Halide perovskites have seen, over the past years, a tremendous increase in photovoltaic power conversion efficiencies as device working mechanisms are unveiled. Lead halide perovskites are at the forefront of halide perovskite research given their intriguing optoelectronic properties. However, their short lifespan under thermal and moisture stress and their intrinsic toxicity both still stand as the main hurdle to their daily life application. Layered perovskites have emerged as a modified version of conventional bulk halide perovskites with the promise of increasing their stability by incorporating bulky organic cations within the structure. These, while serving as moisture ingress blockers, further contribute to tailor the optoelectronic properties of perovskites. Although generally, optoelectronically-innocent organic spacers are used, the first Chapters of this thesis will explore the possibility of incorporating optoelectronically-active functional cations to further tailor the properties of the perovskite. Within this framework, two spacers, namely, naphthalene diimide (NDI) and benzodithiophene (BDT) will be described. While the incorporation of the former into a layered perovskite appeared challenging, BDT could successfully lead to Ruddlesden-Popper-type perovskite structures when the alkyl ammonium side chain was optimized. Following these results, quasi-layered lead halide perovskites were fabricated using BDT. The performance and stability outperformed those obtained using a standard phenylethylammonium (PEA) spacer. The combination of an assortment of techniques revealed that the improved charge transport in the active layer and the passivation of surface iodide vacancies were behind the enhanced photovoltaic response. Being aware of the toxicity and stability concerns of lead halide perovskites for photovoltaic applications, the emerging double perovskite Cs2AgBiBr6 was investigated. With a Shockley-Queisser limit standing at 16.4%, but with much lower reported photovoltaic device efficiencies, current research aims at understanding the defects present in these materials as well as at establishing novel routes to optimize the performance. The work set out in this thesis first explores the effect of tuning the stoichiometry of the double perovskite. Since bromide vacancies are known to be present in the structure causing defects, the aim was to patch this issue by incorporating additional bromides using an excess CsBr in the precursor. The enhanced photovoltaic response with excess CsBr was accounted for by the passivation of surface bromide vacancies, as suggested by solid-state NMR and photoluminescence measurements, as well as the larger crystal grains. To improve on these results, an alternative smaller cation salt, i.e. LiBr, was further tested as an additive to the double perovskite. A clear enhancement in device performance was observed, where a LiBr diffusion treatment, as opposed to a precursor solution additive, was deemed optimal. 7Li solid-state NMR measurements revealed a distinct environment, likely of the Li+ ions at interstitial positions accompanying the bromide anions towards vacancy sites while intensity modulated photovoltage spectroscopy yields longer charge recombination lifetimes. These findings pave the way for the exploration and optimization of defect-mitigating treatments in double perovskites.