title: In-situ Untersuchungen der Kritallisation von Bleihalogenid-Organischen Perowskit-Filmen aus Precursor-Tinten nach Abscheidung mittels meniskusunterstützter Beschichtungsverfahren
title: In-situ Studies on the Crystallization of Lead Halide-Organic Perovskite Films from Precursor Inks Deposited by Meniscus Guided Coating Techniques
Creators: name: Jang, DongJu
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2025

description: The fabrication technology of perovskite photovoltaic(PPV) devices has developed rapidly in the last decade. Incorporating new technologies, concepts, and background theories happen at a breath-taking speed which leads to rapidly rising efficiency records, as the underlying physics and technology of perovskite devices are similar to those of organic photovoltaic devices. However, the mainstream of PPV device research relies on an empirical approach that focuses on varying device fabrication procedures and evaluating the efficiencies of the final devices. While this research approach has undoubtedly contributed to achieving the current highly efficient devices, it often lacked a solid theoretical background or explanation for the improved performance instead relying more on empirical results. Research groups have improved the efficiency of small perovskite research devices but faced the challenges when scaling up to larger modules, due to downgraded crystal quality and thus decreasing device efficiency. Therefore, innovations in coating techniques and a better understanding of the crystallization processes are required for fabricating high-quality perovskite films on expanded substrates. This thesis focuses on the investigation of meniscus-assisted coating(MAC) of perovskite precursor inks by Doctor Blading, and explores different crystallization techniques in chemical and physical approaches. In particular, based on previous reports, volatile precursor solution coating, and gas quenching-assisted deposition were selected as reference recipes for enhancing the quality of coating and crystallization of perovskite films on larger areas. In contrast to conventional coating, volatile precursor coating does not require quenching processes to accelerate solvent evaporation. On the contrary, rapid solvent evaporation leads to macroscopic failures in the perovskite film. To solve this problem, I studied the relationship between meniscus shape on one hand and the evaporation rate of the precursor solution and coating speed on the other hand. A combined experimental and numerical approach provided the ideal conditions for fabricating perovskite films on substrates, emphasizing the importance of maintaining a constant precursor concentration at the meniscus during the coating process for fabricating uniform crystals. In the next step, chemical additives containing chlorine were introduced to enhance the quality of perovskite films formed from volatile precursors. In-situ measurements, including Photoluminescence (PL) and UV-vis absorption (UV-vis) spectroscopy, were equipped to monitor the entire crystallization process to explain the impact of these additives. Both PL and UV-vis were valuable in observing crystal growth and the perovskite ratio in the precursor film turning into perovskite crystal. These approaches allowed for a study on inhibiting localized and uncontrolled crystal growth while achieving uniform crystallization across the entire film. The introduction of chlorides during crystallization was found to create an intermediate state that effectively delayed the crystallization process through various pathways. Notably, these additives prevented localized crystallization but induced simultaneous crystallization at a specific concentration across the entire areas of the film, resulting in uniform grain sizes and improved crystallinity of the perovskite film. The gas quenching method, which utilizes the rapid flow of compressed gas to accelerate solvent evaporation, has been studied in pursuit of expanding the variety of available perovskite types and diversifying the fabrication process. This process effectively deposits perovskite films on substrates even when using precursors with slow evaporation rates. However, the physical and chemical properties of the precursor often caused irregularities in the crystalline film's morphology. Therefore, Solvent additives were introduced to address these issues of the gas-quenching process. These proposed solvent additives strongly interact with lead iodide, forming complex adducts. Thus, even after the domain solvent has evaporated, the precursors retain a stable gel-like intermediate state, which helps mitigating the unfavourable physical effects during the crystallization process. Carefully adjusting the concentration of the additives depending on the strength of the interaction between the solvent additives and lead iodide, effectively suppresses irregularities caused by the coalescence of perovskite materials. In conclusion, not only the coating physics via the MAC method but also the 2-D crystallization direction was properly understood based on different precursors and crystallization mechanisms. In addition, specific explanations were provided on how various additives, in the form of various intermediates, can enhance the perovskite crystals.