Perovskite Solar Cell (PSC) technology has gained a lot of interest in the photovoltaics community due to their excellent opto-electronic properties. The inverted architecture (p-i-n) PSCs are especially promising from a commercialization point of view. Herein, we developed an automated deposition process for Electron Transport Layer (ETL) C60 and interfacial buffer layer BCP using a scalable deposition technique, physical vapor deposition (PVD), to establish a baseline for p-i-n devices based on a wide bandgap Perovskite composition. With the help of a Perovskite Automated Solar Cell Assembly Line (PASCAL) robot and with multiple iterations on devices, we were able to show reproducibility in the fabrication process of single-junction devices, which is typically one of the main advantages of adopting a scalable technique such as PVD. Additionally, devices showed reasonable performance, with a PCE of 17%, Voc of 1.02 V, and Jsc of 20 mA/cm2. To implement these devices into tandems, which require a transparent top cell and therefore a sputtered transparent conductive oxide top contact which necessitates a sputter buffer layer, we investigated PVD SnOx as a replacement ALD SnOx (difficult to commercialize). However, it was shown that the as-deposited SnOx isn’t dense enough to act as a protection layer, requiring an alternative buffer layer and/or alternate TCOs which have a soft-landing deposition process. Lastly, using implied Voc calculations, it is shown that much of the PSC performance is lost at the interfaces due to extensive non-radiative recombination. In this direction, a Lithium Fluoride buffer layer showed promising passivation effect at the PSK/ETL interface and could be the next step towards optimization.

With global efforts transitioning toward renewable energy, solar power holds immense potential for reliable electricity generation. Metal halide perovskites have emerged as promising photovoltaic semiconductors due to their remarkable optoelectronic properties, including high absorption coefficients, tunable bandgaps, and high charge carrier mobility. In addition, low-temperature solution processing offers advantages in cost and scalability. However, their susceptibility to degradation under operating conditions remains a barrier to widespread commercialization. This thesis reports on high throughput research enabled by streamlining and adding capabilities to the existing automated fabrication and characterization setup at the Solar Energy Innovation Lab. Additionally, three distinct approaches were explored to improve perovskite solar cell performance and stability: (1) cationic substitution for lattice stabilization, (2) intermediate phase promoting additives to enhance perovskite film crystallization, and (3) metal oxide nanoparticle-based charge transport layers for optimized band alignment and higher open-circuit voltage. Finally, contributions towards developing international standard compliant degradation testing facilities is presented.

Water accelerates various modes of degradation in silicon photovoltaic (PV) modules including encapsulant yellowing, delamination, and contact corrosion. Moisture primarily enters silicon PV modules through the polymeric module components (encapsulant and backsheet). To mitigate moisture-induced degradation, we must understand the dynamics of moisture in state-of-the-art encapsulants and module architectures. In this work, we present a robust optical method to quantify water content on the front and rear side encapsulants in bifacial silicon PV modules and use such measurements to validate a model for simulating water concentration in these modules. First, we quantify the solubility and diffusivity of water in four modern encapsulants: ethylene vinyl acetate (EVA) and polyolefin (POE) both with and without UV-blocking additives. Second, we measure water concentration within glass-glass and glass-backsheet using water reflectometry detection (WaRD), tracking the diffusion of moisture throughout the modules as a function of time and environmental condition. Crucially we separate the water content from the front encapsulant and rear encapsulant and backsheet within the glass-backsheet modules. Third, we present a model of moisture dynamics in bifacial silicon PV modules and show it to be consistent with our measurements. Finally, we apply this model to simulate the concentration profile of water typical fielded climates as a function of time, environmental conditions, cell size, and module architecture. Overall, our work presents: 1) a quantitative picture of water dynamics in industrially relevant module architectures and field conditions, and 2) a framework to extend this approach other encapsulants and module designs.

With the focus of global energy production shifting towards renewables, efforts have been made to develop sustainable, low-cost technologies to cater to our needs while protecting our environment. The past decade has seen an exponential increase in the popularity of perovskite based solar cells with a push towards commercialization mainly due to its low processing cost, tunability and high efficiency. Despite the attractive advantages offered by perovskite solar cells, one of its shortcomings is its long-term stability. The research community has been invested in tackling issues related to understanding this water-perovskite interaction which poses a threat to device stability. It is known that moisture is one of the most challenging factors to tackle when it comes to degradation of these devices. The presence of water has a detrimental impact on the film’s composition, structure as well as optical properties. Thus, monitoring the uptake of water without destroying the nature of the film altogether is extremely crucial since it directly translates to optimizing the levelized cost of energy efficiency. Researchers have extensively studied the effects of this complex interaction, but there has not been an attempt to isolate the presence of moisture in these films in order to study the cause itself - Water. This thesis aims to study the interaction between water and perovskites using an in-situ and non-destructive technique based on the short-wave infrared (SWIR) spectroscopy.

