Lighting comprises a large part of the global electricity consumption as of today, and the use of lighting in illumination and displays is only projected to grow. It is therefore imperative to meet this energy demand, not only by means of greener energy production, but also with materials that are both more efficient to fabricate as well as to use. Low cost and energy efficient light sources therefore play an important role in minimizing further greenhouse emissions from the way we choose to live.

Metal halide perovskites are a group of semiconductors that have received a great amount of attention during the past years due to impressive - and continuously increasing - performance as active materials implemented in solar cells and light emitting diodes. This is due to highly desirable optoelectronic properties combined with low-cost, solution-processable fabrication methods. Simple bandgap-tunability is easily achieved by compositional and dimensional engineering, allowing perovskite emission to span a broad wavelength region from ultraviolet to near infrared. As with previous technologies, attaining stable, bright, and pure blue light has proven difficult also in metal halide perovskites. This thesis investigates some of the challenges in achieving blue emission in mixed-halide and mixed-dimensional perovskites for light-emitting-diode applications.

Mixed-halide alloying provides the most straightforward way of tuning the bandgap of perovskites. Unfortunately, mixed bromide/chloride-perovskites (used to achieve blue light) suffer from both spectral and temporal instabilities, as well as severe luminescence quenching at the large chloride contents necessary for blue emission. The spectral instability arises from a segregation of halides into regions of differing halide content, and hence different bandgap, resulting in a shift in emission color during operation. Although the origins of the poor temporal stability of perovskite light emitting diodes are manifold, one of the main problems are the low barriers for halide migration under the applied electric field during operation, rapidly degrading the device properties.

We first find that compositional heterogeneities, stemming from rapid uncontrolled film growth, both lowers the threshold for further halide segregation as well as serves as centers for non-radiative recombination, resulting in reduced luminescence yield. We show that by carefully moderating the crystallization dynamics it is possible to achieve films with a homogeneous composition, thereby mitigating the negative effects arising from material inhomogeneities. We identify means of how growth environment, stoichiometric tuning and chelating additives can be used to favorably control film formation and provide guidelines that can be more widely applied in the fabrication of perovskite films and devices. We continue by investigating the role of Br/Cl-alloying on device efficiency and stability in green to blue emitting perovskite LEDs. We find that chloride incorporation, while having only a minor impact on efficiency at moderate levels, detrimentally affects device stability even in small amounts. We ascribe this phenomenon to an increased mobility of halogen ions in the mixed-halide lattice resulting from an increased chemically and structurally disordered landscape with reduced migration barriers. We assign this as the major obstacle towards stable blue-emitting mixed-halide perovskite light emitting diodes.

In the last work we investigate blue emitting mixed-dimensional Ruddlesden-Popper perovskites (RPPs) comprising of multiple-quantum-well-structures of varying bandgap. Successful implementation in LEDs has been attributed to efficient carrier funneling from large bandgap (donor) regions to low bandgap regions (acceptors) resulting in improved luminescence yields due to trap state filling from the locally increased carrier density. However, due to the enhanced carrier concentrations in acceptor domains, Auger recombination quickly outcompetes radiative recombination mechanisms already at moderate pump fluences or carrier injection densities in RPPs. We show that by moderating the inter-well carrier transfer, while at the same time providing adequate defect passivation, high quantum yields can be maintained even at large carrier densities. We thereby show that RPPs can support a large density of carriers without compromising luminescence efficiency, paving the way for their use in high brightness applications by engineering the funneling and recombination processes in these materials.

The work in this thesis provides new insights on various dynamical processes in metal halide perovskites aimed at light emitting applications. The hope is that it will contribute toward the understanding of these systems and help in bringing these materials closer to practical use.

