Ligand Mediated Surface Chemistry in Perovskite Nanocrystals for Light-Emitting Application

Abstract

Metal halide perovskite nanocrystals (PNCs) have emerged as exceptional light-emitting materials, distinguished by their high photoluminescence quantum yields (PLQYs), narrow emission linewidths, and precisely tunable bandgaps spanning the entire visible spectrum. These remarkable optical properties position PNCs as promising candidates for next-generation display technologies and advanced photonic applications. However, realizing their full potential in practical optoelectronic devices requires overcoming several fundamental challenges inherent to their ionic nature and surface dominated properties. The soft ionic lattice of perovskites renders them susceptible to ion migration under electrical bias and thermal stress, leading to operational instability and performance degradation. Furthermore, the insulating organic ligands necessary for colloidal stability and defect passivation simultaneously impede efficient charge transport in solid-state films. Beyond these issues, the anisotropic optical properties arising from quantum confinement effects and the accurate determination of electronic structures at buried interfaces remain poorly understood, limiting rational device optimization strategies. This thesis presents a comprehensive investigation into ligand mediated surface chemistry of colloidal PNCs, establishing systematic strategies to address these challenges through molecular-level engineering. By elucidating the fundamental relationships between ligand structure, surface chemistry, and device performance, this work provides rational design principles for achieving high-efficiency, stable, and functionally advanced perovskite-based light-emitting devices. In Chapter 2, a multisite coordination strategy is developed using tetrafluoroborate (BF4⁻) as a pseudo-halide ligand to achieve robust defect passivation in FAPbBr3 nanocrystals. Unlike conventional monodentate ligands that bind exclusively to undercoordinated Pb2+ sites, BF4⁻ enables simultaneous coordination with both inorganic lead and organic formamidinium cations through hydrogen bonding interactions. This multisite binding significantly strengthens ligand attachment, effectively suppressing ion migration and enhancing PLQY to 98.8% by mitigating nonradiative recombination pathways. Computational analysis confirms that the multiple fluorine atoms in BF4⁻ establish stronger binding energies compared to single halide ligands. Building upon this enhanced surface stability, a post- synthetic ligand exchange using short chain FABr is introduced to replace insulating oleate ligands without compromising optical properties. This synergistic combination of strong BF4⁻ passivation and FABr mediated conductivity enhancement yields high performance green emissive LEDs with a maximum external quantum efficiency (EQE) of 25.2% at 4,474 cd/m², maintaining over 20% EQE up to approximately 8,000 cd/m². Device characterization under applied electric fields demonstrates significantly suppressed ion migration in BF4⁻-passivated films, validating the effectiveness of multisite coordination in achieving both defect suppression and improved charge transport. Chapter 3 establishes a ligand engineering strategy to systematically control the orientation of self- assembled CsPbBr3 nanoplatelets (NPLs) and their transition dipole moment (TDM) alignment. By partially substituting native ligands with ammonium bromide-based ligands of varying chain lengths and steric properties—specifically oleylammonium bromide (OAMBr), phenethylammonium bromide (PEABr), and butylammonium bromide (BABr)—both crystallization kinetics and inter-ligand interactions are precisely modulated. Long-chain OAMBr restricts lateral growth during synthesis, producing smaller NPLs that preferentially assemble in edge-up orientation through enhanced ligand interdigitation. This vertical alignment of NPLs results in out-of-plane TDMs, generating highly linearly polarized emission with a degree of polarization (DOP) of 14.6%. Conversely, short-chain BABr permits faster growth, yielding larger NPLs that adopt face-down orientation due to reduced inter-ligand interactions. Face-down NPLs exhibit in-plane TDMs with minimal polarization (DOP = 3.0%) but superior optical outcoupling efficiency in the surface-normal direction, achieving LED performance with EQE of 3.14%. The aromatic PEABr produces intermediate-sized NPLs with mixed orientations due to π–π interactions. Grazing-incidence wide-angle X-ray scattering (GIWAXS) and angle-dependent photoluminescence measurements confirm the correlation between ligand structure, NPL orientation, and optical anisotropy. These findings establish clear design principles linking molecular engineering to TDM alignment, enabling tailored emission characteristics for polarized light sources and enhanced outcoupling efficiency. In Chapter 4, a refined methodology for constructing accurate electronic structures and interfacial energy diagrams of CsPbBr3 nanocrystals is established. Conventional linear scale ultraviolet photoemission spectroscopy (UPS) analysis is shown to significantly overestimate ionization energies due to the unusually low density of states near the valence band maximum. By introducing logarithmic scale UPS analysis, the intrinsic valence band edge positions are more accurately determined, revealing energy alignments consistent with experimental device characteristics. Device studies demonstrate that turn-on voltage depends primarily on electron transport layer mobility rather than hole injection barriers, validating the corrected energy diagram. Furthermore, ligand induced surface dipoles are identified as critical modulators of vacuum level alignment and interfacial energetics. Didodecyldimethylammonium bromide (DDAB)-capped PNCs, possessing weaker surface dipoles, enable efficient and loss-free electron injection with minimal thermal dissipation. In contrast, native primary ammonium-capped PNCs induce larger vacuum level shifts that, despite permitting charge injection, cause excessive carrier thermalization and heat generation during device operation. Infrared thermal imaging confirms that DDAB-based devices operate at significantly lower temperatures, directly correlating with extended operational stability. This comprehensive framework integrating photoemission spectroscopy with device-level analysis provides essential insights into accurate electronic structure determination and interfacial dipole engineering for optimized PNC-based optoelectronics. Collectively, this thesis demonstrates that ligand chemistry serves as a powerful and versatile tool for controlling the structural, optical, electronic, and interfacial properties of perovskite nanocrystals. Through systematic investigation of ligand binding mechanisms, crystallization dynamics, and interfacial dipole effects, this work establishes fundamental design principles that bridge molecular level surface engineering with macroscopic device performance. The findings not only deepen the fundamental understanding of perovskite surface chemistry but also provide practical guidelines for achieving effective defect passivation, orientation control, charge transport optimization, and energy level alignment. These advances pave the way toward realizing next-generation perovskite optoelectronic devices, including high performance LEDs, linearly polarized light sources for advanced display and photonic applications.