Improving Efficiency of Perovskite and Silicon Solar Cells by Reducing Surface Recombination, Bulk Recombination, and Thermalization Losses

Abstract

Solar photovoltaic (PV) technology stands at the forefront of renewable energy solutions, driven by the urgent need to mitigate climate change and secure sustainable energy sources. As the demand for cleaner energy intensifies, there is a growing emphasis on advancing the efficiency and cost-effectiveness of solar cells. This thesis delves into critical strategies aimed at enhancing the performance of both perovskite and silicon-based solar cells by addressing key efficiency-limiting factors: surface recombination, bulk recombination, and thermalization losses.

Chapter 1 serves as the introduction to this thesis, setting the stage for the exploration and investigation that follows. This chapter provides an overview of the research context, objectives, and significance of the study within the field of photovoltaics. It outlines the key research questions, hypotheses, and the methodology employed to address them.

In Chapter 2, we investigate the dielectric properties of Acetamidinium lead iodide and a composition of Methylammonium lead iodide with ten percent substitution of Acetamidinium, comparing them with pristine Methylammonium lead iodide. The aim is to elucidate the factors contributing to the superior device performance observed with the ten percent Acetamidinium substituted Methylammonium lead iodide. Measurements conducted across various temperatures reveal that Acetamidinium lead iodide exhibits a significantly lower dielectric constant compared to Methylammonium lead iodide. A key finding of this study is the alteration in the curie-like behavior pattern observed in the ten percent Acetamidinium substituted Methylammonium lead iodide, in contrast to the zero percent substitution. Specifically, while the dielectric constant of the zero percent substitution drops steeply after 150 Kelvin, this curie temperature shifts to 138 Kelvin with a slightly broader decrease in the ten percent substituted variant due to the constrained dipolar rotation of the larger Acetamidinium ion within the perovskite matrix. The pristine Acetamidinium lead iodide exhibits an even broader decrease in dielectric constant.These results suggest that the enhanced performance of the mixed-cation lead iodide perovskite is not primarily attributed to a lower exciton binding energy. Rather, it is influenced by the restricted rotation and bulkiness of the Acetamidinium cation, along with increased hydrogen bonding, which collectively hinder ion migration. This effect raises the activation energy required for vacancy-mediated halide migration, thereby reducing interface recombination losses and improving the overall device performance.

Chapter 3 explores the advantages of inorganic metal-oxide transport layers over organic counterparts in organic-inorganic perovskite thin-film solar cells (PSCs), emphasizing their superior photo, thermal, and operational stability. Despite these benefits, oxide films deposited below 150 °C often suffer from poor quality and stoichiometry issues. This chapter introduces a novel method for depositing titanium dioxide (TiO2) thin films at a mere 90 °C using atomic layer deposition (ALD) with titanium (IV) tert-butoxide (TTB) and water as precursors. The study systematically investigates the influence of precursor pulse times and purge times on film quality and growth characteristics. Optimal deposition conditions are identified, yielding a growth rate of 0.1 Å/cycle with films exhibiting high density and a refractive index of 2.2. An ’ALD-window’ between 50-120 °C is identified where deposition is uniform and reproducible. Fourier transform infrared spectroscopy (FTIR) confirms complete TTB decomposition, while X-ray photoelectron spectroscopy (XPS) validates the formation of stoichiometric TiO2 films. Utilizing these low-temperature deposited ultrathin (3nm) TiO2 films as electron transport layers (ETLs) in perovskite solar cells (both N-I-P and P-I-N architectures) achieves champion power conversion efficiencies (PCE) of 13.07% and 8.87%, respectively. This breakthrough enables the seamless integration of ultrathin TiO2 films as efficient ETLs in halide-perovskite solar cells, particularly in inverted architectures, and paves the way for scalable fabrication methods suitable for large-area applications at low thermal budgets.

Chapter 4 is dedicated to overcoming critical efficiency challenges in perovskite solar cells, specifically focusing on mitigating surface and bulk recombination issues and improving band alignment at transport layer interfaces. This research introduces tetraphenylethylene enamine (TPE-en) as a surface modification strategy aimed at enhancing the performance of perovskite solar cells. Through precise manipulation of the perovskite’s surface and grain boundaries using TPE-en, we successfully enhance the alignment of energy bands at the interface between the perovskite layer and the hole transport layer, thereby significantly reducing recombination processes within the perovskite material. Utilizing the solubility of small organic molecules in orthogonal solvents, TPE-en is introduced onto the perovskite surface using methods similar to anti-solvent techniques. Our investigations demonstrate significant enhancements in short circuit current density, fill factor, and open circuit voltage of the surface-modified (SM) perovskite. Specifically, we achieve notable power conversion efficiencies of 18.73% (MA0.9AA0.1PbI3) and 21.61% (CsI0.05[(FA)5/6(MA)1/6]0.95Pb(I0.9Br0.1)3). Comparative analyses reveal that TPE-en out performs other reported TPE derivatives in device performance. Through detailed interface analysis, we observe that TPE-en effectively mitigates surface and grain boundary defects by elevating the highest occupied molecular orbital (HOMO) levels of the perovskite. This modification introduces an interface dipole at the perovskite-spiro-OMeTAD interface, as confirmed by optical measurements including time-resolved photoluminescence, ultraviolet photoelectron spectroscopy, and X-ray photoelectron spectroscopy. The formation of a 0.28 eV surface dipole facilitates effective band alignment, enhancing hole extraction efficiency and overall photovoltaic performance. This study underscores the potential of TPE-en as a promising strategy for advancing perovskite solar cell technology through enhanced interface engineering and improved device efficiency.

Chapter 5 focuses on studying and addressing the issue of thermalization loss in silicon solar cells. As the cornerstone of contemporary solar cell technologies, silicon exhibits 33% of thermalization losses when photons are absorbed, resulting in energy dissipation beyond its bandgap. This inefficiency limits the overall sensitivity to light and reduces the efficiency of silicon-based photovoltaic devices. To tackle this challenge, the chapter explores the concept of sensitizing silicon solar cells using singlet exciton fission (SF). This process involves the conversion of a high-energy singlet exciton into two lower-energy triplet excitons. These triplet excitons are energetically matched to the silicon bandgap, making them capable of generating additional electron-hole pairs when transferred to the silicon material. The study aims to replicate and expand upon previous work by Einzinger et al. by employing a different SF chromophore, specifically 9,10-Bis(phenylethynyl)anthracene (BPEA). This chromophore is investigated for its potential to enhance silicon solar cell performance by efficiently generating triplet excitons and facilitating their transfer to silicon. By integrating BPEA with both silicon and perovskite absorbers, the research seeks to mitigate thermalization losses and potentially increase solar cell efficiencies beyond the traditional single-junction limit of 29%. By systematically addressing these critical aspects through theoretical analyses, experimental studies, and advanced characterization techniques, this thesis seeks to advance the fundamental understanding of photovoltaic performance. The insights gained contribute to the development of next-generation solar cells capable of achieving higher efficiencies and approaching the theoretical limits of energy conversion.