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School of Materials Science and Engineering

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College of Engineering

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Surface passivation for enhanced stability and performance in perovskite solar cells

(Georgia Institute of Technology, 2023-12-13) Sharma, Sakshi

Lead halide perovskite solar cells (PSC) have emerged as promising next generation photovoltaics. Their unique ABX3 stoichiometry- where ‘A’ is a monovalent cation, ‘B’ is a divalent metal cation and ‘X’ is a halogen- provides tremendous potential for composition and bandgap engineering to obtain desired optoelectronic properties, enabling high power conversion efficiencies exceeding 25%. Despite their growing appeal, commercialization of PSC technology faces challenges due to device instabilities in ambient conditions. Particularly, device interfaces between the active perovskite layer and adjacent charge transport layers are vulnerable to defects which can accelerate perovskite degradation under environmental stressors such as heat, moisture, or oxygen, limiting their long-term viability. Interfaces also significantly impact charge transport, collection and recombination mechanisms in devices and thus require optimization. To address these challenges, research has concentrated on interface modification to passivate surface defects, protect the bulk of perovskite from external environment, and tune the charge transfer properties at the surface. Conjugated organic ammonium salts have been used at interfaces to introduce hydrophobicity on the perovskite film and promote charge delocalization brought on by conjugation. However, most surface treatment strategies relying on organic molecules introduce an electrically insulating spacer layer under thermal stress. Heat induced diffusion of molecules can reconstruct the interface into lower dimensional phases, which impedes charge extraction and affects photo-conversion efficiency (PCE) of devices. This brings a tradeoff between the benefits of passivation and charge extraction. For proper interface design, it is essential to study the thermal behavior of these passivation layers and establish their relationship with the optoelectronic properties of solar cells. This work explores the thermal behavior of passivation agents, specifically employing long-chain thiophene-functionalized π-conjugated molecules (2TI and 4TmI, with two and four thiophene rings, respectively) on interfacial structural stability and charge extraction. Tailoring the steric hindrance of the bulky cations used to treat perovskite surfaces presents an opportunity to control cation mobility, and consequently any phase changes resulting at elevated temperatures. Structural studies reveal that the length of the cation backbone regulates the rate of interfacial perovskite structure reconstruction on prolonged heating. Consequently, faster phase conversion is observed in 2TI compared to larger 4TmI, with the formation of a n=1 A’PbI4 two- dimensional phase which consists of inorganic PbI6 octahedra monolayers separated by an organic spacer layer, A’ being either 2T or 4Tm. The oligothiophene tail in these molecules further contributes to spacer layer conductivity, prompting distinct charge extraction and recombination behaviors in 2TI versus 4TmI passivated devices, confirmed by synchrotron-based X-ray measurements. Results show that despite the observed phase changes, 2TI treated devices can tune the surface potential to promote efficient hole extraction to the overlying hole transport layer and reduce carrier recombination. This interfacial steric engineering translates to high performing passivated solar cells, with 2TI/CsFAPbI3 devices exhibiting efficiency exceeding 20%, an open-circuit voltage of 1.07 V and minimal changes under continuous thermal exposure. By identifying the nature and impact of heat induced dynamical structural changes at passivated perovskite interfaces, this work highlights the key to surface functionalization so that solar cell performances can be maintained at high operating temperatures.
Characterization of Commercial Dielectric Zaristo-700 as a Redistribution Layer Material for Advanced Packaging

(Georgia Institute of Technology, 2023-12-06) Madelone, Sophia Marie

This body of work, in detail, outlines the fundamental steps taken to characterize a material for novel use in RDL build-up layers for advanced packaging. The material (Zaristo-700) discussed in this thesis was only used in RF applications, and now we are exploring its use in the wiring layers. In the PRC, research into thin films, spin-on films, and many other dielectrics have been published before. It is essential to understand that this work is necessary to establish a “library” or catalog of information on all the materials we use to provide the correct material, depending on the goals of future projects. The material and electrical properties of Zaristo-700 are characterized through JEDEC adhesion testing (Peel test), leakage current measurements on ITO glass slides before and after Highly Accelerated Stress Testing (HAST) treatment, a series of dose tests to document the most optimized pitch-scalability at 8.0 m L/S, and lastly Shadow-Moire warpage studies of one layer and three-layer RDL samples. Leakage current measurements taken before and after HAST stayed at or below 2.0 nA. As we will explore, the CTE and adhesion of Zaristo-700 are excellent and contribute to making a great material for the RDL wiring layers. Taiyo Ink. has stated that this version of the dielectric film accounts for issues such as stability in how long it can sit, delamination during or after curing, delamination during fabrication processes, and so on. Whereas some of the dielectric films of other companies still have these problems. This research is working towards answering the unknowns about this dielectric and how well it will function as a future RDL build-up material through characterization and analysis of its properties. These results are a positive indication for use as an RDL dielectric in advanced packaging.
Atomic Layer Deposition As A Method Of Fine Tuning The Surface Chemistry Of Oxide Materials