In recent years, hybrid halide perovskite solar cells have attracted much attention due to their excellent optoelectronic performance. Leveraging the structural flexibility of perovskites, composition engineering appears to be an effective approach to improve material and device stability but can also raise durability challenges such as phase separation. In this presentation, I will share insights into the composition-performance relationships in these emerging materials resulting from correlative X-ray, electron, and optical microscopy investigations. Encompassing a wide range of perovskite chemistries, this work reveals the challenges and benefits of increasing compositional complexity and elucidates the roles of substituents and additives in these complex material systems. By identifying potential pitfalls in tuning the materials chemistry of halide perovskites, we provide a path forward for further improving their optoelectronic performance and making them commercially viable.

Electrochemical CO2 conversion into value-added chemical feedstocks, if powered with renewable electricity, is a promising path to approach carbon neutrality. However, developing active and selective catalysts is necessary to enable energy efficient conversion. In this thesis, I provide strategies to design active electrocatalysts with enhanced selectivity for CO2 electrolysis via tuning of the local reaction environment. First, I show that modulating the surface strain of a model Cu (001) catalyst epitaxially grown on a single-crystal Si substrate results in an increase of the Cu 3d band center position. This change in the electronic structure causes a suppression of the CO production pathway, increasing selectivity to multi-carbon products. I demonstrate that multi-carbon products can be further selected for by an operando restructuring of the micro- and nanoscale morphology of Cu-based catalysts. An electrochemically reduced Cu(OH)2 nanowire catalyst enhances the selectivity of multi-carbon products at moderate electrolysis potentials where a hierarchical morphology evolves. A final level is the tuning of the chemical state of the catalyst surface. By switching the initial surface oxidation state of tin oxide catalysts to be SnII-rich, the selectivity and energy efficiency of formate generation are promoted, offering a possible nearest-term path to carbon-negative CO2 electrolysis. Optimizing electrolyzer design is also crucial to facilitate mass transport of CO2 to reach industrial relevance. The importance of the mass transport of CO2 to enhance overall activity is also demonstrated by controlling flow rate of partially concentrated CO2 stream. These findings reveal the importance of surface strain, morphology and chemical state on designing efficient CO2 catalysts, providing fundamental guidelines and direction toward carbon neutral CO2 conversion.

The ascent of perovskite photovoltaics has been one of the most significant and exciting developments in renewable energy research. The myriad of absorber composition blends and compatible device structures offer a wide variety of applications and several paths towards full commercial deployment. However, the rapid decrease in performance of perovskite photovoltaics under operational conditions prohibits their ubiquitous distribution.

In this dissertation, a top to bottom approach is utilized to advance the commercial viability of perovskite photovoltaics. Beginning with encapsulation schemes, large-area graphene stacks with a polyisobutylene edge seal were investigated as potential moisture and oxygen barriers for perovskite films. These materials were found to provide a moderate but insufficient protection from a damp heat environment. However, an all-glass encapsulation scheme enabled over 1000 hours of exposure to damp heat (85 ◦C, 65% RH) without any loss in film quality (validated by photoluminescence, UV-Vis spectroscopy, and X-ray diffraction). Attention was then focused on the selective contact layers, beginning with a presentation of electrochemical measurements that were adapted to study the optoelectronic properties, surface coverage, and reactivity of transport layers. Tin oxide, commonly used as an electron transport layer in perovskite photovoltaics, was highlighted as a case study to demonstrate the screening potential of these tools by correlating cyclic voltammetry on a bottom selective contact to final device performance once the device stack was completed. Chronoamperometry was also used to test the reactivity of tin oxide, at the surface, to acidic perovskite precursors, which can potentially be used to predict the reactivity of perovskites at buried interfaces within a device. Next, a solvent-free dry transfer process was designed and highlighted for its ability to deposit a large, uniform film of conjugated polymers which are frequently used as hole transport layers in perovskite photovoltaics. This technique is especially useful for top selective contacts since it eliminates concerns with solvent compatibility of the underlying perovskite film. Lastly, design considerations of the absorber were comprehensiviely explored through the development of a high throughput Perovskite Automated Solar Cell Assembly Line which was used, in combination with active learning principles, to design and test wide bandgap perovskite absorbers for use in silicon-perovskite tandem photovoltaics. Overall, this dissertation presents the steps that we have taken to advances perovskite photovoltaics from an intriguing research project to an effective and vital technology that aid in addressing the ever increasing global energy demands.