The threshold carrier density, conventionally evaluated from optical pumping, is a key reference parameter towards electrically pumped lasers with the widely acknowledged assumption that optically excited charge carriers relax to the band edge through an ultrafast process. However, the characteristically slow carrier cooling in perovskites challenges this assumption. Here, we investigate the optical pumping of state-of-the-art bromide- and iodine-based perovskites. We find that the threshold decreases by one order of magnitude with decreasing excitation energy from 3.10 eV to 2.48 eV for methylammonium lead bromide perovskite (MAPbBr(3)), indicating that the low-energy photon excitation facilitates faster cooling and hence enables efficient carrier accumulation for population inversion. Our results are then interpreted due to the coupling of phonon scattering in connection with the band structure of perovskites. This effect is further verified in the two-photon pumping process, where the carriers relax to the band edge with a smaller difference in phonon momentum that speeds up the carrier cooling process. Furthermore, by extrapolating the optical pumping threshold to the band edge excitation as an analog of the electrical carrier injection to the perovskite, we obtain a critical threshold carrier density of similar to 1.9 x 10(17) cm(-3), which is one order of magnitude lower than that estimated from the conventional approach. Our work thus highlights the feasibility of metal halide perovskites for electrically pumped lasers.

Bandgap tuning through mixing halide anions is one of the most attractive features for metal halide perovskites. However, mixed halide perovskites usually suffer from phase segregation under electrical biases. Herein, we obtain high-performance and color-stable blue perovskite LEDs (PeLEDs) based on mixed bromide/ chloride three-dimensional (3D) structures. We demonstrate that the color instability of CsPb(Br1-xClx)(3) PeLEDs results from surface defects at perovskite grain boundaries. By effective defect passivation, we achieve color-stable blue electroluminescence from CsPb(Br1-xClx)(3) PeLEDs, with maximum external quantum efficiencies of up to 4.5% and high luminance of up to 5351 cd m(-2) in the sky-blue region (489 nm). Our work provides new insights into the color instability issue of mixed halide perovskites and can spur new development of high-performance and color-stable blue PeLEDs.

One of the most critical challenges in perovskite light-emitting diodes (PeLEDs) lies in poor operational stability. Although field dependent ion migration is believed to play an important role in the operation of perovskite optoelectronic devices, a complete understanding of how it affects the stability of PeLEDs is still missing. Here, we report a unique self-repairing behavior that the electroluminescence of moderately degraded PeLEDs can almost completely restore to their initial performance after resting. We find that the accumulated halides within the hole transport layer undergo back diffusion toward the surface of the perovskite layer during resting, repairing the vacancies and thus resulting in electroluminescence recovery. These findings indicate that one of the dominant degradation pathways in PeLEDs is the generation of halide vacancies at perovskite/hole transport layer interface during operation. We thus further passivate this key interface, which results in a high external quantum efficiency of 22.8% and obviously improved operational stability.

Bright and efficient blue emission is key to further development of metal halide perovskite light-emitting diodes. Although modifying bromide/chloride composition is straightforward to achieve blue emission, practical implementation of this strategy has been challenging due to poor colour stability and severe photoluminescence quenching. Both detrimental effects become increasingly prominent in perovskites with the high chloride content needed to produce blue emission. Here, we solve these critical challenges in mixed halide perovskites and demonstrate spectrally stable blue perovskite light-emitting diodes over a wide range of emission wavelengths from 490 to 451 nanometres. The emission colour is directly tuned by modifying the halide composition. Particularly, our blue and deep-blue light-emitting diodes based on three-dimensional perovskites show high EQE values of 11.0% and 5.5% with emission peaks at 477 and 467nm, respectively. These achievements are enabled by a vapour-assisted crystallization technique, which largely mitigates local compositional heterogeneity and ion migration. Achieving bright and efficient blue emission in metal halide perovskite light-emitting diodes has proven to be challenging. Here, the authors demonstrate high EQE and spectrally stable blue light-emitting diodes based on mixed halide perovskites, with emission from 490 to 451nm by using a vapour-assisted crystallization technique.