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2019 (1)

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Dissertation
Stability enhancement of CsPbI3 LEDs via direct laser writing

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Abstract

Technological advancements are paramount for solving the climate change predicament. Climate change is multi-faceted, requiring local and global solutions. Energy production and consumption are both important topics to address to reduce the load of our society and economy on the ecosystem. Advancements in solar energy production, as well as reducing the energy demand of lighting applications are on the horizon due to the advent of metal halide perovskites (MHP). In less than a decade, MHPs have taken its place in solid-state technology, as a suitable candidate material for photovoltaic (PV) cells and light-emitting diode (LED) lighting as a semi-conductor material. CsPbI3, one of the many perovskite materials, has the material properties worthy for commercialization as a perovskite material, due to its good thermal stability and low fabrication costs. Its small band gap (1.73 eV) is able to capture light from the entire visible spectrum. CsPbI3 has different polymorphs, where the perovskite forms (α-, β- or γ-phase) are only stable at higher temperatures. At ambient temperature, the perovskite structure is instable and quickly transforms to its non-perovskite form (δ-phase). The instability issue is the main hurdle to overcome. Direct laser writing (DLW) has proven to enhance the ambient stability of CsPbI3 perovskite, as shown by Steele et al. (2017). This work investigates the stabilizing effect of DLW on CsPbI3 thin films. Two parameters in particular, the size of the laser written grid and the laser power density, were varied to reveal their influence in the DLW stabilization of α-phase CsPbI3 thin films. It was shown that higher power densities will stabilize the α-phase more effectively. The stabilizing effect of DLW is inversely dependent on the grid size. Furthermore, many characterization methods were used to unveil the structure and elemental composition of the laser written material. No crystallographic or elemental analysis was able to distinguish the laser written material from the as-grown thin film CsPbI3, but SEM imaging revealed that the nano-crystal are no more present. Photoluminescence spectroscopy of the laser written material points to the formation PbO, or possibly highly stabilized γ-phase CsPbI3. Applying the DLW method in LED fabrication yielded no successful devices, however this is not necessarily caused by the DLW treatment, but has more likely to do with the difficulty of the fabrication process as a whole. The enhanced stability shown in this work is an encouragement to further develop the DLW technique, as well to further investigate the microstructural stability enhancements achieved at the interfaces of perovskites. These developments would improve the feasibility of commercial application of CsPbI3 and other MHP materials.

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