In-Situ and Operando Multimodal Microscopy of Metal Halide Perovskite Optoelectronic Devices
Metal-halide perovskites are materials at the forefront of the next generation of optoelectronic materials. Of particular interest is their remarkable power conversion efficiencies when incorporated into thin film solar cells. The properties of next-generation semiconductors such as perovskites are dominated by microscopic variations in their structure, composition and photophysics. Perovskites show extraordinary levels of disorder and this has considerable implications for their function. Gaining a microscopic understanding into how the optoelectronic quality of perovskite thin films and their interfaces with contact layers affects their performance is crucial to enabling solar cells with sufficient performance and stability to commercialise. In this thesis, I detail the development of a multi-modal microscopy toolkit to probe the optoelectronic quality of perovskite thin films and devices and spatially correlate these measurements with microscopic chemistry and structural information. In the first experimental chapter, I detail the capabilities of a hyperspectral, wide-field optical microscope, capable of measuring spatially resolved photoluminescence, reflectance and transmittance spectra with diffraction resolution. With a variety of perovskite thin film samples, I show that thin-film morphology and surface passivation play a huge role in photoluminescence intensity, spectrum and stability. The second experimental chapter applies calibration tools to the hyperspectral microscope, enabling the extraction of device relevant metrics such as the quasi-Fermi level splitting and Urbach Energy microscopically. We spatially correlate these measurements with nanoprobe X-ray diffraction and fluorescence to probe structure and chemistry. Applying this multimodal toolkit to state-of-the-art alloyed perovskites, we find that nanoscale variations in chemical composition dominate the optoelectronic properties of these perovskite films and form energetic funnels that carriers fall down and away from trap states. This study helps to explain the remarkable defect tolerance of these materials. The final experimental chapter augments the optical microscopy setup to measure voltage dependent photoluminescence maps. Voltage dependent photoluminescence allows the extraction of pseudo current-voltage curves of the devices, enabling the recombination and charge transport losses of perovskite solar cells to be mapped microscopically. I show that microscopic performance heterogeneity has a large impact on both macroscopic performance and stability. By mapping the same areas before devices before and after ageing, the microscopic effects of degradation on charge extraction can be imaged. Taken together, the results here show the important microscopic influences on performance from thin films to complete devices and the powerful multi-modal methodologies developed are widely applicable to a wide array of disordered semiconductors.
microscopy, perovskite, renewable energy, solar energy
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