<p>Two-dimensional (2D) halide perovskites are an attractive class of hybrid perovskites that have additional optoelectronic tunability due to their accommodation of relatively large organic ligands. Nevertheless, contemporary ligand design depends on either expensive trial-and-error testing of whether a ligand can be integrated within the lattice or conservative heuristics that unduly limit the scope of ligand chemistries.</p><p>This work begins by investigating simulation-guided ligand design. Employing molecular dynamics (MD) simulations and machine learning (ML) models, systematic ligand exploration unveils the principles governing the stability and structural relationships of these perovskites. Promising ligand candidates undergo a refinement process informed by feasibility considerations, followed by experimental synthesis and characterization to underscore the effectiveness of simulation-informed design strategies.</p><p>Delving into the realm of anionic diffusion, a pivotal factor in 2D perovskite stability, the study examines this property using both experimental methods and simulation techniques. This parallel examination underscores the alignment between simulation predictions and real-world observations, offering nuanced insights derived from molecular simulations. Importantly, simulations serve as potent tools for hypothesis validation when ligands originate from experimental synthesis, affirming conjectures stemming from empirical insightsThe exploration extends to comprehending the molecular insights of experimental observations, thereby shedding light on factors that enhance device efficiency. We explore mechanisms for mitigating phase disproportionation, optimizing ion diffusion, modulating molecular interactions between perovskite and polymeric hole-transporting materials, and uncovering the single-molecule behavior that leads to high photoluminescence quantum yields. Notably, all simulation outcomes align with experimental findings, further validating the utility of MD analysis in the context of 2D perovskite systems.</p><p>Furthermore, this work addresses a crucial aspect of MD simulations, namely the refinement of force field models. Previously developed topology automated fixed-charge force-field interactions (TAFFI) is augmented through the incorporation of polarizability using classical Drude oscillators, resulting in a novel framework termed TAFFI-Drude. This approach enhances electrostatic properties while retaining transferability and consistency from the existing TAFFI model.</p><p>The thesis concludes with a comprehensive discussion of the findings across the aforementioned areas, highlighting the impact of simulation-driven design and insights in advancing 2D perovskite research. The implications of these discoveries for optoelectronic applications and the broader field of materials science are explored, emphasizing the potential for innovation and improvement within the realm of 2D halide perovskites.</p>

<p>As energy demands for the world increase, the necessity for alternate sources of energy are critical. Just in the United States alone, 92 quadrillion British thermal units (Btu) were used in 2020. As political and geographical pressures surrounding oil increase, along with the growing concern for climate, the drive to explore alternative and renewable means for harvesting energy is on the rise. Solar cells, also known as photovoltaics (PVs), are an attractive renewable source and have been developed as an alternative energy means for over 60 years. When considering losses due to atmospheric absorption and scattering, the Earth’s surface gets about 1000 W/m2 of energy from the sun, which is why there are research efforts around the world trying to maximize the efficiency of solar cells.</p> <p>Organic-inorganic halide perovskites provide for ideal absorbing layers that feature long carrier lifetime and diffusion lengths, strong photoluminescence, and promising tunability. Furthermore, the solution-processing methods used to make these perovskites ensure that the solar cells will remain low-cost and have easy scale-up possibilities. The main problem perovskites is that they degrade in the presence of water, thus leading to decreased device performance.</p> <p>In this work two approaches are investigated to increase moisture stability. The first investigates incorporation of thiols as pseudohalides into the 2D perovskite structure. Instead of the theorized perovskite, two novel 2D compounds were created, Pb₂X(S-C₆H₅)₃ (X= I, Br, Cl) and PbI_1.524(S-C₆H₅)_0.476. While not perovskites, this study gives insight into the effect that the thiol may have on determining structure when comparing –S-C₆H₅ with –SCN groups. Future work will explore more electronegative thiols that will be used to make moisture resistant, tunable 2D perovskites.