(Georgia Institute of Technology, 2023-12-05) Yom, Typher

Atomic layer deposition (ALD) is a vapor-phase synthesis method in which a material is deposited onto a surface with precise atomic thickness. Through ALD, ultra-thin monolayers of oxide materials can be deposited onto powders, creating mixed oxide surfaces with tunable surface chemistries, enabling their usefulness towards catalytic processes in the petrochemical and fine chemical industries. ALD holds an advantage over typical solution-phase methods of creating mixed oxide materials due to the latter’s difficulty in controlling the surface composition, making analysis difficult. However, if we can better understand the interactions of the surface in solution, it can be used to design more effective catalysts. One way to observe this is by studying the zeta potential of the surface, which is directly correlated with surface charge and is a product of these acid-base interactions at the interface. Each material can be identified using the isoelectric point, which is the point at which the zeta potential/net surface charge is zero. For mixed metal oxides, their isoelectric points were calculated in the literature to be the summation of each individual component’s isoelectric point multiplied by its surface coverage. However, this calculation assumes that the components do not interact with each other when mixed. In order to investigate this discrepancy, we used ALD to deposit thin layers of titanium oxide onto silicon oxide powders. If we were to assume the equation used in the literature, we can assume that one single monolayer over the surface would be sufficient to convert the isoelectric point from that of silicon oxide to that of titanium oxide. However, our results have indicated that the isoelectric point did not reach that of titanium oxide until multiple monolayers were deposited, indicating that a different model/equation must be utilized to better elucidate the surface behavior. Additionally, during these studies of the isoelectric point, we have formulated an equation that can correlate the thickness of ALD-deposited films with the material’s relative atomic percent. This equation was created by assuming that the shape of the particle + film retains its shape, and therefore its volume formula, allowing it to work for ultra-thin films, but not for much thicker films. Finally, this thesis highlights the importance of being mindful of the precursor used for powder ALD: precursors like TiCl4 can create byproducts like HCl from the reactor walls and the powder itself. These byproducts can then adsorb onto the powder surface, which can block film growth or affect the pH of the resulting solution when the powder is dispersed in water. Extra measures, such as a double dose or a post-process washing step, were implemented, and should be used when performing powder ALD.
In situ functionalization of anion-conducting solid polymer electrolyte membranes

(Georgia Institute of Technology, 2023-08-16) Shah, Parin Nitin

Hydrogen is a viable option for storage and on-spot generation of energy. Alkaline electrolyzers and fuel cells have several advantages over acidic counterparts such as simple fabrication, non-precious metal catalysts and low crossover. It has been shown that crosslinked anion–exchange membranes synthesized by vinyl addition polymerization of norbornene show excellent performance in alkaline electrochemical devices. However, a long reaction time is needed for converting the tethered bromoalkyl moiety in the polymer to a quaternary ammonium head-group because a tertiary amine has to diffuse into the polymer. This amination process is not compatible with the roll-to-roll membrane formation process. In this study, anion exchange membranes have been prepared by in situ amination of the functionalized polymer during membrane casting. The scope of three different tertiary amine: Trimethyl amine, Triethyl amine and N-methyl piperidine was investigated for the in situ membrane casting process. The polymers used in this study were also in situ crosslinked with N,N,N’ ,N’ - tetramethyl-1,6-hexanediamine during membrane casting to prevent excessive water uptake. By changing the reaction solvent, temperature, and concentration, it was possible to balance the reaction kinetics while still maintaining polymer solubility to cast membranes. The conversion was monitored as a function of reaction time (using NMR) and the reaction conditions were optimized to develop a novel process of producing pre-functionalized membranes that is compatible with the current roll-to-roll infrastructure. Membranes having high ion exchange capacity (upto 3.4 meq g-1 ) and high ionic conductivity (upto 56 mS cm-1 at room temperature) were prepared using this process. Precisely controlling the reaction time made it possible to directly cast quaternized membranes on a roll-to-roll timescale, thus avoiding the need for the long-duration, ex situ amination step. Alkaline electrolyzer performance with these in situ aminated membranes showed comparable performance to membranes prepared by the conventional, ex situ amination method.
Machine Learning Helps to Build Drug Release Kinetic Models

