High-throughput exploration of multinary perovskite compositions for solar cell applications




Moradi, Shahram

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A photovoltaic (PV) device or a solar cell converts sunlight directly to electricity. Mechanistically, a solar cell is a sandwich of multiple material layers. The interaction of photons with the absorber layer in the middle of the sandwich leads to the creation of excited electrons; a neighboring layer then extracts these electrons, and this unidirectional transport of electrons is by definition an electrical current. Silicon is the most commonly used material in commercial solar cells. But thin film solar cells generally cost less and are easier to manufacture than silicon. Thin film solar cells are made by coating a thin layer of a highly light-absorptive semiconductor material on a substrate. Typically, the appearance of thin film layer semiconductor is dark which enables absorption of most of the visible spectrum. Recent advancements in PV technologies have greatly improved their light-to-electricity conversion efficiency and led to new solar materials that can be competitive alternatives to commercialized silicon technologies. One such promising material is halide perovskite. Perovskite solar cells (PSCs) can be printed, coated, or vacuum-deposited on a substrate. They are typically easy to fabricate and reach power conversion efficiencies (PCE) of higher than 20%. In the short period, PSCs have demonstrated a fast growth of PCE from 3% in 2009 to over 25% in 2019. However, to be commercially feasible, PSCs have to become stable and durable enough to operate for at least 20 years under outdoor conditions (ISOS protocol)[1–6]. Therefore, the research community is focusing on the stability, as well as scalability of PSCs. Strategies for the fabrication of thin films are constantly developed and have significantly benefited from the advent of high-throughput synthesis (HTS) and exploration methods. HTS methods allow systematic preparation of combinatorial composition libraries to discover targeted properties and functional materials. The HTS strategy provides a platform to combine various ratios of materials by preparing a massive number of samples and screening optoelectronic properties of the prepared library to find the optimized combinatorial ratios of multinary compositions. The goal of this dissertation is to develop HTS methods for optimizing multinary compositions for PSCs. The dissertation introduces two high-throughput methods on how to optimize perovskite materials for perovskite solar cell applications with a compositionally-graded film (CGF) platform. The first method elaborates on the synthesis and characterization of binary halides MAPb(IxBr1-x)3 on a CGF with over 200 compositions getting synthesized in less than one minute and characterized with a robotized spectrometer. The second method develops for the optimization of triple cations on ternary CGF we call it t-CGF for investigating the most stable perovskite CsxMAyFAzPbI3. We utilized the reported results for fabricating PSCs with high durability and capable of being scaled up for commercialized. The dissertation consists of 6 chapters: Chapter 1 will introduce the fundamentals (structure and optoelectronic properties) of the perovskite material. In addition, the objectives of the dissertation will be discussed at the end of this chapter. Chapter 2 will review the strategies for synthesizing and depositing thin films that are essential for PSCs. We elaborate on most existing HTS methods for different applications including solar cells, light-emitting diodes, batteries, thermoelectrics, and superconductors. Chapter 3 will focus on experimental methods of preparation and characterization of perovskite thin films used in this dissertation. Chapter 4 will discuss our HTS of binary alloys. It is based on compositionally-graded films (CGF) for optimizing binary compositions. As a showcase example, we focus on binary perovskite HTS. We will also show how a spectrometer and a robotic arm could facilitate the high-throughput characterization of synthesized materials. Chapter 5 will discuss our HTS of ternary alloys. It is based on triple compositionally-gradient films (t-CGFs) for optimizing ternary compositions. We apply our t-CGF strategy to discover a range of stable ternary perovskites. We then use them to make PSCs and reveal three degradation mechanisms in devices under operation as a function of composition. Chapter 6 provides outlook for the use of these methods toward the improvement of the functional materials for applications beyond solar. We also discuss how PSCs can be shaped by the HTS methods toward commercialization.



High-throughput experimentation, Multinary optimization and compositional engineering, Perovskite solar cells, Material discovery