Synthesis, Characterization, and Passivation of Semiconductor Thin Films for Photovoltaic and Solar Fuel Applications

In the race to secure a renewable energy future, a variety of approaches to harness the power of sunlight have garnered significant research efforts. Semiconductors materials and devices have asserted themselves as a centerpiece in many approaches for harvesting solar energy, the most notable of which being photovoltaic and solar fuel applications. In both cases, the understanding and optimization of semiconductor material properties is paramount for the successful commercialization of next generation energy technologies. The performance of semiconductors is often challenged by the impact of material defects and resulting defect-induced trap states. In this dissertation, the characterization of material defects informs rational defect passivation of perovskite and tantalum nitride thin films for photovoltaic and solar fuel applications, respectively. State-of-the-art, high-performance perovskite solar cells (PSCs) contain a large amount of iodine to realize smaller bandgaps. However, the presence of numerous iodine vacancies at the surface of the film has been broadly shown to induce deep-level defects which limit the efficiency and stability of PSCs. In Chapter 2, modifying the defective surface of perovskite films with cadmium iodide (CdI2) effectively reduces the degree of surface iodine deficiency, and stabilizes iodine ions via the formation of strong Cd-I ionic bonds. This largely reduces the interfacial charge recombination loss, yielding a high efficiency of 21.9% for blade-coated PSCs with an open-circuit voltage of 1.20 V, corresponding to a record small voltage deficit of 0.31 V. The CdI2 surface treatment also improves the operational stability of the PSCs, retaining 92% efficiency after constant illumination at 1 sun intensity for 1000 h. This work provides a promising strategy to optimize the surface/interface optoelectronic properties of perovskites for more efficient and stable solar cells and other optoelectronic devices. Tantalum nitride (Ta3N5) has gained significant attention as a potential photoanode material; however, electrochemical performance and stability are challenged by high densities of defects and related trap states. Passivation of such defects has provided an enormous boost to the development of many semiconductor materials, particularly perovskites, but the understanding of defects and passivation thereof is still in its infancy for Ta3N5 materials and thin films. Utilizing drive-level capacitance profiling (DLCP) we reveal the spatial and energetic distribution of trap states throughout Ta3N5 thin films. Coupling DLCP with compositional and structural characterization we were able to attribute the high density of deep traps at surface of films to oxygen-related defects. This understanding informed our rational passivation via silatrane moieties capable of targeting the oxidized surface and ultimately suppressing trap density at the surface by two orders of magnitude and reducing free-carrier density of films by over one order of magnitude. The successful passivation of Ta3N5 films mitigates defect-induced charge trapping and recombination to ultimately improve material and device performance. Overall, the work reported in this dissertation demonstrates how detailed characterization and understanding of semiconductor thin films can be used to design rational passivation strategies capable of boosting material performance and stability and enable next-generation renewable technologies.Doctor of Philosoph