Interface Engineering and Optimization of Methylammonium Lead Iodide-Based Perovskite Solar Cells: From Fabrication Processes to Performance Enhancement

Abstract

Perovskite solar cells (PSCs) have emerged as a promising technology for efficient and cost-effective photovoltaic energy conversion. Among the various perovskite materials investigated, Methylammonium lead iodide (MAPbI3) has garnered significant attention due to its high efficiency and compatibility with solution processing techniques. However, despite its potential, MAPbI3-based PSCs face challenges related to device performance and stability, stemming from inherent defect properties and ion migration within the material. This research is dedicated to a comprehensive examination of MAPbI3 as an absorber layer in PSCs, aiming to address these challenges and advance the understanding of device physics. The investigation begins by exploring the intricate multilayer structure of PSCs, which comprises the perovskite layer, charge transport layers, and electrodes. These layers introduce challenges associated with charge recombination and trap state formation at various interfaces, impacting device performance. One pivotal facet of this work entails a meticulous analysis of fabrication processes and their profound impact on both the quality of perovskite films and the resultant performance of the devices. Specifically, techniques such as vacuum annealing and methylamine (MA) vapour exposure are meticulously investigated to discern their effects on the crystallographic attributes and optoelectronic properties inherent within the MAPbI3 perovskite films through a systematic examination of the behaviour exhibited by MAPbI3. This affects the intricate mechanisms governing charge transport and degradation phenomena within the material. Such endeavours yield invaluable insights into the optimization strategies requisite for enhancing device performance. Moreover, these fabrication processes are not only employed to elucidate the fundamental mechanisms underlying charge transport and degradation but also to confront the formidable challenge of recovering perovskite films post-degradation. By leveraging techniques such as vacuum annealing and MA vapour exposure, the research endeavours to restore the structural and optoelectronic integrity of the perovskite films. In essence, the exploration of fabrication processes within this study extends beyond mere characterization; it serves as a cornerstone for the development of robust device optimization strategies, while simultaneously offering solutions to the persistent challenge of perovskite film recovery. The absorption spectrum of MAPI exhibits non-uniformity across the visible spectrum, while charge carrier generation in devices is similarly non-uniform. This investigation focuses on the impact of different wavelengths of radiation on the performance characteristics of solar cell devices. The devices are subjected to red, green, and blue (RGB) illumination, and their performance is scrutinized using External Quantum Efficiency (EQE) spectra to discern the varying impacts of different energy levels within the electromagnetic spectrum. Capacitance and impedance measurements are integrated under RGB illumination conditions and varied temperature settings to explore the inherent defect properties of the system. Despite existing reports on wavelength-dependent degradation, these comprehensive investigations are pivotal in scrutinizing alterations in defect states, elucidating the role of interfacial defects, discerning the wavelength's impact on interfacial degradation, and comprehending the intricate dynamics of charge carriers in solar cells. The next section of the investigation delves into the characterisation of defects, both bulk and interfacial, which are identified as pivotal factors influencing device performance. Strategies for mitigating bulk defects through the use of transition metal salts are explored, along with approaches for defect tailoring and band-edge engineering. Additionally, the study employs advanced spectroscopic techniques such as photoluminescence and Deep-level transient spectroscopy to analyse charge carrier dynamics and identify pathways for improvement. Interfacial defects and their impact on charge transport between the absorber and transport layers are thoroughly examined. The roles of self-assembled monolayers (SAM), 2D, and 3D materials in interface defect passivation and charge transport are investigated with meticulous details with the different luminescence and capacitive studies. Furthermore, the study evaluates the effect of interface improvement on device stability, crucial for long-term performance. In summary, this thesis offers a systematic and comprehensive investigation into MAPbI3-based PSCs, leveraging advanced characterisation techniques and device simulations. By addressing fundamental challenges and elucidating key aspects of device physics, this research contributes to the advancement of perovskite solar cell technology, paving the way for more efficient and stable photovoltaic devices.