UC Davis

Next-generation photovoltaic devices based on semiconducting quantum dot materials offer the promise of record-breaking efficiencies, enabled by multiple exciton generation. However, the long-range charge transport necessary for high device performance remains limited. This limitation arises from energetic disorder caused by variations in quantum dot size, spacing, orientation, and level of connection to neighboring dots. On larger length scales, defects such as atomic and superlattice dislocations, zig-zag jitter, and wave-like oscillations further impact transport properties and overall device efficiency. To better understand and ultimately mitigate these structural challenges, we employ electron tomography, a technique that reconstructs a three-dimensional (3D) volume from a series of two-dimensional (2D) images taken at different tilt angles. This approach allows us to directly visualize the internal architecture of quantum dot assemblies with nanometer-scale resolution.Additionally, we extend our characterization methods to a ubiquitous structure in semiconductor fabrication: polycrystalline silicon embedded in amorphous silicon dioxide (SiO2). Here, we introduce a novel approach using valence-electron energy loss spectroscopy (VEELS tomography. By fitting linear models to EELS images, we enhance contrast between Si and SiO2, enabling detailed structural analysis at the nanoscale. Through these advanced imaging and analysis techniques, our work aims to provide critical insights into the microstructural origins of transport limitations in next-generation photovoltaic and semiconductor materials.