Perovskite solar cells have gained significant attention in recent years due to their potential as a low-cost and highly efficient alternative to traditional silicon-based solar cells. In particular, 3D perovskites have shown exceptional performance in solar cells, but their stability remains a concern. To address this issue, researchers have been exploring the use of 2D perovskites, which offer enhanced stability due to their reduced dimensionality. This thesis aims to investigate the role of 2D perovskites in enhancing the efficiency and stability of 3D perovskite solar cells, as well as exploring the combination of 3D/2D perovskites to further enhance the efficiency and stability of the solar cells. The thesis presents a simple and scalable approach for the fabrication of 2D perovskite thin films with a homogenous layer thickness, termed as the "phase-selective method". This method involves the dissolution of single-crystalline powders with a homogeneous perovskite layer thickness in desired solvents to fabricate thin films. In situ characterizations reveal the presence of sub-micrometer-sized seeds in solution that preserve the memory of the dissolved single crystals and dictate the nucleation and growth of grains with an identical thickness of the perovskite layers in thin films. This approach can lead to the production of high-quality 2D perovskite thin films, which can enhance the stability and efficiency of solar cells. In the second part, the thesis presents a study on the development of highperformance 2D perovskite solar cells using Li-doped nickel oxide (NiOX) as a hole transport layer (HTL). The incorporation of Li-doped NiOX significantly improves the morphology, crystallinity, and orientation of 2D perovskite films and affords a superior band alignment, facilitating efficient charge extraction. Furthermore, 2D PSCs with Li-doped NiOX exhibit excellent photostability without the need for external thermal management, which is a significant advantage for practical applications. Finally, the thesis addresses the challenge of achieving solution-processed heterostructures in halide perovskites by developing a new approach to grow phasepure 2D halide perovskite stacks of the desired composition, thickness, and bandgap onto 3D perovskites without dissolving the underlying substrate. The resulting 3D/2D bilayer exhibited a high photovoltaic efficiency and exceptional stability, indicating that the 3D/2D bilayer inherits the intrinsic durability of 2D perovskite without compromising efficiency. Overall, this thesis presents several significant contributions towards the development of highly efficient and stable perovskite solar cells. The phase-selective method for fabricating 2D perovskite thin films can lead to the production of high quality 2D perovskites, while the use of Li-doped NiOX as an HTL can significantly improve the efficiency and stability of 2D perovskite solar cells. Additionally, the development of a new approach to grow 2D halide perovskite stacks onto 3D perovskites can pave the way for future developments in the field of perovskite solar cells. These findings have significant implications for the development of a clean energy future.