Manipulation of crystallization, optical properties and phase distribution of two-dimensional metal halide perovskite through solution-based method

Two-dimensional (2D) metal halide perovskite has attracted great attention with regards to solving the instability issue of the three-dimensional (3D) counterpart due to the use of hydrophobic cations. However, the natural quantum confinement effect and mismatched dielectric coefficient determine that the 2D perovskite comprises multiple microstructural domains with different orientations and distributions. In this thesis, systematic studies of different model systems were implemented, in terms of large organic spacers (Ruddlesden-Popper type, Dion-Jacobson type, Alternating-Cations-in-the-Interlayer type, mixed spacers), small cations (methylammonium, formamidinium), in order to manipulate the crystal orientation and phase distribution (normal gradient, reversed gradient) of the 2D perovskite thin film fabricated through spin-coating, which is a common solution-based fabrication method. Different processing methods (solvent engineering, additive doping, hot-casting, anti-solvent) were carried out for both internal and external control, in order to improve surface morphology and vertical grain alignment. Additionally, patterns and textures were also created by nanoimprinting lithography in both top-down and bottom-up manners, in order to induce light trapping and coupling and to tune the optical properties of 2D perovskite (e.g. to control the light absorption and reflection). The take-home message of this thesis is that the crystallization, optical properties and phase distribution of 2D and quasi-2D perovskite thin films can be controlled and regulated by different solution-based manipulation strategies and processing methods, a combination of which is usually used to make a difference. The comprehensive understanding of crystallization behaviour of 2D perovskites and the innovative highlight of spacer mixing shed light on building a promising future of highly efficient and stable devices such as 2D/3D perovskite solar cells, tandem solar cells.