ABSTRACT OF THE THESIS

Liquid Phase Epitaxy Doping for High-Performance Emitters in Silicon Solar Cells

by

Tulika Rastogi

Master of Science

University of California San Diego, 2018

Professor David Fenning, Chair

Recombination in the emitter is a primary contributor to recombination current in modern industrial silicon solar cells. This loss is primarily due to recombination that occurs because of the inflexible constraints of solid-state diffusion kinetics during conventional emitter formation that result in a heavily-doped, difficult-to-passivate front surface. P-n junction formation by gas diffusion is also one of the costliest steps in the cell process due to equipment capital expenditure, the large thermal budget required, and the low-throughput batch nature of the process [1].

Liquid phase epitaxy doping (LPE-D) is proposed as a facile low-temperature, low-cost replacement for boron and/or phosphorus diffusion. This doping process decouples the emitter formation from the slow solid-state kinetics of gas diffusion. The doping profile formed is largely time-independent – theoretically, doping could be done in a brief firing step or during bulk impurity gettering, allowing throughput to be optimized without constraining cell architecture in next generation solar cells. The liquid phase epitaxy approach results in a nearly uniform doping concentration profile with the potential for significantly lower saturation currents and improved surface passivation in comparison to existing and proposed industrial diffused emitters. We examine the performance of emitters doped via liquid phase epitaxy using simulation and present experimental demonstration.

Perovskite photovoltaics hold promise to accelerate the world’s transition to renewable energy by greatly reducing the cost of solar energy. While promising, perovskite commercialization is hindered by short cell lifetimes, and thus researchers use accelerated degradation testing to quickly engineer improvements. However, commercial test equipment is cost prohibitive, and many research groups instead design custom systems. This work presents the novel design of an accelerated and inexpensive testing enclosure for perovskites, and describes the engineering challenges of light, temperature, and electrical tests. Currently, the system is designed for a capacity of 45, 1 cm2 cells, capable of illuminating at 7.2 times the sun’s peak intensity, up to 85C, with a leak rate of 0.3%/hr, and 4-point electrical connection to a custom maximum power-point (MPP) circuit. Total cost of the system is ~$2.5k and is compact; cells can be transferred directly within a glovebox. This paper presents the thermal design, modeling, and challenges of high temperatures. We then present the structural and electrical design and conclude with a cost and failure analysis. The broader goal of this work is to contribute design guidelines for laboratories designing inexpensive, custom enclosures, in hopes of accelerating perovskites’ commercialization.

Transforming carbon dioxide (CO2) into valuable chemical feedstocks through electrochemical processes powered by renewable electricity shows promise in achieving carbon neutrality. However, the development of effective and selective catalysts is essential for enabling energy-efficient conversion. The activity and selectivity of the reaction towards formic acid production are governed by the stabilization of the *OCHO intermediate on the catalyst surface. This thesis presents strategies for designing active electrocatalysts that exhibit improved selectivity towards formic acid in CO2 electrolysis by manipulating the chemical environment surrounding the catalyst. Introducing a partial positively charged copper species on the surface in the form of copper sulfide stabilizes the *OCHO intermediate on the catalyst surface. The ligand plays a key role in supplying the proton required for the reduction of CO2 to formate, and that an optimum pKa value ligand is beneficial for improved selectivity towards formate. Another aspect involves tuning the chemical state of tin oxide catalyst surfaces. By favoring a Sn (II) rich initial surface oxidation state, the selectivity and energy efficiency of formate generation are improved, offering a potential near-term solution for carbon-negative CO2 electrolysis. Optimal design of electrolyzers is also crucial for facilitating the mass transport of CO2, thereby increasing industrial relevance. These discoveries underscore the importance of chemical environment surrounding the catalyst, and chemical state in the efficient design of CO2 reduction catalysts to formic acid and providing fundamental guidelines and direction towards achieving carbon- neutral CO2 conversion.