(Georgia Institute of Technology, 2023-05-22) Tian, Xuzheng

Long-acting injectables (LAI) are one of the most promising drug delivery systems for the treatment of chronic diseases. Since they can maintain the drug concentration in the target tissue, thus reducing dose frequency and adverse effects as well as improving patient compliance. The use of polymer matrices delivery systems shows an extraordinary diversity in drug development research. But due to the time-consuming experiments and complicated drug release mechanisms, the efficiency of LAI development is strongly restricted. This thesis used machine learning to predict the long-period in vitro test profiles based on the datasets collected from published literature. In addition to comparing the accuracy performance of different machine learning algorithms, a combination of empirical mathematic models and machine learning algorithms is further studied in the case to improve the model evaluability.
Parameters Affecting the Dynamic Shock Response of Aluminum Oxide Under Tamped Richtmyer-Meshkov Instability Conditions

(Georgia Institute of Technology, 2023-05-02) Zusmann, Benjamin L.

The objective of this research is to provide constitutive models for brittle, granular materials, with new experimental data analyzed to determine the effects of various parameters on the strength response. Dynamic shock compression behavior of aluminum oxide, Al2O3, powder is investigated using a powder-tamped Richtmyer-Meshkov Instability (RMI) experimental design, which probes the interface between a well-understood solid driver, annealed oxygen-free high conductivity copper (OFHC Cu) for this study, and a granular material of interest as the tamper, Al2O3 powder for this study. The RMI phenomenon involves the magnification and growth of an initially perturbed surface under high-pressure conditions produced by shock wave propagation. The material strength response to shock compression is investigated by monitoring the resistance to instability growth through measurements of the final jet length of the driver material into the tamper material. For the powder-tamped RMI experiments performed in this work, Al2O3 powders with varying tap densities (1.96 g/cc, 2.30 g/cc, and 2.46 g/cc) were used as the tamper material. The overall objective was to determine (a) the effect of the inherent resistance to instability formation and growth affected by varying initial density of the Al2O3 powders, (b) the projectile impact velocity influencing the intensity of shock loading conditions, and (c) kη0, a value representing the relative geometry of the RMI profile as a function of amplitude and wavelength. Measurements of the jet length captured in each of the multiple frames obtained from the various experiments allowed correlation of the effects of projectile velocity, initial density, and kη0 on the final jet length. The data generated in this study is viewed both as a whole dataset and in relevant subsets in order to isolate each primary experimental parameter to avoid confounding contributions to a particular shock response.
Solid State Chemical Recycling of Polymer by Variable Frequency Microwave

(Georgia Institute of Technology, 2023-04-27) Vichare, Preksha

Fixed frequency microwaves have been studied in various chemical transformation applications. A common issue seen with fixed frequency microwave is non-uniform heating and arcing. The current study highlights the use of variable frequency microwave (VFM) heating to achieve rapid thermal depolymerization of polymer composites. A wide range of additives were investigated to select the best performers for microwave heating at low energy. Carbon nanotubes, carbon nanofibers, carbon black, graphene, reduced graphene oxide were shortlisted for further investigation by incorporating the materials into different polymer matrices. The chemical and thermal influence of additives on polymer decomposition has been studied. The depolymerization of polyphthalaldehyde, polypropylene carbonate, two polyhydroxyalkanoates, and nylon 6 has been achieved. Additional experiment of polymer depolymerization with the combination of a photoacid generator and microwave-absorbing additive resulted in a reduction of total microwave energy required. This study demonstrates the use of VFM systems for chemical recycling and monomer recovery of select polymer matrices.
Stability Studies of ALD Films and Infiltrated Hybrid Materials

(Georgia Institute of Technology, 2022-12-19) Fairach, Selma Raquel

Aluminum oxide (alumina) thin films deposited through atomic layer deposition (ALD) are of great interest in chemical barrier and corrosion protection applications. However, the stability of ALD alumina in aqueous solutions is still not fully understood. Due to its metastable amorphous phase, the hydration and degradation behavior of ALD alumina films behaves differently from its crystalline Al2O3 counterpart. A full understanding of why these films hydrate and/or dissolve requires the exploration of different deposition conditions and ion content in solutions used. This thesis will discuss efforts to further elucidate the hydration and dissolution behavior of ALD alumina films. For this study, alumina thin films were ALD deposited onto silicon substrate at 150 °C using trimethylaluminum (TMA) and H2O. These films were then studied in Type 1 DI water and different concentrations of NaCl solutions at room temperature. Films were gently dried using a nitrogen gun and thickness was measured using a Cauchy ellipsometry model. After 15 days of immersion in Type 1 DI water, significant thickness growth is observed at twice (27 days) and 2.5 times (33 days) the normalized thickness. Similar hydration is not observed in salt-containing aqueous solutions nor upon exposure to air. This thesis will discuss the possible effects of CO2 dissolution and carbonate formation as well as ionic species on the hydration and dissolution processes of these alumina films. Similar stability applications are observed in the infiltration of hybrid organic-inorganic electronic devices through vapor phase infiltration (VPI). Spiro-OMeTAD is one of the most-studied hole transport layer (HTL) materials in perovskite solar cells, but it is known to degrade quickly due to thermal effects as well as gold diffusion into the layer. This thesis will discuss how infiltration on TiCl4 and H2O into Spiro-OMeTAD layers can change the thermal properties and prevent early degradation. Upon infiltration of TiOx, the formation of crystals observed on Spiro-OMeTAD layers decrease, and at 10 hours of infiltration, the glass transition temperature of the film is found to decrease almost 15 °C. Similar behavior is not found in samples that are thermally controlled, indicating that this behavior is not an annealing effect. This thesis will discuss all the different thermal stability results with varying infiltration times and precursors used.
Enhanced Solution Growth of Perovskite Single Crystals Through the Use of Solvent Additives and Dopants

