Stability of 2D metal halide perovskites

Abstract

Two-dimensional (2D) and quasi-2D metal halide perovskites (MHPs) have emerged as promising alternatives to conventional 3D perovskites due to their enhanced stability, tunable optoelectronic properties, and suppressed ion migration. However, their intrinsic instability under operational stressors remains a critical challenge. This thesis investigates the degradation mechanisms of 2D MHPs through the lens of photo/electrochemical redox activity and explores strategies to enhance their stability and functional applications. The degradation mechanisms of 2D Pb-based MHPs were systematically studied in this thesis by comparing Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) phases under controlled illumination and bias. Photostability and halide segregation resistance were also evaluated in mixed-halide systems with varying organic spacers. Multiple characterization methods were performed to validate the proposed mechanism, including analyzing the organic and inorganic component behavior and identifying the various degradation reaction products from diverse angles. Additionally, 2D tin-based halide perovskites (THPs) were engineered for thermoelectric and red-emitting materials with enhanced stability. In 2D Pb-based MHPs, degradation is found to initiate via hole accumulation, leading to structural ionic bond weakening and iodide oxidation (I2, I3-, interstitial defects). These oxidized species drive spacer deprotonation, forming vacancies that accelerate iodide migration. RP perovskites, with monoammonium spacers, exhibit rapid degradation due to single-site deprotonation, while DJ phases, featuring diammonium spacers, resist vacancy formation and iodide migration, enhancing their photostability. Experimental validation confirmed reduced iodine expulsion and gaseous byproducts in DJ systems. This proposed mechanism is supported by the photo-induced halide segregation behavior observation in 2D mixed halide perovskites, where aromatic spacers with ethylammonium tail demonstrated superior resistance to photoinduced halide segregation compared to alkyl-based spacers. FTIR analysis linked this stability to reduced spacer loss and suppressed redox activity between organic cations and oxidized halides, which is in good agreement with the proposed degradation pathway. For 2D THPs, TEA2SnI4 emerged as an ambient-stable material with promising thermoelectric performance. SnI4 doping increased conductivity, achieving a much higher power factor, surpassing reported frontier material PEA2SnI4. As red emitters, TEA-based THPs exhibited enhanced photoluminescence stability when modified with 3-phosphonopropionic acid (3PPA), SnCl2, and aromatic ligands. This work establishes a mechanism framework linking photo/electrochemical redox activity to 2D MHP degradation, guiding the development of stable architectures. The exploration of 2D THPs then opens new avenues for sustainable optoelectronic and energy-harvesting applications, underscoring the transformative potential of 2D perovskites in next-generation technologies.