This dissertation describes our study on ion migration-driven optoelectronic and structural changes of hybrid metal halide perovskites under light illumination or electric bias, and methods to exploit or inhibit these ion migration processes by implementation of effective passivation approaches. We first critically review recent studies on the photobrightening effect, which has a strong correlation with the extent of photo-induced ion migration processes, and has a substantial impact on performance and lifetime of the perovskite optoelectronic devices. We highlight the sensitivity of this effect to experimental considerations such as atmosphere, photon energy, photon dose, perovskite composition, and morphology, and resolve apparent contradictions in the literature. We then explore the impact of light illumination on triple-cation lead mixed-halide perovskite films, with substantially enhanced luminescence efficiency observed in these materials. We perform a range of experimental characterisations to understand these changes in photo-physical properties of the perovskite materials under light illumination. Although the brightening effect in halide perovskites has the advantage of achieving high luminescence efficiencies, the bandgap instability arising from ion segregation, particularly in systems with high bromide fractions, prevents the full exploitation of the continuous bandgap tunability in perovskite materials. To this end, we demonstrate a passivation approach using potassium halides to both enhance the luminescence efficiency and inhibit ion segregation in hybrid perovskite films. Through photoelectron spectroscopy, and chemical mapping measurements, we indicate that the passivation effects mainly originate from formation of potassium halide inclusions with higher concentration at the perovskite surface. We also explore the changes in properties of perovskite films when biased with an electric field in an operating light-emitting device architecture. Using a combination of in-situ and ex-situ measurements, we monitored the degradation of mixed-halide perovskite light-emitting diodes over time. Through chemical mapping, and optical spectroscopy measurements, we revealed that the degraded performance arises mainly from accumulation of halides at one interface. We demonstrate how the potassium passivation immobilizes excess halide species and thus inhibits these ionic migration effects. The passivated light-emitting diodes show enhanced external quantum efficiency from 0.5 to 4.5% and, most importantly, show significantly enhanced operational stability over the control devices. The half-life of decay for the devices under continuous operation is increased from <1 hour to ~10 hours through passivation, and ~200 hours under pulsed bias operation. This work provides an insight into understanding and controlling ion migration processes and highlight routs to improved performance and operational stability in new device structures.