(Georgia Institute of Technology, 2022-12-06) Lawless, Rachel

Perovskites have garnered significant research interest due to their high optoelectronic performance, composition tunability, and diverse applications. Traditional single crystal growth methods require high temperatures and expensive precursors and equipment, but recent work with solution-growth methods has worked to reduce the temperature and cost. This reduction has come at a cost to quality and size. This work uses methodical control of the parameters (concentration, temperature, heating rate, evaporation) to increase the crystal size of the perovskites. This work has seen that the equilibrium growth of the perovskite is essential for increasing the crystal size of the system and making single crystals more widely used. Further enhancement of the perovskite single crystal size and crystal quality was also achieved using dibutyl sulfide as a solvent additive. Dibutyl sulfide helped to control the solution dynamics and slowed down the crystal growth and allowed for larger crystals with a higher crystallinity to form. This enhancement led to an increase in photoluminescence and carrier lifetime. The single crystals were also doped with Ce3+ to gain an understanding of how the Ce affects the properties of the material. It was found that the Ce3+ incorporated well into the crystal and led to a decrease in crystallinity and size due to the increase in defects in the crystal. The band gap and emission were redshifted as Ce was incorporated. The photoluminescence intensity was enhanced, and carrier lifetimes were decreased. It was found that the dibutyl sulfide additive enhanced the crystallinity and performance of the perovskite while Ce doping enhanced the optical performance of the material while introducing defects as well.
Spark-Plasma Sintered SiCw-Al2O3 Composites and the Influence of 3YSZ on Their Microwave Heating Behavior

(Georgia Institute of Technology, 2022-08-01) Rath, Miriam

Ceramic composites consisting of an alumina (Al2O3) matrix and silicon carbide whisker (SiCw) reinforcing phase have found traditional commercial uses as high-performance tooling and wear parts due to their exceptional hardness, thermal shock resistance, and chemical inertness. However, more recently they have also found use as microwave heating elements, primarily due to the microwave absorption properties of SiC, which displays resonance in the microwave frequency range allowing for rapid and uniform volumetric heating. Similarly, 3 mol% yttria stabilized zirconia (3YSZ) displays strong microwave absorption properties, experiencing a thermal runaway effect, causing the material to uncontrollably heat up to temperatures above 1000C. As such, 3YSZ has the potential to increase heating rates and maximum temperatures reached of microwave-based material technologies. The first section of this research establishes the percolation threshold of spark-plasma sintered (SPS) SiCw-Al2O3 composites to compare to other SiCw-Al2O3 composites made with varying densification methods. Once the percolation threshold was established, a composition was chosen to add 3YSZ as an additional filler phase in attempt to influence the microwave heating behavior of the composite. While SiC can transfer electromagnetic energy to thermal energy due to the establishment of polarized and dielectric loss caused by the external electric field, 3YSZ displays thermal runaway due to a small increase in the loss factor. Thus, adding 3YSZ to a SiCw-Al2O3 composite in small volume contents can potentially induce more controlled thermal runaway in the bulk composite behavior. In the later sections of this thesis, numerical simulation work was done to study the electromagnetic microwave heating behaviors of both SiCw-Al2O3 composites of varying compositions as well as the 3YSZ-SiCw-Al2O3 composites. Localized thermal hotspots were observed in eclectically percolated SiCw-Al2O3 composites and more conducting 3YSZ-SiCw-Al2O3 composites demonstrating the tie between microwave absorption and electrical conductivity in these samples. This work provides in the initial groundwork in understanding the bulk electrical and microwave heating behaviors of 3YSZ-SiCw-Al2O3 composites.