Abstract:
Within this cumulative dissertation the perspectives of all-inorganic caesium lead halide perovskites for novel applications in electroluminescent light emitting devices is investigated. The urge to increase the efficiency and stability of this material for devices is one of the most focused research topics in recent years. Especially the field of nanoscience offers completely novel approaches to functionalizing perovskites, and these will herein be exploited to optimize the system. The main focus will be on tailoring the ligand shell to control the properties of the nanocrystals. The underlying scientific phenomena will be investigated and utilized for more efficient light emitting devices.
Firstly, the underestimated aspect of structure in a nanocrystalline assembly will be discussed. By X-ray nanodiffraction the structural coherence of nanocrystals inside a superlattice was investigated. The experiments revealed an increasing distortion upon approaching the edges of the superstructure, which could be correlated to a significant hypsochromic shift of the photoluminescence signal. An increasing strain, induced by disorder, could be held responsible for the shift in luminance. These findings emphasize the importance of structure for the application of perovskites in a device as the preparation of structurally sound emitter layers emerged as a determining factor for the emitted light.
Subsequently, the ligand shell of the nanocrystals came into focus, because the dynamics on a particle surface are of uttermost importance when considering exchange reactions and solution processed fabrication of thin films. To reveal the dynamics, quasielastic neutron scattering and nuclear magnetic resonance experiments were conducted and combined. They revealed a highly dynamic surface equilibrium for nanocrystals functionalized with a zwitterionic surface ligand. Notably, the dynamics and morphology appeared to be tunable with the absolute ligand concentration. This could be explained by investigating the surface binding situation via small angle scattering methods. The zwitterionic character exhibited two possible scenarios, in which either one or both respective functional group(s) is tethered to the particle surface. This drastically affected the surface dynamics, in means of a tunable Fickian diffusion coefficient of the ligand.
During the logical next step, the native ligand shell of perovskites was exchanged with an accordingly functionalized organic semiconductor. The novel ligand partially compensated the surface dipole of the nanocrystal, which effectively lowered the charging energy. This facilitated the charge carrier injection and resulted in a significantly lower electronic resistance. These coupled organic-inorganic nanostructures could be successfully deployed in an electroluminescent light emitting device with increased efficiency.
Lastly, the stability of this novel material was probed because perovskites are intrinsically rather unstable towards external influences. By X-ray photoelectron spectroscopy, the decomposition mechanism of the particles enclosed in the native and exchanged ligand shell was determined. In the native case, the decay into the respective halide salts could be observed. However, the exchanged system exhibited an increased stability, in terms of a slower decomposition, and a different decay mechanism, i.e., a disproportionation of Pb2+ into Pb0 and Pb3+. The underlying phenomenon was again the stabilization through the partially compensated surface dipole. Additionally, it was found that a lattice contraction, induced by halide segregation, preceded the decay and likely also initiated it.
In summary, this thesis provides experimental evidence for novel approaches to functionalize perovskite nanocrystals for an optimized application in light emitting devices in terms of enhanced stability, electrical, and optical properties.
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