scholarworks.unist.ac.kr/handle/201301/91558 Title: Controlling Crystallization of Perovskite Layers and Understanding the Effects of By-products Author(s): Park, Jaewang Abstract: Perovskite solar cells (PSCs) have arisen as a game changer in photovoltaic technology due to their outstanding optoelectronic properties and low-cost processability. However, realizing both high efficiency and long-term stability remains a major challenge, primarily due to uncontrolled crystallization and unstable phase transitions of perovskite layers. My doctoral research has focused on controlling the crystallization kinetics and phase stability to achieve highly crystalline and stable perovskite thin films. Through various strategies based on organic cations and halides, the growth of perovskite layers were effectively regulated and systematically investigated. In Chapter 2, the incorporation of alkylammonium chlorides (RACl) into the formamidinium lead iodide (FAPbI3) precursor enabled precise control over crystallization kinetics, preferred orientation, and film morphology. In particular, propylammonium chloride (PACl) effectively facilitated the phase transition and yielded smooth, defect-minimized films, leading to a record power conversion efficiency of 25.73% in 2022, along with excellent operational stability. This work was published in Nature (Nature, 2023, 616, 724). In Chapter 3, a quasi-2D scaffolding approach was applied to Cs-rich wide-bandgap perovskites, improving film crystallinity and phase stability. The quasi-2D acted as an intermediate scaffold, resulting in high crystallinity, uniform surface morphology, while reducing defect density. This study was published in Small (Small, 2025, 21, 2500197). In Chapter 4, the spontaneous formation of formamidinium-based by-products, such as methylformamidinium iodide (MFAI) and propylformamidinium iodide (PFAI) were identified as key factors influencing the phase transition and stability of FAPbI3. The characteristics of the by-products are governed by the size of the cations, which in turn affects their decomposition kinetics and molecular configuration. Compared with MFAI, PFAI promotes a more favorable phase transition and enhances the phase stability of FAPbI3. Consequently, the adoption of PACl over MACl leads to improved long-term stability of perovskite solar cells, owing to the decreased residual MA+ and MFA+, and increased retention of PFAI. This research is still ongoing state, and the manuscripts are being prepared. Overall, my doctoral research work provides fundamental insights into the crystallization and phase transition mechanisms of perovskites, offering effective design principles for achieving highly efficient and stable PSCs. Major: School of Energy and Chemical Engineering 2026-01-31T15:00:00Z https://scholarworks.unist.ac.kr/handle/201301/91553 Title: Interfacial Engineering for proton transfer pathways in electrochemical Low-voltage H2 storage Author(s): Lee, Jisu Abstract: The global pursuit of carbon neutrality has intensified the transition toward hydrogen as a clean and high-energy-density carrier for sustainable energy systems. This dissertation systematically explores the interfacial engineering principles governing hydrogen generation, storage, and conversion across electrochemical systems. The discussion begins with the fundamentals of conventional water electrolysis, highlighting their operational mechanisms, material designs, and limitations imposed by the sluggish oxygen evolution reaction (OER) and high cell voltages (> 1.8 V). To mitigate these kinetic and energetic barriers, low-energy electrochemical strategies are introduced, where anodic OER is replaced by the oxidation of small organic and inorganic molecules such as formic acid, ethanol, hydrazine, ammonia, and hydrogen peroxide. These reactions, characterized by lower redox potentials (−0.33 to 0.69 VRHE), enable hydrogen production at substantially reduced voltages. Building upon efficient hydrogen generation, the work extends to hydrogen storage coupling with electrochemical systems. Traditional thermochemical hydrogenation of liquid organic hydrogen carriers (LOHCs) such as the toluene/methylcyclohexane pair requires high-temperature and high- pressure operation. In contrast, the electrochemical palladium membrane reactor (ePMR) offers low- voltage alternative by directly coupling water electrolysis with in-situ hydrogen absorption, diffusion, and spillover across a palladium membrane. This electrochemical approach enables controllable hydrogenation under ambient conditions, bridging hydrogen production and storage toward