Stabilizing metastable lead-free double perovskite by morphological and molecular level dimensional reduction

Wang, Shizhe (2024): Stabilizing metastable lead-free double perovskite by morphological and molecular level dimensional reduction. Dissertation, LMU München: Fakultät für Chemie und Pharmazie

Abstract

In the field of perovskite solar cells, explorations of new lead-free double perovskite materials are of great interest to address toxicity issues of lead-based perovskites. Cs2AgBiBr6 is the first and the most-studied double perovskite material as a promising candidate for photovoltaic applications, owing to its long carrier lifetime and outstanding stability. However, reported PCEs of Cs2AgBiBr6 solar cells remain mostly around 3% due to the large indirect band gap of this material and other factors. Therefore, the expansion of absorption to the longer wavelengths is being pursued for double perovskite solar cells. To address this issue, general strategies are either searching for alternative combinations of central metal atoms to replace Ag-Bi or developing its iodide analogue. Some candidates with theoretically promising optoelectronic properties consist of metastable octahedra, which can not be synthesized as bulk crystals but can be stabilized by morphological or molecular level dimensional reduction. In this thesis, we first reviewed the strategy of morphological level dimensional reduction. We summarized the latest advances of the synthesis methods for both lead-based and lead-free perovskite nanocrystals (PNCs). Stability is one of the critical issues for the possible commercialization of PNCs. We reviewed the crystal structural stability, interface-induced stability and environmental stability, such as oxygen, moisture, light and thermal stability. Moreover, strategies for encapsulation to protect PNCs were discussed. Importantly, various applications of the PNCs in optoelectronics, like solar cells, light-emitting diodes and photodetectors, were briefly presented. Finally, the challenges remaining for improving the stability of PNCs and an outlook towards possible directions in developing PNC-based devices with high performance and great operation durability were addressed. In the second part of the thesis, we describe the synthesis of Cs2CuSbCl6 double perovskite nanocrystals (DPNCs) at ambient conditions by a facile and fast method, namely, a modified ligand-assisted reprecipitation (LARP) method. We chose methanol as solvent for precursor salts as it is less toxic and easily removed in contrast to widely-used dimethylformamide. Our computational structure search shows that the Cs2CuSbCl6 structure containing alternating [CuCl6]5- and [SbCl6]3- octahedral units is a metastable phase that is 30 meV/atom higher in energy compared to the ground state structure with [CuCl3]2- and [SbCl6]3- polyhedra. However, this metastable Cs2CuSbCl6 double perovskite structure can be stabilized through the solution-based nanocrystal synthesis. Using an anion-exchange method, Cs2CuSbBr6 DPNCs are obtained for the first time, featuring a narrow band gap of 0.9 eV. Finally, taking advantage of the solution processability of DPNCs, smooth and dense Cs2CuSbCl6 and Cs2CuSbBr6 DPNC films are successfully fabricated. Hence, we establish that these metastable Cs2CuSbCl6 and Cs2CuSbBr6 double perovskite structures can be successfully stabilized by morphological level dimensional reduction. Besides searching for other metal combinations to enhance the optical absorption ability, replacing Br- with I- can significantly reduce the band gap of Cs2AgBiBr6 double perovskites. Unfortunately, Cs2AgBiI6 is unstable due to the lower formation energy of competing Cs3Bi2I9 but can also be stabilized by morphological level dimensional reduction. In this context, we prove that the metastable iodide analogue can also be stabilized by molecular level dimensional reduction, which implies introducing large hydrophobic organic cations into double perovskite structures and “cutting” the 3-dimensional structures normal to a certain axis to form 2-dimensional perovskites. Here, we report that both Ag-Bi-I and Cu-Bi-I systems are thermodynamically stable within the structures of (aromatic-O-linker-NH3)4AgBiI8 and (aromatic-O-linker-NH3)4CuBiI8, respectively, where the aromatic moiety is naphthalene or pyrene and the linker is ethyl or propyl. The intrinsic electronic challenges of double perovskites are investigated and the electronic anisotropy of two-dimensional perovskites is alleviated. With more extended conjugated systems, (pyrene-O-ethyl-NH3)4AgBiI8 was isolated from a total of eight new 2D double perovskites, exhibiting an electronic band structure forming a type IIb multiple quantum well system with favorable intraorganic layer arrangement for out-of-plane conductivity, leading to a photocurrent response ratio of almost three orders of magnitude under AM1.5G illumination. Finally, (pyrene-O-ethyl-NH3)4AgBiI8 was also integrated to construct the first pure n = 1 Ruddlesden-Popper 2D double perovskite solar cell featuring 2D layers parallel to the electrode substrate.

• 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. September 2024
• 1. Bericht­erstatter:in: Bein, Thomas