Perovskite solar cells (PSCs) have emerged as next-generation photovoltaic technologies due to their potential as a low-cost, but highly efficient alternative to conventional silicon photovoltaics. However, the stability of three-dimensional metal halide perovskites (3D-MHPs) remains a longstanding concern, frustrating commercialization. To address this issue, two-dimensional metal halide perovskites (2D-MHPs) have emerged as more durable polymorphs. Nevertheless, their quantum confined nature and unpredictable formation give rise to anisotropic charge transport, short carrier diffusion lengths and uneven energy landscapes in PSC devices. This thesis aims to investigate the formation and degradation dynamics of 2D-MHPs with a focus on tailoring their phase-polydispersity, orientation and energy landscape, and subsequently integrating the excellent carrier properties of 3D-MHPs with the superior stability of 2D-MHPs via epitaxially grown 2D-on-3D heterostructures.First, I studied process-property relationships governing the formation of multilayered 2D-MHP films with uniform phase distribution and favorable crystal orientation. The rate of solvent removal during spin-coating was critical for the crystalline orientation, with faster solvent removal aligning the inorganic framework with the direction of current flow in PSCs, improving carrier mobility. Synergistic use of a strongly-coordinating additive promoted the formation of films with narrow phase polydispersity, reducing non-radiative recombination. As a result, the emergent 2D-MHP films exhibited micron-long electron diffusion lengths and fast carrier funneling, enabling PSCs of ~14% PCE with vastly superior outdoor stability compared to 3D-MHPs. Next, I introduced a new film architecture that synergistically integrates the superior carrier transport of 3D-MHPs with the robustness of 2D-MHPs, representing one of the earliest demonstrations of 2D/3D heterostructures in inverted PSCs. Thin, conformal 2D-MHP capping layers were grown atop a bulk 3D-MHP to form 2D-on-3D (2D/3D) heterostructures. To implement them in inverted PSCs, I designed surface treatments that form spatially uniform 2D-capping layers dominated by n = 3 and 4 2D-MHPs, reducing the conduction band minimum offset at the 3D

2D interface, enabling ultralong electron diffusion lengths. To phase-stabilize the capping layers, I sought to use the more stable formamidinium FA+ as a replacement for the widespread methylammonium MA+ cage cation. For the first time, I synthesized FA-based n = 3 2D-MHP single-crystals through lattice-matching, accommodating the larger internal lattice strain imposed by FA+. As a capping layer, the new FA-based 2D-MHPs exhibited spectacular phase stability with no degradation at all, completely protecting the underlying 3D-MHP under combined light, heat and humidity. Leveraging the superior structural stability and carrier properties, the FA-based 2D/3D stacks enabled inverted PSC devices with 25.33% PCE and 95% PCE retention 1200 h under continuous operation, at the forefront of PSC efficiency and stability. Overall, this thesis presents insights on the crystal growth process, sequence of events in the formation and degradation and new material design principles of 2D-MHPs. By understanding these fundamental aspects, perovskite solar cells with tailored energy landscapes and charge transport are achieved, enabling state-of-the-art perovskite solar cell performance with technologically relevant operational stability. Thus, these contributions propel perovskite solar cells closer to commercialization.