Miele, Lorenzo (2024) Micro and nano structuring of soft composite film for optoelctronics. [Tesi di dottorato]

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Abstract

This thesis centers on the study and the design of novel approaches for the structuring of polymeric composite films on the micro and nano scales fabricated through phase separation processes. We investigated the mechanisms that governs the morphology build up of a dispersed phase within a polymeric matrix enhancing the scientific comprehension and thus developing novel processing tools. Two systems were analyzed, namely polymeric porous membranes, and a tin-based perovskite-polymer composite. \\ The gas foaming technique is a facile and scalable method for production of pores/bubbles in a polymeric matrix. We considered different strategies to tailor the bubbles morphology exploiting the effect of process parameters, in particular the blowing agent and pressure. The use of a blowing agent in its liquid state has been explored, exploiting its transition to the gaseous phase to impose a non-trivial pressure drop reduction. Considering the effect of pressure, we propose the use of non-trivial pressure treatment, named \textit{inverse pressure quench}, designed to manipulate the bubble dynamics. In particular, by releasing the pressure to an intermediate pressure, it is possible to reduce the bubble growth and prevent impingement, thus gaining a tool for studying phenomena involved in polymer foaming and, more generally, in polymer physics. We studied the model PP/N\textsubscript{2} system, and observed bubble growth, Ostwald ripening of close bubbles, bubble interaction causing shape change (from spherical to polyhedral and \textit{vice versa}) up to coalescence, various bubble growth regimes, and bubble relative position variation due to periodic pressure histories. Our method deepen the study of fundamental processes involved in foaming and their application in structuring polymeric materials.\\ Moving towards the applications, this study investigates the use of physical foaming techniques for the fabrication of porous polymeric membranes with nanometric thickness from poly(lactic-co-glycolic) acid (PLGA) thin films. Employing a gas-impermeable barrier layer, we successfully impeded gas diffusion out from the sample, forcing the cellular structure formation. The results highlight the impact of nanoscale confinement on morphology and the procurance of early coalescence. Thus, we propose the so called \textit{Coarsening Driven Gas Foaming} technique, which consists in controlling the cells coarsening as a tool to produce larger, open cells, without compromising film stability.\\ The thesis then extends its attention to tin halide perovskite films for solar cell application, emphasizing the need for morphological control of the crystals structure to enhance optoelectronic quality and long-term stability. We showed that the addition of a pyridine-based polymeric additive can slow down the crystallization process, leading to the formation of crystals encapsulated in a polymeric matrix, namely a perovskite-polymer composite film. Finally, the combination of the pyridine-based polymer with the corresponding small molecule enabled adjustment of the crystallization process, while keeping the polymer content minimal, facilitating electrical transport between the perovskite grains and proving effective in enhancing the power conversion efficiency of the solar cell perovskite devices.\\ This research deepens the analysis of mechanisms driving morphology build up during phase separation process. By leveraging the study and engineering of different systems, resulting in innovative processing tools that open new avenues for fabrication of structured materials.

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