Surface Chemistry of Lead-Free Perovskite Materials in Relation to their Photophysics and Energy Storage


Student thesis: Doctoral Thesis

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Award date16 Aug 2023


Metal halide perovskite materials have attracted widespread attention in the past decade owing to their impressive features such as low-cost, facile synthesis methods, bandgap tunability, long carrier lifetime, and high photoluminescence quantum yield (PLQY, which is defined as the number of photons emitted as a fraction of the number of photons absorbed) of excitonic emission covering the whole visible region. In this thesis, we study the surface chemistry and crystallization mechanisms of various perovskite materials ranging in their dimensions from nanoscale to microscale and propose strategies to fine-tune the surface Van der Waals interactions between perovskites and external factors for several applications, from bright and adjustable self-trapped emission to high mass loading of element iodine and effective immobilization of polyiodide ions with considerable electrochemistry reliability.

Chapter 1 of this thesis provides a literature review on recent advances of synthesis of perovskite nanocrystals and microcrystals, and their applications in optoelectronic devices, information encryption, and energy storage while serving as solid-state electrolytes. We pay special attention to the emerging effective in situ- or post-treatment strategies of perovskite materials such as passivation of surface halide vacancies using proper capping ligands and inert coating shells to improve PLQYs and the overall stability of perovskite materials. We notice that the presence of unfavorable toxic lead component in perovskite materials recently shifted the research focus towards lead-free perovskite alternatives, such as lead-free double perovskites and tin-halide perovskite derivates. Chapter 2 presents high-performance lead-free double perovskite Cs2AgInCl6 nanocrystals (NCs) enabled by a Bi3+ and Ce3+ co-doping strategy by which the uncoordinated chloride ions and silver vacancies were eliminated, which resulted in PL emissions adjustable from 589 to 577 nm under different doping amount of Ce3+ (the feeding ratio of Bi3+ was fixed at 1%). Among them, Cs2AgInCl6 co-doped with 1% Bi and 2% Ce achieved the highest PLQY of 26% for the PL peak centered at 580 nm while Cs2AgInCl6 NCs only doped with 1% Bi showed a low PLQY of 10% at 591 nm. The introduced Ce3+ dopant contributed to the localization of self-trapped excitons and prevented PL quenching, and meanwhile the cerium ion’s 5d excited state provided an energetically favorable indirect route for the radiative relaxation process. This co-doping strategy allowed us to fabricate well-crystallized lead-free halide double perovskite NCs with a strong and tunable emission.

Lead-free 2D perovskites based on tin halide octahedron slabs with Dion-Jacobson (DJ) phases have drawn wide attention due to their improved stability; still, reports on light-emitting DJ lead-free perovskites are scarce. Chapter 3 introduces our room-temperature ligand assisted re-precipitation method used for the synthesis of lead-free 2D ODASnBr4 (ODA is short for 1,8-octanediamine) perovskite microcrystals. The introduced chloroform and dichloromethane molecules acting as proton donors effectively enhanced crystallinity and emission properties of the DJ-phase perovskite microcrystals, owing to the formation of C-H…Br hydrogen bonds between the acidic C-H proton donors and [SnBr6]4- octahedra. As such, ODASnBr4 microcrystals produced using these molecular dopants delivered an impressive PLQY approaching 90% combined with a stable emission, which is able to sustain continuous UV irradiation for up to 6 hours. Besides, after applying controlled exposure to these molecular dopants one could fine-tune the peak emission of ODASnBr4 over a spectral range of 570-608 nm with high PLQYs of 83–88%, which provided a facile strategy to adjust the spectral position of DJ perovskite emission without changing halides or A-site spacers. Related to this, Chapter 4 presents the conventional saturation recrystallization process which was generally used for the synthesis of perovskite single crystals but resulted in a complex mixture instead of pure DJ-phase ODASnBr4 perovskite single crystals due to the strong hydrogen bonding between diammonium cations and the polar hydrogen bromide solution. Primary alcohols such as EtOH, PrOH, BuOH, and PeOH were used to dissolve and remove undesired byproducts, which simultaneously resulted in the molecular doping and improved crystallinity of the DJ-phase perovskites. Specifically, the interaction between the [SnBr6]4− octahedron slabs and the primary alcohols, in forms of O-H...Br hydrogen bonds, improved the lattice coordination of perovskites and promoted removal of undesired surface-absorbed water and the reaction byproducts, thereby achieving high PLQYs close to 90%. The positive impacts on the lattice crystallinity and surface capping further resulted in improved thermal stability and oxidation resistance of DJ perovskite microcrystals, offering prospects for further applications of these materials.

Though low-dimensional organic-inorganic hybrid perovskites have shown much-improved stability, their highly ionic lattice inevitably suffers from structural degradation when exposed to polar solvents. However, this feature may in turn give raise to their applications in rechargeable metal-halogen batteries as conversion-type cathodes which are considered promising for energy storage systems, but suitable and stable electrode materials are scarce. Chapter 5 demonstrates the use of 0D organic-inorganic MXDA2SnI6 (MXDA is short for M-xylylenediamine) perovskite microcrystals as conversion-type cathodes in widely studied aqueous metal-iodine batteries, which suffer from serious intrinsic issues such as the undesired shuttle effect and volatile iodine that hinder their reliable long-term performance. The use of 0D MXDA2SnI6 perovskite microcrystals ensured a high proportion of elemental iodine (46 wt.%) in the whole cathode. Benefitting from the long-chain organic matrix and B-site divalent Sn cations in the MXDA2SnI6 perovskite, iodide anions could be effectively confined on the cathode side to relieve the shuttle process, so that the respective cathodes showed reliable electrochemical activity. Moreover, the formation of triiodide anions was effectively restrained, replaced by the dominating presence of pentaiodide ions during the charging process. According to the DFT calculations, Sn-I…I halogen bonds and N-H…I hydrogen bonds were found to be the main contributors towards the increased formation energy of I3- ions and effective immobilization of iodide species on the cathode. As a result, rechargeable Zn-I2 battery achieved a champion capacity of over 206 mAh g-1I at 0.5 A g-1 (close to the theoretical limit of 211 mAh g-1I) with an outstanding capacity retention of 87% at 3 A g-1 and prolonged cyclic stability. By demonstrating the viability of ionic perovskites as conversion-type cathodes, this research advanced the progress towards high-performance cathode materials for metal-I2 batteries.

Chapter 6 summarizes and highlights the general major outcomes of this study in relation to the surface chemistry of various perovskite materials and offers an outlook into some remaining issues and future perspectives toward broader applications of perovskite materials in optoelectronics and energy storage.

    Research areas

  • Lead-free perovskite, Surface chemistry, Fluorescence, zinc-based battery