Ponti, Corinna (2024) Multiple deposition approaches of tin halide perovskite for solar cells. [Tesi di dottorato]

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Abstract

Climate change is the defining issue of our time, and we are at a critical juncture to ensure our well-being sea levels, and numerous other environmental and socio-economic impacts. Mitigating the causes of climate change is a global imperative, and one of the most promising solutions lies in adopting renewable energy technologies to reduce and gradually phase out fossil fuels. According to the International Energy Agency (IEA), photovoltaic (PV) technology is one of the fastest-growing renewable energy sources. It is on track to become on planet Earth for the forthcoming decades. As the levels of greenhouse gases such as carbon dioxide continue to rise, the planet faces more frequent and severe extreme weather events, rising the world's largest energy source by the end of this decade. In 2023, solar PV alone accounted for three-quarters of renewable capacity additions worldwide. This technology offers multiple advantages from both an environmental and social sustainability perspective: solar energy can be harnessed locally without massive investments in distribution networks, making it particularly suitable for remote areas with limited infrastructure. Moreover, regions such as Sub-Saharan Africa, parts of South Asia, and Latin America receive some of the highest solar irradiance levels globally, meaning that developing countries have a high potential for solar energy production. Research is innovating the PV sector to develop inexpensive and sustainable solar energy. From a practical point of view, this means finding solutions to produce semiconductor materials from abundant raw materials that can deliver solar cells with higher efficiencies. In this context, metal halide perovskite solar cells (PSC) have attracted tremendous interest from many researchers due to their excellent optoelectronic properties, low cost, and versatility. Substantially thinner perovskite layers than conventional silicon (a fraction of micrometer versus 160 micron) can provide solar cells with outstanding photon-to-current efficiency (PCE) of over 26%. Additionally, perovskite materials can be tuned to absorb different wavelengths of light, potentially allowing for the development of multi-junction solar cells that can achieve even higher efficiencies. While perovskite solar cells offer many advantages, one significant challenge associated with their widespread adoption is the presence of lead in their composition. Lead is a toxic heavy metal that poses significant environmental and health risks. If not properly managed, lead from photovoltaic systems can leach into the environment, contaminating soil and water and posing a threat to wildlife and human health. To address these concerns, researchers are actively developing lead-free perovskite materials to align the progress in this technology with a more holistic concept of sustainable energy sources. This approach also aligns more with the 'Safe and Sustainable by Design' (SSbD) concept and the 'Chemical Strategy for Sustainability' within the European Green Deal. These initiatives aim to phase out the most harmful substances without increasing the environmental impact of technology in other life cycle steps. One alternative that has garnered significant attention is using tin-based perovskite materials. Tin (Sn) perovskites are a promising substitute for lead-based perovskites due to their electronic properties and lower toxicity. However, perovskites come with their challenges, particularly regarding stability and processability. Tin is more prone to oxidation than lead, which can result in the degradation of the perovskite material and a consequent drop in the performance of the solar cells. To improve the stability and efficiency of tin perovskites, researchers are exploring various strategies, such as incorporating additives that can inhibit oxidation, using protective coatings to prevent interactions with the surrounding environment and tailoring processing methods and parameters. This thesis is devoted to the optimisation and better understanding of less-explored deposition methods for tin-based perovskite. Particular attention is given to the correlation between the deposition parameters/techniques explored and the microstructural and optoelectronic properties of the obtained perovskite materials, both at the thin film and complete device levels. Since researchers have demonstrated that the oxidation of tin can also occur in the absence of oxygen due to the oxidising nature of some solvents (particularly dimethylsulfoxide, DMSO) used for perovskite deposition, this work explores the use of alternative solvent systems and solvent-free deposition methods. The thesis is structured as follows: Chapter 1 – 'Fundamentals and State of the Art' – briefly introduces the reader to the working principle of a solar cell and the mechanisms that cause performance losses. The main peculiarities of perovskite materials are introduced, along with a short overview of the advances that have allowed rapid progress in this specific PV technology. The focus then shifts to tin-based PSCs, highlighting their main advantages and challenges and distinguishing between organic and inorganic compositions. The state of the art regarding the role of solvents in the oxidation of tin perovskites is summarised, along with our group's first report on DMSO-free deposition of Sn PSCs. Finally, an introduction to solvent-free thermal evaporation and the current status of its application to lead-free perovskite are provided. Chapter 2 – 'Reasons to Go Lead-Free'—reinforces the motivation of this dissertation's general topic. It presents an overview of the historical issues created by adopting lead-containing technologies and a perspective on lead-based PSCs. Chapter 3 – 'Methods' – introduces the experimental methodologies employed in this study to synthesise and characterise the perovskites and the PV devices. Chapter 4 – 'Optimising FASnI3 deposited from DMSO-Free Solvent System on Water-Free PEDOT' – reports the approach and the results of the first experimental project of this PhD: an alternative DMSO-free solvent system for the deposition of organic FASnI3 is combined with a water-free selective contact to exclude the presence of any trace of oxidising species in the processing phase. A step-by-step enhancement of the macroscopic and microscopic properties of the perovskite thin film is achieved, corresponding to a gradual improvement of the photovoltaic performances in inverted p-i-n solar cells up to a PCE of 7%. Chapter 5 – ‘Exploring the role of process parameter on thermally evaporated CsSnI3 thin films ‘– is dedicated to inorganic and lead-free CsSnI3 materials. Given the difficulties in depositing this material from a DMSO-free solvent system, thermal evaporation is employed. Specifically, the work compares two sequential deposition protocols (double layer and multilayers) to evaluate their effect on the thin film crystallinity, morphology, stability, and electric properties. Preliminary attempts at integrating the material in inverted p-i-n devices are also discussed. The results show that without changing any other parameters, the deposition protocol adopted dictates substantially differences in the properties of the final material, creating some trade-off between the two: the grain dimensions, photoluminescence and mobility are better in the double layer case, while multilayer show a more uniform grain size distribution, flat surface and no sign of unreacted precursors. The method has no influence on the doping density, which is in the 1019 range, a too high value for giving working solar cells. Introducing SnF2 there is a photovoltaic response, which in preliminary tests results higher in the multilayer case. Further research is needed to elucidate the role of the deposition method adopted in the full devices.

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