The metal halide perovskite group of materials not only have led to great advances in photovoltaics, but also have shown great potential in light emitting applications. Other than energy efficiency and brightness, perovskite emitters have advantages such as high colour saturation and low processing temperatures to compete with present inorganic and organic light emitting diode (LED) technologies. However, poor device stability continues to be the limiting factor for perovskite LEDs (PeLEDs) to be realised towards commercialisa- tion. In this thesis, the stability study was first demonstrated on methylammonium lead bromide (MAPbBr3) PeLED device under continuous operations. It was characterised in the post-biased device that the applied electric field simultaneously induces the chemical decomposition of MAPbBr3 material and the dissociation of the active Ag metal electrode. The formation of silver bromide (AgBr) marks the electrochemical degradation of MAPbBr3 and the corrosion of Ag electrode. These factors explain the short lifetime, irreversible device performance and poor emission stability during device operation. It is worth mentioning that emission stability (i.e. colour shifting) remains an unsolved problem in mixed-halide perovskites, as phase segregation problem occurs when external optical/electrical energy exceeds the threshold for halide demixing. In the following part of this thesis, vapour-assist crystallisation (VAC) technique was developed to stabilise the bromide/chloride (Br/Cl) mixed-halide phase and give spectrally stable emission. It was dis- covered that VAC-treatment slows-down the crystallisation process and promotes a two-step halide redistribution within the perovskite thin-film. The controlled crystallisation results in a mixed-halide phase with lower defects and homogeneously ordered composition. Spectrally stable emission was achieved when segregation threshold is raised well-above the maximum working condition of PeLED device. As a result, spectrally stable electroluminescence is realised at 478 nm wavelength (CIE coordinates (0.104, 0.124)) in VAC-treated rubidium- caesium-formamidinium triple-cation perovskite (RbCsFA)Pb(Br0.6Cl0.4)3. VAC-treated device luminance shows a 4-fold increase to 1638 cd/m2 and external quantum efficiency shows a 10-fold increase to 8.6 %, compared to the respective control counterparts. Further- more, VAC-treatment was applicable for improving spectral stability and device performance in phenethylammonium (PEA) incorporated PEA-CsPb(Br0.7Cl0.3)3 quasi-2d perovskite system, giving 472 nm wavelength pure-blue emission. Hence, there is potential that emitters made of VAC-treated perovskites could be pushed further to the deep-blue region without losing their colour stability and quantum efficiency. Furthermore, the role of Rb+ in Rb/Cs composition stoichiometry of perovskite was further investigated. The incorporation of Rb+ not only induces further blue-shift in emission wavelength, but also enhances photoluminescence quantum efficiency and compositional homogeneity in perovskite film. The addition of Rb+ also promotes a faster transformation of the stable perovskite phase. Finally, the optimised PeLED devices based on VAC-treated Rb0.1Cs1.2FA0.2Pb(Br1−xClx)3 (x = 0.4 - 0.45) give spectrally stable electroluminescence from blue (478 nm) to deep-blue (468 nm) region. The external quantum efficiencies from PeLEDs developed in this work gives 10.4 % at 478 nm - CIE coordinates (0.104, 0.124), and gives 5.36 % at 468 nm - CIE coordinates (0.130, 0.058), which is close to the Rec.2020 specified primary blue (0.131, 0.046). Both devices exhibit a narrow electroluminescence bandwidth, with full-wavelength at half-maximum (FWHM) given by 15.4 and 15.0 nm for blue and deep-blue PeLEDs, respectively. Spectrally stable emission is confirmed up to maximum luminance of 2485 cd/m2 and 1009 cd/m2 for blue and deep-blue PeLEDs, respectively. The performance of PeLEDs developed in this work is one of the highest reported to date in deep-blue colour regions.