</p> <p>The second approach is to incorporate longer organic ammonium cations into the perovskite structure to produce quasi-2D perovskite films fabricate them into devices. Adding in electronically insulating ligands leads to a stricter requirement for vertically aligned 2D films and special care must be taken to have efficient charge collection. The current field has successfully incorporated short ligands such as butylammonium (BA) into PVs, however the extension to larger and more beneficially hydrophobic ligands has been very scarce. In this work, a novel solvent engineering system is developed to create vertically aligned quasi-2D perovskite absorbing layers based off of a bithiophene ligand (2T). These absorbing layers are then characterized and incorporated into efficient PV devices. Generalizations to solvent conditions related to ligand choice is discussed herein, creating deep insights into incorporating more conjugated ligands into devices.</p>

<p dir="ltr">Two-dimensional (2D) perovskite semiconductors are an emerging family of hybrid materials featuring a built-in quantum well architecture which has gained much interest due to its potential as a promising candidate for next-generation photovoltaic and optoelectronic applications. To successfully integrate 2D perovskites as efficient devices, it is imperative that a thorough understanding of the fundamental properties these materials possess and how their complex heterostructures behave is established. However, to date, the synthetic challenges regarding high-quality crystals of these materials due to the structural complexity and the hybrid nature have impeded further progress in this area. Thus, we demonstrate a general method to construct tunable 2D organic semiconductor-incorporated perovskites (OSiP) by simultaneously manipulating slab thickness of the inorganic layers and conjugation length of the organic substituents. The energy band offsets and exciton dynamics at the organic-inorganic interfaces were elucidated using computational means and ultrafast spectroscopy, while lattice dynamics were quantified via temperature-dependent spectroscopy and X-ray diffraction studies. Results show that longer and more planar π-conjugated organic ligands induce a more rigid inorganic crystal lattice, which leads to suppressed exciton-phonon interactions and superior optoelectronic properties such as efficient lasing.</p><p dir="ltr">Furthermore, understanding ion migration in two-dimensional (2D) perovskite materials is key to enhancing device performance and stability as well. However, prior studies have been primarily limited to heat and light-induced ion migration. To investigate electrically induced ion migration in 2D perovskites, we construct a high-quality single crystal 2D perovskite heterostructure device platform with near defect-free van der Waals contact. While achieving real-time visualization of directional ion migration, we also uncover the unique behavior of halide anions inter-diffusing towards the opposite direction under prolonged bias. Confocal microscopy imaging reveals a halide migration channel that aligns with the crystal and heterojunction edges. After sustained ion migration, stable junction diodes exhibiting up to ~1000-fold forward to reverse current ratio are realized. Unraveling the fundamental properties of 2D OSiPs as well as ion migration in 2D perovskite heterostructures paves the way towards stable and efficient devices.</p>

<p dir="ltr">In response to the ongoing global energy crisis, significant efforts have been made to enhance the efficiency of energy conversion devices that utilize renewable resources. This has spurred the development of multi-component semiconductors, which combine the strengths of both organic and inorganic materials to offset their individual limitations. This dissertation investigates advanced band engineering strategies to manipulate photophysical phenomena in these hybrid systems for specific applications. By focusing on two-dimensional perovskites and heterostructures formed from transition metal dichalcogenides and organic molecules, we explore how the inorganic components can sensitize their organic counterparts, examining the charge and energy transfer processes occurring at their interfaces and giving rise to unique excited states with diverse optical properties which hold significant potential for energy harvesting technologies.</p><p dir="ltr"><b>CHAPTER 1</b> furnishes readers with extensive insights into the semiconducting materials discussed in this dissertation, along with essential knowledge of fundamental concepts (such as excitons, charge and energy transfer, singlet fission, Marcus Theory, etc.) crucial for a deeper understanding of subsequent chapters. It also highlights the unresolved questions addressed later in this dissertation.</p><p dir="ltr"><b>CHAPTER 2 </b>provides a comprehensive overview of the spectroscopic and other characterization techniques used to study these materials.