sustainable, distributed LOHC-based hydrogen systems. In the conversion domain, ammonia emerges as a key carbon-free energy carrier. The sesction compares three promising electrochemical routes such as lithium-mediated nitrogen reduction (Li- NRR), direct N₂ reduction (NRR), and nitrate electroreduction (NO₃RR). Among them, NO₃RR offers a particularly attractive pathway due to its lower activation barriers and high selectivity for ammonia formation under mild conditions. The mechanistic distinction between the electron-transfer route and the hydrogen-atom-mediated route highlights how surface H+/H* relay through co-adsorbed oxoanions governs the selectivity toward NH₃ formation. Finally, the dissertation integrates these concepts through interfacial engineering. The strategic control of molecular orientation and proton dynamics at solid–liquid interfaces. On the anodic side, orientation control through ligand and steric effects tunes adsorption geometry and reactivity of bulky molecular substrates. On the cathodic side, cooperative electronic and electrolyte effects modulate interfacial H⁺/H* behavior, governing proton-coupled electron transfer (PCET) and hydrogen spillover processes. Collectively, these principles establish a unified framework for low-voltage hydrogen production, liquid-phase storage, and selective nitrogen conversion, paving the way for next-generation electrocatalytic energy systems that integrate. Major: School of Energy and Chemical Engineering 2026-01-31T15:00:00Z https://scholarworks.unist.ac.kr/handle/201301/91543 Title: Electrolyte-Driven Regulation of Oxygen Redox and Interfacial Chemistry in Layered Oxide Cathodes Author(s): Lee, Jeongin Abstract: The instability of oxygen redox chemistry in layered oxide cathodes fundamentally limits the achievable energy density and lifetime of lithium-based batteries. The degradation processes associated with oxygen evolution can be viewed from two complementary perspectives. From the cathode surface perspective, highly reactive oxidized oxygen species generated during charging decompose the electrolyte, inducing protonation of the surface and creating oxygen vacancies. These vacancies further drive the migration of bulk lattice oxygen (O²⁻) toward the surface, accelerating structural degradation. From the electrolyte perspective, oxygen released from the cathode exists as reactive oxygen species (ROS), such as singlet oxygen and superoxide radicals, which escape into the electrolyte and trigger secondary decomposition reactions. In this study, electrolyte-driven strategies were developed to regulate oxygen redox and mitigate such degradation processes from both perspectives. First, to deactivate oxygen radicals in the electrolyte, the working principles of proton-donating phenolic antioxidants were analyzed in Li-containing organic solvents, leading to the design of an organic superoxide dismutase mimic (SODm), guaiacol. Through associative binding of its hydroxyl and methoxy groups with LiO₂ radicals, guaiacol efficiently promoted lithium-assisted disproportionation, forming a thin polymeric cathode–electrolyte interphase (CEI) that suppressed electrolyte decomposition and enhanced interfacial stability. Second, beyond radical removal, oxygen evolution was further suppressed by introducing electrolyte components, anthracene, capable of anchoring oxidized oxygen species prior to oxygen dimerization, thereby preventing the formation of molecular oxygen. Third, a reduction-driven strategy was proposed to construct a stable inorganic-rich CEI that restrains surface oxygen release. Incorporation of divalent cations such as Mg²⁺ modulated the solvation structure of anions, elevating their reduction potential above 2 V vs. Li/Li⁺ and enabling controlled formation of LiF-rich CEIs through reduction of anion without inducing cathode overlithiation. Collectively, this work establishes an integrated framework for the electrolyte-driven regulation of oxygen redox and interfacial chemistry in layered oxide cathodes. By combining radical scavenging, oxygen anchoring, and reduction potential modulation, this dissertation provides fundamental insights and design principles for achieving structurally stable, long-life, high-energy lithium batteries. Major: School of Energy and Chemical Engineering 2026-01-31T15:00:00Z https://scholarworks.unist.ac.kr/handle/201301/91534 Title: Understanding and Controlling Intermediates for Stable and Efficient Perovskite Solar Cells Author(s): Kim, Jaehui Abstract: Metal halide perovskites have emerged as a next-generation photovoltaic material due to their remarkable