All Inorganic Colloidal Perovskite Nanocrystals for Optoelectronic Applications

Abstract

This dissertation focuses on the optoelectronic properties and device performance of colloidal all-inorganic metal-halide perovskite nanocrystals (CsPbX3 NCs, where X=Br or I). These types of materials have shown promising performance in applications including photovoltaics (PVs) and light-emitting diodes (LEDs) due to their excellent optoelectronic properties, such as high photoluminescence quantum yields (PLQYs), high carrier mobility, long diffusion lengths, low Urbach energies and tuneable bandgaps across the visible and near-infrared wavelength ranges. In this thesis, I investigate how the surface chemistry of CsPbBrxI3-x NC systems is affected by the polarity of the antisolvent used during purification. I find that higher polarity antisolvents result in an increase in the selective etching of surface iodides, resulting in a decrease in PLQYs and blue-shift in the optical bandgap. Through detailed 1H nuclear magnetic resonance (NMR), X-ray photoemission spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM) measurements, I show that the cause for these effects is that higher-polarity antisolvents favour condensation reactions between the oleylamine and oleate ligands, resulting in their detachment, and the removal of surface iodides hydrogen bonded to these ligands. I show how the resulting increase in halide vacancies leads to not only increased non-radiative recombination, but also faster red-shifts in the electroluminescence (EL) peak from LEDs, due to faster ion migration. This work therefore identifies antisolvent polarity, particularly the use of low polarity antisolvents, as a critical parameter for controlling the surface chemistry of halide perovskite NCs. In the next Chapter, I investigate the effect of surface traps on hot carrier cooling kinetics, making use of polar antisolvents to intentionally introduce surface traps in inorganic CsPbBrxI3x NCs. Through energy-dependent PLQY measurements, I observe the increased tendency of the hot carriers to localise in the trap states as their excess energy is increased, especially for Br-based and mixed Br/I NCs. I extract the hot carrier cooling lifetimes for NCs with different composition and defect densities through pump-probe and pump-push-probe transient absorption spectroscopy measurements. These results, together with density function theory calculations, suggest that the defect tolerance found in band-edge charge-carriers can be extended to hot carriers. That is, the relaxation kinetics of hot carriers in materials with shallower defect states, such as I-based NCs, tend to be less influenced by the traps. This new understanding is important in the future design of systems that can extract hot carriers, and the ultimate realisation of hot carrier solar cells. In the final Chapter, I achieve the synthesis of CsPbI3 nanoplatelets with colour-pure emission. By developing a solution-based route to self-assemble these nanoplatelets and therefore control the isotropy of the transition dipole moment, I achieve the first demonstration of directly-generated linearly polarised electroluminescence (EL) from perovskite nanoplatelet systems. PbI2 precursors are used to passivate surface I-vacancies to reduce trap-assisted recombination and exciton-exciton annihilation. As a result, I achieve CsPbI3 light-emitting diodes (LEDs) with 2.7% external quantum efficiency (EQE; the highest reported for a strongly-confined perovskite nanoplatelet system). This enabled the demonstration of perovskite LEDs with a high degree of optical polarisation (DOP) of 74.4%. By comparing the polarised emission with weakly confined NCs and bulk films, I find that the degree of polarisation is also related to the exciton fine structure splitting energy which can be governed by exciton binding energy. Through these investigations, I offer new insights into how the surface defect chemistry of metal-halide perovskite NCs can be controlled, and how this influences hot-carrier cooling, which is important for ultimately realising hot carrier solar cells. This work opens up the possibility of developing linearly polarised light emitters that could be achievable across the entire visible and near-infrared wavelength ranges, which can be used for 3D displays and optical communication.