Solar cell technology is rapidly establishing itself as a viable option for large-scale power generation. Currently the most popular technology for this is based on Silicon-solar cell technology, which is also known as first generation photovoltaics. Hybrid inorganic-organic perovskite materials based on methylammonium lead iodide are an emerging technology. In the 10 years since their discovery efficiencies have risen from 3 % to 23 %, and as such are on the verge of commercialisation. Yet there are still some unsolved problems. One such problem is the lack of stability, caused by a susceptibility to degradation by water, heat and even light. Using mixed-cation perovskites has been shown to improve the short-term performance and long-term stability of perovskite solar cells. In this work a new and stable alternative cation to methylammonium is proposed, azetidinium. Azetidinium lead iodide exhibits a unique 2.5-Dimensional structure when used on its own, and improves efficiency and stability when used in conjunction with methylammonium. Following this the effect of cation-mixing is assessed in more detail. A wide-ranging study using 8-different cationic additives was performed to analyse their effect on iodide diffusion in methylammonium lead iodide. The distortion caused, expanding perovskite lattice size, increases the barrier for iodide diffusion, and with the largest cationic additives bulk iodide diffusion is no longer observed on the timescale of the experiment. This is then extended to formamidinium-based perovskites and the record efficiency triple-cation perovskite. In this case the cationic additives reduce the activation energy for iodide diffusion, and this is attributed to the distortion to smaller lattice sizes.