optoelectronic properties and low-cost solution processability. However, despite achieving certified power conversion efficiencies exceeding 26%, the large-scale application of perovskite solar cells (PSCs) is still hindered by intrinsic instabilities associated with intermediates and the corresponding unwanted impurities generated during crystallization. These transient species govern nucleation, growth, and phase stability, ultimately determining device reproducibility and durability. The central objective of this work, entitled “Understanding and Controlling of Intermediates for Stable and Efficient Perovskite Solar Cells”, is to elucidate the formation mechanisms of these intermediates and impacts on final device to develop molecular design strategies that control them to achieve long- term stability and high efficiency. In the first part of this work, quasi‐2D scaffolding intermediates were introduced to stabilize 1.67 eV Cs‐rich pure‐iodide perovskites. By integrating low-dimensional phases within the 3D perovskite lattice, the formation of thermodynamically unstable δ-phase was effectively suppressed, while structural rigidity and carrier lifetime were simultaneously enhanced. This strategy demonstrated that controlled intermediate phases can act as beneficial scaffold for suppressing halide segregation, thus enabling more stable perovskite films suitable for tandem applications. In the second study, the role of cation heterogeneity and its influence on final devices were systematically explored. Through precise chemical tracking and spectroscopic analyses, it was revealed that the coexistence of formamidinium (FA⁺) and methylammonium (MA⁺) cations can induce the formation of a benign methylformamidinium (MFA⁺) species via a mild cation-exchange process. Rather than being detrimental, this MFA⁺ acts as a local structural buffer that suppresses δ-phase nucleation and reinforces the stability of the black α-perovskite phase. Moreover, the presence of MFA⁺ subtly modifies the local electrostatic environment and lattice symmetry, leading to reduced trap- assisted recombination and enhanced charge carrier mobility. This cooperative effect improves charge extraction at the perovskite/transporting layer and minimizes nonradiative losses during operation. Consequently, MFA+ not only plays a structural role in stabilizing the desired phase but also contributes electronically by promoting more efficient charge transport and longer carrier lifetimes. These findings highlight that selective cation-mediated interactions can be deliberately tuned to control crystallization pathways, passivate local defect states, and improve both structural and electronic properties of perovskite films. Finally, the third part of this thesis introduces a byproduct-free stabilization strategy employing a neutral amine additive, propylamine (PA). In contrast to conventional ammonium halide additives that trigger iodide oxidation and generate undesirable alkyl-formamidinium residues, PA establishes a dynamic and reversible hydrogen-bonded equilibrium with FA⁺ cations in the precursor solution. This molecular interaction effectively suppresses iodide oxidation in the liquid phase, thereby maintaining chemical integrity during solution aging. During the subsequent annealing process, the absence of chloride and the moderated FA⁺ reactivity prevents the formation of volatile or alkylated byproducts, leading to cleaner crystallization pathways. Consequently, the PA-assisted perovskite films exhibit markedly improved crystallinity, larger grain domains, and lower trap densities, which translate into enhanced charge transport, superior thermal robustness, and enhanced power conversion efficiency. This approach demonstrates that precise molecular control in the solution stage can dictate the solid-state quality of perovskite films, offering a chemically benign route toward highly stable and efficient solar cells. Collectively, these studies provide a unified chemical and mechanistic understanding of how intermediate chemistry governs perovskite crystallization and stability. The findings demonstrate that rational control of precursor interactions—whether through quasi-2D scaffolding, cation heterogeneity, or dynamic molecular buffering—can transform unstable solution chemistry into robust, high- performing materials. This work lays a foundation for the rational design of perovskite compositions and processing routes that decouple efficiency from instability, paving the way toward reliable, scalable photovoltaic technologies. Major: School of Energy and Chemical Engineering 2026-01-31T15:00:00Z