In polycrystalline thin-film cadmium telluride (CdTe) solar cells, atomic defects (dopants: copper (Cu), arsenic (As); and selenium (Se) alloy) have significantly enhanced hole density and minority carrier lifetime. Density functional theory (DFT) has predicted the atomic configurations of relevant defects and their electronic structures. Yet, experimental evidence of the defects, especially their spatial distribution across the absorber, is still lacking. Herein, since it can probe local atomic structure of elements of interest with trace-elemental sensitivity, nanoprobe X-ray absorption near edge structure (XANES) spectroscopy was used to elucidate atomic structures of Cu, As, and Se. After XANES spectra were measured from CdTe devices, the atomic information was extracted from the measured spectra by fitting them with reference spectra, which were simulated from 1) point defects and grain boundaries (GBs) predicted by DFT; 2) secondary phases which could form under processing conditions. XANES analysis of various device architectures revealed structural inhomogeneities across the absorbers from point defects to secondary phases. The majority of the Cu dopant atoms form secondary phases with surrounding atoms even inside the absorbers, explaining the low dopant activation. When entering the target lattice site (Cd), Cu forms a complex with chlorine (Cl) and becomes a donor defect, compensating hole density. Compared to Cu, As dopant tends to enter the target site (Te) more frequently, explaining higher hole density in As-doped CdTe. Notably, As on the Te site forms neutral charged complexes with Cl. Although they are not as detrimental as the Cu-Cl complex, the As-Cl complexes may be responsible for low dopant activation and compensation observed in As-doped CdTe devices. Complementary to the DFT prediction, this work provided the distribution of Se local structures across the absorber, specifically the variation of Se-Cd bond lengths in differently performing areas. Under environmental stressors (heat and light), it showed atomic reconfiguration of Se and Cl at GBs, and Se diffusion into the bulk, co-occurring with device degradation. This framework was also extended to study defect evolution in other thin-film solar cells (CIGS and emerging perovskite). XANES analysis has shed light on atomic defects governing solar cell performance and stability, which are crucial in pushing the efficiency toward the theoretical efficiency limit.

The hierarchical silica structure of the Coscinodiscus wailesii diatom was studied due to its intriguing optical properties. To bring the diatom into light harvesting applications, three crucial factors were investigated, including closely-packed diatom monolayer formation, bonding of the diatoms on a substrate, and conversion of silica diatom shells into silicon.

The closely-packed monolayer formation of diatom valves on silicon substrates was accomplished using their hydrodynamic properties and the surface tension of water. Valves dispersed on a hydrophobic surface were able to float-up with a preferential orientation (convex side facing the water surface) when water was added. The floating diatom monolayer was subsequently transferred to a silicon substrate. A closely-packed diatom monolayer on the silicon substrate was obtained after the water evaporated at room temperature.

The diatom monolayer was then directly bonded onto the substrate via a sintering process at high temperature in air. The percentage of bonded valves increased as the temperature increased. The valves started to sinter into the substrate at 1100°C. The sintering process caused shrinkage of the nanopores at temperatures above 1100°C. The more delicate structure was more sensitive to the elevated temperature. The cribellum, the most intricate nanostructure of the diatom (~50 nm), disappeared at 1125°C. The cribrum, consisting of approximated 100-300 nm diameter pores, disappeared at 1150°C. The areola, the micro-chamber-liked structure, can survive up to 1150°C. At 1200°C, the complete nanostructure was destroyed. In addition, cross-section images revealed that the valves fused into the thermally-grown oxide layer that was generated on the substrate at high temperatures.

The silica-sintered diatom close-packed monolayer, processed at 1125°C, was magnesiothermically converted into porous silicon using magnesium silicide. X-ray diffraction, infrared absorption, energy dispersive X-say spectra and secondary electron images confirmed the formation of a Si layer with preserved diatom nano-microstructure. The conversion process and subsequent application of a PEDOT:PSS coating both decreased the light reflection from the sample. The photocurrent and reflectance spectra revealed that the Si-diatom dominantly enhanced light absorption between 414 to 586 nm and 730 to 800 nm. Though some of the structural features disappeared during the sintering process, the diatom is still able to improve light absorption. Therefore, the sintering process can be used for diatom direct bonding in light harvesting applications.