Halide perovskite semiconductors have rapidly increased in power conversion efficiency in photovoltaic applications, achieving efficiencies comparable to commercially established silicon-based technology in less than ten years since the introduction of halide perovskites in photovoltaics. Lead halide perovskites, in particular, have been increasingly studied due to their excellent optical properties, low-cost solution processability, and ease of fabrication. However, the issue of long-term stability is the main roadblock for their commercialization.This document discusses two main approaches to address the stability issues of lead halide perovskites: 1) reducing their dimensionality from 3D (CsPbI3) to 2D ((C4H9NH3)2Csn 1PbnI3n+1) to form layered halide perovskites, and 2) using mixed cations in the 2D perovskite structure ((C4H9NH3)2((CH(NH2)2)1-xCsx)n-1PbnI3n+1). In the first approach, lead halide octahedral sheets were sandwiched between hydrophobic bulky organic cations to block the diffusion of water molecules into the crystal structure. The effect of solvent additives on crystallization dynamics was also investigated to reduce site energy disorder in layered halide perovskites. We found that processing conditions such as thermal annealing could be manipulated to control crystallization kinetics to reduce site energy distribution. In the second approach, the effect of mixing A-site cations on the stability and crystallographic orientation of layered halide perovskites was studied by systematically changing the ratio of two different cations (formamidinium vs cesium) in precursor solutions. Layered halide perovskites with mixed cations showed higher stability at ambient conditions compared to those with single cations. Furthermore, intermediate compositions of mixed cations resulted in a favorable crystallinity and orientation in layered halide perovskites. Within this dissertation, we have also advised a new technology to remove the need for toxic solvents in thin film fabrication processes. Melting and blade coating of 2D perovskite precursors followed by exposure to cation solutions in benign solvents (e.g., 2-propanol) has been proposed as a proof of concept approach for industrial applications. Even in thick melt-processed films, we achieved successful cation exchange using either solution-based or solid-state techniques, resulting in films with appropriate bandgap for optoelectronic applications such as light-emitting diodes and biosensing. We hope that this dissertation's findings can help the efforts to make these materials more stable and efficient and their processing more economical and environmentally friendly, allowing their implementation in photovoltaics and other optoelectronic applications.