Defect passivation strategies for enhanced longevity and performance in lead and lead-free based photovoltaics

Buyruk, Ali (2023): Defect passivation strategies for enhanced longevity and performance in lead and lead-free based photovoltaics. Dissertation, LMU München: Fakultät für Chemie und Pharmazie

Abstract

Halide perovskites have gained considerable attention in different research areas because of their unique properties and diverse applications in optoelectronics. These materials possess outstanding features, including adjustable and direct bandgaps, high electron and hole mobility, strong light absorption capabilities, and an impressive defect tolerance, resulting in extended carrier lifetimes and diffusion lengths. Halide perovskites stand out because of their ease of production, which extends to methods that can be carried out at room temperature. This combination of exceptional qualities paves the way for developing high-performance thin-film optoelectronic devices that can be manufactured using cost-effective techniques, notably solution-based methods. Consequently, "organic-inorganic electronics" has experienced substantial growth and exploration. Nonetheless, this material's inherent chemical and structural characteristics, coupled with its low-temperature processing methods, unavoidably result in deleterious defects at both the surface and grain boundaries (GBs) of the perovskite's polycrystalline structure. These defects can considerably undermine the solar cell performance and overall material stability. Furthermore, concerns over the environmental impact of lead (Pb) have driven the search for lead-free alternatives. This quest has led to investigating divalent metals like tin (Sn2+) or germanium (Ge2+), although these alternatives have encountered stability issues. Another promising approach involves heterovalent substitution, in which a trivalent cation such as Bi3+ is combined with a monovalent cation like Ag+ to create double perovskites, resulting in compounds with the chemical formula A2M′M″X6 (examples include Cs2AgBiBr6, Cs2AgInCl6, Cs2AgSbCl6, etc.). In contrast to traditional perovskite materials, emerging substances like silver-bismuth halides, characterised by their chemical formula AgaBibXa+3b and rudorffite structures, are gaining attention as potential light absorbers in solar cells. Their appeal is based on several key attributes, including their appropriate band gap energy (Eg) and remarkable light-absorbing properties. In this thesis, our primary focus was to investigate the defects at the surfaces and grain boundaries of polycrystalline lead-based perovskite materials and lead-free silver-bismuth halides. These defects significantly negatively impact the performance and stability of photovoltaic devices using these materials. To address these naturally occurring defects, we explored various materials and concepts to develop potential solutions. The opening section of this thesis, designated as Chapter 1, begins by elucidating the theoretical foundations of the relevant research area. Commencing with an exploration of semiconductors, the chapter subsequently delves into the practical applications of these materials, focusing mainly on solar cells. It then discusses an alternative and promising material used in solar cells: lead-containing perovskite-based materials. Continuing within this class, attention is given to more stable subcategories, notably those characterised by low dimensionality, such as 2D and 1D variations. The chapter concludes by examining the Rudorffite material class, which offers a silver-bismuth halide-based alternative to mitigate concerns related to the toxicity of lead-based perovskite materials. The subsequent second chapter provides introductory information regarding characterisation techniques commonly employed in these studies. Chapter 3 of the thesis delves into applying 1,10-phenanthroline, a bidentate chelating ligand, at the methylammonium lead iodide (MAPbI3) film interface and the hole-transport layer. This compound serves a dual purpose, acting as a passivating agent for surface defects involving under-coordinated lead ions and as a means to convert excess or unreacted lead iodide (PbI2) at interfaces into neutralised and beneficial species (PbI2(1,10-phen)x, with x being 1 or 2). This transformation is crucial for enabling efficient hole transfer at the modified interface and enhancing long-term stability. The efficacy of 1,10-phenanthroline in healing defects is confirmed through a comprehensive set of techniques, including photoluminescence measurements (both steady-state and time-resolved), space-charge limited current (SCLC) measurements, light intensity-dependent JV measurements, and Fourier-transform photocurrent spectroscopy (FTPS). Additionally, advanced X-ray scattering methods, nano-Fourier transform infrared (nano-FTIR) spectroscopy, and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) are employed to provide a detailed analysis of the structure and chemical composition at the nanoscale of the perovskite surface following treatment. Chapter 4 of the thesis introduces the application of a novel organic cation, 2-(thiophene-2yl-)pyridine-1-ium iodide (ThPyI), at the surface of 3D methylammonium lead iodide (MAPI) perovskite films, as well as its incorporation into the bulk structure. ThPyI is used either as a passivator on top of 3D MAPI to create surface-passivated films with an additional stable 1D perovskite phase, formed as a product of the unreacted surface PbI2 and organic cation or independently introduced into the 3D MAPI precursor to achieve bulk passivation. In both approaches, ThPyI leads to improved power conversion efficiency (PCE) and enhanced stability in solar cells. Moreover, advanced characterisation techniques, such as Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS), confirm that ThPyI promotes the preferred orientation of bulk MAPI slabs, enhancing charge transport. Notably, the bulk-passivated 3D configuration demonstrates excellent stability, with 84% PCE retained after 2000 hours of exposure without encapsulation. This study introduces innovative strategies for using organic spacers with diverse binding motifs and passivation techniques to address defects in hybrid 3D/1D perovskite solar cells. The last chapter of the thesis focuses on applying a well-established material, poly(2,5-dimercapto-1,3,4-thiadiazole), to enhance the efficiency and stability of solar cells based on Ag3BiI6. The central strategy in this chapter involves using in-situ polymerisation for bulk treatment to passivate the detrimental defects within the internal structure of Ag3BiI6. The study shows that polymer-treated Ag3BiI6 films can enhance efficiency and maintain stability when utilised in solar applications, underlining their potential as a safer and more sustainable option for solar energy conversion.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 540 Chemie
• Fakultäten: Fakultät für Chemie und Pharmazie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 19. Dezember 2023
• 1. Bericht­erstatter:in: Ameri, Tayebeh