</p><p dir="ltr"><b>CHAPTER 3 </b>illustrates how the relative band alignment and coupling between the organic and inorganic layers of 2D perovskites impact the rates and dynamics of the transfer of triplet excitons across the hybrid interface. It also demonstrates how one can utilize extremely fast triplet transfer times to induce rapid photon upconversion in the perovskite, facilitated by doping of the organic layer. Triplet energy transfer driven photon upconversion is a promising method for enhancing the efficiencies of solar cells; therefore, this chapter makes a stride towards contributing newer insights for optimizing solar energy conversion.</p><p dir="ltr"><b>CHAPTER 4</b> focuses on how tuning the dimensionalities of 2D perovskites can modify their energy landscapes and further impact the interfacial photophysics, leading to the creation of long lived and mobile ‘interfacial’ excitons with enhanced electro-optic properties, promising for potential applications in solar cells and quantum computing.</p><p dir="ltr">In <b>Chapter 5</b>, the focus shifts to organic molecules that can undergo singlet fission, which are sensitized by highly absorbing monolayer transition metal dichalcogenides (TMDCs). This chapter explores how to induce intense emission from triplet pairs of the organic molecules — the critical yet elusive intermediate species in singlet fission, by engineering direct energy transfer into them from the TMDCs. Singlet fission-based technologies hold the potential to significantly enhance solar cell efficiencies, driving extensive research into optimizing the behavior of multi-excitonic triplet pairs. These triplet pairs offer the exciting possibility of multiphoton emission and/or the donation of multiple electrons (or excitons) in a single step. Our work aims to advance the understanding of these prospects and contribute to their practical application.</p><p dir="ltr"><b>CHAPTER 6 </b>summarizes the key findings from the previous chapters and explores potential future research directions. This dissertation, as a whole, contributes to and paves the way for further investigation into optimizing band engineering-based functionalities in 2D organic-inorganic semiconductors. These efforts aim to advance photophysical applications focused on improving energy conversion for a cleaner, more sustainable future.</p>

<p>Developing the next generation soft optoelectronic materials is of great importance for achieving high-performance, low-cost electronics. These novel material systems bring about new chemistry, physical phenomena, and exciting properties. Organic inorganic hybrid two-dimensional perovskites and organic stable radical molecules are two exciting material systems that bear high expectation and await extensive exploration.</p> <p>Organic inorganic hybrid two-dimensional perovskites are considered one of the solutions to the pressing instability issue of halide perovskites toward commercialization. Moreover, dimension reduction of perovskites creates new opportunities for using two-dimensional perovskites as thermoelectric applications due to the ultralow thermal conductivity. However, two-dimensional perovskite thermoelectric is still at its’ incipient stage of development, therefore a timely proof of potential is required to draw further research interests.</p> <p>In earlier part of this work, the two-dimensional perovskites featuring π-conjugated ligands are synthesized and optimized for high thermoelectric performance. With material design, device engineering, intensive measurements, and careful data analysis, we successfully showed that two-dimensional perovskites are competitive candidate for the emerging thermoelectric materials. Furthermore, temperature and carrier concentration dependencies on thermoelectric properties were also established, giving future researchers a generalized optimization strategy. </p> <p>Organic stable radical molecules are promising for organic electronics as stable radicals don’t require high conjugation for efficient solids-state charge transport. Thanks to their unique redox capability and the unpaired electrons, organic radicals have many unique electronic and magnetic properties that could be useful in spin-related applications. However, the understanding in charge transport mechanisms as well as structure-to-properties correlation remain shallow.</p> <p>In later part of this work, we achieved the highest recorded long channel electrical conductivity of non-conjugated radicals. Meanwhile, the important role of close packing between radical sites was demonstrated by slightly changing chemical design that resulted in drastic change in electrical conductivity. Finally, we concluded that the solid-state charge transport in non-conjugated species is governed by variable range hopping mechanisms. </p>